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

✍ Scribed by A.G. Adam; K. Athanassenas; D.A. Gillett; C.T. Kingston; A.J. Merer; J.R.D. Peers; S.J. Rixon

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 414 KB
• Volume: 196
• Category: Article
• ISSN: 0022-2852
• DOI: 10.1006/jmsp.1999.7841

✦ Synopsis

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 X ˜1⌺ ϩ , with the structure r 0 (Y-O) ϭ 1.948 6 Å, r 0 (O-H) ϭ 0.920 6 Å; the bending frequency ( 2 , ) is 313.73 cm Ϫ1 (237.43 cm Ϫ1 in YOD) and the Y-O stretching frequency ( 3 , ϩ ) is 673.83 cm Ϫ1 (655.34 cm Ϫ1 ). Two excited electronic states have been identified; they are assigned as B ˜1⌸ (16 449 cm Ϫ1 ) and C ˜1⌺ ϩ (18 509 cm Ϫ1 ). 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 C ˜1⌺ ϩ state to 457 cm Ϫ1 and reduces that of the lower Born-Oppenheimer component of the B ˜1⌸ state (which has AЈ symmetry in the C s point group) to the extent that the molecule becomes nonlinear, with a potential barrier at the linear configuration of about 120 cm Ϫ1 . The presence of the potential barrier is clearly demonstrated by the level structure of YOD, where the ⌺ ϩ vibronic component of the 010 vibrational level (linear molecule notation) lies 1.4 cm Ϫ1 below the 000 level. The upper Born-Oppenheimer component, which has AЉ symmetry, is unaffected; its bending frequency is similar to that of the ground state. Perturbations occur in both the B ˜1⌸ and C ˜1⌺ ϩ 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.