Radiation and chemi-ionization/recombination processes in atom-atom and ion-atom collisions in the modeling of low-temperature stellar atmospheres A. A. Mihajlov 1) /M,JQMDWRYLü 1) 06'LPLWULMHYLü 2) , D. Jevremoviü 3) , P. Hauschildt 4) 1) Institute of Physics, P. O. Box 57, 11001 Belgrade, Yugoslavia 2) Astronomical Observatory, Volgina 7, 11160 Belgrade, Serbia, Yugoslavia 3) APS Division, Phys. Depart., Queen's University Belfast, BT7 1NN, Northern Ireland,UK 4) Depart. of Phys. and Astronomy and Center for Simulat. Physics, University of Georgia, Athens, GA 30602 USA Abstract Results of our investigations of the influence of radiation, chemi-ionization and chemi-recombination processes in atom-atom and ion-atom collisions (in the case of the symmetric atom-atom and ion-atom systems) in stellar hydrogen and helium plasmas are presented. The considered ion-atom radiation processes influence significantly on the optical characteristics of stellar plasma, and the considered chemi-ionization/recombination processes on the excited atomic energy level populations, as well as on the electron density. The consequence of the obtained results is that they should be taken into account for the modeling of photosohere and lower chromosphere of the Sun and similar star (hydrogen case) and white dwarfs atmospheres (helium case). 1. INTRODUCTION In this contribution, the influence of two groups of closely related collisional processes, on characteristics of the weakly ionized layers of stellar atmospheres has been considered.The first group of processes are symmetrical ion-atom radiation processes. We have in view here the emission processes λ ε + ⇒ + + + 2 A A A , (1a) λ ε + + ⇒ + + + A A A A , (1b) and inverse absorption processes + + + ⇒ + A A A λ ε 2 , (2a) + + + ⇒ + + A A A A λ ε , (2b) where A and A + denotes either H(1s) and H + or He(1s 2 ) and He+(1s), + 2 A - either hydrogen or helium molecular ion in the ground electronic state, and ε λ - the energy of the emitted or absorbed photon with the wavelength λ. Here we consider the range of the ion-atom termal impact energies (lower or close to 1eV), and consequently, the optical part of the EM spectrum. Moreover, we note that here we assume that the molecular ion + 2 A is in highly