Dynamics of excited-state proton transfer systems via time-resolved photoelectron spectroscopy

The use of time-resolved photoelectron spectroscopy for analyzing excited state intramolecular proton transfer (ESIPT) and internal conversion dynamics in a model system was investigated. The photoelectron spectra of both the excited state enol and keto tautomers were presented as a function of pump laser wavelength and pump-probe time delay. It was found that the internal conversion dynamics in o-hydroxybenzaldehyde (OHBA) was influenced by interactions with a close-lying n??* state.open958

Time-resolved configuration interaction in all-trans decatetraene.

International audienceno abstrac

Resonance Raman excitation profiles of 1,3-butadiene in vapor and solution phases

Resonance Raman spectra of 1,3-butadiene have been obtained in the vapor phase at six excitation wavelengths between 2231 and 2140 Å and in cyclohexane solution at six wavelengths between 2310 and 2179 Å. Absolute scattering cross sections have also been measured in the vapor at 2150 Å and in solution at three wavelengths. The experimental absorption spectra and Raman excitation profiles are compared with the results of numerical simulations that explicitly incorporate 18 vibrational modes and include excited-state frequency changes and Duschinsky rotation but consider only a single resonant electronic state (the lowest allowed state of 1Bu + symmetry). The effect of the nearby forbidden 21Ag - state is also explored in separate simulations that properly treat the coupling of the two states by v24, the lowest frequency vibration of bu symmetry, but explicitly consider only three vibrational modes at a time. It is concluded that the forbidden state has a relatively small effect on the resonance Raman intensities of most transitions not involving excitation of v24. However, the sensitivity of the spectra to small changes in the Duschinsky rotation parameters leads to considerable ambiguity in efforts to experimentally determine excited-state vibrational frequencies and geometry changes and solvent effects on these quantities. The data are most consistent with a model in which the extreme diffuseness of the absorption spectrum arises from rapid depopulation of the initially excited 1Bu + state without any large distortion from planarity in that state. © 1993 American Chemical Society.link_to_subscribed_fulltex

Discerning vibronic molecular dynamics using time-resolved photoelectron spectroscopy

International audienceDynamic processes at the molecular level occur on ultrafast time scales and are often associated with structural as well as electronic changes. These can in principle be studied by time-resolved scattering(1-3) and spectroscopic methods, respectively. In polyatomic molecules, however, excitation results in the rapid mixing of vibrational and electronic motions, which induces both charge redistribution and energy flow in the molecule(4,5). This `vibronic' or `non-adiabatic' coupling is a key step in photochemical(6) and photobiological processes(7) and underlies many of the concepts of molecular electronics(8), but it obscures the notion of distinct and readily observable vibrational and electronic states. Here we report time-resolved photoelectron spectroscopy measurements that distinguish vibrational dynamics from the coupled electronic population dynamics, associated with the photo-induced internal conversion, in a linear unsaturated hydrocarbon chain. The vibrational resolution of our photoelectron spectra allows for a direct observation of the underlying nuclear dynamics, demonstrating that it is possible to obtain detailed insights into ultrafast non-adiabatic processes

Electronic relaxation dynamics in DNA and RNA bases studied by time-resolved photoelectron spectroscopy

We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA and RNA bases adenine, cytosine, thymine, and uracil in a molecular beam. We discuss in detail the analysis of our adenine TRPES spectra. A global two-dimensional fit of the time and energy-resolved spectra allows for reliable separation of photoelectron spectra from several channels, even for overlapping bands. Ab initio calculations of Koopmans' ionization correlations and He(I) photoelectron spectra aid the assignment of electronically excited states involved in the relaxation dynamics. Based upon our results, we propose the following mechanism for electronic relaxation dynamics in adenine: Pump wavelengths of 250, 267 and 277 nm lead to initial excitation of the bright S2(????*) state. Close to the band origin (277 nm), the lifetime is several picoseconds. At higher vibronic levels, i.e. 250 and 267 nm excitation, rapid internal conversion (?? < 50 fs) populates the lower lying S1(n??*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(????*) state, previously proposed as an ultrafast relaxation pathway from S2(????*). We present preliminary results from TRPES measurements of the other DNA bases at 250 nm excitation.close16615

Substituent effects in molecular electronic relaxation dynamics via time-resolved photoelectron spectroscopy: pi pi* states in benzenes

We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: C=C (styrene), C=O (benzaldehyde), and C???C (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (??-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second ????* state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the second ????* state, a slower decaying component to the first ????* state. Very fast electronic relaxation constants (< 100 fs) for the second ????* states were observed for all molecules studied and are explained by relaxation to the first ????* via a conical intersection near the planar minimum. Although a "floppy" methyl substitution (??-methylstyrene, acetophenone) leads as expected to even faster second ????* decay rates, a rigid ring substitution (indene) has no discernible effect. The much slower electronic relaxation constants of the first ????* states for styrene and phenylacetylene are very similar to those of benzene in its first ????* state, at the same amount of vibrational energy. By contrast, the lifetime of the first ????* state of indene was much longer, attributed to its rigid structure. The second ????* state of benzaldehyde has a short lifetime, similar to the other derivatives. However, the relaxation of its first ????* state is orders of magnitude faster than that of the non-carbonyl compounds, due to the well-known presence of a lower lying n??* state. Methylation (acetophenone) leads to still faster first ????* state relaxation rates. These results fit very well with the current understanding of aromatic photophysics, demonstrating that time-resolved photoelectron spectroscopy provides for a facile, accurate and direct means of studying electronic relaxation dynamics in a wide range of molecular systems.close393