As an example, our recent work on nitric acid and other important atmospheric species seeks to understand from first principles quantum mechanics why some chemical products are produced and not others. In the photodissociation of nitric acid two chemical bonds may break, producing OH+NO2 and HONO+O respectively. The ability of the electronic wavefunction to change along the reaction coordinate, particularly the orientation of the radical OH p electron's orbital, plays a critical role in determining what products are formed. Our early molecular beam experiments showed that nonadiabatic recrossing of the transition state plays a dominant role in determining the branching between chemical bond fission channels, reversing the expected branching between C-Br and C-Cl fission in the Br(CH2)nCOCl. Suppressing rapid intramolecular electronic energy transfer allows you to preferentially cleave a selected chemical bond. Our experiments and supporting ab initio calculations elucidate the intramolecular distance and conformation dependence of nonadiabatic recrossing of the reaction barriers in the competing reaction channels. Other experiments use molecular photodissociation to directly access both the upper and lower adiabatic potential energy surfaces, respectively, near the transition state region of a excited state bimolecular reaction to probe the influence of nonadiabatic coupling in chemical reaction dynamics. Our molecular beam photofragmentation and emission spectroscopy experiments and collaborative theoretical work on CH3SH investigated how accessing different regions of the CH3S + H → CH3+SH reactive potential energy surfaces changes the branching between the S-H and C-S bond fission channels and how nonadiabatic coupling influences the dynamics. In this system and in H2S, we used the technique of emission spectroscopy of dissociating molecules to investigate the dynamics which occurs during the subpicosecond dissociation event, providing a key link between the absorption spectrum and the final product quantum states. We have recently introduced a method for investigating the competing unimolecular dissociation channels of isomerically-selected radicals as a function of internal energy in the radical. Radical intermediates play a key role in a wide range of chemical processes, yet many key isomeric radical intermediates elude direct experimental probes. Our experiments photolytically produce from an appropriate precursor a selected radical isomer and disperse the radicals by their neutral velocity imparted in the photolysis, thus dispersing them by internal energy. For the unstable radicals, they then measure the branching between C-C and C-H fission products via tunable VUV photoionization of products dispersed by their velocity. This offers the unprecedented ability to measure the branching between isomeric product channels as a function of internal energy in the dissociating radical isomer on the ground state potential energy surface.