High resolution deep-ultraviolet dual-comb spectroscopy of benzene, benzene-d6 and toluene

Accurate broadband, high-resolution spectral data for atmospheric molecules are essential for modelling exoplanet atmospheres and elucidating the complex chemistry of Earth’s atmosphere. However, existing molecular databases are severely lacking in both spectral resolution and coverage, especially in the ultraviolet (UV) regime. In this work, we use free running dual comb spectroscopy with its key strengths of broad spectral coverage, high spectral resolution and short acquisition times in the deep-UV to investigate benzene (C 6 H 6 ), deuterated benzene (C 6 D 6 ) and toluene (C 7 H 8 ). This study focuses on the 37,800 to 38,180 cm −1 (261.78 – 264.55 nm) spectral region with a spectral resolution of ∼ 0.034 cm −1 (= 1 GHz). Numerous weak vibronic transitions of C 6 H 6 are detected with absolute absorption cross sections down to 10 −21 cm² molecule −1 , revealing six previously unobserved spectral features and resolving the rovibrational substructure of several strong peaks. Comparison with previous studies yield line assignments for a majority of transitions. For toluene, the rotational structure of the 1 1 1 0 (37,945 cm −1 ) and 2 9 1 0 (38,019 cm −1 ) bands is resolved. Additionally, our measurements of the C₆D₆ spectrum provide the first high-resolution absorption cross section reported in this spectral region. A comparison of the C₆H₆ and C₆D₆ spectra provides the mode-dependent isotope shift for several transitions. Temperature-dependent measurements of C 6 H 6 across the 300 – 348 K range reveal hot bands that roughly double in intensity while cold bands stay constant. These results provide new spectroscopic insights for the development of rovibronic UV line lists of aromatic hydrocarbons highly relevant for atmospheric modelling.