Resonant two-photon ionization spectroscopy in supersonic beams

Abstract

The use of supersonic beams provides internally ultracold molecules which exhibit sharp spectral features (0.2 Å = FWHM) in UV–visible spectroscopy. In particular, we have studied various disubstituted benzenes in expansions of 1-atm backpressure of Ar. These species have been studied in a time-of-flight mass spectroscopy using resonant two-photon ionization spectroscopy. The focus of our recent work has been to determine problems associated with ionizing specific classes of molecules, in particular, halogenated aromatic species. Ionization in resonant two-photon ionization has been found to strongly depend on the substitution groups present and their position on the benzene ring. Different groups tend to shift the ionization limit and the resonant absorption relative to benzene, often resulting in low efficiencies in resonant two-photon ionization. This appears to be especially true in orthosubstituted compounds due to a combination of coulombic interactions which shift ionization potential, and steric factors which decrease the transition probability of the S0 → S1 transition. We have studied this phenomenon to develop rules to describe general trends in resonant two-photon ionization. Further, we describe several analytical applications of this method including isomer discrimination, trace analysis in mixtures, and isotopic enhancement of chlorinated compounds.