Denis 1. Rousseau Princeton University Princeton, N e w Jersey
I
Laser Chemis+ry
I
In 1960 when Maiman (1) developed the laser, chemists looked forward to having a t their dis~ o s a al new laboratory tool. After six years, however, with a few exceptions, the closest the chemist has come to a laser is his use of the gas laser as a light source in Raman spectroscopy (9). The principal aim of this paper is to discuss several types of laser experiments and acquaint the reader with various laser research problems, thereby creating, hopefully, new interest in laser chemistry. The nature of the chemical problems which may be studied with the laser may be divided into two separate categories-Laser Chemistry and the Chemical Laser. The chemical laser is a means of producing laser action as a result of a chemical reaction. In other words, by making and breaking chemical bonds, species are produced in excited states such that laser action occurs. The chemical laser will not be discussed in this paper; an excellent article on this subject by G. C. Pimentel has recently appeared in ScientificAmerican (3). Laser chemistry involves using a conventional laser as a monochromatic, high-intensity light source for studying chemical properties. The interaction of laser radiation with chemical systems is very complex and may take place via any one of, or a combination of, several mechanisms. For simplicity of discussion, however, laser photochemical processes shall be divided into three distinct sections: Ionization Processes; Thermal Processes; and Multiple Photon Processes. The ruby laser will be discussed briefly to introduce the reader to general laser principles and properties.
can occur. According to the Boltzmann distribution ions in the ground state there are normally more than in any excited state. The relatively long lifetime of the Z E state allows one to pump electrons from the 'Az state until the population is inverted, that is, there are more ions occupying the excited state than the ground state. Laser action takes place when the ions in the 2E state are stimulated to fall back to the ground state, thereby bringing about the normal thermal d i s tribution once again.
I Ground Figure 1.
Sta:e
Electronic energy lovolr of Cr+'ion.
Ruby Laser
The primary component of the ruby laser is the ruby rod, which is fwdy cylindrica1,and has flat parallel ends. The co posltion of ruby 1s AlzOa doped with a small amount of CrzOa. The active component for lasing is the chromium ion which normally has a concentration of about 0.05%. The energy levels for the chromium ion are indicated schematically in Figure 1. Excitation proceeds from the ground state, indicated by its spectroscopic notation 4A2,via absorption of light in the blue and green spectral regions, to the states 'FI and 'F2. Radiationless transitions rapidly take place from these states to the 2E state. Since the spin multiplicity of the doublet state is dierent from the ground state, the lifetime of the %Estate is quite long compared to the other states, and population inversion 1 Editor's NOTE:See also the discussion of safety precautions appropriate to laboratoria involved in laser research: nrrs JOURNAL 43, A335 (1966). R. G., and 1 EnrToR's NOTE: See, for example, LAYTON, EYRING, E. M., TEIS JOURNAL 40, 338 (1963).
566
/
Journal o f Chemical Education
A condition of population inversion does not lead to laser action unless the ruby is placed in a resonant cavity. As sketched in Figure 2, the resonant cavity consists of two mirrors, one a t each end of the ruby, which are carefully aligned parallel to the faces of the ruby. The population inversion gives rise to Buorescence in all directions. Some of this light reaches one of the two mirrors forming the cavity and is reflected back through the ruby. As it passes through the ruby
Lamp
Ruby MI
S
Laser Beam M2
Schematic diagram of resonant cavity for ruby loser. C i s the capacitor bank to supply the energy to the lamp. MI is o totally reflecting mirror. Mnis a partially tranrmitting mirror from which the beam emerges. S is the porition in which a Kerr cell Q-witchmoy be ploced. Figure 2.
it stimulates more fluorescent radiation which is emitt,rd in the same direction as the incident light. More and more light t,hen starts traveling along the direction of the ruby axis, always being reflected back through the ruby by the two parallel mirrors. With each pass through t,he ruby the beam gains intensity by stimulat,ing more emission. The output beam is taken from the cavity by making one of the mirrors partially transmitting. A xenon lamp, placed in one of various configurat,ions around the ruby, is used as the source of blue-green excitation. Up to 10,000 joules of energy are supplied to this xenon lamp by discharging a capacitor bank. The duration of this pumping radiation is approximately one millisecond, giving a laser output duration of about half a millisecond. The output energy of the ruby laser normally ranges from 1 to 100 j a t power levels reaching lo5 W. The entire ouiput is within n band only 0.1 wide, centered a t 6943 A. Thc power of the laser output may be increased another three orders of magnitude by "Q-spoiling" the resonant ravity. In this Q-switching process the resonant ravity which is necessary for lasing to occur is spoiled for a rontrolled period of time while the ruby is being pnmped. This prohibits laser action from occurring when the levels become inverted and means that the population of ions in t,he 2Bstate is built up to an extraordinarily high level. By instantly completing the resona.nt. cavity, hence allowing reflection to occur, the excited ruby emits its stored energy very rapidly (l(t-100 nanoseconds). This gives pulses of very high powerup to lo9 W/rm2. In essence, Q-switching is a means of storing t,he energy in t,he ruhy and removing it in a single high-pourer pulse. The most common Q-switching devices are the rotating mirror and the Kerr cell. By replacing mirror MI in Figure 2 with a very rapidly rotating mirror, the resonant ciwity will he formed only during the short interval when t,he rotut,ing mirror is parallel to Mt. By synchronizing the lat,ation of the mirror with the flash lamp output, the time when the resonant cavity is completed may be selected so as to optimize the power of the laser output. When a Kerr cell is used as the switching element it is placed within the optical cavity a t posit,ion S in Figure 2. When the high-voltage field is applied to t,he Iierr cell t,he polarized output from t,he ruby is r o t a t ~ d(adkiit.iona1polarizers may be necessary) I to spoil t,he optical ravit,y. When the voltage is turned off, a proress whirh may be accomplished very rapidly, the polarizat.ion becomes correct to form a resonant cavity. As with the rotating mirror, the time of switching is optimized 1.0 obtain the desircd power output,. There are t,hree important characteristics of lascr radiation. I t is intense, monochromatic, and coherent. Coherence is the property that all wave packets are in phase wit,h each ot,her. With a conventional source, interferenre pat,terns can only be obtained from any very small area of the source, e.g., a pinhole, as there is no coherence beheen different points on the radiating surface. I n laser radiation, however, all of the wave packets are in phase. This enables one to observe interference patterns from the entire emitting surface. I t is the rohereuce property that enables one to focus the laser to an image t.hat is brighter than the original sonrce. W t h n ronventional incoherent sonrce it is
impossible to focus to an area brighter than the original source. The output beam of the rnby laser at 6943 has a typical line width of 0.1 A. With careful temperature control this can be improved even more. Such narrow line widths could be obtained with a monochromator using a conventional black body source; however these would not have such intensities as are possible with the laser. Consider a typical black body source peaked a t 6943 A. This source would have an output power of 0.016 W/cm2 in the 0.1-A region around 6943 A. A typical ruby laser will have an output of about 50 kws/cm2 in the same spectral region. This six-ordersof-magnitude increase in intensity over a conventional black body source may be still further improved by Q-switching the rnby. For a more detailed analysis of this discussion the reader should consult the book by Lengyel (4). Ionization Processes
The electric field strength associated with an intense laser beam is extremely large. A typical output from a Q-switched laser is 10 Mws over an area of 1 cm2, and by focusing this beam down to an area of cm2 a power density of 10" W/cmZ may be obtained. By classical electromagnetic radiation theory such a power density corresponds to an electric field of 10' v/cm. (Instantaneous variations in the laser output power may, however, cause the field strength to be somewhat greater than calculated here.) Such an electric field puts an oscillating potential gradient of 1 v across a 10-A molecule. Although a field of this magnitude is not large enough to ionize most molecules, it certainly should he strong enough to perturb a molecule such that for the duration of the electric field it may react in a unique way. Experimentally we find that laser-induced ionization of gases is a well known phenomenon. It bas, in fact, become a "parlor trick" among laser scientists to produce a spark in air by focusing the output of a Qswitched laser. At the point of focus a very bright flash is observed accompanied by a sharp sound. The flash lasts much longer than the laser pulse duration and grows to several millimeters in length. Gases which have been successfully ionized, as evidenced by the bright spark, include, in addition to air, Hz, He, Ne, Ar, Kr, N2, C12 (5-9). I t is very unlikely that the electric field strength associated with the laser beam is capable of directly ionizing small gaseous atoms and molecules. Static dc fields of about 10,000 v/cm will readily bring about ionization, so it might be argued that the laser produces breakdown by a similar mechanism. A static high-voltage discharge in a gas is caused by the availability of the free electrons which are always present. (There are about 103free electrons per cma in a typical gas.) The high voltage dc field accelerates some of these free electrons so that they initiate a cascade process which leads to breakdown of the gas. For the laser to interact with gases by such a mechanism, there must be within the region of high field strength a large probability of finding some free electrons during the laser pulse. However, the concentration of free electrons in the focal volume of a laser is much too small to account for the regularity of the gas breakdown phenomenon. Volume 43, Number 1 1 , November 1966
/
567
Several theories have been proposed for laser-induced ionization of gases, ranging from an analogy to classical microwave breakdown theory, to laser interaction with m a l l particles floating in the gas (8-13). Although none of these mechanisms have been satisfactorily verified experimentally, the prevalent feeling among most investigators is that the process is initiated by a small number of multiple photon absorptions, (see discussion below) and propagated by the inverse brastrallen effect (10). The latter effect is often invoked for explaining solar reactions. In this process a photon collides with an electron while the electron is in the field of an atom or ion. In this three body collision, energy from the photon is transferred to the electron. Several of these events take place, thereby producing high-energy electrons which in turn collide with atoms and bring about further ionization leading to complete breakdown. (See Wright (10) for a complete explanation of this process.) Much more careful and detailed research must now be done on this subject in order to determine unequivocally how the focused laser beam interacts with a seemingly transparent gas to bring about ionization. The sensitivity of biological materials to various forms of radiation has led many (14, 15) to expect that these sub$.ances will undergo ionizations in the presence of these strong fields (lo7 v/cm). An investigation conducted on laser irradiation of small samples of mouse tissue (16)showed that free radicals were produced; (ESR spectroscopy confirmed their presence). The problems in evaluating laser excitation of biological materials are enormous due to the complexity of the substances. For example, here one does not know whether the free radicals resulted from dissociating a molecule into fragments, one or more of which was a free radical, or whether the free radicals resulted from ionization of molecules without further fragmentation. Free radicals can be produced in biological samples by heat, ultraviolet radiation, X-radiation, and 7-radiation. Research to examine the laser effects and determine how they are similar to, or different from, the effectsproduced by the more classical sources of excita tion is an important challenge. This difficult problem is one which, when solved, may yield results very important to biological and medical science. Thermal Processes
Irradiation of an opaque substance, such as a metal, with the laser can produce extremely high temperatures in very short time periods. For example, this phenomenon bas been utilized to burn holes in stacks of razor blades. It was at one time such a common ploy that the output of the laser was sometimes referred to as so many "Gillettes," denoting the number of razor blades that could be penetrated by a single laser pulse. The heating effect in metals has been described in detail by J. F. Ready (17). He found that the original reflectivity had little effect on the metal vaporization process. The photon density in the laser beam is great enough so that a very slight absorption in the beginning of the pulse was sufficient to start the melting process, which made the once-reflective surface now a very good absorber. The absorption excites electronic states which are rapidly converted into lattice and vibrational modes giving rise to the extremely high temperatures. Temperatures of about lo4 OK can be obtained at a 568 / Journal o f Chemical Education
rate of change of about 101° deg/sec. Even transparent materials are damaged by focused laser radiation. This presents problems to laser researchers. Lenses and other optical components in their systems often become damaged, rendering an experiment difficult or even impossible.' French scientists (18) have taken advantage of the high temperatures that have become available with laser irradiation and have studied the emission spectra of a whole series of molecules excited in this fashion. They propose that it is an cucrllrnt means of excitation for emission studies sinw it climinatrs the uossibilitirs of electrode contamination, which is a problem in conventional emission spectroscopy. Several researchers have investigated the laser irradiation of graphite (19,30). The rapid heating causes a jet of small particles to be ejected from the point of laser impact. Mass spectrometry (30) demonstrated that these particles ranged from C1 to Cv. By performing the experiment in the open air, Howe (19) was able to obtain strong emission spectra from the species CN in addition to the expected C2 emission. One can imagine future experiments in which the surrounding atmosphere is varied, perhaps making many new compounds possible. Medical applications of such thermal effects are already everyday practice in the ofice of the ophthalmologist. In treating patients with detached retinas, the laser is aligned to be focused by the pupil of the eye on the area to be irradiated. The energy is carefully adjusted to melt the tissue just to the point of coagulation. A single pulse of the laser can, in this process, coagulate a retinal defect in less than a millisecond and do negligible damage to the surrounding area. Cancer researchers are currently investigating the possibilities of using this laser heating effect in the treatment of diseased areas. As yet it has only been used in a few cases of skin cancer, but with intensive research the laser may soon become a useful tool in cancer therapy. Multiple Photon Processes
Electronic transitions normally occur in atoms or molecules by the absorption of light in the visible and ultraviolet spectral regions. The frequency, vo, of this light is such that the energy of the photon is exactly the same as the energy difference between the ground state, En, and the excited state, El. With the high power of the laser, however, the probability that an atom or molecule may simultaneously absorb two photons a t the laser frequency, vL, increases tremendously. The sum of their energies is the energy needed to bring about the transition from Eoto El, ( 2 vL = vO) These two processes are illustrated in Figure 3. The possibility of such two-photon processes was proposed several years ago by Goppert-Mayer (31). There need be no real state between the ground state and the excited state attained by the two photon absorption. For the sake of mathematical convenience however, one may consider a virtual state E' to exist a t the laser frequency. An electronic transition resulting from the absorption of light is restricted by selection rules such that the transition can take place only if the transition moment, Ifio.ll, is nonzero; otherwise the transition is said to be forbidden. 1fi0,~l may be defined by the integral J*l*fi\~rod~,where qo is the wave function of the
E O
Flgure 3. Energy levels for on electronic tronsltlon from Eo to EL. L& Tramition via o single photon .absorption. Right: Transition through o rirluol slate E' via a M-photon ~bsorptiaa
is the wave function ground state energy level, Eo; of the excited state, El, and p is the dipole moment operator. For the integral to be nonzero, the product Tl*pFomust be an even or symmetric function: it must not change sign as it passes through the origin. The dipole moment operator, p, is a function of the coordinate, and hence is an odd function. Thus for the product, to be an even function, G must be antisymmetrical with respect t o 90.A more detailed analysis of these symmetry selection rules may be found in any standard spectroscopy or quantum chemistry textbook (22). If the transition from the state Eo to El occurs via a two-photon absorption, the symmetry selection rule then requires that the ground state wave function, has the same symmetry as the excited state wave function * I . This is readily understood when we consider the wave function W of the virtual state E', and look a t the two-photon absorption as being simply two consecutive one-photon transitions, each obeying the symmetry selection rule. These symmetry selection rules tend to hold rigorously only for small atoms, for in molecules symmetry forbidden transitions become partially allowed due to perturbations. We may therefore expect to be able to observe two-photon transitions in many systems, although those systems which have a strong on+photon absorption a t twice the laser frequency may very likely have only a weak two-photon absorption. The chemist should be rather excited by the possibilities of two-photon transitions. One can even conceive of a two-photon spectrometer which would presumably complement the conventional one-photon spectrometer, and would enable the spectroscopist more easily to determine the symmetry of the wave functions involved in the various transitions. Before such a spectrometer could be created, laser technology will have to be developed considerably. The necessary component is a continuous high-powered laser, the frequency of which can be varied over a wide range. At present the output from a continuous laser is not sufficiently high for such an application, and no laser can be tuned over a wide frequency range. There have been several two- and three-photon absorptions reported (23-26). Since the frequency of the ruby laser is restricted to a narrow range around 6940 A, only those systems can be studied which have energy levels at two or three times the energy of the ruby laser, that is 3470 and 2315 A respectively.
Only a few of the reported multiple-photon absorptions have led to chemical change (27, 28). Pm and Rentzepis have been able successfully to polymerize styrene and several of its derivatives by laser excitation of the solid monomer held at liquid nitrogen temper* ture (27). A free radical was formed and stabilized so that when warmed to room temperature, the conventional polystyrene polymerization took place. Styrene is transparent a t the ruby laser wavelength and is known to undergo polymerization when excited by ultraviolet light, so a multiphoton absorption was proposed as the most plausible mechanism. Polymerization reactions should, in general, be a very sensitive means of detecting chemical change brought about by the laser, since a single photochemical reaction may result in a polymer involving lo5molecular units. A small number of laser events will therefore yield a large amount of detectable material. We have recently studied the laser induced decomposition of silver chloride into colloidal silver and chlorine (28). I n the usual photochemical proces5 ultraviolet light of wavelengths shorter than 4100 A dissociates silver chloride, leaving colloidal silver particles which appear red in color. When crystalline Agsl was irradiated with light from a mby laser at 6943 A, a spectral region where it is normally completely transparent, decomposition was evidenced by red spots of colloidal silver a t the point of focus. A stimultaneous blue-green emission was observed (i.e., at higher energies than the incident laser radiation), and the intensity was dependent on the square of the incident laser intensity, thereby demonstratmg that the fluorescence was caused by a two-photon absorption. Although both of these multiple-photon absorptions resulting in chemical changes are experiments which have been done by conventional ultraviolet photolysis, they do demonstrate that multiple-photon chemical reactions are possible. The next step in this area of research is to find systems in which the multiplephoton absorption will lead to reactions or products which are unattainable by conventional single-photon absorption. Conclusion
Several types of experiments have been briefly d i e cussed to point out the nature of the effects which take place and some of the problems involved, when a laser beam interacts with matter. Not all of these experiments are of general interest to the chemist as very IittIe "chemistry" has as yet been studied with the laser. There are probably two areas which show the most promise. First, advantage may be taken of the monochromatic property of the laser to populate selectively specific energy levels. This may be accomplished by a one-photon absorption where, due to the high intensity, a high population of a specific level may be brought about. On the other hand, specific energy levels which are unattainable by a single-photon transition may be reached by symmetry-allowed two-photon transitions. New energy states may therefore be selectively populated. Secondly, the high concentration of energy makes a new type of flash photolysis possible. Substances which are transparent or very reflective to normal light become very absorbent in the high-intensity laser light, and are thereby heated to thousands of Volume 43, Number
1 I, November 1966
/
569
