Seeded optically driven avalanche ionization in molecular and noble gases

We report experimental and numerical results on the dual laser-pulse plasma excitation in molecular and noble gases at atmospheric pressure. Dilute plasma channels generated through filamentation of ultraintense femtosecond laser pulses in air, argon, and helium are densified through the application of multijoule nanosecond heater pulses. Plasma densification in molecular gases is always accompanied by the fragmentation of the plasma channels into discrete bubbles, while in atomic gases, under certain conditions, the densified channels remain smooth and continuous. The densification effect in atomic gases persists through considerably longer delays between the femtosecond and nanosecond pulses compared to that in molecular gases. Using rate equations we trace this difference in the temporal dynamics of densification to the different cooling mechanisms operative in atomic and molecular cases.