Excitonic coupling in SAMs of azobenzene-functionalized alkanethiols

The functionalization of surfaces with molecular switches is a rapidly growing field in today’s research. Molecules can be used as repeatable building blocks for electronics and sensors and thereby open the perspective to tailor devices on the nanoscale. In this respect self-assembled monolayers (SAMs) have often been considered as ideal platforms to order and align molecules at surfaces. Optical properties and geometric structure of self-assembled monolayers of azobenzenefunctionalized alkanethiols have been investigated by UV/Visible and near edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl endgroup to the chromophore both the molecular tilt and twist angle of the azobenzene moiety are accessible. Based on this detailed structural analysis the energetic shifts observed in optical reflection spectroscopy can be qualitatively described within an extended dipole model. This substantiates sizeable excitonic coupling among the azobenzene chromophores as an important mechanism that hinders trans to cis isomerization in denselypacked self-assembled monolayers.

J. Am. Chem. Soc., 2010 in press.

funded DFG through Sfb658, Elementary Processes in Molecular Switches at Surfaces

Ultrafast demagnetisation of Gd

Gadolinium with its half-filled 4f core shell is a particular prototype system to study magnetisation dynamics. The strongly localised 4f ⁷ electrons carry most of the magnetic moment of 7.55 μ_B per atom. They spin-polarise the 5d valence
electrons resulting in the extra contribution of 0.55 μ_B per atom. This
intra-atomic 4f-5d exchange coupling in turn mediates magnetic order within the 4f system below the Curie temperature of T_C = 293 K. This behaviour differs significantly from the itinerant ferromagnets iron, cobalt, and nickel.

In a recent experiment we combined laser and synchrotron for pump-probe experiments which allows us to directly measure the temporal evolution of the 4f contribution to the magnetisation. Epitaxial Gd(0001) films of 100 Å thickness, with the easy axis in plane, have been grown on W(110). The Gd magnetisation is monitored by magnetic linear dichroism (MLD) of the 4f photoemission line. We established that the
temperature dependence of the MLD signal follows the spontaneous magnetisation M/M(0) and assume that this likewise holds on a picosecond time-scale. The transient magnetisation after laser excitation (hν = 1.5 eV, fluence of 3.5 mJ/cm²) is shown in Fig. 1 as a function of pump-probe delay Δt (open circles). A 20 % drop is followed by a recovery within 1 ns. The drop in the MLD contrast demonstrates that the 4f magnetic order is reduced upon optical excitation of the 5d6s valence electrons.

One may argue that this is a pure thermal effect, since we probe the 4f core-level with a 50 ps time-resolution and electron-lattice equilibration takes 1.5 ps. From the transient lattice temperature and the temperature-dependent magnetisation we calculated the transient magnetisation under the assumption that thermal equilibrium is established at all delays. This thermal scenario of M(Δt) is plotted in Fig. 1 by the solid line which was convoluted with a Gaussian of 50 ps width to account for the synchrotron-radiation pulse-length. 80 ps after laser excitation the measured 4f MLD and the calculated magnetisation agree nicely, which substantiates our modelling.
However, the measured drop of the magnetisation at earlier times is by a factor of two smaller than expected, which is consistently reproduced for different fluences. This striking deviation of the data from the thermal estimate before 80 ps have elapsed shows directly that equilibrium between the Gd spin system and lattice is not established in this time regime. The 4f spin-lattice relaxation thus takes much longer than the electron-lattice equilibration.

Fig. 1: Transient, relative change of the magnetization: measured 4f dichroism (open circles), calculated from lattice temperature and magnetization (dashed line), and convoluted with a Gaussian of 50 ps FWHM (solid line).

Phys. Rev. Lett. 100 (2008) 107202.

funded by DFG through SPP1133 "Ultrafast Magnetization Processes"