*2.1. Absorption Measurements in High-Temperature and -Pressure Environments*

High temperature spreads the population of the target species into a large ensemble of quantum states, and this diluted population means less absorption for any specific optical transition. At the same time, high pressure broadens optical transitions and blends the individual transitions so that an absorption measurement with a narrowband laser often includes contributions from multiple transitions. Thus, the high-temperature, highpressure measurement conditions require understanding of the temperature- and pressuredependent absorption spectrum of the target species and the potential interference from other components of the gas along the optical path. For the sensor design and data analysis, we rely on modeling of the absorption spectrum using the HITRAN/HITEMP database [24,25] augmented with prior laboratory work on the specific laser wavelengths used for detection (see more details and citations in the Introduction section and below).
