Time-resolved photoelectron imaging was used to investigate the electronic relaxation dynamics of gas-phase aniline, N, N-dimethylaniline, and 3,5-dimethylaniline following ultraviolet excitation at 250 nm. Our analysis was supported by ab initio coupled-cluster calculations evaluating excited state energies and (in aniline) the evolution of a range of excited state physical properties as a function of N-H bond extension. Due to a lack of consistency between several earlier studies undertaken in aniline, the specific aim of this present work was to gain new insight into the previously proposed non-adiabatic coupling interaction between the two lowest lying singlet excited states S1(ππ(∗)) and S2(3s/πσ(∗)). The methyl-substituted systems N, N-dimethylaniline and 3,5-dimethylaniline were included in order to obtain more detailed dynamical information about the key internal molecular coordinates that drive the S1(ππ(∗))/S2(3s/πσ(∗)) coupling mechanism. Our findings suggest that in all three systems, both electronic states are directly populated during the initial excitation, with the S2(3s/πσ(∗)) state then potentially decaying via either direct dissociation along the N-X stretching coordinate (X = H or CH3) or internal conversion to the S1(ππ(∗)) state. In aniline and N, N-dimethylaniline, both pathways most likely compete in the depletion of S2(3s/πσ(∗)) state population. However, in 3,5-dimethylaniline, only the direct dissociation mechanism appears to be active. This is rationalized in terms of changes in the relative rates of the two decay pathways upon methylation of the aromatic ring system.

Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond ultraviolet laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the CHNO ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theoretically predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold molecular beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths <250 nm, revealing extremely rapid (<200 fs) relaxation of the bright S(ππ) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility.

We present results from a recent time-resolved photoelectron imaging (TRPEI) study investigating the non-adiabatic relaxation dynamics of N,N-dimethylaniline (N,N-DMA) and 3,5-dimethylaniline (3,5-DMA) following excitation at 240 nm. Analysis of the experimental data is supported by ab initio coupled-cluster calculations evaluating excited state energies and the evolution of several excited state physical properties as a function of N-H/N-CH bond extension - a critical reaction coordinate. The use of site-selective methylation brings considerable new insight to the existing body of literature concerning photochemical dynamics in the related system aniline at similar excitation wavelengths. The present work also builds on our own previous investigations in the same species at 250 nm. The TRPEI method provides highly differential energy- and angle-resolved data and, in particular, the temporal evolution of the photoelectron angular distributions afforded by the imaging approach offers much of the new dynamical information. In particular, we see no clear evidence of the second excited 2ππ* state non-adiabatically coupling to the lower-lying S(ππ*) state or the mixed Rydberg/valence S(3s/πσ*) state. This, in turn, potentially raises some unresolved questions about the overall nature of the dynamics operating in these systems, especially in regard to the 2ππ* state's ultimate fate. More generally, the findings for the aromatic systems N,N-DMA and 3,5-DMA, taken along with our recent TRPEI results for several aliphatic amine species, highlight interesting questions about the nature of electronic character evolution in mixed Rydberg-valence states as a function of certain key bond extensions and the extent of system conjugation. We begin exploring these ideas computationally for a systematically varied series of tertiary amines.

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.