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Probing chemical dynamics at surfaces with ultrafast atom pulses

Periodic Reporting for period 3 - HBEAM (Probing chemical dynamics at surfaces with ultrafast atom pulses)

Reporting period: 2020-10-01 to 2022-03-31

Today, it is well-known in chemical science that chemical reactions involve collisions. The reaction partner have to collide before they can react and form new chemical bonds. However, our ability to study and understand chemical reactions on an atomic scale is still limited. Molecular beam techniques are the state-of-the-art in controlling the conditions of a molecular collision. They provide control over collision energy as well as impact parameter and molecules can be prepared in well-defined quantum states. However, we still cannot control the exact time when a collision happens. Although short-pulse lasers can be used to measure the kinetics of photo-chemical processes, we cannot determine the time of a collision better than a few microseconds, limited by the typical time duration of pulsed molecular beams.

In the HBeam project, we address this issue and aim for the development of neutral matter pulses, i.e. hydrogen atom pulses, shorter than 1 ns. The "bunch compression photolysis" method for producing these sub-ns H-atom pulses has been developed in our department and will be further developed to make it usable for surface scattering and kinetics experiments. Bunch compression photolysis uses an ultra-short UV laser pulse (248.5nm <1ps) to photo-dissociate HI and produce H atoms with a speed of 19.3 km/s. However, the ultra-short laser pulse has a large spectral bandwidth and will therefore produce H-atoms with many different velocities, broadening the pulse until it hits the surface. By introducing a spatial chirp in the photolysis region, we can generate the faster H-atoms at a larger distance from the surface than slower H-atoms. Thus, the initially broad atom pulse will bunch into a sub-ns pulse right at the impact at the surface.

In our experiment, the atom pulse is time synchronized with a picosecond laser pulse that excites the surface, e.g. exciting electronic states or introducing a rapid temperature rise. Consequently, our experiment will be enable the study of surface dynamics and reaction on an at least nanosecond time scale, which is several orders of magnitude faster than current state-of-the-art molecular beam methods.
Since the start of the HBeam project, our work covered the set-up of a new laboratory devoted to the project, the generation and detection of short H-atom pulses and the scattering of H-atoms pulses from epitaxial graphene. The set-up of the laboratory included the identification of suitable equipment, i.e. photolysis laser system, as well as the design of a new ultra-high vacuum machine, which fulfils the requirements of sub-ns resolved surface scattering experiments. In October 2018, we started to assemble the new machine and produced the first bunch compresses H-atom pulses in April 2019. At the current set-up, we were able to produce/ detect H-atom pulses with time duration of 2-2.5ns using velocity map imaging and multi-photon ionization (MPI) of 355nm, 25ps laser pulses. Since the resolvable pulse duration is still larger than the theoretical limit, we tried to identify the limiting factors in the experiment and set up numerical models to simulate the experiment. Measurements of the laser pointing stability indicated that the observable pulse duration is limited by the pointing stability of the photolysis and MPI detection laser at the experimental apparatus. We therefore started to develop a laser beam stabilization system that is able to at least compensate for fluctuations and drifts on a second time scale. In the near future, we plan to test the limits by performing experiments that involve single shot ion detection with synchronized laser beam position detection.

In 2020, we started to study the first scattering experiments and since then we investigates the normal incidence scattering of the ultra-short H-atom pulses from single-crystal epitaxial graphene on Ir(111).

Parallel to the design and construction of the new experiment, we worked on two related projects. First, synthesis of monolayer transition metal dichalcogenides (TMDC) aiming for large area, high coverage monolayer WS2. The prepared samples using chemical vapour deposition (CVD) showed reasonable photo-luminescence yields and biexponential decays with lifetimes of 10-90ps and 150-400ps. Especially the slow components is on the order of the theoretically reachable pulse duration of the H-atom pulses. A challenge for the experiments is the preparation of clean TMDC samples in ultra-high-vacuum (UHV). CVD samples are grown are prepared under ambient pressure and exposed to air. Consequently, a cleaning procedure would be required. We investigated the thermal stability of the CVD-grown TMDC sample in high vacuum and found that the luminescence decreases rapidly upon heating of the samples. As a conclusion, we are looking into possibilities to prepare TMDC samples in-situ in UHV.

Second, we investigated resonant laser-induced desorption of hydrogen from Si(100) und Si(111). While exciting the H-Si stretch vibration on H-terminated silicon surfaces with an intense narrow band ps laser, we looked for enhanced the desorption of H2. However, we could not yet identify any resonant desorption. Nevertheless, the experimental results triggered new ideas on ultra-short laser-induced desorption (LID) of adsorbates and reaction intermediates at metal surfaces. So far we worked on understanding the process by investigating desorption of CO and O/O2 from Pt surfaces. Especially, the observation that femtosecond laser pulses are able to desorb atomic oxygen from Pt(111) and Pt(332) indicated, that the technique might be suitable to study intermediates in surface reactions. Recently, the LID apparatus has been upgraded with a dual molecular beam source and an ion imaging detector. This enabled the study of surface kinetics in a velocity resolved kinetics experiments. We tested the capability of LID in kinetic experiments by measuring desorption kinetic traces of CO and NH3 from Pt(111) and Pt(332) and compared the results to other methods. The good agreement demonstrated that LID well-suited to measure kinetics at surfaces overcoming problems of other methods like conventional velocity-resolved kinetic that have limitation at low temperatures, i.e. low reaction rates, since LID does not rely on a detection of the desorption flux. Moreover, in addition to previous approaches the LID capability of the experiments does not only allow to detect reaction products but also species that form as an intermediate on the surface. The lesson learned from LID experiments on molecular surface reaction are of great value for experiments involving ultra-short H-atom pulse with synchronized laser surface excitation.
We are currently able to reliably detect H-atom pulses with a pulse duration of about 2-2.5ns which is the shortest atom pulse that has been measured and resolved so far. The current limitation are small angular instabilities of the photolysis and detection laser beams caused by the intrinsic laser beam pointing stability, drifts between laser tables and fluctuations in air density in the beam path. Nonetheless, it is the shortest and most intense H-atom beam that is currently available while exhibiting an extraordinary high density. Atom beams from HI photolysis using excimer lasers and dye lasers are typically significantly longer and weaker due to the longer pulse duration of the lasers and the smaller photolysis volume for tight focusing. Using bunch compression photolysis with a Ti:Sa laser alone yields similar pulse durations but only low densities of H-atoms because of the low achievable pulse energy. Our approach allows for bunch compression as well as for H-atom densities since we are able to use ultra-short laser pulses with a large bandwidth and pulse energies up to 20mJ. We recently adapted the focusing conditions of the photolysis laser using appropriate cylindrical lenses, which allowed us to gain at least a factor of 5 in H-atom beam intensity and use a higher amount of photolysis pulse energy.

In addition, we significantly improved the detection scheme of scattered H-atoms from graphene. In detail, we ionize the scattering product less than 1mm in front of the surface and let the ions fly in a field-free region into the ion detector before they are extracted. This close ionization to the surface improved the detection efficiency of scattering products by at least a factor of 10. However, the close ionization results in drawbacks of the proper velocity mapping of scattered H-atoms onto the imaging detector. In experiments where high velocity resolution is required, this problem can be overcome by moving the detection laser further away from the surface. Fortunately, perfect velocity mapping is not necessary in experiment with synchronized laser excitation. In that case, the detection laser will just detect the product of the interaction and we only change the timing between H-atom pulse and surface excitation laser.