On the interaction of continuous wave and pulsed lasers with surface adsorbed molecules: general theory and N 2 on graphite

Abstract

A study is made of the interaction of continuous wave (CW) and pulsed lasers with surface adsorbed molecules. The phases and amplitudes of the two components of the effective field experienced by the adsorbate molecule, taking account of the response of the substrate to the incident radiation, is worked out using Fresnel's theory of reflection of a monochromatic electromagnetic wave at the interface of dispersive dielectric media. Of particular interest are the temporal and frequency effects in the effective field for a pulsed laser, as a function of pulse duration, caused by the response of the substrate to the various frequency components of the pulse. Applications of the expressions for the effective fields to the computation of adsorbate absorption spectra, and of the underlying temporal behaviour of adsorbate excited states, is discussed using two- and three-level models of N 2 adsorbed on graphite as examples. Emphasis is placed on topics not considered in previous work, e.g., spectral effects due to the different phases of the two components of the effective field, and due to the finite duration of a (Gaussian) pulsed laser. The use of pulsed and CW lasers as probes of surface adsorbed molecules is discussed. Analysis of the adsorbate spectra and the temporal behaviour of the adsorbate excited states can probe the dielectric properties of the substrate as well.