Emily J. Brown

Femtosecond transient-grating techniques: Population and coherence dynamics involving ground and excited states

Time-resolved transient grating techniques (TG) arising from four-wave mixing (FWM) processes are explored for the study of molecular dynamics in gas-phase systems ranging from single atoms to large polyatomic molecules. For atomic species such as Ar and Xe, each TG signal shows only a peak at zero time delay when all three incident pulses are overlapped temporally. For diatomic O2 and N2 and linear triatomic CS2 molecules, the TG signals exhibit ground state rotational wave packet recurrences that can be analyzed to obtain accurate rotational constants for these molecules. With heavier systems such as HgI2, ground state vibrational and rotational wave packet dynamics are observed. Resonant excitation allows us to select between measurements that monitor wave packet dynamics, i.e., populations in the ground or excited states or coherences between the two electronic states. To illustrate these two cases we chose the X→B transition in I2. TG measurements yield dynamic information characteristic of vibration...

Quantum control of the yield of a chemical reaction

Order of magnitude enhancement in the concerted elimination pathway leading to I2 product formation in the photodissociation reaction of CH2I2 by the use of positively chirped 312 nm femtosecond laser pulses is demonstrated. The maximum yield is found for chirps of 2400 fs2 while the minimum is found near −500 fs2. Multiphoton excitation with 624 nm pulses results in the opposite effect, where the maximum yield is found near −500 fs2. The enhancement as a function of chirp is found to depend on the wavelength and intensity of the laser pulses. These results offer new experimental evidence for quantum control of chemical reactions.

Population and coherence control by three-pulse four-wave mixing

Control of coherence and population transfer between the ground and excited states is reported using three-pulse four-wave mixing. The inherent vibrational dynamics of the system are utilized in timing the pulse sequence that controls the excitation process. A slight alteration in the pulse sequence timing causes a change in the observed signal from coherent vibration in the ground state to coherent vibration in the excited state. This control is demonstrated experimentally for molecular iodine. The theoretical basis for these experiments is discussed in terms of the density matrix for a multilevel system.

The role of pulse sequences in controlling ultrafast intramolecular dynamics with four-wave mixing

This article seeks to provide a fundamental understanding of time-resolved four-wave mixing (FWM) processes based on a large body of experimental measurements on a model system consisting of isolated iodine molecules. The theoretical understanding is based primarily on a diagrammatic approach. Doublesided Feynman diagrams are used to classify and describe the coherent FWM processes involved in the signal obtained with each pulse sequence. Different pulse sequences of degenerate femtosecond pulses are shown to control the optical phenomena observed, that is transient grating, reverse-transient grating, photon echo and virtual photon echo. The experimental data reveal clear differences between the nonlinear optical phenomena. We find that the virtual photon echo sequence k1 - k2 + k3 is the most efficient for controlling the observation of ground - or excited-state dynamics. The strategy followed to make this assessment was to compare transients when the time delay between two of the three pulses set in or out of phase with the excited-state vibrational dynamics. We have obtained a signal from pulse sequences k1 + k2 - k3 for which FWM signal generation for this two-electronic-level system is forbidden. This signal can be explained by the cascading of a first-order polarization and a second-order process to generate the FWM signal. The implications of our findings are discussed in the context of multiple-pulse methods for the control of intramolecular dynamics.

Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. I. Transition state dynamics and product channel coherence

The direct observation of the photoinduced molecular detachment of halogens X2 from halogenated alkanes RCHX2 is presented. Three-photon excitation at 312 nm produces molecular halogens and a carbene; the halogen products are formed predominantly in the D′ state. Femtosecond pump–probe spectroscopy of the reaction reveals a fast (τ<50 fs) dissociation with no evidence of intramolecular vibrational redistribution. This is consistent with a prompt dissociation without intermediates. The experimental results demonstrate vibrational coherence in the halogen product, which requires that the reaction proceed by a concerted mechanism.

Femtosecond concerted elimination of halogen molecules from halogenated alkanes

The concerted dynamics involved in the molecular detachment of halogenated alkanes, i.e., CX2YZ→CX2+YZ (where X=H, F or Cl and Y, Z=I, Br or Cl) have been studied by femtosecond pump–probe spectroscopy. Particular emphasis has been placed on exploring the role of symmetry in the parent molecule. For experiments carried out on CH2ICl, product fluorescence was observed in two regions of the spectrum: at 430 nm, corresponding to the D′→A′ transition, and at 340 nm, attributed to the G→A transition. Differences in the dissociation time for the two pathways can be understood by considering the energy available for fragment recoil. The elimination process was found to be slower for methylene bromide than for methylene iodide, possibly because of the difference in the enthalpy of reaction. When three gem-dibromo compounds (Y, Z=Br, X=H, F or Cl) were compared, they were found to have significantly different dissociation times: 29.7 fs for the fluorinated species, 58.6 fs for the hydrogenated species, and 80.6 fs for the chlorinated species. The difference in the transition state lifetime between the hydrogenated and chlorinated species can be rationalized in terms of changes in the reduced mass, the thermodynamics of the reactions, and the energy partitioning. The fluorinated compound was found to dissociate much faster than predicted by thermodynamic and reduced mass arguments.

Sequences for controlling laser excitation with femtosecond three-pulse four-wave mixing

Three-pulse four-wave mixing (FWM) is used here to study and control laser excitation processes. For general laser excitation processes, after a molecule interacts resonantly with a laser pulse, the molecule has a probability of being in the ground or in the excited state. Control over this process depends on the phase and amplitude of the electric fields that interact with the molecular system. Here we show how three-pulse FWM can be used to control the excitation of iodine molecules. Depending on the time delay between the first two pulses, the observed signal reflects the dynamics of the ground or excited state. A theoretical formalism based on the density matrix formulation is presented and solved for a four-level system. Experiments are found to be in excellent agreement with the theory. The influence of linear chirp on three-pulse FWM experiments is explored. Spectrally dispersed three-pulse FWM is found to be extremely useful for studying the effect of chirp on laser excitation of molecular systems. Experimental demonstrations of these effects are included.

WHAT ROLE CAN FOUR-WAVE MIXING TECHNIQUES PLAY IN COHERENT CONTROL?

The role of four-wave mixing (FWM) techniques in coherent control is considered from the point of view of some of the most important developments in this field over the past years, namely multiphoton excitation, pump-dump methods, interference between coherent pulses, chirped laser pulses, and optimal control. FWM techniques provide a powerful platform for combining coherently multiple laser pulses. We explore the effectiveness of these techniques in controlling chemical reactions. The phase relationship between the pulses is maintained by detecting the signal in a phase-matching direction. The results presented show control over the observed dynamics from ground and excited state populations. The FWM signal results from the polarization of the sample following three different electric field interactions. The virtual echo sequence is achieved by the interactions of the sample with three consecutive electric fields characterized by exp[i(kx-ω t)], exp[-i(kx-ω t)] and exp[i(kx-ω t)]. This sequence allows control over the observed ground or excited state dynamics. With the photon echo pulse sequence, characterized by interactions with exp[-i(kx-ω t)], exp[i(kx-ω t)], and exp[i(kx-ω t)], we find that control of ground and excited state populations is not achieved. Differences between these two pulse sequences are shown experimentally and illustrated using wave packet simulations. Data obtained using the ‘mode suppression’ technique, in which the timing between the first and third laser pulses is fixed while the second pulse is scanned are presented. We show that this technique does not suppress the observed vibrational coherence from the ground or excited state but it yields an additional component to the signal that is independent of the vibrational coherence of the sample. Spectrally dispersed FWM is shown to be an ideal tool for studying intramolecular dynamics and this idea is applied to understanding the role of chirp in controlling molecule-laser interactions. All coherent control methods are affected by the rate of decoherence of the sample. Here we show how these rates are measured with FWM techniques. The measurements presented here illustrate how photon echo measurements yield the homogeneous relaxation rate while the virtual echo measurements yield the sum between homogeneous and inhomogeneous