Building up BeamER-C
The BeamER-C apparatus comprises three different sections:
• The preparation chamber: which includes the sample manipulator to move the sample between the main and the preparation chamber. The preparation chamber can be separated from the main chamber by closing the interjacent slide valve.
• The main chamber: which contains the ion optics, the multi channel plate (MCP)/Phosphor screen detector, a leak valve/ion gun, an Auger spectrometer, windows for laser access and the residual gas analyzer (RGA).
• The source chamber: which houses two supersonic beam valves and two differential pumping stages.
Ultra High Vacuum (UHV)
An important requirement for gas-surface experiments is keeping the surface free from contaminations. The results of a reaction at a surface might be strongly biased if the surface is poisoned by adsorption of undesired residual gas molecules. Controlling the background gas load in the chamber is an important means of reducing this interference. The definition of a Langmuir is a 1 s exposure at a pressure of 1.3*10-6 mbar, which corresponds to approximately 1 monolayer (ML) exposure. We can thus estimate the time it takes to form a monolayer to be several hours at 1*10-10 mbar. Establishing ultra-high vacuum (UHV) conditions, i.e. a base pressures on the order 10-9 – 10-10mbar.
In order to achieve Ultra High Vacuum conditions, are used UHV-compatible materials.
- The machine was made out of stainless steel, which was annealed prior to construction to outgas H2 from the bulk material.
- Metal-fitted ultraviolet-grade fused silica (UVFS) windows were used where possible.
- Differentially pumped CaF2 windows were used for laser access.
- The sample holder was made out of stainless steel/tantalum. Any cable insulation was made from UHV-compatible polymers,i.e. Kapton or a similar material.
- The nickel grids for the ion optics were glued with graphite.
- Non-conducting spacers on the ion optics and the sample holder were made of either polyether ether ketone (PEEK) or Macor, a machineable glass-ceramic.
Also, a large number of turbo-molecular pumps (TMPs) are used to evacuate the various chambers. All TMPs are backed by dry scroll pumps to obtain foreline pressures of 10−3 – 10−4 mbar. Scroll pumps were chosen over rotary vane pumps to maintain an oil-free environment.
We have built pulsed nozzles based on a popular piezo (PZT) actuator design, but modified to improve performance. These devices produce pulses of molecules as short as 20 μs FWHM (velocity spread limited) with molecular densities of ~1013 cm-3 at the location of the surface. The temporal profile of our molecular beams, which limits the time resolution of the measured kinetics traces, we measure using the analogous procedure described by Fig. 1, this time performed on the incoming molecular beam rather than the reaction product.
The piezo valve is composed of a main body (e) with gas inlet (f) and a baseplate (d) holding the disk translator. The piezo disc translator works as follows: The back of the lead zirconate titanate (Pb[ZrxTi1−x]O3) (PZT) crystal (a) is glued to the 0.6–0.75 mm thick steel membrane (b) with conductive silver epoxy. When a negative voltage between 500–1300 V is applied to the front of the piezo crystal it contracts radially and the radial shear between piezo crystal and the steel membrane causes the membrane to curve. The combination acts as a bimorph. The stamp (c), which is screwed to the steel membrane, retracts to the right. The O-ring on the tip of the stamp sealing the nozzle orifice (g) retracts as well and allows the gas in the volume surrounding the stamp in the main body to expand through the orifice.
Strong field ionization
For some molecules such as water or carbon dioxide we use focused high intensity fs-laser pulses to do non-resonant strong field ionization. A high-power fs-laser (Coherent Astrella, 1 kHz Ti:Sapphire, up to 6 mJ per pulse in <100 fs) is installed in the laboratory. For molecules with a high ionization potential such as CO2, a fs-laser increases the ionization efficiency. This allows the measurement of systems where the flux is very low. The higher signal-to-noise ratio allows the measurement range to be extended to conditions where slower kinetics are present,i.e. lower temperatures and concentrations.