Populations Of B (2)Sigma(+)(U) And X (2)Sigma(+)(G) Electronic States Of Molecular Nitrogen Ions In Air Determined By Fluorescence Measurement

We carry out a combined experimental and numerical investigation on the population distribution between the ground state and the excited state of molecular nitrogen ions in an 800-nm laser field. Efficient population transfers between the ground state N-2 (+)(X (2)Sigma(+)(g), nu = 0) and the excited state N-2 (+)(B (2)Sigma(+)(g), nu' = 0) occur with a 400-nm time-delayed pulse, resulting in an enhanced fluorescence at 391 and 428 nm. The population distribution between the ground state N-2 (+)(X (2)Sigma(+)(g), nu = 0) and the excited state N-2 (+)(B (2)Sigma(+)(g), nu' = 0) is extracted from the fluorescence enhancement factor. The results show that the population of N-2 (+)(X (2)Sigma(+)(g), nu = 0) decreases when the laser exceeds 2.1 mJ in our experiment. Our study provides a method to check the population information of both excited and ground states experimentally in a strong-field-induced plasma of ionized nitrogen molecules.