title: Coherent phase control of internal conversion in pyrazine.
abstract: Shaped ultrafast laser pulses were used to study and control the ionization dynamics of electronically excited pyrazine in a pump and probe experiment. For pump pulses created without feedback from the product signal, the ion growth curve (the parent ion signal as a function of pump/probe delay) was described quantitatively by the classical rate equations for internal conversion of the S2 and S1 states. Very different, non-classical behavior was observed when a genetic algorithm (GA) employing phase-only modulation was used to minimize the ion signal at some pre-determined target time, T. Two qualitatively different control mechanisms were identified for early (T < 1.5 ps) and late (T > 1.5 ps) target times. In the former case, the ion signal was largely suppressed for t < T, while for t ≫ T, the ion signal produced by the GA-optimized pulse and a transform limited (TL) pulse coalesced. In contrast, for T > 1.5 ps, the ion growth curve followed the classical rate equations for t < T, while for t ≫ T, the quantum yield for the GA-optimized pulse was much smaller than for a TL pulse. We interpret the first type of behavior as an indication that the wave packet produced by the pump laser is localized in a region of the S2 potential energy surface where the vertical ionization energy exceeds the probe photon energy, whereas the second type of behavior may be described by a reduced absorption cross section for S0 → S2 followed by incoherent decay of the excited molecules. Amplitude modulation observed in the spectrum of the shaped pulse may have contributed to the control mechanism, although this possibility is mitigated by the very small focal volume of the probe laser.
authors: Department of Chemistry (m/c 111), University of Illinois at Chicago, Chicago, Illinois 60680-7061, USA.
authors: Robert J
authors: RJ
authors: Gordon
authors: Institute of Atomic and Molecular Physics, Jilin University, Changchun 130021, People's Republic of China.
authors: Zhan
authors: Z
authors: Hu
authors: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
authors: Tamar
authors: T
authors: Seideman
authors: Department of Chemistry (m/c 111), University of Illinois at Chicago, Chicago, Illinois 60680-7061, USA.
authors: Sima
authors: S
authors: Singha
authors: Science and Mathematics Faculty, College of Letters and Sciences, Arizona State University, Mesa, Arizona 85212, USA.
authors: Maxim
authors: M
authors: Sukharev
authors: Department of Chemistry (m/c 111), University of Illinois at Chicago, Chicago, Illinois 60680-7061, USA.
authors: Youbo
authors: Y
authors: Zhao