Two-Dimensional Raman Echoes: Femtosecond View of Molecular Structure and Vibrational Coherence

Conventional vibrational spectroscopies such as infrared absorption and spontaneous or coherent Raman scattering provide a one-dimensional projection of intermolecular and intramolecular vibrational structure and motions onto a single frequency (or time) axis. For simple molecules with well-separated eigenstates, this gives direct information on energy levels and their oscillator strengths. The situation is very different in complex systems with highly congested levels such as biological molecules, liquids, polymers, and glasses.1-5 Here the microscopic information is highly averaged, and is often totally buried under broad, featureless lineshapes, whose precise interpretation remains a mystery. Typical examples are the amide band in proteins,6,7 and hydrogen-bonded liquids.8,9 For decades, optical spectroscopists have dreamt about using multidimensional coherent nonlinear spectroscopic techniques to overcome these difficulties and unravel the microscopic picture underlining complex spectra. Coherent spectroscopy uses several laser beams to generate signals that propagate along new directions, different from those of the incoming pulses. “Coherent” implies that phase information plays an important role. In this context, coherence has a dual meaning corresponding to both the radiation field and the molecules. First, the signal field is generated by many molecules whose externally driven charge distributions oscillate in phase, creating a signal field with a well-defined phase. This is in contrast to incoherent techniques such as fluorescence where individual molecules contribute additively to the signal’s intensity rather than to the amplitude, and there is no phase relation between the light emitted by different molecules. In addition to the phase of the field, the laser beams create wavepackets of molecular states; coherence thus further implies a definite phase relationship between different states. Multiple pulses can create, manipulate, and probe this coherence which carries important signatures of molecular structure, coupling patterns, and dynamics. Many modern techniques in spectroscopy make use of coherence. For example, one of the major accomplishments of nuclear magnetic resonance has been its capacity to disentangle hopelessly complicated spectra by spreading them in more dimensions onto several frequency or time axes.10 Carefully designed pulse sequences make it possible to eliminate dipolar interactions which dominate the line widths. Since their inception in the 1970s, multidimensional NMR techniques have turned into a powerful tool which provides structures with resolution comparable to X-rays, and dynamics of very complex molecules, crystals, and proteins with up to a few thousand atoms. Multidimensional spectroscopy probes correlations between signals and offers an unusual sensitivity and selectivity. Magnetic resonance is the elder brother of coherent optical spectroscopy: the two share many basic concepts. Since the ability to shape and control radiowave pulses predated similar advances in laser technology, many of the optical techniques had been invented in NMR 20-30 years earlier. By extending these ideas to the optical regime, it should be possible to come up with novel classes of spectroscopies which could probe complex vibrational motions. Picosecond coherent spectroscopy of atoms and small molecules in the gas phase is very similar to that of nuclear spins.11,12 The extension to complex molecules in the condensed phase has been a much more difficult task which requires some significant advances. The large transition dipoles and strong interactions induce line broadenings resulting from ultrashort dephasing processes, which the excitation pulses need to overcome. The necessary impulsive excitation requires laser pulses shorter than the corresponding vibrational periods and dephasing timescales. In recent years, femtosecond laser techniques have made giant strides, and Shaul Mukamel, Professor of Chemistry at the University of Rochester, NY, and at the Rochester Theory Center for Optical Science and Engineering, received the Ph.D. degree in chemical physics in 1976 from Tel-Aviv University, Israel, and held postdoctoral positions at MIT, Cambridge, and the University of California, Berkeley. His research interests include theoretical studies of ultrafast nonlinear optical spectroscopy in condensed phases, electronic excitations in conjugated and aggregated molecules and semiconductor nanostructures, and biological electron and energy transfer. He is a coauthor of 350 publications and a graduate-level textbook, Principles of Nonlinear Optical Spectroscopy; Oxford University Press, 1995. Dr. Mukamel is a fellow of the American Physical Society and of the Optical Society of America, and a recipient of the Fulbright, Alfred P. Sloan, Camille and Henry Dreyfus, Guggenheim, and Alexander von Humboldt Senior Awards.