Electronic spectroscopy of the CaCCCH[subscript 3] and SrCCCH[subscript 3] free radicals

The first spectroscopic observation of the free radicals CaCCCH[subscript 3] and SrCCCH[subscript 3] 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 [superscript 2]E–[X with combining tilde] [superscript 2]A[subscript 1] and [B with combining tilde] [superscript 2]A[subscript 1]–[X with combining tilde] [superscript 2]A[subscript 1] electronic transitions. The vibrational structure in each system is limited to a short progression in the metal–carbon stretching mode. The Ö[X with combining tilde] 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 [B with combining tilde] states of both CaCCCH[subscript 3] and SrCCCH[subscript 3] 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 [B with combining tilde] state out of the predissociative region.