Ultrafast surface dynamics

STM has not as yet proved to be easily applicable to the area of ultrafast surface phenomena. Nevertheless, some success has been achieved in the direct observation of dynamic processes with a larger timescale. Kitamura et al [23], using a high-temperature STM to scan single lines repeatedly and to display the results as a time-ver.sn.s-position pseudoimage, were able to follow the difflision of atomic-scale vacancies on a heated Si(OOl) surface in real time. They were able to show that vacancy diffusion proceeds exclusively in one dimension, along the dimer row. 

Bonn M, Denzler DN, Eunk S, Wolf M. 2000. Ultrafast electron dynamics at metal surfaces Competition between electron-phonon coupling and hot-electron transport. Phys Rev B 61 1101-1105. 

Fig. 1.1. Schematic view of the Coulomb explosion imaging of nuclear dynamics. Molecules exposed to an intense laser field undergo structural deformation in response to the formation of light-dressed potential energy surfaces, and decompose into fragment ions after multiple ionization. Since the momentum vectors of fragment ions sensitively reflect the geometrical structure just before the Coulomb explosion, the ultrafast nuclear dynamics of a molecule in an intense laser field can be elucidated through measurements of the momenta of fragment ions...

Abstract A challenging task in surface science is to unravel the dynamics of molecules on surfaces associated with, for example, surface molecular motion and (bimolecular) reactions. As these processes typically take place on femtosecond time scales, ultrafast lasers must be used in these studies. We demonstrate two complementary approaches to study these ultrafast molecular dynamics at metal surfaces. In the first, the molecules are studied after desorbing from the surface initiated by a laser pulse using the so called time-of-flight technique. In the second approach, molecules are studied in real time during their diffusion over the surface by using surface-specific pump-probe spectroscopy. 

Lane IM, Liu Z-P, King DA, Arnolds H (2007) Ultrafast vibrational dynamics of NO and CO adsorbed on an iridium surface. J Phys Chem C 111 14198... 

Surface Femtochemistry and Ultrafast Carrier Dynamics, led by Christian Frischkom (PhD in Physics 1997, Universitat Gottingen, adviser Udo Buck at FHI since 2008 previously Group Leader at the Freie Universitat Berlin)... 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

A second problem in these studies concerns cavitation dynamics on the nanometer length scale [86]. If sufficiently energetic, the ultrafast laser excitation of a gold nanoparticle causes strong nonequilibrium heating of the particle lattice and of the water shell close to the particle surface. Above a threshold in the laser power, which defines the onset of homogeneous nucleation, nanoscale water bubbles develop around the particles, expand, and collapse again within the first nanosecond after excitation (Fig. 9). The size of the bubbles may be examined in this way. 

Imaging of surface plasmon and ultrafast dynamics in gold nanorods by near-field microscopy. J. Phys. Chem. B, 108, 16344-16347. 

Matsumoto, Y. (2007) Photochemistry and photo-induced ultrafast dynamics at metal surfaces. Bull. Chem. Soc.Jpn., 80,842-855. 

Recently, Eisenthal and coworkers have developed time-resolved surface second harmonic techniques to probe dynamics of polar solvation and isomerization reactions occurring at liquid liquid, liquid air, and liquid solid interfaces [22]. As these experiments afford subpicosecond time resolution, they are analogous to ultrafast pump probe measurements. Specifically, they excite a dye molecule residing at the interface and follow its dynamics via the resonance enhance second harmonic signal. 

The ab initio molecular dynamics study by Hudock et al. discussed above for uracil included thymine as well [126], Similarly to uracil, it was found that the first ultrafast component of the photoelectron spectra corresponds to relaxation on the S2 minimum. Subsequently a barrier exists on the S2 surface leading to the conical intersection between S2 and Si. The barrier involves out-of-plane motion of the methyl group attached to C5 in thymine or out-of-plane motion of H5 in uracil. Because of the difference of masses between these two molecules, kinematic factors will lead to a slower rate (longer lifetime) in thymine compared to uracil. Experimentally there are three components for the lifetimes of these systems, a subpicosecond, a picosecond and a nanosecond component. The picosecond component, which is suggested to correspond to the nonadiabatic S2/S1 transition, is 2.4 ps in uracil and 6.4 ps in thymine. This difference in the lifetimes could be explained by the barrier described above. 

As a last example of a molecular system exhibiting nonadiabatic dynamics caused by a conical intersection, we consider a model that recently has been proposed by Seidner and Domcke to describe ultrafast cis-trans isomerization processes in unsaturated hydrocarbons [172]. Photochemical reactions of this type are known to involve large-amplitode motion on coupled potential-energy surfaces [169], thus representing another stringent test for a mixed quantum-classical description that is complementary to Models 1 and II. A number of theoretical investigations, including quantum wave-packet studies [163, 164, 172], time-resolved pump-probe spectra [164, 181], and various mixed... 

Finally, we consider the performance of the MFT method for nonadiabatic dynamics induced by avoided crossings of the respective potential energy surfaces. We start with the discussion of the one-mode model. Model IVa, describing ultrafast intramolecular electron transfer. The comparison of the MFT method (dashed line) with the quantum-mechanical results (full line) shown in Fig. 5 demonstrates that the MFT method gives a rather good description of the short-time dynamics (up to 50 fs) for this model. For longer times, however, the dynamics is reproduced only qualitatively. Also shown is the time evolution of the diabatic electronic coherence which, too, is... 

Finally, we discuss applications of the ZPE-corrected mapping formalism to nonadiabatic dynamics induced by avoided crossings of potential energy surfaces. Figure 27 shows the diabatic and adiabatic electronic population for Model IVb, describing ultrafast intramolecular electron transfer. As for the models discussed above, it is seen that the MFT result (y = 0) underestimates the relaxation of the electronic population while the full mapping result (y = 1) predicts a too-small population at longer times. In contrast to the models... 

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