Electronic spectra of YOH and YOD in the visible region: Strong vibronic coupling between the (B)over-tilde(1)II and (C)over-tilde(1)Sigma(+) states

Laser excitation spectra of yttrium monohydroxide, YOH, have been recorded in the 500-625 nm wavelength region. Rotational analysis of bands of YOH an d YOD has shown that the ground state is (X) over tilde(1)Sigma(+), with th e structure r(0)(Y-O) = 1.948(6) Angstrom, r(0)(O-H) = 0.920(6) Angstrom; t he bending frequency (nu(2), pi) is 313.73 cm(-1) (237.43 cm(-1) in YOD) an d the Y-O stretching frequency (nu(3), sigma(+)) is 673.83 cm(-1) (655.34 c m(-1)). Two excited electronic states have been identified; they are assign ed as (B) over tilde(1)Pi (16 449 cm(-1)) and (C) over bar(1)Sigma(+) (18 5 09 cm(-1)). Unusually strong vibronic coupling through the bending vibratio n occurs between these two states, which causes their vibrational structure s to be highly irregular; assignments have only been possible following ext ensive wavelength-resolved fluorescence experiments. The vibronic coupling raises the bending frequency of the (C) over tilde(1)Sigma(+) state to 457 cm(-1) and reduces that of the lower Born-Oppenheimer component of the (B) over tilde(1)Pi state (which has A' symmetry in the C, 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 ba rrier is clearly demonstrated by the level structure of YOD, where the Sigm a(+) vibronic component of the 010 vibrational level (linear molecule notat ion) lies 1.4 cm(-1) below the 000 level. The upper Born-Oppenheimer compon ent, which has A " symmetry, is unaffected; its bending frequency is simila r to that of the ground state. Perturbations occur in both the (B) over til de(1)Pi and (C) over tilde(1)Sigma(+) states; some of these represent local interactions between the two of them, but others are caused by higher vibr ational levels of lower-lying "dark" electronic states. Over 40 ground stat e vibrational levels have been identified for both YOH and YOD from the wav elength-resolved fluorescence spectra. (C) 1999 Academic Press.