1,720,985 research outputs found
Sodium doped hydrogen bonded clusters: Solvated electrons and size selection
No full textNeutral, sodium doped clusters feature special properties related to solvated electron formation which allow for a wide range of applications. In case of water, methanol and ethanol - but not for ammonia - the evolution of the ionization energy stops at small cluster sizes, reaching values similar to binding energies of solvated electrons in the liquid phase. Because of the appreciable lowering of the ionization energy, their photoionization can be performed free of fragmentation by standard lasers. This provides a very effective method for detection and, in combination with infrared excitation, for taking OH-stretch spectra of size selected, neutral clusters. (C) 2013 Elsevier B.V. All rights reserved.Deutsche Forschungsgemeinschaft [GRK 782, ZE 890 1-1
Photodissociation of hydrogen halide molecules in different cluster environments
No full textThe photodissociation of molecules and the interaction of the products with the surrounding cage atoms is an important field of research in solvation dynamics. The targeted generation of clusters with known size distributions and the placement of molecules on the surface or in the interior of these clusters allows us to carry out such investigations for finite systems as function of the size. We will present results of the photodissociation of HBr and HI molecules at 243 nm interacting with different rare gas clusters Nen, Arn, Krn, and Xen in the size range from n = 50 to 830. We mainly measure the kinetic energy of the outgoing H atoms in a time-of-flight mass spectrometer. The amount of fast, unperturbed or slow, caged and recombined H atoms depends strongly on the site, the special surface state, the cage material, the cluster size, and the kinetic energy available. The results are compared with calculations using mixed quantum-classical methods. In the photodissociation of HI in Xen clusters we observed the formation of HXeI that belongs to a recently discovered class of ionically bound systems. It is detected by the orientation in combined strong laser and weak electric fields. In small complexes (HBr), that were deposited in and on large rare gas clusters either vibrations or rotations were exited depending on their site and size
Search for oriented HXeX molecules from the photolysis of HCl and HBr in xenon clusters
No full textThe orientation of free HRgX molecules arising from photodissociation of hydrogen halide molecules HX on rare gas clusters Rgn is investigated in a new experimental arrangement: the rare gas cluster beam with HX molecules in the surface is intersected by two focussed laser beams at one point, where it additionally interacts with a weak electrostatic field. The possibility to generate and successively orient this new class of molecules is analyzed for a couple of new candidate molecules. We observe oriented HXeCl molecules, while HXeBr and HKrCl escaped our detection scheme, as was expected from the analysis. Details of the measured kinetic energy distributions (KED) of H-fragment atoms from the HXeCl photodissociation are analyzed in terms of the Xe–H vibrational structure on the calculated potential energy surface of HXeCl. Finally, a strong indication for observation of HXeH molecules is found in the fragment KED peak around 0.35 eV. These symmetric molecules arise from the dissociation of more than one HX molecule on the cluster and are consistently observed for all HX–Xen systems with X=Cl,Br,I
Infrared spectroscopy of sodium doped water clusters: Interaction with the solvated electron
No full textThe measured vibrational OH-stretch spectra of size-selected Na(H2O)(n) clusters for n = 8, 10, 16, and 20 are compared with first-principle calculations, which account for the interaction of the sodium cation, the electron, and the water molecules with the hydrogen-bonded network. The calculated harmonic frequencies are corrected by comparing similar results obtained for pure water clusters with experiment. The experimental spectra are dominated by intensity peaks between 3350 and 3550 cm(-1), which result from the interaction of the H atoms with the delocalized electron cloud. The calculations, which are all based upon the average spectra of the four lowest-energy isomers, indicate that most of the peaks at the lower end of this range (3217 cm(-1) for n = 8) originate from the interaction of one H atom with the electron distribution in a configuration with a single hydrogen-bonding acceptor. Those at the upper end (3563 cm(-1) for n = 8) come from similar interactions with two acceptors. The doublets, which arise from the interaction of both H atoms with the electron, appear in the red-shifted part of the spectrum. They are with 3369/3443 cm(-1) quite pronounced for n = 8 but slowly vanish for the larger clusters where they mix with the other spectral interactions of the hydrogen-bonded network, namely, the fingerprints of the free, the double, and the single donor OH positions known from pure water cluster spectroscopy. For all investigated sizes, the electron is sitting at the surface of the clusters
Sodium Microsolvation in Ethanol: Common Features of Na(HO-R)<sub><i>n</i></sub> (R = H, CH<sub>3</sub>, C<sub>2</sub>H<sub>5</sub>) Clusters
No full textEthanol clusters are generated in a continuous He seeded supersonic expansion and doped with sodium atoms in a pick-up cell. By this method clusters of the type Na(C2H5OH)(n) are formed and characterized by determining size selectively their ionization potentials (IPs) for n = 2-40 in photoionization experiments. A continuous decrease to 3.1 eV is found from n = 2 to 6 and a constant value of 3.07 +/- 0.06 eV for n = 10-40. This IP evolution is similar to the sodium-water and the sodium-methanol system. Quantum chemical calculations (B3LYP and MP2) of the IPs indicate adiabatic contributions to the photoionization process for the cluster sizes n = 4 and 5, which is similar to the sodium-methanol case. The results of the extrapolated IPs and the vertical binding energies (VEBs) of cluster anions are compared with the recently reported VEBs of solvated electrons in liquid water, methanol, and ethanol solutions in the range of 3.1-3.4 eV. The new results imply that the extrapolated VBEs of solvated electrons in anionic clusters match the VBE in liquid water, while they are about 0.5 eV too low for methanol. The influence of the presence of counterions on these findings is discussed.DFG [GRK 782, ZE 890-1-1
Experimental and theoretical study of the microsolvation of sodium atoms in methanol clusters: Differences and similarities to sodium/water and sodium/ammonia
No full textMethanol clusters are generated in a continuous He-seeded supersonic expansion and doped with sodium atoms in a pick-up cell. By this method, clusters of the type Na(CH3OH)(n) are formed and subsequently photoionized by applying a tunable dye-laser system. The microsolvation process of the Na 3s electron is studied by determining the ionization potentials (IPs) of these clusters size-selectively for n = 2-40. A decrease is found from n = 2 to 6 and a constant value of 3.19 +/- 0.07 eV for n = 6-40. The experimentally-determined ionization potentials are compared with ionization potentials derived from quantum-chemical calculations, assuming limiting vertical and adiabatic processes. In the first case, energy differences are calculated between the neutral and the ionized cationic clusters of the same geometry. In the second case, the ionized clusters are used in their optimized relaxed geometry. These energy differences and relative stabilities of isomeric clusters vary significantly with the applied quantum-chemical method (B3LYP or MP2). The comparison with the experiment for n = 2-7 reveals strong variations of the ionization potential with the cluster structure indicating that structural diversity and non-vertical pathways give significant signal contributions at the threshold. Based on these findings, a possible explanation for the remarkable difference in IP evolutions of methanol or water and ammonia is presented: for methanol and water a rather localized surface or semi-internal Na 3s electron is excited to either high Rydberg or more localized states below the vertical ionization threshold. This excitation is followed by a local structural relaxation that couples to an autoionization process. For small clusters with n < 6 for methanol and n < 4 for water the addition of solvent molecules leads to larger solvent-metal-ion interaction energies, which consequently lead to lower ionization thresholds. For n = 6 ( methanol) and n = 4 ( water) this effect comes to a halt, which may be connected with the completion of the first cationic solvation shell limiting the release of local relaxation energy. For Na(NH3)(n), a largely delocalized and internal electron is excited to autoionizing electronic states, a process that is no longer local and consequently may depend on cluster size up to very large n
Size-selective vibrational spectroscopy of methyl glycolate clusters: comparison with ragout-jet FTIR spectroscopy
No full textA comprehensive experimental study of OH-stretching vibrations of size selected methyl glycolate clusters is presented. A depletion spectroscopy experiment in a crossed molecular beam apparatus was employed to scrutinize the cluster size assignment based on pressure dependence studies in a jet-FTIR experiment. First, the dimer to tetramer size assignments of the FTIR spectrum are confirmed by depletion signal angular dependencies measured at the FTIR absorption maxima. Then, independent depletion spectra of the size selected dimers through tetramers are presented. The depletion spectra exhibit peak broadening and blue-shifts with respect to the FTIR spectrum. These differences are discussed and partially explained by cluster heating through energy transfer in the scattering collisions with Ne atoms
Solvent-Induced Photostability of Acetylene Molecules in Clusters Probed by Multiphoton Dissociation
No full textWe have studied the multiphoton photodissociation of (C(2)H(2))(n) and (C(2)H(2))(n)center dot Ar(m) clusters in molecular beams. The clusters were prepared in supersonic expansions under various conditions, and the corresponding mean cluster sizes were determined, for which the photodissociation at 193 nm was studied. The measured kinetic energy distributions (KEDs) of the H fragment from acetylene in clusters peak around 0.2 eV, in agreement with the KED from an isolated C(2)H(2) molecule. However, the KEDs from the clusters extend to kinetic energies of over 2 eV, significantly higher than the maximum fragment energies from an isolated molecule of about I eV. The photofragment acceleration upon solvation is a rather unusual phenomenon. The analysis based on ab initio calculations suggests the following scenario: (i) At 193 nm, photodissociation of acetylene occurs mostly in the singlet manifold. (ii) The solvent stabilizes the acetylene molecule, preventing it from undergoing hydrogen dissociation and funneling the population into a vibrationally hot ground state. (iii) The excited C(2)H(2) absorbs the next photon and eventually dissociates, yielding the H fragment with a higher kinetic energy corresponding to the first C(2)H(2) excitation. Thus, the H-fragment KED extending to higher energies is a fingerprint of the cage effect and the multiphoton nature of the observed processes. The photon-flux dependence of the KEDs reflects the rate of the vibrational energy flow from the hot ground state of acetylene to the neighboring solvent molecules
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