Ejected electron spectrum of sodium autoionizing levels obtained by electron impact excitation 'at 500 eV incident energy

Ejected electron spectrum of sodium autoionizing levels obtained by electron impact excitation 'at 500 eV incident energy

Volume 56A, number 4 PHYSICS LETTERS 5 April 1976 EJECTED ELECTRON SPECTRUM OF SODIUM AUTOIONIZING LEVELS OBTAINED BY ELECTRON IMPACT EXCITATION AT...

184KB Sizes 0 Downloads 31 Views

Volume 56A, number 4

PHYSICS LETTERS

5 April 1976

EJECTED ELECTRON SPECTRUM OF SODIUM AUTOIONIZING LEVELS OBTAINED BY ELECTRON IMPACT EXCITATION AT 500 eV INCIDENT ENERGY T.W. OTTLEY and K.J. ROSS Department of Physics, University of Southampton, Southampton, UK Received 28 November 1975 The ejected electron spectrum of sodium vapour has been observed at 90°to the direction of an incident electron beam with kinetic energy 500 eV. Comparisons are made with the ultraviolet absorption data.

Continuing our present studies of autoionzing transitions in alkali metal vapours we wish to present preliminary data for sodium vapour obtained by observing ejected electron spectra at 90°to the direction of an incident electron beam of kinetic energy 500 eV. The apparatus used for this work is identical to that used for the study of potassium (Ottley and Ross [1]), and comprises a Superior Electronics 3k/5u electron gun operating at 500 eV, and a hemispherical electrostatic electron velocity analyser operating at a resolution of 150 meV throughout the spectrum. ~n the case of these sodium studies it was found neces-

sary to sacrifice resolution for intensity. We do not feel that this indicates a lower cross section for the processes which we observed, but rather that it reflects the mode of operation of the present apparatus and the serious magnetic field problems associated with it. 99.9% sodium was used for the present investigation. Fig. I shows the ejected electron spectrum obtamed for sodium vapour at the incident energy of 500 eV. Due to unknown contact potentials it is not possible for us to establish an absolute energy scale, and the spectrum was calibrated using the data of Ejected Electron Energy (eV)

25

26

27

28

29

30

31

32

33

34

0

Sodium 500 eV

Excited State Energy (eV) Fig. 1. Ejected electron spectrum of sodium obtained at 90°to the direction of the incident electron beam of energy 500 eV.

270

Volume 56A, number 4

PHYSICS LET1~ERS

5 April 1976

Table 1 Connerade et al. [2]

Wolff et al. [3]

Energy (eV) a b c d e f g h

k 1 m n o p q

30.77 t 30.93 31.65 31.78 32.83 33.06 33.26 33.57 33.99 34.42 34.95 35.41 35.64 35.98 t 36.21 36.30 36.86

r

37.23 36 56

S

.

u

38.12 38.23

Energy (eV)

Relative strength *

Energy (eV)

Relative strength

30.767 30.933

VS VS

30.768 ~30.934

VS VS

B B

34.468

W

B

35.402 35.631 35.985 36.217

W W VS S

VB FB B B

p37.544 ~-37.560 38.125 38.232

W VW FS S

Sh Sh FB B

FB FB

35.982 36.215

VS S

B FSh

37.224

FW FW FS

Sh Sh D

W

Sh B

r

37 546 .

38.126 38.233

*

VS Very strong, S Strong, FS Fairly Strong, FW Fairly Weak, W Weak, VW Very Weak; VB Very Broad, B Broad, FB Fairly Broad, D Diffuse, FSh Fairly Sharp, Sh Sharp. t Calibration points (see text). *

Connerade et a!. [2], and in particular, using the transitions at 30.77 eV and 35.98 eV as calibration lines, In this way the ejected electron energy scale was established. The lower excited state energy scale is ohtamed by adding the ionization potential of 5.14 eV to the ejected electron energy, assuming that autoionizing transitions take place into the continuum of Na!. At the high incident energies used for the present investigation excitation to higher continua is, of course, energetically possible. Table I lists those lines which have been consistently observed at 500 eV incident energy. Also listed are the corresponding transitions taken from the ultraviolet absorption data of Connerade et al. [2], and Wolff et al. [3] ; agreement is found between all the strong lines observed in our spectra and that of Wolff, and of the weak lines in the latter spectrum only those lying within 50 meV of our observed line centres are tabulated. The uncertainty in normalising to the cen-

tre of our line positions, together with non-linearities in the electronic scan mechanism result in an error of 25 meV in the determination of the energies in table 1. The data are the average of three independent spectra. The feature o is not indicated in fig. 1 as it did not show up as a prominent peak in that particular scan, although it did show itself as a small peak on the side of feature p in the two other spectra. There is a most striking difference between the present spectrum and the spectra of potassium, rubidium, and caesium, which we have reported in earlier publications (Ottley and Ross [1], Ross and Ottley [41and Ross et al. [5], respectively). In the case of potassium we observed two very weak transitions at 500 eV incident electron kinetic energy which were not present in the ultraviolet absorption spectra; for rubidium we did not observe any such transitions, while in the case of caesium we observed ten weak transitions not present in the optical studies. In the 271

Volume 56A, number 4

PHYSICS LETTERS

present work we observe nine transitions not present in the ultraviolet absorption spectrum, and in contrast to the other alkali metal vapours, a number of these, particularly those lines labelled d, h, i, and p, are seen to be very strong relative to the line labelled n, which is classified as strong by Connerade et a!. Also, in contrast to the caesium data, most of these lines lie in a region which does not contain any strong optical transitions. The possibility exists that molecular formation may be contributing to the spectrum. However, such features generally exhibit themselves in the region close to the leading doublet. On this basis only the structure centred at 31.78 eV can be considered as possibly deriving from molecular formation. The spectrum of Wolff et al. shows adsorption at 31.42 and 31.58 eV which may be associated with the structure in the same region of our spectrum. Work on the apparatus used for the present investigation has had to be discontinued while we complete the construction of a variable angle ejected electron spectrometer, and we do not have any low energy

272

5 April 1976

data on sodium. Clearly, the spectrum shown in fig. 1 indicates that work at low energies will be most important, particularly for an understanding of the features not observed by ultraviolet absorption spectroscopy. We acknowledge the receipt of a Research Fellowship (TWO) from S.R.C. Thanks ar~due to the Professors of Physics for providing the apparatus and laboratory facilities.

References [1] T.W. Ottley and K.J. Ross, J. Phys. B8 (1975) L249. Connerade, W.R.S. Garton and M.W.D. Mansfield, Astrophys. J. 165 (1971) 203. [3] H.W. Wolff, K. Radler, B. Sonntag and R. Haensel, Z. Physik 257 (1972) 353.

[21 J.P.

[4] K.J. Ross and T.W. Ottley, Phys. Lett. 54A (1975) 57. [5] K.J. Ross, W.R. Newell, T.W. Ottley and S.H. Al-Shamma, J. Phys. B8 (1975) L113.