Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an additional reagent is also well established. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place. Here we explore the transition from the weak- to the strong-field regimes of laser control for the dissociation of a polyatomic molecule, methyl iodide. The control over the yield of the photodissociation reaction proceeds through the creation of a light-induced conical intersection. The control of the velocity of the product fragments requires external fields with both high intensities and short durations. This is because the mechanism by which control is exerted involves modulating the potentials around the light-induced conical intersection, that is, creating light-induced potentials.

Vinylidene-acetylene isomerization is the prototypical example of a 1,2-hydrogen shift, one of the most important classes of isomerization reactions in organic chemistry. This reaction was investigated with quantum state specificity by high-resolution photoelectron spectroscopy of the vinylidene anions HCC- and DCC- and quantum dynamics calculations. Peaks in the photoelectron spectra are considerably narrower than in previous work and reveal subtleties in the isomerization dynamics of neutral vinylidene, as well as vibronic coupling with an excited state of vinylidene. Comparison with theory permits assignment of most spectral features to eigenstates dominated by vinylidene character. However, excitation of the ν in-plane rocking mode in HCC results in appreciable tunneling-facilitated mixing with highly vibrationally excited states of acetylene, leading to broadening and/or spectral fine structure that is largely suppressed for analogous vibrational levels of DCC.

The correlation between chemical structure and dynamics has been explored in a series of molecules with increasing structural complexity in order to investigate its influence on bond cleavage reaction times in a photodissociation event. Femtosecond time-resolved velocity map imaging spectroscopy reveals specificity of the ultrafast carbon-iodine (C-I) bond breakage for a series of linear (unbranched) and branched alkyl iodides, due to the interplay between the pure reaction coordinate and the rest of the degrees of freedom associated with the molecular structure details. Full-dimension time-resolved dynamics calculations support the experimental evidence and provide insight into the structure-dynamics relationship to understand structural control on time-resolved reactivity.

Femtosecond time-resolved photoelectron spectroscopy experiments have been used to compare the electronic relaxation dynamics of aniline and d7-aniline following photoexcitation in the range 272-238 nm. Together with the results of recent theoretical investigations of the potential energy landscape [M. Sala, O. M. Kirkby, S. Guérin and H. H. Fielding, Phys. Chem. Chem. Phys., 2014, 16, 3122], these experiments allow us to resolve a number of unanswered questions surrounding the nonradiative relaxation mechanism. We find that tunnelling does not play a role in the electronic relaxation dynamics, which is surprising given that tunnelling plays an important role in the electronic relaxation of isoelectronic phenol and in pyrrole. We confirm the existence of two time constants associated with dynamics on the 1(1)πσ* surface that we attribute to relaxation through a conical intersection between the 1(1)πσ* and 1(1)ππ* states and motion on the 1(1)πσ* surface. We also present what we believe is the first report of an experimental signature of a 3-state conical intersection involving the 2(1)ππ*, 1(1)πσ* and 1(1)ππ* states.

Vibrationally state selected predissociation lifetimes for the methyl radical in the 3pz Rydberg state have been measured by femtosecond velocity map imaging and the results explained by ab initio theoretical calculations.