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Stolte, ibid., p. 413. 36. T. Rettner and R. N. Zare, J. Chem. Phys. 77, 2416 (1982). 37. Z. Karny, R. C. Estler, R. N. Zare, ibid. 69, 5199 (1978). 38. K. Kleinermanns and J. Wolfrum, ibid. 80, 1446 (1984). 39. P. R. Brooks, R. F. Curl, T. C. Maguire, Ber. Bunsenges. Phys. Chem. 86, 401 (1982). 40. P. Arrowsmith, S. H. P. Bly, P. E. Charters, J. C. Polanyi, J. Chem. Phys. 79, 283 (1983). 41. C. Jouvet and B. Soep, Chem. Phys. Lett. 96 426 (1983). 42. I am grateful for support from the National Bureau of Standards, National Science Founda- tion, Air Force Office of Scientific Research, Department of Energy, and Army Research Office. Spectroscopy of Transient Molecules this issue, Leone (1) discusses bimolecu- lar reactions in the gas phase. Our article deals with unimolecular processes, in- cluding the nature of excited states and reaction intermediates as revealed by their spectra, the dynamics of intramo- Summary. It is now possible to resolve completely the initial and final quantum states in chemical processes. Spectra of reactive intermediates, of highly vibrationally excited molecules, and even of molecules in the process of falling apart have been recorded. This information has led to greater understanding of the molecular structure and dynamics of small gas-phase molecules. Many of the concepts and spectroscop- ic techniques that have been developed will be valuable throughout chemistry. it is possible to resolve individual quan- tum states in the observation of molecu- lar spectra, in the preparation of reactant molecules, and in the analysis of reaction products. Such information, in combination with theory, reveals a great deal about the dynamics of atomic and molecular mo- tions and about the potential energy sur- faces that govern them. In an article in lecular vibrational energy redistribution as probed by spectroscopy, and the dynamics of photofragmentation as re- solved quantum-state by quantum-state. We have selected a few examples to illustrate the power of some of the new types of experiments and of the tools now available, but much equally impor- tant and interesting work is not dis- cussed. Warren D. Lawrance is an Adolf C. and Mary Sprague Miller Institute postdoctoral fellow in the Chemistry Department at the University of California, Berkeley, and a guest scientist at the Materials and Molecular Research Division of the Lawrence Berkeley Laboratory, Berkeley, California 94720. C. Bradley Moore is professor and chairman of the Chemistry Department at the University of California, Berkeley, and faculty senior scientist with the Materials and Molecular Research Division of the Lawrence Berkeley Laboratory. Hrvoje Petek is a National Science Foundation predoctoral fellow in the Chemistry Department at the University of California, Berkeley. 22 FEBRUARY 1985 Lasers have made it possible to ob- serve the spectra and study the dynamics and chemical kinetics of many free radi- cals (2, 3), ions (2, 4), molecular excited states, and other transient species (2). Frequency resolution as high as 1 part in 108 is possible. Lasers with pulses as brief as 10-14 to 10-13 second access the shortest chemically significant time scales, for which the energy uncertainty, AE - hiAt, is comparable to chemical bond energies (3, 5). Sensitivities suffi- cient to detect single molecules have been demonstrated for more modest lim- its of spectral and temporal resolution (6). Methylene (CH2), the prototype for divalent carbon intermediates, has been the focus of many experimental and the- oretical studies aimed at determining the structure and the energy separation of the two low-lying electronic states, the "metastable" singlet ('CH2) and the ground triplet (3CH2) states. Spectro- scopic detection of methylene eluded experimentali-sts for many years during which the only evidence for the theoreti- cally postulated electronic structure of methylene was the vastly different chem- istry exhibited by the two electronic spin states (3). In an article in this issue, Goddard (7) discusses the interaction between experiment and theory that pro- duced accurate determinations of both the singlet and triplet structures, as well as the value of 9 kcal/mol for the energy separation between the singlet and triplet states (A.-). The pioneering flash-kinetic spectros- copy work of Herzberg provided the first spectra and structure for triplet methylene and showed that it is the ground state (8). Laser magnetic reso- nance, a technique whereby rotational or 895 Understanding Molecular Dynamics Quantum-State by Quantum-State Warren D. Lawrance, C. Bradley Moore, Hrvoje Petek The process of energy transfer within a molecule or group of molecules is closely related to the making and break- ing of chemical bonds. Although it is not possible to photograph step-by-step mo- tions of individual atoms and molecules,