The present invention relates to an apparatus for focusing electromagnetic, preferably pulsed laser, radiation, onto a sample with the aid of an optical component for focusing the radiation and a mirror for reflecting the radiation.
It is known to use laser radiation, in the microanalysis of materials, to evaporate the sample material and to produce plasma. In such procedures, it is desirable to focus the radiation to extremely small areas of the order of magnitude of a few microns, in order to make it possible to obtain not only information about the chemical composition of the sample material but also about its structure. The sample is generally accommodated in a vacuum chamber in which the devices for analyzing the evaporated sample material, e.g. a time of flight mass spectrometer among others, are disposed. If the sample is very thin, e.g. 0.1-0.5.mu., focusing of the laser beam on the side of the sample facing the mass spectrometer may be effected through the sample. If the sample is thicker and not transparent to the laser radiation the radiation must be introduced into the vacuum chamber, for example via a window, and must there be deflected onto the sample with the aid of a mirror.
The present invention relates to such an apparatus for focusing the reflected beam of electromagnetic radiation onto a sample.
A paper by J. F. Eloy and J. L. Dumas in "Methodes Physiques d'Analyse (GAMS)", July through September 1966, at pages 251 et seq., discloses the introduction of laser radiation into a vacuum chamber through a window, the provision of a lens in the vacuum chamber for focusing a beam of the laser radiation and the deflection of this focused laser beam in the direction of the sample with the aid of a planar mirror. This arrangement has the drawback that the lens must have a long focal length since it is arranged relatively far away from the sample. This makes it impossible to produce focal spots of less than 20.mu. diameter.
It is further known, as disclosed in a paper by R. A. Bingham and P. L. Salter in "International Journal of Mass Spectroscopy and Ion Physics", 21 (1976), page 133, to initially deflect the laser beam in the direction of the sample with the aid of a planar mirror and to arrange a lens system between this planar mirror and the sample. This prior art arrangement has the advantage that it can operate with lenses having short focal lengths, but it also has the drawback that relatively large optical components must be arranged in the immediate vicinity of the sample, which complicates the structure of the device as a whole. Moreover, when operating with various laser light wavelengths, the lens system must be achromatic, i.e. it must be independent of the wavelength of the laser radiation. Such an achromatic system, however, is very expensive.