Ultrafast relaxation electronically excited states

Since the relaxation of the higher exciton states occurs on an ultrafast timescale of about 100 fs [23,26], the absorption spectrum for a closed structure, Fig. 26.4, top and centre, consists of either one or a few relatively broad spectral bands, respectively. For both cases, i.e., circular and elliptical arrangement, the transitions from the k = 1 exciton states are polarized perpendicular with respect to each other. Moreover, the lowest exciton state is optically forbidden, because a C2-type symmetry reduction alone, i.e., an ellipse, does not give rise to oscillator strength in the k = 0 state. This situation is reminescent to the electronically excited states of the B850 BChl a... 

Our objective is to understand how the noncovalent interactions responsible for nucleic acid secondary structure (i.e. base stacking and base pairing) affect the photophysics of these multichromophoric systems. Here we describe initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides... 

After UV photoexcitation the DNA and RNA bases return to the electronic ground state at an ultrafast time scale of about one picosecond [1], Their short excited-state lifetimes imply an intrinsic stability against structural photoinduced changes. The characterization of the excited-state energy surfaces by means of stationary points, conical intersections and relaxation paths has been of fundamental importance for the understanding of the mechanisms taking place in the ultrafast deactivation of these bases [2-10], In particular, theoretical investigations have shown the existence... 

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]. 

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