In this work, we present some preliminary experimental and theoretical results along this direction of research. Furthermore the properties of these atoms, and Rubidium in particular, are well known, not only in free space but also when embedded into solid crystals. The points mentioned above are essential for the proposed axion detection scheme, since the linewidth of the guest-atom energy levels involved in the transitions should be smaller than the separation of the two Zeeman sublevels, in order to be able to separate them. This is not an easy task to accomplish. For example, as we will see later on, our spectroscopy measurements of Rubidium atoms in a Argon matrix show that the linewidth in that case can be relatively larger. Let us consider for example a peak of the fluorescence spectrum obtained from our measurements, shown in the next section, with \(\lambda_{c}\simeq 1155\) nm for the peak wavelength and a width \(\Delta\lambda\simeq 39\) nm. The energy width of the line is thus \(\Delta E\simeq 3.6\cdot 10^{-2}\) eV, which is quite a bit larger than what is necessary for resolving the two Zeeman sublevels. A main motivation of this work is to provide some hints for finding different schemes that significantly reduce the linewidth, for example different combinations of guest and matrix atoms or molecules, or different transition schemes. Very small optical linewidths have been recently found for thulium atoms trapped in argon and neon solid matrices [32], but they involve a magnetic-dipole transition, which is not a useful one for our proposed detection scheme. Matrix Isolation Spectroscopy seems to be an appropriate experimental technique for this purpose, because it allows us to efficiently control the interactions between the atoms and the environment, to strongly reduce the mutual interaction between the atoms due to their large separation, as well as suppressing the thermal broadening. It consists of trapping the alkali atoms in a solid matrix of inert materials at cryogenic temperatures, such as rare gases (RG) or inert molecules such as, for example, p-H\({}_{2}\), O\({}_{2}\), CH\({}_{4}\). Rb atoms trapped in a magneto optical trap could in principle be also used to detect the axions. However, one of the advantages of using the MIS is related to the high density of target atoms that can be probed with respect to spectroscopic studies in the gas phase. This fact is important also in the particle detection view since a high number of target atoms is necessary in order to maximize the interaction probability. Other axion detection schemes have been proposed, for example based on microwave cavities. The advantage of using a specific detection schemes depends also on the assumed (unknown) mass of the axion. In the axion mass range we are considering, larger than \(\sim\)10\({}^{-4}\) eV, the axion-electron interaction upon which our detection scheme is based is more efficient, while a scheme based on microwave cavities is more advantageous for smaller masses [29]. Moreover, differently from other schemes exploiting, for example, microwave cavities in the GHz region to detect DM axions, our approach can be easily tuned to a large energy range in the meV scale by varying the magnetic field, thus offering the possibility to investigate more sectors in the DM scenario in a complementary way. In the cases we consider in this paper, possibly relevant for DM search, the free atoms or molecules considered, rare gases, alkali atoms or parahydrogen, do not have electric dipole moments and their wavefunctions have spherical symmetry. Furthermore, the guest-atom-matrix interactions, mainly dispersion interactions, are very weak, and thus we expect that this property is maintained also when the atoms are inside the solid matrix. Figure 1: The proposed scheme for the axion detection. The axion induces a transition between the two Zeeman sublevels \(a\) and \(b\), that can be tuned to the axion mass by the external magnetic field controlling the energy separation \(\Delta E\).