Photoionization of epichlorohydrin enantiomers and clusters studied with circularly polarized vacuum ultraviolet radiation.

The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.

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