Probing multiphoton excitation mechanisms of argon using quantum beat spectroscopy*

Multiphoton excitation of the Ar atoms contained in air can create a population of atoms which are inverted with respect to an intermediate state, leading to cavity-free lasing in air which is of interest for possible remote sensing applications. We used a 261 nm laser to pump the 3-photon resonant states of Ar and observed superfluorescence at 1327 nm in pure Ar and 1409 nm in air. In air, the increased collisional dephasing rate suppresses superfluorescence, but the presence of nitrogen molecules opens a new pathway for pumping Ar atoms, allowing the generation of a strong 1409 nm emission. We used a pump-probe technique to investigate the mechanism behind this new pathway for pumping the Ar atoms in air. We observed quantum beats in Ar at frequencies detuned from the natural beat frequency between atomic energy levels, which can be attributed to a population of electrons excited by third-harmonic radiation generated from the large nonlinear susceptibility near the single-photon resonances of the third-harmonic radiation. Measured beat frequencies closely matched with theoretical predictions of third-harmonic generation. We demonstrate that the addition of nitrogen enhances this pathway, enabling observation of 1409 nm air lasing originating from electronic states whose energies are modified by the third-harmonic radiation.