Thermodynamics of an atomic monolayer with diatomic substitutional impurities

Abstract

The low-temperature thermodynamics of a two-dimensional monatomic crystalline matrix containing a diatomic molecular impurity is investigated theoretically. Typical examples of this type of system are monatomic layers of rare gases (Ne, Ar, Kr, Xe) with included molecules of the type N2 and O2. Another example is a hydrogen film, which is a mixture of ortho and para components. Expressions are obtained which describe the crystalline field for a diatomic impurity with allowance for both the contribution of the atoms of the 2D matrix and the field created by the atoms of the substrate. Thus the effective crystalline field is a complicated function of the orientation of the diatomic rotator. In particular, the equilibrium orientation of the rotator depends substantially on the relative amplitudes of the crystalline fields of the matrix and substrate. For example, if the attraction exerted by the substrate is dominant, then the rotator in the equilibrium state will be oriented perpendicular to the layer, and in the opposite case the equilibrium orientation of the rotator will correspond to one of its positions in the plane of the layer. In these two cases the spectra of rotational states of the diatomic impurities and, hence, the thermodynamic characteristics of the system are substantially different. The temperature dependence of the impurity specific heat of the system exhibits a low-temperature peak, the position of which corresponds to temperatures T∼B/2 (B is the rotational constant of the impurity) for rotators lying in the plane of the layer, and T∼KB (K is the amplitude of the crystalline field) for rotators perpendicular to the layer. Such behavior of the system is in principle amenable to experimental observation.