The Experiments: experimental results
LIAD as a function of the cell temperature at different buffer gas pressures

When the cell is illuminated the alkali concentration in the gas phase increase several times above the equilibrium value, it gets a peak value after a certain time depending on the cell size and the buffer gas pressure, and then start a very slow decreasing, due to progressive depletion of the alkali concentration in the coating.

The peak value of the alkali concentration in the vapour phase was measured in a standard illumination condition using two identical cells filled with buffer gas at different pressures, by varying the temperature over a wide range. The data are shown in this plot

A transition temperature is observed, which is higher for the cell with the higher pressure of buffer gas. This is in agreement with a macroscopic model taking into account diffusion processes both in the coating and in the gas.

Actually, when the light is on, two fluxes of diffusing atoms,
J₊,  from the coating to the gas phase  (surface desorption)  and
J_-,  from the gas phase to the metallic source (condensation)
compete in modifying the vapour concentration.
The diffusion in the coating has a strong dependence on the cell temperature, while the diffusion coefficient in the gas phase mainly depends on the gas pressure.

As a consequence, the flux from the coating to the gas can prevail on the condensation flux only when the cell temperature is high enough. When the condensation flux is depressed due to a higher buffer gas pressure, the transition temperature is lowered.

The results shown here above are in quantitative agreement with models developed in this frame, where explicit dependence of diffusion coefficients on the experimental parameters are taken into account.

A peculiar result of the model is that the concentration of atoms inside the coating, though impossible to extimate absolutely (it keeps related to the absolute diffusion coefficient in the coating, not precisely known), is found to be very large.

This confirms the possibility of using such a kind of coatings as very efficient and light-controllable atom sources (see also Mariotti et al. Opt.Comm 134 1997), to be used in a wide range of atomic physics experiments. In particular, we plan to build up a LIAD sourced cell for application in laser cooling of atoms in a magneto optical trap.

A simple macroscopic model >