The HF chemical laser as an irreversible heat engine

A detailed kinetic code of an HF chemical laser is used to evaluate the thermodynamic properties characterizing the laser as an irreversible heat engine. The time-dependent thermodynamic analysis is demonstrated for the two limiting cases of negligible and enhanced relaxation rates, especially rotational relaxation. The main finding is the laser efficiency is appreciably larger in the second case, despite the irreversible energy losses by relaxation processes. This is due to the entropy increase of the heat bath which compensates for the entropy decrease of the lasing molecules. Consequently an efficient conversion of internal energy into laser light (thermodynamic work) can take place. In the 'frictionless' (no relaxation) laser the entropy of the lasing molecules cannot decrease. Hence lasing terminates with entropy and energy rich populations, and the laser efficiency is relatively poor.