Variable Angle Photoelectron Spectroscopy

Citation

Mintz, Donald Mills (1976) Variable Angle Photoelectron Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n30x-qw67. https://resolver.caltech.edu/CaltechTHESIS:02242017-160501611

Abstract

This thesis describes the design, construction, and performance of a variable angle photoelectron spectrometer. The experiment consists of a stationary resonance line lamp, a sample chamber, and a rotatable electrostatic electron energy analyzer. Experiment operation is extensively automated through several interfaces to a small on-line digital computer.

Results are presented for a variety of atomic, diatomic, and polyatomic sample gases. In the case of Ar, Ne, N₂, and CO, higher order lines in the resonance line lamp produce many peaks which do not overlap extensively with those of the principal vacuum-ultraviolet output. The additional lamp output lines allow for measurement of the variation of the photoelectron angular distribution asymmetry parameter, β, with photoelectron energy. Results for Ar and Ne are in good agreement with previous theoretical calculations.

For N₂, we find a discontinuity in the variation of β near 5 eV photoelectron energy for the v' = 0 peak of the X²Σ⁺_g ionic state. The result is interpreted in terms of the possible mechanisms: autoionization, variation of the electric dipole transition moment with internuclear distance, and resonance phenomena. For peaks of the A²∏_u ionic state, the variation of β with ionic vibrational level matches the variation with photoelectron energy. For CO, anomalous structure is assigned to higher peaks of the X²Σ⁺ vibrational progression. The dominant mechanism is probably autoionization.

Photoelectron angular distributions were measured for the series of olefins: ethylene, isobutylene, 2-methyl,2-butene, and 2, 3-dimethyl, 2-butene. The variation of β with electron energy was inferred by a comparison of data for the four structurally similar molecules. In the σ2p orbital ionization region of the spectrum, we find a similar variation of β with electron energy over bands corresponding to widely varying orbital symmetries. In the π orbital ionization region of the spectrum, β increases with increasing electron energy, taken from the variation of β across the vibrational envelopes. Accompanying the drop in β for π ionization with increasing methyl substitution is an anomalous drop in β at the high ionization potential end of the σ2p region which may indicate the presence of a σ/π orbital mixing.

Photoelectron angular distributions were measured for a series of di-olefins. In the case of allene, we propose the ordering 2e, 3b₂, 1e for the three lowest-lying orbitals on the basis of a deconvolution of the band in the ionization potential range 14-16.5 eV. A re-examination of 1,3-butadiene supports the π, π, σ energy ordering of low-lying molecular orbitals. The variation of β across the σ2p region in the photoelectron spectra of 1,4-pentadiene and 1, 5-hexadiene supports an earlier notion that through-space interaction of the π-bonds dominates through-bond interaction. Spectra of the π region suggest that the interaction between π moieties decreases in going from 1,4-pentadiene to 1,5-hexadiene.

In an examination of low-lying structure of 1,4-cyclohexadiene, the anomalously low β value for the lowest π-ionization band probably arises from σ/π mixing.

In an examination of the two low-lying photoelectron bands of pyridine below 11 eV IP, the sharp rise of β across the 9-10 eV band is interpreted in terms of the orbital ordering n, π, π.

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