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

Abstract

We report the results of an experimental study of electron ionization of large helium nanodroplets doped with formic acid (FA). Several homologous series of cluster anions are observed, including [FA n -H] − , undissociated FA n − , and these ions complexed with one or more H 2 O. Some major features resemble those observed upon sputtering of frozen FA films but they differ significantly from results obtained by electron attachment to bare FA clusters in the gas phase. The FA n − and (H 2 O)[FA n -H] − series show abrupt onsets above n = 2 and 5, respectively. A prominent resonance in the anion yield occurs at 22.5 eV due to the formation of an intermediate He −* . Also observed are homologous series of [FA-H] − or [FA 2 -H] − complexed with helium. The cation chemistry is dominated by the production of protonated formic acid clusters, [FA n H] + , but various other homologous cluster ion series are observed as well. Graphical Abstract Electronic supplementary material The online version of this article (10.1007/s13361-018-02124-z) contains supplementary material, which is available to authorized users.

Helium Droplet Mass Spectrometry

“…Two recent studies by Mahmoodi-Darian et al investigated the interaction of HNDs doped with FA [389] as well as FA and C 60 with low-energy electrons (Fig. 2.25) [390].…”

“…The most abundant anion cluster series in the purely FA-doped droplet experiment was dehydrogenated [FA n -H] -, whereas for cations, protonated [FA n +H] + dominates the mass spectrum. Less abundant, but also observed are undissociated [FA n ] ± , deprotonated [FA n -H] + , and the more abundant ions complexed with one or more water molecules [389].…”

“…© copyright Elsevier. All rights reserved large amount of water in the mass spectra arises from intra-cluster ion-molecule reactions [389]. The findings are in good general agreement with a study utilizing sputtering of frozen FA films by energetic 252 Cf fission fragments-the anion mass spectra are dominated by [FA n -H]with minor contributions of (H 2 O) m [FA n -H] -(m ≤ 2), similarly the cationic spectra feature mostly [FA n +H] + , with (H 2 O) m [FA n +H] + (m ≤ 2) also being detected [393,394].…”

Chemistry and physics of dopants embedded in helium droplets

“…It was shown that despite fragmentation of molecular dopants not being quenched by energy dissipation, fragmentation intermediates can be stabilized within the He matrix (Bartl et al, 2013). EI and PI were used in the past to analyze the behavior of a variety of atomic and molecular dopants such as Au (Martini et al, 2018), haloalkanes (Yang et al, 2006), rare gases (Kim et al, 2006), SF 6 (Peterka et al, 2006), H 2 O (Yang et al, 2007), acetic acid (da Silva, Jaksch, et al, 2009), CH 4 (Leidlmair, Bartl, et al, 2012), NaF (Weinberger et al, 2016), valine (Weinberger et al, 2016), C 60 (Goulart et al, 2017), or formic acid (Mahmoodi‐Darian et al, 2019). For a more comprehensive overview of the results of these investigations see ref.…”

Electron ionization of clusters containing the formamide molecule

“…Since the ion source chamber pressure corresponds to that of single collision conditions, FAH + is formed by the break-up of an ionized formamide cluster, most probably from the ionized dimer. The observation of protonated parent (cluster) ions is characteristic for hydrogen-containing molecules and a highly abundant species upon electron ionization of clusters [37,[40][41][42][43][44][45]. The structures and energies of the neutral formamide dimer were the focus of few theoretical and experimental studies previously.…”