Ultraviolet absorption spectroscopy of dissociating molecules: Effects of cluster formation on the photodissociation of CH3I

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Abstract
The ultraviolet absorption spectra of jet-cooled CH3I, (CH3I)2, and (CH3I)n, n>2 are measured in the valence A state and the Rydberg B state. A significant blue shift of the valence state is observed upon cluster formation. The magnitude of this shift yields a dimer bond strength of at least 500 cm−1 which implies extensive dimer formation at room temperature and moderate pressures. The methyl iodide B-state spectrum consists of a single progression in the ν2 mode. Dimerization retards the predissociation and therefore increases the excited state lifetime; consequently many new modes are observed which in the monomer are coupled to the dissociation. The implications of this result to the photodissociation dynamics are discussed. The Rydberg spectra of (CH3I)n, n>2 show asymmetric line shapes which may be due to interferences between the B state and an underlying continuum. The nature of these interferences and the possible indentity of this continuum are discussed.