Electronic Spectra of YOH and YOD in the Visible Region: Strong Vibronic Coupling between the B1Π and C1Σ+ States

Abstract

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 X1Σ+, with the structure r0(Y–O) = 1.9486 Å, r0(O–H) = 0.9206 Å; the bending frequency (ν2, π) is 313.73 cm−1 (237.43 cm−1 in YOD) and the Y–O stretching frequency (ν3, ς+) is 673.83 cm−1 (655.34 cm−1). Two excited electronic states have been identified; they are assigned as B1Π (16 449 cm−1) and C1Σ+ (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 C1Σ+ state to 457 cm−1 and reduces that of the lower Born–Oppenheimer component of the B1Π 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 Σ+ vibronic component of the 010 vibrational level (linear molecule notation) lies 1.4 cm−1below 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 B1Π and C1Σ+ 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.