Advances In Multi-photon Processes And Spectroscopy, Volume 23

(Volume 23)

S H Lin, A A Villaeys;Y Fujimura

• Publisher: WSPC
• Year: 2016
• eBook ISBN: 9789814749770
• Topic: Scienze fisiche
• Subtopic: Fisica atomica e molecolare

This volume presents recent progress and perspectives in multi-photon processes and spectroscopy of atoms, ions, and molecules. The subjects in the series cover the experimental and theoretical investigations in interdisciplinary research fields in nat

CHAPTER 1

CONTROL OF RADIATIONLESS TRANSITIONS

Robert J. Gordon^*,† and Tamar Seideman^‡

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.

1.Introduction

Radiationless transitions between different electronic states of a molecule are manifest throughout nature and are of great interest in both pure and applied science. These processes result from the strong correlation between nuclear and electronic motion near the intersection (or avoided crossing) of pairs of electronic states, where the Born–Oppenheimer (BO) approximation breaks down. A few examples of current interest are the cis–trans isomerization of retinal, which is the initiating reaction in vision, deactivation of β-carotene in light harvesting complexes, ultrafast conversion of electronic to vibrational energy in DNA bases, which protects them from degradation by UV radiation, ring-opening of cyclohexadiene, which has been proposed for constructing an ultrafast optical switch and is a prototype reaction in vitamin D synthesis, and collisional quenching of electronic energy in combustion reactions.

Light-initiated radiationless transitions may be characterized as either photophysical or photochemical. The former entails transitions between electronic states without altering the bond structure or conformation of the molecule. Spin-conserving transitions of this type are called internal conversion, whereas spin-changing transitions are referred to as intersystem crossing. A hallmark of internal conversion is extremely rapid (sub-ps) energy transfer. Examples of unimolecular photochemical processes include isomerization, bond rearrangement, photodissociation, and molecular fragmentation. Radiationless transitions are also involved in bimolecular processes such as photoassociation, charge transfer (“harpooning”) reactions, and collisional quenching.

Radiationless transitions have been studied since the early days of quantum mechanics. Several recent reviews are given in Refs. 1–10. Interest in recent years has focused on controlling these processes, both for practical application as well for the fundamental knowledge that such studies provide. A wide variety of control schemes have been developed by diverse, and often non-overlapping, scientific communities. One methodology, sometimes referred to as “environmental control”, involves chemical modification of the reactant as well as modification of the solvent properties in condensed phase reactions. An alternative method, referred to loosely as “optical control”, includes modification of the radiation used to initiate the reaction (the “actinic” light) and the introduction of additional control fields that interact with the excited molecule. Within the last category, we may distinguish between the use of light tuned to excite specific states that lead to different products (sometimes referred to as “eigenstate control” or “passive control”) and light that has been tailored to guide the molecule along selected reaction paths (sometimes referred to as “active control”). While these methodologies embody very different conceptual approaches to controlling chemical reactions, strong similarities exist between their underlying mechanisms. A goal of this perspective is to reveal the synergistic connections between these methods, using selected examples from each, and in the process discovering new directions for research.

This chapter is organized as follows: In Sec. 2, we briefly review the properties of adiabatic and non-adiabatic potential energy surfaces (PESs) and the topography of their intersections. In Sec. 3, we give a few illustrations of chemical and environment control. These examples will serve as a segue to optical control by external fields, discussed in Secs. 4–6. These sections constitute the core of this chapter, where we discuss strategies for control in weak, intermediate, and strong fields. The three regimes correspond to (1) fields that are weak enough for perturbation theory to apply, (2) fields strong enough to induce cycling between electronic states without substantially perturbing the PESs, and (3) fields strong enough to dress the electronic states without ionizing the reactant. Although the intensity ranges of these regimes overlap and are molecule-dependent, there are distinct control strategies characteristic of each. In Sec. 7, we explore the role of coherence in controlling radiationless processes in a condensed phase, and in Sec. 8 we conclude with an outlook for future research.

2.Adiabatic and Diabatic Transitions

2.1.Breakdown of the BO approximation

We start our discussion of controlling radiationless transitions with a brief review of the BO approximation and its breakdown.¹¹ The most general formulation of the molecular wave function is obtained by solving the complete time-independent Schrödinger equation for a molecule with n electrons and N nuclei,

where r denotes the coordinates of all the electrons, R denotes the coordinates of all the nuclei, and the various terms in the Hamiltonian are the electronic kinetic energy, the electron–electron potential energy, the nuclear kinetic energy, the nuclear–nuclear potential energy, and the electron–nucle...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Preface
• Contents
• 1. Control of Radiationless Transitions
• 2. Optimal Control Approaches for Aligning/Orienting Linear Molecules
• 3. Femtosecond Laser-Induced Coulomb Explosion Imaging
• 4. Development of Ultrashort Pulse Lasers and their Applications to Ultrafast Spectroscopy in the Visible and NIR Ranges
• 5. Nonlinear Optical Properties in Molecular Systems with Non-Zero Permanent Dipole Moments in Four-Wave Mixing under Stochastic Considerations
• Index