Electronic spectroscopy of the CaCCCH3 and SrCCCH3 free radicals

Abstract

The first spectroscopic observation of the free radicals CaCCCH₃ and SrCCCH₃ is reported. Vibrationally-resolved laser-induced fluorescence spectra, which were recorded under supersonic jet conditions, reveal two band systems for each molecule which have been assigned to the à ²E– ²A₁ and ²A₁– ²A₁ electronic transitions. The vibrational structure in each system is limited to a short progression in the metal–carbon stretching mode. The Ö origin is blue-shifted relative to the monoacetylides of Ca and Sr. Furthermore, in contrast to CaCCH and SrCCH, where predissociation seems to reduce the fluorescence quantum yield to near zero, the states of both CaCCCH₃ and SrCCCH₃ show strong fluorescence. Although relatively remote from the chromophore (the metal atom), the methyl group perturbs the electronic structure sufficiently to shift the low lying vibrational levels of the state out of the predissociative region.