Published: January 12, 2011 r2011 American Chemical Society 1305 dx.doi.org/10.1021/jp1029486 | J. Phys. Chem. A 2011, 115, 1305–1312 ARTICLE Photodissociation Dynamics of Acetophenone and Its Derivatives with Intense Nonresonant Femtosecond Pulses Xin Zhu, † Vadim V. Lozovoy, † Jay D. Shah, † and Marcos Dantus* ,†,‡ † Department of Chemistry and ‡ Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, United States b S Supporting Information ABSTRACT: 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 mole- cules 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 10 15 W/ cm 2 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. ’ INTRODUCTION It has been over two decades since ultrafast lasers were first used to explore the photodissociation dynamics of molecules. During this time, important lessons have been learned about direct and indirect bond breakage, curve crossing dynamics, and dynamics over saddle points. Over a decade ago, some interest shifted from probing the dynamics to controlling the dynamics. Of particular interest were studies in which an intense nonreso- nant laser field is shaped prior to interacting with isolated molecules, and the subsequent yield of fragment ions, detected using a mass spectrometer, was found to vary according to the phase and amplitude of the shaped laser pulses. Recently, experimental evidence indicates that pulse duration can account for the majority of the changes in fragmentation observed. 1-6 Here, we turn our attention to the dynamics occurring soon after isolated molecules interact with an intense nonresonant laser field. Our study reveals dynamics taking place over a range of four orders of magnitude in time, and these provide information about how molecules interact with intense laser fields and how pulse shaping influences the yield of different fragment ions. Early femtosecond photodissociation dynamics studies were carried out using a pump laser that was resonant with an electronic potential energy surface (PES), and the dynamics occurring on the PES were subsequently probed by a probe laser pulse tuned to detect the photofragments. 7-9 Since the early days it was noted that femtosecond lasers, even when not resonant with an electronic transition, would be able to cause excitation, fragmentation, and ionization. A distinction was made between long pulse excitation, where the process could be characterized by multiphoton transitions, and short pulse excita- tion, where the process was characterized by fast, single optical cycle, field ionization followed by fragmentation. The former process became known as ladder switching to indicate that photodissociation takes place at a comparable rate with transi- tions among upper excited states. 10,11 Direct ultrafast excitation to selected PES allowed direct probing of wave packet dynamics. Bound and quasibound states arising from the crossing of electronic states led to the observa- tion of coherent oscillations. Extrapolating from the early studies, one would expect that a shaped pulse could be used to time a number of discrete transitions among two or more PES in order to direct the photodissociation reaction and control the product formation. Control of a wave packet requires a collection of sub- 50 fs pulses at a number of different wavelengths in the 260-532 nm spectral region. A laser source capable of creating three or more pulses by pulse shaping a single input pulse in the UV-vis wavelength range is still outside of present technical capabilities, although significant progress is being made on shaped UV sources and their use for controlling chemical reactions. 12-14 The widely available near-IR femtosecond shaped sources have Received: April 1, 2010 Revised: November 18, 2010