Chirped pulse multiphoton ionization of nitrogen: Control of selective rotational excitation in N2+(B 2Σu+)

We report on fluorescence spectra of N-2(+)(B (2)Sigma(+)(u))-> N-2(+)(X (2)Sigma(+)(g)) obtained from multiphoton ionization of molecular nitrogen by 804 nm femtosecond laser pulses. The analysis of the fluorescence spectra reveals that the vibrational levels v=0 and v=4 in the B (2)Sigma(+)(u)-state of N-2(+) are primarily populated. The rotational state distribution of N-2(+)(B (2)Sigma(+)(u), v=0) is determined from the rotationally resolved fluorescence spectra. It is demonstrated that the linear chirp of the 804 nm femtosecond laser pulse has a strong influence on the rotational state distribution of the vibrational ground state of the molecular cation N-2(+)(B (2)Sigma(+)(u), v=0). Possible mechanisms leading to the experimental results are discussed. The particular population of the vibrational levels as well as the linear chirp dependence of the fluorescence signal gives evidence for the importance of a resonant intermediate state. The N-2 a (1)Pi-state is likely involved in a resonant multiphoton excitation process. This permits to selectively control the rotational population of the cation that is formed via chirped pulse multiphoton ionization.