Pure vibrational spectroscopy of S-0 formaldehyde by dispersed fluorescence

Dispersed fluorescence spectra from the 0(00) rotational level of 4(0), 4(1), 5(1), and 3(1)4(1) S-1 formaldehyde (H2CO) have been recorded. From these spectra, 198 new vibrational states have been assigned with energies up to 12500 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 cm 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 (<9500 cm(-1)) and relatively pure (zero-order coefficient squared greater than 0.75 or largest in a given cm 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). (C) 1996 American Institute of Physics.