Transient absorption spectroscopy in molecular beams

Molecular beams are the ideal environment for observing and understanding molecular dynamics on ultrafast timescales. The lack of solvent interactions and lowered vibrational temperatures make experimental signals easier to interpret and theoretical modeling more tractable enabling more direct comparisons. Unfortunately, optical ultrafast measurements of gas-phase samples are typically not possible due to the low sample densities. To address this gap, we have developed a new spectrometer using femtosecond enhancement cavities and frequency comb lasers with a demonstrated detection limit of ΔOD = 1 x 10^-9/√Hz, making transient absorption measurements in molecular beams possible. This technique also allows for direct comparison to conventional solution phase transient absorption studies as well as opens up new sample avenues possible only in molecular beams such as clusters or radicals. In this talk, we will discuss ultrafast transient absorption measurements in molecular beams and direct comparison to solution-phase data. We explore the effect of argon clustering on excited-state proton transfer and internal conversion in salicylideneaniline and the competition between intersystem crossing and internal conversion in 2-thiouracil.