Mercury is a heavy metal with relatively high volatility. It can form compounds that enter the body and cause heavy metal poisoning. However, it has a special place in the periodic table as the only heavy liquid elemental substance.
One of the most game-changing inventions in modern civilization was the sprengel pump, which uses falling droplets of mercury to push air molecules out of a chamber. This produced high vacuums containing only rarified mercury vapor. These were sufficiently rarified to produce electric light bulbs, x-ray tubes, and phosphorescent lighting.
Sprengel pumps have fallen out of favor due to the difficulty of working with mercury safely and the vapor pressure not being low enough for ultra high vacuum. Also problematic is the creation of a monolayer of mercury on surfaces.
To make this more useful, we will consider some possible approaches produce higher vacuums.
One approach is to use a cryo trap. For this to be be effective, we need to limit the amount of exposed mercury liquid to the vacuum. The piston pipe of the sprengel pump leads to a reservior of liquid, which prevents air from entering. Gas molecules pressed down by the falling drops are pushed into the liquid and exit into the surrounding air.
To ensure the vacuum is improved, we can close the valve leading to the reservior once the equilibrium is reached (10^-8 torr) where mercury vapor pressure is the sole contaminant to the vacuum.
This point will be lower if the mercury reservior is chilled below its freezing point. The precise temperature to reach UHV (10^-11 torr) by chilling mercury is poorly studied and unknown, but likely in the range of temperatures achieved by evaporating dry ice. At this point in the process, it is not removing any more air molecules, but condensing (or rather, depositing) mercury atoms. As they touch the solid cold surface of the reservior, they lose momentum and stick to it.
The temperature of the main chamber should be kept higher, or perhaps cycled up and down, during this step in order to ensure the mercury condensed against the surfaces is converted to vapor.
The thermal conductivity of metal makes it harder to have part of it be warm with another part ver cold, so it is likely more achievable in a glass system with thin walls. Borosilicate (formerly known as Pyrex) is likely ideal for this, given its high strength and ability to tolerate temperature differentials without cracking.
Ion trapping may make this unnecessary, however. We can keep the whole system at room temperature and reduce to the point where it is only mercury vapor at the equilibrium pressure, then close the valve leading to the liquid reservior.
In a side chamber, a charged plate with a negative charge can be used to collect mercury ions that are positively charged. Mercury can be charged with ultraviolet light, which can be produced with a mercury vapor lamp without a phosphor coating (or with a gap in it, if you are working with fluorescent tubes). There are now UV LEDs, and the ultraviolet in sunlight could also be used.
Rather than UV, the usual approach to this (which also works with air molecules) is to use a positively charged grid. As mercury vapor atoms pass near the grid, they lose electrons. Being positively charged, they are repelled from this grid, and attracted to the negatively charged plate.
The connector leading to the ion trap chamber can be equipped with a valve, and closed when the pressure in the chamber drops sufficiently. This can be monitored by observing the re-radiation of UV being absorbed by the mercury vapor, using a photosensor. This is perhaps a reason to prefer UV as the ion charge mechanism.
Note that the sprengel pump during its operation tends to replace air molecules in the chamber with mercury vapor. So it should be left running with this valve open to purify the vapor for some time to ensure purity. Once the process is complete, the valve to this is closed and the ion trap is opened and charged. The valve for that is then closed and the charged plate is turned off.
A cathode with a higher surface area would be better, so we can imagine a curled up plate for this.
UV light imparts momentum when it impacts and charges Hg vapor atoms, so we can direct the UV towards the connector, charging ions and moving them in the direction of the connector.