Spectroscopic measurements of vapor clouds due to oblique impacts

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Abstract
Vapor clouds generated by oblique impacts were observed spectroscopically. The observations here concentrate on the earliest downrange‐moving component, among multiple components of vapor clouds generated by hypervelocity impacts of quartz projectiles into dolomite targets. The spectrum of impact vapor simultaneously exhibits blackbody radiation, molecular band emission, and atomic line emission, but their relative ratios change with time and space. The spectrum of the earliest component is dominated by the line/band emissions. The strong band/line emissions demonstrate the presence of a gas phase. The ratio of line/band emission to blackbody radiation is higher in near‐vertical impacts than shallower angle impacts. Ratios of normalized intensities of emission lines indicate that a Boltzmann distribution with an equilibrium temperature ranging from 4000 K to 6000 K approximates well the distribution of calcium atoms in energy levels in the impact vapor. Furthermore, the temperature of impact vapor appears to be controlled by the vertical component of the impact velocity. The degree of self‐absorption of a calcium emission line, which comes from vaporized dolomite target, indicates that only a very small mass of impact vapor is involved in the observed atomic radiation process. This observation, as well as the short lifetime and extremely high temperature, suggests that jetting may be the dominant source of the line emissions in the earliest stage vapor component at impact velocities less than 6 km/s. The experimental techniques and analysis methods presented in this study are applicable to other components of impact‐generated vapor and will provide new information on vaporization mechanisms in hypervelocity impacts.