Beyond the Born-Oppenheimer Approximation: High-Resolution Overtone Spectroscopy of H2D+ and D2H+
M Farnik, S Davis, David Nesbitt
Transitions to overtone 2upsilion}2 and 2upsilion}3 and combination upsilion}2+upsilion}3 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(<0.2 cm-1) with theoretical predictions of Tennyson and co-workers (J. Tennyson, private communication), with small but systematic deviations for 2upsilion}2, upsilion}2 + upsilion}3, and 2upsilion}3 excited states indicating directions for further improvement in such non-Bom-Oppenheinier treatments. Spectroscopic assignment and analysis of the isotopomeric transitions reveals strong Coriolis mixing between near resonant 2upsilion}3 and upsilion}2 + upsilion}2 + upsilion}3 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+, but with statistically significant discrepancies noted for H2D+. Finnally H2D+ vs. D2H+ isotopomer populations are analyzedas a function of D2/H2 mixing ratio and can be well described by steady state kinetics in the slit discharge expansion.
, Davis, S.
and Nesbitt, D.
Beyond the Born-Oppenheimer Approximation: High-Resolution Overtone Spectroscopy of H<sub>2</sub>D<sup>+</sup> and D<sub>2</sub>H<sup>+</sup>, Journal of Chemical Physics
(Accessed December 6, 2023)