Radial Coupling at Conical Intersection Governs Competing Fragmentation Pathways in Halomethane Cations

Controlling selective bond cleavage in polyatomic molecules remains a fundamental challenge in photochemistry, primarily due to nonadiabatic dynamics at conical intersections. By combining time-resolved Coulomb explosion imaging with quantum wave packet simulations, we report a striking reversal in bond selectivity for two structurally similar halomethane cations under strong laser fields, i.e., CH_{3}Cl^{+} predominantly undergoes C-Cl cleavage, whereas CH_{3}F^{+} favors C-H fission. We attribute this contrast to the halogen-dependent strength of radial coupling at the conical intersection between the A and B electronic states. The strong coupling in CH_{3}Cl^{+} diverts the wave packet to the repulsive A state, enhancing C-Cl cleavage and opening an indirect resonant Raman excitation pathway for C-H rupture. In contrast, the weaker coupling in CH_{3}F^{+} restricts the dynamics primarily to direct Raman excitation, resulting in dominant C-H bond fission. Our findings establish radial coupling strength at conical intersections as a key control parameter for multibond fragmentation and suggest new avenues for laser-driven reaction control through targeted manipulation of nonadiabatic couplings.