Multiphoton ionization studies of C6H6–(CH3OH)n clusters. II. Intracluster ion–molecule reactions

Abstract

The neutral C6H6–(CH3OH)n clusters, which have been spectroscopically characterized in Paper I, serve here as precursors for the study of intracluster ion chemistry initiated by resonant two-photon ionization. Resonant enhancement allows ion chemistry product yields to be determined as a function of cluster size by selective excitation of a single size cluster with the laser. Most of the work presented here uses one-color resonant ionization via the 610 transition of the C6H6 chromophore in the cluster. No ion chemistry is observed for the 1:n clusters with n≤2. At n=3, dissociative electron transfer (DET) to form C6H6+M+3 (M=CH3OH) is observed with a product yield of 6%. The remaining 94% of ionic products result from fragmentation of the 1:3 cluster by loss of a single CH3OH molecule. The unprotonated M+3 product ion is unusual in that electron bombardment or photoionization of pure methanol clusters yields exclusively protonated methanol cluster ions. The attachment of a C6H6 molecule to the methanol cluster provides an extremely gentle photoionization mechanism which produces M+3 with little enough internal energy to preclude its breakup to M2H++CH2OH (or CH3O ). The opening of this product channel at n=3 is consistent with estimates of the ionization potential of Mn clusters which predict an endothermic ET reaction at n=2 which becomes exothermic at n=3. Despite the increasingly exothermic DET and dissociative proton transfer product channels, larger clusters (n≥4) continue to predominantly undergo unreactive fragmentation. For the 1:4 and 1:5 clusters, in addition to DET products, dissociative proton transfer (DPT) products are also observed. The MnH+ product arises from proton transfer from C6H+6, while Mn−1H+ probably occurs by DPT within the M+n cluster following loss of C6H6 in DET. Scans of the 1:2–1:5 clusters through their 610110 transitions yield a broader set of products which reflect the 5 kcal/mol increase in the reactant (1:n)+ cluster internal energies.