Excited-state dynamics, time-resolved

We turn now to a more detailed description of the photoionization probe step in order to clarify the ideas presented above. Time-resolved photoelectron spectroscopy probes the excited-state dynamics using a time-delayed probe laser pulse that brings about ionization of the excited-state wave packet, usually with a single photon... 

A time-resolved ion yield study of the adenine excited-state dynamics yielded an excited-state lifetime of 1 ps and seemed to support the model of internal conversion via the nn state along a coordinate involving six-membered ring puckering [187]. In order to determine the global importance of the tict channel, a comparison of the primary photophysics of adenine with 9-methyl adenine will be useful, as the latter lacks a tict channel at the excitation energies of concern here. The first study of this type revealed no apparent changes in excited-state lifetime upon methylation at the N9 position [188] a lifetime of 1 ps was observed for both adenine and 9-methyl adenine. This was interpreted as evidence that the tict is not involved in adenine electronic relaxation. 

Another approach of great importance for studies of excited state dynamics is sub-picosecond time resolved spectroscopy. A number of authors have reported femtosecond pump-probe measurements of excited state lifetimes in A, C, T, and G [13-16] and base pair mimics [17]. Schultz et al. have reported time resolved photoelectron spectroscopy and electron-ion coincidence of base pair mimics [18]. these studies can also be compared with similar measurements in solution [19-24], While time resolved measurements provide direct lifetime data, they do have the limitation that the inherent bandwidth reduces the spectral resolution, required for selecting specific electronic states and for selecting single isomers, such as cluster structure and tautomeric form. 

It is possible that the absence of some of the most stable structures of base pairs in the gas phase is due to short excited state dynamics. Direct experimental proof of this explanation requires fast time resolved measurements. For an indirect measure one can consider the linewidths in UV spectra. Of the 27 base pairs analyzed in the gas phase so far, 24 structures do not form WC type pairs and all of those exhibit sharp structured REMPI spectra, as shown in Figure 12.9(a). Only three... 

The excited state dynamics that are probed are expressed in terms of differential absorbance (AA), that is the difference between the absorbances of the excited (Ap) and unexcited (A) sample. The experiments provide the spectra of the probe beams which have travelled respectively through the sample (Is) and reference (Jr) cell in the presence the pump pulse or without. The time resolved differential absorption across the explored spectral range is ... 

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

Takezaki, M., Hirota, N., Terazima, M., Excited State Dynamics of Nitrobenzene Studied by the Time resolved Transient Grating Method Prog. Nat. Sci. 1996, 6, S453 S456. 

The spectroscopy and dynamics of the excited state double proton transfer in 2-amino-4,6-dimethylpyrimidine and 2-amino-4-methoxy-6-methylpyrimidine has been studied by steady-state and time-resolved fluorescence spectroscopy (01CP(265)233). 

Figure 13 Comparison of the experimental and a quantum mechanically computed (by exact wave packet propagation using an ab initio computed potential energy) spectrum of a nonrotating Na, molecule pumped to its B electronic state. (Courtesy of Experiment by S. Rutz, E. Schreiber, and L. Woste Computations by B. Reischl, all of the Free University of Berlin) (a) The short time dynamics Shown is the population of the excited state vs. time as determined by a pump-probe experiment and by the computation (points connected by a straight-line segments). The periodicity (about 320 fs) is due to the symmetric stretch motion, (b) A frequency spectrum. The long time dynamics (as reflected in the well-resolved spectrum) show the contribution of a different set of vibrational modes. The dominant peaks can be identified as the radial pseudorotation motion of Na,(B) while the splittings are due to the angular pseudorotational motion. (Adapted from B. Reischl, Chem. Phys. Lett., 239 173 (1995) and V. Bonacic-Koutecky, J. Gaus, J. Manz, B. Reischl, and R. de Vivie-Riedle, to be published.)...

In the time-resolved experiments described so far, the pump laser simply populates the intermediate excited state. The experiment, therefore, becomes a means to study that excited state. Dynamic processes like catalysis and theoretical studies, however, are often more concerned about the electronic ground state than about excited states. For this purpose, it is useful to investigate vibrational wave packet dynamics from that ground state. 

---