Rotation-induced vibrational mixing in X̃ 1A1 formaldehyde: Non−negligible dynamical consequences of rotation

Abstract

Individual rotation-vibration levels of the formaldehyde X̃ 1A1 state with 7400<Evib<8600 cm−1 have been examined by the stimulated emission pumping (SEP) technique. At low values of the rotational quantum number (J≤3), the SEP spectra were simple. The only vibrational levels which appeared in the spectra were those expected either to have large Franck–Condon overlap with the à 41 level or to have appreciable Fermi resonance with a nearby Franck–Condon allowed level. At higher J and Ka values, the spectra rapidly became more complex and the observed level densities at J≊10, Ka≊2 were several times larger than the known total density of vibrational levels. This increase in the density of spectrally accessible vibrational levels was a result of rotation-induced mixing of the anharmonic vibrational basis functions (Coriolis coupling) which compromised the ‘‘goodness’’ of both vibrational and Ka quantum numbers. Coriolis matrix elements computed in a harmonic normal mode basis set qualitatively confirmed the importance of rotation-vibration mixing. The failure to obtain quantitative agreement is attributed to anharmonic effects. The rotation-dependent vibrational mixing effects observed in the SEP spectra indicate the importance of rotation in intramolecular vibrational dynamics and mode-selective vibrational excitation. Rotation significantly diminishes the structural differences (manifest in Franck–Condon factors, rotational constants, electric dipole moments) between rotationless vibrational levels and promotes an averaging of the character of near degenerate vibrational levels together with a partial destruction of the Ka rotational quantum number. This means that the onset of the quasicontinuum in infrared multiphoton dissociation and the inhomogeneous widths of high overtone bands would be very different for excitation out of a single low vs high J level.