Walter H. Wurster

Measured Transition Probabilities for the Schumann‐Runge System of Oxygen

Transition probabilities for the Schumann‐Runge system of O2 have been measured in absorption in the wavelength region 2650–3900 A. Oxygen was heated in a shock tube to temperatures up to 4500°K, thereby populating high vibrational and rotational levels. Absorption spectra were photographed using a high‐speed flash lamp and a large Littrow quartz spectrograph. Transition probabilities for the bands of three sequences, associated with the zero, 1st, and 2nd vibrational levels of the excited electronic state, yield an f value of 0.048±0.008 which, when corrected for wavelength dependence, is about ⅓ that determined in the vacuum ultraviolet. This corresponds to a decrease in transition probability with increasing internuclear separation. Line‐width variations with temperature and density have also been measured. The optical diameter for O2–O2 collisions was determined as 6 A and for O2–O collisions as 10 A.

Measured Transition Probability for the First‐Positive Band System of Nitrogen

The electronic transition probability for the nitrogen first‐positive system has been obtained from absolute spectral‐intensity measurements of the infrared radiation from shock‐heated nitrogen. A 12‐channel spectrometer was used, whose resolution was sufficient to separate the vibrational band sequences. The temperature dependence of the spectrum was shown to agree with that determined for nitrogen in thermodynamic equilibrium. An electronic transition moment | Re |2 equal to 0.096 a.u. and independent of internuclear separation gave the best fit to the data. The measured intensities are shown to be lower by a factor of 5 than those reported for measurements in shock‐heated air, and the corresponding f number, evaluated at 9512 cm—1 (v′=v″=0), was found to be (2.8±0.7)×10—3. A second component of radiation was detected, and shown to be attributable to either the CN red or N2+ Meinel band systems.

Nitric Oxide Bands near 1 μ in Shock‐Heated Air

The banded radiation from a strong radiating system has been observed in the near‐infrared spectrum of shock‐heated air. The dependence of the radiant intensity upon species concentration and temperature, as well as an estimate of the oscillator strength, indicate that the radiation results from transitions between excited electronic states of nitric oxide. This system is shown to be a significant contributor to the infrared emission spectrum of high‐temperature air.

Measurement of atomic nitrogen and carbon photoionization cross sections using shock tube vacuum ultraviolet spectrometry

Spectrally resolved vacuum ultraviolet radiation measurements of the continuum arising from the recombination of electrons with N+ and C+ ions have been made using a high-purity shock tube to generate the radiating source. Mixtures of neon and nitrogen or carbon monoxide were used as the test gas, with the radiation observed behind the reflected shock at temperatures of 12,500–13,000°K. Experiments were performed wherein the shocked gas ranged from optically thin to blackbody conditions, from which photoionization cross sections for both N and C were obtained between 700 A and 1100 A. In this wavelength range, the use of windows is precluded. The attendant problems and unique features of the instrumentation designed for these experiments are described. They include a multi-channel spectrometer and an explosively driven windowless plunger which couples the spectrometer to the shock tube. The measured cross-section values are compared with other available experimental values and theoretical predictions. Agreement in the results for nitrogen is taken to validate the technique. No other experimental values were found for carbon, where the measurements yield a cross section approximately twice that obtained from theoretical calculations.

Quantitative spectroscopic studies with the shock tube

Abstract The limitations and inherent precision of shock tube spectroscopy is discussed in relation to the measurement of molecular transition probabilities. The techniques used in measurements on the O 2 Schumann-Runge and the N 2 first positive band systems are presented, and the errors in the results are discussed. Finally, the i.r. spectrum of air is shown, and evidence is presented which indicates that the radiation results from transitions between excited electronic states in the NO produced in shock-heated air. The need for the positive identification of radiating species is stressed, and the examples were chosen to illustrate the various techniques used to achieve such identification, upon which the quantitative aspect of shock tube spectroscopy must be based.

Nitric oxide radiation in the near I.R. spectrum of shock-heated air

Abstract Quantitative spectroscopic measurements have been made of the infrared spectrum of shock-heated air and nitrogen between 0·9 and 1·3 μ. The measurements for air were obtained in the reflected shock region of a shock tube, covering the temperature range 6500–7200°K. The nitrogen data were obtained behind incident shock waves for temperatures between 4600–5700°K, and in the reflected shock region for temperatures from 6800–7500°K. In a previous study it was shown that air radiates much more significantly than nitrogen in this spectral range, and that the radiation could be attributed to transitions between excited electronic states of the nitric oxide molecule. The present measurements confirm these results and also show that the observed excitation energy of the radiation is inconsistent with energy levels in nitrogen. The data from both studies are reviewed, and it is concluded that the NO hypothesis is consistent with the experimental evidence.

Shock‐Tube Spectroscopy of Ablative Species

A technique has been developed which permits shock‐tube spectrometric measurements of radiation to be made from molecules normally occurring as solids prior to decomposition. Use is made of a metallic‐aerosol generator, in which a wire of suitable material is exploded in a controlled gaseous environment. The resultant aerosol serves as the test gas upon expansion into the shock tube. The particles are vaporized by shock waves, and spectra are recorded behind the reflected shock. Several advantages accrue from this technique. Most important for spectroscopic purposes is the fact that the metals are introduced in pure form. The metallic‐aerosol technique and its experimental validation are discussed.

Radiative Diagnostics in Nonequilibrium Flows

Conventional shock tube spectroscopy generally proceeds along two lines. One of these has been stressed in the previous paper, in which the shock tube is used to provide a method for processing gases to high temperature equilibrium conditions, under which the radiation from the gases can be measured, and the spectroscopic parameters of the radiating species thereby deduced. The second aspect is that of using known radiating systems as diagnostics to deduce the kinetics of various gasdynamic or molecular processes. In this paper, examples of each will be discussed, with the emphasis placed on nonequilibrium measurements. These examples are taken in part from a current research program at this laboratory, wherein the objective of the work is to obtain the data necessary to calculate the vacuum ultraviolet (VUV) radiation flux behind strong shock waves in air. To do this, the problem involves two tasks: (a) to obtain the spectral distribution and the associated transition probabilities for the radiation, and (b) to measure the excitation rates that govern the populations of the relevant electronic states.