VIBRATIONAL ECHOES - A NEW APPROACH TO CONDENSED-MATTER VIBRATIONAL SPECTROSCOPY

This review describes the first ultrafast infrared vibrational echo ex periments, which are used to examine liquids, glasses and proteins. Li ke the nuclear magnetic resonance (NMR) spin echo and other NMR pulse sequences, the vibrational echo can extract dynamical and spectroscopi c information that cannot be obtained from a vibrational absorption sp ectrum. The vibrational echo measures the homogeneous vibrational line width even if the absorption line is massively inhomogeneously broaden ed. When combined with pump-probe (transient absorption) experiments, the homogeneous pure dephasing (energy level fluctuations) is obtained . Conducting these experiments as a function of temperature provides i nformation on dynamics and intermolecular interactions. The nature of the method and the experimental procedures are outlined. Experimental results are presented for the metal carbonyl solutes, W(CO)(6) and Rh( CO)(2)acac, in several glassy and liquid solvents. The dynamics of the CO ligand bound at the active site of the protein myoglobin are exami ned and compared with those in several myoglobin mutants. The results provide insights into protein dynamics and how protein structural fluc tuations are communicated to a ligand bound at the active site. In add ition, two new vibrational echo methods are reviewed. One method invol ves using multilevel vibrational coherences, which gives rise to vibra tional echo beats, to measure vibrational anharmonicities and excited- state dephasing. The other method, in which a vibrational echo spectru m is taken, is demonstrated to be capable of the suppression of unwant ed background that dominates the normal vibrational absorption spectru m.