Ag. Adam et al., 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, J MOL SPECT, 196(1), 1999, pp. 45-69
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.