MULTIPHOTON IONIZATION STUDIES OF C6H6-(CH3OH)N CLUSTERS .2. INTRACLUSTER ION-MOLECULE REACTIONS

The neutral C6H6-(CH3OH)n clusters, which have been spectroscopically characterized in Paper 1, 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 6(0)1 transition of the C6H6 chromophore in the cluster. No ion chemistry is observed for the 1:n clusters with n less-than-or-equal-to 2. At n = 3, dissociative electron transfer (DET) to form C6H6 + M3+ (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 M3+ 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 M3+ 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 M(n) 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 greater-than-or-equal-to 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 M(n)H+ product arises from proton transfer from C6H6+, 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 6(0)1 1(0)1 transitions yield a broader set of products which reflect the 5 kcal/mol increase in the reactant (1:n)+ cluster internal energies.