Department of Chemistry

It is often assumed that the anisotropy in time-resolved fluorescence and pump-probe experiments is always smaller than or at most equal to 2/5. From resonance Raman experiments it is well known, however, that the anisotropy can range anywhere from 1 to -l/2 (depolarization ratio between 0 and 00) depending on the symmetry of the final state in the Raman transition. Since the resonance Raman intensity depends on the short-time vibrational dynamics in the excited state potential, it is to be expected that similar anisotropies will be observed in femtosecond pump-probe experiments. In this article the anisotropies in ultrashort pulse pump-probe experiments will be calculated and the original results, well known from the theory of resonance Raman, are recovered for zero delay times. In time-resolved pump-probe experiments there is a new effect, however, that is not present in resonance Raman scattering: for increasing delay times the anisotropy is expected to decay due to the electronic dephasing of the degenerate electronic states involved. Therefore measurement of the femtosecond anisotropy decay enables one to obtain electronic dephasing times in condensed phases.

The low-frequency (1-200 cm ÿ1 ) vibrational spectra of peptides and proteins in solution have been investigated with ultrafast optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). Spectra have been obtained for di-L-alanine (ALA(2)) and the a-helical peptide poly-L-alanine (PLA) in dichloroacetic acid solution. The poly-L-alanine spectrum shows extra amplitude compared to the di-L-alanine spectrum, which can be explained by the secondary structure of the former. The globular proteins lysozyme, a-lactalbumin, pepsin, and b-lactoglobulin in aqueous solution have been studied to determine the possible influence of secondary or tertiary structure on the low-frequency spectra. The spectra of the globular proteins have been analyzed in terms of three nondiffusive Brownian oscillators. The lowest frequency oscillator corresponds to the so-called Boson peak observed in inelastic neutron scattering (INS). The remaining two oscillators are not observed in inelastic neutron scattering, do therefore not involve significant motion of hydrogen atoms, and may be associated with delocalized backbone torsions.

In femtosecond terahertz-pulse (T-ray) imaging of metal structures with dimensions of the order of the wavelength, it is observed that the T rays propagate faster than the vacuum speed of light. In the case of apertures this can be understood as a waveguide effect in which superluminal velocities are expected close to the cutoff frequency. However, the effect is also observed close to knife edges and in propagation past thin metal wires.

Ultrafast time-resolved anisotropy experiments in which electronically excited [5-( 1',4'-benzoquinonyl)-10,15,20-triphenylporphinato]magnesium (MgPQ) was probed with polarization both parallel and perpendicular to the pump pulse have been carried out in a variety of solvents. These results show that the photoinduced electron transfer process involving the locally-excited MgP*Q state is solvent independent, while the thermal charge recombination reaction is solvent dependent. Moreover, these studies indicate that the photoinduced charge separation reaction may be rate-limited by electronic population equilibration between near degenerate porphyrin Q and Q, levels. @ (a) Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A,; Lehman, W. R.; Nemeth, G. A.; Moore, A. L. Nature 1984, 307, 630-632. (b) Gust, D.; Moore, T. A.; Liddell, P. A.; Nemeth, G. A,; Makings, L. R.; Moore, A. L.; Barrett, D.; Pessiki, P. J.; Bensasson, R. V.; Rougee, M.; Chachaty, C.; De Schryver, F. C.; Van der Auweraer, M.; Holzwarth, A. R.; Connolly, J. S.

High-power femtosecond laser pulses have been used to excite surface plasmons in 500 Å silver and gold films. Nonlinear excitation results in the emission of electron bunches through multiphoton excitation at low power and laser-induced field emission at high power. The energies of photoelectrons are found to extend as high as 0.4 keV. Calculations show that these high energies are due to ponderomotive acceleration in an evanescent field extending from the metal film out into the vacuum. The theoretical calculations suggest that femtosecond electron pulses with relativistic energies can be generated using longer wavelengths or by developing the surface morphology.

Propagation of single-cycle terahertz pulses through and past wavelength-sized metal structures has been studied experimentally. In waveguides close to cutoff, it is found that the phase velocity can become superluminal and even negative. Multiple reflections of evanescent waves inside the waveguide are found to be the cause of a negative phase velocity below the cutoff frequency. The centroid delay of terahertz pulses propagating past a thin metal wire is found to be advanced or delayed depending on the polarization with respect to the wire. In all cases of superluminal propagation described here, the principle of causality is preserved. In a restricted sense, exchange of information faster than the speed of light is found possible, however, the principle of causality ensures that information cannot advance by more than the inverse bandwidth of the signal. This eliminates causal-loop paradoxes and ensures that faster-than-light communication is not practical. q 2000 Elsevier Science B.V. All rights reserved. PACS: 42.25.Bs; 73.40.Gk; 42.65.Re; 03.65.Bz 0030-4018r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 3 0 -4 0 1 8 0 0 0 0 5 4 2 -3 ( ) K. Wynne et al.r Optics Communications 176 2000 429-435 430

We have developed a method of measuring the growth of nanoparticles during sol-gel glass formation based on labelling the particle with a fluorescent dye and determining the multiphoton excited decay of fluorescence anisotropy due to Brownian rotation. Multiphoton excitation is shown to give a higher dynamic range of measurement than one-photon excitation. We illustrate the sub-nanometre resolution and stability of our approach by detecting a 0.8-1.1 nm silica particle hydrodynamic mean radius increase in a tetramethylorthosilicate sol at pH 2.3 labelled with rhodamine 6G and observed over ≈4 weeks and also with a stable silica colloid of radius 6 nm, pH 8.9, labelled with a 6-methoxyquinoline-type dye.

Changes in the ultrafast dynamics and terahertz Raman spectrum accompanying a helix-to-coil transition of a homo-polypeptide have been observed for the first time. Formation of the R-helix is associated with a shift to lower frequency of a broad Raman band attributable to solvent-peptide intermolecular hydrogen bonding. This band facilitates direct spectroscopic observation of so-called hydration water near a peptide and yields the first quantitative estimate of the time scale of the ultrafast dynamics in the solvation shell, which range from 0.18 to 0.33 ps (185-100 cm -1 ) depending on the secondary structure of the peptide. Such fast motions of solvent molecules have been referred to as the "lubricant of life" and are thought to play key roles in determining structure and activity of proteins.

The photodissociation dynamics of the acetone S 2 (n, 3s) Rydberg state excited at 195 nm has been studied by using femtosecond pump-probe mass-selected multiphoton ionization spectroscopy. For the first time, the temporal evolutions of the initial state, intermediates, and methyl products were simultaneously measured and analyzed for this reaction to elucidate the complex dynamics. Two mechanisms were considered: (1) the commonly accepted mechanism in which the primary dissociation occurs on the first triplet-state surface, and (2) the recently proposed mechanism in which the primary dissociation takes place on the first singlet-excitedstate surface. Our results and analyses supported the validity of the new mechanism. On the other hand, the conventional mechanism was found to be inadequate to describe the observed dynamics. The temporal evolution of methyl products arising from the secondary dissociation of hot acetyl intermediates exhibited a very complex behavior that can be ascribed to the combination of a nonuniform initial vibrational distribution and the competition between dissociation and slow intramolecular vibrational redistribution.

Structural relaxation in the peptide model N-methylacetamide (NMA) is studied experimentally by ultrafast optical Kerr-effect spectroscopy over the normal-liquid temperature range and compared to the relaxation measured in water at room temperature. It is seen that in both hydrogen-bonding liquids, relaxation is present and in each case it is found that this can be described by the Cole-Cole function. For NMA in this temperature range, the and relaxations are each found to have an Arrhenius temperature dependence with indistinguishable activation energies. It is known that the variations on the Debye function, including the Cole-Cole function, are unphysical, and we introduce two general modifications: one allows for the initial rise of the function, determined by the librational frequencies, and the second allows the function to be terminated in the relaxation.

When salts are added to water, the viscosity generally increases suggesting the ions increase the strength of the water's hydrogen-bond network. However, infrared pump-probe measurements on electrolyte solutions have found that ions have no influence on the rotational dynamics of water molecules implying no enhancement or breakdown of the hydrogen-bond network. Here we report optical Kerr-effect and dielectric relaxation spectroscopic measurements, which have enabled us to separate the effects of rotational and transitional motions of the water molecules. These data show that electrolyte solutions behave like a supercooled liquid approaching a glass transition in which rotational and translational molecular motions are decoupled. It is now possible to understand previously conflicting viscosity data, nuclear magnetic resonance relaxation, and ultrafast infrared spectroscopy in a single unified picture.

A resonant "incoherent" rectification process is presented relying on the excitation of surface plasmons on a nanostructured metal surface. Excitation of gold and silver films with 800-nm femtosecond laser pulses results in the emission of terahertz radiation with an angle-dependent efficiency and an approximately third-order power dependence. It is shown that the source of this terahertz pulse generation is the surface-plasmonassisted multiphoton ionization and ponderomotive acceleration in the evanescent field of the surface plasmon. Simple models are used to understand the forces and dynamics near the surface.

We report the generation of ultra-violet fluorescence from linear and cyclic saturated hydrocarbons in neat solution when excited with femtosecond near-infrared laser pulses at 800 nm. Fluorescence decay, spectra and power dependence studies are reported. Steady-state and time-resolved experiments are in agreement with previously reported data obtained with one-photon vacuum ultra-violet, X-ray, electron impact and two-photon excitation. Our results indicate that the mechanisms leading to the fluorescing excited state are multi-photon absorption and ionisation with the latter followed by diffusion-controlled geminate cation-electron recombination. q 1999 Elsevier Science B.V. All rights reserved. 0009-2614r99r$ -see front matter q 1999 Elsevier Science B.V. All rights reserved.