Chirped pulse multiphoton ionization of nitrogen: Control of selective rotational excitation in N2+(B Σ2u+)

Abstract

We report on fluorescence spectra of N2+(B Σ2u+)→N2+(X Σ2g+) 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 Σ2u+-state of N2+ are primarily populated. The rotational state distribution of N2+(B Σ2u+, 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 N2+(B Σ2u+, 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 N2 a Π1-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.