On the Mechanism of the Lewis-Rayleigh Nitrogen Afterglow

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Abstract
The relative intensities in the nitrogen afterglow, from 5000 to 11 000 A, have been measured in pure nitrogen and after addition of several foreign gases. Most of the bands in the recently discovered Y — B 3Πg system are overlapped by B 3Πg — A 3Σu+ bands but intensity assignments have been made. Changes in relative intensities accompanying changes in external conditions show that the bands can be classified into five groups of different kinetic origins. The vibrational levels of the B 3Πg and Y states just below De, the energy of two separated ground state nitrogen atoms, are not populated. At low temperature the highest populated levels of the B 3Πg and Y states tend to a limit about 850 cm—1 below De. It is suggested that this is approximately equal to the dissociation energy of the 5Σg+ state of the nitrogen molecule. Some of the emission from the low vibrational levels of the B 3Πg state is associated with the radiation cascade Y→B 3Πg→A 3Σu+, but the rest has a different kinetic origin. The emission from B 3Πg (v=7, 6, 5) is probably the second step in the cascade 3Δu→B 3Πg→A 3Σu+. Foreign gases modify the vibrational population distribution of the upper levels of the B 3Πg state, each in its characteristic fashion. Some gases quench the emission from the lowest vibrational levels of B 3Πg more effectively than the rest of the spectrum. The quenching of the emission from the higher levels depends on the pressure of nitrogen. A mechanism accounting for most of the afterglow emission is discussed.