Process and arrangement for laser-induced spectral analysis

Details Process and arrangement for laser-induced spectral analysis Process and arrangement for laser-induced spectral analysis

1998-11-30

2000-05-30

Font, Frank G.

Optics: measuring and testing

By dispersed light spectroscopy

With sample excitation

356316, G01J 330, G01J 3443

Patent

active

060696958

DESCRIPTION:

BRIEF SUMMARY
The invention relates to methods and arrangements for spectral analysis by means of laser emission of non-metallic or at most partially metallic materials containing halogen, in particular chlorine, a pulsed laser beam being directed at the material to generate a plasma and the light emitted by the plasma in an expansion direction in conical form being focused optically, being passed to a spectrometer and being analyzed after a predefined delay after the laser beam has been triggered, and the pulsed laser beam having an energy density between about 10.sup.8 and about 10.sup.12 W/cm.sup.2.
Such a method and an arrangement for this purpose have previously been disclosed, for example by EP-B-176 625, for the spectral analysis of steel, where the light emitted by a quasi-pointlike plasma volume is detected one-dimensionally via a photofilm, a photodiode or a photomultiplier tube and is then evaluated for analysis. This known method requires inert gas to be introduced to improve the accuracy of the analysis results. Method and arrangement are unsuitable for the spectral analysis of halogens.
In principle, spectral analysis is based on the fact that the intensity of the spectral lines is proportional to the density n.sub.i of the emitting atom of the materials in state i and the Boltzmann factor e.sup.-Ei/kT, where E.sub.i is the excitation energy and T is the temperature. In a gas or plasma of constant density and temperature this provides, in a simple manner, the option even of quantitative analysis. In the plasma generated by a pulsed laser beam the situation is considerably more complicated, since the plasma has large gradients both in density and in temperature over time as well as over space, and is therefore not homogeneous, the gradients depending not only on the generating conditions but also on the individual materials.
It is therefore an object of the present invention to improve, in the abovementioned respect, the methods and arrangements for laser-induced spectral analysis of nonmetallic and at most partially metallic materials containing halogen, in particular chlorine.
This object is achieve by means of the method of the type mentioned at the outset, by means of averaging being carried out temporally and spatially over a range of density gradients and temperature gradients of the plasma in order to obtain stable, reproducible spectral line information for analysis, by means of all of the luminous intensities emanating from at least one volume slice of the expansion cone being summed and averaged.
Since this procedure takes place in a short, temporally predefined time during which the temperature of the dilute plasma remains sufficiently high, as does, consequently, the specific light emission of chlorine, it is possible to use all of a large plasma volume, especially for the purposes of quantitative spectral analysis.
A reliable and reproducible, relatively simple, rapid and flexible detection of halogens and in particular of chlorine in compositions of materials is a special advantage of the present method and the corresponding arrangement.
The main reason that laser-induced spectral analysis of chlorine has hitherto not been successful is that the excited states of the halogen atoms are at very high energy, at about 10 eV. In accordance with the Boltzmann factor, strong emissions therefore occur only in the region of the hot plasma focus having temperatures of about 10.sup.5 .degree. K. In this region, however, the electron density of the plasma is very high, so that the line emission is completely dominated by the intense bremsstrahlung of identical wavelength of the electrons present there in high density, an analysis thus being prevented. Since the resonance transitions for halogens, moreover, are consequently in the wavelength range of typically 100 nm, they can be detected only by means of the considerably less convenient VUV spectroscopy. (VUV=vacuum ultra violet region 1<250 nm).
It is an advantage of the method that the volume slice (in the form of a conical section or angled) can be

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