The photolysis of (H(2)O)(n) nanoparticles of various mean sizes between 85 and 670 has been studied in a molecular beam experiment. At the dissociation wavelength 243 nm (5.10 eV), a two-photon absorption leads to H-atom production. The measured kinetic energy distributions of H-fragments exhibit a peak of slow fragments below 0.4 eV with maximum at approximately 0.05 eV, and a tail of faster fragments extending to 1.5 eV. The dependence on the cluster size suggests that the former fragments originate from the photodissociation of an H(2)O molecule in the cluster interior leading to the H-fragment caging and eventually generation of a hydronium H(3)O molecule. The photolysis of surface molecules yields the faster fragments. At 193 nm (6.42 eV) a single photon process leads to a small signal from molecules directly photolyzed on the cluster surface. The two photon processes at this wavelength may lead to cluster ionization competing with its photodissociation, as suggested by the lack of H-fragment signal increase. The experimental findings are complemented by theoretical calculations.
"The details of ice nanoparticles generation in supersonic expansions has been recently investigated by Kim et al. (2004), Manka et al. (2012), and Li et al. (2013). The individual particles can be investigated under controlled conditions in vacuum by various means: e.g., ionization (electron, photon) and mass spectrometry (MacTaylor and Castleman, 2000; Lengyel et al., 2012b); infrared (IR) spectroscopy (Yacovitch et al., 2011, 2012; Preston et al., 2012; Fujii and Mizuse, 2013) or ultraviolet (UV) photodissociation experiments (Kreher et al., 1999; Li and Huber, 2001; Poterya et al., 2007, 2008a, 2011; Ončák et al., 2008, 2011); particle (electron , photon, neutron) scattering (Heath et al., 2003; Kim et al., 2004; Manka et al., 2012); special methods such as sodium doping and subsequent spectroscopies (Bobbert et al., 2002; Yoder et al., 2011; Pradzynski et al., 2012). Such experiments provide unprecedented molecular-level insight into the small particle generation , their (photo)chemistry and (photo)physics and detailed dynamics of the processes on/in these particles. "
[Show abstract][Hide abstract] ABSTRACT: THIS REVIEW SUMMARIZES SOME RECENT EXPERIMENTS WITH ICE NANOPARTICLES (LARGE WATER CLUSTERS) IN MOLECULAR BEAMS AND OUTLINES THEIR ATMOSPHERIC RELEVANCE: (1) Investigation of mixed water-nitric acid particles by means of the electron ionization and sodium doping combined with photoionization revealed the prominent role of HNO3 molecule as the condensation nuclei. (2) The uptake of atmospheric molecules by water ice nanoparticles has been studied, and the pickup cross sections for some molecules exceed significantly the geometrical sizes of the ice nanoparticles. (3) Photodissociation of hydrogen halides on water ice particles has been shown to proceed via excitation of acidically dissociated ion pair and subsequent biradical generation and H3O dissociation. The photodissociation of CF2Cl2 molecules in clusters is also mentioned. Possible atmospheric consequences of all these results are briefly discussed.
Frontiers in Chemistry 02/2014; 2:4. DOI:10.3389/fchem.2014.00004
[Show abstract][Hide abstract] ABSTRACT: Vertical ionization energies of the nucleosides cytidine and deoxythymidine in water, the lowest ones amounting in both cases to 8.3 eV, are obtained from photoelectron spectroscopy measurements in aqueous microjets. Ab initio calculations employing a nonequilibrium polarizable continuum model quantitatively reproduce the experimental spectra and provide molecular interpretation of the individual peaks of the photoelectron spectrum, showing also that lowest ionization originates from the base. Comparison of calculated vertical ionization potentials of pyrimidine bases, nucleosides, and nucleotides in water and in the gas phase underlines the dramatic effect of bulk hydration on the electronic structure. In the gas phase, the presence of sugar and, in particular, of phosphate has a strong effect on the energetics of ionization of the base. Upon bulk hydration, the ionization potential of the base in contrast becomes rather insensitive to the presence of the sugar and phosphate, which indicates a remarkable screening ability of the aqueous solvent. Accurate aqueous-phase vertical ionization potentials provide a significant improvement to the corrected gas-phase values used in the literature and represent important information in assessing the threshold energies for photooxidation and oxidation free energies of solvent-exposed DNA components. Likewise, such energetic data should allow improved assessment of delocalization and charge-hopping mechanisms in DNA ionized by radiation.
Journal of the American Chemical Society 05/2009; 131(18):6460-7. DOI:10.1021/ja8091246 · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have studied the multiphoton photodissociation of (C(2)H(2))(n) and (C(2)H(2))(n) x 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 1 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.
The Journal of Physical Chemistry A 05/2009; 113(26):7322-30. DOI:10.1021/jp811073j · 2.69 Impact Factor
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