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Annual Meeting Optical Society of America

Observation of a vibrational-wave-packet-induced photon echo

Thomas J. Dunn, John N. Sweetser, Czesław Radzewicz, Ian A. Walmsley

Published Jan 1, 1992• 0 citations
Publisher

Abstract

Most experiments that study the dynamics of vibrational wave packets in small molecules utilize geometries in which propagation plays little or no role.1 In contrast, we have developed an apparatus for performing time-resolved fluorescence upconversion from an extended sample of sodium dimers that are excited by a resonant short optical pump pulse, with a temporal resolution of about 70 fs.

DOI
10.1364/oam.1992.mdd4

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Most experiments that study the dynamics of vibrational wave packets in small molecules utilize geometries in which propagation plays little or no role.1 In contrast, we have developed an apparatus for performing time-resolved fluorescence upconversion from an extended sample of sodium dimers that are excited by a resonant short optical pump pulse, with a temporal resolution of about 70 fs.
A method is presented for reconstructing the quantum state of vibrational wave packets in diatomic molecules by a heterodyne experiment. After the preparation of a wave packet, a femtosecond laser pulse interferes with the induced polarization and the resulting signal is frequency filtered and measured by a photodetector. The conditions are analyzed under which the resulting time- and frequency-resolved photocurrent contains the information needed to recover the quantum state of the prepared wave packet. It is shown that the vibrational wave function in the excited electronic state can be measured directly without the need of involved numerical reconstruction. Simulations of the proposed experiment are performed for sodium dimers.
Numerous studies to date are aimed at a better understanding of the optical dynamics of dissolved dye molecules. Most dyes exhibit prominent vibronic features in their absorption spectra. For impulsive excitation the initially excited state is a vibronic wave packet, that strongly influences the observed dynamics. Shank and co-workers were the first to recognize the importance of vibrational mode suppression towards optical dephasing measurements.[1,2] However, mode suppression based on time-integrated stimulated photon echo works only in systems that exhibit Bloch-like dynamics.
Recent progress has been made in the theoretical and experimental study of the amplification of optical signals without population inversion (AWI) of the participating electronic levels. Induced coherence among appropriate levels in the active medium is the necessary condition for obtaining AWI. Similarly, the generation of wave packet states in atoms and molecules also relies on the coherent superposition of several states. It is feasible, therefore, that AWI is possible in a system which has been excited into a wave packet state. We have thus demonstrated experimentally such amplification in an extended sample of sodium dimers and we term this process wave packet amplification (WPA). An important characteristic of WPA is that the gain is doubly phase-sensitive—it is modulated at both the wave packet oscillation period and the optical period. The former time-dependence is a result of the periodic revivals of the pump-induced electronic polarization and the latter optical phase-sensitivity is a manifestation of the coherent nature of the molecular wave packet. The experiments to be described use a time-resolved upconversion method to detect the change in energy of a 50 fs probe pulse after it passes through a sodium heat pipe that has been pumped with another 50 fs pulse. The maximum saturated energy gain in this system is four and corresponds to all of the available energy being transferred from the wave packet to the probe pulse.
We have performed transient multi-photon ionization experiments on small alkali clusters of different size in order to probe their wave packet dynamics, structural reorientations, charge transfers and dissociative events in different vibrationally excited electronic states including their ground state. The observed processes were highly dependent on the irradiated pulse parameters like wavelength range or its phase and amplitude; an emphasis to employ a feedback control system for generating the optimum pulse shapes. Their spectral and temporal behavior reflects interesting properties about the investigated system and the irradiated photo-chemical process. First, we present the vibrational dynamics of bound electronically excited states of alkali dimers and trimers. The scheme for observing the wave packet dynamics in the electronic ground state using stimulated Raman-pumping is shown. Since the employed pulse parameters significantly influence the efficiency of the irradiated dynamic pathways photo-induced ioniziation experiments were carried out. The controllability of 3-photon ionization pathways is investigated on the model-like systems NaK and K2. A closed learning loop for adaptive feedback control is used to find the optimal fs pulse shape. Sinusoidal parameterizations of the spectral phase modulation are investigated in regard to the obtained optimal field. By reducing the number of parameters and thereby the complexity of the phase moduation, optimal pulse shapes can be generated that carry fingerprints of the molecule's dynamical properties. This enables to find "understandable" optimal pulse forms and offers the possiblity to gain insight into the photo-induced control process. Characteristic motions of the involved wave packets are proposed to explain the optimized dynamic dissociation pathways.