Intracluster ion-molecule reactions have been studied by dissociative photo
ionization of mixed acetylene-ethane van der Waals clusters in a supersonic
molecular beam. The studies were carried out by recording threshold photoe
lectron-photoion coincidence mass spectra of the clusters at different phot
on energies between 10.8 and 11.3 eV. Intracluster reactions have been obse
rved in mixed dimer, trimer, pentamer, and hexamer clusters. In the case of
the mixed dimer ion [(C2H2)(C2H6)](+) two fragment ions result from intram
olecular reactions: C3H5+ (CH3 loss), and C2H4+ (C2H4 loss). This indicates
that the dimer ion isomerizes into the 2-butene ion prior to dissociation,
as is the case for ethylene dimer ions. Mixed trimer ions also dissociate
by CH3 loss. No intracluster hydride transfer prior to dissociation is obse
rved, although this reaction channel is important in bimolecular C2H2+ + C2
H6 collisions. Two very specific intracluster reactions leading to C2H6 los
s were observed for pentamer and hexamer mixed cluster ions. A model involv
ing evaporation and intracluster reactions leading to CH3 and C2H4 loss is
proposed to explain this surprising result. (Int J Mass Spectrom 199 (2000)
201-209) (C) 2000 Elsevier Science B.V.