High-resolution absorption spectrum of jet-cooled CH3Cl between 70 000 and 85 000 cm-1: New assignments
Résumé
The absorption spectrum of jet-cooled CH3Cl was photographed from 165 to 117 nm (or 60 000-85 000 cm-1, 7.5-10.5 eV) at a resolution limit of 0.0008 nm (0.3-0.6 cm-1 or 0.04-0.08 meV). Even in the best structured region of the spectrum, from 70 000 to 85 000 cm-1 (8.7-10.5 eV), observed bandwidths (full width at half maximum) are large, from 50 to 150 cm-1. No rotational feature could be resolved. The spectrum is dominated by two strong bands near 9 eV, 140 nm, the D and E bands of Mulliken [J. Chem. Phys. 8, 382 (1940)] or the spectral region D of Price [J. Chem. Phys.4, 539 (1936)]. Their relative intensity is incompatible with previous assignments, namely, to a triplet and a singlet state belonging to the same configuration. On the basis of the present ab initio calculations, those bands are now assigned to two singlet states, the 1A1 and 1E excited states resulting from the 2e34pe Rydberg configuration. The present calculations also reveal that the two 1E states issued from 2e34sa1 and 2e34pa1 are quasidegenerate and strongly mixed. They should be assigned to the two broad bands near 8 eV, 160 nm, the B and C bands of Mulliken and Price. Three vibrational modes are observed to be active: the CCl bond stretch nu3(a1), and the CH3 umbrella and rocking vibrations, respectively, nu2(a1) and nu6(e). The fundamental frequencies deduced are well within the ranges defined by the corresponding values in the neutral and ion ground states. The possibility of a dynamical Jahn-Teller effect induced by the nu6(e) vibrational mode in the 1E Rydberg states is discussed.