An analytical approach for the description of photoabsorption by a gas or plasma medium containing atomic and molecular components in thermodynamical equilibrium is developed. Continuous absorption of radiation is due to the photodissociation of a diatomic molecule from a manifold of excited rovibrational states and free-free transitions between the two electronic terms of a quasimolecule temporarily formed during a collision of atomic particles. The formulae are obtained for individual photodissociation cross sections from a given rovibrational state and for the Bolztmann-averaged cross section. Particular attention is paid to the derivation of a general analytical expression for the total absorption coefficient including the integral contribution of bound-free and free-free radiative transitions. The consideration is based on the theory of nonadiabatic transitions combined with the approximation of a quasicontinuum for rovibrational states. The theory is applied to the investigation of photoabsorption by the H₂⁺ ion in the IR, visible and UV spectral regions. It is shown that our results are in good agreement with available ab initio quantal calculations of photodissociation cross sections and with semiclassical calculations of absorption coefficients. Special attention is paid to the investigation of the relative contributions of the H₂⁺ and H^- ions to the total absorption in a wide range of wavelengths and temperatures.
Some of the results obtained can be found in Refs. [1,2,3].

References