The fast decay of ionized nitrogen molecules in laser filamentation investigated by a picosecond streak camera

A column of bright filament is generated when a strong 800 nm femtosecond laser propagates in pure nitrogen gas, from which some fluorescence emission lines are observed. These emission lines can be assigned to the transitions of N-2(+)( B-2 Sigma(+)(u) -> X-2 Sigma(+)(g)) and N-2(+) ( (CIIu)-I-3 ->(BIIg)-I-3). By using a picosecond streak camera, we investigated the fluorescence decay dynamics of N-2(+)( B-2 Sigma(+)(u)-> X-2 Sigma(+)(g)). It was found that the decay time is in the sub-nanosecond range when the gas pressure is several millibar, more than two orders magnitude shorter than its spontaneous emission lifetime. Through monitoring the population evolution of N-2(+)( B-2 Sigma(+)(u)) as a function of free electron density, we concluded that the fast decay originates from the collision between N-2(+)( B-2 Sigma(+)(u)) and free electrons in the plasma filament.