Femtosecond visible and infrared analogues of multiple-pulse nuclear magnetic resonance techniques provide novel snapshot probes into the structure and electronic and vibrational dynamics of complex molecular assemblies such as photosynthetic antennae, proteins, and hydrogen-bonded liquids. A classical-oscillator description of these spectroscopies in terms of interacting quasiparticles (rather than transitions among global eigenstates) is developed and sets the stage for designing new pulse sequences and inverting the multidimensional signals to yield molecular structures. Considerable computational advantages and a clear physical insight into the origin of the response and the relevant coherence sizes are provided by a real-space analysis of the underlying coherence-transfer pathways in Liouville space.

Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations.

The observation of coherent dynamics ensuing from the excitation of molecular systems by femtosecond laser pulses is at the heart of femtochemistry. The time-dependent evolution of coherent superpositions of quantum states, the physical basis for the observation of coherent dynamics and their manipulation are of central importance to coherent control of physicochemical processes. In the early days of femtochemistry, there was significant skepticism regarding the type of information that could be learned from spectroscopic experiments using very short pulses. There were arguments that one could infer the dynamics from frequency-resolved experiments and that the femtosecond experiments did not offer new information. It was also assumed that experiments with very short pulses would smear the available spectroscopic information because of their broad bandwidths. Approximately two decades after the initial experiments, it has become clear that femtosecond experiments have opened an extremely valuable window into the dynamic behavior of atomic and molecular systems that is influencing how we think about physics, chemistry, and biology. In particular, we highlight the fact that some of the highest resolution spectroscopic measurements being carried out employ femtosecond laser pulses. These experiments, specifically those taking advantage of rotational coherence, provide resolution that rivals microwave spectroscopy. The reason spectroscopic information is not lost in femtochemistry experiments is coherence, a property that can be manipulated to control physicochemical processes by a number of different approaches, which will be reviewed here. This review presents a summary of some of the most salient contributions to the field of coherent laser control from an experimentalist’s perspective. While a number of theoretical papers have made key contributions to the field, it is from the experimental successes, as well as failures, that we can best learn how to implement new strategies and develop future applications. Coherent laser control, in the context of this review, encompasses experiments in which the coherent properties of the laser and/or the molecule are required for controlling a particular physicochemical process. We distinguish for each of the experiments between coherence in the laser field(s), * To whom correspondence should be addressed. Phone (517) 3559715 (ext-315). Fax (517) 353-1793. E-mail dantus@msu.edu. 1813 Chem. Rev. 2004, 104, 1813−1859

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Experimental coherent laser control of physicochemical processes.

Optical pulse shaping approaches to coherent control

A control system and apparatus for use with an ultra-fast laser is provided. In another aspect of the present invention, the apparatus includes a laser, pulse shaper, detection device and control system. A multiphoton intrapulse interference method is used to characterize the spectral phase of laser pulses and to compensate any distortions in an additional aspect of the present invention. In another aspect of the present invention, a system employs multiphoton intrapulse interference phase scan. Furthermore, another aspect of the present invention locates a pulse shaper and/or MIIPS unit between a laser oscillator and an output of a laser amplifier.

Control system and apparatus for use with ultra-fast laser

Nonlinear optical processes are controlled by modulating the phase of ultrafast laser pulses taking advantage of multiphoton intrapulse interference. Experimental results show orders of magnitude control over two- and three-photon excitation of large organic molecules in solution using specific phase functions. We show simulations on the effect of phase modulation on the second- and third-order amplitude of the electric field spectrum, and demonstrate that the observed control is not caused by simple changes in peak intensity.

Multiphoton intrapulse interference. II. Control of two- and three-photon laser induced fluorescence with shaped pulses

We survey the possible time-resolved three pulse nonlinear spectroscopic techniques and their application to gas-phase samples. The role of each of the interacting electric fields is specified, and the nature of the signal is interpreted using the density matrix representation. Simulations of these nonlinear optical signals are based on the perturbative solution of the Liouville equation for the density matrix to third order in the applied fields. We present closed expressions for the integrated and frequency dispersed signals that identify the type of molecular response for each technique and give the signal dependence on the various time delays between laser pulses. We choose a simple experimental system, two-electronic states coupled to a single vibrational mode of diatomic iodine, with weak dephasing, to illustrate various molecular polarization responses in four-wave mixing experiments including pump−probe, reverse transient grating, and photon echo. These signals and their simulations illustrate how...

Ultrafast Nonlinear Spectroscopic Techniques in the Gas Phase and Their Density Matrix Representation

The roles of pulse sequence and pulse chirp were explored using femtosecond three-pulse four-wave mixing (FWM). The experiments were carried out on gas-phase I2 and the degenerate laser pulses are resonant with the transition between the X (6gY ) ground and B (0uY ) excited electronic states. Impulsive excitation leads to the observation of vibrational coherence in the ground and the excited states. Control over the observed population and vibrational coherence is achieved by using specific pulse sequences. Using chirped pulses results in changes in vibrational coherence. When the FWM signal is spectrally dispersed, the two-dimensional data (wavelength and time delay) provide important spectroscopic information about the intramolecular dynamics of both electronic states. This information is not typically available in time or spectrally integrated measurements. A theoretical foundation for these observations based on the density matrix formalism is briefly discussed. Copyright  2000 John Wiley & Sons, Ltd.

Femtosecond spectrally dispersed three-pulse four-wave mixing: the role of sequence and chirp in controlling intramolecular dynamics

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.

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Population and coherence control by three-pulse 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 ...

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

Experimental control and characterization of intramolecular dynamics are demonstrated with chirped femtosecond three-pulse four-wave mixing (FWM). The two-dimensional (spectrally dispersed and timeresolved) three-pulse FWM signal is shown to contain important information about the population and coherence of the electronic and vibrational states of the system. The experiments are carried out on gas-phase I 2 and the degenerate laser pulses are resonant with the X (ground) to B (excited) electronic transition. In the absence of laser chirp, control over population and coherence transfer is demonstrated by selecting specific pulse sequences. When chirped lasers are used to manipulate the optical phases of the pulses, the two-dimensional data demonstrate the transfer of coherence between the ground and excited states. Positive chirps are also shown to enhance the signal intensity, particularly for bluer wavelengths. A theoretical model based on the multilevel density matrix formalism in the perturbation limit is developed to simulate the data. The model takes into account two vibrational levels in the ground and the excited states, as well as different pulse sequences and laser chirp values. The analytical solution allows us to predict particular pulse sequences that control the final electronic state of the population. In a similar manner, the theory allows us to find critical chirp values that control the transfer of vibrational coherence between the two electronic states. Wave packet calculations are used to illustrate the process that leads to the observation of ground-state dynamics. All the calculations are found to be in excellent agreement with the experimental data. The ability to control population and coherence transfer in molecular systems is of great importance in the quest for controlling the outcome of laser-initiated chemical reactions.

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Bruna I. Grimberg

Papers

Ultrafast Nonlinear Spectroscopic Techniques in the Gas Phase and Their Density Matrix Representation

Femtosecond spectrally dispersed three-pulse four-wave mixing: the role of sequence and chirp in controlling intramolecular dynamics

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

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

Control and Characterization of Intramolecular Dynamics with Chirped Femtosecond Three-Pulse Four-Wave Mixing