Local laser superheating due to quenching determined by degenerate four-wave-mixing and absorption thermometry

We used degenerate four-wave-mixing (DFWM) spectroscopy to measure the rotational distribution of transient formed NaH molecules in the Na(3p)-H2 collision system applying a recently developed new method for the correct consideration of the transition dipole moment. Considering the NaH molecule as a thermometric species, we found high rotational temperatures differing significantly from the oven temperature by up to 80%. For comparison the revealed temperatures were also determined with linear absorption measurements giving excellent agreement between the two methods. The increased temperatures are explained by a spatially localized heating mechanism which is caused by kinetic energy transfer between Na(3p) and H2. A rate equation model, which includes quenching and reaction processes, agrees satisfactorily with all types of chosen parameters, including different sodium excitations as well as different cell temperatures.