Fig. 1. Diagram for measuring NO generated by atmospheric chemical reactions induced by femtosecond filaments

Fig. 2. Direct absorption spectra and wavelength modulation absorption spectra (WMS) of NO

Fig. 3. Change of the wavelength modulation absorption spectral intensity of NO with reaction time under different air pressure. (a) 100 Pa; (b) 1000 Pa; (c) 1×10⁴ Pa; (d) 5×10⁴ Pa

Fig. 4. (a) Transmission spectra of NO and the corresponding fitting profile; (b) NO concentration after 30 min of reaction in different air pressure

Fig. 5. Absorption spectrum of NO superimposed on the background of laser power at atmospheric pressure

Fig. 6. Change of transmission spectral intensity and number density (the insert in the bottom left corner) of NO with air pressure