Photoexcited molecules in the vacuum ultraviolet region electronically relax via Rydberg and intra-valence excited states. As an example of such relaxation processes, ion-pair formation of ArHCl + hv --> ArH+ + Cl- from the third (1)A' electronic state of ArHCl is studied theoretically. The potential energy surfaces are calculated at the level of multi-reference single and double excitation configuration interaction theory. Detailed energetics of the ion-pair formation is discussed. It is found that the present molecular system has a potentiality to enhance the ion-pair formation owing to the large electronic polarizability and high ionization threshold of Ar. (C) 2002 Elsevier Science B.V. All rights reserved.