Photodissociation Dynamics of Acetophenone and Its Derivatives with Intense Nonresonant Femtosecond Pulses

Recent work from our group (Lozovoy, V. V.; Zhu, X.; Gunaratne, T. C.; Harris, D. A.; Shane, J. C.; Dantus, M. J. Phys. Chem. A 2008, 112, 3789) using shaped nonresonant femtosecond pulses to ionize and fragment polyatomic molecules indicated that pulse duration is the most important parameter for controlling the relative yield of different fragment ions. Here we explore the time-resolved dynamics that ensue following the interaction of the molecules with a strong 10¹⁵ W/cm² nonresonant near-infrared laser field. The data reveal that most of the fragmentation processes occur well after ionization. The molecular dynamics are followed in the 10⁻¹⁴−10⁻¹⁰ s time scale. Studies carried out on acetophenone derivatives are used to assign the observed modulation in the benzoyl product ion yield, which is found to correlate with further ionization and fragmentation through electronic coordination. The resulting experimental data, together with photoelectron spectra and the electron-ionization mass spectra of these compounds, allow us to propose ladder switching processes taking place in this family of compounds which regulate the different fragment ions observed. This analysis sheds light on how pulse duration influences the yield of different fragment ions.