I. Experimental measurements of the population temperaturebehind the reflected shock in a shock tube are presented. Emissionfrom two wavelength intervals of the OH ^2Ʃ → ^2π electronic bandsystem was measured photoelectrically, the signals observed beinggenerated by a narrow core of hot gas in the reflected shock regionlooking axially up the tube. The ratio of the rate of increase ofintensity with unit increase of optical depth in the two spectral regionsis a unique function of the temperature for a transparent gas. Thelinearity of the signal increase with time represents an experimentalverification of the transparency and equilibration of the test gas.
In the temperature range of 3300-4300° K(M_s ~ 4), themeasured spectroscopic temperature was in good agreement with thecalculated equilibrium temperature, the estimated accuracy of thespectroscopic temperature being ±75°K. A relaxation time of about25 µsec was observed for the (2,2) and (3,3) vibration bands to reachstatistical equilibrium with the lower (0,0) and (1,1) vibrational levelsin the ^2Ʃ state from which the emission occurred.
II. Previous shock tube measurements of the oscillator strengthof the OH ^2Ʃ → ^2π band system made in this Laboratory^(1) have beencorrected. Light scattering in the absolute intensity calibration hasbeen eliminated and a continuous flushing technique was used for preparationand introduction of the water vapor-argon test gas mixtureinto the tube. The experimental technique remains essentially thesame as in the earlier studies: hot gas samples at 3100-3500° K wereproduced behind the reflected shock and the linear rate of increase ofabsolute spectral intensity in the transparent gas region was measuredby monitoring emission from axial observations in the shock tube.
The absorption electronic f-number for the ^2Ʃ → ^2π bandsystem has been determined from the measurements as (3.9 ± 0.9) x10^(-3). This value should be compared with Carrington's ^(2) result of1.4 x 10^(-3) from absorption measurements in flames, and Oldenbergand Rieke's^(3) value of 1.3 x 10^(-3) and Dyne's ^(4) value of 0.7 x 10^(-3)from measurements in absorption cells.
