This paper reviews the diode laser applications in the field of high resolution molecular spectroscopy (determination of transition frequencies, intensities, collisional broadening, and shift of spectral lines) as well as their use in the areas environmental protection, medicine, and high technologies. Recent achievements in methods and techniques of diode laser spectroscopy are described.

The current status of the lead salt diode lasers developed at Lebedev Physical Institute is described. Diffused lasers, LPE lattice-matched PbSnSeTe lasers, PbSSe diode lasers with controlled carrier concentration, and PbS/PbSSes/PbSnSe heterostructure lasers with a quantum-well active region are discussed. The lasers have improved performance due to their superior electrical and optical confinement.

Semiconductor lasers and photodiodes for the 1.8-2.5 micron spectral range are described which can be used in gas analysis systems. The characteristics of pulsed lasers with output optical power reaching 1 W and low-threshold single-mode lasers are presented. The photodiodes have high quantum efficiency (0.6 without antireflection coating) and high speed or response (0.5 ns at reverse bias of several volts).

The linewidth of GaInAsSb diode lasers operating at 1.8-2.4 micron has been measured using gas absorption line as a frequency discriminator. The linewidth of the lasers varied from 3 to 60 MHz, could alter at least by an order of magnitude, and decrease as the temperature decreases and emission power increases. The results show that GaInAsSb lasers are suitable for high-resolution molecular spectroscopy.

The stimulated emission frequencies of laser QW heterostructures are analyzed here as a function of the size and composition of the active region and wide bandgap emitters, the excitation level, and the spectral characteristics of low-dimensional systems with varying thickness of QW layers. The simultaneous lasing conditions at two remote frequencies are determined for varying widths of the reactive region. In a GaAs-AlGaAs system with layer thickness of 50 and 100 A, the wavelength shift can attain 170-250 A.

The use of mid-IR fiberoptic components in spectroscopy and analysis using A4B6 tunable diode lasers has been studied, and the results are reported. Spectral parameters of the fibers and complete spectroscopic systems based on the fibers and Pb-salt diode lasers are presented. The further development of the presented systems for applications in remote monitoring, multicomponent gas mixtures analysis, and evanescent wave spectroscopy of gases is addressed.

Several sources of frequency scale errors in tunable diode laser spectroscopy are discussed: optical feedback, diode laser field distribution instabilities, baseline uncertainties, diode laser radiation frequency noise, and data reduction errors. In order to achieve frequency scale precision of 10^-5 cm^-1, these sources should be taken into account.

A system for handling diode laser control operations and data processing routines for a wide range of applications is described. The data acquisition technique is summarized and the diode laser temperature controller, laser current source, mechanical drive unit, data acquisition system, and digital part of the system are examined. The power supply and noise immunity of the system are briefly considered.

The high resolution spectra of the 1 - 2 and 2 - 3 transitions between excited vibrational levels of SF₆ molecules cooled to T equals 30 K in supersonic jets have been experimentally investigated by the double resonance method. A tunable high pressure CO₂ laser was used to excite molecules and the excited transitions were probed with a tunable diode laser. A computer simulation of the experimental spectra has been made. The agreement of the experimental and calculated spectra was attained with an accuracy of 10^-3 cm^-1, while the values of anharmonicity constants for the (nu) ₃ mode of the SF₆ molecule were refined. The matrix elements of the dipole moment of the excited transitions and the wide range spectra of 1 - 2, 2 - 3, 3 - 4 transitions in SF₆ were also calculated.

An absorption spectrum of the SnH₄ molecule has been measured with Doppler limited resolution in the region of 1903 - 1960 cm^-1 by means of a tunable diode laser spectrometer. Infrared transitions of the Q- and R-branches of allowed (nu) ₃ (F₂) and 'forbidden' (nu) ₁ (A₁) bands were observed. These two states were found to be strongly mixed by the rotational-vibrational interaction, and as a consequence the (nu) ₁ band appears in absorption. Wave numbers of transitions to the (nu) ₁, (nu) ₃ energy levels with the quantum number of angular momentum J equals 1 - 15 have been obtained with an accuracy of about 0.001 cm^-1. More than 230 transitions of the ¹²⁰SnH₄ isotopic species were used for simultaneous analysis of the (nu) ₁, (nu) ₃ resonance states. In this way, 21 spectroscopic parameters of the upper states have been determined using a model Hamiltonian, which explicitly takes into account the resonance interaction. The parameters obtained reproduce the experimental spectrum with r.m.s. discrepancy of 0.002 cm^-1.

The qualitative modifications of the rovibrational energy-level structure under rotational excitation are studied for the (nu) ₁, (nu) ₃ bands of ¹²⁰SnH₄. Two types of qualitative changes are shown to exist: the critical phenomena corresponding to the modification of the cluster structure and the so-called diabolic points associated with the redistribution of energy levels between different branches as the rotational quantum number increases. A simple model interpretation of energy-level redistribution is given to show that such a phenomenon is typical for tetrahedral molecules.

The results of investigation of nu1 and nu3 SO2 absorption bands using tunable diode laser spectroscopy are presented. Positions and strengths of more than 560 lines have been measured. More than 80 percent of the detected lines were assigned with help of calculations based on the Watson type of the Hamiltonian for the isolated state. The fundamental and hot bands of various sulfur dioxide isotope species were taken into account. The first derivative of the dipole momentum of the observed transition was estimated.

The absorption spectra of the NH₂ radical have been recorded with a resolution of 10^-3 cm^-1 in the 1343 - 1605 cm^-1 range. The formation of the radicals took place as a result of pulsed photolysis of NH₃ in the 190 - 220 nm region. The total number of NH₂ radicals formed was 10¹⁷ - 10¹⁸ for a 5 microsecond(s) ec duration of the photolyzing pulse and a 250 J energy supplied to the photolysis lamp. Nineteen lines of vibrational-rotational transitions with N -4 cm^-1.

Collisional broadening of absorption lines in (S-32)F6 and CF2Cl2 spectra has been investigated by using diode-laser spectroscopy for different buffer gases. New methods are proposed to obtain average information on line widths in polyatomic molecular spectra that are characterized by high spectral line densities.

A diode laser spectrometer has been used for high accuracy line profile measurements in the 1.7-2.4 micron region. Measured lineshapes have been least-squares fitted by Voigt profile with flating Gaussian component. Gaussian component pressure dependence resulting from the Dicke narrowing effect is observed. Line intensities, self-induced broadenings, and shifts of five water vapor lines near 5475/cm are also presented.

The application of high-resolution tunable diode laser spectroscopy to the trace gas analysis of polyatomic molecules is experimentally investigated. A trace detection limit of 10 exp -8 vol pct was obtained for Freon-12, allowing the concentration of this gas in the atmosphere to be measured.

A simple diode laser system has been developed for carrying out routine monitoring of atmospheric CO concentrations as low as the natural background or higher. The system's principles of operation, electronics, software, and specifications are presented.

Continuous CO monitoring in Karadag National Park in the Crimea was performed from 1988 to the present. Correlations have been observed over the system's long term operation between CO concentration, weather, biosphere, and Sun activity parameters measured simultaneously.

Characteristic diurnal variations of CO concentration obtained with high-altitude TDL- sounding in the boundary layer of urban atmosphere are discussed with respect to atmospheric conditions and variations of traffic intensity. Standard deviation of the measured concentrations appeared to be highly correlated with the concentration value. The regional background of atmospheric CO outside Moscow was estimated using this correlation as 200 ppbv. Simultaneous operation of two TDL-analyzers showed the coincidence of the concentrations within 10 - 12%.

Results on the application of tunable diode laser gas analysis to determining the trace components of human breath are presented. Schemes of the analyzers specially developed for measurement of both carbon oxides in expiration are described. A few results illuminating possible applications of TDL in high sensitive medical diagnostics have been obtained. For nonsmokers, the expired concentration of CO is slightly higher than inhaled air. Specific surplus value depends on the person's age. The surplus CO content increased significantly just after intensive physical exercises like jogging. For smokers, the farmacokinetical curve of abundant CO removal from the organism could be investigated. The smoking status of tested individuals becomes easy available. Breath-hold simultaneous measurements of CO and CO₂ have shown the difference in the dependencies of their concentrations on breath-holding time. The possibility to investigate phenomena like molecular pulmonary diffusion of the alveolar-capillary membrane and an organism's compensation reactions to oxygen shortage seems to become real. Perspective leads for development and the application of diode laser spectroscopy methods to the analysis of gaseous microimpurities in medicine are also discussed.

Vibrational super-equilibrium excitation and the simultaneous effect of rotational cooling have been measured for CO₂ molecule desorption from a titanium foil surface using diode-laser spectroscopy. CO₂ molecules were desorbed when the foil was heated by a current impulse. CO₂ molecules binding energy with the titanium surface is equal to approximately 0.22 eV. The vibrational and rotational temperatures are equal to 100 +/- 10 K and 220 +/- 20 K accordingly, for the surface temperature range of 120 K - 180 K.

The excitation of ammonia via one- and two-photon transitions by TEA CO₂-laser radiation has been studied under collisionless conditions. It has been shown that the experimental results of two-photon excitation of ¹⁴NH₃ molecule in the 2 (nu) ₂ a(1, 1) $IMP a(1, 1) transition by the 10P(24) CO₂-laser line, the frequency offset of which is approximately equals 0.02 cm^-1, can be described well by modeling the laser radiation as a broadband chaotic field. The experimental results of one-photon excitation in ¹⁴NH₃ and ¹⁵NH₃,⁴ with the offset values exceeding the CO₂- laser linewidth, can be also described well with the use of a chaotic field with a continuous spectrum. In the case of a nearly exact one-photon resonance, however, the excitation efficiency of molecules depends critically on the real mode structure of the CO₂-laser radiation.

The results of measurements of an electron density in a microwave plasma filament in dense gas (argon) are reported. The electron density has been determined on the basis of Stark broadening of lines detected in the absorption spectrum. A high-resolution spectrometer incorporating GaAlAs diode laser operating at 870 nm has been used to measure Stark broadening and shifts of the argon line. The electron density in the filament was found to increase from the initial level of 10 exp 12/cu cm to value n sub e greater than 10 exp 16/cu cm. The dependencies of the electron density on gas pressure and microwave power density are presented.

Grating properties of level population in resonant atomic Rb ((lambda) equals 7800 angstroms) and Cs ((lambda) equals 8521 angstroms) vapor both in thick vapor cell and near the dielectric- vapor boundary are investigated in reference to the problem of the creation of spectral- selective elements for semiconductor lasers with external cavities.

A new technique to measure spectral linewidths of pulsed IR lasers is described. The results for the linewidth Gamma of a TEA CO2 laser are presented. The value of Gamma = 0.013/cm +/- 0.002/cm obtained experimentally is in a good agreement with the results of model calculations.

A qualitative pattern of cooling molecules in a pulsed supersonic jet is given on the basis of literature data. A procedure of measurements is described and known relations are presented which allow the determination of rotational temperatures and concentrations of molecules in a jet using the measured intensities of infrared absorption lines. The parameters of SF₆ and NH₃ jets have been determined for various pulsed valves by means of tunable diode lasers. We used pulsed supersonic jets to study collisionless infrared excitation of SF₆ and NH₃ molecules by an intensive laser field. An important point in the treatment of these experiments was to know with sufficient accuracy the concentration of molecules in a jet and their distribution over rotational levels of the ground vibrational state (characterized by the rotational temperature T_rot in the case of Boltzman distribution). We determined these parameters from the measurements made by means of tunable diode lasers. Various pulsed valves were tried which allowed us to obtain jets with different gas temperatures and different concentrations of molecules.

The use of far-IR injection lasers in pulsed molecular high-resolution spectroscopy is addressed. Far-IR radiation generation in a magnetoplasma spectral window is reviewed. The frequency tuning of long-wavelength diode lasers under combined temperature and magnetic field influence is examined.

A setup is reported for molecular analysis of ultrapure volatile substances based on absorption two-channel computerized IR spectrometry with pulsed tunable injection lasers operating within the wavelength range from 4 to 12 microns. The setup is equipped with a heated vacuum system for gas supply and for gas mixtures preparation. Analytic characteristics are estimated in case of water detection in oxygen, argon, and monogermane; the detection limit is 8 x 10 exp -7 vol pct, 4 x 10 exp -7 vol pct, and 4 x 10 exp -7 vol pct, respectively.