Gas temperature measurements through broadband femtosecond nitric oxide laser-induced fluorescence in low temperature plasmas

This study demonstrates the use of nitric oxide (NO) laser-induced fluorescence (LIF) to measure gas temperature using femtosecond (fs) laser pulses. The technique is established by temperature measurements in a low pressure (100 − 1700 mTorr) gas phase over the surface of a thermocouple monitored resistive wire, as well as in a low-pressure non-equilibrium plasma. A broadband fs laser was tuned to excite several rovibrational NO A-X (0, 0) transitions at the Center Wavelengths (CWLs) of 225 nm and 227 nm to collect their respective LIF signals. It is shown that the ratio of LIF signals at these two CWLs can be used for gas thermometry. LIFBASE simulations were used to identify the sensitive CWLs for temperature measurements. The experiments were conducted in two stages. Firstly, a linear relation between the ratio of LIF signals at 225 nm and 227 nm from the gas phase (2 % NO+N₂) as a function of the thermocouple measured temperature is established experimentally for the 300 − 600 K range. Secondly, it is shown that the temperature measurements obtained by the NO LIF ratio closely agree with the hot wire temperature setpoint in a low pressure non-equilibrium air Capacitively-coupled plasma. The present work shows that fs NO LIF can be reliably used to measure gas temperature to study plasma-surface interactions.