Ultrafast radiationless decay

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

Zgierski MZ, Fujiwara T, Kofron WG, Lim EC (2007) Highly effective quanching of the ultrafast radiationless decay of photoexcited pyrimidine bases by covalent modification photophysics of 5,6-trimethylenecytosine and 5,6-trimethyleneuracil. Phys Chem Chem Phys 9 3206-3209... 

The qualitative picture suggested by these calculations has recently been confirmed by new experiments on the UV photochemistry of phenol. Lee and collaborators have shown that loss of H atoms is the major process in phenol at 248 nm excitation [36], Ashfold and co-workers have applied high-resolution photofragment translational spectroscopy to obtain uniquely detailed insight into the photodissociation dynamics of phenol [37], The phenoxy radical is produced in a surprisingly limited subset of its available vibrational states [37], Such vibrational mode-specific dynamics is a signature of ultrafast radiationless decay through directly accessible conical intersections. Similar results have been obtained for pyrrole, imidazole and indole [38 10],... 

ULTRAFAST RADIATIONLESS DECAY IN NUCLEIC ACIDS INSIGHTS FROM NONADIABATIC AB INITIO MOLECULAR DYNAMICS... 

Fig. 1.1 Schematic view of an ultrafast radiationless decay process through a conical intersection...

It is well known from previous experimental [9-12] and theoretical works [13, 21, 24-26] that after excitation to the bright B2u t t ) state, pyrazine undergoes an ultrafast radiationless decay process in a few tens of femtoseconds. This process was directly observed recently by TRPES using sub-20 fs pulses [11, 12]. In this section, we present quantum dynamics simulations of the excited state dynamics of pyrazine triggered by a 14 fs sine-squared shaped laser pulse resonant with the transition from the ground to the B2u Tnr ) state. Specifically, the total Hamiltonian operator reads... 

As seen in second part of these thesis, the development of efficient quantum dynamical computational methods also opened the door to the study of the laser control of polyatomic molecular systems. In a first application, we used a simplified model Hamiltonian describing the excited state dynamics of pyrazine to investigate the laser control of its ultrafast radiationless decay by a strong non-resonant laser... 

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