Ultrafast non-resonant multiphoton preparation of ion-molecule reactions within clusters

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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

Intramolecular charge transfer induced by solvent interaction

The effect of solvation on the appearance of a red shifted Twisted Intramolecular Charged Transfer emission is reviewed. Comparison between condensed phase and small cluster red fluorescence is discussed in the particular case of methyl 4-N,N-dimethylaminobenzoate (DMABME). Using a supersonic expansion DMABME-(H2O)n clusters are studied by monitoring at the same time mass spectra and dispersed fluorescence spectra as well as by lifetime measurements. For DMABME-(H2O)n clusters, a clear red shifted fluorescence is observed readily when one or two water molecules are clustered to the molecule. The results of this work are compared with those obtained on the model compound 4- Ν,Ν-dimethylaminobenzonitrile (DMABN), where no TICT emission is observed in small DMABN-(Solvent)n clusters

Observation of the first vibrations of the newly born Cs–CH

The femtosecond photodissociation of CsI in the CsI–CH3CN complex has been studied. Recurrences are observed in the detection of the Cs+–CH3CN ion complex. They are assigned to the first vibrations of the Cs–NCCH3 bond formed after the CsI dissociation. This result is in good agreement with calculations on the similar NaI–CH3CN system which predict a linear structure for the ground state complex