Vacuum ultraviolet excited state dynamics of small amides

Non-adiabatic dynamics of Rydberg-excited diethylamine

Femtosecond time-resolved photoelectron spectroscopy was employed to investigate the ultrafast non-adiabatic dynamics of diethylamine (DEA). Following the direct excitations of the two main conformational structures (i.e., TT and TG) of DEA to the 3p Rydberg states at 200 nm, DEA undergoes internal conversion to the 3s state with ∼68 fs, which is similar to previously relaxation pathways of several secondary and tertiary amines. Subsequent dynamics on the 3s Rydberg state evolves along the NH stretching coordinate and is then accompanied by the breaking of the NH bond in about 120 fs, rather than the cleavage of the NC, or CC bonds as well as the conformational transformations between the TT and TG conformers on the 3s states. Theoretically, we reveal the pre-dissociative nσ* character evolving along the NH stretching coordinate within the 3s state as DEA dissociates to yield H atom products. As contrasted to the well-known conformational NC bond rotation motions in systems such as tertiary amines, the non-adiabatic relaxation of the 3s state of DEA is predominantly characterized by the NH bond dissociation process.

Nonstatistical Photoinduced Processes in Gaseous Organic Molecules

Processes that proceed in femtoseconds are usually referred to as being ultrafast, and they are investigated in experiments that involve laser pulses with femtosecond duration in so-called pump probe schemes, where a light pulse triggers a molecular process and a second light pulse interrogates the temporal evolution of the molecular population. The focus of this review is on the reactivity patterns that arise when energy is not equally distributed on all the available degrees of freedom as a consequence of the very short time scale in play and on how the localization of internal energy in a specific mode can be thought of as directing a process toward (or away from) a certain outcome. The nonstatistical aspects are illustrated with examples from photophysics and photochemistry for a range of organic molecules. The processes are initiated by a variety of nuclear motions that are all governed by the energy gradients in the Franck-Condon region. Essentially, the molecules will start to adapt to the new electronic environment on the excited state to eventually reach the equilibrium structure. It is this structural change that is enabling an ultrafast electronic transition in cases where the nuclear motion leads to a transition point with significant coupling between to electronic states and to ultrafast reaction if there is a coupling to a reactive mode at the transition point between the involved states. With the knowledge of the relation between electronic excitation and equilibrium structure, it is possible to predict how the nuclei move after excitation and often whether an ultrafast (and inherently nonstatistical) electronic transition or even a bond breakage will take place. In addition to the understanding of how nonstatistical photoinduced processes proceed from a given excited state, it has been found that randomization of the energy does not even always take place when the molecule takes part in processes that are normally considered statistical, such as for example nonradiative transitions between excited states. This means that energy can be localized in a specific degree of freedom on a state other than the one that is initially prepared. This is a finding that could kickoff the ultimate dream in applied photochemistry; namely light excitation that leads to the rupture of a specific bond.

Ultraviolet Excitation Dynamics of Nitrobenzenes

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.

VUV excited-state dynamics of cyclic ethers as a function of ring size

The ultrafast vacuum ultraviolet (VUV) Rydberg state dynamics of cyclic ethers reflects ring strain.