MULTIPHOTON PROCESSES IN CYCLOHEXANE AND TRANS-DECALIN AND THE FORMATION OF HIGH-MOBILITY CATIONS

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.