The phenomenon of "light amplification by stimulated emission of radiation" (the laser), discovered over twenty years ago, has become one of the most exciting developments in science. Providing a source of intense, coherent, narrowly monochromatic radiation, with the capability of either continuous generation or periodic flashes of extraordinarily short duration (down to pico-seconds), the laser has already had a remarkable effect on research in many diverse areas of engineering and the physical and biological sciences.
The most important factor determining the response of systems to high-intensity infrared radiation is the quantity of vibrational energy deposited in the absorbing molecules. Determination of this quantity is complicated by spatial nonuniformities in the infrared radiation fluence and by distributions over energy in the system. In this paper, we obtain expressions for the infrared fluence distribution in a focused Gaussian beam. This is combined with an empirical model for the excitation function (0) to calculate energy deposition for a range of parameters. These results are compared with available experimental data on dissociation yields for sulfur hexafluoride, and also to various approximate results. Finally, we consider the advantages and limitations of several possible experimental approaches to determining the absolute excitation level of molecules and microscopic energy distributions in the focal volume of the laser beam.
Short Communications is a section of this journal which publishes, rather quickly, short papers containing new, significant material in rapidly advancing areas of optical engineering. Optical Engineering is a good place (it may be the best place) to publish your work, because we can publish it quickly. Here is a publication schedule:
An example of good usage of nomenclature. In the November/December 1979 Optical Engineering (page SR-164) is a review of the book Radiometric Calibration: Theory and Methods by Professor Clair L. Wyatt. I was especially impressed by Wyatt's use of the nomenclature' that I have been advocating for the last six years:
We discuss the possibility of selective bond breaking induced by the multiple photon absorption of infrared (IR) radiation. The rate at which a reactant molecule's internal vibrational energy distribution relaxes to a random distribution (intramolecular vibrational relaxation [IVR] rate) plays the central role in this discussion. It is pointed out that statistical theories of unimolecular decay, like the Rice Rampsburger Kassel (RRK) and Rice Rampsburger Kassel Marcus (RRKM) theories, preclude any possibility of selectivity because of their assumption of very rapid IVR rates. A new model of IVR and unimolecular decay which does not assume very rapid IVR rates, the restricted IVR model, is presented. Within the framework of this model the conditions necessary for selectivity are discussed quantitatively. A review of the available evidence concerning IVR rates suggests that selectivity is possible with sufficiently short and intense laser pulses. In the light of current knowledge about IVR rates we see no reason to doubt that Hall and Kaldor's experiment using separately two laser pulses of widely differing frequencies to induce the simultaneous isomerization and fragmentation of cyclopropane is a genuine example of selective bond breaking.
This article is meant to serve as a brief introduction/review for non-laser chemists of the application of intense, pulsed infrared laser radiation to organic molecules. The topics discussed demonstrate that pulsed infrared laser radiation in many instances is a unique method of inducing or augmenting organic reactions and does not merely resemble conventional heating processes. Examples are presented of laser-specific molecular selectivity, laser-controlled chemical equilibria, inducement of the high energy reaction channel in bifunctional reactants, sensitized organic reactions, and reactions in solids and at gas-solid interfaces. It is obvious from the relatively few literature reports that the application of infrared laser photochemistry to organic chemical systems is in its infancy.
The infrared photolysis of polyatomic molecules is discussed. Several examples are presented, with products being detected either by chemiluminescence or by laser-induced fluorescence. The technique allows a study both of the dynamics of unimolecular decomposition and of the reaction kinetics and dynamics of the resulting free radicals. Information on the former is deduced from such features as product energy partitioning, product yield as a function of incident laser fluence, and the effect of collisions on the degree and rate of the dissociation. The latter aspect of the technique is powerfully illustrated by the reactions of C2 (X,a) radicals (produced by photolysis of several organic molecules) with such gases as 02 and NO, from which chemiluminescent reaction products have been detected.
High order infrared multiphoton excitation in the ground electronic state of collision free CrO2Cl2 has resulted in visible fluorescence. The prompt fluorescence has been studied as a function of laser fluence, pulse duration, under collisional and collision-free conditions and was shown to arise from a spontaneous one-photon radiative decay of molecular eigenstates. Intramolecular scrambling of vibronic levels corresponding to the ground state electronic manifold with a discrete level(s) belonging to the low lying excited electronic state is believed to be the origin of such eigenstates. This fluorescent channel is shown to compete with a dissociative channel forming CrO2Cl + CI; the radical further dissociates to the stable product Cr02. The threshold for fluorescence versus dissociation is discussed and a branching ratio, and consequent chemical mechanism are suggested.
Using Floquet's theorem, the elements of the time propagation matrix for a class of matter-radiation Hamiltonians are resolved into products of functions periodic over the optical cycle and aperiodic exponential functions of time. The methodology is applied to nonrotating HF, with particular emphasis upon the determination of dynamic Stark shifts and power broadening parameters.
Resonance ionization spectroscopy (RIS) which is a selective multistep photoionization process using tunable pulsed lasers has provided a new sensitive method for the study of a variety of processes in chemical physics. These include the selective detection of very small numbers of atoms and molecules, thus providing very sensitive probes of trace pollutants, chemical reactions, diffusion, and density fluctuations. Also RIS proved to be far more sensitive than the conventional absorption or fluorescence methods for the study of binary atomic collisions. The extra sensitivity allows the investigation on the far wing of optically thin samples where the high density and high temperature effects (dimer absorption, three body effects, and self broadening), are essentially eliminated. In particular the method will be crucial in studies of S-S and S-D transitions.
Results of experiments to determine the laser potential of diatomic Group IV-A fluorides SiF, GeF, and SnF are presented. This class of molecules is characterized 1/2,3/2 ground state terms. The amount of spin-orbit splitting of the two terms ranges from 162 cm-1 in SiF to 2317 cm-1 in SnF. The first excited state in these molecules is an AzI state, which in GeF and SnF is slightly metastable. Several reaction schemes are known which chemically produce excited states in SiF, GeF, and SnF. Thus, the possibility exists for laser action on the A2E X2 3/2 systems in GeF and SnF. In SiF, there is a highly metastable a4E level which lies above the A2E state. Results of efforts to produce laser action on the A4E A2W system in SiF are presented. Techniques described in this work include chemiluminescence and laser induced fluorescence.
This overview addresses the rapidly expanding use of lasers for spectroscopic studies of alkali metal vapors. Since the alkali metals (lithium, sodium, potassium, rubidium and cesium) are theoretically simple ("visible hydrogen"), readily ionized, and strongly interacting with laser light, they represent ideal systems for quantitative understanding of microscopic interconversion mechanisms between photon (e.g., solar or laser), chemical, electrical and thermal energy. The possible implications of such understanding for a wide variety of practical applications (sodium lamps, thermionic converters, magnetohydrodynamic devices, new lasers, "lithium waterfall" inertial confinement fusion reactors, etc.) are also discussed.
Recent progress in the study of chemical reactions of laser pumped electronically excited atoms is presented. A brief discussion of the design of these types of experiments is given and is based largely on the constraints imposed by presently available laser systems. A new tunable laser source developed to increase the range of experiments accessible to workers in the field is also described. Several examples of recent experiments on the reactions of laser excited atoms are given. Emphasis is given to the interpretation of the experimental results with various surface model calculations. Finally, a quick consideration of the importance of laser photochemistry to near-term industrial applications is outlined.
Irradiation of gaseous NO2 with the 488 nm line of an argon ion laser during its reaction with C2H4 over a Pt catalyst at 250 C resulted in up to a fourfold increase in the CO2 product yield. This enhancement is believed to result from the reaction of vibrationally excited NO2 with adsorbed C2H4 or a species derived from it. The observed effect disappeared after a period of time due to surface poisoning. Hydroxyl radicals have been detected leaving the surface of Pt and Rh-Pt catalysts during the reaction of H2 and 02 at 600-800 C. The OH radical was detected by its fluorescence at 340 nm induced by a dye laser, both in the gas phase and in an Ar matrix at 10 K. The activation energies for OH production from Pt and Rh-Pt surfaces have been determined.
Recent experiments indicate that laser radiation can have significant nonthermal effects on molecular dynamical processes occurring at a solid surface. These processes include unimolecular decomposition and desorption. It has also been suggested that the interaction with laser radiation involves multiphoton absorption. This is particularly interesting since the power density of the radiation is only 10 watts/cm2, which is orders of magnitude less than the power densities typically needed to induce multiphoton absorption in the gas phase. In an effort to understand the mechanisms for such processes and to further explore the novel area of heterogeneous catalysis with lasers, theoretical studies have been undertaken for several different types of processes occurring at a solid surface: 1) laser-stimulated surface phenomena (migration, recombination and desorption), 2) laser-controlled heterogeneous rate processes, and 3) atom-surface collisions in the presence of laser radiation. This last type includes diffractive scattering, energy transfer and collisional ionization of an adatom by a gas-phase projectile atom.
It is shown that a liquid crystal material such as MBBA may be encapsulated in a wedge type cell to form a liquid crystal wedge. Such a wedge can split an incident laser beam into two orthogonally polarized beams and hence can find applications where such splitting is required. It is also shown that such a wedge can also be used as a lateral shearing interferometer. Two simple methods are indicated to distinguish between well-crystallized regions and badly crystallized regions.
Small scale fluctuations in refractive index can affect visibility and image quality in ocean optics. Such fluctuations are a result of temperature and salinity microstructure. Ocean mixing proceeds by the stirring together of dissimilar water types at finer and finer scales until diffusion creates a water type intermediate to the original components. Optically, the most important scale in the mixing cascade is microstructure because it consists of the highest gradient and smallest scale structures. Two classes of mixing process have been distinguished by shadowgraph images made in conjunction with profiles of temperature, salinity, and velocity shear. One class is diffusive and depends on the vertical distribution of temperature and salinity. The other class is turbulent and depends on velocity shear.
Linear systems methodology is proposed which quantifies flare as part of the spread function or Modulation Transfer Function (MTF) of a microdensitometer with general application to any optical detection system. Because the irradiance system, the sample's scattering properties and the optical components alter the flare portion of these functions, an empirical method, such as edge-gradient analysis, should be used to determine their profiles. Applying straightforward linear systems theory, one has a method of correcting for flare which is usually considered to be a "nonlinear and unpredictable" factor. More importantly, this method is applicable for any object distribution (for a given optical system and sample scattering profile). Partial spread functions and partial MTFs are introduced which permit quantifying the degradation produced by flare, this quantity being denoted as the partial flare factor. The partial flare factor is compared to a more restrictive black-overlay flare factor. Examples are discussed for optical detection systems with and without flare.
A Rotating Aperture Wheel (RAW) technology is under development which involves replacing a grid for scatter elimination with an assembly of one fore and two aft lead aperture wheels which are rotated to maintain the alignment between the slit or aperture pattern of each. Such a design has the unique feature that its motion can be made independent of the x-ray exposure time and duration allowing for the first time the practical use of a moving slit anti-scatter technique in rapid sequence and dynamic radiographic procedures. The technology is flexible as to lend itself toward use with varying source-to-image receptor distances (SIDs) such as are involved in fluoroscopy.
A novel technique called photometric stereo is introduced. The idea of photometric stereo is to vary the direction of incident illumination between successive images, while holding the viewing direction constant. It is shown that this provides sufficient information to determine surface orientation at each image point. Since the imaging geometry is not changed, the correspondence between image points is known a priori. The technique is photometric because it uses the radiance values recorded at a single image location, in successive views, rather than the relative positions of displaced features. Photometric stereo is used in computer-based image understanding. It can be applied in two ways. First, it is a general technique for deter-mining surface orientation at each image point. Second, it is a technique for determining object points that have a particular surface orientation. These applications are illustrated using synthesized examples.
Abstract. Performance in area step-stare detection is measured primarily by the sensitivity and revisit time. The various steps of the detection process are modeled by simple equations, resulting in approximations for the performance figures of a sensor as a function of the design parameters. A second more difficult problem is to arrive at the basic sensor design parameters given the performance figures of detectable target size and required revisit time. The procedure outlined for doing this is to consider the footprint size as a parametric variable. A family of sensor designs described by simple formulas is the result. A trade-off study can then be used to arrive at the optimum footprint size and optimum design. This process must be repeated for each of the candidate wavebands with a final trade-off selection between these optimum designs.