Luminescence of the (O₂(a¹Δ_g))₂ collisional complex in the temperature range of 90-315 K: Experiment and theory

Experimental and theoretical studies of collision induced emission of singlet oxygen molecules O₂(a¹Δ_g) in the visible range have been performed. The rate constants, half-widths, and position of peaks for the emission bands of the (O₂(a¹Δ_g))₂ collisional complex centered around 634 nm (2) and 703 nm (3) have been measured in the temperature range of 90–315 K using a flow-tube apparatus that utilized a gas-liquid chemical singlet oxygen generator. The absolute values of the spontaneous emission rate constants k₂ and k₃ are found to be similar, with the k₃/k₂ ratio monotonically decreasing from 1.1 at 300 K to 0.96 at 90 K. k₂ slowly decreases with decreasing temperature but a sharp increase in its values is measured below 100 K. The experimental results were rationalized in terms of ab initio calculations of the ground and excited potential energy and transition dipole moment surfaces of singlet electronic states of the (O₂)₂ dimole, which were utilized to compute rate constants k₂ and k₃ within a statistical model. The best theoretical results reproduced experimental rate constants with the accuracy of under 40% and correctly described the observed temperature dependence. The main contribution to emission process (2), which does not involve vibrational excitation of O₂ molecules at the ground electronic level, comes from the spin- and symmetry-allowed 1¹A_g←¹B_3u transition in the rectangular H configuration of the dimole. Alternatively, emission process (3), in which one of the monomers becomes vibrationally excited in the ground electronic state, is found to be predominantly due to the vibronically allowed 1¹A_g←2¹A_g transition induced by the asymmetric O–O stretch vibration in the collisional complex. The strong vibronic coupling between nearly degenerate excited singlet states of the dimole makes the intensities of vibronically and symmetry-allowed transitions comparable and hence the rate constants k₂ and k₃ close to one another