Ultrafast relaxation

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]. 

Also, the potential dependence of ultrafast relaxation at electrochemical interfaces of 1-dodecanethiol- and 1-HT-modified Au(lll) electrodes has been investigated in HCIO4 and H2SO4 solutions, applying transient reflectivity measurements ]143]. 

SIZE-DEPENDENT ULTRAFAST RELAXATION PHENOMENA IN METAL CLUSTERS... 

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... 

That electrostatic forces could be crucial to vibrational energy relaxation was amply demonstrated by the liquid water simulations of Whitnell et al. (34). They noted that since the electrostatic portion of the force between their solvent and a dipolar solute was linear in the solute dipole moment, Equations (12) and (13) implied that the electrostatic part of the friction ought to scale as the dipole moment squared. When they then found that their entire relaxation rate scaled with the square of the solute dipole moment, it certainly seemed to be convincing evidence that electrostatics forces were indeed the primary ingredients generating ultrafast relaxation. Subsequent theoretical work on relaxation rates in such manifestly protic solvents as water and alcohols has largely served to reinforce this message (37,38,60,61). 

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. 

DNA/RNA nucleobases and several tautomers and derivatives that only the natural systems have barrierless MEPs connecting the FC region to the (gs/TnT )CI. In all the other studied purine derivatives, we have found different minima and energy barriers along the 1 (tttt HL) MEP and thus hindering the ultrafast relaxation. 

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.

Modern Time-Resolved Studies Photodissociation and Ultrafast Relaxation... 

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-... 

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