Field-induced surface hopping method for probing transition state nonadiabatic dynamics of Ag3

We present the simulation of time-resolved photoelectron spectra of Ag(3) involving excitation from the linear transition state, where nonadiabatic relaxation takes place in a complex manifold of electronic states. Thus, we address ultrafast processes reachable by negative ion-to neutral-to positive ion (NeNePo) spectroscopy starting from the linear Ag anionic species. For this purpose we use our newly developed field-induced surface hopping method (FISH) augmented for the description of photoionization processes. Furthermore we employ our method for nonadiabatic molecular dynamics "on the fly" in the framework of time-dependent density functional theory generalized for open shell systems. Our presented approach is generally applicable for the prediction of time-resolved photoelectron spectra and their analysis in systems with complex electronic structure as well as many nuclear degrees freedom. This theoretical development should serve to stimulate new ultrafast experiments.

Ultrafast photodynamics of furan

Ultrafast photodynamics of furan has been studied by time-resolved photoelectron imaging (TRPEI) spectroscopy with an unprecedented time resolution of 22 fs. The simulation of the time-dependent photoelectron kinetic energy distribution (PKED) has been performed with ab initio nonadiabatic dynamics "on the fly" in the frame of time-dependent density functional theory. Based on the agreement between experimental and theoretical time-dependent photoelectron signal intensity as well as on PKED, precise time scales of ultrafast internal conversion from S(2) over S(1) to the ground state S(0) of furan have been revealed for the first time. Upon initial excitation of the S(2) state which has π-π* character, a nonadiabatic transition to the S(1) state occurs within 10 fs. Subsequent dynamics invokes the excitation of the C-O stretching and C-O-C out of plane vibrations which lead to the internal conversion to the ground state after 60 fs. Thus, we demonstrate that the TRPEI combined with high level nonadiabatic dynamics calculations provide fundamental insight into ultrafast photodynamics of chemically and biologically relevant chromophores.

A comparative picosecond transient infrared study of 1-methylcytosine and 5'-dCMP that sheds further light on the excited states of cytosine derivatives

The role of N1-substitution in controlling the deactivation processes in photoexcited cytosine derivatives has been explored using picosecond time-resolved IR spectroscopy. The simplest N1-substituted derivative, 1-methylcytosine, exhibits relaxation dynamics similar to the cytosine nucleobase and distinct from the biologically relevant nucleotide and nucleoside analogues, which have longer-lived excited-state intermediates. It is suggested that this is the case because the sugar group either facilitates access to the long-lived (1)n(O)π* state or retards its crossover to the ground state.

Relaxation mechanisms of UV-photoexcited DNA and RNA nucleobases

A comprehensive effort in photodynamical ab initio simulations of the ultrafast deactivation pathways for all five nucleobases adenine, guanine, cytosine, thymine, and uracil is reported. These simulations are based on a complete nonadiabatic surface-hopping approach using extended multiconfigurational wave functions. Even though all five nucleobases share the basic internal conversion mechanisms, the calculations show a distinct grouping into purine and pyrimidine bases as concerns the complexity of the photodynamics. The purine bases adenine and guanine represent the most simple photodeactivation mechanism with the dynamics leading along a diabatic ππ* path directly and without barrier to the conical intersection seam with the ground state. In the case of the pyrimidine bases, the dynamics starts off in much flatter regions of the ππ* energy surface due to coupling of several states. This fact prohibits a clear formation of a single reaction path. Thus, the photodynamics of the pyrimidine bases is much richer and includes also nπ* states with varying importance, depending on the actual nucleobase considered. Trapping in local minima may occur and, therefore, the deactivation time to the ground state is also much longer in these cases. Implications of these findings are discussed (i) for identifying structural possibilities where singlet/triplet transitions can occur because of sufficient retention time during the singlet dynamics and (ii) concerning the flexibility of finding other deactivation pathways in substituted pyrimidines serving as candidates for alternative nucleobases.