J . Phys. Chem. 1985,89, 1861-1865 in a Fermi resonance is given byz3
where I+ (I-) is the intensity of the higher (lower) frequency component, K = (Y&/ffb is the ratio of the unperturbed polarizability of the double quantum vibration, v,, to the single quantum vibration, vb, and 2( 2 12) u
Y=
'
(A'
+ 8V2)'12 - A
where A is the unperturbed frequency difference, 2v, - vb, and U is the perturbation energy, usually 3-10 cm-'. Of these parameters, only K depends on the laser wavelength, by virture of resonance effects on the polarizabilities. K is expected to have a small value, since the unperturbed polarizability of the overtone should be smaller than that of the breathing mode. It cannot be zero, however, since then I + / L = y-', independent of the laser wavelength. We expect K to be smaller with UV excitation, where the ring-breathing mode is in resonance, than with visible excitation, where off-resonance contributions may be relatively more important for the overtone of the out-of-plane mode. If K
