Single and multiphoton ionisation of short lived molecules in the gas phase

Baker, Jacob (1991) Single and multiphoton ionisation of short lived molecules in the gas phase. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The work presented in this thesis is concerned with the study of small, unstable molecules by photoelectron spectrosocpy and multiphoton spectrosocpy. The first half of this thesis is concerned with the photoelectron spectra of some unstable molecules produced by rapid atom-molecule and molecule-molecule reactions in the gas phase. The experimental techniques used are described, as are the theoretical ab-initio methods used to aid in the assignment and interpretation of the photoelectron spectra. The Hel photoelectron spectra of the nitroxide radical NH₂O, and nitrosyl hydride HNO, generated in the F + NH₂OH reaction have been recorded. The photoelectron spectra and ab-initio calculations performed in this study also indicate an effectively planar structure for the NH₂O(X²B₁) state and the a³A₂ and A¹A₂ states of the cation. The Hel photoelectron spectra of molecules formed from the reactions of atomic and molecular fluorine with Me₂S and Me₂S₂ are presented. The photoelectron spectra of primary reaction products CH₃S, CH₃SCH₂ and CH₃SSCH₂ are presented together with photoelectron spectra that can be assigned to the HFCS, HCF and CH₃SF molecules. Ab-initio calculations have been performed on all the above molecules (their neutral ground states and low lying cationic states), which include computation of ionisation energies, equilibrium geometries and vibrational frequencies, to aid in assignment of photoelectron bands. The second half of this thesis is concerned with the REMPI spectra of some sulphur containing diatomic molecules. The experimental techniques used are described, as are the theoretical methods employed in calculation of rotational line strengths to aid in assignment of resonance state symmetries and rotational structure of the observed bands. The REMPI spectrosocpy of the diatomic molecules SO(X^3Σ-), S₂(X³Σg^-) and NS(X²II) have been investigated. Several low lying Rydberg states have been observed and characterised, many for the first time, from their REMPI spectra. A variety of techniques has been employed in the characterisation of each transition and these include; recording spectra with different photon orders, laser polarisation dependence studies, measurement of vibrational and spin-orbit structure, rotational analysis and band contour simulations. For SO(X^3Σ-) three new Rydberg states have been observed and these are assigned to the (4p)^{3Δ, (3d)}^3 Φ and (3d)³II states. For S₂(X³ Sigmag _g^-), the first Rydberg state of gerade symmetry, assigned to the (3d)^3Δg state, is reported.