RIANS · Rydberg Interactions at Nanostructured Surfaces

Rydberg Interactions at Nanostructured Surfaces

The main interest of the present project is to study Rydberg-atom interactions with nanostructured surfaces. In particular, the electron transfer from a beam of laser-excited H atoms to a nanoparticle surface is studied. By carrying out such experiments with Rydberg atoms or molecules we can vary the electronic orbital size (by spectroscopic selection of the principal quantum number) of the incident Rydberg species relative to the size of the nanostructures on the surface. It is expected that the results of these measurements will be sensitive to the electronic and geometrical structure of the nanostructured surface and reveal novel physics associated with the detailed nature of the charge transfer process in these systems. According to the original plan, we needed to prepare and characterize a range of gold nanoparticle coated surfaces. Then, following training of the Fellow on the experimental apparatus and a redesign of apparatus as required, we needed to perform both experimental and theoretical research on the Rydberg-surface interactions with a sequence of gold nanoparticle surfaces of different nanoparticle diameter and material type and varying principal quantum numbers using H atoms or H2 molecules. The main results achieved so far are as follows: - Nanoparticle synthesis We have produced gold nanoparticles, following the recipe reported in [1] and measured the UV-VIS spectrum of our nanoparticles, comparing it with that of the commercial products of Sigma-Aldrich and for the nanoparticles supplied from Professor Eah’s group [1] nominally of the same size (5nm) . The Plasmon absorption peaks are observed for all of the three cases and occur in a similar position, which is a common label for checking the size of the nanoparticles [2]. If we change the parameters of the synthesis, the size of nanoparticle will show a modest tuning range (5nm ~9nm). - Gold Nanoparticle surface preparation We have produced surfaces with gold nanoparticles of different sizes (20nm, 50nm, 80nm diameter etc., purchased initially from BBI international) deposited on a 1-inch diameter silicon wafer, as used in previous experiments [3, 4]. Monolayer surfaces using home-made nanoparticles (5nm) have been produced as well; these are mostly covered by the nanoparticle monolayer and show roughness in the order of 10nm (surface from my work) and 2nm (surface from Eah group). - Redesign of the experimental apparatus We redesigned the experimental apparatus and finished the assembly of new parts to improve the system performance and correct earlier design flaws. Ion optics were added for focusing the ion flux properly and thus increasing the signal that can be received by the MCP (Micro channel plate). Second, we introduced an in-vacuum stepper motor mechanism and modified the corresponding design of the TOF (time of flight) part: This enabled the TOF (time of flight) part to be moved out of the way, and made it possible to perform surface science analysis (XPS and LEED measurements) and surface cleaning processes with an ion gun in the extra chamber below the main chamber. - Rydberg-surface experiments In initial experiments with 20nm, 50nm and 80nm nanoparticle surfaces, we found that the Rydberg atom (H atom) surface-ionization signal from the surfaces with nanoparticles is slightly greater than those without nanoparticles, whereas the differences between surfaces with nanoparticles of varying sizes are less clear within the regime of measurement error. The possible cause is that the percentage coverage of nanoparticles on the surface may be not enough to observe effects of surface corrugation on the signal above the noise level. The size of nanoparticles and the energy levels of Rydberg atoms should be investigated further and selected carefully.

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