Picosecond ionization dynamics in femtosecond filaments at high pressures

Author(s): Gao, X; Patwardhan, G; Schrauth, S; Zhu, D; Popmintchev, T; Kapteyn, HC; Murnane, MM; Romanov, DA; Levis, RJ; Gaeta, AL | Abstract: © 2017 American Physical Society. We investigate the plasma dynamics inside a femtosecond-pulse-induced filament generated in an argon gas for a wide range of pressures up to 60 bar. At higher pressures, we observe ionization immediately following a pulse, with up to a threefold increase in the electron density within 30 ps after the filamentary propagation of a femtosecond pulse. Our study suggests that this picosecond evolution can be attributed to collisional ionization including Penning and associative ionizations and electron-impact ionization of excited atoms generated during the pulse. The dominance of excited atoms over ionized atoms at the end of the pulse also indicates an intrapulse inhibition of avalanche ionization. This delayed ionization dynamics provides evidence for diagnosing atomic and molecular excitation and ionization in intense laser interaction with high-pressure gases.