No abstract

No abstract

No abstract

Vibrational spectra of liquid nitrogen shock compressed to several high pressure/high temperature states were recorded using single-pulse multiplex coherent anti-Stokes Raman scattering. Vibrational frequencies, third-order susceptibility ratios, and linewidths are presented for the fundamental and several excited-state transitions. Vibrational frequencies were found to increase monotonically up to ≈17.5 GPa single shock and ≈30 GPa double shock. Above these pressures, the vibrational frequencies were observed to decrease with further increases in pressure. The decrease in vibrational frequency occurs in a pressure regime where the shocked nitrogen is becoming optically opaque. The consequence of the decrease in vibrational frequency on the Grüneisen mode gamma and its effect on the N2 equation of state is discussed. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these high pressures and temperatures. Finally, the measured linewidths exhibit an almost linear dependence on shock temperature, and also suggest that the vibrational dephasing time has decreased to less than 1 ps at the highest pressures.

Gaseous volcanic emissions provide important insights on volcanic processes and may aid in predicting eruptive activity, but traditional in situ measurements are prohibitively dangerous at many volcanoes. Here we describe techniques for remotely measuring volcanic gas composition, at ranges up to tens of kilometers, using ground-based passive infrared spectroscopy. The principles of passive in frared spectroscopic remote sensing are discussed, and the effects of thermal contrast on the detectability of gases in various viewing geometries are quantita tively examined. This examination reveals that, for ground-based measurements, a sky background usually provides the best results. Our initial remote sensing campaign (April-May, 1996) at the White Island and Ruapehu volcanoes in New Zealand, combining passive Fourier transform infrared (FTIR) spectroscopy with traditional COSPEC and direct sampling measurements, confirmed this and demonstrated spectroscopic detection of S0 2 , H 2 0 and HC1 at ranges of up to 6 km, with good agreement between FTIR-derived and direct sampling based gas ratios. In measurement campaigns at Mexico's Popocatepetl volcano (February 1997 and February 1998), we monitored variations of S0 2 , SiF 4 , HC1, and HF, and extended the range of sky-background passive infrared techniques to over 17 km. In the highest temporal resolution study to date of short-term compositional variations associated with an explosive eruption (25-26 February 1997), we ob served a steady increase in SiF 4 /S0 2 over several days preceding the eruption, followed by a tenfold decrease in this ratio over a few hours immediately after wards, a signature suggesting an adiabatic expansion prior to and during a gasdriven explosion. We examine the ultimate range limits of ground-based passive FTIR spectroscopy and possible future extensions to imaging and airborne in struments.