Beyond the Born–Oppenheimer approximation: High-resolution overtone spectroscopy of H2D+ and D2H+

Abstract

Transitions to overtone 2ν2 and 2ν3, and combination ν2+ν3 vibrations in jet-cooled H2D+ and D2H+ molecular ions have been measured for the first time by high-resolution IR spectroscopy. The source of these ions is a pulsed slit jet supersonic discharge, which allows for efficient generation, rotational cooling, and high frequency (100 KHz) concentration modulation for detection via sensitive lock-in detection methods. Isotopic substitution and high-resolution overtone spectroscopy in this fundamental molecular ion permit a systematic, first principles investigation of Born–Oppenheimer “breakdown” effects due to large amplitude vibrational motion as well as provide rigorous tests of approximate theoretical methods beyond the Born–Oppenheimer level. The observed overtone transitions are in remarkably good agreement (<0.1 cm−1) with non-Born–Oppenheimer ab initio theoretical predictions, with small but systematic deviations for 2ν2, ν2+ν3, and 2ν3 excited states indicating directions for further improvement in such treatments. Spectroscopic assignment and analysis of the isotopomeric transitions reveals strong Coriolis mixing between near resonant 2ν3 and ν2+ν3 vibrations in D2H+. Population-independent line intensity ratios for transitions from common lower states indicate excellent overall agreement with theoretical predictions for D2H+, but with statistically significant discrepancies noted for H2D+. Finally, H2D+ versus D2H+ isotopomer populations are analyzed as a function of D2/H2 mixing ratio and can be well described by steady state kinetics in the slit discharge expansion.