The B←X spectrum of ArCl2: linear and perpendicular isomers Original Research Article

The B←X excitation spectra for ArCl2 are calculated for both the linear and perpendicular X state isomers. The intensity of the perpendicular isomer spectrum is concentrated in the narrow 0–0 transitions of the van der Waals modes, especially for excitation to lower vibrational levels of the Cl–Cl stretch. For excitation to higher vibrational levels, IVR results in much more complicated spectra since doorway states borrow considerable intensity. For the linear isomer the qualitative appearance of the spectrum is completely different. Most of the intensity is spread into a fairly smooth continuum. The only narrow resonances are relatively weak excitations to high bending vibrational levels in the B state. The calculated spectra can be qualitatively understood in terms of Franck–Condon arguments, and they show why experimental observation of the linear isomer has been unsuccessful using optical spectroscopy. The results also show that van der Waals Franck–Condon factors must be taken into account when using the continuum intensity to estimate the relative populations of the two isomers. Finally, it is suggested that the product rotational distribution can be used to confirm the assignment of continuum excitation intensity to the linear isomer.