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 (<9500 cm−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.
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