Advances in Multi-Photon Processes and Spectroscopy

The following sections are included:

• PREFACE
• CONTENTS

Radiationless transitions provide a mechanism for ultrafast conversion of electronic energy into nuclear motion of a molecule. This effect results from strong vibronic mixing at geometries where two or more electronic states are degenerate (or nearly degenerate in regions of avoiding crossing).We review in this chapter different approaches to controlling such transitions. General control strategies include (1) tuning the frequency of the actinic light to enhance the yield of a specific product, (2) tailoring the properties (spectral phase and amplitude) of the excitation source to guide the generated vibrational wave packet to a specific region of the conical seam between the potential energy surfaces, and (3) modifying the shape and location of the conical intersection either by altering the chemical environment of the molecule or by introducing a strong electromagnetic field. The goals of this chapter are to reveal underlying mechanistic similarities among these general methods and to outline areas for future research.

Molecular alignment/orientation control is a key technique to manipulate or observe molecular dynamics in a molecule-fixed frame. In the present chapter, we explain optimal control approaches that provide numerically synthesized control pulses as well as clues that would lead to a better understanding of control mechanisms. After introducing the simulation procedures in a general and fundamental form, three case studies of linear molecules are adopted to illustrate how the simulation works in practical applications, which will demonstrate the effectiveness of the optimal control approaches in studying molecular alignment/orientation.

We review recent progress in the field of Coulomb imaging using femtosecond laser pulses of variable length, referred to as Femtosecond Multiple Pulse Length Spectroscopy (FEMPULS). This method introduces a multi-dimensional approach to the study of the molecular dynamics of the multiply ionized triatomic molecules: CO₂, OCS, and N₂O. We describe the experimental setup used and the approaches needed to optimize the multi-particle detection, coincidence technique. The results show the degree of high resolution imaging which can be achieved with few cycle pulses, and how the onset of charge resonance enhanced ionization (CREI) can be observed as pulse length is increased. By coupling pulse length variation with Dalitz and Newton plotting techniques, stepwise processes can be identified for all three molecules, giving insight into the dynamics, particularly on the 3+ state, which has been revealed as the doorway state to CREI. Finally, in the case of OCS, pulse length variation is shown to have the potential as a control mechanism, as it modulates the ratio of stepwise to concerted processes.

The use of ultrafast spectroscopy to gain detailed information about nonlinear processes can disclose the key physical mechanisms in the processes. The information promises to be a useful means to enhance the necessary nonlinearity for optical devices such as optical switches and optical manipulations of quantum states. Our group has generated stable visible to near-infrared laser pulses with a sub-5-fs duration using a non-collinear optical parametric amplifier. We also developed a detection system composed of 128 lock-in detectors, which provides an indispensable means of obtaining ultrafast nonlinear responses simultaneously in a very broad spectral range.

The developed ultrashort pulse laser is used to study the electronic relaxation and molecular vibration dynamics in molecular systems through the interaction of the ultrashort pulses with the systems. Ultrashort pulses can stimulate Raman scattering in a molecular system. Observed time-resolved spectrum was analyzed to separate the electronic and vibrational contributions to the transient difference absorbance. The probe wavelength dependence of the vibrational amplitude is explained in terms of the coupling between the two pairing components among the pump field, Stokes field, anti-Stokes field, and coherent vibrational coordinates.

The solvent effects over nonlinear optical properties of a two-level molecular system in presence of a classical electromagnetic field were modeled in this work. The collective effects proper of the thermal reservoir are modeled as a random Bohr frequency, whose manifestation is the broadening of the upper level according to a prescribed random function. A technique of work, based in the use of the cumulant expansions to obtain the average in the Fourier components associated with the coherence and populations, evaluated by the use of the Optical Stochastic Bloch Equations (OSBE), is employed. Analytical expressions for susceptibility, optical properties and non-degenerate Four-Wave Mixing (nd-FWM) signal intensity, were obtained. Numerical calculations were carried out to construct surfaces corresponding to these magnitudes as a function of the pump-probe frequency detuning, values of the permanent dipole moments (PDM), noise parameters and relationships between the longitudinal and transversal relaxation times. Our results show that it is necessary to neglect the Rotating-Wave approximation (RWA) in order to measure the effect of the permanent dipole moments and that the inclusion of these favors two-photon transitions over those with one-photon. In general, the effect of non-zero permanent dipole moments, are reflected in the appearance of new and more complex signals associated with new multiphoton processes.

The following section is included:

• INDEX