The Experiments: cell preparation

A typical cell consists of a pyrex tube about 2 cm in diameter, about 5-10cm in length, closed by two sealed windows at the ends, connected to a glass bulb containing the alkali reservoir.

The cell preparation is as follows: after a careful cleaning, the cell is rinsed with an ether solution of PDS or OCT (see the page on siloxanes for details) and placed for a few hours in an oven heated to about 200°C. This is enough to produce a siloxane film that, if the operation is correctly made, leaves the cell perfectly transparent and without halos or deposits.

The cell is then connected to a bulb, which in future will be the cell metal reservoir, and then to a vacuum system. The coated cell has to be evacuated to 10^-5 Torr for several days to completely eliminate any trace of chemical active impurities that might be present inside of coating

Warming up the cell improves and speeds up this cleaning process. When the cell starts to show very low degassing the it is filled with a buffer gas (typically Argon or Krypton) at a chosen pressure and then by gently shaking of the cell a melted Rb metal drop is inserted in the cell reservoir.

It is important to note that in order to avoid contamination of the cell and the effect by impurities coming from the reservoir (due to unwanted chemical reactions between metal drop and organic compound) the vapour source must be not coated

Once the cell is prepared, it requires a curing time of few days at room temperature to be operative. After that time its behavior shows constant features, allowing us to repeat the observations with a high reproducibility. This is an important issue because it confirms the stability of the coating even after a long aging period.

Longer curing periods were observed with different geometry cells, including capillary connections and/or other features causing an increase in diffusion time.

A typical experimental apparatus >