CHAPTER 11: An Introduction to Laser-field Effects on Chemical Reactivity

This chapter is meant as an introduction for chemists by chemists to the field of laser–molecule interaction. Such an intermediate level introduction appears to be scarce in the literature. In this domain of research the fields are generally treated classically (as non-quantized oscillating electric fields) while the atoms and molecules are treated quantum mechanically. It is common to adopt the “dipole approximation”, which takes the wavelength of the field to be infinite compared to molecular dimensions, and to also neglect magnetic effects. These considerations, when adopted within the bounds of applicability of the Born–Oppenheimer approximation, yield an effective laser-molecule potential governed by three dominant terms: the field-free potential, a dipolar term, and a polarizability term. Except in some excited states, the polarizability term is always stabilizing (energy lowering), but the magnitude of the stabilization depends on the magnitude of the relevant tensor component at a given reaction coordinate. The dipolar term can be either stabilizing or destabilizing depending on the phase of the radiation and the direction of the field-free dipole moment with respect to the incoming radiation. The interplay of these two field-dependent (and time-dependent) terms can completely change the shape of the potential energy surface and provide us with tools to tune and control chemical reaction by the proper choice of laser intensity and phase. Ultrashort laser pulses (of the order of hundreds of atto-seconds) can drive time-dependent oscillation in the electron density itself since this is the time scale of the electronic motion within atoms and molecules.