Time-resolved photoelectron imaging was used to investigate nonadiabatic processes operating in the excited electronic states of nitrobenzene and three methyl-substituted derivatives: 3,5-, 2,6-, and 2,4-dimethylnitrobenzene. The primary goal was evaluating the dynamical impact of the torsional angle between the NO2 group and the benzene ring planesomething previously implicated in mediating the propensity for branching into different photodissociation pathways (NO vs NO2 elimination). Targeted, photoinitiated release of NO radicals is of interest for clinical medicine applications, and there is a need to establish basic structure–dynamics–function principles in systematically varied model systems following photoexcitation. Within our 200 ps experimental detection window, we observed no significant differences in the excited-state lifetimes exhibited by all species under study using a 267 nm pump and ionization with an intense 400 nm probe. In agreement with previous theoretical predictions, this suggests that the initial energy redistribution dynamics within the singlet and triplet manifolds are driven by motions localized predominantly on the NO2 group. Our findings also imply that both NO and NO2 elimination occur from a vibrationally hot ground state on extended (nanosecond) timescales, and any variations in NO vs NO2 branching upon site-selective methylation are due to steric effects influencing isomerization prior to dissociation.

Time-resolved photoelectron spectroscopy using a vacuum ultraviolet probe brings new insight to the excited state dynamics operating in acetylacetone.

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We review new light source developments and data analysis considerations relevant to the time-resolved photoelectron imaging technique. Case studies illustrate how these themes may enhance understanding in studies of excited state molecular dynamics.

We present a numerical modelling study employing a kinetic model based on rate equations to investigate the role of excited state lifetime and laser pulse duration on effective relative detection efficiency in time-resolved pump-probe spectroscopy. The work begins to address the critical outstanding problem of photochemical branching ratio determination when excited state population evolves via competing relaxation pathways in molecular systems. Our findings reveal significant differences in detection sensitivity, which can exceed an order of magnitude under typical experimental conditions for excited state lifetimes ranging between 10 fs and 1 ps. We frame our discussion within the widely used approach of ultrafast photoionization for interrogating excited state populations, but our overall treatment may be readily extended to consider a broader range of experimental methodologies and timescales.