The intermolecular potential energy surface (PES.) of Ar interacting with the acetylene cation in its (2)Pi(u) ground electronic state is characterized by infrared photodissociation (IRPD) spectroscopy and quantum chemical calculations. In agreement with the theoretical predictions, the rovibrational,analysis of the IRPD spectrum of C2H2+-Ar recorded in the vicinity of the antisymmetric CH stretching fundamental (v(3)) is consistent with a vibrationally averaged T-shaped structure and a ground-state center-of-mass separation of R-c.m. = 2.86 +/- 0.09 Angstrom. The v(3) band experiences a blueshift of 16.7 cm(-1) upon complexation, indicating that vibrational excitation slightly reduces the interaction strength. The two-dimensional intermolecular PES of C2H2+-Ar, obtained from coupled cluster calculations with a large basis set, features strong angular-radial coupling and supports in addition to a global pi-bound minimum also two shallow side wells with linear H-bound geometries. Bound state rovibrational, energy level calculations are carried out for rotational angular momentum J=0-10 (both parities) employing a discrete variable representation-distributed Gaussian basis method. Effective spectroscopic constants are determined for the vibrational ground state by fitting the calculated rotational energies to the standard Watson A-type. Hamiltonian for a slightly asymmetric prolate top. (C) 2004 American Institute of Physics.