Time-resolved Diffraction Techniques Research Articles

Monitoring molecular nonadiabatic dynamics with femtosecond X-ray diffraction

Ultrafast time-resolved X-ray scattering, made possible by free-electron laser sources, provides a wealth of information about electronic and nuclear dynamical processes in molecules. The technique provides stroboscopic snapshots of the time-dependent electronic charge density traditionally used in structure determination and reflects the interplay of elastic and inelastic processes, nonadiabatic dynamics, and electronic populations and coherences. The various contributions to ultrafast off-resonant diffraction from populations and coherences of molecules in crystals, in the gas phase, or from single molecules are surveyed for core-resonant and off-resonant diffraction. Single-molecule [Formula: see text] scaling and two-molecule [Formula: see text] scaling contributions, where N is the number of active molecules, are compared. Simulations are presented for the excited-state nonadiabatic dynamics of the electron harpooning at the avoided crossing in NaF. We show how a class of multiple diffraction signals from a single molecule can reveal charge-density fluctuations through multidimensional correlation functions of the charge density.

Diffraction-Detected Sum Frequency Generation: Novel Ultrafast X-ray Probe of Molecular Dynamics.

We propose a novel time-resolved diffraction technique based on sum frequency generation that combines an optical pump with an X-ray stimulated Raman probe. Simulations are presented for formyl fluoride, which is nonchiral in the ground state and evolves into a chiral nonplanar structure in the first excited state upon excitation by a circularly polarized UV pump. A coherently controlled elliptically polarized pump is used to prepare the molecule in a selected enantiomer and the chiral interconversion dynamics is then monitored by the probe diffraction.

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Solvent exchange in a metal-organic framework single crystal monitored by dynamic in situ X-ray diffraction.

Understanding the processes by which porous solid-state materials adsorb and release guest molecules would represent a significant step towards developing rational design principles for functional porous materials. To elucidate the process of liquid exchange in these materials, dynamic in situ X-ray diffraction techniques have been developed which utilize liquid-phase chemical stimuli. Using these time-resolved diffraction techniques, the ethanol solvation process in a flexible metal-organic framework [Co(AIP)(bpy)0.5(H2O)]·2H2O was examined. The measurements provide important insight into the nature of the chemical transformation in this system including the presence of a previously unreported neat ethanol solvate structure.

Optically excited structural transition in atomic wires on surfaces at the quantum limit.

Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.

Streak-camera reflection high-energy electron diffraction for dynamics of surface crystallography

A new technique for ultrafast dynamics of surface crystallography was developed by combining reflection high-energy electron diffraction with the electron deflectors of a streak camera system. A one-dimensional distribution of electrons scattered by a crystal surface is selected by a linear slit on a screen, and then the electrons are quickly deflected by the sweep electrodes behind the slit. Thus, a temporal evolution of the one-dimensional diffraction pattern can be displayed as a streak image on a screen. This is a unique method of time-resolved electron diffraction, as a pulsed electron beam is not required to obtain a temporal evolution. The temporal evolution of the diffraction pattern can be projected on a screen from single-shot measurements. The technique was tested on an Si(111)-7 × 7 surface, and the dynamics of the surface structure were successively obtained from changes in spot intensities. Although the present time time-resolution was limited by the present pumping laser ~ 5 ns, the nominal resolution of the streak system is expected to be ~100 ps. • We have developed a new time resolved diffraction technique by combining RHEED and the streak camera. • The technique is very unique because temporal evolution of the diffraction is available without a short pulse electron beam. • Time evolution of the 1D diffraction pattern for Si(111)-7 × 7 surface was successfully obtained as streak image.

X-ray Fingerprinting of Chemical Intermediates in Solution

Diffraction methods involving the scattering of particles or photons have been widely successful in determining the structures of molecules. Time-resolved diffraction techniques have been especially useful for observing fast structural changes and short-lived chemical intermediate states in solids and in the gas phase. The solution environment, which is of greatest interest in biology and industrial chemical processing, has not been amenable to diffraction methods, however. In their Perspective, [Anfinrud and Schotte][1] discuss results reported in the same issue by [ Ihee et al .][2] in which short pulses of x-rays were used to take the fingerprint of an intermediate molecular structure in solution. The ability to probe molecules in solution should offer many new insights into previously unexplored liquid-phase phenomena. [1]: http://www.sciencemag.org/cgi/content/full/309/5738/1192 [2]: http://www.sciencemag.org/cgi/content/short/309/5738/1223

Excited-state structure by time-resolved X-ray diffraction.

X-ray crystallography has traditionally been limited to the study of the ground-state structure of molecules and solids. Recent technical advances are removing this limitation as demonstrated here by a time-resolved stroboscopic study of the photo-induced 50 micros lifetime excited triplet state of the [Pt(2)(pop)(4)](4-)ion [pop = pyrophosphate, (H(2)P(2)O(5))(2-)], performed at helium temperatures with synchrotron radiation. The shortening of the Pt-Pt bond by 0.28(9)A upon excitation is compatible with the proposed mechanism involving promotion of a Pt-Pt antibonding dsigma* electron to a weakly bonding p orbital. The contraction is accompanied by a 3 degree molecular rotation. The time-resolved diffraction technique described here is applicable to reversible light-driven processes in the crystalline solid state.

Changes in the X-ray diffraction pattern from rigor muscles by application of external length changes

The effect of external force on the X-ray pattern from frog muscles in rigor was studied by a time-resolved diffraction technique. When sinusoidal length changes (1.5-3% of the muscle length, 5 Hz) were applied to the muscle, the 14.3 nm intensity decreased during the releasing phase and increased during the stretching phase. The intensity ratio of the equatorial 1.0 and 1.1 reflections did not change, nor were there any appreciable intensity changes in the 5.9 nm and 5.1 nm reflections during the length change. Experiments were also done with the relaxed muscles and no change was seen in any reflection, indicating that the rigor linkages are needed to produce the 14.3 nm intensity change. Thus the distinct effect of the length change was detected only on the 14.3 nm reflection. These results suggest no large conformational changes are induced in both the distal part of the myosin head attached to actin and the actin filament during the oscillation. It is therefore most probable that the proximal portion of myosin heads including S-2 contributes to the intensity change in response to the length change (see, also ref. 21). When the muscle was stretched beyond the filament overlap, the 14.3 nm intensity change was suppressed to less than 50% of that of the slack length. It was also found that the tension change delayed the intensity change during the length oscillation. However, this delay of the tension change as observed in the muscle at the slack length was lacking in the overstretched muscle, indicating that the 14.3 nm intensity change may arise partly from a portion other than the crossbridges.

X-ray-diffraction studies of acoustoelectrically amplified phonons

X-rays diffracted by nearly perfect crystals of $n$-type InSb have been investigated in the presence of intense acoustoelectrically (AE) amplified phonons. The fact that these phonons are restricted to a relatively narrow frequency band and have a well-defined propagation and polarization direction presents an excellent opportunity to investigate the nature of x-ray-photon---phonon scattering in a diffracting crystal. In a Bragg-reflection scattering geometry, the intense amplified phonons give rise to satellite peaks symmetrically located about the central elastic Bragg peak in a rocking profile, from which the phonon wave vectors are obtained. In the Laue transmission geometry the attenuation of anomalous transmission is used as a sensitive probe of nonequilibrium phonons. It is found that for AE phonons it is not adequate to multiply the structure factor by a Debye-Waller factor, unlike the case for thermal phonons. In addition to the fast-transverse-acoustic (FTA) branch, the only one predicted to be amplified by the theory of the AE effect, other weaker modes are found to be present, with polarization directions corresponding to the slow-transverse-acoustic branch and the longitudinal-acoustic branch. A time-resolved diffraction technique, with a resolution time of 0.5 \ensuremath{\mu}sec, has allowed us to monitor the growth of the acoustic flux as a function of time, and to conclude that the non-[110]-FTA phonons result in part from mode conversion at the end wall of the crystal, and in part from direct conversion within the forward-traveling acoustic domain.