The nCC + 3nCH rovibrational manifold of acetylene. II. Intramolecular perturbations and symmetry-breaking processes

We identify perturbing rovibrational states that are responsible for local J-dependent interactions in the nu(CC)+3 nu(CH) rovibrational manifold of acetylene (C2H2) at similar to 11 600 cm(-1), observed by infrared-ultraviolet double resonance (IR-UV DR) spectroscopy. These comprise: firstly, the set of vibrational eigenstates (0 1 3 0 0)(I)(0) that are involved in an avoided crossing with the primary (0 1 3 0 0)(I)(0) states, as designated in a previous report [Milce and B. J. Orr, J. Chem. Phys. 106, 3592 (1997)]; secondly, a Pi(u) state locally perturbing (0 1 3 0 0)(I)(0) in the range J=3-5; the adjacent (0 1 3 0 0)(I)(0), J=6 state, which is locally perturbed but with no obvious mixed-in Pi-character; and finally, the local perturber of the (0 4 0 3 3)(+)(0) level at J=12. These three vibrational states are now identified and relabeled, according to their most prevalent zero-order normal-mode basis states, as (0 1 3 0 0)(0) [previously (0 1 3 0 0)(I)(0)]; (0 4 0 3 3)(2) [previously (0 1 3 0 0)(II)(0)]; (0 2 1 5 2)(1) (previously an unidentified perturbing Pi(u) state); and (0 1 1 8 2)(2) [previously the unidentified local perturber of (0 4 0 3 3)(+)(0), J=12]. This analysis is achieved with the aid of the well-established anharmonically coupled polyad model, adapted from a set of generalized quantum numbers for C2H2. The model has been expanded to include rotational structure, first, in the form of l-resonance off-diagonal elements and, second, in the form of a variety of resonant Coriolis-type interactions. We also predict likely identities for the perturber states involved in unusual odd-Delta J symmetry-breaking effects that have been characterized dynamically. It is now postulated that these effects are due to resonant Stark mixing induced by electric fields arising in either molecular collisions or the infrared excitation pulse itself. Coincident ultraviolet probe transitions from doublet levels of opposite e/f parity also contribute to the observed odd-Delta J energy transfer ascribed to symmetry breaking. (C) 2000 American Institute of Physics. [S0021-9606(00)01121-1].