Photogeneration of the singlet sigma state of oxygen O2( b 1Σ g +) via double spin–flip transition in van der Waals complex of butadiene with oxygen C4H6 −

The photogeneration of the singlet sigma state of oxygen O2b1Σg+ from van der Waals complexes of butadiene with oxygen C4H6−O2 was revealed and investigated under the molecular beam conditions. The action spectrum of O2b1Σg+ photogeneration, detected by resonance-enhanced multiphoton ionization (2 + 1), is measured with the two-laser pump–probe approach. The spectral location of the center near 250 nm and red-side edge of the action spectrum near 290 nm coincide very well with the numbers expected for the double spin-flip process C41H6−O2+hν→3C43H6+O2(Σg+1, as they are estimated within a weak-coupling model. Velocity map imaging of the singlet sigma state of oxygen indicates formation of two ensembles (translationally “cold” and “hot”) with the increase in the “hot” component contribution with rising photon energy. The “cold” component with the translational temperature independent of the photon energy is provided by direct complex dissociation after “vertical” excitation on the repulsive wall of the van der Waals potential. The presence of the translationally “hot” component is attributed to partial redistribution of the energy of vibrationally excited triplet butadiene to the van der Waals bond on a time scale of complex dissociation. The revealed photoinduced double spin-flip is expected to take place for any weakly bound complexes of oxygen (encounter complexes X − O2 in gas phase and contact complexes in condensed phase). The results obtained also indicate the accessibility of numerous supramolecular double spin-flip transitions in complexes X − O2, giving rise to the pair-states, involving any singlet state of oxygen and any triplet state of a complex partner.