Atomic line emission and high-order harmonic generation in argon driven by 4-MHz sub-μJ laser pulses

Abstract

We report on the coexistence of atomic line emission (ALE) and high-order harmonic generation (HHG) from argon with experimental conditions bridging the multiphoton and tunnel ionization regimes. Driven by sub-$\ensuremath{\mu}$J femtosecond laser pulses in tight-focusing geometry, characteristic spectra of ALE from highly excited neutral argon as well as from singly ionized argon are detected in the presence of the harmonics. The results are discussed with respect to the electronic structure of argon and the phase-matching condition of the HHG process.