High-resolution infrared overtone spectroscopy of ArHF via Nd:YAG/dye laser difference frequency generation

Abstract

The first high-resolution spectra of ArHF excited to the vHF=2←0 manifold near 7800 cm−1 are recorded via direct infrared absorption in a slit supersonic expansion. The tunable difference frequency light is generated via nonlinear subtraction of a cw Nd:YAG laser from a tunable cw ring dye laser in temperature phase matched LiNbO3, and permits continuous single-mode access to the 1–2 μm near-IR region. Rotationally resolved spectra are presented for the pure HF stretching overtone (2000)←(0000), as well as for combination band excitation into the Σ bend (2100)←(0000) and Π bend (2110)←(0000) internal rotor levels built on the vHF=2 overtone stretch. Local perturbations in the Π bend spectrum are observed which arise from a resonant crossing of rotational levels with the (2002) van der Waals stretch and allow spectroscopic analysis of this state. Nonresonant coupling between the Σ and Π bend vibrational levels is evidenced by anomalous P branch/R branch transition intensities and is analyzed as Coriolis interactions in a tumbling, hindered rotor. The spectra reveal Doppler limited line shapes [Δν=79(11) MHz] characteristic of the temperature and geometry of the slit expansion. An upper limit of Δνprediss≤2 MHz Lorentzian broadening is established, indicating an 80 ns lower limit to the predissociation lifetime. Comparison of intermolecular vibrational levels in ArHF vHF=0, 1, and 2 indicates a systematic increase in both angular anisotropy and radial well depth upon excitation of the high-frequency HF stretch. In conjunction with previous results from the vHF=1 and vHF=0 vibrational levels, these studies provide the necessary data for fitting an atom+diatom potential energy surface as a function of all intermolecular and intramolecular internal degrees of freedom.