Radiative transitions in the molecular H2+ ion

A theoretical analysis was made of continuous light absorption in the visible, IR, and UV spectral ranges in a quasi-equilibrium hydrogen plasma with an effective temperature between 1500 and 25000 K. The light absorption is caused by the photodissociation of molecular H₂⁺ ions from a large group of excited vibrational – rotational levels of the ground electronic term and by free – free radiative transitions at the collisions of protons (H⁺ ) with hydrogen atoms H(1s). The phototransitions under consideration take place with a change of the electronic state ²Σ⁺_g → ²Σ⁺_u of the molecular (quasi-molecular) H₂⁺ ion. Quantum and quasi-classical descriptions of partial photodissociation cross sections and effective photoabsorption cross sections at particle collisions were accomplished on the basis of the theory of nonadiabatic transitions between the potential energy curves of a diatomic molecule. An analytical approach is set forth to calculate the integral contribution from all possible vibrational – rotational levels to the photodissociation cross section averaged over the Boltzmann distribution function. A detailed study was made of the contribution of bound – free and free – free transitions to the total optical absorption coefficient by the H₂⁺ system as a function of temperature and wavelength. The recently calculated values of effective phototransition cross sections and photoabsorption coefficients are presented and compared with previously available data. Emphasis is placed on considering the relative contribution of positive molecular H₂⁺ ions and negative atomic H^– ions to the total photoabsorption coefficient of quasi-equilibrium hydrogen plasmas.