ROTATIONAL STATE SELECTIVITY IN N(2)(+)X(2)SIGMA(+)(G)(UPSILON(+)=0) BY DELAYED PULSED-FIELD IONIZATION SPECTROSCOPY VIA THE A''(1)SIGMA(+)(G)(UPSILON'=0) STATE

The pulsed field ionization (PFI) spectrum of nitrogen is recorded involving overall transitions from the neutral ground state, X (1) Sigma(g)(+) (v '' = 0), to the lowest vibrational slate of the ion, N(2)(+)X(2) Sigma(g)(+) (v(+) = 0). Tn our apparatus, designed for the study of state selected ion-molecule reactions, it is demonstrated that clean populations of ions with rotational angular momentum quantum number N+ = 0-6 can be produced by the PFI of high-n Rydberg states. These Rydberg states are populated here in a (2 + 1') two-colour excitation scheme, resonant al the two-photon level with the a '' (1) Sigma(g)(+) (v' = 0) Rydberg state. More than 10(2) state selected ions are produced per laser pulse. Conclusive evidence is presented for rotational channel interactions being responsible for the enhanced relative intensities of the branches corresponding to transitions involving N+ - J' < 0 in the final step. Several interloper states contributing to this process are identified unambiguously. Detailed experiments on one threshold (N+ = 1), using pulsed extraction fields and de discrimination fields of different magnitudes, confirm that, in our experiment, PFI occurs diabatically. Evidence is presented of long lived states above the classical adiabatic threshold. A comparison of measured intensities with those calculated using the model of Buckingham, Orr and Sichel is made.