13310 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J. Phys. Chem. zyxwvut 1995, 99, zyxwvu 13310-13320 FEATURE ARTICLE Infrared Vibrational Photon Echo Experiments in Liquids and Glasses A. Tokmakoff? D. Zimdars, R. S. Urdahl, R. S. Francis, A. S. Kwok, and M. D. Fayer* Department of Chemistry, Stanford University, Stanford, Califomia 94305 Received: May 12, zyxwvutsrqp 1995@ The vibrational dynamics of polyatomic solutes in polyatomic liquid and glassy solvents are examined using picosecond infrared photon echo experiments and pump-probe experiments from room temperature to 10 K. The photon echo experiments measure T2, the homogeneous dephasing time (homogeneous line shape), while the pump-probe experiments measure the vibrational lifetime, TI, and the orientational relaxation dynamics. By combining these measurements, a complete analysis of vibrational dynamics is obtained in the liquid, in the supercooled liquid, through the glass transition, and in the glass. Experiments were conducted on the asymmetric CO stretching mode of tungsten hexacarbonyl zyxwv (- 1980 cm-I) in 2-methylpentane (2-MP), 2-methyltetrahydrofuran, dibutyl phthalate (DBP), carbon tetrachloride, and chloroform. The experiments were conducted using the picosecond IR pulses from a zyxwv superconducting-accelerator-pumped free electron laser. The absorption line widths for all glasses are massively inhomogeneously broadened at low temperature. In the room temperature liquids, while the vibrational line in 2-MP is homogeneously broadened, the line in DBP is still extensively inhomogeneously broadened. The temperature dependences of the homogeneous line widths in the three glasses are a zyxwvu Tz power law. The contributions of vibrational pure dephasing, orientational relaxation, and population lifetime to the homogeneous line shape are examined in detail in the 2-MP solvent. The complete temperature dependence of each of the contributions is determined. In addition, the temperature dependence of zyxwvutsr TI is observed to be "inverted" in most of the solvents; Le., the lifetime becomes longer as the temperature is increased. of the anharmonic coupling matrix elements. I. Introduction Vibrational spectra of even large molecules reveal detailed structure, a large number of lines that can be assigned to the various types of motions that occur in a molecule. The position of the spectroscopic peak associated with a particular mode yields the energy of the vibration. However, in general, even a well-resolved vibrational line does not provide information on dynamics. In principle, dynamical information is contained in a spectroscopic line shape. Vibrational line shapes in condensed phases contain all of the details of the interactions of a normal mode with its environment. These interactions include the important microscopic dynamics, intermolecular couplings, and involve the time scales of solvent evolution that modulate the energy of a transition. However, the line shape can also include essentially static structural perturbations associated with the distribution of local solvent configurations (inhomogeneous broadening). An infrared absorption spectrum or Raman spectrum gives frequency-domain information on the ensemble-averaged interactions that couple to the states involved in the tran~ition.'-~ Line shape analysis of vibrational transitions has long been recognized as a powerful tool for extracting information on molecular dynamics in condensed pha~es.4.~ The difficulty with determining the microscopic dynamics from a spectrum arises because linear spectroscopic techniques have no method for separating the various contributions to the * To whom correspondence should be addressed. ' Present address: Physik Department, Technische Universitat Miinchen, @ Abstract published in Advance ACS Abstracts, August 15, 1995. DE5748 Garching, Germany. 0022-365419512099-133 10$09.00/0 Analysis shows that this is caused by temperature dependence vibrational line shape. The IR absorption or Raman line shape represents a convolution of the various dynamic and static contributions to the observed line shape. In some cases, polarized Raman spectra can be used to separate orientational and vibrational dynamics from the line shape, yet as with all linear spectroscopies,contributions from inhomogeneousbroad- ening cannot be eliminated.6 To completely understand a vibrational line shape, a series of experiments are required to characterize each of its static and dynamic components. These experiments can be effectively accomplished in the time domain, where well-defined techniques exist for measuring the various quantities. Nonlinear vibrational spectroscopy can be used to eliminate static inhomogeneous broadening from IR and Raman line shapes.6 Techniques such as the infrared photon and Raman echoI0-l3 can determine the homogeneous vibrational line shape which contains the important microscopic dynamics, when this line shape is masked by inhomogeneous broadening. Below we present the temperature-dependent vibrational dynamics of the triply degenerate T I , CO stretching mode (-1980 cm-l) of tungsten hexacarbonyl (W(CO)6) in the molecular glass-forming liquids 2-methyltetrahydrofuran (2- MTHF), 2-methylpentane (2-MP), and dibutyl phthalate (DBP). Two aspects of the vibrational line shape in these systems are discussed in detail. Initially, we compare the behavior of the homogeneous line widths in the glasses and the transition to the room temperature liquids for the three solvents, using picosecond IR photon echo experiments. The temperature dependence of the vibrational dephasing and the degree of 0 1995 American Chemical Society