Two-color double resonant multiphoton ionization of N₂ and the LIF detection of N₂⁺ ion produced by multiphoton ionization

Highly excited states of the N₂ molecule around 104 000 cm^-1 have been measured by two-color double resonant MPI via the a¹Π_g state in a supersonic free jet. The (b′ and c′) ¹Σ_u⁺ states and the (b and c) ¹Π_u states were observed and the spectra show clearly the parity selection rule for the one-photon dipole transition. The relative intensities of the rotational branch of the ¹Π_u ← a¹Π_g spectra show considerable mixing of the ¹Π_u states with the ¹Σ_u⁺ state. Also, laser-induced fluorescence detection was applied to measure the N₂⁺ ions produced by the (2 + 2) MPI through the a¹Π_g state at room temperature. It was suggested that the produced N₂⁺ ion is rovibrationally highly excited.