Analysis of the ultrafast photodissociation of electronically excited CF2I2 molecules by femtosecond time-resolved photoelectron spectroscopy

Applying the femtosecond pump-probe technique combined with the photoelectron-photoion coincidence detection we have studied the time-resolved photoelectron spectra of CF2I2 and its fragments after excitation with 4.65 eV photons. The time-dependent photoion signals reflect the complete dissociation of the CF2I2 molecules with a time constant of (100 +/- 30) fs which is preceded by an ultrafast relaxation process with (30 +/- 10) fs. The analysis of the electron spectra reveals that three electronic states with different vibrational energies are populated by one photon excitation during the pump pulse. Furthermore, the number of absorbed pump and probe photons for higher order excitation, the ionization potential of CF2I2 and its binding energies in the ionic state have been determined by the electron spectroscopy. Both the ion signals as well as the electron spectra demonstrate that the observed products CF2, I-2, and I are formed by dissociation of the excited CF2I2 molecules, but no CF2I has been detected in all experiments with widely spread laser parameters. Thus, we conclude the concerted reaction mechanism to be the dominant dissociation channel while the sequential decay with the CF2I intermediate is negligible. The measured long-living signals for I-2(+) are suggested as due to molecular detachment after absorption of two pump photons. The detected electron spectra for I+ at longer delay times reflect the formation of highly excited neutral iodine atoms by absorption of at least three pump photons. (C) 2000 American Institute of Physics. [S0021- 9606(00)00529-8].