The first 1.5 years of the HBeam project covered the design and built of the scientific apparatus as well as identification and purchase of the suitable equipment, especially laser systems that match the requirements of the project. First bunch compressed H-atom pulses were produced in April 2019. In parallel, we evaluated the feasibility of a the proposed time-resolved surface scattering experiment: (1) action spectroscopy of H2 formation in H-atom interaction with H-terminated silicon and (2) H-atom scattering from photo-excited monolayer transition metal dichalcogenides TMDC. We prepared H-terminated Si(100) and Si(111) surfaces and studied the possibility for resonant enchancement of laser-induced desorption (LID) of H2. The surfaces were irradiated with tunable picosecond IR laser pulses. A time-delayed UV laser beam aimed to detect possibly desorbing H2. Unfortunately, it was not possible to desorb H2 from H-Si surfaces although ablation of Si clusters was observed. Later experiments on LID of CO from metal surfaces showed that the laser beam profile is critical to the LID process, a condition that was not achievable with the available picosecond IR laser sources. Nevertheless, the work stimulated further experiments aiming to apply short-pulse LID to study intermediates in surface reactions, leading to the development of velocity-resolved laser-induced desorption kinetics [Chemistry Methods 2022, 2, e202200017]. In addition, TMDC sample of WS2 and MoS2 were prepared using chemical vapour deposition (CVD). Unfortunately, CVD did not allow the production of homogeneous monolayer TMDCs with high coverage over a reasonable surface area (2x2mm²). Time-correlating single photon counting measurement of samples in UHV showed that sample cleaning by annealing leads to irreversible depletion of fluorescence, indicating fast quenching of long-lived excitonic states.
As a consequence, we choose H-atom scattering from epitaxial graphene on Ir(111) as a first system to demonstrate the scattering capability of the generated short H-atom pulses [J. Phys. Chem. A 2022, 126, 43, 8101–8110]. The experimental results stimulated theoretical calculations based on high-level quantum theory (MCTDH) which for the first demonstrated 75 dimensional quantum calculation in good agreement with experimental results [J. Chem. Phys. 2023, 159, doi: 10.1063/5.0176655].
Current experiments focus on the search for model systems that allow for time-synchronized laser excitation of the surface that lead to a change in the H-surface interaction. A promising candidate is H-atom scattering off Ger(111), which shows a surface temperature dependent change in the final H-atom kinetic energy distribution. A short laser pulse can cause a rapid change of the surface temperature and the decay might be probed by a time-delayed H-atom pulse.
