Fig. 1. Scheme of probing JT deformation in a photoionized ${\mathrm{CH}}_4^{+}$ cation.

The ${\mathrm{CH}}_4^{+}$ cation in a C_3v geometry (at the peak of the potential energy surface) undergoes JT distortion to a C_2v geometry (at the bottom of the potential energy surface) and the identified structural evolution pathway is indicated by the arrows. By recording the coincident fragments from Coulomb explosion of those geometries as a function of time between the pump-probe laser pulses, the dynamics of the JT deformation can be revealed.