Angular distribution of plasma luminescence emission during filamentation in air

We experimentally measure the angular distribution of plasma luminescence emission during the filamentation of linearly polarized femtosecond laser pulses. It is found that the luminescence from N2 shares the same intensity in all directions, while that from N 2 + is more intense in the direction parallel to that of the laser polarization. The isotropic emission behavior of luminescence from N2 illustrates that the formation of excited N 2 ( C 3 Π u + ) does not result from the dissociative recombination; on the other hand, the linear increase of the strength of 337 nm signal with pressure indicates that the collision plays a negligible role, excluding the collision assisted intersystem crossing scheme. However, the intersystem crossing does not rely on the collision, making it a possible scheme. This study will be helpful to the understanding of mechanism of plasma luminescence emission during femtosecond filamentation.