  Absorption spectroscopy studies the absorption of electromagnetic wave in a
material sample under test. The interacting electromagnetic wave can be
propagating in free space or in a waveguide. The waveguide-based absorption
spectroscopy method has several advantages compared to the free space setup.
The effect of the waveguide cross section on the interaction between the
waveguide mode and the sample can be expressed by a factor, called interaction
factor. In this article, a new formulation for the interaction factor is
derived. It is shown that this factor is inversely proportional to the energy
velocity of the waveguide mode.
