Threshold photoelectron spectroscopy of vibrationally excited nitrogen

Threshold photoelectron spectroscopy (TPES) has been used to study flowing nitrogen subjected to a microwave discharge. The first three photoelectron (PE) bands of nitrogen corresponding to the ionizations N-2(+) (X-2 Sigma(+)(g)) v(+) <- N-2 (X-1 Sigma(+)(g)) v '', N-2(+) (A(2)Pi(u)) v(+) <- N-2 (X-1 Sigma(+)(g)) v '' and N-2 (+) (B-2 Sigma(+)(u))v(+) <- N-2 (X-1 Sigma(+)(g)) v '' were investigated. An analysis of the vibrationally resolved threshold photoelectron (TPE) spectra shows evidence of population of the vibrational levels v '' = 0-5 in the N-2 X-1 Sigma(+)(g) neutral state. By a comparison with the PE spectrum recorded under the same conditions, use of computed Franck-Condon factors for each ionization and evidence from vacuum ultraviolet absorption spectroscopy, the relative intensities of vibrational components in a TPE band can be qualitatively explained using the Franck-Condon factors for each ionization as well as the gain in intensity from autoionization from Rydberg states that are degenerate with an ionization threshold or lie just above a threshold. The enhancement in intensity obtained in the TPE spectra, relative to the intensity in a PE spectrum recorded under the same conditions, was estimated as at least one order of magnitude. The first band of atomic nitrogen was also observed in the discharge-on TPE spectra. The experimental resolution was sufficiently good to allow the three ionizations N+(P-3(0,1,2)) <- N(S-4(3/2)) to be resolved and their relative component intensities were measured as 1: 0.95 +/- 0.10: 0.70 +/- 0.10. The complementary nature of the TPES and PES techniques has been outlined and the extra information obtained from studying a vibrationally excited small molecule such as N-2 with these methods has been demonstrated.