Lasers: The Light Fantastic prepared b) J. Chem. Educ. Staff
Moon Beams, Atom Spotters and Sperm Watchers Lasers were once described as "the light fantastic," and they seem to he living up to this billing. A laser beam has been to the moon and hack. A laser system has detected single atoms of cesium in an environment of 1.0 X loL9atoms of other elements; another has measured the swimming rate and details of the swimming motions of spermatozoa in mucus from the human cervix. Others may hold the key to controlled nuclear fusion and almost limitless energy supplies for the entire earth.
Intense, Monochromic and Coherent
A laser is a device that can produce a narrow, extremely intense beam of coherent light of a single frequency (or wavelength). The beam may be quite narrow, of the order of 1 milliradian or 0.05 degrees. The intensity can exceed 2 X 1019 W/cm2 (Compare this with theintensity of the energy from the brightest flash lamn that is less than ten thousand watts per squarecentimeter of lump iurtkce.) In c ~ n t r mtoorrlinnry sun or lamo linht. which t i incoherent or disordered in terms of the phase ofitskaves, laser light is coherent or in phase over the entire length and width of its beams. Also, laser light unlike ordinary light can be virtually monochromic, consisting of photons of a single (or very narrow hand) wavelength or frequency. The ruby laser-the first to be made-produces red light of wavelength 694.3 nanometers, having a hand width (variation of wavelength) of about 0.01 Am. he spectral purity, coherence, and directivity of the laser beam nives it manv advantages over ordinary light. A wide variety of pulsed and continuous wave (cw) lasers having fixed or tunable wavelengths throughout the visible, ultraviolet, and infrared regions is available. The term laser is an acronym for light amplification by stimulated emission of radiation. In effect, this means amplifying or increasing the output of light by stimulating the release of photons from atoms or molecules that are in excited states. The released photons may arise as a result of electrons dropping to lower electronic energy states or from transitions among vihrational-rotational or purely rotational energy states. The First Ruby Laser The first laser was made in 1960 by T. H. Maiman using a ruby rod about 2 in. long. The rod consisted of high purity aluminum oxide, A1203, doped with about 0.05% chromium(II1) oxide, Cr203. The ends of the rod were polished and coated with a semitransoarent reflective coating that served as a mirror Lo reflect yh;,tons I)ack and forth wiihin the rod. When the rod was exposed to an intense green-blue light from a xenon Ilashtuhe, a large fraction of the chromium rlllj ions absorbed enerrv. (Certain of their electrons moved to hieher energy states.j~hortlyafter exposure to the exciting so&e, there was a burst of intense red light emanating as a beam from the ends of the rod. Figure 1is a schematic diagram of the ruby laser modified slightly from the original design to increase the intensity of the laser heam and to provide a pulsed energy output.
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Lamp Ruby
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LASER BEAM
S
M2
Figure 1. Schematic diagram of a ruby laser. The region between the minors MI and &is known as rexnance cavny: its design, especially t b swkhing device S, permits larger numbers of stimulating photons to move through ma rod, thereby increasing energy output. MI is a totally reflectingmirror; M2 is a partially bansminingmirror
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20 Energy
lo3 cm-'
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10
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0
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Ground State
Figme 2. Diagram of m g y 1-1s in C? showing metastable 2E state inwlved in the laser transition. Adapted from reference ( la).
Excitation and Stimulated Emission Figure 2 is a diagram of the energy states of a chromium(II1) ion. I t illustrates the transitions that lead t o excitation and laser emission. As photons of blue and green light from the xenon lamp are absorbed hy the chromium ions, electrons move from the ground state, represented by 4A2 to excited states, represented by 4F, and 4Fz.Both of these excited states are verv unstable. and the ions auicklv lose some of the ahsorbed-energy, m&ng to the mecastahie state represented by 2fi:.'rhi~state has a relativelv long lifetime. with a result that a large fraction of the chromiumions in the irradiated ruby Laser beams ere extremely dangerous; they can kill on contact with vital oreans. - Thev should be used onlv after proper training. Volume 55, Number 8. August 1978 1 529
state are stimulated to release their additional energy and return to the ground state. The process by which a large fraction of the chromium ions is excited to and remains for a time in the metastable 2E state is known as population inversion. (The population of the ions is inverted from most being in the ground state to most being in the metastahle excited state.) Laser designers sometimes say that photons from the xenon lamp "pump the chromium electrons" from the ground state until population inversion has occurred. Thus the term "optical pumping to effect population inversion" is common. An excited chromium ion is stimulated to release energy and return to the mound state when it encounters a ohoton of frequency (or kavelength) identical t o that of the iaser light. Let us suppose then that after population inversion has occurred in the ruby rod, a photon of red light enters the rod. If this photon encounters an excited chromium ion, the chromium ion will he stimulated to emit a second photon of red light. Two photons now are moving inside the rod. Each time one of them encounters an excited chromium ion it stimulates emission of another photon. The reflective coating a t the ends of the rod prevent8 photons from leaving it. As photons reflect back and forth inside the rod, more and more excited ions are stimulated to release photons. Ultimately, the photon huildup is great enough that an intense, narrow, coherent beam of monochromatic light emerges from the ends of the rod.
The He-Ne Gas Laser This. and certain other eas lasers. consists of a mixture of two or more gases in which"populatibn inversion in one gas is obtained by transferring energy t o i t from one of the other gases. In the He-Ne laser, the helium atoms are pumped to a higher state, and they transfer their excess energy to the neon atoms through collisions. In the process, large numbers of neon atoms exist in a metastable state. Stimulation of emission from these neon atoms is achieved by reflection of photons of appropriate frequency between mirrors placed a t both ends of a cavity surrounding the tube containing the gases. Ultimately, the laser beam emerges from the resonance cavity through one of the end mirrors that has been made semitransparent for this purpose,
Chemical Lasers In chemical lasers, the energy needed t o bring about population inversion comes from the making and breaking of chemical bonds within the laser medium. In effect, a chemical reaction is allowed to proceed so that one of the products formed has a large fraction of its molecules in a metastable state. Stimulation of emission from these excited products produces the laser beam. An example of a chemical laser system is
The HCI is vihrationallv excited. Uoon stimulation hv.ohotons . of appropriate wavelen&h back and forth through the reaction mixture. the excited HCI will release laree numbers of photons of identical wavelength.
0 Tunable Lasers
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Manv lasers can be "tuned" t o narrow the waveleneth of radiation emitted or to select from a range of wavelength oossibilities. Amone the most versatile and useful of these are dye lusers Most dye laqers contain solutions of colored organic substances used or classdied ns dvm. These substances al~sorh a broad range of wavelengths, and they can assume metastahle excited states havinz relativelv. lone - lifetimes. If a solution of a dye is exposed to intense pumping source, a large fraction
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530 1 JOurnal of Chemical Education
PUMP RADIATION
1 1 1 1 1
SchemallCdlagram of a dye lsoer M, sthe tmlly reflectingm l r r a M, s mr pan~alhlwansmmmg o w minor F s tta hequnq sslscm Adaptad
F gwe 3
of the dve molecules or ions will move t o metastable excited states. 111these excited states, the dye molecules may be capnhle of emitting the same broad range of wavelenahn they can absorb. T o stimulate emission of only a single wavelength (or a very narrow band, -4.01 nm), the laser is "tuned" by allowing photons of only the desired wavelength to reflect back and forth among the excited dye molecules. A commonly-used wavelength.selecting device is a diffraction grating armnged so that photonsof the desired wavelength are focused on the dvesolutwn. Toavoid overhentine. ". thesolution iscirculated oier a cooling unit. Figure 3 is a diagram of a tunable dye laser.
0 Lasers in Chemistry Three recent applications of lasers in chemistry are: isotope separation, laser picosecond spectroscopy, and resonance ionization spectroscopy. Removal of fissionable isotooe U-235 from natural uranium is an example of laser isotope separation. A gaseous sample of natural uranium (99.2% U-238,0.7% U-235) was irradiated with light from a xenon laser which excited electrons in U-235 atoms hut not in those of U-238. Then photons from a krypton laser stripped the excited electrons from the U-235 atoms, leaving positively-charged ions. These ions were collected on a negatively-charged electrode, reduced to atoms, and the U-235 metal isolated. Lasers also have been used to separate isotopes of boron, chlorine, and sulfur. Laser picosecond spectrosco~vis used to studv ultrafast moleculai processes in liquids~processesthat bccur in a trillionth of a second or less (or between one and several hundred picoseconds). Examples of such processes are the orientational motions of polyatomic molecules, the rates of recombination of atoms or ions inside solvent cages, and the transfer of energy to and from a chlorophyll molecule during photosynthesis. The basis for this technique is that certain types of lasers can fire off light pulses that last no longer than the events thev are attemotine . to measure. Resonance ionization spectroscopy has been used to detect single atoms of cesium in the presence of 1 X 10'9 atoms of other elements. Tiny amounts of any alkali metal can be detected by this technique which involves tuning a laser beam so that i t ionizes only one type of atom in a gaseous mixture. The electrons released when the targeted atoms ionize are collected and counted in a proportional counter, giving a very accurate measure of the number of these atoms present in the sample.
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0 Lasers in Biochemistry
An excitine new use of lasers is to determine the velocitv of important contents or components in living cells. The b k i s for these determinations is that radiation reflected from a moving object is shifted slightly in wavelength: the magnitude of this shift enahles us to determine the velocitv of theobiect. Laser velocimetry, as this procedure is known,-has been &ed to measure the swimming speed of rabbit, fish, hull, and human spermatozoa in biological fluids, and to determine both the speed and details of motion of swimming microorganisms. It also has been used to determine details of blood flow in large
and small human blood vessels-in the retinal vessels and in major veins and arteries. Laser light scattered from the surface of skin gives information about microcirculation near the surface.
Other Uses of Lasers Lasers are used in medicine in a number of ways including correcting retinal detachment, facilitating healing of ulcers, coagulating blood vessels to obstruct the flow of blood. They provide the most accurate means of measuring distances and frequency. Thus they are used to measure earth movements. to determine distances to the moon surface or to determine the moon's orbit with a precision of 42 cm or 1part in lo9. In industry, lasers are used in precision cutting, drilling, and welding. The garment industry, for example, uses a laser system for cutting fabrics.
laser ~ r o i e c is t the the most dupiicate in a controlid manner, the major energy-producing reactions of the sun and the stars. The most promising approach to controlled nuclear fusion involves the use ofhigh powered lasers focused on pellets of deuterium or a deuterium-tritium mixture. The temperature inside these laser heated pellets is estimated a t 108K. If laser fusion proves successful the world will have the energy it needs for millenia. References
EoUC.53.13 11976).1b) Roub.eau,DeniaI..,J.CHEM.EDUC., 43,566(1966l.(cl Won&D. Pan, J.CHEM. EDUC.,48,654(1976I. 121 E x p a r ~ r n ~ /or n l high school loboretory. Castka,JosephE.. 3. CHEM. EDUC.,S1.573 ,,om, ,.",..,. (31 Dye loser instrumenlolion, Quick, Jr..C.R.,andWittig,C..3. CHEM.EDUC.,54.70S 11971).
volume 55, Number 8, August 1978 1 531
