INVESTIGADORES
ROMANO Rosana Mariel
congresos y reuniones científicas
Título:
Chemistry at the LNLS-2: Photoionization Dynamics of Complex Systems: from closed shell to radical and cationic species
Autor/es:
MAURICIO F. ERBEN; MARIANA GERONÉS; ROSANA M. ROMANO; LUCAS RODRÍGUEZ PIRANI; CARLOS O. DELLA VÉDOVA
Lugar:
Campinas, Brasil
Reunión:
Workshop; 2nd Workshop LNLS-2 new source: Scientific Cases; 2009
Resumen:
Introduction Photoionization techniques are well-suited methods for studying the spectroscopy of isolated molecules and the dynamic of the ionic species formed under the action of vacuum ultraviolet irradiation. Recently, photoionization is becoming increasingly important as a probe of chemical systems, from complex molecules, and transition states to clusters and radicals. Thus, the combined use of photoionization and photoelectron spectra techniques allowed obtaining a quite complete data set of the photoionization cross sections and photoelectron spectra of stable molecular species. These data are of prime interest, for example, for encourage atmospheric chemistry models. In particular, sulfur gases are known to be precursors of aerosol particles and could also act as a feedback mechanism in climate regulation which affects the Earth’s radioactive balance by direct scattering of solar radiation. In this context, dimethylsulfide DMS[1] is the most abundant sulfur-containing compound found in seawater and plays an important role in the global sulfur cycle, releasing into the atmosphere different sulfur species, e.g. sulfur dioxide, methane–sulfonic acid, sulfate and sulfuric acid, all of which can contribute to the acidity of rain. Synchrotron-based studies on sulfur containing molecules of atmospheric relevant interest have been accomplished.[2-5] For instance, Kivimäki et al. reported recently the photoelectron spectra of the “super greenhouse” gas SF5CF3 at the sulfur 2p level.[6] Our research group has studied systematically the electronic properties of shallow- and inner-core level in carbonylsulfenyl-chloride compounds, with general formula XC(O)SCl. Thus, FC(O)SCl,[7, 8] ClC(O)SCl,[9] and CH3OC(O)SCl[10] species have been studied by using synchrotron radiation in the 100-1000 eV range and their ionic fragmentation after electronic decay has been analyzed. Most recently, we have succeeded in analyzing the electronic structure and ionic dissociation induced by photon absorption in the outermost valence region of chlorine and sulfur containing species. This study used a combined experimental approach that includes HeI photoelectron spectroscopy and photoionization under the action of synchrotron radiation in the 10-22.5 eV region.[11-13] Other studies on divalent sulfur compounds were devoted to the ionic dissociation of S 2p excited thioacetic acid [CH3C(O)SH],[14] and more recently methyl thiocyanate (CH3SCN)[15] and perchloromethyl mercaptan (CCl3SCl). On the other hand, studies of transient molecular/radical species and molecular clusters have proven to be more difficult since it is hard to prepare targets with sufficient number densities for conventional photoelectron and photoionization experiments. Very recently experiments on such species have been conducted at new-generation synchrotron facilities and provided valuable data on their spectroscopy and dissociation pathways. Thus, relevant works in the chemistry of hydrocarbon combustion have been done by groups at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory and the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China.[16, 17] Such experiments are foundational examples of a new generation of experiments that may be performed on next generation light sources in which synchrotron radiation is used as a probe of chemical dynamics. Proposal It would be of great interest to obtain vibrationally resolved spectra on closed shell polyatomic systems over a broad energy range for both valence- and core-electron ejection in order to study how the ionization process derives in changes of the molecular geometry. Second, it would be informative to study how the ionization dynamics behave far from threshold for radical species. Both experiments tend to understand the chemical dynamics of processes involved in atmospheric chemistry. It is important to note that such an information is not available by other experimental means so far. The use of high resolution and intensity should open exciting studies on such complex systems. High brilliance is especially important to study compounds possessing a very low vapor pressure as well as for radical species. Thus, considerable opportunities would exist for the investigation of fundamental reactions involved in atmospheric chemistry. In a more extended perspective, the availability of soft x-ray radiation would be also used to characterize aerosols, and the tunability in the ultraviolet could be used to generate selective ionizations. Whit these data branching ratios in key reactions could be determined. Taking into consideration the high photon flux prospected for the new machine, another interesting aspect that deserves attention is the chance of planning the construction of a line devoted to the study of chemical reactions. Molecular beams and cluster-based experiments could be of great interest. Once more, cross-section measurements for ion-radical reactions are an unexplored field. High VUV fluxes are required for the preparation of state-selected neutral atoms and radicals for reactive studies, a requisite that might be realistic in the LNLS-2. Atoms and radicals in specific internal states can be prepared by photodissociation of appropriate precursor molecules. The knowledge of the photodissociation cross sections would allow a reliable determination of the number density for the acquired photoproduct. This methodology could be expanded to the study of plasma chemistry, a field of increasing importance from both, industrial and academic purposes. The study of processes in the plasma state needs both high sensitivity and resolution. The study of the photoionization achieved using multi-coincidence spectrometries would allow obtaining  data for reactivity studies of ions in excited states. In effect, the state-of-the-art for reaction cross sections and dynamics involving simple species attains the measure at the rotationally state-selected level.