RESEARCH
Chemical Laser Efforts Broaden Scope of Laser Operational chemical laser developed at Berkeley is based upon excitation by a chemical reaction The successful operation of a laser pumped by the energy of a chemical reaction tops a series of chemical laser developments occurring in the past three months. Jerome V. V. Kasper and Dr. George C. Pimentel of the University of California, Berkeley, say they have obtained laser emission in the infrared energized by the reac tion: H + CL -» HC1* + Cl. Chemical lasers are important to chemists as they provide a new tech nique for determining energy distri bution and energy transfer in chemical reactions. The wave length, intensity, and lifetime of the laser emission also give valuable information about the nature of the excited species. Among other comparatively recent developments in chemical lasers are those in which the exciting or pump ing energy comes from outside sources, such as light or electric discharges. For instance: • Mr. Kasper and Dr. Pimentel have successfully operated what they call a photodissociation laser in which photons initiate a chemical reaction that produces excited iodine atoms.
In going back to the ground or unexcited state, the iodine produces laser emission in the very near IR. • Dr. W. J. Witteman and Dr. R. Bleekrode at Philips Research Labora tories (Eindhoven, the Netherlands) have obtained both pulsed and con tinuous laser operation in the far IR with an electric discharge which breaks down water vapor into hydrogen atoms and rotationally excited OH radicals. • D r . C. Κ. Ν. Patel and his co workers at Bell Telephone Laboratories have achieved laser action in the IR by passing an electric discharge through carbon dioxide and carbon monoxide at very low pressure. Mr. Kasper and Dr. Pimentel have observed stimulated emission and laser oscillation in a few vibrational-rotational (P-branch) transitions of hy drogen chloride. The hydrogen chlo ride forms in reactions initiated by flash photodissociation of chlorine in a molecular chlorine-hydrogen mixture. The California chemists' finding con firms the 1961 prediction made by Dr. J. C. Polyanyi of the University of Toronto that the vibrationally excited
hydrogen chloride formed in this re action would show stimulated emission. The Berkeley pair believes that their hydrogen chloride laser is the first operating laser based upon excitation by a chemical reaction. In their experiments, Mr. Kasper and Dr. Pimentel fill a laser tube with a mixture containing one volume of chlorine and two volumes of hydro gen. The mixture is then exposed to the flash from a xenon-filled quartz flash tube. The resulting laser emis sion is centered near 3.8 microns in the IR, they find. Study on this system is still in progress at Berkeley. Aside from its interest as possibly the first laser ex cited by a chemical reaction, the sys tem points to a new way to determine energy distribution in this as well as other chemical reactions, Dr. Pimentel says. Recently, Mr. Kasper and Dr. Pi mentel have observed laser action dur ing flash photolysis of gaseous CF 3 I and gaseous CH 3 I [Appl. Phys. Let ters, 5, ( 1964 ) ]. The stimulated emis sion is due to the 2¥i; -> 2 P 3 / transi1
/2
V2
tion of atomic iodine at 1.315 microns.
Chemical Lasers May Take Several Different Forms A chemical laser converts the free energy change of a chemical reaction into specific excitation of some product species leading to critical population inversion and laser action. This is in contrast to the well-known successfully op erating lasers of today such as the ruby laser and the heliumneon laser, whose energy comes from outside sources such as light and electric discharges. In this definition of a chemi cal laser, the energy required for pumping is produced by the chemical reaction itself. An extension of this definition would include an external energy source (photons, gamma rays, electrons, and the like) as the initiator of the chemical reaction. This doesn't violate the commonly accepted use of the expression chemi cal reaction—photolytic and radiolytic processes are con sidered chemical processes. Within this extended definition, the essential factor is the making and breaking of chemical
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bonds and not the origin of the energy required for these processes. In the organization of the Conference on Chemical Lasers (which met last September at the University of California, San Diego) the extended version of the definition of a chemi cal laser was used. In other words, the discussions included the use of outside energy sources in " p u m p i n g " the chemical reaction. The making or breaking of chemical bonds was used as the criterion. It was in 1961 that scientists first thought about the possi bilities of producing lasers by directly using the energy of chemical reactions. That year, Dr. J. C. Polanyi of the University of Toronto suggested that several reactions known to produce vibrational excitation in product molecules might be effective for infrared laser action. Subsequently, Dr. Martin Hertzberg of Republic Aviation Corp. discussed the ex-
Research
This is the first laser in which popula tion inversion is achieved by photodissociation, the California pair says. The photodissociation seems to pro duce enough of an excess of excited iodine atoms over the number of ground state atoms to permit laser oscillation. Past workers have shown that photolysis of CH 3 I gives mainly 2 P 1/2 iodine atoms. No such study has been made for CF 3 I, the Berke ley pair notes. Known rate con stants show that the products, C F 3 and CH 3 , are consumed rapidly, producing C 2 F e and C 2 H e . By contrast, iodine atoms have a recombination half-time greater than 100 microseconds. The rapid losses of C F 3 and CH 3 and the slow combination of iodine atoms are consistent with iodine atom emission over a 50-microsecond interval, Mr. Kasper and Dr. Pimentel point out. The California chemists have found extremely high stimulated emission gain in their photodissociation laser, especially with CF 3 I. Water Vapor. Dr. Witteman and Dr. Bleekrode of Holland's Philips Research Laboratories have obtained both pulsed and continuous laser op
erations in the far IR using a laser filled with water vapor at about 0.4 torr. Using a laser 1 meter long and having an internal diameter of 3.2 cm., they obtained a pulsed laser reaction. Operation was with a peak voltage of about 4 kilovolts at about 40 milliamperes. The Philips scientists find that con tinuous laser operation in the far IR can be obtained with a similar system with a laser tube 4 meters long. Dis charge voltage is 7 kilovolts at about 100 milliamperes. The laser action disappears suddenly after a few min utes: This disappearance is probably related to the dissociative recombina tion of water into hydrogen and oxy gen, the Philips pair says. Using a Golay cell equipped with a quartz window, three lines were de tected in the IR; two occurred at 118 ± 1 microns and were difficult to separate, the third line had a wave length of 78 ± 1 microns. An addi tional strong line was found at 28.1 ± 1 microns by using a Golay cell with a KBr window and a Michelson interferometer. Dr. Witteman and Dr. Bleekrode attribute some of the observed wave lengths to transitions between rota tional levels of the excited OH radi cal. By spectroscopically analyzing the UV emission of the water vapor under laser conditions, they show the presence of excited OH radicals. Carbon Dioxide. Dr. Patel of Bell labs (Murray Hill, N.J.) has ob tained continuous wave laser action on a number of rotational transitions of a vibrational band of carbon di
citation of selective levels of atoms in luminescent exo thermic chemical reactions. He applied his general treat ment to two specific cases: alkali metal-halogen and alkali metal-group II metal halide reactions. Dr. A. N. Oraevskii of the U.S.S.R. showed that chemical reactions can result in population inversion (excess population of electronically, vibrationally, or rotationally excited products), using the alkali-halide reactions as an example. His estimates show that in such a system, maser action is more promising if there are chain reactions, and that metastable states favor inversions. Dr. R. A. Young of Stanford Research Institute discussed atomic association processes leading to molecules in excited electronic states with possible critical inversion density with respect to the vibrational-rotational states of the ground electronic state. A number of scientists have recently gathered a wealth of
oxide. Strongest laser transition oc curs at 10.6324 microns. An electric discharge supplies the original energy for the excitation. The carbon di oxide gas is at about 0.2 torr. From time-dependence studies, Dr. Patel concludes that the dominant process leading to excitation of carbon dioxide molecules may be recombina tion or cascade: CO + Ο -> C 0 2 * + h v (Recombination) C O / * -> C O / + hv (Cascade) CO and Ο are dissociation products of C 0 2 , and C O / * and C O / refer to vibrationally excited states of carbon monoxide. More recently, Dr. Patel has ob tained laser action with mixtures of nitrogen and carbon dioxide and nitro gen and nitrous oxide. In both cases, laser action is due to vibrational energy transfer from nitrogen. In the region where laser action is produced, there is no electric discharge, he finds. Impetus. Major impetus to re search on chemical lasers was given by a conference organized by Dr. W. R. Bennett, Jr. (Yale University), Dr. K. A. Brueckner (University of Cali fornia, San Diego), and Dr. Κ. Ε. Shuler ( National Bureau of Standards, Washington, D.C.) which was held at the University of California, San Diego, last September. The papers presented at this conference have been compiled into a special Applied Optics Supplement on Chemical Lasers (C&EN, Jan. 25, page 23) edited by Dr. Shuler and Dr. Bennett.
information vital to anyone doing research on chemical lasers. Dr. Kurt E. Shuler and Dr. Tucker Carrington of the National Bureau of Standards, Washington, D.C, and Dr. John C. Light of the University of Chicago have surveyed nonequilibrium chemical excitation and chemical pumping of lasers. Dr. H. P. Broida (University of California, Santa Barbara) has gathered information on inverted population distributions produced by chemical reactions^ Dr. Polanyi has reviewed vibrational-rotational population inversion. Dr. D. R. Herschbach of Harvard University has surveyed molecu lar beam studies of internal excitation of reaction products. Dr. A. B. Callear of the University of Cambridge (England) has reviewed energy transfer in molecular collisions. These re view papers are included in the 20-paper Applied Optics Sup plement on Chemical Lasers published by the Optical Society of America (Washington, D.C).
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As this supplement emphasizes, a number of scientists are studying chemical reactions which may produce population inversion (excess popula tions of electronically, vibrationally, and rotationally excited atoms or mole cules) which may, in turn, give rise to laser action. Most examples of popu lation inversions produced by chemical reactions have been found in atomic flame reactions at low pressures. In these cases, the diatomic molecules usually must be first dissociated in an electrical discharge to obtain the re active atoms that enter into the flame reactions. One example of population inver sion has been found in an acetyleneoxygen flame, according to experiments by Dr. Bleekrode and Dr. W. C. Nieuwpoort, also of Philips Research Laboratories. Part of the radiation from the reaction zone of this flame is nonthermal—it's chemiluminescent in origin. In studying low-pressure diffu sion or premixed oxyacetylene flames burning at 1 to 15 torr, these Philips workers paid special attention to the determination of populations of species such as C 2 and CH. Dr. Bleekrode and Dr. Nieuwpoort have come up with a simple model of a flame which reflects the characteristic requirements to start laser action. By choosing the proper experimental parameters in the model, they have shown that it should be possible to attain laser action. Excited CN. The use of electroni cally excited CN (produced by reac tion of atomic nitrogen with CC14 in a flame) to initiate laser action has been suggested by Dr. T. T. Kikuchi of General Motors Defense Research Laboratories (Santa Barbara, Calif.) and Dr. H. P. Broida of the Univer sity of California, Santa Barbara. So far, however, their calculations indicate that the electronically excited CN produced would show only limited possibility of laser action. Such re actions are not a likely source of laser radiation for room temperature opera tion because there are too many energy levels into which reaction products are formed. By contrast, if the flame reaction were kept at low temperatures (about 77° K.), laser action at IR and red wave lengths may be possible, their calculations show. The number of available energy levels is reduced at low temperatures, and laser action at IR wave lengths is possible if the vibrational levels of the ground elec
tronic state can be depleted or "thermalized" fast enough. Experiments are still needed to evaluate the neces sary parameters for laser action at low temperature before a chemically ex cited laser can be achieved, Dr. Broida feels. Explosions. The only example of population inversion by explosion is that of the H 2 -C1 2 explosion initiated by a flash studied by Mr. Kasper and Dr. Pimentel. Dr. John A. Howe of Bell labs has examined exploding mixtures of oxy gen with carbon monoxide, with hy drogen, with methane, and with ethyl ene. He has looked for evidence of net optical gain (laser oscillation) in the 0.3- to 30-micron wave length region. Net optical gain would indi cate inverted populations. So far, however, Dr. Howe has observed no laser oscillation. Interesting results using chemical explosions have come from a group at Interphase Corporation-West, Palo Alto, Calif. There, Dr. Irwin Wieder, Dr. R. R. Neiman, and Dr. A. P. Rodgers have studied the IR and UV radiation emitted by excited species in low-pressure gaseous oxygen-acetylene explosions. Their aim was to estab lish the population distribution in selected energy levels. In the UV, they used cavity techniques, and found a relative enhancement of several elec tronic transitions in CH and OH radi cals. The Palo Alto scientists also studied the emission in IR from excited carbon dioxide molecules which form behind a fast chemical detonation wave. They have found evidence for an en hanced population of the high vibra tional levels of carbon dioxide mole cules. Many scientists have expressed pes simism as to the feasibility of develop ing chemical lasers—particularly those pumped by the free energy of the reaction itself. Others, however, re main optimistic. For example, NBS' Dr. Shuler, one of the organizers of the Chemical Lasers Conference, be lieves that the chemical laser has some thing in common with the four-minute mile. "Once the barrier is broken," he predicts in the conference foreword, "successful operation of chemical lasers will be announced regularly in the literature." Now that an HC1 chem ical laser has been successfully oper ated, this prediction would seem to be borne out and further achievements in this field may be expected.
