Laser Spectroscopy

a branch of optical spectroscopy whose methods are based on the use of laser radiation. By employing monochromatic laser radiation, quantum transitions can be induced between specific atomic and molecular energy levels. By contrast, spectroscopy based on nonlaser light sources studies spectra resulting from transitions between an enormous number of quantum states of atoms and molecules.

The first serious laser experiments in spectroscopy were carried out after sufficiently powerful lasers were developed with an output radiation of a fixed frequency in the visible region. Such lasers were used to produce Raman spectra. The advent of the frequency-tunable laser opened up fundamentally new possibilities for laser spectroscopy. Through laser spectroscopy it became possible to solve or to attempt to solve important problems that could not be handled by spectroscopy based on conventional light sources.

The high monochromaticity of the radiation from tunable lasers permits the measurement of the true shape of the spectral lines of a substance—that is, the shape undistorted by the spread function of the spectroscopic device. This development is particularly important for the spectroscopy of gases in the infrared region, where the resolution of the best industrial devices of the usual type is 0.1 cm, which is 100 times greater than the width of narrow spectral lines.

The methods of nonlinear laser spectroscopy are based on the time and spatial coherence of laser radiation. This coherence makes possible the study of the spectral-line structure usually concealed by the Doppler broadening due to the thermal motion of the gas particles.

Owing to its high monochromaticity and coherence, laser radiation causes a substantial number of particles to move from the ground state to an excited state. As a result, the sensitivity of registration of atoms and molecules is enhanced: impurities consisting of 10² atoms or 10¹⁰ molecules per cm³ of a substance can be registered. Methods are being developed for registering individual atoms and molecules.

Extremely short laser pulses are employed to investigate the rapidly occurring (~10^–6–10^–12 sec) processes of excitation, deex-citation, and transfer of excitation in a substance. Through the use of pulses of directional laser radiation, the scattering and fluorescence spectra of atoms and molecules in the atmosphere can be studied at substantial distances (-100 km), information can be obtained on the composition of the atmosphere, and environmental pollution can be monitored.

By focusing laser radiation, the composition of small amounts of a substance (with dimensions of the order of a wavelength) can be studied. This technique has been successfully applied to the analysis of local emission spectra.

The devices employed in laser spectroscopy are fundamentally different from conventional spectroscopic devices. In devices using tunable lasers there is no need to separate the radiation into a spectrum by means of dispersing elements (prisms or diffraction gratings), which are an important part of conventional spectroscopic devices. Sometimes laser spectroscopy makes use of devices in which the radiation is separated into a spectrum by means of nonlinear crystals (see Figure 3 in NONLINEAR OPTICS).