Vibrational echoes: a new approach to condensed-matter vibrational spectroscopy

This review describes the first ultrafast infrared vibrational echo experiments, which are used to examine liquids, glasses and proteins. Like the nuclear magnetic resonance (NMR) spin echo and other NMR pulse sequences, the vibrational echo can extract dynamical and spectroscopic information that cannot be obtained from a vibrational absorption spectrum. The vibrational echo measures the homogeneous vibrational linewidth even if the absorption line is massively inhomogeneously broadened. 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 information 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 examined and compared with those in several myoglobin mutants. The results provide insights into protein dynamics and how protein structural fluctuations are communicated to a ligand bound at the active site. In addition, two new vibrational echo methods are reviewed. One method involves using multilevel vibrational coherences, which gives rise to vibrational echo beats, to measure vibrational anharmonicities and excited-state dephasing. The other method, in which a vibrational echo spectrum is taken, is demonstrated to be capable of the suppression of unwanted background that dominates the normal vibrational absorption spectrum.