VIBRATIONAL POTENTIAL ENERGY SURFACES IN ELECTRONIC EXCITED STATES
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Potential energy functions concentrate on the use of the harmonic oscillator. Potential functions that are not harmonic, however, can be much more informative, especially if their energy minima do not correspond to the coordinate origins. Several spectroscopic methods, including infrared and ultraviolet absorption, Raman, jet-cooled laser-induced fluorescence, and cavity ringdown, have been utilized to map out the vibrational quantum states of molecules in their ground and excited electronic states. Data on the higher excited vibrational levels for large-amplitude vibrations such as ring-puckering, ring-twisting, ring-flapping, and internal rotation allow one- or two-dimensional potential energy surfaces to be accurately determined. In many cases, ab initio and/or DFT computations are utilized to complement the experimental work. Following a discussion of theory, experimental methods, and computational methods, the spectroscopic results and PESs for several types of molecules are presented. First, the PESs for the carbonyl wagging vibration of seven cyclic ketones in their S1(. n, *) excited states are reviewed. Except for 2-cyclopentenone (2-CP), which is conjugated and planar, the PESs have a barrier to planarity, which increases with angle strain. PESs for the ring-bending and ring-twisting vibrations were also determined for these ketones in both their ground and excited states. The LIF study of trans-stilbene and two substituted stilbenes allowed two-dimensional PESs for the internal rotations of the phenyl groups to be calculated for both ground and S1(, *) states. 2009 Elsevier B.V. All rights reserved.
