Laser Spectroscopy of Wire-Like Molecules

Abstract

Cavity-enhanced Laser-Induced Fluorescence

With the incorporation of a new optic mount and focussing lens a cavity ring-down set-up was altered to collect fluorescence signal from molecules seeded in a molecular beam. Running both the CRDS and fluorescence measurements simultaneously produced a technique with significantly enhanced sensitivity compared to either of the two individual methods. Preliminary analysis with SO2 and a selection of polyaromatic molecules point to the method been best suited for measuring the fluorescence spectra of fast fluorescence lifetime (< 1 ns) molecules and in confined environments i.e. molecular beams. This has been characterised using a known sample and also compared to the CRDS and fluorescence measurements ran as individual techniques.

The method was found to increase the sensitivity of the current experimental set-up by two orders of magnitude. This was then applied in the analysis of two new molecular samples to obtain spectra for use in analysing the torsional motions within the molecule.

Torsional Motion Analysis of 2,5-bis(phenylethynyl)thiophene

Torsional motions in polyphenyls, potential molecular wires, are important in the charge carriage capability of the species. Using a combination of UV-cavity ring-down and fluorescence detection the absorption and photoexcitation spectra of 2,5-bis(phenylethynyl)thiophene were successfully measured. Using a simple cosine potential and simulation program the spectra were reproduced and were found to be close to the values determined in previous research on the two ring system tolane and 1,4-bis(phenylethynyl)benzene.