Hendrik Ulbricht

Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons

We have studied the interaction of polyaromatic hydrocarbons ~PAHs! with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene, and ovalene at submonolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV, and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of two-dimensional adsorbate islands up to the desorption temperature. Preexponential frequency factors are found to be in the range 10 14 ‐10 21 s 21 as obtained from both FalconerMadix ~isothermal desorption! analysis and Antoine’s fit to vapor pressure data. The resulting binding energy per carbon atom of the PAH is 5265 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is 6165 meV/atom, which is considerably larger than previously reported experimental values.

Models of Wave-function Collapse, Underlying Theories, and Experimental Tests

We describe the state of the art in preparing, manipulating and detecting coherent molecular matter. We focus on experimental methods for handling the quantum motion of compound systems from diatomic molecules to clusters or biomolecules. Molecular quantum optics offers many challenges and innovative prospects: already the combination of two atoms into one molecule takes several well-established methods from atomic physics, such as for instance laser cooling, to their limits. The enormous internal complexity that arises when hundreds or thousands of atoms are bound in a single organic molecule, cluster or nanocrystal provides a richness that can only be tackled by combining methods from atomic physics, chemistry, cluster physics, nanotechnology and the life sciences. We review various molecular beam sources and their suitability for matter-wave experiments. We discuss numerous molecular detection schemes and give an overview over diffraction and interference experiments that have already been performed with molecules or clusters. Applications of de Broglie studies with composite systems range from fundamental tests of physics up to quantum-enhanced metrology in physical chemistry, biophysics and the surface sciences. Nanoparticle quantum optics is a growing field, which will intrigue researchers still for many years to come. This review can, therefore, only be a snapshot of a very dynamical process.

A Kapitza–Dirac–Talbot–Lau interferometer for highly polarizable molecules

Research on matter waves is a thriving field of quantum physics and has recently stimulated many investigations with electrons1, neutrons2, atoms3, Bose-condensed ensembles4, cold clusters5 and hot molecules6. Coherence experiments with complex objects are of interest for exploring the transition to classical physics7,8,9, for measuring molecular properties10, and they have even been proposed for testing new models of space-time11. For matter-wave experiments with complex molecules, the strongly dispersive effect of the interaction between the diffracted molecule and the grating wall is a major challenge because it imposes enormous constraints on the velocity selection of the molecular beam12. Here, we describe the first experimental realization of a new set-up that solves this problem by combining the advantages of a so-called Talbot–Lau interferometer13 with the benefits of an optical phase grating.

Interaction of molecular oxygen with single-wall carbon nanotube bundles and graphite

The adsorption of oxygen on highly oriented pyrolytic graphite (HOPG) and bundles of single-wall carbon nanotubes (SWNTs) at 28 K is studied using thermal desorption spectroscopy and by a measurement of sticking probabilities. The low-coverage binding energy of oxygen adsorbed on SWNT bundles, 18.5 kJ/mol, is 55% higher than the low-coverage binding energy on HOPG, 12.0 kJ/mol. Molecular mechanics calculations reveal that such an increase can be attributed to the higher effective coordination of binding sites on SWNT bundles. The character of the oxygen-SWNT interaction should therefore be van der Waals type which suggests that the observed oxygen species is physisorbed and does not facilitate bulk doping of SWNT samples.

Wave and particle in molecular interference lithography

The wave-particle duality of massive objects is a cornerstone of quantum physics and a key property of many modern tools such as electron microscopy, neutron diffraction or atom interferometry. Here we report on the first experimental demonstration of quantum interference lithography with complex molecules. Molecular matter-wave interference patterns are deposited onto a reconstructed Si(111) 7x7 surface and imaged using scanning tunneling microscopy. Thereby both the particle and the quantum wave character of the molecules can be visualized in one and the same image. This new approach to nanolithography therefore also represents a sensitive new detection scheme for quantum interference experiments.

Desorption kinetics and interaction of Xe with single-wall carbon nanotube bundles

We present a study on the kinetics of xenon desorption from single-wall carbon nanotube (SWNT) bundles using thermal desorption spectroscopy (TDS). Desorption features from SWNT samples are broadened and peaked at higher temperature if compared to graphite. This can be explained using a coupled desorption–diffusion (CDD) model, which yields the low-coverage binding energy for Xe adsorption on SWNT bundles, 27 kJ mol−1. The latter is about 25% higher than the monolayer binding energy on graphite, 21.9 kJ mol−1. Using molecular mechanics calculations we find that this increase is consistent with adsorption in highly coordinated groove-sites on the external bundle surface or in endohedral sites inside of SWNTs.

Theory and experimental verification of Kapitza–Dirac–Talbot–Lau interferometry

Kapitza–Dirac–Talbot–Lau interferometry (KDTLI) has recently been established for demonstrating the quantum wave nature of large molecules. A phase space treatment permits us to derive closed equations for the near-field interference pattern, as well as for the moire-type pattern that would arise if the molecules were to be treated as classical particles. The model provides a simple and elegant way to account for the molecular phase shifts related to the optical dipole potential as well as for the incoherent effect of photon absorption at the second grating. We present experimental results for different molecular masses, polarizabilities and absorption cross sections using fullerenes and fluorofullerenes and discuss the alignment requirements. Our results with C60 and C70, C60F36 and C60F48 verify the theoretical description to a high degree of precision.

Slow beams of massive molecules

Abstract.Slow beams of neutral molecules are of great interest for a wide range of applications, from cold chemistry through precision measurements to tests of the foundations of quantum mechanics. We report on the quantitative observation of thermal beams of perfluorinated macromolecules with masses up to 6000 amu, reaching velocities down to 11 m/s. Such slow, heavy and neutral molecular beams are of importance for a new class of experiments in matter-wave interferometry and we also discuss the requirements for further manipulation and cooling schemes with molecules in this unprecedented mass range.

Near-field interferometry of a free-falling nanoparticle from a point-like source.

Matter-wave interferometry performed with massive objects elucidates their wave nature and thus tests the quantum superposition principle at large scales. Whereas standard quantum theory places no limit on particle size, alternative, yet untested theories--conceived to explain the apparent quantum to classical transition--forbid macroscopic superpositions. Here we propose an interferometer with a levitated, optically cooled and then free-falling silicon nanoparticle in the mass range of one million atomic mass units, delocalized over >150 nm. The scheme employs the near-field Talbot effect with a single standing-wave laser pulse as a phase grating. Our analysis, which accounts for all relevant sources of decoherence, indicates that this is a viable route towards macroscopic high-mass superpositions using available technology.

Proposal for a Noninterferometric Test of Collapse Models in Optomechanical Systems

The test of modifications to quantum mechanics aimed at identifying the fundamental reasons behind the unobservability of quantum mechanical superpositions at the macroscale is a crucial goal of modern quantum mechanics. Within the context of collapse models, current proposals based on interferometric techniques for their falsification are far from the experimental state of the art. Here we discuss an alternative approach to the testing of quantum collapse models that, by bypassing the need for the preparation of quantum superposition states might help us addressing nonlinear stochastic mechanisms such as the one at the basis of the continuous spontaneous localization model.