Excited States of Proton-bound DNA/RNA Base Homo-dimers: Pyrimidines

We are presenting the electronic photo fragment spectra of the protonated pyrimidine DNA bases homo-dimers. Only the thymine dimer exhibits a well structured vibrational progression, while protonated monomer shows broad vibrational bands. This shows that proton bonding can block some non radiative processes present in the monomer.Comment: We acknowledge the use of the computing facility cluster GMPCS of the LUMAT federation (FR LUMAT 2764

Is NaI soluble in water clusters?

${\rm NaI{-}(solvent)}_n$ clusters (solvents being ${\rm NH_3}$, ${\rm H_2O}$ or ${\rm CH_3CN}$) have been studied by resonance enhanced two photons ionization, leading to the detection of ${\rm Na^+}{-}{\rm (solvent)}_n$ clusters. When water is the solvent, large clusters up to n> 50 can be observed, whereas for ${\rm NH_3}$ and ${\rm CH_3CN}$ no clusters larger than 10 could be evidenced. Because the first step in the ionization process is the excitation from the ground solvated (${\rm Na^+}{-}{\rm I^-}$) ion pair state to a covalent excited state, the differences in the cluster size distribution for different solvent may be interpreted as a difference in cluster structures leading to a difference in the charge separation in the ground state

Catching the collision complex through a femtosecond coherently controlled pump/probe process

International audienceWe propose a very simple and efficient way to stabilize ions issued from a collision complex through a femtosecond coherently controlled pump/probe process. Starting from a van der Waals complex, one can initiate a collision at a well-defined time and with a restricted impact parameter. Formation of stable ionic complex can be achieved by ionizing the collision complex at the "right time." We present in this paper its application to the NaI–(CH3CN)1–2 system. Na+–CH3CN ion formation is coherently controlled by ionization of colliding Na atom on CH3CN molecules issued from the dissociation of NaI within NaI–(CH3CN)1–2. Classical mechanic calculations using simple ionization/dissociation conditions can reproduce the experimental data and give an insight into the control of such a reaction

Hydrogen transfer in excited pyrrole–ammonia clusters

The excited state hydrogen atom transfer reaction (ESHT) has been studied in pyrrole–ammonia clusters [PyH–(NH3)n+hν→Py'+'NH4(NH3)n-1]. The reaction is clearly evidenced through two-color R2P1 experiments using delayed ionization and presents a threshold around 235 nm (5.3 eV). The cluster dynamics has also been explored by picosecond time scale experiments. The clusters decay in the 10–30 ps range with lifetimes increasing with the cluster size. The appearance times for the reaction products are similar to the decay times of the parent clusters. Evaporation processes are also observed in competition with the reaction, and the cluster lifetime after evaporation is estimated to be around 10 ns. The kinetic energy of the reaction products is fairly large and the energy distribution seems quasi mono kinetic. These experimental results rule out the hypothesis that the reaction proceeds through a direct N–H bond rupture but rather imply the existence of a fairly long-lived intermediate state. Calculations performed at the CASSCF/CASMP2 level confirm the experimental observations, and provide some hints regarding the reaction mechanism

Une ligne VUV au Serveur Laser du Centre Laser de l'Université Paris-Sud

Au Centre Laser de l'Université Paris-Sud (CLUPS), un serveur laser (SELA) pour la physico-chimie permet d'accueillir des expérimentateurs autour de sources laser nanoseconde synchronisées à 10 Hz dans des domaines de fréquences variés. Des lasers commerciaux à colorants et OPO pompés par des Nd-YAG couvrent les domaines IR, VIS et UV. Ce domaine a été étendu dans le VUV entre 6 et 19 eV (200–65 nm) par mélange non-linéaire à 4 ondes dans des gaz rares (Xe, Kr, Ar) en suivant les deux schémas résonants ωVUV = 2ω1 ± ω2 qui s'appuient sur un état excité du gaz rare. Un monochromateur sépare le VUV souhaité des autres fréquences et le refocalise au centre de l'expérience. La génération de VUV accordable a été caractérisée en flux (qq 108 à 109 photons/pulse) et en pouvoir résolvant (E/ΔE = 500000 à 15.8 eV), ce dernier étant mesuré sur les raies d'autoionisation très fines de l'argon 3s23p5(2P1/2)ns couramment appelées ns'. Le pouvoir résolvant de cette source laser VUV et la possibilité de la coupler avec les autres lasers dans des expériences à plusieurs couleurs (IR-VIS-UV + VUV) la rend très intéressante et complémentaire des lignes synchrotron VUV, comme DESIRS sur SOLEIL