An axial molecular-beam diode laser spectrometer

A mid-infrared tunable diode laser molecular-beam spectrometer for the purpose of trace gas sensing and the study of van der Waals complexes is described. The spectrometer employs a Herriott multipass cell with up to 72 passes. The sample gas is injected parallel to the optical axis through a hole at the center of the far mirror. The molecular absorption is Doppler split, resulting from the laser beam propagating parallel and antiparallel to the molecular-beam expansion. The axial expansion leads to narrower line widths and increased sensitivity, compared to the traditional vertical injection method, as a result of selective sampling of the central part of the molecular expansion with reduced Doppler broadening and longer residence time of the molecular sample in the laser beam. The molecular expansion leads also to selective signal enhancement of low-J transitions, as demonstrated for the ν 3 antisymmetric stretch vibration of CO 2 . A microwave horn antenna was implemented into the spectrometer to enable microwave–infrared double-resonance experiments. The spectrometer performance was evaluated by recording spectra of the CO 2 – Ar , ( CO 2 ) 2 , CO 2 – He , and CO 2 – SO 2 van der Waals complexes near the R(0) transition of the ν 3 band of CO 2 around 2349 cm−1. The feasibility of using a pulsed molecular expansion for trace gas sensing is explored.