Humidity measurement in air using filament-induced nitrogen monohydride fluorescence spectroscopy

Strong fluorescence emissions from nitrogen monohydride (NH) free radical at 336 nm were observed in air, when water vapor was present in a 400 nm femtosecond laser filament. The generated NH radical is ascribed to chemical reaction between nitrogen and water vapor during laser filamentation. The backscattered fluorescence from NH shows an exponential increase with increasing filament length, indicating amplified spontaneous emission. The fluorescence intensity of NH was found to be linearly dependent on the relative humidity around the filament region, and a record of NH fluorescence detection at 2 m was achieved, which opens up the possibility for remote humidity measurements.