The heating of matter by a source of intense x rays is studied under conditions where the energy transport in the hot material is carried by the flux of incident photons. This leads to the formation of a self-similar heating wave. Particular emphasis is given to the ablative heating wave that exists for a time interval in between supersonic heating initially and diffusive heat transport at later times. An approximate, analytic solution of the similarity equations describing the ablative heating wave is given. A power law dependence of the opacity on temperature and density is assumed, which is strictly valid for a fully ionized plasma and monochromatic incident radiation, but possibly also useful to model more complicated situations in terms of effective parameters.

The use of lasers as light sources in IR spectroscopy allows considerable improvement in measuring sensitivity since most lasers serving as light sources can emit much higher powers than conventional IR generators. This property is of particular advantage in measuring aqueous solutions because it counteracts the intrinsic attenuation of water, which is high in most IRwavelength ranges.The sensitivity of a laser absorption spectrometer in measuring aqueous solutions is a factor of about 100 as high as that of a conventional instrument. The detection limit for K2SO4, NaH2PO4 , Na2HPO4 , KC1O3 and (NH4)2Cr2O7 was between 10 and 30 mg /1 (equivalent to 10-30 ppm), while that for glucose and ethanol was about 35 mg/1 (equivalent to 35 ppm).Since, however, tunable lasers have so far only been available in very few wevelength ranges, these light sources are used at present mainly with one wavelength for measuring concentrations and tracing variations of concentration of certain molecules or ions as a function of time.Besides the higher power of the laser, its monochromaticity is a great advantage since it appreciably improves the resolution. This property is particularly useful in measurements on multi-component systems, e. g. biological tissues, tissue solutions and bacteria cultures. Combining a laser light source with an ATRprism such as is used in IR technique makes the use of a transmission cell superflous. It is sufficient to place or attach the test specimen, without an intervening space, against an ATR prism irradiated by the laser in order to obtain results with the same sensitivity as in the transmission case.Test samples with high intrinsic attenuation such as aqueous solutions or biological specimens are thus saved from exposure to harmful heating.These instruments are suitable for, for example, continuous monitoring of various ions in sewage, for process supervision and control and for in vivo recording of metabolic processes in biological tissues.Schematic layout of the ATR spectrometer

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The radiation hydrodynamics of a 125-/tm thin carbon layer illuminated by thermal radiation of 7" rad = 200 eV temperature is studied within a multigroup radiation model. Whereas a major part of the incident photons deposit their energy by A'-shell absorption close to the surface, soft photons below the K edge and hard photons (hv > 800 eV) penetrate deeper into the material and drive a heat wave with a sharp front. A nonablated mass fraction of 20% is accelerated with a hydrodynamic efficiency of 11%. About half of the incident radiation flux is reemitted by the heated carbon plasma. The heat front trajectory can be reproduced by the analytical heating wave model when effective opacity parameters corresponding to the most penetrating components are used.

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