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 photoelectr on-photoion coincidence detection we have studied the time-resolved photoel ectron spectra of CF2I2 and its fragments after excitation with 4.65 eV pho tons. The time-dependent photoion signals reflect the complete dissociation of the CF2I2 molecules with a time constant of (100 +/- 30) fs which is pr eceded by an ultrafast relaxation process with (30 +/- 10) fs. The analysis of the electron spectra reveals that three electronic states with differen t vibrational energies are populated by one photon excitation during the pu mp pulse. Furthermore, the number of absorbed pump and probe photons for hi gher order excitation, the ionization potential of CF2I2 and its binding en ergies 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 excite d CF2I2 molecules, but no CF2I has been detected in all experiments with wi dely spread laser parameters. Thus, we conclude the concerted reaction mech anism to be the dominant dissociation channel while the sequential decay wi th the CF2I intermediate is negligible. The measured long-living signals fo r I-2(+) are suggested as due to molecular detachment after absorption of t wo 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. [S 0021- 9606(00)00529-8].