Experimental determination of vibrational potential energy surfaces and molecular structures in electronic excited states

For more than three decades far-infrared and Raman spectroscopies, along wi th appropriate quantum mechanical computations, have been effectively used to determine the potential energy functions which govern the conformational ly important large-amplitude vibrations of nonrigid molecules. More recentl y, we have utilized laser-induced fluorescence (LIF) excitation spectroscop y and ultraviolet absorption spectroscopy to analyze the vibronic energy le vels of electronic excited states in order to determine the potential energ y surfaces and molecular conformations in these stares. Transitions from th e ground vibrational state in an So electronic state can typically be obser ved only to several excited vibronic levels. Hence, the LIF of the jet-cool ed molecules generally provides data on only a few excited state levels. Ul traviolet absorption spectra recorded at ambient temperatures, however, oft en provide data on many additional excited vibronic levels. However, these can only be correctly interpreted if the electronic ground state levels hav e been accurately determined from the far-infrared, Raman, and dispersed fl uorescence studies. In this article, we will first present our results for bicyclic molecules in the indan family in their S-0 and S-1(pi,pi*) electro nic states. Two-dimensional potential energy surfaces in terms of the ring- puckering and ring-flapping vibrations were utilized for the analyses. Next , we review our work on trans-stilbene in its S-0 and S-1(pi,pi*) states an d examine the data from which two-dimensional. potential energy surfaces we re determined for the phenyl torsions and one-dimensional functions were ca lculated for the torsion about the C=C bond, which governs the photoisomeri zation. Finally, we consider seven cyclic ketones in their S-0 and S-1(pi,p i*) states. The carbonyl wagging vibration of each was studied in its elect ronic excited state in order to determine the barrier to inversion and the wagging angle. The barrier to inversion was found to increase with angle st rain. Conformational changes between the ground and excited electronic stat es were also examined in terms of the out-of-plane ring motions.