Z. Physik 257, 43-- 50 (1972) 9 by Springer-Verlag 1972

Cross Sections for the Excitation of the 4 ~S-, 4 ~Dand 3 ~P-Levels of Helium Excited by Fast He+-Ions D. Hasselkamp, R. Hippler, A. Scharmann, and K. H. Schartner I. Physikalisehes Institut der Universitgt Giegen Received September 25, 1972 An experimental study has been made of the excitation of the 41S-, 41D- and 31P-levels of helium by fast He+-ions in the energy range of 80 to 1100 keV. Absolute cross sections have been obtained by normalization on proton-impact results. A pronounced broad maximum has been found in the three excitation functions. The results are compared with proton-impact results of equal velocities.

I. Introduction We have studied the excitation of the helium 4aS -, 4~D - and 3XPlevels by He+-impact in the energy range from 80-1 100 keV. Excitation cross sections have been calculated from the measured emission cross sections of the 4 ~ S - 2 1 p - , 4 ~ D - 2 ~ P - and 3aP-2~S-transitions of the He-atom. Absolute cross sections for the above mentioned processes have been reported by de Heer and van den Bos 1, Wolterbeek Muller and de Heer 2, and Blair and Gilbody 3 in the energy range up to 150 keV. Measurements of Dworetsky et al. 4 in the low energy range (below 5 keV) are not considered in this work. The purpose of this work is to extend the measurements of the above mentioned groups up to 1 100 keV. N o data are yet existing in this energy range. The apparatus has been described elsewhere 5. However, in order to separate radiation from direct and exchange processes by Doppler shift, the Leiss-monochromator was mounted at an angle of 70 ~ with respect to the beam, thus shifting emission lines of the fast particles to longer wavelengths. The slit widths of the monochromator varied between 0.15 and 0.3 mm. 1 Heer, F. J. de, Bos, J. van den: Physica 31, 365 (1965). 2 Wolterbeek Muller, L., Heer, F. J. de: Physica 48, 345 (1970). 3 Blair, W. F. G., Gilbody, H. B.: Abstracts of papers of the VIIth ICPEAC, p. 837. Amsterdam: North Holland 1971. 4 Dworetsky, S. H., Novick, R., Smith, W. W., Tolk, N.: Phys. Rev. Letters 18, 939 (1967).- Dworetsky, S. H., Novick, R. : Phys. Rev. Letters 23, 1484 (1969). 5 Hasselkamp, D., Hippler, R., Scharmann, A., Schartner, K. H.: Z. Physik 248, 254 (1971).

44

D. Hasselkamp et al.:

The ion beam traversed 10 cm of the collision chamber before it reached the observation region. The distance from the observation region to the entrance window of the optical device was about 0.5 cm. A polarization foil could be entered into the optical system. The single photon counting technique was applied. Background pulses amounted to 1-2 counts per second. Signal rates varied between 0.5/sec and 50/sec. Typical counting times were 100 and 1000 sec. The residual gas pressure was 4 . 1 0 .7 Torr. Helium pressures were 1 . 4 . 1 0 . 4 Torr in the case of the 41S - and 4~D-levels and 7 . 1 0 .5 Torr in the case of the 31P-level. The beam current varied between 0.5 and 50 gA. At energies below 100 keV and above 1000 keV very small beam currents and beam instabilities might give rise to additional errors.

H. Results

All measurements have been performed at single collision conditions. Signal-pressure-plots have been taken for the three emission lines in question. The emission radiation of the 4 1 S - 2 1 P - and 4 1 D - 2 1 P transitions is linear with pressure up to pressures of several 10 -a Tort. Data have been taken at 1 . 4 . 1 0 -4 Tort. Special care has been taken of the 3 1 P - 2 1 S - t r a n s i t i o n . Fig. 1 shows the corresponding signalpressure-plot. Linearity is maintained up to 9 . 1 0 -5 Torr. The data

COUNTS/Io0sec 1500

He§

He 175keV

//f

_

LLI n-

1000

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x

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12

5

10

15

20

PIle Fig. 1. Signal-pressure-plotof the 31p_ 2 zS-transition

x

10-5Torr

Cross Sections for the Excitation of the 4 1S-, 4 1D- and 3 1P-Levels Table 1. Excitation cross sections in units of 10 -19

crrl2

E (keV)

4 1S

4 1D

3 1p

C.C. (%)

80 100 150 200 300 400 500 600 700 800 900 1000

1.01 1.26 2.35 2.81 2.95 2.85 2.58 2.36 2.24 2.04 1.89 1.74 1.61

1.18 1.30 1.57 1.85 1.84 1.52 1.28 1.11 0.92 0.85 0.72 0.65 0.51

3.50 7.52 13.9 17.7 23.9 28.9 30.7 29.5 30.8 31.6 28.9 28.4

27.7 17.3 14.6 14.3 11.3 8.0 7.5 7.1 6.3 5.6 5.6 5.3

1100

45

C. C. : Cascade corrections to the uncorrected 3 1P-excitation cross section have been taken at a pressure of 7 . 1 0 - s Torr, which should be low enough to avoid resonance capture. The cross section results are presented in Table 1 and in Figs. 2, 3, and 4. The evaluation of the absolute cross sections was performed in the following way: First the emission cross sections were measured as a function of energy. At 360 keV the ratios of the light yields resulting from proton impact and He-ion impact have been measured for the 4 1 S - 2 1 P and 31P-21S-transitions under equal conditions. Knowing very well the absolute excitation cross sections for the 41S - and 31P-levels of helium for proton impact s, the cross sections for He +-impact on helium for these two levels were easily obtained (normalization on proton impact results). It seemed not favourable to use this procedure for the normalization of the excitation cross sections of the 41D-level as well, as for the proton impact results of the 41D-level greater errors must be accepted s. Absolute data were obtained by measuring the ratio of the emission cross sections for the 4 1 D - 2 1 P - and 41S-21P-transitions at 100 keV and then calculating the ratio of the corresponding excitation cross sections which is easily done when the lifetimes and the transition probabilities are known 6 (normalization on o-41s). (The different quantum efficiencies of the optical system for the three wavelengths have been taken into account.) 6 Gabriel, A.H., Heddle, D.W.O.: Proc. Roy. Soc. (London), Ser. A258, 124 (1960).- Bos, J. van den: Thesis, Amsterdam 1967.

46

D. H a s s e l k a m p et al.:

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present results MuUer. de Heer van den Bos et oL. Hasselktamp et al.

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1.0 84

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EHe+,/'EH+ Fig. 2. Excitation cross section a41 s as a f u n c t i o n of energy, x xx present results, ooo Wolterbeek Muller a n d de H e e r 2, A~, v a n d e n Bos, Winter a n d de Heer 7 ( H +impact), o ~ 1 7H6 a s s e l k a m p , Hippler, S c h a r m a n n , a n d Schartner s (H+-impact)

The cross sections have been corrected for the polarization of the emitted radiation and for the polarization due to the optical equipment. As there are yet no informations about higher levels of the He-atom excited by He + in the energy range considered here, cascade correction factors had to be calculated approximately by applying a n-3 dependence to the measured uncorrected cross section values up to n = 8. The results for the 31P-cross sections are given in Table 1 in column C.C. The so calculated cascade corrections for the 41S - and 4aD-cross sections were less than 3 and 1.5 700respectively in the whole energy range. These cross sections have not been corrected. It should be noted that due to the special mounting of the optical equipment the emission radiation of the 4~S-2~P-transition of the target gas will be disturbed by the Doppler shifted emission radiation of the 3XP-21S-transition of the impacting neutralized He-atom in the energy range from about 500 to 900 keV. In order to estimate the error caused by this effect we measured the ratio of the emission cross sections of both transitions at 200 and 400 keV, where resolution was

Cross Sections for the Excitation of the 4 IS-, 4 1D- a n d 3 !P-Levels

47

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EHe* j4EH* Fig. 3. Excitation cross section tr4~o as a function of energy, x x x present results, ooo Wolterbeek Muller a n d de Heer 2, , , , v a n den Bos, W i n t e r a n d de Heer 7 ( H +impact), o ~ o H a s s e l k a m p , Hippler, S c h a r m a n n , a n d Schartner s ( H + - i m p a c t )

10"19cm2

z 3(] o

• o zx =

present results de Heer, van den Bos 4~x I van den Bos et oL H+I Hasselkomp et OL t~ ~ /

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EHe§ 14EH+ Fig. 4. Excitation cross section 0"31P as a function of energy, xxx present results, ooo d e H e e r a n d v a n den Bos 1, ~ v a n den Bos, W i n t e r a n d de Heer 7 (H+-impact), o~o H a s s e l k a m p , Hippler, S c h a r m a n n , a n d Schartner s ( H + - i m p a c t )

48

D. Hasselkamp et al.:

yet possible. These measurements assured us that in the critical energy region this disturbance will not exceed 1 70 which can be neglected. Errors in the here presented cross section values are mainly resulting from the uncertainties in the proton impact results taken for normalization. Taking also into consideration the measuring process we believe our data to be accurate to 20~o in the case of the 41S - and 3 ~P-levels and to 30 700in the case of the 4 ~D-level. IH. Discussion

As is clearly seen, all three curves exhibit a pronounced broad maximum in the investigated energy region, at 260 keV for the 41S-level, at 250 keV for the 41D-level and at 650 keV for the 31P-level. Taken together with the results of Wolterbeek Muller and de Heer z, and de Heer and van den Bos ~ we find that the main feature in the cross sections for the considered processes is the existence of two well separated maxima at energies > 10 keV. For the excitation of the 3 ~P- and 41S-levels we find the high energy maximum in the cross sections to be considerably higher than the low energy maximum found by the other groups. This is different in the case of the 4~D-excitation where the low energy maximum dominates clearly the high energy maximum. The difference in the excitations functions for optically allowed (31P) and optically forbidden (41S, 41D) transitions, namely the shift of the cross section maxima for optically allowed transitions to higher energies is indeed found in the case of the high energy maxima. Moreover the prediction of the Bethe-Born-approximation in the high energy range, a41s> o41D, is confirmed. The comparison of the present results with the data obtained by the above mentioned groups in the overlapping energy region shows a systematic deviation which amounts to 25 ~o in the case of the 4~Sand 41D-levels at 100 keV. The agreement in the case of the 3~P-level seems to be accidental and may be explained by too high pressures used in their measurements. Measurements of Blair and Gilbody 3 for the 4~S- and 4~D-levels fit at 100 keV to our curves with a deviation of only 5 70. The data of this group however have been quoted as preliminary. To our knowledge no corrected data have been published by this group in the meantime. It is of considerable interest to compare the cross section ratios of the different groups in the overlapping energy region. This has been done in Table 2. The values of column A give the measured cross section ratios of our group. Column B shows the corresponding ratios obtained by calculating the ratios of the absolute cross sections of Table 1 (ob-

Cross Sections for the Excitation of the 4 1S-, 4 1D- and 3 1P-Levels

49

Table 2 Cross section ratios

A

80 keV

a41D/~41S

100 keV

1.03

80 keV

B

C

D

1.17

1.11

1.27

1.03

0.96

1.03

0.34

0.19

E

1.15

0"41D/O'31P

100 keV

0.17

80 keV ~4~s/az119

A: B: C: D: E:

100 keV

0.17 0.29

0.16

0.17

0.165

own cross section ratio measurements. determination of the ratios out of Table 1. de Heer, van den Bos1. Wolterbeek Muller, de Heer 2. Blair, Gilbody 3.

tained by the methods described in Chapter II). The agreement is extremely good. Comparing the results of the different groups there is also a good agreement in the case of a41D/a4,s. The comparison of the other ratios is of no use since no reliable data are available at 100 keV. There exist no theoretical calculations to compare our data with. It seemed therefore to be of particular value to compare proton- and He+-impact results at the same impact velocities. This has been done in Figs. 2, 3 and 4, where proton-impact results of van den Bos 7 and of our own group 5 are included. The relative course of the two excitation functions is very much alike. Both curves show two maxima though the low energy m a x i m u m in the proton-impact case is not always clearly separated. In both cases the high energy m a x i m u m dominates for the 4 ~S- and 3 ~P-levels while the low energy m a x i m u m dominates in the case of the 4aD-level. It is interesting to compare the positions of the maxima on the velocity scale. The high energy m a x i m u m of the three excitation functions is located at the same impact velocity for both impacting species (there seems to be a slight deviation in the case of the 41S-level). Concerning this point the predictions of the Bethe-Born-approximation are confirmed, namely that the magnitude of the cross section is determined by charge and velocity of the impacting particle and the oscillator strength. 7 Bos, J. van den, Winter, G. J., Heer, F. J. de: Physica 40, 357 (1968). 4 Z. Physik, Bd. 257

50

D. Hasselkamp et al. : Cross Sections for the Excitation of the 4 1S-Levels

For the low energy maximum the close correlation found above is only existing for the 41S-excitation function. In the case of the 31P as well as of the 41D-excitation functions the low energy maximum for He+-impact is located at smaller impacting velocities than those for proton-impact. This indicates a different excitation mechanism for proton- and He+-impact. We are indebted to Mr. Trylat for his assistance in constructing the target chamber and in taking the experimental data. Prof. Dr. A. Scharmann Dr. K.-H. Schartner Dipl.-Phys. D. Hasselkamp Dipl.-Phys. R. Hippler I. Physikalisches Institut der Justus Liebig-Universit/it D-6300 GieBen, Leihgesterner Weg 104 Federal Republic of Germany