Spectrum‐line reversal measurements of the time variation of Na excitation temperatures behind shock waves in N2 over the range 1720°—4340°K are described. From a comparison of the results with recent interferometric and infrared measurements of N2 vibrational relaxation, made also behind shock waves, it is shown that the observed Na excitation temperature is identified with the vibrational temperature of the relaxing nitrogen molecules. The present measurements thus confirm that the thermal excitation of Na by energy transfer from the N2 vibrational mode occurs with a much higher probability than by transfer from the rotational or translational modes. This result removes recent doubts concerning the interpretation of Na‐line reversal temperatures in terms of N2 vibrational temperatures in earlier shock‐tube studies. The probability of Na excitation by vibrationally excited N2 at the temperatures concerned is shown to be close to unity. The Na–N2 interaction process in various environments is briefly discussed in the light of this and other quenching measurements at low temperatures.

The measurements were made with a double‐beam spectrum‐line reversal system in which the sodium emission and absorption intensities are both recorded in the same optical path. The high‐temperature measurements were made using a calibrated xenon arc as the background source for reversal. For the low‐temperature measurements a tungsten strip lamp was employed. The system is able to measure Na‐line reversal temperatures from 1500° to 5500°K to within an accuracy of ±2% and with a time resolution of 1 μsec. The optical arrangement and xenon source characteristics are described in detail.

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