Dissociative excitation of N2 by electron impact: Translational spectroscopy of long-lived high-Rydberg fragment atoms

Abstract

Nitrogen atoms in long-lived high-Rydberg states have been produced in the dissociative excitation of N2 by electron impact. Four principal features were found in the time-of-flight distributions of the dissociation fragments and in the corresponding translational (kinetic) energy distributions. Appearance potentials and excitation functions were measured for high-Rydberg atoms with known and well-defined translational energies; for the slowest high-Rydberg fragments the excitation function exhibits sharp, resonancelike structure near threshold. The core-ion model of high-Rydberg dissociation, which treats the Rydberg electron essentially as a spectator in the dissociation process, is described and used to interpret the data. Observed dissociation limits are assigned as one fragment being an N+(3P) core surrounded by a high-Rydberg electron and the other fragment being a nitrogen atom or ion. In addition, the high-Rydberg translational energy distributions are in reasonable qualitative agreement with kinetic energy distributions of N+ measured by dissociative ionization experiments. These observations provide substantial support for the core-ion model. The available data on the dissociative core-ion states of N2+ are discussed extensively. However, no unique assignments can be made for the molecular high-Rydberg states which participate in the dissociation processes.

The dissociative ionization of nitrogen

“…Threshold energies have been measured up to 6 eV retarding potential and a straight line with the proper slope of 0.5 could be drawn and extrapolated to 24.34 eV. This is in agreement with Appell [15] and Smyth [16]. No particular feature is found in the ion energy range scanned between 0 and 6 eV kinetic energy carried away by N + .…”

“…The comparison of our results with the ion energy distributions deduced from time-of-flight experiments [16,28] is uneasy at low energy (<2 eV) [23]. At higher ion energies structures are observed in the same kinetic energy range in both the present curve and in that obtained by Smyth et al [16].…”

“…At higher ion energies structures are observed in the same kinetic energy range in both the present curve and in that obtained by Smyth et al [16]. The present ion energy distribution curve shows more details than that obtained by Kieffer and Van Brunt [14].…”

“…One possible interpretation of this could be the transition to the repulsive wall of the stable D 2 ∏ g state. This interpretation has been proposed previously by Wankenne [5] for collision induced phenomena and by Smyth [16]. By direct transition from the X 1 Σ g + state of N 2 , it is not possible to excite the bound vibrational levels of the D 2 ∏ g state of N 2 + which lie outside the Franck-Condon region.…”

“…The dissociative ionization of nitrogen has been studied over a wide ion energy range by means of kinetic energy analysing techniques applied to an N + ion beam formed by low energy electron impact on N 2 [13][14][15][16][17][18][19]. The energy distributions have been investigated at fixed angle (mostly at 90°) [11,13,14] and as a function of the angle θ formed by the electron beam and the ion beam analyzing direction [13,[17][18][19].…”