State-to-state photodissociation dynamics of CO 2 at 157 nm.

State-to-state photodissociation of CO ₂ ( v ₂ = 0 and 1) at 157 nm via the O( ¹ D) + CO( X ¹ Σ ⁺ ) channel was studied by using the sliced velocity map imaging technique. Both the O( ¹ D) and CO( X ¹ Σ ⁺ ) products were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI). Detection of CO via the B ¹ Σ ⁺ ←← X ¹ Σ ⁺ transition allowed ro-vibrational state-selective detection, and combined with imaging, the fragment energy and angular distributions have been derived. For CO( v = 0 and 1| j ) products from the CO ₂ ( v ₂ = 0) molecule, the angular distributions of low- j CO display positive anisotropic parameters (about 0.8); with j increasing, the product anisotropic parameters gradually reduce to zero. While for CO( v = 0 and 1| j ) products from the vibrational excited CO ₂ ( v ₂ = 1) molecule, the angular distributions of low- j CO also display positive anisotropic parameters; with j increasing, the product anisotropic parameters first decrease to zero and then become negative (about -0.5). Experimental results show that the observed variation of the product angular distribution with the rotational quantum number of CO is consistent with trends predicted by a classical model for non-axial fragment recoil. The results support advanced theoretical predictions of a predominantly parallel transition to the bent 2 ¹ A' excited state of CO ₂ , where bending introduces torque during the direct dissociation process.