The B <- X spectrum of ArCl2: linear and perpendicular isomers

The B <-- X excitation spectra for ArCl2 are calculated for both the linear and perpendicular X state isomers. The intensity of the perpendicular isom er spectrum is concentrated in the narrow 0-0 transitions of the van der Wa als modes, especially for excitation to lower vibrational levels of the Cl- Cl stretch. For excitation to higher vibrational levels, IVR results in muc h more complicated spectra since doorway states borrow considerable intensi ty. For the linear isomer the qualitative appearance of the spectrum is com pletely different. Most of the intensity is spread into a fairly smooth con tinuum. The only narrow resonances are relatively weak excitations to high bending vibrational levels in the B state. The calculated spectra can be qu alitatively understood in terms of Franck-Condon arguments, and they show w hy experimental observation of the linear isomer has been unsuccessful usin g optical spectroscopy. The results also show that van der Waals Franck-Con don factors must be taken into account when using the continuum intensity t o estimate the relative populations of the two isomers. Finally, it is sugg ested that the product rotational distribution can be used to confirm the a ssignment of continuum excitation intensity to the linear isomer. (C) 1998 Elsevier Science B.V. All rights reserved.