THE LONG PATH, LOW TEMPERATURE ABSORPTION SPECTRUM OF FORMALDEHYDE
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Date
1957
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Publisher
Ohio State University
Abstract
“The spectrum in the near ultraviolet region obtained at high resolution with a multiple pass cell cooled to dry ice temperature exhibited a number of interesting new features. Collision broadening of the rotational lines was reduced to a near minimum so that the limitations in resolving power were essentially those imposed by the instrument itself, in this case about 240,000. The absence of high temperature structure permitted important new bands to be found. A progression of six very weak, double-headed, temperature insensitive hands was found to the long wave length side of the $n-\pi^{*}$ system. While some of these have been reported previously, this is the first indication that they belong to a new electronic transition. In addition to these bands, the absence of high rotational lines in the vibronic transition at $28,313 cm^{-1}$ (A-band) permitted identification of a relatively weak parallel type band at $28,190 cm^{-1}$. This is the ($0-0$) band of the $n-\pi^{*}$ transition which is forbidden under the strict total vibronic $C_{2v}$ selection rules. Parallel bands of similar appearance are observed throughout the longer wave length regions of the system and are due to transitions from the ground vibronic state to symmetric members of higher inversion doublets, alone or in combination with $v_{2}$. The relative intensity of this system as compared with that of the “allowed” part of the electronic transition gives an indication of the deviation from $C_{2v}$ symmetry of the electronic wave functions in the non-planar excited state. This work supported in part by OOR, Project No. TB2-0001 of Contract No. DA-36-034-ORD-2109.”
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Author Institution: Department of Chemistry, The Johns Hopkins University