Strong-field induced vibrational coherence in hot iodine molecules

Date of Completion

January 2009


Physics, Molecular|Physics, Atomic|Physics, Optics




We present studies of the generation, observation and manipulation of vibrational coherence with ultrafast lasers and investigations of molecular structure and dynamics using vibrational wave packets. In a pump-probe experimental scheme, vibrational wave packets are created in molecular potential wells with femtosecond laser pulses; the wave packets are further ionized at a delay time to a dissociative potential curve and the evolution of the wave packets as a function of time is mapped through detection of the dissociative fragment ions. The wave packets we focus on are generated in the ground electronic state of neutral I2, the B state of neutral I2 and the ground and excited states of I+2 . The experimental time-of-flight results show vibrations in ion signals and the kinetic energy of the fragment ions. We determine the host state for the vibrational wave packet through frequency analysis and the mechanisms of creating them mainly through the phases of the vibrations. We also simulate the motion of the wave packets to provide theoretical pictures of the vibrations, which are used to confirm the states responsible for the vibrations, map the measured kinetic energy to the internuclear separation and give predictions of the vibrations' dependence on laser pulse parameters. We show the results of the vibrations for hot molecules compared with cold molecules and find that initial excitation of vibrational levels is not detrimental to vibrational coherence for Lochfrass or the “R-dependent ionization” scheme. In fact, Lochfrass can leads to vibrational cooling, as well. We compare bond-softening with Lochfrass through calculations and show that initial incoherence is detrimental to producing coherence through bond-softening. We present the experimental results of intensity and wavelength dependence of the induced vibrations and theoretical results of the pulse-width and temperature dependence. Using the vibrational wavepacket, we probed the potential curves of excited states of I+2 and I2+2 and a highly charged ionic state I3+2 . Furthermore, we investigate the ionization rate from the intermediate state and the angular dependence of the ionization by rotating the polarization of the probe pulse. We calculate the kinetic energy as a function of the internuclear separation reached by the pump and the probe pulses. The calculations show, in general, the dynamics of trapping molecules in potential wells using intermediate states.^