Electronic Relaxation in Large Molecules

Abstract

We shall be concerned with intramolecular, nonreactive, interstate electronic relaxation processes in electronically excited states of collision-free isolated large molecules. These radiationless electronic relaxation processes involve the intramolecular conversion of electronic energy into vibrational energy. It is appropriate to start with a

• I PROLOGUE, where we discuss some of the general features of the acquisition, storage and disposal of energy in molecular systems, as explored from the microscopic point of view. This will be followed by another introductory section, where we dwell on the

• II HISTORY of the development of the experimental background, which provided conclusive evidence for the occurrence of electronic relaxation within a bound level structure of isolated large molecules. As this experimental work opened up some challenging theoretical issues, we proceed to outline the

• III METHODOLOGY underlying the theoretical description of time evolution within a bound level structure. This will be followed by the introduction of the

• IV BASIC MOLECULAR MODEL for interstate coupling and electronic relaxation, which provides the basis for outlining the theory of

• V TIME EVOLUTION involving the decay of a metastable state into a continuum or a quasicontinuum, which results in the exponential decay law and the golden rule decay rate. I Next, deviations from the exponential decay law are considered, going

• VI BEYOND THE GOLDEN RULE, introducing the effective Hamiltonian formalism to handle the decay of a manifold of discrete states into a common continuum and bringing up the notion of quantum beats in the radiative decay of some complex molecular level structures. While up to this point only time-resolved experimental observables were considered, we proceed to describe the diverse sources of experimental information discussing

• VII OBSERVABLES, which pertain to determination of populations and to the interrogation of retention of phase relationships, involving time-resolved observables, energy-resolved observables and observables pertaining to coherent optical effects. As an example for energy-resolved observables, we consider

• VIII ABSORPTION LINESHAPES, focusing attention on the information regarding interstate coupling which stems from optical spectroscopy. The last stage of the exposition will discuss

• IX THE RISE AND FALL OF EXCITED STATES of isolated polyatomic molecules, where we distinguish between implications of interstate coupling and relaxation, briefly discuss excitation modes and digress on criteria for practical irreversibility in a bound level structure. We conclude with a discussion of

• X FUTURE TRENDS, where some novel and intriguing problems in the area of intramolecular dynamics are exposed.