Laser spectroscopy

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The term laser spectroscopy covers various methods of spectroscopy in which lasers are used to examine atomic or molecular spectra. The methods can be classified according to the lasers they use or according to the object to be examined or the area of ​​application.

Most methods in common is that with the laser an electron of the electron shell of the to be tested atom or molecule from a lower energy level is raised to a higher level, wherein the energy difference between the two levels just the energy of the laser radiation corresponds (= photon energy). This absorbs some of the radiation. If this absorption is measured with a detector, this process is called absorption spectroscopy . The excited electron of the atom or molecule then falls back to a lower energy level, with radiation being sent out (“emitted”) in a random direction. The detection of this radiation is called emission spectroscopy .

Areas of application

Laser spectroscopic methods are used in various areas of analysis . On the one hand, laser spectroscopy is used in basic physical research, for example, as a precision tool in atomic physics to investigate the properties of atoms and their electron shells or to determine density in plasmas . On the other hand, laser spectroscopic methods are used in trace analysis to detect substances in a gaseous environment. The areas of application range from environmental analysis to process control in the semiconductor industry.

Laser types used

The type of laser used in laser spectroscopy depends on three main aspects:

  • the spectral range to be measured,
  • the line width and
  • the tunability of the laser .

The order of magnitude of the spectral range for laser spectroscopy of atoms in the ground state can typically be in the range from one to a few electron volts , which corresponds to wavelengths of approx. 1000-100 nm ( IR radiation or UV radiation ). For precision experiments are still often dye laser (Engl. Dye Laser ) application that can cover a very wide spectral range with a small line width. To get into the UV range, this type of laser is often used together with laser frequency doublers . For the area of ​​the visible part of the electromagnetic radiation spectrum (“ light spectrum ”) laser diodes are often used these days , which offer a very good price-performance ratio and are less expensive to operate and maintain than dye lasers. Lasers with lower emission frequencies are used for transitions within higher energy levels than that of the ground state.

The line width of the emitted laser radiation is decisive for the precision of the spectroscopic measurement. The smaller this is, the more precisely the atomic or molecular transition to be spectroscoped can be measured. Without further measures, a free-running laser typically has line widths of many gigahertz, depending on the structure of the resonator . In the case of dye lasers (also ring lasers ), frequency-selective elements (e.g. prisms , gratings , Lyot filters ) must therefore be introduced into the resonator in order to reduce the line width to the few megahertz required for spectroscopic precision experiments . In the case of laser diodes , these are built into external resonators in which part of the primary laser radiation is fed back into the diode through optical grids . Frequently used external resonator designs in use with laser diodes are the Littrow design and the Littmann-Metcalf design .

Almost all lasers used in spectroscopy have in common that the frequency of the light emitted by the laser can be freely selected within a certain range in order to match the laser frequency exactly to that of the atomic or molecular transition to be spectroscoped. One also speaks of the tunability of the laser. But it is also possible, with a fixed laser frequency, to adjust the system to be examined by varying an exactly known parameter, e.g. B. a magnetic field in resonance (physics) .

If molecules are the object of investigation, then in addition to the electronic excitation, there are also excitations of various vibration and rotation states. The excitation energies here can range from tenths to thousandths of an electron volt, which corresponds to the spectrum of the near to far infrared . In the mid-infrared in particular, many molecules have very characteristic absorption lines and can be identified based on the distribution of these lines in the spectrum. Special laser diodes made of lead salts or quantum cascade lasers are used to generate the laser radiation required for this .

Special procedures

In absorption spectroscopy , the object to be examined (usually a gas or plasma ) is located between the laser source and the detector. The radiation that is not absorbed is recorded with the detector. The absorption lines caused by the absorption become visible in the spectrum .

With laser-induced fluorescence , the atoms or molecules are excited by the laser and the radiation emitted during de-excitation is detected. This is usually done with a detector, the opening of which is perpendicular to the beam path of the laser. The emission lines measured are deliberately not the same as those of the absorption.

In addition, there are a number of methods in which the resonance is detected indirectly. This includes B. the resonance ionization spectroscopy , in which the atoms or molecules are ionized in the case of resonance , which then electrically z. B. can be detected with a mass spectrometer , as well as cavity ring-down spectroscopy .

Due to the high light intensities available, lasers also allow spectroscopy at multiphoton transitions, with an atom simultaneously absorbing two or more photons from the same laser beam. The return to the basic state then usually takes place without radiation or through several photons in succession, whereby absorbed and emitted photons can have different frequencies.

The most important method in this context is Doppler-free two-photon spectroscopy. Here, the laser beam is reflected back after passing through the gas, so the atoms are exposed to a back and forth beam. If the laser is tuned in such a way that the frequency of the laser light corresponds to just half the energy difference of a two-photon transition between the atoms, then an atom can absorb one photon each from the outward and return beams. If the atom is moving, one ray is red and the other blue shifted from its perspective due to the Doppler effect . The Doppler effect then compensates for the two-photon transitions, which is what makes very high-resolution spectroscopy possible.

The Doppler effect is also used in collinear laser spectroscopy . This method is particularly suitable for use on heavy ion accelerators for spectroscopy of fast ion beams , especially short-lived, radioactive isotopes of a certain chemical element . In collinear laser spectroscopy , a laser beam with a fixed frequency is superimposed in parallel on an ion beam, the kinetic energy of which is variable. By changing the ion beam energy, the Doppler shifted laser frequency in the moving system of the ions changes. For observation of the optical resonance , optical methods (. For example, detection of the fluorescence - photons with photomultipliers ), or non-optical procedures (utilization of optical pumping and selective ionization ) can be used.

List of laser spectroscopic methods

literature

Textbooks:

Publications in specialist journals:

Web links

  • Laser in the Encyclopedia of Laser Physics and Technology (Engl.)