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Resonance excitation of the 3p6-subshell in potassium: Contribution to the single ionization
Nuclear Instruments and Methods in Physics Research B 233 (2005) 280–283 www.elsevier.com/locate/nimb
Resonance excitation of the 3p6-subshell in potassium: Contribution to the single ionization M.J. Evrij a, A.A. Borovik Jr. a, L.L. Shimon b, J.E. Kontros a, A.A. Borovik a
a,*
Institute of Electron Physics, 21 Universitetska vul., Uzhgorod 88017, Ukraine b Uzhgorod National University, Uzhgorod 88000, Ukraine Available online 26 April 2005
Abstract The total autoionization cross-section of potassium has been studied as a function of incident electron energy over the range from the lowest autoionization threshold at 18.72 eV to 24.4 eV and at an energy resolution of 0.25 eV. The cross-section has been obtained by determining the normalized total intensities of ejected-electron spectra accurately measured at the magic angle of 54.7. The structure observed in the autoionization function has been assigned to the negative-ion resonances based upon the lowest autoionizing configurations 3p54s2 and 3p53d4s. The quartet levels from the 3p53d4s configuration give the bulk of the autoionization contribution in the single ionization cross-section in the energy range 20.5–24.4 eV. Comparison of the present results with earlier available experimental and theoretical results is presented. 2005 Elsevier B.V. All rights reserved. PACS: 32.80D; 34.80D Keywords: Atom; Ionization; Autoionization; Negative-ion resonance; Cross-section
1. Introduction Numerous experimental works devoted to the investigation of the impact ionization of heavy alkali metals have revealed an abrupt increase of the ionization cross-section above the np6 excitation *
Corresponding author. Fax: +38 031 224 3650. E-mail address: [email protected] (A.A. Borovik).
threshold (n = 3, 4, 5 for K, Rb and Cs, respectively) (see e.g. [1–3] and references therein). None of the calculations performed to describe this effect has been able to determine correctly the position and magnitude of the autoionization contribution (see e.g. [4,5] and references therein). In regard to potassium atoms, both the earlier data [6,7] and the recent data [8–11] on the near-threshold excitation of the lowest autoionizing configurations predict a resonance character of the effect of
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.122
M.J. Evrij et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 280–283
Excited-state energy, eV 18
19 5
20
22
3p 4s PJ 3/2
21 61
22
23
24
-
3p S0 + e
1/2
(a)
3p5n1l1n2l2 2,4L
Intensity (arb.u.)
autoionization in electron impact ionization. This is consistent with the assumption made earlier by authors [4,5] that the autoionization contribution would be significant only at the threshold of excitation of autoionizing levels. With the aim of directly determining the magnitude of the autoionization contribution and, in particular, for clearing up the role of the 3p6 resonance excitation in the process, we have precisely measured the total autoionization cross-section in potassium in an impact energy range from the lowest autoionization threshold at 18.72 eV to 24.4 eV. The use of the energy-selected electron beam and of the small increment step of the incident energy has enabled several resonance structures in the autoionization cross-section to be observed for the first time.
281
3p61S0 + e-
(b)
(c)
(d)
14
15
16
17
18
19
20
Ejected-electron energy (eV)
2. Experiment The ejected-electron spectra of potassium atoms have been measured on an apparatus and by the method both described elsewhere [12,13]. In short, the apparatus involves an incident electron beam source, a 127 electrostatic cylindrical electron analyzer and a source of potassium beam. A 127 electrostatic cylindrical electron monochromator [14] has been used in the present measurements for producing an incident electron beam with a typical intensity of 1–3 · 10 7 A and an energy spread of about 0.25 eV (FWHM). The resistively heated single-channel oven produced a potassium beam with a density in the interaction region of about 1011 cm 3 and an angular spread of about 90. The ejected-electron spectra have been measured in succession, step-by-step for different impact energy values in an ejected-electron energy region 13.6–20.7 eV and at the magic observation angle of 54.7. A ‘‘current-to-frequency’’ converter was employed to normalize the ejected-electron intensity [15]. Fig. 1 shows an example for a sequence of four spectra measured at the impact energies 18.96, 19.63, 22.10 and 24.15 eV. We note that all measured spectra contain only the lines corresponding to the single-channel decay of the 3p5n1l1n2l2-autoionizing states located below the
Fig. 1. The ejected-electron spectra of potassium atoms between 13.6 and 20.7 eV observed at 54.7 for four sets of electron impact energy: (a) 18.96 eV; (b) 19.63 eV; (c) 22.10 eV; (d) 24.15 eV. The energy width per channel is 20 meV. In spectra a, b, c the high-energy region containing the elastic and energy-loss peaks is not shown in order to simplify the picture. All spectra have been ‘‘flattened’’ by subtracting a non-linear background function.
lowest excited 3p54s[3/2]1 ionic state at 24.44 eV [16]. In this case, the total number of ions created by autoionization or, in other words, the total autoionization cross-section of potassium atoms is proportional to the sum of total excitation cross-sections of decaying autoionizing states. The latter cross-section is proportional to the total spectral intensity determined as the sum of normalized line intensities observed in the spectra. The set of such sums obtained at different impact energy values reflects the impact energy dependence of the total autoionization cross-section of the potassium atoms. For deriving the line intensities we used the data processing procedure described earlier [12,13]. The relative error in determining the total spectral intensities being of statistical character did not exceed the maximum value of 15%. The relative line intensities were put on an absolute scale by normalization of the intensity of the line corresponding to the decay
282
M.J. Evrij et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 280–283
of the 3p54s22P3/2 state, to a theoretical value of the cross-section for this state at 500 eV [17]. The incident electron and ejected-electron energy scales have been calibrated by using the photoabsorption data for the excitation threshold of the 3p54s22P3/2 state at 18.722 eV [18]. The uncertainty of both energy scales has been estimated to be ±50 meV. All spectra have been measured with an ejected-electron energy resolution of about 0.15 eV.
3. Results and discussion The total autoionization cross-section of potassium atoms is presented in Fig. 2. Also displayed are the assignments and positions of the lowest 3p5n1l1n2l2-autoionizing levels [18,19]. From the figure it may be seen that for the impact energies below 20.5 eV the autoionization cross-section is determined by the decay of the 3p54s22P and 3p53d4s4P levels. However, the cross-section does not exceed the value of 2.2 · 10 17 cm2 in this energy region. The dominant feature of the autoionization function above 20.5 eV is an abrupt increase in the cross-section up to the maxi.
mum value of 1.5 · 10 16 cm2. This is caused by the strong resonance excitation of the lowest 3p53d4s4F, 2F and 4D autoionizing levels [8– 11,20]. These levels and, in part, the autoionizing quartets from the 3p54s4p configuration determine in main the autoionization of the potassium atoms in the whole impact energy region 20.5–24.4 eV. The measured autoionization function exhibits four well-resolved features a–d located at 19.15, 19.55, 21.0 and 22.0 eV, respectively. Features a, b may be definitely related to the negative-ion resonances at 19.12 and 19.47 eV observed recently in excitation of the 3p54s22P3/2 level [11]. The scatter in the data around 21 eV (see feature c) reflects the presence in this region of several overlapping resonances. Their origin may be associated with the negative-ion resonances based upon the 4FJ and (3F) 2FJ levels (J 6 7/2) from the dominating 3p53d4s configuration [11,20]. Feature d corresponds to the negative-ion resonance observed at 22.09 eV in excitation function of the 3p53d4s4D level [7,11]. Finally, considering that the measured total autoionization cross-section is equal to the total autoionization contribution in the impact ionization cross-section, we have compared our results with earlier impact ionization data. Fig. 3 shows such a comparison with the experimental results
. . . . .
Ionization cross-section (10-16 cm2)
10
1
8
3
2
6
4
2
0 5
Fig. 2. The total autoionization cross-section of potassium atoms over the electron impact energy range 18.5–24.5 eV. The energy positions and assignments of the autoionizing levels marked on top of the autoionization function are taken from [17,18]. A vertical broken line marks the excitation threshold of the 3p53d4s4F7/2 autoionizing level at 20.48 eV.
10
15
20
25
30
Electron impact energy (eV) Fig. 3. Electron impact ionization cross-sections in potassium: 1 – theory [4], 2 – present data (total autoionization crosssection), 3 – experimental data [3]. A vertical broken line marks the lowest autoionization threshold at 18.72 eV.
M.J. Evrij et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 280–283
[3] obtained at the best energy resolution of 0.1 eV, and with the theory [4] that is the only work where the autoionization processes were taken into account. As can be seen from the figure, all data confirm the resonance character of the autoionization contribution in an energy region between 18.72 eV and 20.5 eV. The magnitude of this contribution is comparable in both experiments but the theory [4] overestimates evidently the effect of autoionization in this energy region. Above 20.5 eV the theory – although it is still too large – describes the resonance shape of the measured autoionization cross-section reasonably well. However, none of the foregoing experiments [1–3] has been able to observe this feature in the ionization function of potassium atoms. Such a discrepancy seems to be strange, because the autoionization resonances have been observed experimentally in the ionization function of sodium [21] where the effect of autoionization is much weaker than in potassium [1–3]. To clarify the problem further detailed investigations of the electron impact ionization in potassium atoms would be highly desirable.
Acknowledgment The authors are grateful to Professor O.B. Shpenik for favourable encouragement of this work. One of us (AAB) gratefully acknowledges the support by the INTAS (Grant 03-51-4706).
References [1] Yu.P. Korchevoi, A.M. Przonski, Sov. Phys.: JETP 24 (1967) 1089.
283
[2] I.P. Zapesochny, I.S. Aleksakhin, Sov. Phys.: JETP 28 (1969) 41. [3] K.J. Nygaard, Phys. Lett. A 51 (1975) 171. [4] B.N. Roy, D.K. Rai, Phys. Rev. A 8 (1973) 849. [5] A.W. Pangantivar, R.J. Srivastava, J. Phys. B 20 (1987) 5881. [6] A.A. Borovik, E.A. Breza, I.P. Zapesochny, in: Proceedings of the XVI International Conference on the Physics of Electronic and Atomic Collisions, NY, USA, 1989, p. 875. [7] A.A. Borovik, in: Proceedings of the International Conference on The Centenary of Electron, Uzhgorod, Ukraine, 1997, p. 232. [8] B. Feuerstein, A.N. Grum-Grzhimailo, W. Mehlhorn, J. Phys. B 32 (1999) 4547. [9] A.N. Grum-Grzhimailo, K. Bartschat, J. Phys. B 33 (2000) 1843. [10] A.A. Borovik, A.N. Grum-Grzhimailo, K. Bartschat, in: Proceedings of the XXIII International Conference on the Physics of Electronic and Atomic Collisions, Stockholm, Sweden, 2003, p. We084. [11] A.A. Borovik, A.N. Grum-Grzhimailo, O.I. Zatsarinny, K. Bartschat, J. Phys. B 38 (2005) 1081. [12] A.A. Borovik, V.N. Krasilinec, J. Phys. B 32 (1999) 1941. [13] A.A. Borovik, Ukr. Phys. J. 45 (2000) 1270. [14] A.A. Borovik, Prib. Tekh. Eksp. (PTE): Sov. Phys. IET A 3 (1991) 124. [15] V.B. Gusev, A.I. Schaks, Prib. Tekh. Eksp. (PTE) N2 (1982) 80 (in Russian). [16] C.E. Moore, Atomic Energy Levels, Vol. 1, NBS Circular no 467, 1958. [17] B. Feuerstein, A.N. Grum-Grzhimailo, W. Mehlhorn, J. Phys. B 31 (1998) 593. [18] M.W.D. Mansfield, Proc. Roy. Soc. Lond. A 346 (1975) 539. [19] M.W.D. Mansfield, T.W. Ottley, Proc. Roy. Soc. Lond. A 365 (1979) 413. [20] A.A. Borovik, H. Rojas, G.C. King, E.Yu. Remeta, J. Phys. B 32 (1999) 4225. [21] P. Marmet, M. Proulx, in: Proceedings of the 4th International Symposium on Resonant Ionization Spectroscopy, Gaithersburg, MD, USA, 1988, p. 89.
Resonance excitation of the 3p6-subshell in potassium: Contribution to the single ionization M.J. Evrij a, A.A. Borovik Jr. a, L.L. Shimon b, J.E. Kontros a, A.A. Borovik a
a,*
Institute of Electron Physics, 21 Universitetska vul., Uzhgorod 88017, Ukraine b Uzhgorod National University, Uzhgorod 88000, Ukraine Available online 26 April 2005
Abstract The total autoionization cross-section of potassium has been studied as a function of incident electron energy over the range from the lowest autoionization threshold at 18.72 eV to 24.4 eV and at an energy resolution of 0.25 eV. The cross-section has been obtained by determining the normalized total intensities of ejected-electron spectra accurately measured at the magic angle of 54.7. The structure observed in the autoionization function has been assigned to the negative-ion resonances based upon the lowest autoionizing configurations 3p54s2 and 3p53d4s. The quartet levels from the 3p53d4s configuration give the bulk of the autoionization contribution in the single ionization cross-section in the energy range 20.5–24.4 eV. Comparison of the present results with earlier available experimental and theoretical results is presented. 2005 Elsevier B.V. All rights reserved. PACS: 32.80D; 34.80D Keywords: Atom; Ionization; Autoionization; Negative-ion resonance; Cross-section
1. Introduction Numerous experimental works devoted to the investigation of the impact ionization of heavy alkali metals have revealed an abrupt increase of the ionization cross-section above the np6 excitation *
Corresponding author. Fax: +38 031 224 3650. E-mail address: [email protected] (A.A. Borovik).
threshold (n = 3, 4, 5 for K, Rb and Cs, respectively) (see e.g. [1–3] and references therein). None of the calculations performed to describe this effect has been able to determine correctly the position and magnitude of the autoionization contribution (see e.g. [4,5] and references therein). In regard to potassium atoms, both the earlier data [6,7] and the recent data [8–11] on the near-threshold excitation of the lowest autoionizing configurations predict a resonance character of the effect of
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.122
M.J. Evrij et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 280–283
Excited-state energy, eV 18
19 5
20
22
3p 4s PJ 3/2
21 61
22
23
24
-
3p S0 + e
1/2
(a)
3p5n1l1n2l2 2,4L
Intensity (arb.u.)
autoionization in electron impact ionization. This is consistent with the assumption made earlier by authors [4,5] that the autoionization contribution would be significant only at the threshold of excitation of autoionizing levels. With the aim of directly determining the magnitude of the autoionization contribution and, in particular, for clearing up the role of the 3p6 resonance excitation in the process, we have precisely measured the total autoionization cross-section in potassium in an impact energy range from the lowest autoionization threshold at 18.72 eV to 24.4 eV. The use of the energy-selected electron beam and of the small increment step of the incident energy has enabled several resonance structures in the autoionization cross-section to be observed for the first time.
281
3p61S0 + e-
(b)
(c)
(d)
14
15
16
17
18
19
20
Ejected-electron energy (eV)
2. Experiment The ejected-electron spectra of potassium atoms have been measured on an apparatus and by the method both described elsewhere [12,13]. In short, the apparatus involves an incident electron beam source, a 127 electrostatic cylindrical electron analyzer and a source of potassium beam. A 127 electrostatic cylindrical electron monochromator [14] has been used in the present measurements for producing an incident electron beam with a typical intensity of 1–3 · 10 7 A and an energy spread of about 0.25 eV (FWHM). The resistively heated single-channel oven produced a potassium beam with a density in the interaction region of about 1011 cm 3 and an angular spread of about 90. The ejected-electron spectra have been measured in succession, step-by-step for different impact energy values in an ejected-electron energy region 13.6–20.7 eV and at the magic observation angle of 54.7. A ‘‘current-to-frequency’’ converter was employed to normalize the ejected-electron intensity [15]. Fig. 1 shows an example for a sequence of four spectra measured at the impact energies 18.96, 19.63, 22.10 and 24.15 eV. We note that all measured spectra contain only the lines corresponding to the single-channel decay of the 3p5n1l1n2l2-autoionizing states located below the
Fig. 1. The ejected-electron spectra of potassium atoms between 13.6 and 20.7 eV observed at 54.7 for four sets of electron impact energy: (a) 18.96 eV; (b) 19.63 eV; (c) 22.10 eV; (d) 24.15 eV. The energy width per channel is 20 meV. In spectra a, b, c the high-energy region containing the elastic and energy-loss peaks is not shown in order to simplify the picture. All spectra have been ‘‘flattened’’ by subtracting a non-linear background function.
lowest excited 3p54s[3/2]1 ionic state at 24.44 eV [16]. In this case, the total number of ions created by autoionization or, in other words, the total autoionization cross-section of potassium atoms is proportional to the sum of total excitation cross-sections of decaying autoionizing states. The latter cross-section is proportional to the total spectral intensity determined as the sum of normalized line intensities observed in the spectra. The set of such sums obtained at different impact energy values reflects the impact energy dependence of the total autoionization cross-section of the potassium atoms. For deriving the line intensities we used the data processing procedure described earlier [12,13]. The relative error in determining the total spectral intensities being of statistical character did not exceed the maximum value of 15%. The relative line intensities were put on an absolute scale by normalization of the intensity of the line corresponding to the decay
282
M.J. Evrij et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 280–283
of the 3p54s22P3/2 state, to a theoretical value of the cross-section for this state at 500 eV [17]. The incident electron and ejected-electron energy scales have been calibrated by using the photoabsorption data for the excitation threshold of the 3p54s22P3/2 state at 18.722 eV [18]. The uncertainty of both energy scales has been estimated to be ±50 meV. All spectra have been measured with an ejected-electron energy resolution of about 0.15 eV.
3. Results and discussion The total autoionization cross-section of potassium atoms is presented in Fig. 2. Also displayed are the assignments and positions of the lowest 3p5n1l1n2l2-autoionizing levels [18,19]. From the figure it may be seen that for the impact energies below 20.5 eV the autoionization cross-section is determined by the decay of the 3p54s22P and 3p53d4s4P levels. However, the cross-section does not exceed the value of 2.2 · 10 17 cm2 in this energy region. The dominant feature of the autoionization function above 20.5 eV is an abrupt increase in the cross-section up to the maxi.
mum value of 1.5 · 10 16 cm2. This is caused by the strong resonance excitation of the lowest 3p53d4s4F, 2F and 4D autoionizing levels [8– 11,20]. These levels and, in part, the autoionizing quartets from the 3p54s4p configuration determine in main the autoionization of the potassium atoms in the whole impact energy region 20.5–24.4 eV. The measured autoionization function exhibits four well-resolved features a–d located at 19.15, 19.55, 21.0 and 22.0 eV, respectively. Features a, b may be definitely related to the negative-ion resonances at 19.12 and 19.47 eV observed recently in excitation of the 3p54s22P3/2 level [11]. The scatter in the data around 21 eV (see feature c) reflects the presence in this region of several overlapping resonances. Their origin may be associated with the negative-ion resonances based upon the 4FJ and (3F) 2FJ levels (J 6 7/2) from the dominating 3p53d4s configuration [11,20]. Feature d corresponds to the negative-ion resonance observed at 22.09 eV in excitation function of the 3p53d4s4D level [7,11]. Finally, considering that the measured total autoionization cross-section is equal to the total autoionization contribution in the impact ionization cross-section, we have compared our results with earlier impact ionization data. Fig. 3 shows such a comparison with the experimental results
. . . . .
Ionization cross-section (10-16 cm2)
10
1
8
3
2
6
4
2
0 5
Fig. 2. The total autoionization cross-section of potassium atoms over the electron impact energy range 18.5–24.5 eV. The energy positions and assignments of the autoionizing levels marked on top of the autoionization function are taken from [17,18]. A vertical broken line marks the excitation threshold of the 3p53d4s4F7/2 autoionizing level at 20.48 eV.
10
15
20
25
30
Electron impact energy (eV) Fig. 3. Electron impact ionization cross-sections in potassium: 1 – theory [4], 2 – present data (total autoionization crosssection), 3 – experimental data [3]. A vertical broken line marks the lowest autoionization threshold at 18.72 eV.
M.J. Evrij et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 280–283
[3] obtained at the best energy resolution of 0.1 eV, and with the theory [4] that is the only work where the autoionization processes were taken into account. As can be seen from the figure, all data confirm the resonance character of the autoionization contribution in an energy region between 18.72 eV and 20.5 eV. The magnitude of this contribution is comparable in both experiments but the theory [4] overestimates evidently the effect of autoionization in this energy region. Above 20.5 eV the theory – although it is still too large – describes the resonance shape of the measured autoionization cross-section reasonably well. However, none of the foregoing experiments [1–3] has been able to observe this feature in the ionization function of potassium atoms. Such a discrepancy seems to be strange, because the autoionization resonances have been observed experimentally in the ionization function of sodium [21] where the effect of autoionization is much weaker than in potassium [1–3]. To clarify the problem further detailed investigations of the electron impact ionization in potassium atoms would be highly desirable.
Acknowledgment The authors are grateful to Professor O.B. Shpenik for favourable encouragement of this work. One of us (AAB) gratefully acknowledges the support by the INTAS (Grant 03-51-4706).
References [1] Yu.P. Korchevoi, A.M. Przonski, Sov. Phys.: JETP 24 (1967) 1089.
283
[2] I.P. Zapesochny, I.S. Aleksakhin, Sov. Phys.: JETP 28 (1969) 41. [3] K.J. Nygaard, Phys. Lett. A 51 (1975) 171. [4] B.N. Roy, D.K. Rai, Phys. Rev. A 8 (1973) 849. [5] A.W. Pangantivar, R.J. Srivastava, J. Phys. B 20 (1987) 5881. [6] A.A. Borovik, E.A. Breza, I.P. Zapesochny, in: Proceedings of the XVI International Conference on the Physics of Electronic and Atomic Collisions, NY, USA, 1989, p. 875. [7] A.A. Borovik, in: Proceedings of the International Conference on The Centenary of Electron, Uzhgorod, Ukraine, 1997, p. 232. [8] B. Feuerstein, A.N. Grum-Grzhimailo, W. Mehlhorn, J. Phys. B 32 (1999) 4547. [9] A.N. Grum-Grzhimailo, K. Bartschat, J. Phys. B 33 (2000) 1843. [10] A.A. Borovik, A.N. Grum-Grzhimailo, K. Bartschat, in: Proceedings of the XXIII International Conference on the Physics of Electronic and Atomic Collisions, Stockholm, Sweden, 2003, p. We084. [11] A.A. Borovik, A.N. Grum-Grzhimailo, O.I. Zatsarinny, K. Bartschat, J. Phys. B 38 (2005) 1081. [12] A.A. Borovik, V.N. Krasilinec, J. Phys. B 32 (1999) 1941. [13] A.A. Borovik, Ukr. Phys. J. 45 (2000) 1270. [14] A.A. Borovik, Prib. Tekh. Eksp. (PTE): Sov. Phys. IET A 3 (1991) 124. [15] V.B. Gusev, A.I. Schaks, Prib. Tekh. Eksp. (PTE) N2 (1982) 80 (in Russian). [16] C.E. Moore, Atomic Energy Levels, Vol. 1, NBS Circular no 467, 1958. [17] B. Feuerstein, A.N. Grum-Grzhimailo, W. Mehlhorn, J. Phys. B 31 (1998) 593. [18] M.W.D. Mansfield, Proc. Roy. Soc. Lond. A 346 (1975) 539. [19] M.W.D. Mansfield, T.W. Ottley, Proc. Roy. Soc. Lond. A 365 (1979) 413. [20] A.A. Borovik, H. Rojas, G.C. King, E.Yu. Remeta, J. Phys. B 32 (1999) 4225. [21] P. Marmet, M. Proulx, in: Proceedings of the 4th International Symposium on Resonant Ionization Spectroscopy, Gaithersburg, MD, USA, 1988, p. 89.