The invention concerns a Raman microscope, in particular for a Fourier transform (FT) spectrometer with a lens for magnified imaging of a point-shaped region on the surface of a measuring sample, with means for irradiation of laser radiation onto the point-shaped region and with means for detecting the emitted Raman radiation.
A Raman microscope of this kind is, for example, known in the art from the conference contribution "Holographic Optical Components for Laser Spectroscopy Applications" by Harry Owen, distributed at the conference "Holographics International '92", 26th-29th Jul. 1992, London, England.
Raman scattering is a very weak process with which a photon interacts with a molecule and is inelastically scattered. The photon loses or gains energy in this process which results in a frequency shift of the scattered photon. This frequency shift corresponds to rotational, vibrational or electronic state transitions of the molecule from which the photon was scattered. For vibrational transitions the scattering probability is approximately 10.sup.-7. In contrast thereto, the known Rayleigh elastic scattering process with which no energy transfer takes place during the interaction between the photon and the molecule, is substantially stronger. The Raman spectroscopy has become a very sensitive and highly predictive method for the determination of chemical and molecular structures in fluids, solids, and on surfaces. It provides statements which are complementary to the usual infrared (IR) spectroscopy since with Raman processes, molecular transitions are possible which are forbidden in the processes of IR spectroscopy.
In recent years near-infrared (NIR) lasers, for example, the Neodym-YAG laser have also found increasing application for Raman spectroscopy, in particular in Fourier transform (FT) spectrometers. Such a FT-IR spectrometer is, for example, described in the company publication "IFS 66" of Bruker Analytische MeBtechnik GmbH. The fluorescence light in the visible region, which is extremely disruptive to classical Raman experiments, can be largely avoided by the utilization of NIR lasers for the excitation of the Raman processes.
For the investigation of very small or inhomogeneous samples with the assistance of Raman spectroscopy microscopes are utilized in Raman spectrometers with appropriate lenses for the magnified imaging of a point-shaped region on the surface of the measuring sample. Such a microscope lens, usually constructed from optical lenses, is described in the two publications mentioned above. Such a commercially available glass lens, which is optimized for the visible wavelength region and utilized for the introduction of focused laser light onto the surface of the sample as well as for the detection of the emitted Raman spectrum, has, however, the disadvantage of chromatic aberration leading to a distortion of the spectra taken. For this reason these glass lenses are only usable in a very limited optical region.
To compensate for the chromatic aberrations within a particular wavelength region, conventional glass lenses are given an anti-reflecting coating. This only works, however, in the visible wavelength region and the lens leads to imaging errors in the NIR region. Although an anti-reflecting coating along with a correction for the known glass lenses in the IR region would be in principle possible, such a lens would, however, be very difficult to produce and the effectiveness of the correction would be limited to a small section of the IR wavelength region. For this reason, IR glass lenses with anti-reflecting coatings are rarely available commercially and are extremely expensive.
It is therefore the purpose of the present invention to introduce a Raman microscope of the above mentioned kind which is constructed in a simple fashion from commercially readily available optical components and which does not exhibit chromatic aberration.