Isotope Effect in D2O Negative Ion Formation in Electron Transfer Experiments: DO–D Bond Dissociation Energy

H2O/D2O negative ion time-of-flight mass spectra from electron transfer processes at different collision energies with neutral potassium yield OH–/OD–, O–, and H–/D–. The branching ratios show a relevant energy dependence with an important isotope effect in D2O. Electronic state spectroscopy of water has been further investigated by recording potassium cation energy loss spectra in the forward scattering direction at an impact energy of 205 eV (lab frame), with quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom supporting most of the experimental findings. The DO–D bond dissociation energy has been determined for the first time to be 5.41 ± 0.10 eV. The collision dynamics revealed the character of the singly excited (1b2–1) molecular orbital and doubly excited states in such K–H2O and K–D2O collisions.

may delay autodetachment leading to a "stabilization" of the temporary negative ion (TNI), which can result in energy redistribution through the different available degrees of freedom leading either to a stable parent anion or different fragmentation channels. As far as potassiumwater electron transfer process is concerned, in the unimolecular decomposition of the TNI, momentum transfer may provide OHconsiderable velocity to escape the collision complex, whilst in DEA, momentum transfer provides to Hsubstantial velocity to escape the TNI. For further discussion, see main manuscript OHand Hformation.

Experimental Method
The Lisbon laboratory equipped with a crossed molecular beam set up used to investigate the anionic fragmentation pattern of K collisions with H2O and D2O together with K + energy loss measurements, has been described elsewhere. [4][5][6] Briefly, it consists of two vacuum chambers both differentially pumped and interconnected by a gate valve with a 0.5 cm wide aperture, where the base pressure in the potassium chamber was 4×10 -5 Pa and in the collision chamber was 5×10 -5 Pa. The working pressure in the collision chamber after H2O and D2O effusion was 1×10 -3 Pa (for K + energy loss measurements) and 5×10 -4 Pa for (Time-of-Flight mass spectrometry). The neutral beam of potassium atoms is produced in the potassium chamber, where a commercial ion source (HeatWave, US) generates hyperthermal potassium cations (K + hyp) that are accelerated to a set kinetic energy towards the entrance of an oven. Here, these ions are resonantly charge exchanged in the charge exchange oven (CEO) with thermal potassium atoms (K o th), obtained by heating solid potassium at 393 K, yielding K o hyp. The resultant beam comprises K + hyp ions that did not charge exchange and are removed from the K o hyp beam by two deflecting plates placed at the exit of the CEO, before passing into the collision region. From the resonant charge-exchange process and the CEO slits apertures, the K o hyp beam is mainly composed of potassium atoms in the ground state configuration with its outermost electron as 4s. Thus, the experimental thresholds of formation are in assertion that K* in a 4p state would result in values at lower energies than those reported here (see Sec. III).
Such has been shown in the past in other energy loss data from potassium collisions with pyrimidine, 5 halothane, 7 tetrachloromethane 6 and more recently with hexachlorobenzene 8,9 and nimorazole. 10 The K o hyp beam intensity is monitored at the entrance of the collision chamber by a surface ionization detector of the Langmuir-Taylor type. Thereafter, the K o hyp beam crosses at right angles with an effusive target beam, which is admitted to vacuum through a 1 mm diameter capillary from an external sample holder. In the collision region, the negative ions formed were extracted by a pulsed electrostatic field (380 Vcm -1 ), and mass analysed by a reflectron TOF spectrometer (r-TOF) with a mass resolution m/Δm ≈ 800. The beam energy resolution for TOF mass spectra collection in the collision energy range investigated was ∼0.6 eV. The r-TOF mass calibration was performed from the well-known fragmentation patterns from collisions of potassium atoms with CH3NO2 and/or CCl4 molecules. 6,11 Note that comprehensive background spectra (without the sample) were obtained and subtracted from the sample measurements. Branching ratios (BRs) for the fragment anions from H2O and D2O have been obtained and result from the fragment anion yield divided by the total anion yield at a given collision energy.
Potassium cations formed post-collision experiments were energy loss analysed in the forward scattering direction (θ ≈ 0°), while experiments were not performed in coincidence with r-TOF mass spectrometry. The analyser was operated in constant transmission mode, hence keeping the resolution constant throughout the entire scans. The estimated energy resolution during the experiments was ~1.2 ± 0.2 eV. The energy loss scale was calibrated using the K + beam profile from the potassium ion source serving as the elastic peak. H2O and D2O were supplied by Sigma-Aldrich with a stated purity 99.9% and were degassed through repeated freeze-pump-thaw cycles.

Theoretical Method
Electronic structure investigations of the molecular orbitals (MOs) formed in collisions between potassium (K) atoms and water (H2O) have been performed to provide insight into the electron transfer process up to 30 eV. In particular, the analysis of the computed lowest unoccupied molecular orbitals (LUMOs) is crucial to assess the nature of the different electronic states that result in the detected negative ions of the current experiments.
The Minnesota hybrid-meta GGA functional M06-2X 12 family has been shown to be very sensitive to the integration grid employed and generally requires finer grids than other functionals in order to get reasonable numerical stability. Gaussian 16 automatically includes an ultrafine integration grid in the density functional theory (DFT) calculations in order to improve the accuracy of the results. The grid greatly enhances the accuracy at reasonable additional cost. This functional has been shown to be reliable in particular in computing energies. 13 Two gaussian basis sets have been used in our calculations, i.e., 6-311++G(2d,p), and 6-311++G(3df,3pd). 14,15 The geometry of the K + H2O was initially fully optimized at the M06-2X/6-311++g(2d,p) level of theory and the equilibrium distance between the potassium K and oxygen O atoms is 2.67 Å. The resulting optimized system upon the collision of K atoms with H2O resulted in the structure shown in Figure S1. All quantum chemical calculations have been performed with the Gaussian 16 program package. 16 The calculation has been carried out in Cartesian coordinates, with no symmetries. All electrons have been taken into account for potassium, oxygen and hydrogen atoms with the 6-311++g(2d,p) basis set during optimisations calculations. The natural molecular orbitals for K−H 2 O have been calculated by M06-2X/6-311++g(3df,3pd) methodology.

Oformation
We note that features resulting from the Gaussian fittings in the energy loss spectra of In potassium-water/deuterium oxide collisions, formation of Omay proceed through the following reactions: where The oxygen anion has been reported in DEA experiments to H2O/D2O with three resonances at 6.5 (7.0), 8.6 (9.0) and 11.8 (12.0) eV (Table S1) with increasing intensity as the electron energy is increased. 17 At its peak maximum in DEA experiments through the 2 B1 resonance, the cross section for O -+ H2 production is ~ 40 times less intense than H -+ OH formation. 18 However, such experimental evidence is not consistent with the energetics of the product channels but rather with the dynamics of the DEA process. 19 Haxton et al. 19 have also reported that OH produced via the 2 B1 resonance is accompanied by extensive vibrational excitation. The oxygen anion produced via the 2 A1 resonance, was suggested to proceed resonances (Table S2).  (Table S2).

H -/Dformation
However, from the time-of-flight mass spectra of K + H2O and K + D2O collisions, such thresholds have been obtained at 10.24 and at 10.98 eV ( Figure 1 and Table S2), thus meaning that H -/Dformation can proceed through the 2 A1 resonance. The energy difference can be attributed to the translational and internal energies of the fragments formed. A kinetic-energy release distribution (KERD) of hydrogen anion from H2O (at 100 eV collision energy in the lab frame) has been obtained from a linear TOF mass spectrometer and is depicted in Figure S3.
The methodology used to obtain the KERD has been described elsewhere for different polyatomic molecular targets. [28][29][30] For further details on the methodology, the interested reader should consult Limão-Vieira et al. 28  The distribution maximum at low kinetic-energy release is related to a statistical process where the excess energy is channelled into the available degrees of freedom of the OH radical.
Note that OH formation via the 2 B1 resonance is accompanied by extensive vibrational 32 (Table II) Table 1).
Regarding the statistical process in the KERD of Figure S3, it was fitted with a function form Ref. 28 (and references therein) as: with εd the kinetic energy release, C a constant independent of the energy, s the adapted degree of freedom and Ee the available excess energy. The dashed line in Figure S3 was obtained for an excess energy of 4.1 eV from the difference between the resonance at 8.5 eV and the dissociation limit of the HO-Hconfiguration. The fitting procedure corresponds to s = 3 and C = 4.0×10 -7 , the latter with no physical meaning since the distribution is in arbitrary units. The value of three degrees of freedom seems appropriate as to those expected in water, yet the reaction path energy that is channelled into the accessible state in the collision process and to direct dissociation, is a quite complex intramolecular process within the TNI formed.