The Experiments: a typical experimental setup

The typical experimental set-up is schematically reported in this figure (see "Experiment in progress" for a detailed one) :

A glass cell (see details) is illuminated either partially or totally by suitable light sources (we used alternatively lasers, tungsten lamps, Hg discharge lamps, IR leds, visible leds, flash-lamps, halogen lamps...).

A diode laser beam filtered down to few microwatt power to prevent direct LIAD emission and optical pumping crosses the cell and impinges onto a photodiode.

The diode laser (available e.g. for Rb and Cs) is tuned to a definite hyperfine transition of the alkali atoms, thus allowing the atomic density to be inferred by measuring the light absorption, through the Beer's law.

The laser wavelength is modulated at very low frequency, thus further preventing optical pumping and allowing precise LIAD measurements by means of a box-car technique. Two typical Rb spectra shown shown in this picture, where four peaks are observed (two hyperfine ground levels by two Rb isotopes produce four peaks whose Doppler bandwith does not allow the hyperfine structures of the excited level to be resolved).

The red line is obtained in dark condition, while the blue one is obtained after the cell is illuminated. The boxcar technique allows to get measurements during a 100Hz scanning of this spectrum, by opening a 1 microsecond time window in coincidence of one of the maxima.

Either fast or slow data acquisition allow for the quantitative analysis of fast and long-lasting dynamics of the desorption process. We studied the dynamics and its dependence on some experimental parameters, and in particular as a function of the cell temperature and the buffer gas pressure. A model including diffusion processes both in the gas phase and in the coating bulk give good description of both the time behavior and the amount of observed atoms in the gas phase.

Two plots of atom absorption as a function of time after cell illumination are reported here below. The first plot is recorded over 15sec, with low illumination power. The atom concentration get a maximum about three seconds after the light is switched on, corresponding to a value more than double of the equilibrium one.

This second plot shows the behaviour over a longer period, with a more powerful desorbing light. At t=t₁ the cell is illuminated, the absorption increases by a factor 15 in few seconds, then it starts a slow decrease. At t=300sec the desorbing light is switched off, and the atom density drops with a fast decrease to the equilibrium value.

In both graphs, at t=t₀ the diode laser frequency was put out of resonance for few seconds, in order to measure the absolute transparency of the cell, so getting the "zero" signal.

The the peak value of the atom concentration depends on the illumination intensity, but other experimental parameters have large effects. In particular, we put in evidence (both experimentally and in a theoretical model) that the cell temperature (even when the atom source is kept at a fixed temperature) and the buffer gas pressure play peculiar roles in the process.

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