The CO2 laser that was used in this work was a Coherent DEOS GEM Select
100. The principle of operation of a CO2 laser is not discussed in detail here
as it is covered in many textbooks, e.g. reference [154]. Briefly, there is a
mixture of CO2, N2 and He gases contained between electrodes in the gain
region of the cavity. An RF field is applied between the electrodes which
creates a discharge. The first vibrational mode of the N2 molecules (which
has a very large excitation cross-section) is excited. This state is metastable
and close in energy to the first asymmetric stretch mode of the CO2 molecules.