Slow metastable decay process of (C3H7+

Abstract

Besides the usual single-monomer evaporation, the {CiW^)niC^^i^) cluster ions produced by electron-impact ionization of neutral propane clusters have been observed to decay by an additional channel leading to evaporation of up to four C3H8 monomers. It is shown that this additional fragmentation channel is initiated by a new decay mechanism, i.e., the isomerization of an /i-propyl ion to a more stable 5^^r-propyl ion within the cluster.PACS numbers: 36.40.+d Studies of the fragmentation processes following electron-impact ionization of van der Waals (vdW) clusters of small molecules (e.g., nitrogen [1]) have provided valuable information on the dynamics of energized cluster ions (energy flow, mechanisms of dissociation, rearrangement processes within ionized clusters, etc.). In particular, slow fragmentation processes, usually referred to as the metastable decompositions of cluster ions, provide an important source of information [2]. Although there exist many studies of neat or mixed clusters of atoms and/or small molecules [2,3], much less is known about the metastable fragmentation of ionized clusters of polyatomic molecules (e.g., see [4]). Because prompt and metastable fragmentations of the ionized single molecules have been thoroughly studied and are well understood [5], simple hydrocarbons such as ethane and propane are especially suitable systems for dissociation studies.In this communication we report the results of our studies of metastable fragmentation of ionized clusters of propane and ethane. The dominant metastable dissociation process observed for stoichiometric and nonstoichiometric cluster ions is single-monomer evaporation, a well known and characterized (by the quasiequilibrium theory) process in the case of ionized vdW cluster ions [2]. None of the metastable decay reactions which occur for monomer ions (e.g., C3H8^-^ C3H7"'", etc.[5]) occurs in the cluster environment, apparently due to the fast energy transfer from intramolecular modes to intermolecular vibrations of cluster constituents. Moreover, besides the usual monomer evaporation, however, an additional metastable decay reaction involving the loss of up to four monomers from the (C3H7"*")(C3H8)« ion has been observed. Taking into account the various features (temporal evolution, energetics, fragmentation pattern) of this novel metastable decay process we have concluded that this decay reaction is driven by the isomerization reaction within the cluster of /i-propyl ions to isopropyl ions in the /is time regime. This reaction sequence constitutes a new mechanism for metastable dissociation of cluster ions (i.e., isolated-state-driven reactions), thereby adding a new variety to the two previously known metastable decay mechanisms of this kind, i.e., excimer-induced dissociation of rare-gas cluster ions [6] and the release of in-tramolecular vibrational quanta from N2 molecules within an ionized nitrogen cluster [1].The supersonic-beam-electron-impact ionization source and double-focusing mass spectrometer system has been described in det...

Propane clusters in Titan’s lower atmosphere: insights from a combined theory/laboratory study

“…Various aspects of the fragmentation dynamics of ionized molecular clusters have been explored in the past, in particular for propane (Foltin et al 1992), but many pieces of the puzzle are still missing. We provide below some elements on the the energetics and the dynamics of the processes involved.…”