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Electron spectroscopy using metastable helium atoms (2¹S,2³S) and 584 Å photons Yee, Derek Sui Chang

Abstract

Electron spectroscopy using metastable helium atoms (2¹S,2³S) and 584 Å photons Using a high resolution 127 degree electron analyzer, a quantitative comparison has been made of the He* (21S,23S) Penning electron and the 584 % photoelectron spectra of thirty molecules (H₂, HD, D₂, N₂, CO, NO, O₂, NH₃, PH₃, C₂H₄, HCN, (CN)₂, CH₃CN, BrCN, ICN, H₂O, CH₃OH, CH₃CH₂OH, (CH₃)₂CHOH, (CH₃)₃COH, CH₃OCH₃, CH₂OCH₂, CH₃CH₂OCH₂CH₃, CH₂CH₂OCH₂CH₂, OCH₂CH₂OCH₂ CH₂, HCHO, CH₃CHO, CH₃COCH₃, HCOOH and CH₃COOH). Where vibrational structure has been resolved, the vibrational spacings are observed to be the same (within experimental error) for both modes of ionization. In addition, the relative vibrational intensities for ionic states of the diatomic molecules (except for O₂) were found to be the same for both modes of ionization. For the process He*(2³S)/O⁺₂ (X²πg), an observed difference of the vibrational envelope for He*(2³S) Penning ionization and 584 Å photoionization has been traced to an autoionizing level of O₂ which is essentially resonant with the He*(2³S) metastable energy. Differences between the vibrational envelopes for Penning ionization and photoionization have also been observed for the ground ionic state of a number of oxygen containing molecules. These differences have been explained by pertubations of the potential surface of the target molecule due to the presence of the metastable atom. When comparing the relative electronic state populations for the two modes of ionization, large differences were observed. Some of the difference has been explained by the fact that the state populations are a function of the excitation energy, since comparisons were made between photons with the energy 21.22 eV and metastable atoms with the energies 20.62 eV and 19.82 eV. For the -C≡N containing molecules it was observed that the ratios of the relative populations of states corresponding to the removal of π bonding to nitrogen lone pair were significantly greater for photoionization than for Penning ionization. Finally, the energy shifts ΔE[sub obs] were measured for the obs He*(2¹S,2³S) Penning ionization process. When vibrational structure was apparent in the electron spectra, it was possible to evaluate the true AE energy shift ΔE[sub obs]. The magnitude of the true ΔE energy shift was found to be of thermal energies.

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