Sensing Positional Isomers Of Propyl Alcohols By Filament-induced Nonlinear Spectroscopy

Femtosecond laser technique has attracted an extensive scientific interest and been a hot research area since it emerged in 1995. When powerful femtosecond laser pulses are propagating in transparent materials, it can produce self-guided structures, which are called filaments. Rayleigh length can’t limit its propagation any longer. It is based on such filamentation in air that can be controlled to occur at a distance as far as a few kilometers. The physical origin of filament is ascribed to self-focusing resulting from optical Kerr effect and plasma defocusing resulting from strong-field ionization. Laser intensity inside filament is clamped to 5×1013W/cm2. Abundant nonlinear effects are involved in the high powerful femtosecond laser filamentation, such as the conoscopic radiation, long distance and stable propagation, supercontinuum generation by filamentation and so on. Additionally, the physical properties can find extensive and practical applications, such as laser lightning, rain and snow making, remote sensing of atmospheric pollution, nuclear reaction induced by laser, high-order harmonics generation assisted by filament and attosecond pulses etc.We report an approach for remote recognition of positional isomers of propyl alcohol vapor through nonlinear fluorescence induced by the high-intensity femtosecond laser filaments in air. By measuring characteristic fluorescence of n-propyl and isopropyl alcohol vapors produced by femtosecond filament excitation, it is found that they show identical spectra, namely those from molecular bands CH, C2 and CN, but the relative intensities are different, which can be used to differentiate the two propyl alcohol isomers. The different signal intensities were ascribed to the different ionization potential of the isomer molecules, leading to different efficiency of fluorescing fragments.