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

Abstract

Electron impact (70 eV) mass spectra of a series of C 1 -C 6 alcohols encased in large superfluid liquid helium nanodroplets (B60 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 a -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.

Ion formation upon electron collisions with valine embedded in helium nanodroplets

“…order of magnitude more intense from the doped He nanodroplets than the gas phase which agrees well with previous observations from reference [38]. The present mass spectrum agrees also very well with the results obtained by Denifl et al [20] with a sector field mass spectrometer and HNDs with an average size ten times smaller.…”

Photochemical processes in doped argon-neon core-shell clusters: The effect of cage size on the dissociation of molecular oxygen

“…Droplets of liquid helium provide the softest matrix material in nature, and surprisingly even for this extreme case caging is observed in the fragmentation upon electron impact ionization. 31,32 However, here the observation of intact cluster ions of the embedded molecular clusters was attributed to efficient removal of the excess energy due to rapid cooling by the superfluid helium.…”

Electronic Spectroscopy of Toluene in Helium Nanodroplets: Evidence for a Long-Lived Excited State

“…Similarly, the enhancement signal suggests that formation of the fragment ion C 5 H 5 + increased in the presence of the laser at the appropriate 9 resonant frequency when compared to its absence. Consequently, the laser must deposit the toluene molecule into an excited state that affects the outcome of the subsequent ionization event.…”

“…9 One important difference in the current experiments is the use of a continuously flowing helium droplet In order to record electronic transitions of toluene, ions in specific mass channels were monitored as a function of laser wavelength. The UV laser light (2 mJ per pulse) was generated by frequency doubling the output from a dye laser (Sirah Cobra Stretch) pumped by a Continuum Surelite II-10 Nd:YAG laser.…”