Since the development of the laser, beginning only about 12 years ago, the conduct of the entire field of spectroscopy has undergone major and far-reaching changes. The laser is far more than just an improved light source; its impact has rather been qualitative, in the same way that the development of nuclear magnetic resonance has altered the conduct of synthetic organic chemistry, or as the digital computer has made modern systems analysis possible. Some indication of this impact may be gained from the increase in the number of articles published in the Journal of Chemical Physics which make experimental use of lasers, or deal directly with laser problems; this is shown, for the period 1963-1974, in Figure 1 (following page). Since this particular journal is a basic one in its field, not particularly devoted to new developments in quantum electronics, the increase shown is a good measure of the extent to which laser techniques have permeated such classical areas as molecular spectroscopy and reaction dynamics. Following an induction period of several years, in which the technology was developed by physicists and electrical engineers and filtered down from these to chemists and spectroscopists, the usage of lasers showed a sharp increase - far more than could be accounted for simply by an increase in the overall volume of publications - until today 60 to 90 laser-related articles appear in each semi-annual volume of this journal alone.
Holography has had profound effects in many fields of optics. It has even affected spectroscopy as a means of producing ghost-free dif-fraction gratings of unusually high quality. Holography can also be a means for recording, modifying, or recognizing spectra. The techniques used in accomplishing these goals are jointly called "holographic spectro-scopy".
Optoacoustic spectroscopy refers to the measurement of optical absorption by a sample using acoustic methods to measure the degree to which the sample has been heated by the absorbed radiation. This technique has numerous applications, including monitoring trace con-taminants in the atmosphere. It is especially useful in situations requiring the quantitation of weak absorption spectra or circumstan-ces where a large dynamic range is required. This paper summarizes the results of recent experiments in which acoustic resonance has been used to enhance signal levels and improve detectivity. Scaling laws are given which allow the dynamic range and detectivity of this technique to be determined. Consideration is given to the use of both coherent and incoherent light sources, and comparisons are made to conventional techniques such as absorption spectroscopy. Finally, the application of optoacoustic spectroscopy to the measurement of collision-broadened absorption lineshapes is discussed.
Today, we have a new generation of spectroscopic tools that have 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 will review the design of low-FM jitter lasers that can be precisely tuned, line-narrowing techniques including saturation, molecular beam and two-photon methods; and possible schemes for calibrating precisely the visible and infrared regions of the electromagnetic spectrum. Recent data in ultrahigh resolution spectroscopy will be presented.
One of the more chalenging problems in spectroscopy is the detailed determination of the composition of the upper atmosphere. A traditionally fruitful technique has been solar spectroscopy in the visible and infrared regions of the spectrum (Ref. 1), where the attenuation spectrum of direct sun light is analysed. More indirect solar spectroscopy has been used to monitor ozone concentrations via the Umkehr effect. Some years ago a survey was made of the airglow emission spectrum in the infrared using balloon borne fourier transform spectrometers (Rars. 2,3,4). This means of observation is highly complementary to solar spectroscopy in that it provides good sensitivity for detection of small concentrations of excited molecules such as OH. In optimizing the absorption path through the atmosphere by means of balloon borne solar spectroscopy and very low sun positions, it has been possible to observe very small traces of HNO3 (Rai% 5). Since attention was drawn to the possible role of NO as a catalytic agent in the destruction of ozone by Crutzen (Ref. 6), this gas as well as NO2 has been observed in long path solar spectra (Refs. 7, 8) and NO by chemiluminescent analysis of air samples (Ref. 9). In attempting to understand the different processes of chemistry, photochemistry and circulation taking place in the upper atmosphere it is necessary to make more than a few measurements to identify suggested molecules. We have initiated a program of measurements that is intended to provide a general survey of minor constituents in the upper atmosphere that may be ob-servable by means of improved ground based spectroscopic techniques. With the ground based technique it becomes possible to consider relatively economical means of monitoring these constituents both temporally and geographically.
The fields of laser physics and chemical kinetics have benefited greatly from their mutual interaction. Laser methods have extended the scope of chemical kinetics in both the time and energy domain and have provided measurements of rate constants which were unobtainable before the advent of the laser. Conversely, developments in the field of chemical kinetics have been instrumental in predicting and explaining the operation of lasers.
The basic goal of experimental spectroscopy is the precise determination of the strength and energy associated with the transitions of various atoms and molecules. It is also desirable to measure these parameters under a multitude of conditions such as varying pressures, temperatures and admixtures. If appropriate data of sufficient precision can be obtained various models of molecular and atomic structure can be refined and more fully verified. Further, the interaction and energy exchange process between species can be more fully understood.
In this paper we review the design and operation of tunable lasers which have outputs in the visible and ultraviolet portions of the spectrum, and which employ organic dyes as amplifying media. The em-phasis will be on continuously operating lasers, and in considering the generation of UV radiation we will restrict our attention to non-linear sum-frequency generation using a tunable visible laser as the fundamental light source.
There has been a rapid development of tunable infrared lasers in the past four years, spurred in part by their need in spectroscopic applications. The purpose of this talk is to review the present state of continuously tunable infrared lasers which operate in the 1 to 20 4,m wavelength region, the so-called "fingerprint" region of the infrared. Since it is not possible to discuss each of the ten types of lasers developed to date due to limitation of time, we will only identify them, discuss their general characteristics, and illustrate their capabilities with a few examples of spectral data. Some of the experimental techniques unique to tunable laser spectroscopy will also be described.
Recently single-frequency dye lasers have been used in several new spectroscopic applications such as isotope separation and two photon absorption in which the doppler effect is eliminated by the use of two oppositely directed beams. (Ref. 1,2) In these applications it is necessary to scan the single longitudinal mode of the dye laser in a continuous frequency sweep over several GHz. Furthermore, it is necessary that any frequency can be tuned to with a minimum of inconvenience.
The dye laser has become the dominant tool in the field of tunable laser spectroscopy. Its wide continuous tuning range, from the ultraviolet to the infrared, offers a potential unequaled by any other source. This very tunability, however, makes narrow-band operation difficult since one optical element (grating, prism, wedge filter or etalon) is required to tune within the operating range of any dye (typically 20-50 nanometers). With this single element, band-widths down to 0.01 nm have been achieved but more typical values are .05-2 nm. Further narrowing requires an additional element, usually an etalon, requiring that two wave-length selecting elements be tuned synchronously to scan the laser. Furthermore, longitudinal cavity modes become important in some lasers so that cavity length also must be varied.
In order to perform ultrahigh-resolution spectroscopy, a tunable laser with a narrow spectral width is required.In this paper we describe a single-frequency jet stream cw dye laser that is carefully designed to min-imize high-frequency laser jitter. The re-sidual low-frequency jitter is reduced to 200 kHz rms by locking the laser frequency to an external reference cavity. The stability of the laser has been verified by observing extremely narrow resonances (700 kHz FWHM) in an iodine molecular beam. The laser frequency can be linearly and smoothly tuned by varying the length of the external cavity. In certain precision spectroscopic applications long-term laser stabilization is needed for the generation of secondary wavelength standards throughout the visible region. By locking the dye laser to a hyperfine transition in a molecular beam of I2 we have demonstrated a long-term stabilization of 6 parts in 1013.
The Environmental Protection Agency by virtue of the Federal Water Pollution Control Act Amendment of 1972 and the Clean Air Act of 1970 is mandated to: restore and maintain the chemical, physical and biological integrity of the Nation's waters; and reduce air pollution to levels considered safe for man and the environment. Environmental monitoring is of paramount importance and absolutely necessary to achieve the above objectives. Because of the various types and extent of the monitoring required the task is formidable and challenging, requiring innovative techniques and instrumentation. The extent to which monitoring is required includes the entire country in the three media-air, water and land. The magni-tude of the problem is realized considering that: (1) over 32,000 major air pollution point sources must be brought into compliance with final emission limitations as prescribed in approved State implementation plans, (2) regulation of 40,000 industrial water users and 13,000 municipal sewage treatment plants is necessary to control pollutant discharge into the Nation's waterways and (3) the Agency has a mandate to monitor land quality and assess the use of land as they impact air and water pollution. Due to the above requirements for monitoring, the Agency has a continuing monitoring methods development program. The program supports the development of new techniques for monitoring the environment. The necessity to develop new techniques is predicated upon the Agency's mandate to monitor large geographical areas in air, water and land. One class of monitoring which holds great promise for cost effectively meeting this challenge is remote monitoring. The term remote moniterin9 is defined as sensing qualitatively and/or quantitatively a specific chemical, biological and/or physical parameter of the environment where the monitoring instrument and the parameter under investigation are separated by some distance. This paper presents an overview of remote monitoring instrumentation, and includes a discussion of the role remote monitoring may play in the Agency's overall monitoring program.
Recent developments in laser technology, electronics and data-processing have greatly. enhanced, the potential application of lasers to the remote sensing, of atmospheric, pollutants and contaminants. Although reiote sensing of contaminants goes back some 20 years by infrared, techniques, at least in conceptual feasibility studies, recent advances in laser technology have opened, new vistas. The older technology required coop erative reflectors or remote active radiation sources, which limited their applicability.
The quest for increased sensitivity and specificity is common to all remote sensing techniques. At the present time there is no single method which can meet all of the operational requirements for even some of the many tasks amenable to remote detection and analysis. Individual techniques can be optimized for handling special problems and this is the basic reason why remote sensing can be utilized with the present state of the technology. Today's paper discusses a particular problem - the remote analysis of simple gases.
Studies made of the temporal behaviour of laser induced fluorescence as a function of emission wavelength for a variety of materials such as: crude oils, refined petroleum products, fish oils, and rock and mineral samples,lead us to believe that this information represents a new kind of spectral signature. The specificity of this "fluorescence decay spectrum" appears to be somewhat superior to that associated with the normal fluorescence spectrum. Several examples are presented to illustrate the improved identi-fication capability of this new approach. We believe that a significant improvement to the ground truth evaluation capability of the new form of environmental probe currently under development, called a Laser fluorosensor, might result from this advance.
A new atmospheric NO2 monitor with a detectability of O. 6 ppbv is described. A compact He-Cd 10 mIN 442nm laser excites the NO2 molecules and the resultant fluorescence was monitored by photon counting over an 80 sec integration time. The high sensitivity was achieved by development of an optimal NO2 fluorescence bandpass liquid solution filter which did not fluoresce upon absorption of scattered laser light.
The purpose of this paper is to report experimental laser induced Raman and fluorescence measurements of combustion exhausts. The work was motivated by a desire to develop an optical means of performing gas analysis of combustion products of aircraft turbine engines in the field which does not require the placement of a physical probe in the exhaust volume and which could be fully automated.
The composition of mixed gas flames has been studied by many in-vestigators using a variety of spectroscopic methods. Spectra of molecular species in a flame are usually determined by infrared emission spectroscopy. In the study of the gaseous combustion products directly above the flame, however, this otherwise prom-ising method has two drawbacks. Homo-nuclear diatomic molecules, whose dipole moments do not change with vibration, do not generate infrared spectra. In addition, the region above the flame often has too low a temperature to allow the use of any but the most sensitive (least energy limited) infrared instruments (e.g., Fourier transform spectrometers).
In the years immediately following its discovery in 1928, the Raman effect gained wide use among chemists and physicists in the solution of structural problems that could not be handled then by other available techniques. The simplicity of photographic spectroscopy compared with the more difficult procedures of infrared spectroscopy in that period contributed to this state of affairs. Consequently, when automatically recording infrared instruments became available in the late 1940's, the situation was reversed and the Raman effect was relatively little used. Today, however, technical advances in Raman instrumentation, particularly the advent of the laser, have elevated Raman spectroscopy once more to a par with infrared methods.
During the past year, much speculation has centered on the possibility of separating isotopes by the use of lasers. The technique seems to promise very efficient separation processes that could revolutionize the economic aspects of the preparation and the use of isotopes. 1 The most important application of such a method would be in the enrichment of uranium for use in nuclear power reac-tors. 2 There are, however, other applications for pure isotopes if an enrichment technique can be found that can produce large quantities cheaply. We shall describe the various laser separation processes, review the existing work, and attempt an assessment of its economic possibilities.
In this paper we report precision studies of I2127 hyperfine structure using laser molecular-beam techniques. A single-frequency 5145 Å argon laser tunable over 0.1 Å was used to excite a molecular beam of 12 at right angles. The induced fluorescence was collected by a lens system and focused onto a photomultiplier. The I2 transitions excited were the hyperfine compo-nents of the P(13) and R(15), 43-0 lines between the z, and 31-1 electronic states. go u The hyperfine frequency spacings were precisely determined by counting the beat frequency between two lasers that were long-term stabilized to various hyperfine lines. The standard deviation of the measurements was approximately 25 kHz. The measured spectrum was fitted to obtain a quadrupole coupling strength difference AeQq= 1894.48 . 36 MHz and a spin-rotation interaction strength difference AC, = 190.7 ± 1.5 kHz between the upper and lower levels of the P(13) transition. For the R(15) transition, we obtained AeQq = 1894. 66 ± .40 MHz and ACI = 187.1 ± 2. 0 kHz. High-resolution line - shape studies were performed using a long-term stabilized laser and an acousto-optic tuning method. The measured width of the individual I2 transi-tions was 140 kHz (FWHM). By taking into account broadening from the geometric Doppler width, laser jitter, and other small effects, an estimate of 70 kHz (FWHM) for the natural linewidth was obtained which was in excellent agreement with lifetime mea-surements. Attempts to fit the observed line shapes showed clear Lorentzian features. Taking account of the natural linewidth, the instrumental resolution was ≈70 kHz or one part in 1010.
The feasibility of remotely measuring acidity of aqueous solution is demonstrated. Profile changes in the Raman band for water, which occurs over the Raman shift range between 2800 cm-1 and 3800 cm-1 are attributed to modifications of the hydrogen-bonding by acid protons. It is shown that these profile changes are directly related to acid concentrations. Measurements have been made for aqueous solutions of HC1, HBr, and H2SO4. In addition, Raman spectra of an acid aerosol are shown and estimates of the detectivity of acidity by Remote Raman Spectroscopy made.