Enhancement of Laser-Induced Fluorescence by Intense Terahertz Pulses in Gases

Abstract

The enhancement of laser-induced fluorescence by intense terahertz pulses was studied both theoretically and experimentally using selected gases. Semiclassical physical picture incorporating photoionization, electron heating, impact excitation, and dissociative recombination was used to explain the plasma dynamics under terahertz radiation in picosecond scale. The dependences of enhanced fluorescence on the terahertz field, laser intensity, and atomic properties were systematically investigated with neon, argon, krypton, xenon, methane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{CH}_{4}$</tex></formula> ), ethane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $\hbox{C}_{2}\hbox{H}_{6}$</tex></formula> ), propane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{C}_{3}\hbox{H}_{8}$ </tex></formula> ), and n-butane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $\hbox{C}_{4}\hbox{H}_{10}$</tex></formula> ) gases. In noble gas plasma, both narrow line emission and broad continuum emission were found to be enhanced by the terahertz field. Their enhancement ratios and time-dependent enhanced fluorescence are greatly influenced by the scattering cross section and ionization potential of atoms. In alkane gas plasma, the enhancement of the fluorescence emission from various molecular fragments was also observed.