Beyond the Born-Oppenheimer approximation: High-resolution overtone spectroscopy of H_2D~+ and D_2H~+

摘要

Transitions to overtone 2v_2 and 2v_3, and combination v_2 + v_3 vibrations in jet-cooled H_2D~+ and D_2H~+ 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 2v_2, v_2 + v_3, and 2v_3 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 2v_3 and v_2 + v_3 vibrations in D_2H~+. Population-independent line intensity ratios for transitions from common lower states indicate excellent overall agreement with theoretical predictions for D_2H~+, but with statistically significant discrepancies noted for H_2D~+. Finally, H_2D~+ versus D_2H~+ isotopomer populations are analyzed as a function of D_2/H_2 mixing ratio and can be well described by steady state kinetics in the slit discharge expansion.