Negative ions of polyatomic molecules.

In this paper general concepts relating to, and recent advances in, the study of negative ions of polyatomic molecules area discussed with emphasis on halocarbons. The topics dealt with in the paper are as follows: basic electron attachment processes, modes of electron capture by molecules, short-lived transient negative ions, dissociative electron attachment to ground-state molecules and to "hot" molecules (effects of temperature on electron attachment), parent negative ions, effect of density, nature, and state of the medium on electron attachment, electron attachment to electronically excited molecules, the binding of attached electrons to molecules ("electron affinity"), and the basic and the applied significance of negative-ion studies.

metastable atoms, Rydberg states) will not be discussed. The basic processes to be considered, then, can be classified as electron attachment to ground state molecules, to "hot" molecules, or to electronically excited molecules.
Electron attachment to ground state molecules predominantly in their lowest state(s) of internal excitation may be represented by the scheme of Eqs. AX-+ energy (3) ther the field of the ground (G) or excited (E) electronic state with a capture cross section ao(E), and E' (sE) is the energy of the scattered electron; the asterisk indicates excess internal energy and the asterisk in parenthesis indicates possible increase in the internal energy of the corresponding species. Reaction (1) is indirect elastic and inelastic electron scattering. Reaction (2a) is dissociative attachment leading to stable fragment negative ions (for a polyatomic molecule this process can lead to simultaneous multiple fragmentation). Reaction (2b) is dissociative attachment leading to metastable negative ion fragment(s) which are subject to autodetachment and/or autodecomposition. Reaction (3) is parent negative ion formation which is possible when the electron affinity, EA, of AB .. CD is positive (>O eV) and the excess internal energy is removed, principally by collision with another body. All three reactions have been studied extensively especially over the last decade. At room temperature the preponderance of AB .. CD molecules are in the , = 0 level. However, depending on the molecule, even a small population of higher vibrational and/or rotational levels can affect significantly the cross sections and the onsets, especially for reaction (s) 2. Electron attachment to "hot" molecules is summarized by Eq. (4).
e(E) + AB *.-CD* (G; v>0; j >0) 00,v,jLe) AB ...CD-* (G or E) -* decay (4) Here the molecule AB ... CD* is in the ground electronic state but in higher rotational, j, and/ or vibrational, v, states. In this case the cross section for formation G0,j(E) and the probability of the ensuing decomposition(s) depend on the vibrational and/or rotational quantum states. Reaction (4) is appropriate to "hot" gases and it has been investigated recently. Electron attachment to electronically excited molecules is summarized by Eq. (5). e(E) + AB ... CD*(E) AE() AB CD*(E) 4 decay (5) Here the electron is captured by an electronically excited molecule with a cross section aE (e) forming AB ... CD* -(E) in the field of an excited electronic state. Very little is known about this process.

Modes of Electron Capture by Molecules
The various ways via which slow electrons attach to molecules have been well reviewed (1)(2)(3)(4)(5)(6). In general, four such mechanisms leading to the formation of a transient negative ion [negative ion resonance (NIR) ] can be distinguished: shape resonances; nuclear-excited Feshbach resonances; core-excited resonances (type I); and core-excited resonances (type II).
In Figure  Environmental Health Perspectives 4 the molecule is negative (<0 eV), and the incident electron is trapped in the potential well which arises from the interaction between the electron and the neutral molecule in its ground electronic state. This potential barrier is the combined effect (Veffective in Fig. la) of the attractive polarization potential between the neutral molecule and the incident electron (Vattrive in Fig. la) and the repulsive centrifugal potential which arises from the relative motion of the two bodies (V,pU,,,,1 in Fig. la); this latter varies as 1(1 + 1) /2p, where 1 is the angular momentum quantum number and p is the electronmolecule separation. Since the negative ion potential energy curve/surface for a shape resonance lies above that of the neutral molecule, the transient ion is subject to autodetachment, decaying back to the neutral molecule in its ground electronic state plus a free electron, leaving the neutral molecule with or without vibrational and/or rotational energy. The lifetime 'Ta for this autodetachment process is a function of both the size of the barrier and the energy of the anion (i.e., the relative height and thickness of the barrier through which the electron has to penetrate). The former is strongly dependent on the 1 of the state occupied by the captured electron, and the latter largely on the attractive portion of the potential. If energetically possible, the NIR can undergo dissociative attachment. Such NIRs may involve an excited electronic state of the neutral molecule (i.e., electron capture is concomitant with electronic excitation of the capturing molecule) and in this case they are called core-excited (type II) resonances. In Fig. lb we illustrate schematically the nuclear-excited Feshbach resonance. Here EA is positive (>0 eV), and the NIR lies energetically below the ground state of the neutral. Thus, unless the anion is in vibrational levels v' higher than the lowest vibrational level v = 0 of the parent neutral state, the NIR cannot decay into the parent state. This mode of electron capture can involve an electronically excited neutral molecule, in which case the NIR is called coreexcited (type I).
Molecular NIR states are abundant. Often they can be described [and their energies (positions) approximated] in terms of the unoccupied molecular orbitals of the neutral molecule. At times also molecular geometrical changes concomitant with electron impact and strong electrophore sites in a polyatomic molecule can constitute effective modes of electron trapping (6).

Short-Lived Transient Negative Ions
Short-lived (< 10-l' sec) transient negative ions are a common phenomenon in low-energy electron-molecule collisions. In the incident electron energy range in which the temporal trapping of the electron occurs, the magnitude of the electron-scattering cross section changes profoundly, and the "resonance" signifies the existence and gives the position of the negative ion state. Such short-lived negative ion stateslying above the ground state of the neutral molecule-are abundant and decay by autodetachment and/or by dissociative attachment.
It is beyond the scope of this paper to discuss the theoretical treatments of NIR states of polyatomic molecules or to summarize the rather extensive recent experimental results. However, a few comments will be made on the former and a brief discussion will be given on the latter with specific reference to benzene and its substituted derivatives. For these molecules, as a rule, the nr-electron orbitals were considered to interpret the experimental observations. Consider, therefore, a ground state neutral molecule with N electrons forming an isolated negative ion resonance by capture of a slow electron. The captured [(N + 1)th] electron normally enters an unoccupied orbital and the wavefunction irN+1 of the resonant state resembles that of a bound state with N + 1 electrons. It can be expressed (1,7,8) as: In Eq. (6), 4'+ describes a bound (N + 1)-electron state embedded in and degenerate with the continuum (E; E-0), and a (K) is the coefficient for the continuum. If the continuum portion of 1N+1 is neglected, The (vertical) electron affinity (VEA) and the vertical attachment energy (VAE) can be approximated, respectively, by VEA = (1NIHN (8b) In expressions (8a) and (8b), 4N is the N-electron wavefunction for the target ground state, HN and H+l are the corresponding Hamiltonians, 5 EN is the total energy (less the center of mass translation) of the N-electron ground state, and EN+l is the total energy of the anion with the neutral ground state nuclear configuration. Grant and Christophorou (8) pointed out that although it is tempting to express eq. (8b) (via Koopmans' theorem) simply as VAEM E`N+l-EN = EmN (9) where EmN iS the Hartree-Fock (HF) orbital energy of the m-th unoccupied orbital of the Nelectron target, a careful consideration should be given to the effects of reorganization and correlation. If Ereorg is the reorganization energy associated with changes in the N-electron system due to the captured electron, and AEcorr is the change in correlation energy, VAEm =EmN + Ereorg + AEcorr (10) In attempts to find out how well semiempirical approaches explain the experimental results on the NIR states of aromatic molecules, it was found that a qualitative relationship exists between the experimental positions of the NIRs and the CNDO virtual-orbital energies for benzene and substituted benzenes (5,8), and the theory was successful in predicting the number and relative positions of the negative ion states of these systems (5,(8)(9)(10)(11)(12)(13)(14)(15). Semiempirical molecular orbital methods, however, provide only a modest understanding of the shape resonances of organic molecules. They need to incorporate the effects of correlation and reorganization and to consider the a-electrons along with the 7T-electron system. Ab initio calculations are indicated.
Let us now turn our attention to the experimental results on the shape resonances of benzene and substituted benzenes which can serve as a model group for other organic structures.
We designate the six nT-molecular orbitals of benzene (C6H6) T1, ..., TiT6; the three virtual (empty) 'ff-molecular orbitals are 1T,, 'T5, and 'fi6; in the D6, and DSh symmetry the orbitals .2 and 'ri3 and 'its and 'i5 are degenerate. The negative ion resonances of benzene can be generally understood by considering the capture of the incident electron into the 'r4,7it, and 'it6 orbitals.
Thus the observed (Table 1) NIRs of benzene were ascribed to the configurations 'n12n1227'32'n41 (first NIR), T2'7T22,'7T,2'7T1 (second NIR), and tl27t22T327T6i (third NIR) and were all characterized as shape resonances associated with the ground state molecule (the third NIR could contain an admixture of a core-excited resonance).
The introduction of a substituent onto ben-zene breaks the degeneracy of the 'T4, 7T, orbitals and allows two NIRs to be observed experimentally (Table 1) as opposed to only one for benzene. The magnitude of the splitting of the first and second NIRs depends on the electronegativity of the substituent. For electron-donating substituents (e.g., F, OH, CH3, NH2) the orbital energy of the antisymmetric 't-orbital remains virtually the same as in benzene or decreases while that of the symmetric 7T-orbital is raised relative to benzene. For electron-withdrawing substituents (e.g., CHO, NO2, COOH) three NIRs, rather than two, were observed below -2.5 eV ( Table 1). As indicated by Grant and Christophorou (8), in this case the antisymmetric 7T-orbital behaves essentially as described in the case of electron donating substituents, but the symmetric 'i-orbital interacts in-phase and out-of-phase with the carbonyl 7T* orbital to split further the energies and to provide an additional resonance (see Table 1). It seems, nonetheless, that for an understanding of the NIRs of benzene, benzene derivatives, and other organic molecules one must consider the a-electrons along with the 'T-electrons in both the negative ion and the neutral molecule. This can perhaps be illustrated by the recent results on the NIR states of fluorobenzenes (14) ( Table 1). Frazier et al. (14) kept the substituent (F) the same but changed the number of substituents around the benzene periphery from 0 to 6. Such an increase in halogen substituents has a profound effect on the basic properties of the molecular negative ions. The electron affinity and the parent negative ion lifetime, for example, increase, the former from c50 eV (5) to +1.8 eV (17) and the latter from picoseconds to microseconds (6) in going from C6H6 to C6F6. As expected, Frazier et al. (14) found the first and the second 'i-NIRs to be degenerate for benzene and hexafluorobenzene with DSh symmetry and for 1,3,5-trifluorobenzene with D3h symmetry. The energy of the lowest two NIRs is systematically lowered with increasing number of fluorine atoms (this is especially evident if one considers the weighted average position of the first and second NIR; see Table 1). The energy of the third NIR is little affected in going from C6H6 to C6F6; it is also little affected by the nature of the substituent (5,14). This indicates that in all cases the effect of the substituent on the carbon p orbital is minimal. On the basis of the data in Table 1, the first 't-electron affinity of the isolated molecules of all fluorobenzenes in Table 1 is negative. As in-Environmental Health Perspectives  (16,17) to be .+0.42 eV) and +1.8 eV, respectively. b The vertical position of the NIRs is given. When available, the transition energies from the lowest vibr,ational level, v = 0, of the neutral molecule to the lowest vibrational level, v' -0, of the negative ion is given in parentheses and can be considered to give the adiabatic value of EA.
'The first three NIRs are associated with the three unoccupied ir orbitals. The IT*CO is associated with an additional orbital resulting from an interaction of the carbonyl -r* orbital with one (syrmmetric) of the two lowest degenerate ir orbitals of benzene.
The lowest two NIRs for benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene are degenerate (see text). d Data of Frazier et al. (14). e Data of Christophorou et al. (9). f Data of Jordan et al. (18). g Data of Jordan and Burrow (13). h On the basis of the data in Table II of Frazier et al. (14). dicated above, however, the electron affinity of C6F6 is positive (+1.8 eV) and the parent ion C6F6-* is long-lived (Ta 12 psec) and is formed with a large cross section at thermal electron November 1980 energies (19). This would imply either that the lowest negative ion state of C6F6 (and perhaps of C6HF5) is not a 'i but a a, as suggested by Yim and Wood (20) from their electron spin resonance work in the liquid phase, or that it is associated with a molecular distortion. The former seems to be a more likely possibility and the involvement of the a-electron network is thus indicated. For the purpose of illustrating the effect of molecular structure on the position of the shape resonances of polyatomic molecules, we now draw attention to the experimental data selected in Table 2. These indicate that the presence of a double bond lowers the position of the NIR (i.e., it increases EA). The presence of an additional double bond further lowers the position of the NIR, but this lowering is a function of the distance between the two double bonds. It is evident from the data in Table 2 that the replacement of an H atom by a CH3 group raises the NIR (lowers EA). For both aliphatic ( Table 2) and aromatic ( Table 1) molecules, replacement of an H atom by an electron withdrawing group (or a halogen) lowers the position of the NIR. This lowering increases with increasing number of electron withdrawing groups (or halogens) so that EA eventually becomes >0 eV and long-lived parent negative ions can form principally by a nuclear-excited Feshbach resonance mechanism. It is thus seen that, by changing the number and the position of double bonds, especially by changing the number and the nature of the substituent(s) to a basic structure (ethylene in Table 2, benzene in Table 1), NIRs can be found to occur anywhere in the subexcitation energy region.
Dissociative Electron Attachment to Ground-State Molecules As indicated above, resonant dissociative electron attachment is visualized to proceed through a negative ion intermediate-formed by capture of an electron in a restricted energy range defined by a Franck-Condon transition between the initial (neutral molecule and electron at infinite separation) and the final state represent-  (23).
f Data quoted by Jordan and Burrow (13).
g Equated to the electron affinity, EA, of the molecule, and thus to the position of the lowest NIRs which can be a shape resonance or a nuclear-excited Feshbach resonance; EA values: 02 (25), (CN) 2C = C (CN)2 (26), C12C = CC12 (27).
Environmental Health Perspectives 8 ing the transient negative ion-which dissociates into (often multiple) neutral and negative-ion fragments. The reaction had been studied extensively in recent years both experimentally and theoretically. In this section we elaborate briefly on certain aspects of this most interesting process.
The cross section, da.(E), for dissociative attachment as a function of electron energy, E, is equal to the product of the cross section, a0 (E), for capture of the electron by the molecule to form the metastable negative ion intermediate and the probability, p (E), that the transient negative ion will decay via dissociative attachment (rather than via autodetachment or the other possible decay channels), viz., Expressions for a, (E) and p (E) have been given for diatomic molecules. Following O'Malley (28) we write for ada (E) for a diatomic molecule AX initially in its lowest vibrational level (v = 0) and with a purely repulsive negative ion state (Fig. 2): In Eq. (12) m is the electron mass, h is Planck's constant divided by 2-r, g is a statistical factor, ra is the total autodetachment width, rP is the partial auto-detachment width, Pd is the dissociative attachment resonance width, 4o = E0 + lhh (see Fig. 2), E, is the electron energy at the peak of da (E), ½hhw is the zero-point energy and e-P`E), referred to as the survival probability, is now explicitly defined as Re RC FIGURE 2. Schematic potential energy diagram illustrating the process of dissociative electron attachment for a diatomic molecule. and ta= h/ra Although Eq. (12) is limited, strictly speaking, to situations as pictured in Fig. 2, it relates in a general way Gda to a0, E, Tre, Ta, and the reduced mass Mr of the A -Xsystem. It explains nicely the various isotope effects observed in dissociative attachment studies (2) such as those in Figure 3 on hydrogen halides and their |=exp-RC a (R) dR In Eq. (13), RE is the interatomic separation at which electrons of energy E reach the negative ion state (within the Franck-Condon region;    (29). The cross section for the deuterated analog in Fig. 3 are lower, since is is longer (i. o Mr"'2) and the AX-* has a larger chance to decay by the competing process of autodetachment. Negative ion potential energy curves/surfaces have a variety of shapes, and Ad5(E) is affected by these and the competitive decay channels. This limits the applicability of Eq. (12) especially in the case of polyatomic molecules. However, in certain cases of dissociative attachment to polyatomic molecules such as n-CnH2n+,Br, where the formation of Br-involves a direct fast cleavage of Br-along the C-Br coordinate, and the complex molecule may be visualized, as far as this process is concerned, as a diatomiclike R (n-CnH2n+1) -Br system, Eq. (12) may describe the process satisfactorily. Indeed, the relative magnitudes and the widths of the cross sections for the formation of Brvia resonance dissociative attachment to n-CnH2n+1Br (n = 2 to 6) shown in Figure 4 are consistent with their predicted dependences on the reduced masses (30). It should, of course, be noted that In the case of polyatomic molecules, many types of fragment ions can be produced and dis-  sociative attachment processes may not be fast; the dissociation time would depend on the distribution of the internal energy in the transient intermediate, the structural rearrangements following electron capture, and the orbital in which the electron is captured. The modes of decomposition and the nature of the fragment negative ions depend, thus, strongly on the details of molecular structure. We may identify four types of dissociative attachment fragment negative ions: directly cleaved, complementary, multiple, and rearrangement. These can be stable or metastable, subject to subsequent autodetachment and/or autodissociation.
An example of directly cleaved negative ions are the directly cleaved atomic halogen negative ions from the aliphatic hydrocarbons ( Fig. 4) (30,31), the production of which involves no internal rearrangement of the negative ion intermediate following electron capture. Additional examples are the ions Ffrom CF4 and H-from CH4 shown in Figure 5. In Figure 5 we can see also examples of complementary fragment negative ions (e.g., F-and C3F7-, and CF3and C2F5from n-C3F8; Fand CF3from CF4), as well as examples of multiple fragment ions (F-, CF3-, C2F5-, C3F7from n-C3F8 since they all seem to originate from the decay of the same NIR state). It should be observed that category of complementary fragment negative ions is a special case of the category of multiple ions. This category is particularly important, in that it signifies an extensive molecular decomposition which requires concentration of internal energy in more than one coordinate.
The data collected in Figure 5 help us make a few other observations. (a) The ion F-from CF4 and n-C3F8, and more generally the atomic halogen negative ion (s) from saturated aliphatic halocarbons (31) are the most abundant. Parent negative ions were not observed in electron-impact studies of the halogenated lower members of the saturated aliphatics. (b) The energy onsets and yields of complementary ions seem to show some correlation with the electron affinity of the fragments. (c) Perfluorination of a hydrocarbon lowers the positions of the negative ion states (this is supported by a large body of other information) (Table 1) (6,14,43). (d) The absence of CH3is consistent with EA(CH3) < 0 eV. (e) From the positions of the lowest excited electronic states of CF4 and CH4 shown in Figure 5 it seems that the observed dissociative attachment resonances are associated with the ground-electronic state of CF4 and with an excited electronic state of CH4.
Although the F-is the predominant ion for the Environmental Health Perspectives  FIGURE 5. Dissociative electron attachment cross section as a function of electron energy for n-C3F8, CF4 and CH4. The data plotted are from the following sources. n-C3F8: The energy dependence and the relative yield of the F, CF3-, C2F5-and C3F7 ions are from Lifshitz and Grajower (32). These relative cross sections were put on an absolute scale by using the value (33) 5 x 10-11 cm2, for the peak of the Fresonance. CF4: The energy dependence and the relative yields of the F-and CF3ions are from Lifshitz and Grajower (34). These were normalized (33) to 6 x1iO19 cm2 at the peak of the Fresonance at -~6.2 eV. CH4: Data are from Sharp and Dowell (35). The positions of the lowest two electronic states for CH4 and CF4 are from Harshbarger (36)  lower members of the saturated aliphatic perfluorocarbons*, this is not so for the unsaturated or cyclic perfluorocarbon compounds (44). For the latter, EA > 0 eV, and the parent negative ion is by far the most abundant. This can be seen from Figure 6, where the parent and fragment negative ions produced in collisions of slow electrons with perfluoro-2-butyne (2-C4F6) are shown (44). The 2-C4F6 molecule is linear with freely rotating CF3 groups (46). The directly cleaved F-and the complementary C3F3-and CF3ions are very weak compared with the parent ion 2-C4F6-. This is the case for other recently studied unsaturated and cyclic perfluorocarbons (44). Thus it is seen from Figure 7 that the parent ion of c-C4F8 is *The parent negative ion of n-C4F1O has been observed (45).
No parent ion of CF4, C2F6, or n-C3F8 has been reported. much more abundant than any of the observed fragments. The F-and (M -F) -(parent molecule less one F atom)-have low yields [actually the (M -F) -ion was not observed] for c-C4F8 (44), but they are the most intense ions for linear saturated aliphatic perfluorocarbons. For c-C4F8 the C2F3ion, which is characterized as a rearrangement ion, was observed; its production requires geometrical changes in the transient negative ion.
All six types of fragment negative ions seem to be produced when slow electrons collide with 2-C4F8 (perfluorobutene-2) (see It is worth pointing out again that for cyclic and unsaturated perfluorocarbons the parent ion is the predominant ion while for saturated perfluorocarbons the predominant ion is F-. From Figures 6 to 8 and our data published elsewhere (44) it is also indicated that the fragmentation of linear and coplanar perfluorocarbons is lower compared with the cyclic and twisted compounds. This, in turn, indicates the importance of configuration and geometrical rearrangements; in this regard it is worth observing that the work of Sauers et al. (44) on 2-, 1,3-, and c-C4F6 and 2and c-C4F8 indicated that double-bonded structures undergo more extensive fragmentation at low energies compared to cyclic and triple bonded systems.
The multiplicity of negative ion states and the production of a variety of negative ion fragments with large cross sections clearly show the fragility of metastable polyatomic negative ions and the effectiveness of slow (including thermal) electrons to cause extensive and often multiple molecular decompositions tantamount to a molecular "explosion." This and the delicate dependence of the type and abundance of negative ions on the details of molecular structure can, perhaps, be il- be metastable with an autodetachment lifetime of 14 + 3 ,usec). The Cl-ion was by far the most abundant. The intensities of the Cl2-and (M -Cl)-ions with respect to Cl-depended very strongly on the number and relative positions of the Cl atoms in the molecule. The yield of Cl2was, as a rule, very much lower when the two Cl atoms in the Cl2ion originated from the same C atom. The relative cross sections for Cl-production (corrected for the finite width of the electron pulse) are shown in Figure 9 for the chloroethylenes. They indicate the existence of at least five NIRs below -2 eV which may be associated with orbitals dominated by the p-orbitals of the Cl atoms (similar results were obtained for chloroethanes) (31). Since the observed resonances in the Cl-cross-section functions seem to be associated with the Cl-atom-dominated molecular characteristics, the observed fragmentation patterns for these molecules may be common  atmospheric products and elucidates the reaction pathways of such pollutants in the atmosphere (2, 47). The decomposition mechanisms responsible for specific fragment negative ions in complex molecules and the amount of translational and internal energy in the fragments depends on the decaying negative ion state and the amount and distribution of the internal energy in the excited negative ion. In spite of recent work in this area, much knowledge is still needed concerning the patterns of multiple fragmentation of molecular negativeion states and their relation to molecular structure. Of particular interest in this connection is the work of Franklin and co-workers (48-50) on the measurement of the translational energy of a number of negative (and positive) ion fragments (51). This work has shown that for a transient polyatomic molecular negative ion with N vibrational degrees of freedom only a fraction a is effective, so that the sum, Z,, of the translational energies of the fragments is it = E*/aN (14) In Eq. (14), E* is the total excess (of the heat of the reaction) energy in the negative ion intermediate, and aN may be assumed to be the number of active vibrational modes. Franklin et al. reported values of a for various molecules ranging from 0.35 to 0.57 with an average of -0.40. This was taken to indicate that the effective number of vibrational degrees of freedom in the metastable intermediate is less than the available number N. Although this may be, in part, attributed to a dissociation time which is much shorter than the time for complete internal energy equilibration, it is also apparent that the effective number of vibrational degrees of freedom-and hence the value of a-must depend on the spatial distribution of the wavefunction describing the orbit occupied by the captured electron. Franklin's findings on a values less than one are in accord with independent work on the decomposition of transient negative ions not by autodissociation but by autodetachment (6,54). In the latter studies basically the same model for statistical distribution of E* was used, and although the transient negative ion considered (C6F6-*) lives for -12 ,sec (6, 52) the effective number of degrees of freedom was found to be -1/3 of the available N.
Studies on the internal energy distribution within-and partitioning of excess energy among the decomposition products of-the transient negative ion are necessary for the deduction of accurate thermodynamic data, and the elucidation of the fragmentation processes themselves. Relevant, in this regard, is the recent conclusion of Goursaud et al. (53) that in the dissociation of triatomic negative ions, the partitioning of the excess energy favors the kinetic energy if the surface is attractive. That is, the translation to rotation-vibration coupling is weak for repulsive surfaces and strong for attractive surfaces whatever the details of the shape of the surfaces.
Finally, attention is drawn to Figure 10, where dissociative attachment cross sections as a function of electron energy are presented for a number of species in the energy range from 0 to -16 eV. We point out three of the salient features of the behavior of ada: the magnitude of ada-varying by more than seven orders of magnitude-depends on the position Emax of the NIR; the higher the Emax the smaller the ada (2, 54); as -m-* thermal, ada * X792; many molecules possess a multiplicity of NIRs in the subexcitation energy region.
Dissociative Electron Attachment to "Hot" Molecules (Effects of Temperature on Electron Attachment) Electron attachment processes are often affected by the gas temperature T.  (70), who assumed that the direct effect of T on 02 is to produce a Maxwellian distribution of vibrational (v) and rotational (j) states The cross section cda.vj (e) is (28,70) GJdatj(E) = 472g r1ax X (R' -ir-12e-P (16) where Xv is the vibrational wave function, ra,x is the partial autodetachment width for the state X, and the rest of the symbols are as defined earlier.
In his treatment, O'Malley considered the effect of rotational states to be negligible and the excellent agreement of his predictions on the threshold, magnitude, width, and energy position of the 0°-02 resonance with the experimental results justifies this assumption. As T increases, higher vibrational levels are populated, the internuclear distances increase (and hence the Franck-Condon region broadens) significantly and although even at 2000°K there is only a limited amount of vibrational excitation, there is a profound change in the survival factor e-P in Eq. (16) which dominates the temperature dependence of the cross section. The insignificance of rotational excitation seems to be at variance with earlier theoretical predictions (71) of a strong effect of rotational excitation on the yield of H-/H2. In this regard recent experimental (72) and theoretical (73) work on H-/H2 in the 3.75 eV H2-resonance region are quite relevant. Thus the threshold cross section for the 3.75 eV H-/H2 resonance was reported to have shown (72) an increase by a factor 210 with each increase in vibrational quantum and a threefold increase for rotational level increase from j = 0 to j = 7. The results of a recent calculation employing resonance scattering theory with semiempirical parameters (73) shown in Table 3, indicate that although the effect of rotational excitation is not as high as that of vibrational excitation and not as significant as suggested earlier (71) it is, nonetheless, considerable.
Two additional examples can help demonstrate the effects of T on electron attachment processes. The first is on the effect of T on the production of 0from N20 below -4 eV and the second is on the effect of T on the total attachment cross section for various polyatomics.
It is seen from Figure 12 that the production of 0from N20 is very sensitive to T in one region (close to thermal and epithermal energies) and insensitive in another (-2.3 eV). It appears that two states of N20are involved in the production of 0from N20 below -4 eV. The strongly temperature sensitive portion of the cross section involves the lowest (ground) state of N20and is   due to excitation of the bending mode of vibration (60); it is thought to arise from the dependence of bond angle of the energy separation of the electronic ground states of N20 and N20 (the potential energy of the lowest N20state depends significantly on bond angle) (60, 66, 74). The temperature-independent peak at 2.25 eV is ascribed to dissociative attachment via the second N20state. These findings, along with those on the temperature dependence of dissociative attachment to 02, demonstrate the significance of temperature studies in determinations of true onsets for specific product ions and the deduction therefrom of thermodynamic data. In Figure 13 is shown the variation with T of the integrated total attachment cross section for various polyatomics studied by Spence and Schulz (63). Three observations can be made with respect to Figure 13: (1) the spread in the magnitude of the total attachment cross section seen at room temperature decreases as the temperature Environmental Health Perspectives increases; (2) the smaller the magnitude of the attachment cross section at room temperature the larger seems to be its temperature dependence; and (3) at near thermal energies the attachment cross section for polyatomics approaches the theoretical maximum value (Fig. 10)  deal of information about the dynamics of electron attachment processes and the influence of the environment on them (see below). In the present section we deal briefly with long-lived and in the next with moderately long-lived parent negative ions.
The cross sections, the lifetimes, and the dependence of these two quantities on the energy of the captured electron for long-lived parent negative ions depend strongly on molecular structure. Long-lived parent negative ions are usually formed via a nuclear-excited Feshbach resonance mechanism and their cross sections are very large at thermal and epithermal electron energies (2, 6, 75, 76). The electron swarm method has been shown (2, 6, 75) to be uniquely suited for the detection of these ions and for the accurate determination of the NIR states that lead to their production along with their respective cross sections. This capability of the swarm method has been greatly enhanced by the swarm-unfolding technique (78). To illustrate these points we refer below to some recent work on perfluorinated hydrocarbons (PFHs) at the author's laboratory (75,76,79). In Figure 14 the attachment cross sections, aa(6), as a function of electron energy E, and the attachment rate, aw(<E>), as a function of the mean electron energy <E>, are presented for three PFHs with double and triple bonds. It is evident from these data that at least three NIR states exist below -1.2 eV for these molecules. 18 magnitude of Ga (E) for cyclic PFHs (compare data in Figs. 15 and 16). The cyclic structure of the latter seems to be itself the important factor in increasing their ability to attach slow electrons. This can be seen also from the data on cyclic and linear C7Fj4 in Figure 17.
Although an increase in the size of the perfluorocarbon molecule could increase Ga (E), any such effect for the compounds in Figures 15-17 seems to be overshadowed by the much larger increase in Ga (E) at near thermal energies when CF3 is substituted for F ( Fig. 15; C6F6 vs. C6F5CF3 in Fig. 16). For the cyclic PFHs the number of double bonds or the aromatic character of the molecule have no noticeable effect on Ga (E). From the data listed in Table 4 on the electron-attachment properties of the PFHs in Figures 15 to 17, it is apparent that for these molecules the number and the energy positions of the NIRs vary little from one structure to another in contrast to the  Table 4 with those of their nonfluorinated analogs it is clear that perfluorination of a hydrocarbon structure dramatically increases Ga (E) at low energies. Actually the attachment cross sections and the attachment rates for the PFHs in Table 4  where Ntorr is the number of attaching gas molecules per cubic centimeter at 1 torr pressure, fN2 (E, (E)) is the electron energy distribution function at (E), A is the electron de Broglie wavelength divided by 21T, and the rest of the symbols are as defined earlier. The cross sections and the attachment rates in Figures 14 to 17 are total, i.e., for all ions produced. At thermal and near ther-IERGY ev) mal energies they are mostly due to parent ions, but at higher energies (especially around the third NIR) they may contain varying degrees of contributions from dissociative attachment processes (depending on the molecule) as indicated by the electron beam studies.
A number of experimental techniques em-z_ ployed in electron attachment studies do not pro-4.2 1.4 vide-as the electron swarm method of the Oak Ridge National Laboratory Group (2, 6) doesa function of the cross section and the attachment rate as a -C5F8), per-function of electron energy, but rather only the (C7F8) , and "thermal value," (aw)th, of the attachment rate. or the same The thermal attachment rate is a strong function   (92). ' Data of Gant and Christophorou (19). v Data of Christophorou (6).
hydrocarbons it is a strong function of the nature and number of the halogen (s) in the molecule. As can be seen from the selected data on (aw) th for halomethanes in Table 5, (aw) th increases in the order F < Cl < Br < I and with the number of the halogens in the molecule. The thermal value of aw can be determined accurately when Ga (E) iS known from where fM (E) is a Maxwellian function characteristic of the temperature of the experiment. Values of (aW)th determined this way for the PFHs in Figures 14-17 are given in Table 4. In Table 4 are also listed the autodetachment lifetimes of the parent ions of these PFHs.
The time the captured electron is retained by a polyatomic molecule reveals a great deal about the interplay between the two. Knowledge of 1Ta and its dependence on the electron energy can be used as a probe of the dynamics of molecular structure and the details of the electron attachment processes. In this regard attention is drawn to the recent review by the author (6) of the autodetachment lifetimes of metastable negative ions, and to the data of Johnson et al. (102) on nitrobenzenes (Table 6). Nitrobenzenes attach strongly thermal and epithermal electrons and form long-lived parent negative ions (unless a fast dissociative attachment process effectively competes). The autodetachment lifetimes of these parent ions show an increase with increasing EA, as predicted by the theory (6). Also, they show a distinct dependence on the electron donoracceptor properties of the substituent X and the intramolecular interaction between NO2 and X. The Ta values for electron accepting substituents Environmental Health Perspectives  are about ten times larger than those for electron donor. The large values of Ta for o-nitrophenol, 0donating substituents. Actually, CNDO calcu-nitrobenzaldehyde, and o-nitroaniline (Table 6) lations by Johnson et al. (102) indicated that the indicate the high sensitivity of r. to the intramagnitude of Ta correlates with the amount of 7r-molecular interaction of NO2 and X at the ortho electron charge removed from the benzene ring position. by X when X is an electron acceptor and is little The measured lifetimes of long-lived parent affected by the amount of 'r-electron charge do-negative ions and their dependence on e exnated to the benzene ring when X is an electron amples of the latter are shown in Figure 18-have  been discussed and rationalized (6) within the framework of the quasi-equilibrium theory. The theory restricts itself to the intermediate state (i.e., the transient negative ion) and does not consider in detail the entrance (initial electron capture process) and the exit (final autodetaching process) steps both of which depend strongly on selection rules and the symmetry of the state (s) involved (6). These aspects require theoretical attention.
Effect of Density, Nature, and the State of the Medium on Electron Attachment It has been pointed out by the author earlier that a crucial step in our efforts to link physics with chemistry and both with biology is to know how "isolated-molecule" properties change as the 22 molecule finds itself in gradually denser and denser gaseous and finally, in condensed-phase environments. The interfacing of the disciplines of physics, chemistry and biology requires linking physical to chemical and biological environments, i.e., it requires interphase studies.
Electron attachment to molecules and other processes and molecular properties involving separated charges, are the ones most dramatically influenced by both the density and the nature of the medium in which the electron-molecule interaction takes place. Of the electron attachment processes referred to above, the nondissociative electron attachment process involving a transient ion AB ... CD-* which requires collisional stabilization, is obviously the one (though not the only one) to show a strong dependence on the medium. Consider, then, the e, AB ... CD system to be em- Although reaction (20) has been found to describe reasonably well the [M] dependence of R for some systems (e.g., the formation of°2in C2H4) over the entire density range from a few torr to the liquid density (106) in many other cases (77,(106)(107)(108)(109) cases the formation of AB ... . CDdepends on [M] in a complex fashion. Not only is M involved in distant collisions as a stabilizing third body but it may also be involved in the initial electron capture process affecting k, both in magnitude and energy dependence via its effect on the negative ion states of the attaching molecules. Also, both Ta and k8t may be functions The effect of the density of a gaseous medium on the rate of attachment of electrons to molecules embedded in it can be seen from the examples in Figures 19 and 20 for, respectively, 02 Environmental Health Perspectives in N2 (forming 02-) and SO2 in N2 and C2H4 (forming SO2-). The effect of the density of the medium is a function of the nature of both the capturing species and the gaseous medium in which it is embedded as can be seen from these data and also from Figures 21 and 22, where the rate of attachment at a fixed electron energy is plotted against the density (compressibility-corrected pressure) of the medium.
Measurements, such as those in Figures 19-22, taken over a large density range can be modeled and can allow deduction of information on (1) the competitive decay channels of the metastable negative ions and their respective rate constants,  Figure 23, where the rate, k, for formation of the C2H,Br-* ion, and the ratio of the rate constant, k2, of dissociation of C2H,Br-* to the rate constant k3 of autodetachment of C2H5Br-* as a function of the mean electron energy (e) are plotted. For this molecule autodetachment, dissociation and stabilization of C2H,Br-* were assumed to proceed via a single NIR state and to be in competition (108). It is interesting to see that the autodetachment of C2H5Br-* predominates below and the dissociation of C2H5Br-* predominates above the peak of the negative-ion resonance at -0.8 eV.
To illustrate the second point, we list in Table 7 the lifetimes of three parent negative ions determined from such studies. No direct method is currently available for the measurement of lifetimes of negative ions shorter than -1 ,sec. The lifetimes in Table 7 were determined by assuming that the rate constant for stabilization of the transient negative ion in collisions with the molecules M, of the gaseous medium is equal to 27r (e2a/ Mr) 1/2p, where a is the static polarizability of M, e the electron charge, Mr the reduced mass of the negative ion and M, and p the probability of stabilization of the negative ion per collision with M. In certain cases, p is close to 1 (111) but since in general, p is likely to be <1, the lifetimes in Table  7 are lower limits to their true values.
In connection with the third point above, the rates of attachment of thermal electrons for 02 in C2H4, SO2 in N2 or C2H4, and C6H6 in N2 at densities equal to those of the respective liquids, are given in Table 7. The thermal attachment rate for 02 in C2H4 at liquid ethylene density compares well with the rate of attachment of thermal electrons to 02 in nonpolar liquids (77).
A full account of these important aspects of negative ion studies and also of the relevance and significance of electron attachment processes in gases to those in liquids can be found in the literature (77,(112)(113)(114). The involvement of vertical transitions in electron attachment to molecules dissolved in liquids in a manner analogous to that in gases is clearly indicated. existing measurement is that of Burrow (115) on the dissociative attachment to 02 excited in the lowest (metastable) a'Ag state which lies 0.98 eV above the ground state, X'3g , of 02. Burrow found the dissociative attachment cross section (at its maximum) for the reaction e + 02 (a'A) 02-(21j) 0-(2P) + 0 (3P) (22) to be 3.5 + 1 times larger than the maximum from the ground state 2, i.e., e + 02 (X3g-) 2-(2l1u) 0-(2p) + 0 (3p) (23) Dissociative attachment to electronically excited molecules naturally have lower energy onsets and should be significant in cases where the population of low-lying metastable states is high.
Binding of Attached Electrons to Molecules ("Electron Affinity") The electron affinity (EA) of a molecule is normally defined as the difference in energy between the neutral molecule plus an electron at rest at infinity and the molecular negative ion when both, neutral molecule and negative ion, are in their ground electronic-, vibrational, and rotational states (2). The electron affinity can be positive (>O eV) or negative (<0 eV) (see Fig. 24). Two other quantities, the vertical detachment energy (VDE) and the vertical attachment energy (VAE) are closely related to EA, the former when EA is positive and the latter when EA is negative. The VDE is defined (2) as the minimum energy required to eject the electron from the negative ion in its ground electronic and nuclear state without changing the internuclear separation, and the VAE is defined (2) as the difference in energy between the neutral molecule in its ground electronic vibrational and rotational states plus the electron at rest at infinity, and the molecular negative ion formed by addition of the electron to the neutral molecule without allowing a change in the internuclear separation of the constituent nuclei. The schematic diagrams in Figure 24 clarify the physical significance of these quantities and their relation to the various modes of electron capture discussed above. Figures 24a, b, c refer to an electron captured in the field of the ground electronic state of an atom (Fig. 24a) and a diatomic molecule (Fig. 24b and  c). In all three cases the EA is positive and the traditional measurements of EA are typical of these cases. In Figure 24a EA (A) = IVDE (A-) I; this is always the case for atoms. For molecules, however, EA (AX) = IVDE (AX-) only if the equilibrium internuclear separations for AX and AXare the same (as shown in Fig. 24c). Although for some small molecular species this is Environmental Health Perspectives sponding excited species positive (2)(3)(4). It would be interesting to measure the vertical detachment energy VDE (AX*-) of AX*-. In Fig. 24f a case is depicted which is appropriate for a shape resonance. It should be noted that although in this case VAE (AX) c -EA (AX), the electron is temporarily bound to the molecule with a "transient binding energy spectrum" shown by the shaded area in Figure 24f. The coordinate sys-0 tem depicting the potential well is indicated in the figure by RAXe, the radial distance of the electron which is different from that of the internuclear separation.
The electron affinities of atoms and molecules and the methods used for their determination have been reviewed by many authors (2,(116)(117)(118)(119). It is apparent from these reviews and from relevant (and abundant) literature that up until the last decade EA-values were quite inaccurate. Recently, however, accurate determinations of electron affinities have become possible. It should be pointed out that depending on the sign of EA and the species, different experimental methods are appropriate and accurate. In our opinion, to date three experimental methods (see below) pro- 28 vide accurate values of EA, each appropriate to the specific cases exemplified in Figure 24. Equi-; a H4; the case (2,116), in the majority of molecular species a situation similar to that in Figure 24b  Figures 24d and e illustrate the case of electronexcited Feshbach resonances (core-excited type II) for atoms and molecules, respectively. Here the electron is captured in the field of an excited atom A* or an electronically-excited molecule AX*, forming, respectively, A* and AX* (note our nomenclature: A* and AX* indicate, respectively, a negative ion formed by electron capture by an electronically excited atom or molecule, while A* and AX* indicate, respectively, an atomic and molecular negative ion with excess internal energy). EA (A*) and EA (AX*) now refer to the electron affinity of the excited atom A* and the electronically excited molecule AX*; they are both positive and can be greater than those of the corresponding ground-state species. Actually the electron affinity of the ground state atom or molecule can be negative while that for the corre- librium methods may allow accurate determination of EA also, in cases which are free of uncertainties due to other unaccountable electron attachment processes.

Photodetachment Methods
Photodetachment methods (especially those employing laser techniques) are by far the most accurate (2,116). Generally, these methods are restricted to the cases shown in Figure 24a and 24c, and their use to determine the electron affinity of polyatomic species (where situations such as depicted in Figure 24b are likely) is, as a rule, quite uncertain (40,120). Recent work however, ,o (121,122) indicates that even for polyatomic molecules photodetachment methods can often provide accurate values of EA.

Charge-Transfer Techniques
Charge-transfer techniques (17,123,124) and collisional ionization of molecules, M, using fast neutral alkali atoms, A (the EA value is deduced from the threshold of initiation of the reaction A' + M --A+ + M-where A' is usually Na, K, Cs) seem to be most appropriate, especially the latter, for polyatomic molecules (125)(126)(127)(128). (Proper internal energy and Doppler effect corrections are necessary for accurate determination of EA by charge-transfer technique.) Recent results on some halomethanes obtained by using alkali metal neutral atom beams are given in Table 8.
Substantial as the theoretical work has been in this area, its results, presently, lack the accuracy of the recent experimental techniques. Attention is drawn to a recent review by Simons (129).

Basic and Applied Significance of Negative Ion Studies
Negative ion studies-encompassing a wide range of processes and novel reactions through which slow electrons and molecules embrace each other in rich and delicate ways-have attracted intense recent interest and have resulted in stimulating new advances in our knowledge of the  structure of matter and the interaction processes involving electrons and molecules. This is so because such studies are of unique intrinsic value and by their very nature they are trans-disciplinary covering and illuminating broad areas of physics, chemistry and biology. Of equal importance is the unique (and often direct) application they find in many fundamental areas of technology, chemical and electrical engineering, life and radiation sciences, the atmosphere and the environment. In physics, negative ion studies provided basic knowledge on the properties of atoms and molecules, and the dynamics of electronic structure and molecular interactions. They aided the understanding of photophysical processes, the fates of electronic excitation energy, charge-transfer states and complexes. Similarly, they helped in the identification of basic reactions in upper atmosphere, space and the interaction of radiation with matter. They contributed fundamentally to the advancement of electron physics and gaseous electronics, led to new concepts of possible practical utility such as the ion and electron lasers, and the physics of instrumentation especially in the area of radiation detectors and particle accelerators.
In chemistry, negative ion studies contributed substantially to the understanding of the quantum and electronic structure of molecules, chemical reactions, the nature of the intermediates and fast reactions in radiation chemistry. They provided a test of and a challenge for molecular orbital theory and a probe of both the gaseous and the condensed phase of matter. They yielded useful thermochemical and other data and served as the basis for a rich variety of analytical methods including those for detection, identification, quantification and removal of impurities in a variety of systems.   (127).
In biology, negative ion studies played a basic role in clarifying the concepts used in theoretical treatments of biological action of molecules and provided key inputs to recent advances in radiation protection and sensitization, especially in the development of so-called "high affinic" drugs and radiopharmaceuticals. They have potential application in the area of biological and toxic action of molecules by properly using electron attachment processes and their products and reactions as probes of specific mechanisms and functions. They may, hopefully, lead to what we call an "electron capture-based toxicity index" for screening of toxic substances. Reactions involving parent negative ions, radicals and fragment negative ions from dissociative attachment processes, dynamics of electron capture, electron release and electron transfer and the role of these in interrupting the electron transfer cycles in the cell can all be grouped together under what we consider a potentially promising future research area which might appropriately be called bio-ionics.
In engineering and applied science, negative ion studies constitute the foundation of a wide spectrum of areas ranging from extinguishing of plasmas, arcs and flames (in this regard multicomponent gas mixtures may be properly tailored to optimize their function) to combustion and polymer construction; from radio, television, and other communications systems, space-and defense-related applications, to injection beams in fusion machines, tornado bombing (with electron attaching gases), precipitators and health-related effects; from gas discharges to the rich and rapidly developing area of gaseous dielectrics where the concept of the multicomponent gaseous insulator shows a great promise. In most of these technological areas, it is important to realize the significance of tailoring (combining) of gases to optimize electron and ion densities (as well as electron energies) in order to enhance or to inhibit the critical reactions involved.
Finally, attention is drawn to the direct role of negative ions and their reaction processes in the rich and relatively new area of atmospheric and environmental science.