- Email: [email protected]
Electron affinities of some elements
Volume
45A, number
5
PHYSICS
ELECTRON
I. Institut
AFFINITIES
LETTERS
22 October
OF SOME ELEMENTS
D. FELDMANN and R. RACKWITZ f. Experimentalphysik, Universitiit Hamburg,
E. HEINICKE and H.J. KAISER II. Physikalisches Institut. Universitat Heidelberg, Received
1973
3 September
German)
Germany
1973
Photodetachment cross sections of several atomic negative ions have been measured in order to supply a fairly complete set of experimental values for the electron affinity EA of the elements of the short periods.
The measurements were performed using a crossedbeam technique. The negative ions were formed in a Penning ion source with radial extraction [ 11. This source has proved most suitable for the production of negative ions, ion beams of almost all elements have been obtained with reasonable intensity ( 1O-9 to 10p5A). A high pressure xenon arc lamp combined with a prism monochromator serevd as a light source in the photon energy range from 0.5 eV to 3 eV. The detachment chamber was modified as to be able to detect the neutral atoms produced along with the detached electrons. A chopped light beam and Lock-In technique was employed to discriminate the signal against background [2]. The electron affinities EA were determined from the threshold energies of the photodetachment cross sections [3] . The value for Rb and Cs were obtained by extrapolating the cross section below 0.5 eV. For the negative ions of the third group: BB, Caa, In-, Tl- not threshold was found between 0.5 eV and 3 eV, therefore we can give only an upper limit for the EA. The cross sections of Gee and Sn- show low energy tails in accordance with the assumption that our ion beams contained ions in the metastable 2D state. Bi- shows a steep rise of the cross section curve at 0.9 eV with a resonance-like structure peaking at 1.05 eV, which could not be explained yet Since this peak might obscure the true onset, we can only give an approximate value of the electron affinity. In all other cases a rather unambiguous determination of the threshold energies and electron affinities was possible. 404
Table 1
_ Atom
EA (eV)
Li Na K Rb CS
0.61 0.53 0.50 0.48 0.47
B, Ga, In, Tl
less than 0.50
Ge
1.20 1.25 0.66 0.77 0.80 1.05 0.9 1.9
Sn Cr P As Sb Bi Te
f; * + + +
f; -t * t f i +
0.05 0.05 0.05 0.05 0.05
0.1 0.1 0.05 0.05 0.05 0.05 1.2 0.15
A compilation of our results is given in table 1. A complete set of other data concerning electron affinities of these and other elements was given by Steiner 141 We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft.
References [l]
H.J. Kaiser, E. Heinicke, H. Baumann and K. Bethge, Z. Physik 243 (1971) 46. [2] D. Feldmann, Z. Naturforsch. 25a (1970) 621. [3] H.J. Kaiser, E. Heinicke, R. Rackwitz and D. Feldmann, to be published. [4] B. Steiner, in Case studies in Atomic Collion Physics II, eds. E.W. McDaniel and M.R.C. McDowell (North-Holland, 1972) p. 483.
45A, number
5
PHYSICS
ELECTRON
I. Institut
AFFINITIES
LETTERS
22 October
OF SOME ELEMENTS
D. FELDMANN and R. RACKWITZ f. Experimentalphysik, Universitiit Hamburg,
E. HEINICKE and H.J. KAISER II. Physikalisches Institut. Universitat Heidelberg, Received
1973
3 September
German)
Germany
1973
Photodetachment cross sections of several atomic negative ions have been measured in order to supply a fairly complete set of experimental values for the electron affinity EA of the elements of the short periods.
The measurements were performed using a crossedbeam technique. The negative ions were formed in a Penning ion source with radial extraction [ 11. This source has proved most suitable for the production of negative ions, ion beams of almost all elements have been obtained with reasonable intensity ( 1O-9 to 10p5A). A high pressure xenon arc lamp combined with a prism monochromator serevd as a light source in the photon energy range from 0.5 eV to 3 eV. The detachment chamber was modified as to be able to detect the neutral atoms produced along with the detached electrons. A chopped light beam and Lock-In technique was employed to discriminate the signal against background [2]. The electron affinities EA were determined from the threshold energies of the photodetachment cross sections [3] . The value for Rb and Cs were obtained by extrapolating the cross section below 0.5 eV. For the negative ions of the third group: BB, Caa, In-, Tl- not threshold was found between 0.5 eV and 3 eV, therefore we can give only an upper limit for the EA. The cross sections of Gee and Sn- show low energy tails in accordance with the assumption that our ion beams contained ions in the metastable 2D state. Bi- shows a steep rise of the cross section curve at 0.9 eV with a resonance-like structure peaking at 1.05 eV, which could not be explained yet Since this peak might obscure the true onset, we can only give an approximate value of the electron affinity. In all other cases a rather unambiguous determination of the threshold energies and electron affinities was possible. 404
Table 1
_ Atom
EA (eV)
Li Na K Rb CS
0.61 0.53 0.50 0.48 0.47
B, Ga, In, Tl
less than 0.50
Ge
1.20 1.25 0.66 0.77 0.80 1.05 0.9 1.9
Sn Cr P As Sb Bi Te
f; * + + +
f; -t * t f i +
0.05 0.05 0.05 0.05 0.05
0.1 0.1 0.05 0.05 0.05 0.05 1.2 0.15
A compilation of our results is given in table 1. A complete set of other data concerning electron affinities of these and other elements was given by Steiner 141 We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft.
References [l]
H.J. Kaiser, E. Heinicke, H. Baumann and K. Bethge, Z. Physik 243 (1971) 46. [2] D. Feldmann, Z. Naturforsch. 25a (1970) 621. [3] H.J. Kaiser, E. Heinicke, R. Rackwitz and D. Feldmann, to be published. [4] B. Steiner, in Case studies in Atomic Collion Physics II, eds. E.W. McDaniel and M.R.C. McDowell (North-Holland, 1972) p. 483.