Structure and energy transfer within isolated (O2)2 dimers via high resolution electronic spectroscopy

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Abstract
Polarized, high resolution 1Δ g +1Δ g ↔3Σ g −+3Σ g −(1 A g ) and 3Σ g −+1Σ g +→3Σ g −+3Σ g − (3 B 3u ) (O2)2 dimer spectra have been observed in solid neon host at 4.2 K. Excited state electronic fine structure components are resolved and assigned. The 1Δ g +1Δ g transition is both allowed and vibronically induced by b 3u antisymmetric O2 stretch quanta. The 1Δ g +1Δ g geometry is identical to that of the ground 3Σ g −+3Σ g − state; the 3Σ g −+1Σ g + state appears to have slightly different structure. The spectra are entirely consistent with D 2h structure, and less likely with D 2d structure. The dimer triplet (3 B 3u ) ground state subcomponents lie ∼55 cm−1 above the dimer singlet (1 A g ) ground state. The dimer vibrational constants are unperturbed from the O2 in vacuum values. The nearest neighbor vibrational energy transfer time from one O2(1Δ g ) to another is ∼14 ps, as revealed by zero phonon line splittings in emission. The nearest neighbor electronic energy transfer time of the 1Σ g +(v=0) state appears to be 0.6 ps, as revealed by spectral shifts. Both these processes appear to result from electron exchange, and not multipole–multipole interactions. Isotopic substitution experiments show that unfavorable Franck–Condon factors ’’trap’’ the 1Σ g + (v) state on one side of the dimer for v?1. Energy transfer of 1Σ g + (v) from dimers to nearby monomers also occurs. Comparison is made with the calculations of Bhandari and Falicov. Values for the intermolecular exchange integrals are obtained. Gas phase energy transfer is also discussed.