Vibrational and electronic excitation of ionized nitrogen molecules in intense laser fields

Nitrogen molecules are ionized when an intense 800-nm femtosecond laser propagates through a nitrogen gas cell. We experimentally probe the relative population in different vibrational states and electronic states of ionized nitrogen molecules by using an external seed with various wavelengths. It is found that the seed is amplified or attenuated depending on its wavelength when the seed passes through the ionized nitrogen gas. Through measuring the temporal profile, the amplified emission is attributed to the seed-triggered superradiance. Combining with theoretical calculation, our results demonstrate that both vibrational and electronic excitation occurs for ionized nitrogen molecules in 800-nm femtosecond laser fields. The population inversion is built between some quantum states of ionized nitrogen molecules, through which the seed-triggered superradiance is achieved.