A new generation of spectroscopic tools have emerged with such extremely high resolution capabilities that we are no longer limited by the so called "instrumental linewidth." In today's optical spectroscopy, the limit is more likely to be the natural or, in certain cases, the transit-time linewidth. To perform ultrahigh resolution spectroscopy one generally requires a tunable laser with a narrow spectral width, a means of reducing any mechanism that tends to broaden the spectral line under observation (e.g., Doppler and collisional broadening) and, finally, a precise method of calibrating the tuning range of the laser. This paper presents a brief review of the various techniques used to achieve high resolution spectroscopy with lasers.

A discussion of several spectroscopic techniques using two lasers for semiconductor impurity studies is presented. The frequency of one laser is in or near the visible and the other in the far-infrared (10 to 100 microns). The second infrared laser offers several possibilities for spectroscopy in addition to the usual laser properties of high intensity, monochromaticity, and coherence. For example, an infrared laser frequency can be selected which is resonant with or near a donor, acceptor, or exciton (intrinsic or bound) binding energy in a semiconductor. The infrared laser then provides an intense and very precise probe, the effect of which may be studied in the fluorescence or absorption spectra produced by the visible laser. This might be compared to a temperature dependent fluorescence study, with the difference being that a broad thermal effect is replaced by a sharp photodissociative or optical pumping effect produced by the infrared laser. This has application to learning the role of specific impurities in recombination in devices such as an LED.

A method is described for eliminating the Doppler effect by the counterpropagating-beam two-photon technique. Emphasis is placed on gas-phase molecular electronic-state spectroscopy. Past accomplishments with this method are summarized, and applications of the method are discussed.

The triplet state is an energy trap for dye lasers. It is related to the intermediate state in degradation of the dye molecule. Scavengers of the triplet state remove the energy traps but aid in the degradation of the dye since the new singlet state formed by combination of scavenger and triplet state dye molecule is a degraded dye molecule. The return from the triplet state to the singlet state by phosphorescence will not degrade the dye molecule though it is still a trap that decreases the efficiency of the lasing action. The formation of the triplet state with the splitting of the ring structure of the dye molecule (ring containing the carboxyl group) will create new side chains, that can absorb energy due to the rotation vibration modes of the side chains. Hydrogen bonding of the solvent molecules and the dye molecules can restrict this rotation. The hydrogen bonding represents a dipole bond that is an order of magnitude lower in energy than a covalent bond. The solvents used in dye lasers are hydrogen bond forming liquids: water, ethanol, ethylene glycol, glycerol. The chemical stability of the dye laser will depend on the choice of dye molecule and its ability to resist triplet state formation. This is really the chemical stability of the molecule. Those molecules that are most stable will have the least tendency to form the triplet state. The use of triplet scavengers is questionable in terms of degradation of the dye molecule while increasing efficiency of lasing action. An explanation will be given of dimer formation of the dye molecule and carbon dioxide formation as one of the degradation products of the dye.

A new system of the quantitatively analyzing spectroscopy by using a continuously scanning tunable dye laser is proposed. This system can detect simultaneously many components of pollutant gases by one scan. The maximum number of the detectable gas and the sensitivity of the detection is analyzed. The absorption spectrum is calibrated to the output of the laser and to the standard spectrum(Na) and the interpolating Fabry-Plrot interferometer. The gas concentration is calculated by the method of least squares. A foundamental experiment by using a rhodamine 6G dye laser is done. Three kinds of gases, NO2, I2 and Br2, are separately analyzed quantitatively.

Tunable diode lasers have a wide range of applications based on their unique combination of features, including high spectral purity, high spectral brightness, tunability, ease of modulation, small size and complete coverage of the important 3-30 μm spectral range. Demonstrated applications include rapid spectral measurements with a resolution better than 10-4cm-1 and sensitive detection of atmospheric pollutants over kilometer path lengths. Tunable diode lasers are forming the basis of a new class of infrared analytical instruments with applications in many areas of measurement and analysis. This paper will review the properties and uses of tunable diode lasers and describe a number of recent developments of particular significance, including CW operating temperatures as high as 100K for many spectral regions, temperature tuning ranges greater than 300 cm-1 and CW power outputs in excess of 100 milliwatts. Potential applications in a number of new areas will be discussed.

A new technique is presented for measuring the incoherent resonance decay (IRD) and the coherent optical ringing of selectively (5 MHz) prepared electronic states. The method utilizes electro-optic switching of a single laser mode that is on or off resonance with respect to the homogeneous molecular packets in the excited ensemble. The technique was applied to a variety of systems (gases at low pressures, gases at relatively high pressures, and solids at low temperatures) to give information about their phase memory (optical T2 processes), radiative and radiationless decay (optical T1 processes), and to measure directly their optical transition moment between the ground state and the prepared electronic state. The theory of coherent and incoherent states is also given.

With the realization of simultaneous temporal and spectral multiplexing in the infrared using Fourier Transform Spectrometers, it is possible to study entirely new classes of problems which were not generally accessible to infrared techniques before. Now it is possible to simultaneously obtain high spectral resolution and realize spectral signal to noise normally associated with spectrally multiplexed systems on transient samples whose lifetimes are orders of magnitude shorter than the time required to collect one interferogram. Results of time resolved spectral studies are presented with a discussion of the gains in signal to noise one realizes over rapid scan devices.

Time resolution in the range from less than 50 nanoseconds to 1 millisecond is readily achieved by use of a Gated Optical Multichannel Analyzer (OMA). The OMA has high sensitivity from the vacuum ultra violet to the near infra red. Shutter ratios in excess of 106 are obtained under some conditions. Linear operation has been shown for CW and pulsed sources. The pulsed source case was investigated for rapidly pulsed sources and for sources pulsed at intervals. Overall system gamma was unity within 2%.

A rapid-scan spectrometer employing a silicon-target vidicon detector was used to study the time-resolved emission spectra of laser-ignited metals. Bulk specimens of Ca, Mg, Zr, Ti and several Ti alloys were ignited with a 90 W cw CO2 laser in air or under a gentle flow of oxygen. Line and band emissions observed between 300 and 1100 nm during combustion help to identify vapor phase reactants and products and their locations in the flame. Disappearance of discrete spectra during the transient combustion of Ti and Zr gives information on the accumulation of molten oxide products. Observations of the continuum radiation emitted by laser-irradiated flames indicate a laser-stimulated luminescence from condensed metal oxide particles.

The LLL Laser-Fusion Program includes an active experimental effort to measure the distributions in energy, space and time of pulses of soft X-rays (.2-10 keV) radiated by laser-induced plasmas. Requirements for high resolution in these measurements clearly preclude the use of conventional X-ray diagnostic techniques. We will describe the following as examples from our program of advanced X-ray instrument development. 1. Wavelength-dispersive spectrographs with fractional-electron volt resolution at several keV and imaging systems with 3 micron resolution over a 100 micron source. 2. Advanced solid state X-ray sensors that recover data from these instruments in a more expedient manner than photographic film. 3. Microcomputer-controlled stand-alone spectrograph data systems and minicomputer-based integrated systems that can deal with the large array of data generated by the high resolution instruments. 4. An electrostatically-deflected X-ray streak camera with moderate spectral and spatial resolution and a timing resolution of better than 15 ps.

Mode-locked laser techniques for obtaining spectroscopic information about ultrafast molecular processes occurring on a picosecond time scale are described. The results of picosecond measurements on several systems are presented as examples of the method. The advantages of employing two dimensional vidicons and recently developed dispersive techniques to obtain time resolved transient spectra with a single laser pulse rather than with multiple pulse techniques are discussed.

We observed the two-photon excitation of the ground state, 1So, of Xenon to the 1D2 state at 79213 cm-1. A frequency doubled, nitrogen laser pumped dye laser producing approximately 1 kW at 39606.5 cm-1 was the exciting source. The excited state produced was observed by photomultiplier detection of the fluorescent decay to the intermediate 3P2 state in the near infrared. With Xenon at 6 torr, an absorption bandwidth of about 0.50 cm -1 was observed. Due to Doppler broadening, no isotopic shift or hyperfine structure was resolved.

Luminescence methods have been established as very promising for continuous oil-in-water monitoring because of their high sensitivity and relative immunity to particulate backgrounds. The greatest problem with luminescence has been the highly variable response of different oil types. Previous studies have shown that it is possible to roughly equalize response to many oils by weighted summing of multiple emissions resulting from multiple excitations. The use of a large number of filter pairs or mechanically scanned monochromators is undesirable both because instrument response time is too long for a continuous monitor, and because the mechanical scanning of critical optical components does not lend itself to incorporation in a rugged shipboard instrument. This paper discusses an optical summing and weighting technique which retains the advantages of luminescence methods, retains a short response time, but eliminates mechanical scanning of wavelengths. The operation of the optical breadboard is discussed in terms of the oil-in-water problem.

Direct reading intensified vidicon systems suitable for recording Raman spectra have recently become available. A chief point of interest in such systems is their speed. The full multiplex (Fellget's) advantage -- the ability to record a full spectrum with the speed of photoelectric detection -- means an increase in speed equal to the spectrum interval divided by the resolution. If 180 cm -1 are simultaneously recorded and the resolution is 3 cm -1, a 60 fold increase in speed is theoretically and practically attained. In this paper we will present details of an optical system well suited to detection of Raman spectra with a vidicon system, as well as details of the software controlled vidicon spectrum analyzer, the Television Optical Multi-Channel Analyzer or TOMCA. We will also show the applicability of the system to real time spectroscopic measurements of transient phenomena by presenting measurements of the classic iodine clock reaction.

Acoustically tunable narrow band optical filters are based on the phase-matched interaction of light and sound in certain crystals. With such filters, scanning in wavelength and adjustment of bandwidth can be done completely electronically, without mechanical devices. Such an approach can lead to reductions in instrument size and complexity, and facilitate incorporation of the spectrometer into computer-controlled systems. Some advantages over mechanical systems include rapid scanning, multiwavelength sampling, and fast, random access to spectral regions of interest. We review the basic interaction phenomena involved in the process. Parameters such as required acoustic power densities, bandwidths and angular apertures attainable, and the trade-offs among them are discussed. Acoustooptic filters based on presently available materials can now cover the visible and mid-infrared spectral regions. We describe the details of such systems, with emphasis on our own work in the mid-infrared. The potential of such filters for application in rapid scanning, random access and imaging spectroscopy is also discussed.

This is a tutorial paper on the SIMS. The SIMS is an acronym for the spectrometre interferentiel a modulation selective (selective modulation interference spectrometer), recently introduced by Marechal and Fortunato. The paper reviews the basic principles of operation, the properties, the implementation configurations, and the current investigations of the SIMS at Utah State University. The following properties make the SIMS a powerful spectroscopic instrument. It has an extremely large optical throughput. For example, the SIMS can be configured with a light gathering capability, throughput, thousands of times larger than that of a conventional slit spectrometer. The SIMS does not require a computer to recover the spectrum; it has moderate resolving power, and it has relatively rugged implementation configurations.

A heterodyne correlation radiometer is proposed for the sensitive detection of radiating species whose Doppler shift is known, but whose presence we wish to affirm. Such radiation (which may be actively induced) can arise, for example, from remote molecular emitters, impurities and pollutants, trace minerals, chemical agents, or a general multiline source. A radiating sample of the species to be detected is physically made a part of the laboratory receiver, and serves as a kind of frequency-domain template with which the remote radiation is correlated, after heterodyne detection. The system is expected to be especially useful for the detection of sources whose radiated energy is distributed over a large number of lines, with frequencies that are not necessarily known. Neither a stable nor a tunable local oscillator is required. It is shown that the minimum detectable power is expressible in a form similar to that for conventional heterodyning (for both quantum-noise-limited and Johnson-noise-limited detectors). The notable distinction is that the performance of the proposed system improves with increasing number of remotely radiating signal lines and increasing locally produced radiation power. Performance degradation due to undesired impurity radiation is considered and shown not to be a problem in general. The technique should be applicable over a broad frequency range from the microwave to the optical, with its most likely use in the infrared.

Laser based spectroscopic diagnostic tools offer the exciting possibility of spatially and temporally resolved measurements of species concentrations in complex reacting gas flows of engineering interest. In our laboratory we are beginning to examine using laser induced fluorescence to measure species concentration in such devices as aircraft gas turbine combustors. The major problem associated with such measurements is the effect of quenching reactions on the fluorescence signal. To overcome this difficulty we have proposed operating in the saturation mode. For suitable systems the fluorescence signal is then no longer a function of quenching rates or laser power. Very low detectability limits appear possible.

Infrared fluorescence has been widely used to detect kinetic rates for chemical reactions and energy transfer. With the use of special gaseous and interference filters it is possible to detect numerous isotopic species or single excited states of atoms and molecules. Detailed state-to-state kinetic studies using laser excitation and infrared detection is an important new spectroscopic tool. Electronic-to-vibrational energy transfer events involving Br(2P1/2) transfer to diatomic and polyatomic molecular vibrations has been found to be very efficient. In many cases, the electronic-to-vibrational excitation produces population inversions among vibrational levels and many new laser transitions have also been observed. Isotopically selective photochemical studies in bromine have been carried out while continuously detecting the reaction enrichment using infrared vibrational chemiluminescence. The reaction of HC1 (v=2) with Br atoms has been studied. Infrared fluorescence from HC1 (v=1 and 2) is used to determine the extent of reaction versus deactivation pathways. Infrared fluorescence has become a powerful means of studying small molecule dynamics and offers many promising spectroscopic techniques for the future.

A new technique for measuring diffusion coefficient and dynamic friction coefficient in gas sample at low pressure is described. By varying the distance between two spatially separate antiparallel laser beams in a saturable absorber, we are able to follow the dynamics of the Bennett hole and the resonant spike, caused by the non-linear absorption, in the molecular velocity distribution. Systems such as pure I2, I2 in the presence of argon buffer gas, and B02 radicals are studied. The present technique is also capable of measuring the diffusion coefficient of an excited state species. Because of insufficient information on the van der Waals coefficients the question to the nature of the interaction potential for I2 and non-polar quenchers remains to be answered. Finally, further application of technique echnique in the Infrared region is discussed.

A computer-operated optical spectroscopy laboratory and programming for the systematic evaluation of new Nd laser materials is described. The computer controls absorption spectrophotometer and grating mono-chromators used for fluorescence measurements. Fluroescence decay and other data are collected and reduced via the computer to complete the characterization of a potential laser glass.. In this spectroscopy, flexibility is required in order to handle special samples and to redirect data collection or reduction procedures at any time. Since people of different backgrounds operate the system, the amount of interaction between the computer and operator is variable from almost none ("standard measurements" run by technicians) to very high (unusual measurements run by spectroscopists). This variable interaction is provided by a single program for a given task. The use of a computer language (BASIC) and a programming style (command structured) which satisfies these requirements is discussed.