Ion formation upon electron collisions with valine embedded in helium nanodroplets

Abstract

Abstract. We report here experimental results for the electron ionization of large superfluid helium nanodroplets with sizes of about 10 5 atoms that are doped with valine and clusters of valine. Spectra of both cations and anions were monitored with high-resolution time-of-flight mass spectrometry (mass resolution >4000). Clear series of peaks with valine cluster sizes up to at least 40 and spaced by the mass of a valine molecule are visible in both the cation and anion spectra. Ion efficiency curves are presented for selected cations and anions at electron energies up to about 40 eV and these provide insight into the mode of ion formation. The measured onset of 24.59 eV for cations is indicative of valine ionization by He + whereas broad resonances at 2, 10 and 22 eV (and beyond) in the formation of anions speak to the occurrence of various modes of dissociative electron attachment by collisions with electrons or He* − and the influence of droplet size on the relative importance of these processes. Comparisons are also made with gas phase results and these provide insight into a matrix effect within the superfluid helium nanodroplet.

Electron Attachment and Electron Ionization of Formic Acid Clusters Embedded in Helium Nanodroplets

“…The main features in the cluster anion series resemble those seen upon sputtering of FA cryofilms by fission fragments [ 16 ] but several additional homologous series could be identified thanks to the high resolution and dynamic range of the mass spectrometer. The dependence of the anion yield on electron energy confirms that the largest cross section for anion formation is around 22.5 eV, thanks to an indirect process that involves the formation of an electronically excited He −* followed by its migration and electron transfer to the dopant [ 28 ]. On the other hand, the stoichiometry of cluster anions other than intact and dehydrogenated FA n − differs considerably from anions observed upon electron attachment to bare gas-phase FA clusters at 1 eV [ 20 ].…”

“…Therefore, it is not surprising that all anions shown in Figure 4 exhibit this resonance. Furthermore, large HNDs suppress anion formation at low electron energies because of the formation of electron bubbles and the reduced ejection probability of dopant anions [ 28 ].…”

“…Electron bombardment of doped droplets leads to various intermediates, including He + , electronically excited He * and He *− , and electron bubbles. These species may migrate (or resonantly hop) to the dopant and transfer their energy and/or charge, resulting in negative or positive dopant ions [ 25 – 28 ].…”

Electron scattering from molecules and molecular aggregates of biological relevance

“…We also mention briefly that a few experiments have been performed for amino acid clusters in helium nanodroplets (da Silva et al 2010, Weinberger et al 2016.…”

Advances in positron and electron scattering*

“…Screening effects of plasmas on the formation of antihydrogen (H) in an arbitrary s-state from the ground state of the positronium atom (Ps) by antiproton (p) impact have been studied within the framework of charge-conjugation and time-reversal invariance [10]. Mass spectra of electron ionized valine doped helium clusters for both cations and anions and up to 40 valine molecules have been reported by Weiberger et al [11] whereas calculations of the ionization rate of the sodium atom in strong fields within the quasistaic regime were presented by Bunjac et al [12]. The photodetachment of the negative ion of boron, B − 3 P, was investigated by employing the B-spline R-matrix method for photon energies ranging from threshold to 12 eV [13] and new and original experimental data on the dissociative electron attachment (DEA) to 2-nitrophenol molecule, 2-nitrophenol clusters and heterogeneous clusters of 2-nitrophenol, argon and water was presented by Kocisek et al [14].…”