Rovibrational Wave-Packet Dispersion during Femtosecond Laser Filamentation in Air

An impulsive, femtosecond filament-based Raman technique producing high quality Raman spectra over a broad spectral range (1554.7-4155 cm(-1)) is presented. The temperature of gas phase molecules can be measured by temporally resolving the dispersion of impulsively excited vibrational wave packets. Application to laser-induced filamentation in air reveals that the initial rovibrational temperature is 300 K for both N2 and O2. The temperature-dependent wave-packet dynamics are interpreted using an analytic anharmonic oscillator model. The wave packets reveal a 1/e dispersion time of 3.9 ps for N2 and 2.8 ps for O2. Pulse self-compression of temporal features to 8 fs within the filament is directly measured by impulsive vibrational excitation of H2.