Observation of hot bands in the infrared spectrum of H2CO

Two hot bands in the infrared spectrum of formaldehyde (H2CO) have been identified by means of tunable infrared laser spectroscopy using a jet-cooled sample. One band falls in the region 2760-2800 cm−1; it follows a-type selection rules and it has been assigned as the ν1 + ν4 − ν4 hot band. The other band falls in the region 2800-2860 cm−1; it follows b-type selection rules and it has been assigned as the ν5 + ν4 − ν4 hot band. The observations are restricted to low J and Ka levels. It has consequently been possible to ignore the effects of the extensive Coriolis couplings involving these levels in the analysis of the spectra and to model the rotational structure as that of a simple asymmetric top. Least-squares fits of the data have provided values for the band origins: 2774.2706(11) cm−1 for the ν1 + ν4 − ν4 and 2829.2621(8) cm−1 for the ν5 + ν4 − ν4 band. Term values for the upper vibrational levels involved in the transitions have been determined by use of the previously reported term values for the v4 = 1 level.