A. Liu et al., MULTIPHOTON PROCESSES IN CYCLOHEXANE AND TRANS-DECALIN AND THE FORMATION OF HIGH-MOBILITY CATIONS, Journal of physical chemistry, 97(43), 1993, pp. 11265-11273
The dependence of the absolute efficiencies of production of free elec
trons and HM+ (positive ions with anomalously high mobility) on the in
tensity of 248- and 308-nm laser pulses has been measured for neat cyc
lohexane and trans-decalin and for solutions containing aromatic compo
unds. In the neat solvents, the yields of electrons and HM+ have the s
ame intensity dependence; for ionization of these two alkanes, two pho
tons are required at 248 nm and three photons at 308 nm. In solutions
containing aromatic solutes, where the major fraction of the light abs
orption is by the solute, yields of both free electron and HM+ are mar
kedly higher, and at both 248 and 308 nm the intensity dependences ind
icate two photons are required for ionization but that three photons a
re required to create HM+. This is consistent with the explanation, ba
sed on previously reported product analysis studies from this laborato
ry, that the aromatic solute is ionized when its excited state, create
d by the first photon, absorbs a second photon and the radical cation
absorbs a third photon, which enables it to react with the solvent, cr
eating HM+. Examination of previously reported results on anthracene i
n 2-propanol supports a similar explanation for the observed decrease
in the quantum yield of the anthracene radical cation with increasing
intensity. In the neat solvents the production of HM+ can be ascribed
to the reaction of the radical cation of the solvent, produced by simu
ltaneous 2- or 3-photon absorption, with a solvent molecule, which is
consistent with a body of results bearing on the fate of alkyl radical
cations in radiolysis.