Reinvestigation of the HCP electronic spectrum: Experimental determination of D for the X̃ state and observation of hyperfine quantum beats in the B̃ state

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Abstract
In the laser-induced fluorescence spectrum of HCP, a sharp decrease in fluorescence intensity was observed at 41 680 cm−1. This diminuation in emission intensity is due to a rotationally mediated coupling of the excited vibronic level with the dissociative continuum of the X̃ state; this has allowed us to place an upper limit on D0 for the ground electronic state. A lower limit for D0 can be placed at 41 650 cm−1 since strong emission was observed below this excitation energy. Using the assignments of Johns et al. [Can. J. Phys. 47, 893 (1969)] as a guide, fluorescence decays were recorded from the two B̃ state vibronic bands to determine lifetimes. In addition to determining an unusually long lifetime for both states, the decays were seen to be modulated at four frequencies resulting from the coherent excitation of four nuclear hyperfine levels. This increase in lifetime and magnetic hyperfine interaction arises from a mixing of three singlet and triplet states. The contributing states are tentatively identified.