High-resolution spectra of [formula omitted] overtone of trans-formic acid in the supersonic jet

Abstract

First high-resolution low-temperature spectrum of the 2νOH overtone of trans-formic acid have been measured using direct-absorption laser spectroscopy. The supersonic cooling in the jet expansion to 37 K has dramatically simplified the complex ro-vibrational structure of the band due to the thermal depopulation of higher-lying rotational states. Altogether, 597 lines have been measured with a line-position accuracy better than 0.001 cm−1, and 441 of them have been reliably assigned using a custom-build program for a line-by-line assignment using the ground state combination differences. In total, 39 low-lying upper rotational states of the overtone vibration have been calculated from these line assignments. Moreover, some additional states have been also identified and tentatively assigned to a bath of dark states present in this spectral region. A global fit using A-reduced Watson’s type Hamiltonian in Ir representation has been performed to obtain several ro-vibrational constants for the 2νOH vibration. In addition, the fit demonstrates the effect of numerous perturbations in the rotational structure.