Electronic Spectra of YOH and YOD in the Visible Region: Strong Vibronic Coupling between the &Btilde;1Pi and &Ctilde;1Sigma+ States.
Laser excitation spectra of yttrium monohydroxide, YOH, have been recorded in the 500-625 nm wavelength region. Rotational analysis of bands of YOH and YOD has shown that the ground state is &Xtilde;1Sigma+, with the structure r0(Y-O) = 1.9486 Å, r0(O-H) = 0.9206 Å; the bending frequency (nu2, pi) is 313.73 cm-1 (237.43 cm-1 in YOD) and the Y-O stretching frequency (nu3, varsigma+) is 673.83 cm-1 (655.34 cm-1). Two excited electronic states have been identified; they are assigned as &Btilde;1Pi (16 449 cm-1) and &Ctilde;1Sigma+ (18 509 cm-1). Unusually strong vibronic coupling through the bending vibration occurs between these two states, which causes their vibrational structures to be highly irregular; assignments have only been possible following extensive wavelength-resolved fluorescence experiments. The vibronic coupling raises the bending frequency of the &Ctilde;1Sigma+ state to 457 cm-1 and reduces that of the lower Born-Oppenheimer component of the &Btilde;1Pi state (which has A' symmetry in the Cs point group) to the extent that the molecule becomes nonlinear, with a potential barrier at the linear configuration of about 120 cm-1. The presence of the potential barrier is clearly demonstrated by the level structure of YOD, where the Sigma+ vibronic component of the 010 vibrational level (linear molecule notation) lies 1.4 cm-1 below the 000 level. The upper Born-Oppenheimer component, which has A" symmetry, is unaffected; its bending frequency is similar to that of the ground state. Perturbations occur in both the &Btilde;1Pi and &Ctilde;1Sigma+ states; some of these represent local interactions between the two of them, but others are caused by higher vibrational levels of lower-lying "dark" electronic states. Over 40 ground state vibrational levels have been identified for both YOH and YOD from the wavelength-resolved fluorescence spectra. Copyright 1999 Academic Press.