Femtosecond Photodissociation Dynamics of van der Waals Cationic Clusters: a tool for detecting metastable isomers of organic cations

A femtosecond pump (266 nm)–probe (800 nm) experiment as been performed on small clusters of tetrakis(dimethylamino)ethylene (TDMAE) with argon (TDMAE(Ar

Dynamics of excited tetrakis(dimethyl amino)ethylene solvated by argon atoms

The supersonic expansion of a mixture of the title molecule (TDMAE) with argon generates a beam carrying a log-normal distribution of TDAME(Ar)n clusters, broadly centered at . The femtosecond pump–probe technique is used to investigate the excited state dynamics of these clusters up to a 220 ps delay between the pump and the probe. This documents the effect of the argon environment on the TDMAE dynamics. The TDMAE molecule is excited in the valence state V within the cluster by the pump laser at 266 nm. It undergoes deformation in the excited potential energy surface that brings the initial wavepacket to a conical intersection (CI), where the electronic configuration of the molecule switches to a zwitterionic configuration Z. Compared to the behaviour of free TDMAE, the effect of the argon environment is a slow down of the wavepacket movement and an increase of the time scale of the V–Z energy transfer from 300 to 400±50 fs. This slow down effect, that we call a chistera effect, differs from a standard cage effect. Here, the deforming molecule does not experience a hard sphere collision with the argon cage, rather it pushes it away. Furthermore, umbrella oscillations of the dimethylamino groups are excited when the initial wavepacket passes the CI region. Because of the argon environment, the sharp 250 fs oscillation period of the free molecules is transformed into a broad structure of 40 fs width (FWHM) centred at about 240 fs. In addition, breathing oscillations of the argon environment with respect to the TDMAE molecule are observed with a period of 410±40 fs. Finally, long delays between the pump and the probe lasers allow us to investigate the nonradiative energy transfer from the Z electronic configuration of TDMAE(Ar)n to a charge transfer state. The effect of the evaporation of argon atoms in the neutral and the ionised clusters has been taken into account, as its time scale accompanies that of the observed phenomena

Characterizing filaments in regions of high-mass star formation: High-resolution submilimeter imaging of the massive star-forming complex NGC 6334 with ArTeMiS

Context. Herschel observations of nearby molecular clouds suggest that interstellar filaments and prestellar cores represent two fundamental steps in the star formation process. The observations support a picture of low-mass star formation according to which filaments of ~0.1 pc width form first in the cold interstellar medium, probably as a result of large-scale compression of interstellar matter by supersonic turbulent flows, and then prestellar cores arise from gravitational fragmentation of the densest filaments. Whether this scenario also applies to regions of high-mass star formation is an open question, in part because the resolution of Herschel is insufficient to resolve the inner width of filaments in the nearest regions of massive star formation. Aims. In an effort to characterize the inner width of filaments in high-mass star-forming regions, we imaged the central part of the NGC 6334 complex at a resolution higher by a factor of >3 than Herschel at 350 μm. Methods. We used the large-format bolometer camera ArTéMiS on the APEX telescope and combined the high-resolution ArTéMiS data at 350 μm with Herschel/HOBYS data at 70–500 μm to ensure good sensitivity to a broad range of spatial scales. This allowed us to study the structure of the main narrow filament of the complex with a resolution of 8″ or <0.07 pc at d ~ 1.7 kpc. Results. Our study confirms that this filament is a very dense, massive linear structure with a line mass ranging from ~500 M⊙/pc to ~2000 M⊙/pc over nearly 10 pc. It also demonstrates for the first time that its inner width remains as narrow as W ~ 0.15 ± 0.05 pc all along the filament length, within a factor of <2 of the characteristic 0.1 pc value found with Herschel for lower-mass filaments in the Gould Belt. Conclusions. While it is not completely clear whether the NGC 6334 filament will form massive stars in the future, it is two to three orders of magnitude denser than the majority of filaments observed in Gould Belt clouds, and has a very similar inner width. This points to a common physical mechanism for setting the filament width and suggests that some important structural properties of nearby clouds also hold in high-mass star-forming regions

Femtosecond to nanosecond relaxation time scales in electronically excited tetrakis(dimethylamino)ethylene: identification of the intermediates

In the TDMAE molecule (title molecule), the time evolution has been analyzed from the very initial excitation step down to a fluorescent state, over widely different time scales. Pump probe measurements have been performed at 3 different excitation wavelengths 400, 266 and 200 nm. The decay has been followed over the femtosecond and subnanosecond ranges with this method and the decay of the final charge transfer state has been detected by its fluorescence emission. This allows an overview of the complete decay mechanism. The initial relaxation pathway is interpreted in a similar way to ethylenic molecules, where the initial wavepacket is quickly trapped in a doubly excited state Z with charge transfer character. Then the Z state decays slowly (10–100 picoseconds) into the final state. In difference to monoalkenes the final stage of this evolution is a charge transfer state. The decay of transient Z state to the charge transfer state is a further assessment of the partial ionic character of the Z state. This type of molecule with low ionization potential can be viewed as a demonstrative example of the interrelation between the charge induced forces and the deformations in excited state reaction dynamics

Wave Packet Movements near the Conical Intersection between Two Excited Potential Surfaces May Create Observable Molecular Oscillations

International audienceA femtosecond pump-probe experiment is performed on tetrakis(dimethylamino)ethylene. The evolution resulting from the π-π* excitation of the CC double bond corresponds to movement along a single adiabatic potential surface with deformation along several coordinates and passage near a conical intersection. Surprisingly, this movement excites the umbrella mode of the amino groups, resulting in a measurable oscillation regime

Cluster Isolated Chemical Reactions : Medium effects on reactivity

International audienceCluster Isolated Chemical Reaction (CICR) is a new method developed in our laboratory, which enables quantitative studies of heterogeneous chemical processes at a microscopic level. CICR technique consists in depositing in a controlled way reactants on free nanometric van der Waals clusters, that play the role of a reaction medium. It provides a way to study the influence of the reaction medium on the trapped reactants, on the reaction path and on the products. The present paper reviews some of the CICR results obtained in our laboratory concerning the reactivity of barium atoms and small barium aggregates with molecular reactants like N2O, CO2, SF6... on argon, methane or nitrogen clusters. Keywords: barium, clusters, chemiluminescence, chemical reactions, heterogeneous chemistry, mass spectrometr

Binding energies of first and second shell water molecules in the Fe(H

The fragmentation cross-section of the Fe(H2O)$_{1,2}^+$, Co(H2O)$_{1,2}^+$ and Au(H2O)$_{1,2}^+$ ions were measured, as a function of the collision energy. Threshold energies of $1.4\pm0.2$ eV, $1.4\pm0.2$ eVand $1.7\pm0.1$ eVwere measured for the monohydrated $\rm Fe^+$, $\rm Co^+$ and $\rm Au^+$ ions respectively, in fair agreement with the existing literature. Small threshold energies of $0.7\pm0.2$ eV, $0.7\pm0.2$ eVand $0.5\pm0.1$ eV were found for the Fe(H2O)$_{2}^+$, Co(H2O)$_{2}^+$ and Au(H2O)$_{2}^+$ clusters respectively. Secondary thresholds were observed on the cross-section, respectively at $1.7\pm0.3$ eV and $2.0\pm0.2$ eV for the Co(H2O)$_{2}^+$ and Au(H2O)$_{2}^+$ clusters. This double threshold behavior could be attributed to the presence of two kinds of isomers in the beam. The upper threshold is associated with clusters where both water molecules are linked to the metal ion (first solvation shell), whereas the lower threshold corresponds to clusters with one water molecule in the first solvation shell and the other in the second shell. Such an analysis documents the binding energy of either a first shell or a second shell water molecule in the M(H2O)$_{2}^+$ cluster ions

Micro solvation dynamics at the passage of conical intersection abserved in argon clusters of excited tetrakis (dimethylamino) ethylene

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