integrated intensities as a function of temperature have been measured for one NO_2 and for four N_2O_4 combination bands in the spectral region from one to five microns. The temperature was varied from 50 to 100°C for the gas-phase studies and from 25 to 100°C for the liquid-phase experiments. In the spectral region from 5 to 15 microns, integrated intensities at 25°C were measured for one NO_2 and for three N_2O_4 fundamental bands. Results from a series of absorption spectra were interpreted in accordance with the Wilson–Wells-Penner-Weber method. Saturated vapors were used in all experiments; the optical depth was varied by using a series of spacers in a specially designed infrared absorption cell capable of handling both liquid and gas. Measured intensities for N_2O_4 combination bands in the liquid and gas phases were compared and found to differ by less than 16% for three out of four combination bands studied; for the fourth band, the observed difference was about 50%. Results for all of the combination bands investigated indicate that the integrated intensities vary approximately as 1/T in the temperature range under consideration.
Absolute intensity data may be used for a spectroscopic determination of the heat of dissociation for gases in chemical equilibrium. For the reaction N_2O_4 -> 2NO_2, we have found the value of ΔH° ,using the temperature dependence of the absorption bands, to be 13.1 Kcal/(mole N_2O_4);this value is in fair agreement with the value of 13.7 Kcal/(mole N_2O_4); obtained by techniques utilizing density measurements.
In Chapter II, shock tube studies are descri.bed of a diffuse emission band, centered near the resonance lines of sodium and attributed to van der Waals molecules Na_2. A small quantity of finely ground sodium salt (e.g., NaCl, NaBr, or Na_2CO_3) was placed at the end of the low-pressure section of a shock tube containing an argon atmosphere. Spectra were recorded photographically with a 1.5 m grating spectrograph.
