Soft or hard ionization of molecules in helium nanodroplets? An electron impact investigation of alcohols and ethers

Electron impact (70 eV) mass spectra of a series of C[subscript 1]-C[subscript 6] alcohols encased in large superfluid liquid helium nanodroplets (approximately 60,000 helium atoms) have been recorded. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others, most notably cleavage of the C[subscript α]-H bond in the parent ion to form the corresponding oxonium ion. Parent ion intensities are also enhanced by the helium, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. To demonstrate that these findings are not unique to alcohols, we have also investigated several ethers. The results obtained for both alcohols and ethers are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels. The set of molecules explored in this work suggest that electron impact ionization of doped helium nanodroplets does not provide a sufficiently large softening effect to be useful in analytical mass spectrometry

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