Observations of ion-molecule reactions and magicnumbers within ionized clusters of methane

Abstract

Electron impact ionization of methane cluster beams has shown to produce a number of ionic reaction products. These are interpreted as series of cluster ions of the form $(CH_4)_nX^+$ (X = $CH_4$, $CH_5$, $C_2H_7$,$C_2H_6$, $C_2H_5$,$C_2H_4$ $C_2H_3$, $C_2H_2$, $C_2H_5$, $C_2H_4$, and $C_2H_3$ and $n ＞ 0$). For all product ions except for two series (X = $C_2H_7$ and $C_2H_6$ ), the reactive processes can be rationalized in terms of intracluster analogues of known gas-phase ion-molecule reactios by measuring the appearance potentials of these ions. The occurrence of these two exceptions represents an example of a new class of ion-molecule reactions that can only occur within the unique "solvated" environment of the cluster. In addition, the investigations on the relative cluster ion intensity distributions of $(CH_4)_nX^{+v}$ (X = $CH_4$, $CH_5$, $C_2H_7$ ) suggest that $(CH_4)_2X^+$ ions have enhanced structual stabilities. In the case of X=$CH_5$, this stability agrees with that in theoretical and experimental studies. For the other two cases, it was proposed that the stabilities arise from the favorable structures which are similar to the stable structure of $(CH_4)_2CH_5^+$.