Vibrational relaxation studies of matrix isolated C−2

Abstract

Absorption studies of C⁻₂ formed by 1216 Å photolysis of C₂H₂ in Ar at 14 °K indicate that the C⁻₂ concentration approaches a limit of ∼1 ppm, possibly due to electron return tunneling to the unknown cation counterpart X⁺ for separations closer than ∼175 Å. A dual pulsed laser method was used to produce vibrationally excited C⁻₂ and to monitor the v^″=1 nonradiative decay. Lifetime values were strongly dependent on C₂H₂ concentration and increased to 147 msec at an Ar:C₂H₂=20 000 ratio. This value was reduced to 93 msec for a Ar:C₂D₂ matrix, presumably because of near resonance of the C⁻₂ energy (1779 cm⁻¹) with the C≡C stretch (1762 cm⁻¹). Decay times were longer in Kr and Xe than in Ar. The absence of any temperature effect over the range 14 to 30 °K indicates that few lattice phonons are involved in the rate determining step, the transfer to C₂H₂. The rate constant for this transfer decreases as the third to fifth power of an average C⁻₂–C₂H₂ distance, a much slower drop than the tenth power dependence predicted for the first applicable electrostatic coupling term, a quadrupole–quadrupole interaction. A more direct coupling through the intermediate argon atoms is implied but ¹³C studies show that this coupling does not extend so far as to permit ’’resonant’’ transfer of vibrational energy from one C⁻₂ to the next. Direct decay to lattice phonons is slow, with a lower lifetime limit of ∼160 msec in Ar.