Pure vibrational spectroscopy of S0 formaldehyde by dispersed fluorescence

Abstract

Dispersed fluorescence spectra from the 0₀₀ rotational level of 4⁰, 4¹, 5¹, and 3¹4¹ S₁ formaldehyde (H₂CO) have been recorded. From these spectra, 198 new vibrational states have been assigned with energies up to 12 500 cm⁻¹, and their positions have been determined to within an uncertainty of 1 cm⁻¹. The assignment of vibrational lines to specific vibrational states becomes increasingly difficult at the higher energy regions of the spectra (≳9000 cm⁻¹) due to extensive state mixing. Harmonic and first‐order anharmonic vibrational constants were extracted from fits to these vibrational states. For states with highest zero‐order coefficient squared greater than 35%, the standard deviation of the spectroscopic fit is 6.9 cm⁻¹. For states which are lower energy (−1) and relatively pure (zero‐order coefficient squared greater than 0.75 or largest in a given normal mode combination), the standard deviation is 1.7 cm⁻¹. Good agreement with ab initio vibrational constants calculated by Martin et al. [J. Mol. Spectrosc. 160, 105 (1993)] is achieved, except in cases where all observed states contributing to the determination of a particular constant are significantly mixed. These deviations are readily explained by a consideration of anharmonic vibrational interactions that occur among specific combinations of normal modes. The average mean deviation between all experimentally determined energies and a recent theoretical calculation by Burleigh et al. [J. Chem. Phys. 104, 480 (1996)] is 2.6 cm⁻¹.