The role of electron delocalization in the ionization of C6 hydrocarbons using intense 780 nm laser pulses of femtosecond duration

The photoionization/dissociation mass spectra are reported for the series of molecules benzene (C6H6), 1,3,5-hexatriene (C6H8), cyclohexane (C6H12), and n-hexane (C6H14) as a function of laser power intensity from 1 to 3.8 x 10(13) W cm(-2) using a pulse duration of 170 fs and wavelength 780 nm. The ionization orders are localized around 8.3 for benzene, 1,3,5-hexatriene, and cyclohexane and the relative ionization probabilities are measured to be 1,79, and 0.15, respectively. No ion current was observed for n-hexane. The dissociation yield is observed to increase exponentially as a function of the number of atoms in the molecule with cyclohexane undergoing the most dissociation and benzene undergoing essentially no dissociation. These observations are interpreted in light of a field ionization model that incorporates both the ionization potential and the electronic and nuclear structure of the molecule. (C) 1998 American Institute of Physics.