The relative cross section for the gas‐phase photodetachment of electrons has been determined for in the wavelength region 1195–1695 nm (1.04–0.73 eV) and for in the wavelength region 620–1010 nm (2.00–1.23 eV). An ion cyclotron resonance spectrometer was used to generate, trap, and detect the negative ions. A 1000‐W xenon arc lamp with a grating monochromator was used as the light source, except for one series of experiments in which a tunable laser was employed. Single sharp thresholds were observed in both cross sections, with the result that the following electron affinity values have been determined: E.A. eV and E.A. eV.
REFERENCES
1.
L. M. Branscomb, in Atomic and Molecular Processes, edited by D. R. Bates (Academic, New York, 1962), pp. 100–140.
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4.
K. C.
Smyth
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56
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K. C.
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Pressure was recorded at the Vac‐Ion pump; the error for estimating the pressure actually in the cell may be as large as a factor of 2.
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R. N.
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See also
J. A. D.
Stockdale
, R. N.
Compton
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Reinhardt
, Intern. J. Mass Spectry. Ion Phys.
4
, 401
(1970
); these authors attribute the production of to a reaction at the filament.12.
H.
Ebinghaus
, K.
Kraus
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Müller‐Duysing
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Neuert
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19a
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14.
See Ref. 6 and references cited therein.
15.
The subsequent development assumes constant drift velocities in the source and analyzer regions; the net drift motion of the ions is collinear with the light beam.
16.
17.
For the work on (Ref. 4) the residence time of the ions was found to be on the order of seconds. The hard‐sphere collision frequency for molecules at 288° K and torr is approximately [E. W. McDaniel, Collision Phenomena in Ionized Gases (Wiley, New York, 1964), p. 35]. Total reduced collision frequencies for ionic species are expected to be much larger;
and C. A. Lieder, R. W. Wien, and R. T. McIver, Jr., “Ion‐Molecule Collision Frequencies in Gases Determined by Phase Coherent Pulsed ICR,” J. Chem. Phys. (to be published).
18.
Values used: maximum fractional signal decrease is 0.10; analyzer residence time is one‐tenth the source residence time;
19.
For the conversion to product ion, namely via an impurity giving was not so great as for giving (Ref. 4);
nevertheless, the photodetachment signal to noise was a good deal poorer. Since is reported to have a high electron affinity [
J.
Berkowitz
, W. A.
Chupka
, and T. A.
Walter
, J. Chem. Phys.
50
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(1969
)], an optical double resonance experiment was performed in which was monitored while photodetaching The resulting curve shows the same structure as Fig. 1 and is actually smoother.20.
D. M.
Golden
, R. K.
Solly
, N. A.
Gac
, and S. W.
Benson
, J. Am. Chem. Soc.
94
, 363
(1972
); Ref. 10 summarizes previous determinations.21.
Both kinetic energy measurements (Refs. 9 and 10) employed a retarding field technique which estimates only the maximum kinetic energy of the most energetic ions formed at a given electron energy.
22.
The first excited electronic state of lies 1.27 eV above the ground state: G. Herzberg, Electronic Spectra and Electronic Structure of Polyatomic Molecules (Van Nostrand, Princeton, N.J., 1966), p. 584.
23.
J. I.
Brauman
et al., J. Am. Chem. Soc.
93
, 6360
(1971
); this paper presents a study of the thermochemistry for proton transfer reactions of the binary hydrides.24.
Reference 22, pp. 117, 221, and 493.
25.
Reference 22, p. 585.
26.
However, the ammonia bond strength derived in Compton’s study disagrees with that in Ref. 20.
27.
For an electronic transition in which the equilibrium internuclear distances do not change and for a given normal mode, the fractional intensity of the 0‐0 band exceeds 0.94 even when the vibrational frequencies in the two states differ by a factor of 2; see Ref. 22, p. 150 and H. M. Rosenstock and R. Botter, in Recent Developments in Mass Spectroscopy, edited by K. Ogata and T. Hayakawa (University Park, Baltimore, Md., 1970), p. 797.
28.
29.
Experimental determinations: (a)
V. I.
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and V. M.
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37
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)[
V. I.
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and V. M.
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10
, 465
(1960
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S. J.
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(1959
);(d) ,
Astrophys. J.
D.
Feldmann
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25a
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(1970
).30.
For example, in experiments subsequent to those discussed in Ref. 5, the photodetachment of at 1000 nm reveals a substantial signal decrease due to the photodetachment of the precursor ion
31.
32.
1.12 eV, F. M. Page and G. C. Goode, Negative Ions and the Magnetron (Wiley‐Interscience, London, 1969), p. 137;
(c)
I. P.
Fischer
and E.
Henderson
, NASA Star
6
, 1493
(1968
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G. C.
Bayley
, Advan. Mass Spectrometry
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, 726
(1968
).33.
34.
This value is the highest currently proposed; see Ref. 10. Note that for a lower ammonia bond strength the upper limit for the electron affinity of is also lower.
35.
R. N.
Dixon
, G.
Duxbury
, and H. M.
Lamberton
, Proc. Roy. Soc. (London)
305a
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(1968
).36.
Reference 22, p. 116.
37.
Reference 22, p. 587.
38.
The 949‐, 959‐, and 969‐nm points were used from Fig. 2 and the 948‐, 958‐, and 968‐nm points from Fig. 3.
39.
Using the threshold wavelengths cited in the text, a plot of ln(σ) vs where is the ejected electron energy, gives the following results: (a) for the 929–969‐nm points of Fig. 2 the slope is
(b) for the 928–968‐nm points of Fig. 3 the slope is These values should be compared with a similar plot for the monochromator data; for the 947.5–987.5‐nm points and the threshold wavelength of 994.3 nm, the slope was found to be See Ref. 4 for a discussion of threshold laws.
40.
41.
K. C. Smyth and J. I. Brauman, “Photodetachment of an Electron from Selenide Ion; the Electron Affinity and Spin‐Orbit Coupling Constant for SeH,” J. Chem. Phys. (to be published).
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