Chapter 8 - Transmission (Absorption)

Abstract

Transmission through, and reflection from, a medium both depend on the refractive (n) and absorptive (k) properties of the medium. Consequently, it is not always straightforward to classify interactions into the separate categories of transmission, reflection, and absorption spectroscopies. Transmission clearly monitors absorption in gases (except for the reflection from the chamber windows). In probing solids, however, transmission depends on the reflections at the ambient/material interfaces as well as on absorption in the material. Absorption and excitation laser-induced fluorescence in gases and excitation photoluminescence in solids probe the same properties of a system. Each has its strengths and weaknesses. When nonradiative relaxation processes are very fast relative to radiative decay, fluorescence is very weak and LIF (and PL) cannot be used; rapid relaxation has only a minor effect on absorption. This is particularly significant in the infrared because spontaneous emission rates are very slow in the IR. Because absorption measures the difference in the incident and transmitted beams, laser- and detector-related noise must be small in absorption measurements when the absorbed fraction is small; LIF is usually measured relative to a zero background and is therefore more sensitive.