Photoionization and photofragmentation of gaseous toluene using 80-fs, 800-nm laser pulses

This paper presents ion yields resulting from multiphoton ionization and fr agmentation of gaseous toluene (C7H8) in the focus of an 80 fs Ti:sapphire laser beam (lambda=800 nm) with a sufficiently small B-integral [Siegman, L asers (University Science Books, Mill Valley, CA, 1986)]. The peak intensit y was varied between 1.9x10(13) and 2.8x10(14) W cm(-2), and both linear an d circular polarization were used. Over the whole range of intensities stud ied, only the singly charged parent ion and its fragment, C7H7+, are found. Although the Keldysh adiabaticity parameter equals 0.86 for the saturation intensity of similar to 1x10(14) W cm(-2), there is no indication of tunne ling. The parent ion yield is found to be effectively proportional to the s ixth power of the peak intensity. This is shown to be in good agreement wit h a multiple lowest-order perturbation multiphoton ionization model which t akes into account successive channel closing for increasing peak intensitie s and orders up to 11 inclusive. On the assumption that the excess energy a cquired by the toluene cation as a result of the interaction with the elect romagnetic field is of the order of the ponderomotive energy for the intens ity prevailing at the moment of the ionization, the internal energy distrib ution of the toluene cations created that is brought about by this multiple -order multiphoton ionization model is calculated. This internal energy dis tribution is in perfect agreement with the measured C7H7+ yield, if the rat e-energy curve for the fragmentation of excited toluene cations as given by Golovin [Sov. J. Chem. Phys. 2, 632 (1985)] is moderately reduced by a fac tor of 4.5. (C) 2000 American Institute of Physics. [S0021-9606(00)00121-5] .