A New Met Hod to Measure the Pre Ssure of Impact-induce D Vapor Clouds

Introduction: Impact vaporization process may have played an important role in the formation and evolution of the atmospheres and oceans of planets e.g.[1,2] and have had significant consequences to planets’ surface environment e.g.[3,4]. Although a mechanical aspect of this process has been investigated extensively, its chemical aspect has not been studied to a great depth. One of the reasons for this is that there has been no method to measure the pressure of an impact vapor cloud in a laboratory. Temperature, line-of-sight column density, the degree of ionization, and chemical composition can be measured by existing methods [5], but a thermodynamic state of a vapor cloud cannot be determined only from these parameters. For complete thermodynamic description of a vapor cloud, measurement of pressure is indispensable. In this study, we propose a method of pressure measurement using spectral broadening and examine its feasibility. Laser Experiments: We used a high-energy pulse laser (Nd:YAG, 1064 nm) to simulate the impact vaporization process. In order to observe hydrogen emission lines, we used a hydrous mineral, Gypsum (CaSO4·2H2O), as the target. The YAG laser was irradiated vertically on the target in a vacuum chamber filled with Ar at 40 torr. The laser beam, with pulse energy of 240 mJ and pulse duration of 15 ns, was focused with a quartz lens to a 0.7 mm-diameter spot. The impact flash from a resulting vapor cloud is observed with a highspeed spectrometer (focal length = 30 cm). The exposure is triggered by a photodiode, which is placed near the irradiation point. Spectroscopic measurements were made for a variety of exposure times. The wavelength range in this study was from 420 nm to 680 nm. We observed a significant line broadening of hydrogen emission line (Fig. 1). Spectral Analysis: Any spectral line is observed with a finite spectral width, partly due to the finite resolution of the spectrometer and partly due to its intrinsic line width. The intrinsic spectral line broadening has two components: Doppler broadening and collision broadening. Doppler broadening is due to the thermal motion of the light-emitting atoms or ions. The resulting line profile at a frequency is Gaussian.The line width of the Gaussian breadth Dwd of Doppler broadening is given by