Vacuum-Ultraviolet Absorption of Shock-Heated Vibrationally Excited Nitrogen

Abstract

Absorption coefficients of shock‐heated nitrogen have been measured over a temperature range from 3500° to 10 000°K at four wavelengths: 1086, 1176, 1247, and 1334 Å. The experiments were performed behind the reflected shock about 1 cm from the end wall of a 3‐in‐diam shock tube which was equipped with lithium fluoride windows. Light from a pulsed hydrogen discharge lamp passed across the shock tube at the window station and entered the entrance slit of a 0.5‐m Seya—Namioka monochromator. The intensity at the exit slit was recorded using an oscilloscope and a photomultiplier which monitored the fluorescence emitted from a thin coating of sodium salycilate on a quartz screen which was placed immediately behind the exit slit. Absorption of the transmitted light was measured behind the reflected shock at times after vibrational equilibration but prior to significant dissociation. The temperature dependence of the absorption coefficient obtained at each wavelength indicated that the absorption was due to photoexcitation of molecules from within one or two adjacent vibrational levels of the ground electronic state X ¹Σ_g⁺ [to the b′ ¹Σ_u⁺(12.849 eV)] the b ¹π_u(12.575 eV), and probably to other states of comparable energy. Upper limits for the values of the electronic f numbers for the b′←X and b←X transitions were deduced from the experimental data using previously calculated values of the absorption coefficients per unit f number for these bands. Approximate measurements of vibrational relaxation times at 1334 Å (v″≃13) have also been obtained which agree fairly well with the lower temperature measurements of previous investigations.