MICHAEL R. SLABAUOH
chem I fupplement
HELENJ. JAMES Weber State College Ogden, Utah 84408
Lasers: A Valuable Tool for Chemists E. W.Findsen and M. R. Ondrias University of New Mexico. Albuquerque, NM 87131 Lasers have, in recent years, become an increasingly important tool for chemists. The word laser is an acronym that stands for Lieht Amolification hv Stimulated b.'mission of Radiation. ~ { first e iaser was a &by laser built in 1960 by Maiman. Since that time manv varieties of lasers have been commercially developed. Their diverse applications range from initiating chemical reactions to probing the structure and properties of complex biological molecules. In this paper we vresent a brief survev of lasers and some of their applica- tions in chemistry. All lasers share the basic property of producing a collimated beam of monochromatic light. T o appreciate why laser light is so useful to chemists, it is instructive to compare the of a laser to those of the common incandes. - nronerties ~ ~ cent lamp. A lamp is a much more efficient light-producing device than a laser but lacks the s ~ a t i acoherence l and monochromaticity that make lasers sokxtreme~yuseful. The light from a tungsten lamp is emitted in a broad frequency range from the near UV to the infrared. In the process of isolating a narrow frequency range of the lamp's emission much of the energy of the lamp is lost. The monochromaticity of the remaining light is limited by the instrument (generally a monochromator) used to isolate the frequency desired. Most lasers emit all of their stored energy a t one frequency (or a select few), and their monochromaticity is limited by the physical properties of the excited state transition induced in the lasine material. Furthermore. when lieht is emitted from a lamp tlkre is no distinct relationship between one photon's momentum and another's (even if both have the same energy). Laser beams on the other hand, possess the property that all photons within the beam have the same direction of travel and phase relationship. The latter quality is termed ohase coherence. I t is a necessarv consequence of stimulated (rather than spontaneous) emission and is an important characteristic of lasers. A lack of coherence leads to beam divergence and a subsequent loss of intensity. Thus, while a laser's efficiency in converting electrical energy into radiation is very low (generally actthat many diverse molecular interactions contribute to the electronic absorntion snectrum of a molecule. Thus transient absorption changes can be difficult to interpret on a direct molecular level. Time-resolved Raman spectroscopy is a technique that can suoply direct molecular information concerning kinetic proces&:ln thin technique a photochemical reactibn is initiated but instead of moniu~rinathechange in transmittance of a probe beam, the laser light scattered by the sample molecules is detected. Raman scattering occurs when photons are inelastically scattered by a molecule (9).The resulting scattered photons are shifted in energy relative to that of the incident laser beam bv amounts that are eaual to the spacing between vibratioial energy levels in the molecule. Raman scattering is an inherently weak (inefficient) process that requires very intense monochromatic light sources. Indeed, it is one form of soeetroscoov that has blossomed as a direct result of advances in laser ~chnology. Raman studies complementary to the transient absorption study mentioned above have been performed to determine the changes in the heme vibrational structure which occur rapidly after the photolysis of CO ligands from human hemoglobin (10). Picosecond laser pulses both initate the nhotochemical event (in this case the ohotolvsis of the CO molecule from the heme) and probe t
