Some Unexpected Results of Shock-Heating Xenon

Abstract

Results from a detailed experimental study of the structure of pressure-driven incident shock waves in very pure xenon contained in a thoroughly pumped Pyrex shock tube are presented and discussed. As a result of some optical studies of the luminous structure of the shocks, the following features of the luminosity delay time were discovered: independence of the pressure in the undisturbed xenon in the range 0.75 to 4.0 mm Hg, dependence on shock velocity in a way not explainable on the basis of reasonable volume processes alone, and dependence on shock tube diameter. The visible luminosity from the shock was found to terminate well in advance of the measured position of the xenon-driver interface. This is indicative of severe radiation cooling. The visible luminosity was also found to be profoundly altered by the addition of impurities either in the xenon itself or in the driver. The spectrum of the delayed luminosity in the region from 3000 to 10 000 A was studied with an electronic-recording time-resolving spectrometer and found to consist of xenon atom lines superimposed on a strong continuum. The continuum may reasonably be attributed to dissociative transitions from bound excited states of the Xe2 molecule related to the xenon atom levels 7 pKJ and above to unbound Xe2 states related to the atomic levels 6s11 and 6s12. Positive electrical signals, observed during the passage of the shock through external metal rings, are attributed to ejection of electrons from the shock tube walls by photoelectric action and/or metastable atoms. In addition, two different types of electrical precursors were observed. The first was observed under the usual conditions, namely that the shock was sufficiently strong to cause the delayed luminosity. The second was observed in some experiments in which the shock was too weak to cause the delayed luminosity. Both could be due to a photoelectric effect on the shock tube walls, but the precursor observed in the absence of the delayed luminosity may be also due to diffusion of electrons ahead of the shock front. The present experiments strongly indicate that shock tube experiments of others may need reinterpretation.