8/18/2023 0 Comments Sodium emission spectrum![]() However, his research was limited to relatively low densities and pressures 10, 11. His complementary experiments into sodium and other alkali metal emissions demonstrated this linear dependence. Margenau’s early theories on the effect of collisions on atomic spectra, based on differences between the potential energies of an emitter with distance from a perturbing particle, predicted a linear dependence of frequency shift on gas density 8, 9. This particular shift does not have its origin in the Doppler effect, rather it is instead due to the perturbation of electron orbitals by collision. Yet in high pressure environments, a small red-shift occurs, which has been noted during sonoluminescencent investigations of salt solutions 5, 6, 7. At atmospheric pressures, the familiar orange colour is owing to a well-calculated doublet centred at 589.3 nm. Sodium is a very bright spectrally emissive element whose signature often appears in optical emission spectra of combustion, even for seemingly ‘pure’ materials where its presence is unexpected 3, 4. They did however remark on how surprisingly bright the sodium lines appeared. They were not able to confirm their hypothesis something they in part ascribed to obfuscation by spectral broadening and the defuse manner in which the lines manifested. The authors hypothesised that they ought to see distinct blue- and red-shifts of the sodium yellow lines in explosions, depending on whether the explosion was receding from or advancing towards the observer. Among their many pioneering experiments was a series on the spectral lines of the metals developed by exploding gases 2. ![]() George Liveing, Professor of Chemistry, and Sir James Dewar, Jacksonian Professor of the University of Cambridge, formed a decades long scientific partnership researching the nature of spectral lines culminating in the issuance of their 1915 book of collected papers 1. Using the serendipitous presence of sodium, this optical technique allows for fast measurements of both pressure and temperature from the same light source in one measurement. The red-shift exhibits the predicted PT −0.7 dependence with a constant of proportionality of (950 ± 30) GPa -1 Lower deflagration pressures, of 0.5 to 0.9 GPa, are achieved in a fallhammer test, with temperatures of circa 4000 K. ![]() Deflagration at these pressures is achieved using a split Hopkinson pressure bar apparatus, with temperatures of circa 2900 K from the greybody continuum away from spectral features. Here we show that the conditions reached during deflagration of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) permit the red-shift of the sodium D-line to be calibrated to 1.5 GPa. Collision theory predicts the spectral peak to have a red-shift dependent on pressure, P, and temperature, T. The sodium D-line is often present in optical spectra of combustion due to its high prevalence and emissivity. ![]()
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