Role of Helium Droplets in Mass Spectra of Diatomics: Suppression of Dissociative Reactions

Abstract

It is generally accepted that electron impact of doped helium nanodroplets initially produces a positively charged helium atom, which then ionizes the dopant if the two come into contact. In effect the He + can initiate ion-molecule reactions. However, the effect of the surrounding helium on ion-molecule reactions remains ambiguous. To explore this, electron-induced chemistry has been investigated for the diatomic molecules O 2 , CO and N 2 . The helium is found to significantly suppress dissociative ion product channels.3

Helium Droplet Mass Spectrometry

“…The findings further support the authors' conclusion that EI of HNDs doped with small molecules and their cluster cannot be considered a soft ionization route [110]. In contrast to the small molecules discussed so far, EI of HNDs doped with the diatomics O 2 , CO and N 2 show a significant reduction of dissociation (fragmentation) compared to the gas phase reaction between the diatomic species and He + [109]. The authors consider two possible explanations: suppression of the dissociation channel in the ion-molecule reaction by the HND or acting as a reservoir, allowing for recombination of the products after initial dissociation.…”

“…The ionization of small dopant molecules via He + charge transfer and subsequent fragmentation and dissociation products were investigated for several alcohols (C 1 -C 6 ) and ethers [107], haloalkanes (C 1 -C 3 ) [108] and diatomic molecules (O 2 , CO and N 2 ) [109]. The authors found that for the small to medium-sized alcohols and ethers as well as the haloalkanes studied, the HND environment did alter the fragmentation patterns compared to the gas phase, but mostly quantitatively, enhancing certain channels like H-abstraction.…”

“…In order to determine which is the case, the energetics of the ion-molecule reactions and the corresponding amount of He evaporation to dissipate the energy difference were calculated. The authors conclude that while the dissociation reaction itself must be suppressed in the case of O 2 and CO, no conclusion can be drawn for N 2 from the calculated energetics [109]. Shepperson and co-workers investigated the formation of small He n + cluster ions from HNDs with different sizes (average droplet size <N> between 4000 and 90000) and dopants (pristine, H 2 O and Ar) [111].…”

“…Curiously, the natural isotope ratio of silver is not conserved in AgHe, which forms more efficiently with the 107 Ag isotope. The authors suggest that tunneling is responsible for the effect which manifests in an abundance ratio of 107 AgHe to 109 AgHe that is ~20-50% above the natural abundance ratio, being higher in larger HNDs. Taking full advantage of the previously described setup's capabilities, Braun and Drabbels conducted an extensive, threepart study about photodissociation (PD) of alkyl iodides in HNDs, investigating in detail the kinetic energy transfer [185], solvation dynamics [188] and fragment recombination [189].…”

Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K

“…He + + Mn  Mn + + He (15)  (Mn-X) + + X + He (16) The influence of energy transfer to the He matrix or intermolecular relaxation in a cluster on the extent of stabilization of short-lived intermediates (either parent cations or early dissociation products) can be assessed by comparisons with product-ion formation by electron impact ionization of the isolated molecules in the gas phase. For example, a recent He droplet study of the small diatomic molecules O2, N2, and CO was able to demonstrate a significant suppression of the formation of dissociative ion products according to reaction (15) and the reaction scheme depicted in Figure 25 compared to observations of the known gas-phase reactions of these molecules with He + at room temperature [499].…”