3063
J. Phys. Chem. 1991, 95, 3063-3071
Pyrolysis Jet Spectroscopy: Laser- Induced Phosphorescence of Thioformaldehyde and the Triplet Excited-State Bending Potential James R. Dunlop, Jerzy Karolczak? Dennis J. Cloutbier,* Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
and Stephen C. Ross Department of Physics, University of New Brunswick, Fredricton, NB E3B 5A3 Canada (Received: July 31, 1990)
Rotationally cold laser-induced phosphorescence spectra of thioformaldehyde have been obtained by using the pyrolysis jet technique. Rotational constants, including the major spin constants, have been obtained for three excited-state rotational levels. The jet data have been combined with previously available data from room-temperature spectra of H2CS and DICS and fitted to a semirigid invertor Hamiltonian. The results show that it is not necessary to invoke an excited-state inversional barrier in the potential function in order to accurately fit the data. Thioformaldehyde adopts a planar excited triplet state equilibrium geometry, as was also concluded in our previous studies of the excited singlet state.
I. Introduction
In the preceding paper (ref 1, hereafter referred to as part l), we applied the pyrolysis jet spectroscopic technique2 to the SI-So band system of the transient molecule thioformaldehyde (H2CS). A detailed picture of the rovibrational energy levels with Evibup to 5000 cm-' in SIwas obtained. The available excited-state data for H2CS and DzCS were fitted to a semirigid bender model3 to obtain a description of the large-amplitude motion bending potential. The equilibrium excited-state geometry was found to be planar, in agreement with an earlier study using a more limited data set! Injhe present work, we have applied the same technique to the 13A2-X'AI band system of H2CS. Rotationally resolved laser-induced phosphorescence has been observed for a limited number of bands, and the excited-state bending potential has been determined. There have been very few rotational analyses of tripletsinglet bands of polyatomic molecules reported in the literature. This is because such transitions are usually inherently weak, due to the strong AS = 0 selection rule. However, some data are available for a limited set of molecules. The triplet-singlet bands of formaldehyde and various isotopomers have been studied in detaiL5v6 The excited-state rotational constants and structure' were found to be similar to those of the SI state. The electron spin is only weakly coupled to the inertial axes, yielding modest spin splittings ( < I cm-I). The three components of the triplet state have AI, BI, and B2 spin-orbital symmetries in the absence of rotation. The main source of the tripletsinglet oscillator strength in formaldehyde has been shown to be due to mixing of the AI component with the 'Al(n,n*) excited ~ t a t e . ~The , ~ resultant intensity is shared among the three spin components by rotational mixing. The principal transitions arise from the selection rules AK, = 0, AJ = 0, f l , and AN = 0, f l , f2. As shown by Hougen,Io mixing of the B, and B2 components with higher singlet states of the same symmetry can give rise to branches with Ma = +2. These have been observed in the 420band of HzCO.IIJ2 Rotational analyses of the 3BI-1AItransition of SOz have also been teported.13914Small spin splittings (
