Intramolecular V-V transfer between the components of the (3₁/2₁4₁5₁) and (3₁4₁/2₁4₂5₁) Fermi dyads in acetylene in C₂H₂-C₂H₂ collisions

Infrared-ultraviolet double resonance experiments have been performed to determine rate coefficients for collisional intramolecular vibration-vibration transfer in pure acetylene within two Fermi dyads: (a) that formed from almost equal mixtures of the zero-order, normal-mode states |3₁〉 and |2₁(4₁5₁)⁰〉 and (b) that which is composed predominantly of the |3₁4₁ ¹〉, |2₁4₂ ²5₁ ^-1〉 and |2₁4₂ ⁰5₁ ¹〉 states. Pulses of tunable IR excited molecules to the lower component (II) of one of the Fermi dyads and the evolution of population in the upper component (I) was observed by exciting laser-induced fluorescence (LIF) from this level using tunable UV laser radiationIn this Letter, we employ the notation for vibrational states whereby, for example, 3₁ denotes one quantum of excitation in the ν₃ normal mode. (3₁/2₁4₁5₁)_{I, II} denotes the Fermi dyad in which the principal contributions to the vibrational eigenstates are from the normal mode wavefunctions describing the zeroth-order 3₁ fundamental and 2₁4₁5₁ combination vibrational states. In each dyad, the subscript II is used to denote the lower of the two component states, I the higher. Analysis of the LIF signals vs. time yields the following rate coefficients (k_id) for intradyad transfer at 295 K: for (3₁/2₁4₁5₁)_I↔II, k_id=(2.1±0.2)×10^-10 cm³ molecule^-1 s^-1; for (3₁4₁/2₁4₂5₁)_I↔II, k_id=(3.2±0.6)×10^-11 cm³ molecule^-1 s^-1. These results are discussed in terms of the matrix elements for such collision-induced transitions and differences in the mixing of the zero-order states in these two pairs of Fermi dyads.