Fragmentation Dynamics of Acetonitrile and Its Oligomers (Dimers to Nonamers) at Ultralow Temperatures

We have studied the electron ionization-induced fragmentation dynamics of acetonitrile clusters isolated in superfluid helium nanodroplets. The obtained mass spectra reveal a rich fragmentation pattern, including a series of the most prominent (CH₃CN)_k-1H⁺ fragments, along with a series of weaker signals arising from ${\left({\text{CH}}_3\text{CN}\right)}_k^{•+}$ , (CH₃CN)_k-1CH₂CN⁺, mono ¹³C isotopologues of (CH₃CN)_k-1H⁺, He_nCH₃ ⁺, and He_nCH₂ ^•+ fragments. Additionally, in the mass range 10-82 u, peaks corresponding to (CH₃CN)CH_x ⁺ (x = 0-3), (CH₃CN)CN⁺, and $\left({\text{CH}}_3\text{CN}\right){\text{CHCN}}^{•+}$ fragments were observed. Compared with molecular beam experiments, fragments in the mass range 42-82 u indicate the presence of additional direct fragmentation channels in helium droplet experiments. To determine the origin of the observed fragments, their intensity variation with acetonitrile partial pressure was analyzed. The assignments were further validated using CD₃CN isotopologues. The (CH₃CN)_k-1H⁺ and (CH₃CN)_k-1CH₂CN⁺ fragments both originate from the (CH₃CN)_k clusters of the same size. Moreover, for the clusters with k > 2, the ${\left({\text{CH}}_3\text{CN}\right)}_k^{•+}$ fragments also share the origin with (CH₃CN)_k-1H⁺ and (CH₃CN)_k-1CH₂CN⁺ fragments. These findings provide detailed, crucial insights into the chemistry of acetonitrile clusters, initiated by ~24.6 eV of He^•+ ions, at ultralow temperatures.