Kilohertz laser ablation for doping helium nanodroplets

A new setup for doping helium nanodroplets by means of laser ablation at kilohertz repetition rate is presented. The doping process is characterized and two distinct regimes of laser ablation are identified. The setup is shown to be efficient and stable enough to be used for spectroscopy, as demonstrated on beam-depletion spectra of lithium atoms attached to helium nanodroplets. For the first time, helium droplets are doped with high temperature refractory materials such as titanium and tantalum. Doping with the non-volatile DNA basis Guanine is found to be efficient and a number of oligomers are detected

Inelastic scattering of H atoms from surfaces

We have developed an instrument that uses photolysis of hydrogen halides to produce nearly monoenergetic hydrogen atom beams and Rydberg atom tagging to obtain accurate angle-resolved time-of-flight distributions of atoms scattered from surfaces. The surfaces are prepared under strict ultrahigh vacuum conditions. Data from these experiments can provide excellent benchmarks for theory, from which it is possible to obtain an atomic scale understanding of the underlying dynamical processes governing H atom adsorption. In this way, the mechanism of adsorption on metals is revealed, showing a penetration–resurfacing mechanism that relies on electronic excitation of the metal by the H atom to succeed. Contrasting this, when H atoms collide at graphene surfaces, the dynamics of bond formation involving at least four carbon atoms govern adsorption. Future perspectives of H atom scattering from surfaces are also outlined

Alkali Atoms Attached to 3^3He Nanodroplets

We have experimentally studied the electronic $3p-> 3s$ excitation of Na atoms attached to $^3$He droplets by means of laser-induced fluorescence as well as beam depletion spectroscopy. From the similarities of the spectra (width/shift of absorption lines) with these of Na on $^4$He droplets, we conclude that sodium atoms reside in a ``dimple'' on the droplet surface and that superfluid-related effects are negligible. The experimental results are supported by Density Functional calculations at zero temperature, which confirm the surface location of Na, K and Rb atoms on $^3$He droplets. In the case of Na, the calculated shift of the excitation spectra for the two isotopes is in good agreement with the experimental data.Comment: 6 pages, 3 figures, sent to JLT

Spectroscopy of PTCDA attached to rare gas samples: clusters vs. bulk matrices. I. Absorption spectroscopy

The interaction between PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) and rare gas or para-hydrogen samples is studied by means of laser-induced fluorescence excitation spectroscopy. The comparison between spectra of PTCDA embedded in a neon matrix and spectra attached to large neon clusters shows that these large organic molecules reside on the surface of the clusters when doped by the pick-up technique. PTCDA molecules can adopt different conformations when attached to argon, neon and para-hydrogen clusters which implies that the surface of such clusters has a well-defined structure and has not liquid or fluxional properties. Moreover, a precise analysis of the doping process of these clusters reveals that the mobility of large molecules on the cluster surface is quenched, preventing agglomeration and complex formation

Inelastic H and D atom scattering from Au(111) as benchmark for theory.

Efficient transfer of translational energy to electron-hole pair excitation involving multiple collisions dominates H atom collisions with metal surfaces. For this reason, H atom interaction with metal surfaces cannot be modeled within the commonly used Born-Oppenheimer approximation (BOA). This fact makes H atom scattering from metal surfaces an ideal model system for dynamics that go beyond the BOA. We chose the H/Au(111) system as a model system to obtain a detailed dataset that can serve as a benchmark for theoretical models developed for describing electronically nonadiabatic processes at metal surfaces. Therefore, we investigate the influence of various experimental parameters on the energy loss in detail including isotopic variant, incidence translational energy, incidence polar and azimuthal angles, and outgoing scattering angles

Multibounce and subsurface scattering of H atoms colliding with a van der Waals solid

We report the results of inelastic differential scattering experiments and full-dimensional molecular dynamics trajectory simulations for 2.76 eV H atoms colliding at a surface of solid xenon. The interaction potential is based on an effective medium theory (EMT) fit to density functional theory (DFT) energies. The translational energy-loss distributions derived from experiment and theory are in excellent agreement. By analyzing trajectories, we find that only a minority of the scattering results from simple single-bounce dynamics. The majority comes from multibounce collisions including subsurface scattering where the H atoms penetrate below the first layer of Xe atoms and subsequently re-emerge to the gas phase. This behavior leads to observable energy-losses as large as 0.5 eV, much larger than a prediction of the binary collision model (0.082 eV), which is often used to estimate the highest possible energy-loss in direct inelastic surface scattering. The sticking probability computed with the EMT-PES (0.15) is dramatically reduced (5 × 10–6) if we employ a full-dimensional potential energy surface (PES) based on Lennard-Jones (LJ) pairwise interactions. Although the LJ-PES accurately describes the interactions near the H–Xe and Xe–Xe energy minima, it drastically overestimates the effective size of the Xe atom seen by the colliding H atom at incidence energies above about 0.1 eV

Surface location of sodium atoms attached to He-3 nanodroplets

We have experimentally studied the electronic $3p\leftarrow 3s$ excitation of Na atoms attached to $^3$He droplets by means of laser-induced fluorescence as well as beam depletion spectroscopy. From the similarities of the spectra (width/shift of absorption lines) with these of Na on $^4$He droplets, we conclude that sodium atoms reside in a ``dimple'' on the droplet surface. The experimental results are supported by Density Functional calculations at zero temperature, which confirm the surface location of sodium on $^3$He droplets, and provide a microscopic description of the ``dimple'' structure.Comment: 4 pages, 5 figure

Boosting hot electron flux and catalytic activity at metal-oxide interfaces of PtCo bimetallic nanoparticles

Despite numerous studies, the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs) remains elusive because of the ever-changing surface structures, compositions, and oxidation states of NPs under reaction conditions. An effective strategy for obtaining critical clues for the phenomenon is real-time quantitative detection of hot electrons induced by a chemical reaction on the catalysts. Here, we investigate hot electrons excited on PtCo bimetallic NPs during H-2 oxidation by measuring the chemicurrent on a catalytic nanodiode while changing the Pt composition of the NPs. We reveal that the presence of a CoO/Pt interface enables efficient transport of electrons and higher catalytic activity for PtCo NPs. These results are consistent with theoretical calculations suggesting that lower activation energy and higher exothermicity are required for the reaction at the CoO/Pt interface

How Can We Experimentally Determine Why Hydrogen Atoms Are Absorbed on Metal Surfaces?

Although very light weight, hydrogen atoms have a high probability to be absorbed by a metal surface upon collision. In this video, OLIVER BÜNERMANN explains collision experiments carried out to determine why this is the case. During the experiment, they shot a hydrogen atom beam at a gold surface and at an insulator, measured the speed and direction of the atoms bouncing back from each surface and compared the results. The hydrogen atoms scattered from the gold surface suffered a greater energy loss than the one scattered from the insulator. This difference indicates that translational energy carried in the hydrogen atom is transferred into electronic excitations in the metal leading to the high probability of absorption. The experiment results match the predictions of the theoretical model explained by Alexander Kandratsenka