Manho Lim

Vibrational energy relaxation of the cyanide ion in water

The vibrational relaxation time of the cyanide ion in H2O and in D2O was measured by IR-pump–IR-probe experiments. The isotopic composition of the ion was varied in order vary the oscillation frequency of the CN− vibrational mode. In D2O, the vibrational relaxation rate is accelerated from 120 to 71 ps when increasing the vibrational frequency from 2004 cm−1 (13C15N) to 2079 cm−1 (12C14N−). In H2O, time constants between 31 and 28 ps were observed. The systematic dependence of the relaxation rates on the vibrational frequency provides a small portion of the friction spectrum. A significant correlation between vibrational relaxation time of the solute and the IR absorption cross section of the solvent was found, providing experimental evidences for a dominating contribution to vibrational relaxation of Coulomb interactions and the importance of coupling to internal solvent modes. In addition, the infrared bandwidths and the orientational diffusion times are reported. All experimental observables T1, T2, an...

Pump/probe self heterodyned 2D spectroscopy of vibrational transitions of a small globular peptide

Pump/probe self-heterodyne experiments on the amide I band of a small de novo cyclic pentapeptide were utilized to demonstrate a novel form of two-dimensional (2D) vibrational spectroscopy. Spectrally resolved cross peaks are observed, which measure the coupling between different peptide units and which can be related to the structure of the peptide, in analogy to 2D-NMR spectroscopy. In contrast to our previous work, these experiments work in the time domain in the semiimpulsive limit, employing two intense ultrashort infrared laser pulses. A theoretical formalism is presented in order to model the interstate coherent wave packet generated by the excitation pulse and the resulting spectroscopic signal. The observed coherences provide an independent proof of excitonic coupling within the amide I manifold of the peptide backbone.

Unusual vibrational dynamics of the acetic acid dimer

The vibrational relaxation of the C=O stretching mode of the CH3CO2H cyclic dimer, the CH3CO2D cyclic dimer, and CH3CO2CH3 were measured in CCl4 solution at room temperature. The population relaxation of the v=1 state of the C=O mode is nonexponential, modeled with a biexponential decay having a fast time constant in the subpicosecond regime and a slow time constant of a few picoseconds. For the cyclic dimers of the acetic acids, the fast component dominates the population decay, whereas the slow component dominates the decay of the CH3CO2CH3, the model compound for the monomeric acetic acid. Deuteration of the dimer increases the relaxation time constant. The non-hydrogen-bonding monomer methyl acetate also has a subpicosecond decay constant. The pump–probe anisotropy decay reveals that the orientational dynamics of these molecules also occurs on the subpicosecond time scale and is reasonably well described by rotational diffusion in the slip hydrodynamic limit. Stimulated infrared photon echo decay expe...

Spectrally resolved three pulse photon echoes in the vibrational infrared

Abstract Spectrally resolved vibrational photon echo measurements of the N 3 − ion in D 2 O and carbonmonoxy hemoglobin in D 2 O have been made using 100 fs infrared pulses in the 1800–2100 cm −1 region. These measurements allow the separation of contributions from different Feynman pathways to the echo signal, allowing comparisons of the time dependence and correlation functions for the 1–0 and 2–1 coherence terms. The spectra show good agreement with a theoretical description of the data fitted to previously determined multiexponential correlation functions. Fluctuations in the anharmonicity are found to be negligible for HbCO when the harmonic vibrational relaxation model is used.

CHARGE SHIFTING IN THE ULTRAFAST PHOTOREACTIONS OF CLO- IN WATER

The reaction dynamics of ClO− in water following femtosecond ultraviolet photolysis is investigated by measuring time-resolved absorption and anisotropy. Ab initio calculations show that light absorption induces charge shifting from the O− atom to the Cl atom. Molecular dynamics simulations predict that the charge shift is followed by the destruction of the solvent structure around the O atom and its formation around newly formed negative charge on the Cl atom. An ultrafast (∼60 fs) transient absorption change is observed and likely corresponds to the inertial part of the destruction of the solvent structure around the newly formed neutral O atom of the excited state OCl−. The early time anisotropy of −0.13±0.05 decays on the 230 fs time scale and is attributed to the dissociation along a new reaction path toward Cl+O− that is seen independently through the evaluation of the spectrum of Cl. The remaining anisotropy decays within 6 ps due to rotational diffusion of the ion. Probe wavelength dependence of t...

Chirped wavepacket dynamics of HgBr from the photolysis of HgBr2 in solution

Abstract Wavepacket dynamics of the HgBr radical produced by the impulsive photodissociation of HgBr 2 in solution was investigated using femtosecond absorption spectroscopy. Whereas the photolysis in CH 3 CN shows conventional damped wavepacket dynamics of HgBr, in dimethyl sulfoxide a chirped wavepacket with a nonexponential decay is observed. The chirp, which is attributed to the modification of the HgBr potential as a result of solvation occurring on the timescale of the wavepacket motion, provides microscopic insight into the structural and dynamical properties of the solvation. The effects on the observed dynamics from the presence of two wavepackets is also considered.

The fifth-order contribution to the oscillations in photon echoes of anharmonic vibrators

Abstract Vibrational photon echo experiments on the CO stretching mode of carbon monoxide bound to hemoglobin revealed pronounced oscillations with a beat frequency which corresponds to the anharmonicity of the vibrator. Intensity-dependent measurements show that the oscillatory signal is due to interference between the third- and fifth-order polarization. Such oscillations cannot occur in the third-order signal unless the inhomogeneous width exceeds the anharmonicity. An expression is derived which simultaneously describes the intensity dependence, the phase, and the damping of such oscillatory signals.

Energy and phase relaxation accompanying impulsive reactions in liquids

Experiments made possible by new laser methods explore ultrafast features of solution phase reaction dynamics. The results from two examples are given. Photodissociation of small molecules such as and in solution yields highly excited vibrational state distributions and narrow bond length distributions. The dynamics in the transition state region and the ensuing dynamics can be followed by femtosecond laser experiments that probe the phase and energy loss processes. The mercuric iodide experiments show that the phase relaxation is dominated by coherence transfer processes which have a strong analogy to classical motions. The population relaxation dynamics gives results similar to those from classical molecular dynamics simulations which suggest, notwithstanding the significant polarity of the molecules and solvents, that the relaxations are dominated by Lennard - Jones repulsion at sufficiently low frequencies.

Chemical reaction dynamics of some simple molecules in solution

Solvent is a critical component of reactive processes occurring in solutions. Examples of solvent effects in charge shifting (CIO−→CI−O), vibration–rotation interactions (HgI) and geminate dynamics following disulphide bond dissociation are described.

Femtosecond Dynamics, Two Dimensional Infrared Spectroscopy and Echoes of Protein Vibrations

Two approaches that use femtosecond infrared pulses to examine protein dynamics by means of nonlinear IR spectroscopy are described. In the first we dissect the featureless peptide amide-I band and introduce two-dimensional IR spectra [1]. In the second, the correlation function of the frequency fluctuations of a vibrational mode in the active site of carbonic anhydrase and heme pocket of hemoglobin is determined by three-pulse IR photon echoes [2]. These experiments permit the study of ultrafast dynamics without the requirement of chromophores.