## 2 Dilute Rubidium Atoms in a Solid Argon and Neon Matrix: Experimental Results  
In this section we report about the growth and spectral measurements of dilute Rubidium atoms in Argon and Neon solid matrices at cryogenic temperatures. Dependence of the spectra of solid Argon doped with Rubidium atoms from crystal-growth temperature and annealing history has been reported in [6]. The core of the experimental apparatus is the cold finger of a pulse tube refrigerator (Sumitomo RP062B) that could reach a minimum temperature of 4 K, thus allowing rare gas crystals to be grown and maintained. The cold finger is placed in the middle of a six-hole cross stainless-steel chamber (SSC) and at its center is mounted a 25 mm diameter sapphire window (SW) that holds the crystal and allows optical transmission measurements. The SW temperature is monitored through a silicon diode (Lakeshore DT670), and its precise control is ensured using a 15 W heater placed nearby the SW. A quartz and a BK7 window are placed in the two ports of the SSC along the normal direction to the surface of the SW, while the port in the transverse axis is equipped with a mechanical manipulator where both a gas nozzle and an alkali metal dispenser are installed. The manipulator can be placed in front of the SW or 10 cm backwards depending if the growth or the measurements are ongoing. A sketch and a picture of the apparatus described above are shown in Figure 2. Both vacuum and pressure of gas into the SSC are measured using pressure gauges in the range [10\({}^{-9}\)-100] mbar (Pfeiffer PKR251). Crystal growth occurs through spraying the purified rare gas mixed with alkali atoms onto the cold surface of the SW. Prior to being sprayed, the gas passed within a purification system that allows an impurity contamination below the ppb level especially for high-electronegativity elements. This system is described with more specific details in a previous paper [31]. The Rubidium dispenser (SAES Getter) is connected through two vacuum-feedthroughs to a power supply that delivers up to 6 A necessary to sublimate the metal. The parameters that have been used to control crystal formation are the partial pressure of the gas and of the dopant during the growing (P\({}_{g}^{gas}\), P\({}_{g}^{dop}\)), the SW temperature (T\({}_{g}\)) and the time of growing (t\({}_{g}\)). Initially, we grew a\(\sim\)100 \(\upmu\)m slab of pure rare-gas with a flow rate maintained at about 1 mbar\(\cdot\)L/s that leads in P\({}_{g}^{gas}\sim 5\cdot 10^{-5}\) mbar. Only after \(\sim\)30 min is the doping dispenser activated, with a current set to \(\sim\)3.5 A which gives P\({}_{g}^{dop}\sim 5\cdot 10^{-7}\) mbar. In such a way, Rubidium atoms have been embedded into Argon and Neon matrices respectively, with a ratio between the two species of about 1%. Finally, also in the last \(\sim\)100 \(\upmu\)m we deposited only RG, indeed during this step the dispenser current has been set to zero. Given the total time of growth \(t_{g}\sim 4\) h, the thickness of the Rb-Ar and Rb-Ne films are about 1 mm. During the growth, T\({}_{g}\) has been set to \(\sim\)60 K and \(\sim\)10 K for Argon and Neon respectively, while after crystals formation, an annealing process of \(\sim\)1 h at a temperature T\({}_{g}+10\) K has been performed. Finally, for the measurements, the temperature was lowered to about 5 K for all the crystals. Three different samples have been grown to check the reproducibility of any matrix. The optical part of the experimental set-up is mainly composed of a Titanium-sapphire (Ti:Sa) CW laser tunable in the near-infrared (NIR) region, whose output power varies between 150 mW and 800 mW depending on the wavelength. This laser system has been used to cover the broadest range for crystal excitation. Appropriate quartz lenses have  
Figure 2: Sketch of experimental setup. The inset shows a picture of the cold finger with the sapphire window placed at its centre.