Production of N +∗ from N 2 + hv: Effective EUV emission yields from laboratory and dayglow data

Abstract

Photodissociative ionization of N 2 into the unbound N + 2 H 2Σ g + state is identified as the parentage of a number of N + and N excited states that contribute to the u.v. dayglow. Yields have been obtained for the production of extreme ultraviolet emission lines of N + (and N) from a laboratory experiment using a broad-band synchrotron radiation source with fluorescence spectroscopy. The yields are termed “effective”, in that they are constant percentages of the H state cross section. These are compared with effective yields needed to reproduce NII 1085 and 916 Å lines from four dayglow observations using a model of solar energy deposition and photoelectron production and loss in the Earth's thermosphere. The 1085 Å effective yield measured in the laboratory (18%) agrees with that from the dayglow data (average of 17%) to well within experimental uncertainties. Thus, we conclude that photodissociative ionization of N 2 is the primary source of the Nil 1085 Å dayglow. However, there is an order of magnitude discrepancy among the various dayglow observations of the 1085/916 intensity ratio, only one of which is consistent with the laboratory observation of 4.4. Neither contamination by other dayglow features nor atmospheric extinction can account for the disparities. Laboratory measurements of N 2 and O 2 absorption cross sections at these wavelengths are also reported.