Ultrafast relaxation states

It has been shown that photoexcitation of the guanine-cytosine (G-C) base pair leads to proton transfer [231], Watson-Crick (WC) base pairs have excited state lifetimes much shorter than other non-WC base pairs indicating once again that the natural occurring WC base pairs are more photostable than other alternative configurations [115, 118, 232-235], Much work has been done in the gas phase where many different base pair isomers exist. The ultrafast relaxation of the WC base pair has also been confirmed in solution using fluorescence up-conversion measurements [117]. 

In Fig. 2, the rate constants of S2 fluorescence decay (A =1/t ) and the inverses of S, fluorescence rise by S, state formation Xp) of ZP-I systems in Tol and THF are plotted against-AGCS. In Tol solutions, Xp at the top regions are little bit delayed relative to X, which shows that the charge recombination (CR) to the S[ state after charge separation from the S2 state is the main process for the S, formation in these systems. The results in MCH basically showed the same features with those in Tol (data not shown). On the other hand, X, and Xp values are rather close in THF which seems to suggest the ultrafast S, state formation by CR from the vibrationally unrelaxed CS state in the course of the vibrational redistribution relaxation. 

The classical trajectory simulations of Rydberg molecular states carried out by Levine ( Separation of Time Scales in the Dynamics of High Molecular Rydberg States, this volume) remind me of the related question asked yesterday by Prof. Woste (see Berry et a]., Size-Dependent Ultrafast Relaxation Phenomena in Metal Clusters, this volume). Here I wish to add that similar classical trajectory studies of ionic model clusters of the type A B have been carried out by... 

Two paradigms have been widely used in the past decade to describe the ultrafast relaxation of optically excited tttt states in purine molecules, through internal conversions [69], One of them relies on the existence of a conical intersection (Cl) between the excited state and the ground state, accessible on the excited state surface from the Franck-Condon region [69, 70], The second one, Lim s proximity effect , stems from vibronic coupling between the tttt state and nearby mr states found in these heteroatomic molecules [71]. Excited state quantum calculations have therefore focused recently on a precise characterisation of the strong perturbations and interactions undergone by these tttt or nit states. 

Figure VE-1 Ultrafast relaxation of photoexcited carriers into polaronic states with their associated midgap electronic states. We show schematically the band diagrams and corresponding absorption spectra for a non-degenerate ground state polymer.

The vibronic spectra of Do — Di — D2 electronic states recoded by da Silva Filho et al. [45] revealed resolved vibrational structures of the Do and D2 electronic states and a broad and structureless band for the Di state. A slow ( 3-20 ps) and fast k, 200 fs) relaxation components are estimated for the Dq D2 transition in a (femto)picosecond transient grating spectroscopy measurements [16]. The fast component is attributed to the Do D2 transition and a nonradiative relaxation time of 212 fs is also estimated from the cavity ringdown (CRD) spectroscopy data [42]. Electronic structure results of Hall et al. [107] suggest that the nonradiative Do D2 relaxation occurs via two consecutive sloped type CIs [66,108]. We developed a global model PESs for the Do — Di— D2 electronic states and devised a vibronic coupling model to study the nuclear dynamics underlying the complex vibronic spectrum and ultrafast excited state decay of N +[20]. 

Following the time-resolved results of FuP et al in the previous section, we shall presently describe computational efforts to describe and explain the ultrafast relaxation phenomena and dynamics inferred by experimental study. We shall focus on two paradigm systems of chemical importance, namely Cr(CO)5 and Fe(CO)5 (both 18 electron complexes). The photodissociation, and subsequent ultrafast relaxation to the singlet electronic ground state surface are fundamental to the photochemistry of these species. 

Interest in the study of higher energy states has increased in recent years and weak fluorescence has been observed from the S2 level of anthracene crystals using a two-step excitation approach (Katch et al.). Ultrafast relaxation from higher-lying excited states has been reported for several systems and the well-... 

Cyclobutanones in acetic acid undergo a regiospecific photoconversion into 2-acetoxy-5-alkoxytetrahydrofurans with retention of configuration at the migrating a-position, and an investigation of ultrafast relaxation processes in A, A -dimethylaminobenzylidene indan-l,3-dione as a molecular film has shown that formation and vibronic relaxation of the exciton states occurs in less than 100 fs. Equilibration of the two trapped exciton states seems to occur within 20 ps. 

In what follows, we wish to investigate in some more detail the monitoring of the ultrafast excited-state relaxation dynamics, which is exhibited, for... 

Fig. 6.8 Femtosecond pump-probe spectroscopy of diiodoBODIPY (10). (a) Inverted absorption spectrum (6) and transient absorption spectra at 0.5 (1), 20 (2), 50 (3), 110 (4), and 350 (5) ps delay between pump and probe pulses, (b) Differential absorption spectra of intermediate states obtained after modeling the experimental data by a three-exponential equation (see text) that is convoluted with the instrumental response function (1) zero delay (sum of all amplitudes, S2 state), (2) ultrafast relaxation of the S2 state (sum of A2, A3, A4 amplitudes), (3) thermalized Sj state (sum of A3 and A4 amplitudes), (4) Tj state (A4 amplitude). Reprinted with permission from [24]...

The luminescence techniques have already become standard tools to investigate not only the ultrafast relaxation processes, but also the ultrafast transport processes in condensed matters. The interpretation of luminescence results may in some cases be simpler than that of pump-probe results because the latter is a nonlinear technique while luminescence is a linear technique. In other words, the observed results do not contain information of transient dynamics of the initial states of materials. 

Hudock et al. [126] used the ab initio molecular dynamics multiple spawning method to go beyond the static picture based on PES and include the time dependent dynamical behavior and predict time-resolved photoelectron spectroscopy results. According to these results the first ultrafast component of the photoelectron spectra of uracil corresponds to relaxation on the S2 minimum rather than nonadiabatic transitions to the Si state. The authors suggest that the radiationless relaxation from... 

Rini M, Dreyer J, Nibbering ETJ et al (2003) Ultrafast vibrational relaxation processes induced by intramolecular excited state hydrogen transfer. Chem Phys Lett 374 13-19... 

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