Role of rotation–vibration interaction in vibrational relaxation. Energy redistribution in excited singlet formaldehyde

Abstract

The rotationally resolved fluorescence emission spectrum from S₁ H₂CO(ùA₂) following single rotational level (SRL) excitation has been studied in the “collision-free” regime of pressures. Mechanisms of intramolecular and intermolecular vibrational energy flow by vibration–rotation interaction are of interest, more specifically Coriolis coupling. An anomalous rotational line intensity distribution in the absorption spectrum with a strong r-form (ΔK=+1) sub-band and an accompanying weak p-form (ΔK=–1) sub-band, or vice versa, has been observed previously in asymmetric-top molecules due to Coriolis coupling. The presence of Coriolis coupling in asymmetric-top molecules can also be manifested by the emission spectrum, and the first such example is presented here with the rotationally resolved fluorescence emission from the 5¹ and 1 ¹4¹ vibronic levels of S¹ H₂CO with excess vibrational energies of 2968 and 2971 cm^–1, respectively. In the collision-free region of pressure the vibronic emission lines are mainly of p-form from the 5¹ level and r-form from the 1 ¹4¹ level. Coriolis coupling can also lead to the appearance of transitions which are forbidden in the zero-order approximation. Some emission to ν^″₄ is observed owing to mixing of the zero-order vibrational levels by Coriolis coupling.