Periodic Reporting for period 4 - HBEAM (Probing chemical dynamics at surfaces with ultrafast atom pulses)
Reporting period: 2022-04-01 to 2023-09-30
It is well-known in chemical science that chemical reactions require collisions between the reaction partners. Unfortunately, our ability to study chemical reactions on an atomic scale is still quite limited. State-of-the-art molecular beam experiments allow control of the initial impact parameters and detection of reaction products with quantum-state resolution. However, we still lack in the ability to control the exact time of the collision. While Ultra-short laser pulses allow the study of photo-chemical processes with femtosecond resolution, collision induced reactions can only be studied with microsecond resolution.
The HBeam project addressed this issue and aimed for the development of neutral matter pulses. Bunch compression photolysis of hydrogen iodie (HI) is used for generation of sub-ns H-atom pulses with a velocity of up to 19.3 km/s. A KrF short-pulse excimer amplifier is used to generate up to 20mJ photolysis laser pulses to ensure sufficient intensity for surface scattering experiments. By using a spatially chirped photolysis laser focus, we ensure the shortest H-atom pulse duration to be present at surface impact.
The experimental work has shown that generation of 2.5ns H-atom pulses is possible. Limitations result from the experimental capability to resolve shorter pulses due to unavoidable pointing instabilities of photolysis and detection laser beams which can only partially be compensated by active laser beam stabilization. H-pulse intensities are sufficiently high to enable scattering experiments. However, velocity map imaging detection causes significant background making detection of small scattering signals time challenging. Thus, further improvement of H-atom pulse intensities might be required.
The HBeam project addressed this issue and aimed for the development of neutral matter pulses. Bunch compression photolysis of hydrogen iodie (HI) is used for generation of sub-ns H-atom pulses with a velocity of up to 19.3 km/s. A KrF short-pulse excimer amplifier is used to generate up to 20mJ photolysis laser pulses to ensure sufficient intensity for surface scattering experiments. By using a spatially chirped photolysis laser focus, we ensure the shortest H-atom pulse duration to be present at surface impact.
The experimental work has shown that generation of 2.5ns H-atom pulses is possible. Limitations result from the experimental capability to resolve shorter pulses due to unavoidable pointing instabilities of photolysis and detection laser beams which can only partially be compensated by active laser beam stabilization. H-pulse intensities are sufficiently high to enable scattering experiments. However, velocity map imaging detection causes significant background making detection of small scattering signals time challenging. Thus, further improvement of H-atom pulse intensities might be required.
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.
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.
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. We developed a CMOS sensor based active laser beam stabilization to compensate the effects, but the rather low 50Hz repetition rate limits the achievable stabilization improvement.
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. Adaption of focusing conditions of the photolysis laser using appropriate cylindrical lenses allowed us to gain a factor of 5-10 in H-atom beam intensity and use a higher amount of photolysis pulse energy without affecting the pulse duration due to saturation.
For the first time, we demonstrated scattering experiments using ultra-short H-atom pulses using velocity map imaging (VMI) for detection. Previous efforts applied Rydberg atom tagging (RAT), which did not allow for normal incidence and simultaneous detection of multiple scattering angles. VMI allows for rather simple detection of all scattering angles within the acceptance range of the detector but suffers from significantly more background than RAT experiments. In fact, H-atom background represents a significant experimental challenge for detection of low scattering signals and requires the development of a geometry that allows for background suppression while maintaining signal sensitivity. Ionization close to the sample surface turned out to suppress background efficiently, however care have to be taken to extract the generated ions in the center of the detector for proper velocity mapping.
An important outcome of the project was the establishment of femtosecond laser induced desorption (fs-LID) to study reaction intermediates in velocity-resolved kinetics (VRK) experiments. Proof-of-concepts showed the capability to study kinetics of CO, NO and NH3 desorption from Pt surfaces at low temperatures. Current attempt aim to apply LID-VRK for detection of reaction intermediates that normally do not leave the surface.
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. Adaption of focusing conditions of the photolysis laser using appropriate cylindrical lenses allowed us to gain a factor of 5-10 in H-atom beam intensity and use a higher amount of photolysis pulse energy without affecting the pulse duration due to saturation.
For the first time, we demonstrated scattering experiments using ultra-short H-atom pulses using velocity map imaging (VMI) for detection. Previous efforts applied Rydberg atom tagging (RAT), which did not allow for normal incidence and simultaneous detection of multiple scattering angles. VMI allows for rather simple detection of all scattering angles within the acceptance range of the detector but suffers from significantly more background than RAT experiments. In fact, H-atom background represents a significant experimental challenge for detection of low scattering signals and requires the development of a geometry that allows for background suppression while maintaining signal sensitivity. Ionization close to the sample surface turned out to suppress background efficiently, however care have to be taken to extract the generated ions in the center of the detector for proper velocity mapping.
An important outcome of the project was the establishment of femtosecond laser induced desorption (fs-LID) to study reaction intermediates in velocity-resolved kinetics (VRK) experiments. Proof-of-concepts showed the capability to study kinetics of CO, NO and NH3 desorption from Pt surfaces at low temperatures. Current attempt aim to apply LID-VRK for detection of reaction intermediates that normally do not leave the surface.