Ultrafast Structural Changes Research Articles

The experimental visualisation of molecular structural changes during both photochemical and thermal reactions by real-time vibrational spectroscopy

Have you ever hoped to observe transition states? Chemists have long desired to monitor the deformation of molecular structures via transition states to understand the mechanisms of complicated reactions. Detailed knowledge of transition states helps find strategies to develop novel reaction schemes for introducing new functionalities to chemicals. Molecular structural changes via transition states can be observed by real-time vibrational spectroscopy using sub-5 fs laser pulses. In this paper, I report the direct observation of time-dependent frequency shifts of relevant molecular vibrational modes, which allowed for the clear visualization of ultrafast structural changes in molecules during bond breaking and bond reformation steps. Various mechanisms for photochemical reactions were clarified using sub-5 fs laser pulses. Moreover, a non-thermal vibrational excitation method for efficiently driving chemical reactions in the electronic ground state in solution with the use of broadband visible sub-5 fs laser pulses has been developed. The respective chemical reaction processes were directly observed, including transition states during not only "photochemical" but also "thermal" reactions. Time-resolved spectroscopy with a time resolution of a few femtoseconds enables observation of real-time vibrational amplitudes of complicated molecules and opens up new ways for clarifying reaction mechanisms and developing new chemical transformations.

Photo-Induced Pyridine Substitution in cis-[Ru(bpy)2(py)2]Cl2: A Snapshot by Time-Resolved X-ray Solution Scattering

Determination of transient structures in light-induced processes is a challenging goal for time-resolved techniques. Such techniques are becoming successful in detecting ultrafast structural changes in molecules and do not require the presence of probe-like groups. Here, we demonstrate that TR-WAXS (Time-Resolved Wide Angle X-ray Scattering) can be successfully employed to study the photochemistry of cis-[Ru(bpy)(2)(py)(2)]Cl(2), a mononuclear ruthenium complex of interest in the field of photoactivatable anticancer agents. TR-WAXS is able to detect the release of a pyridine ligand and the coordination of a solvent molecule on a faster timescale than 800 ns of laser excitation. The direct measurement of the photodissociation of pyridine is a major advance in the field of time-resolved techniques allowing detection, for the first time, of the release of a multiatomic ligand formed by low Z atoms. These data demonstrate that TR-WAXS is a powerful technique for studying rapid ligand substitution processes involving photoactive metal complexes of biological interest.

Ultrafast structural changes in SrTiO3 due to a superconducting phase transition in a YBa2Cu3O7 top layer

We investigate the structural response of SrTiO3 when Cooper pairs are broken in an epitaxially grown YBa2Cu3O7 top layer due to both heating and optical excitation. The crystal structure is investigated by static, temperature-dependent and time-resolved x-ray diffraction. In the static case, a large strain field in SrTiO3 is formed in the proximity of the onset of the superconducting phase in the top layer, suggesting a relationship between both effects. For the time-dependent studies, we likewise find a large fraction of the probed volume of the SrTiO3 substrate strained if the top layer is superconducting. Upon optical breaking of Cooper pairs, the observed width of the rocking curve is reduced and its position is slightly shifted towards smaller angles. The dynamical theory of x-ray diffraction is used to model the measured rocking curves. We find that the thickness of the strained layer is reduced by about 200 nm on a sub-ps to ps timescale, but the strain value at the interface between SrTiO3 and YBa2Cu3O7 remains unaffected.

Picosecond electron diffraction from molecules aligned by dissociation

In gas electron diffraction an averaging over statistical directions of the molecules takes place. This results in diffraction patterns in the form of isotropic rings which yield information only on the radial distribution function displaying the inter-atomic distances. We demonstrate that by dissociating molecules with linearly polarized light the pattern becomes anisotropic. In the experiments the iodide C 2F 4I 2 is dissociated and molecular difference intensities and difference radial distribution curves are measured for directions parallel and perpendicular to the direction of polarization. With picosecond temporal resolution the curves clearly demonstrate transient anisotropy and its decay by molecular rotation. This experiment is a first step towards the determination of structure and of ultrafast structural changes by electron diffraction from aligned molecules.

フェムト秒レーザー励起を用いたGeSbTeにおける超高速相変化の誘起

フェムト秒レーザー励起を用いたGeSbTeにおける超高速相変化の誘起

Femtosecond characterization of vibrational optical activity of chiral molecules

Optical activity is the result of chiral molecules interacting differently with left versus right circularly polarized light. Because of this intrinsic link to molecular structure, the determination of optical activity through circular dichroism (CD) spectroscopy has long served as a routine method for obtaining structural information about chemical and biological systems in condensed phases. A recent development is time-resolved CD spectroscopy, which can in principle map the structural changes associated with biomolecular function and thus lead to mechanistic insights into fundamental biological processes. But implementing time-resolved CD measurements is experimentally challenging because CD is a notoriously weak effect (a factor of 10(-4)-10(-6) smaller than absorption). In fact, this problem has so far prevented time-resolved vibrational CD experiments. Here we show that vibrational CD spectroscopy with femtosecond time resolution can be realized when using heterodyned spectral interferometry to detect the phase and amplitude of the infrared optical activity free-induction-decay field in time (much like in a pulsed NMR experiment). We show that we can detect extremely weak signals in the presence of large achiral background contributions, by simultaneously measuring with a femtosecond laser pulse the vibrational CD and optical rotatory dispersion spectra of dissolved chiral limonene molecules. We have so far only targeted molecules in equilibrium, but it would be straightforward to extend the method for the observation of ultrafast structural changes such as those occurring during protein folding or asymmetric chemical reactions. That is, we should now be in a position to produce 'molecular motion pictures' of fundamental molecular processes from a chiral perspective.

Singlet Excited-State Dynamics of Nitropolycyclic Aromatic Hydrocarbons: Direct Measurements by Femtosecond Fluorescence Up-Conversion

Understanding the dynamics of the electronically excited states of nitrated polycyclic aromatic hydrocarbons (NPAHs) is of great importance since photochemical reactions determine the atmospheric stability of these toxic pollutants. From previous studies, it is known that electronically excited NPAHs evolve through two parallel pathways: The formation of the first triplet state and the dissociation of nitrogen (II) oxide. In this contribution, we present the first time-resolved emission measurements of the singlet excited states which are the precursors in the aforementioned photoprocesses. We analyzed 1-nitronaphthalene, 9-nitroanthracene, 1-nitropyrene, 6-nitrochrysene, and 3-nitrofluoranthene in solution samples. Although these compounds are considered nonfluorescent, with the frequency up-conversion method it was possible to detect the emission from the S1 states despite their femtosecond and picosecond lifetimes. Except for 1-nitronapthalene, where a single exponential is observed, for the rest of the compounds, the emission shows double-exponential decays indicating ultrafast structural changes in the excited states. From anisotropy measurements, we conclude that no significant internal conversion occurs in the singlet manifold after excitation in the first absorption band. In accord with El-Sayed rules and with previous calculations, the highly efficient intersystem crossing implied by the large triplet yields and the ultrafast S1 decays is accounted by the pi-pi* nature of the S1 and T1 states together with the existence of higher triplet configurations which act as receiver states. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule.

Recent Progress in Ultrafast X‐ray Diffraction

X-ray diffraction with femtosecond time-resolution represents a direct probe of ultrafast structural changes in condensed matter. The generation of ultrashort X-ray pulses in laser-driven plasma and/or accelerator-based sources has made substantial progress, and has allowed for studies of transient structures with an unprecedented accuracy. Herein, recent work on transient crystalline structures is reviewed, with the focus on laser-based experiments.

Selective cap opening in carbon nanotubes driven by laser-induced coherent phonons.

We demonstrate the possibility of a selective nonequilibrium cap opening of carbon nanotubes as a response to femtosecond laser excitation. By performing molecular dynamics simulations based on a microscopic electronic model we show that the laser-induced ultrafast structural changes differ dramatically from the thermally induced dimer emission. Ultrafast bond weakening and simultaneous excitation of two coherent phonon modes of different frequencies, localized in the spherical caps and cylindrical nanotube body, are responsible for the selective cap opening.

Femtosecond silicon K alpha pulses from laser-produced plasmas.

Ultrashort bursts of silicon K alpha x-ray radiation from femtosecond-laser-produced plasmas have been generated. A cross-correlation measurement employing a laser-triggered ultrafast structural change of a CdTe crystal layer (320 nm) shows a K alpha pulse duration between 200 fs and 640 fs. This result is corroborated by particle in cell simulations combined with a Monte-Carlo electron stopping code and calculations on the structural changes of the crystal lattice.

Femtosecond time-resolved Raman spectroscopy using stimulated Raman scattering

Femtosecond Raman spectroscopy has been developed to investigate ultrafast photoinduced structural changes of materials. Vibrational modes in the photogenerated transient species are measured by stimulated Raman scattering using a Raman pump pulse with narrow bandwidth and a femtosecond supercontinuum probe pulse. The Raman signal can be measured without slowing the temporal response and broadening the spectrum, because the temporal and spectral resolutions of the present method can be improved independently without the restriction of the transform limit. The transient Raman spectra of the trans- cis photoisomerization process in the DCM (4-dicyanomethylene-2-methyl-6-$p$-dimethylaminostyryl-4H-Pyran) dye solution were observed with the resolutions of 250 fs and $25{\mathrm{cm}}^{\ensuremath{-}1}.$

Ultrafast structural changes measured by time-resolved X-ray diffraction

High-intensity X-ray pulses from third-generation synchrotron sources make it possible to study the temporal dynamics of rapidly evolving materials. We report a study of rapid and reversible disordering of the structure of an InSb crystal induced by an ultrashort laser pulse. A novel cross-correlation detection technique is described, which allowed us to observe rapid changes in X-ray diffraction that occur on a time-scale of less than 2 ps.

Clocking transient chemical changes by ultrafast electron diffraction

With the advent of femtosecond (fs) time resolution in spectroscopic experiments, it is now possible to study the evolution of nuclear motions in chemical and photobiochemical reactions. In general, the reaction is clocked by an initial fs laser pulse (which establishes a zero of time) and the dynamics are probed by a second fs pulse; the detection methods include conventional and photoelectron spectroscopy and mass spectrometry. Replacing the probe laser with electron pulses offers a means for imaging ultrafast structural changes with diffraction techniques, which should permit the study of molecular systems of greater complexity (such as biomolecules). On such timescales, observation of chemical changes using electron scattering is non-trivial, because space-charge effects broaden the electron pulse width and because temporal overlap of the (clocking) photon pulse and the (probe) electron pulse must be established. Here we report the detection of transient chemical change during molecular dissociation using ultrafast electron diffraction. We are able to detect a change in the scattered electron beam with the zero of time established unambiguously and the timing of the changes clocked in situ. This ability to clock changes in scattering is essential to studies of the dynamics of molecular structures.

Theory for the ultrafast structural response of optically excited small clusters: Time dependence of the ionization potential.

Combining an electronic theory with molecular-dynamics simulations we present results for the ultrafast structural changes in small clusters. We determine the time scale for the change from the linear to a triangular structure after the photodetachment process ${\mathrm{Ag}}_{3}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${\mathrm{Ag}}_{3}$. We show that the time-dependent change of the ionization potential reflects in detail the internal degrees of freedom, in particular coherent and incoherent motion, and that it is sensitive to the initial temperature. We compare with experiment and point out the general significance of our results. \textcopyright{} 1996 The American Physical Society.