Molecular Wavepacket Dynamics at a Jahn-Teller Conical Intersection

According to the Jahn-Teller theorem, any symmetric configuration of atoms in an electronically degenerate molecular state is unstable and distorts to a configuration of lower symmetry, hence lifting the degeneracy. Jahn-Teller dynamics feature in numerous, highly symmetric systems, such as fullerenes and in doped rare-earth magnetites where they underly the phenomenon of ‘colossal magneto-resistance’. Here, we use ultrafast time-resolved photoelectron velocity-map imaging (VMI) to study the fundamental Jahn-Teller dynamics in an excited state of isolated ammonia (NH$_3$) molecules. Supersonically cooled NH$_3$ is resonantly excited by a 160 nm, 80 fs pump pulse. A time delayed 400 nm, 40 fs probe pulse photoionizes the molecule and the kinetic energy and angular distribution of the ejected photoelectron is measured as a function of time. Dramatic changes in the time-dependent angular distributions are observed, which reveal details of non-adiabatic wavepacket propagation on the coupled Jahn-Teller potential surfaces.