EXCITATION OF AUTOIONIZING STATES IN POTASSIUM BY ELECTRON IMPACT

The ejected-electron excitation functions for the 3 p3d4s P3/2, DJ lowest quartet and 3p3d4s(P)P3/2 upper doublet autoionizing states in potassium hav e been measured with an energy resolution of 0.2 eV over t h electron impact energy region from the lowest excitation threshold up to 500 eV. The detailed picture of the excitation dynamics of the P3/2, Dj and P3/2 levels in the near-threshold impact energy region has been achieved due to the use of a small increment of the incident electron energy. An analysis of the data, including the cons ideration of resonance and cascade excitation processes, has been performed on th e base of available experimental and theoretical data on the 3p -excitation in potassium atoms.


INTRODUCTION
The pioneering experimental studies of electron impact ionization of alkali atoms performed by Aleksakhin and Zapesochny [1] have revealed an important role of autoionization in this process.As ionization processes play the dominant role in the different kinds of plasma, including laser plasma [2], their intense experimental and theoretical studies have been started in the middle of seventies (see e.g.[3,4] and references therein).However, only a few works are known on measuring the electron impact excitation functions of core-excited autoionizing states in potassium.First such data were obtained for the (3p 5 4s3d) 4 F and (3p 5 4s4p) 4 D metastable states at the energy resolution of 0.3 and 0.08 eV, respectively [5,6].The strong resonances revealed in the threshold impact energy region were preliminarily attributed by the authors to the negative-ion states.Also, the role of cascade processes has been discussed in the excitation of the quartet states.The total excitation cross-section for the (3p 5 4s 2 ) 2 P 1/2,3/2 lowest autoionizing doublet has been measured recently in [3,7] with the energy spread of the incident electrons equal to 0.7 and 0.25 eV, respectively.Using the results of extended R-matrix calculations, a strong near-threshold structure has been attributed by the authors also to the negativeion resonances.Later, these data have been analyzed with the aim to find the role of cascade processes in excitation of the (3p 5 4s 2 ) 2 P levels [8].
In the present work, we have studied the ejected-electron excitation functions for the (3p 5 3d4s) 4 P 3/2 , 4 D J lowest quartet and 3p 5 3d4s ( 1 P) 2 P 3/2 upper doublet autoionizing states in potassium atoms in the impact energy region from the excitation threshold of levels up to 500 eV.By using an improved incident electron energy resolution of 0.2 eV and a small incremental step of the incident electron beam energy, the dynamics of electron impact excitation of the 4 P 3/2 , 4 D J and ( 1 P) 2 P 3/2 states was studied in detail, with analyzing the role of resonance and cascade excitation processes.

EXPERIMENTAL
Current measurements were performed on apparatus which consisted of a 127 o electrostatic cylindrical monochromator, electron analyzer, and an atomic beam source (fig.1).The monochromator with the mean radius of electron trajectory of 30 mm allowed obtaining an electron beam in the 16 ÷ 100 eV energy region with the intensity of 0.1 mkA and energy spread of 0.2 eV (FWHM) [9].The ejected-electron analyser with the mean radius of 12.7 mm and the energy resolution of 0.15 eV was located at the observation angle of 75 o .The resistively heated source of potassium vapours [10] provided their density in the interaction region of about 10 12 at⋅cm -3 .The ejectedelectron excitation functions have been obtained as an incident-electron energy dependence of line intensities corresponding to the electron decay of the 4 P 3/2 , 4 D J and ( 1 P) 2 P 3/2 autoionizing states.Note an essential peculiarity of such measurementsinstead of measuring the intensity of a single line (as it is done in traditional optical measurements [11]), the whole ejectedelectron spectrum (or its part) is measured for particular impact energy value.In the present work the spectra were measured step-by-step for different values of the incident electron energy.The intensity of the spectra was automatically normalized on the corresponding incident electron beam intensity by a "current-to-frequency" converter.All data acquisition procedure has been automatically controlled by the computerized detection system [12].An uncertainty in estimation of the line intensity depended on the detected signal level providing the necessary statistics of the data acquisition.In present measurements, it did not exceed 20% for the lowest line intensities in potassium spectra.The incident-electron and ejected-electron energy scales were calibrated by using photoabsorption data [13] for the excitation threshold of the (3p 5 4s 2 ) 2 P 3/2 state at 18.722 eV.The uncertainties of both energy scales were estimated as ± 100 meV and ± 50 meV, respectively.

RESULTS
In figure 2, the ejected-electron excitation functions for the (3p 5 3d4s) 4 P 3/2 , 4 D J and 3p 5 3d4s( 1 P) 2 P 3/2 autoionizing states in potassium atoms are shown in an incident electron energy region from the excitation thresholds of levels up to 500 eV.The excitation thresholds for these states are known to be 19.79;21.42 and 22.42 eV, respectively [14] (marked in figure 2 by dashed lines).Note, as it follows from the comparison of ejected-electron spectra measured at low impact energies [15], the 4 P 3/2 line dominates in the spectra.Therefore, the measured excitation function of this level has a minimal influence from the close lying fine-structure components.The same is valid for the ( 1 P) 2 P 3/2 state.The excitation function for the 4 D J states, on the other hand, reflects the summary excitation of all J-components, due to their much closer energy position and comparable excitation cross sections.As it can be seen, the excitation functions for the quartet states possess a resonance-like shape with main maxima at 27.7 eV ( 4 P 3/2 ) and 30.6 eV ( 4 D J ).Both functions start with well resolved near-threshold resonances at 21.1 and 21.5 eV, respectively.On the other hand, the excitation function of the 3p 5 4s3d( 1 P) 2 P 3/2 doublet state starts with a step-like rise of the cross section just above the excitation threshold (see arrow).Above 25 eV there is a smooth rise of the cross section up to the maximum value at about 140 eV.The data in the region 40÷50 eV were omitted from the present consideration due to the strong influence from the energy loss-spectra.
As it follows from the comparison of functions, their general shape agrees well with the character of corresponding excitation transition -spin-exchange for the 4 P 3/2 , 4 D J quartet states and dipole for the ( 1 P) 2 P 3/2 state.Hence, process (1) determines completely the excitation of the (3p 5 3d4s) 4 P 3/2 , 4 D J and ( 1 P) 2 P 3/2 states over the whole impact energy region studied in the present work.In this case processes ( 2) and ( 3) may be considered only as some contributions to the main direct excitation process.
It is well known that in the excitation functions the cascade transitions manifest themselves as a distinct rise of the cross section located close to the excitation threshold of cascading levels.In accordance with our previous analysis [6,8], the contribution from cascade transitions (process 2) to excitation of the low-lying quartet states in potassium may be expected only from the radiative decay of the 3p 5 4s4p quartet levels lying between 20 eV and 20.6 eV.Due to the high excitation energy of the (3p 5 3d4s) 4 D J level at 21.42 eV, such transitions can not influence the excitation of this level.The present data also do not reveal any remarkable cascade features at higher impact energies (see figure 2).The cascade contribution into the (3p 5 3d4s) 4 P 3/2 level, on the other hand, could be revealed in the initial part of its excitation function, in particular in the low-energy wing of the nearthreshold resonance at 21.1 eV.However, in order to reveal this process, the additional measurements should be performed with a The presence of the resonance features in all measured excitation functions just confirms the earlier observations [5][6][7] of the important role of negative-ions in electron impact excitation of the 3p 6 -subshell in potassium.The threshold resonance at 21.1 eV, observed in the excitation function for the (3p 5 3d4s) 4 P 3/2 , level may reflect the formation of a negative-ion state with a tentative configuration 3p 5 3d4s 2 .The same negative-ion configuration could also be responsible for the presence of the threshold resonance at 21.5 eV in the excitation of the (3p 5 3d4s) 4 D J level.
The near-threshold behaviour of the excitation cross section for the 3p 5 3d4s ( 1 P) 2 P 3/2 high-lying level points out the competitive role of the resonance and cascade processes.Indeed, the observed resonance-like rise of the cross-section may be attributed both to the mixture of closelying negative-ion resonances and to the cascade transitions, including those of resonance type, lying in the energy region 23÷30 eV.Unfortunately, at present there are no data on such processes in potassium at these energies.

CONCLUSIONS
In the present work, the electron impact excitation of the (3p 5 3d4s) 4 P 3/2 , 4 D J and ( 1 P) 2 P 3/2 autoionizing states in potassium atoms has been studied over the impact energy region from the lowest excitation threshold up to 500 eV at the incident electron energy resolution of 0.2 eV.The analysis of the data has shown that for all states considered the direct excitation process dominates over the whole impact energy region.The resonance features observed in the near-threshold region of the excitation functions point out the presence of strong negative-ion resonances in the excitation of all considered levels.In case of the (3p 5 3d4s) 4 P 3/2 , 4 D J quartet levels, these resonances can be tentatively attributed to the 3p 5 3d4s 2 configuration of negative potassium ion.For the 3p 5 3d4s( 1 P) 2 P 3/2 level the cascade transitions may also influence its excitation at threshold energies.
This work was supported, in part, by the INTAS under grant 03-51-4706.

Figure 1 .
Figure 1.Electron spectrometer for measuring the ejected-electron spectra of metal vapours: M -incident electron monochromator; A -analyzer of ejected electrons; BS -vapour source; FC -Faraday cup; CH -secondary electron multiplier; T -vapour trap.

Figure 2 .
Figure 2. The ejected-electron excitation functions for the 3p 5 4s3d autoionizing states.Vertical dashed lines mark the excitation thresholds of the states.
arb.u.) smaller incremental step of the electron impact energy.