Ionic and Rydberg states of CF₃I studied by high resolution photoelectron (ZEKE-PFI) and resonance-enhanced multiphoton ionisation spectroscopy

Abstract

Ionic and Rydberg states of CF₃I have been studied using both zero kinetic energy pulsed-field ionisation (ZEKE-PFI) photoelectron and resonance-enhanced multiphoton ionisation (REMPI) spectroscopy. The ground state of the ion (X̃²E_3/2) was characterised using coherent two-photon (one-colour) ZEKE-PFI spectroscopy and extended progressions in the symmetric vibrational modes, similar to those observed in the conventional He(I) photoelectron spectrum, were observed at high resolution. The [²E_3/2]6p Rydberg states were studied using two- and three-photon REMPI spectroscopy with linearly and circularly polarised light. This resulted in the assignment of at least three states with distinct Ω values (2, 1 and 0) with the Ω=2 components being the most intense. The strongest members of the [²E_3/2]6p;2 system were used as resonant intermediates in two-colour (2+1′) ZEKE-PFI experiments which allowed the unambiguous assignment of the majority of the vibrational structure of the intermediate states. The adiabatic ionisation energy of CF₃I was determined as 83652±2 cm^-1. The vibrational structure observed for one of the Ω=2 components of the [²E_3/2]6p cluster in CF₃I differs considerably from that observed for the lower lying 6s states and the analogous 6p states of CH₃I. In particular, ν₃ (the carbon–iodine stretch) shows a vibrational frequency substantially reduced from that of the neutral and ionic ground states. It is concluded that at least one of the [²E_3/2]6p states of CF₃I is distorted, possibly through an interaction with an ion-pair state.

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