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

Transitions to overtone 2nu(2) and 2nu(3), and combination nu(2)+nu(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 (<0.1 cm(-1)) with non-Born-Oppenheimer ab initio theoretical predictions, with small but systematic deviations for 2nu(2), nu(2)+nu(3), and 2nu(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 2nu(3) and nu(2)+nu(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+. Finally, H2D+ versus D2H+ 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. (C) 2002 American Institute of Physics.