We present a direct determination of photofragment alignment produced by circularly polarized light in photolysis of a planar polyatomic molecule. This alignment arises via a new mechanism involving coherent excitation of two mutually perpendicular in-plane transition dipole moment components. The alignment is described by a new anisotropy parameter ${\gamma }_2^{\prime }$ that was isolated by a unique laser polarization geometry. The determination of the parameter ${\gamma }_2^{\prime }$ was realized in ozone photolysis at 266 nm where dc slice images of $\mathrm{O}({}^1{D}_2)$ atomic fragments were acquired. A model developed for interpretation of the photolysis mechanism shows that it can exist only in case of failure of the Born-Oppenheimer approximation when electronic and vibrational (vibronic) interactions have to be taken into account. This finding suggests that determination of the alignment parameter ${\gamma }_2^{\prime }$ can be used as a key for direct insight into vibronic interactions in photolysis of polyatomic molecules. The results obtained for ozone photolysis via the Hartley band showed significant ${\gamma }_2^{\prime }$ alignment but little recoil speed dependence, consistent with the notion that, as opposed to the situation for derivative coupling, under our experimental conditions, the vibronic contributions to the nonadiabatic dynamics are not dependent on recoil speed.

• Received 6 November 2018
• Revised 9 January 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.083403