Influence of autoionizing states on the pulse-length dependence of strong-field Ne+ photoionization at 38.4 eV

International audienceUsing the time-dependent R-matrix approach, we investigate ionization of ground-state Ne +, irradiated by laser light with a photon energy of 38.4 eV at intensities 10 13 W cm −2, 2 × 10 13 W cm −2 and 10 14 W cm −2 as a function of pulse length. Although the photon energy is below the threshold for single-photon ionization, we obtain a significant contribution from single-photon ionization to the ionization probability due to the finite duration of the pulse. The two-photon ionization rates deduced from the calculations are consistent with those obtained in R-matrix-Floquet rate calculations. The ionization probability oscillates with pulse length, which is ascribed to population and depopulation of autoionizing states just above the Ne 2+ ground state, reached after absorption of a single photon. At an intensity of 10 14 W cm −2, pulse lengths longer than 50 cycles are required for two-photon ionization to dominate the ionization probability. Letter to the Editor 2 The development of free-electron lasers operating in the VUV and the X-ray domain has given experimentalists new ways of investigating multi-electron dynamics in strong laser fields. These new laser facilities have, for example, enabled experimentalists to investigate two-photon double ionization of Ne in the photon-energy regime where direct two-photon double ionization is energetically allowed, but sequential (Ne → Ne + → Ne 2+) double ionization is energetically not allowed since the photon energy is not sufficient to ionize Ne + with a single photon (eg. Sorokin et al 2007). At larger photon energies, sequential double ionization is allowed, but also in this process one can find signatures of the fact that the two different emission processes are not independent of each other (Fritzsche et al 2008). One of the grand challenges in theoretical atomic physics is the description of multielectron dynamics in complex atoms irradiated by intense laser pulses. Over the last 15 years, great progress has been made in the description of pure two-electron systems in intense laser fields, for example for two-photon double ionization processes (see, for example, Colgan et al 2002, Feng and van der Hart 2003, Laulan et al 2005, Feist et al 2008) as well as for multiphoton double ionization at 390 nm (Parker et al 2006). These calculations require substantial computational resources, such that the direct extension of these techniques to systems with more than two electrons, like Ne, is unfeasible at present. Other approaches are required to describe the behaviour of complex atoms in intense light fields. The most successful approach for the description of complex atoms in intense laser light at present is the R-matrix-Floquet approach. This approach was designed from the outset to treat complex atoms in intense light fields by combining the R-matrix approach with the Floquet Ansatz (Burke et al 1991). It has been applied to a wide range of problems, ranging from strong-field ionization of Ne and Ar at 390 nm, requiring absorption of at least eight and six photons respectively, (van der Hart 2006) to twophoton emission of the inner 1s electron from ground-state Li − (van der Hart 2005). More recently, the R-matrix-Floquet approach has been instrumental in indicating the importance of detailed atomic structure in two-photon ionization of Ne + (Hamonou et al 2008, Hamonou and van der Hart 2008). The theoretical investigation of ionization processes in Ne + is of particular relevance at the moment, due to the large number of strong-field multiple ionization studies on Ne at photon energies in the range between 38 and 50 eV. Ionization yields of various Ne ions were obtained by Sorokin et al (2007) at photon energies below the Ne + ionization threshold and above this threshold. Moshammer et al (2007) obtained detailed recoil momentum spectra for two-photon double ionization of Ne at 44 eV. Rudenko et al (2008) found that these recoil-ion momentum distributions differed strongly from the recoil-ion momentum distributions for two-photon double ionization of He at 44 eV. At 44 eV, sequential double ionization is allowed for Ne, whereas it is not allowed for He. The dominance of sequential double ionization of Ne was demonstrated experimentally by Kurka et al (2009). Whereas in previous work (Hamonou et al 2008, Hamonou and van der Hart 2009)

Theory and applications of atomic and ionic polarizabilities

Atomic polarization phenomena impinge upon a number of areas and processes in physics. The dielectric constant and refractive index of any gas are examples of macroscopic properties that are largely determined by the dipole polarizability. When it comes to microscopic phenomena, the existence of alkaline-earth anions and the recently discovered ability of positrons to bind to many atoms are predominantly due to the polarization interaction. An imperfect knowledge of atomic polarizabilities is presently looming as the largest source of uncertainty in the new generation of optical frequency standards. Accurate polarizabilities for the group I and II atoms and ions of the periodic table have recently become available by a variety of techniques. These include refined many-body perturbation theory and coupled-cluster calculations sometimes combined with precise experimental data for selected transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index measurements in microwave cavities, ab initio calculations of atomic structures using explicitly correlated wave functions, interferometry with atom beams, and velocity changes of laser cooled atoms induced by an electric field. This review examines existing theoretical methods of determining atomic and ionic polarizabilities, and discusses their relevance to various applications with particular emphasis on cold-atom physics and the metrology of atomic frequency standards.Comment: Review paper, 44 page

Overview of the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Forcing on the Mediterranean Climate (ChArMEx/ADRIMED) summer 2013 campaign

The Chemistry-Aerosol Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr) is a collaborative research program federating international activities to investigate Mediterranean regional chemistry-climate interactions. A special observing period (SOP-1a) including intensive airborne measurements was performed in the framework of the Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region (ADRIMED) project during the Mediterranean dry season over the western and central Mediterranean basins, with a focus on aerosol-radiation measurements and their modeling. The SOP-1a took place from 11 June to 5 July 2013. Airborne measurements were made by both the ATR-42 and F-20 French research aircraft operated from Sardinia (Italy) and instrumented for in situ and remote-sensing measurements, respectively, and by sounding and drifting balloons, launched in Minorca. The experimental setup also involved several ground-based measurement sites on islands including two ground-based reference stations in Corsica and Lampedusa and secondary monitoring sites in Minorca and Sicily. Additional measurements including lidar profiling were also performed on alert during aircraft operations at EARLINET/ACTRIS stations at Granada and Barcelona in Spain, and in southern Italy. Remote-sensing aerosol products from satellites (MSG/SEVIRI, MODIS) and from the AERONET/PHOTONS network were also used. Dedicated meso-scale and regional modeling experiments were performed in relation to this observational effort. We provide here an overview of the different surface and aircraft observations deployed during the ChArMEx/ADRIMED period and of associated modeling studies together with an analysis of the synoptic conditions that determined the aerosol emission and transport. Meteorological conditions observed during this campaign (moderate temperatures and southern flows) were not favorable to producing high levels of atmospheric pollutants or intense biomass burning events in the region. However, numerous mineral dust plumes were observed during the campaign, with the main sources located in Morocco, Algeria and Tunisia, leading to aerosol optical depth (AOD) values ranging between 0.2 and 0.6 (at 440 nm) over the western and central Mediterranean basins. One important point of this experiment concerns the direct observations of aerosol extinction onboard the ATR-42, using the CAPS system, showing local maxima reaching up to 150Mm(-1) within the dust plume. Non-negligible aerosol extinction (about 50Mm(-1)) has also been observed within the marine boundary layer (MBL). By combining the ATR- 42 extinction coefficient observations with absorption and scattering measurements, we performed a complete optical closure revealing excellent agreement with estimated optical properties. This additional information on extinction properties has allowed calculation of the dust single scattering albedo (SSA) with a high level of confidence over the western Mediterranean. Our results show a moderate variability from 0.90 to 1.00 (at 530 nm) for all flights studied compared to that reported in the literature on this optical parameter. Our results underline also a relatively low difference in SSA with values derived near dust sources. In parallel, active remote-sensing observations from the surface and onboard the F-20 aircraft suggest a complex vertical structure of particles and distinct aerosol layers with sea spray and pollution located within the MBL, and mineral dust and/or aged North American smoke particles located above (up to 6–7 km in altitude). Aircraft and balloon-borne observations allow one to investigate the vertical structure of the aerosol size distribution showing particles characterized by a large size (> 10 μm in diameter) within dust plumes. In most of cases, a coarse mode characterized by an effective diameter ranging between 5 and 10 μm, has been detected above the MBL. In terms of shortwave (SW) direct forcing, in situ surface and aircraft observations have been merged and used as inputs in 1-D radiative transfer codes for calculating the aerosol direct radiative forcing (DRF). Results show significant surface SW instantaneous forcing (up to (-90)Wm(-2) at noon). Aircraft observations provide also original estimates of the vertical structure of SW and LW radiative heating revealing significant instantaneous values of about 5 K per day in the solar spectrum (for a solar angle of 30 ) within the dust layer. Associated 3-D modeling studies from regional climate (RCM) and chemistry transport (CTM) models indicate a relatively good agreement for simulated AOD compared with observations from the AERONET/PHOTONS network and satellite data, especially for long-range dust transport. Calculations of the 3-D SW (clear-sky) surface DRF indicate an average of about -10 to -20Wm(-2) (for the whole period) over the Mediterranean Sea together with maxima (-50Wm(-2)) over northern Africa. The top of the atmosphere (TOA) DRF is shown to be highly variable within the domain, due to moderate absorbing properties of dust and changes in the surface albedo. Indeed, 3-D simulations indicate negative forcing over the Mediterranean Sea and Europe and positive forcing over northern Africa. Finally, a multiyear simulation, performed for the 2003 to 2009 period and including an ocean–atmosphere (O–A) coupling, underlines the impact of the aerosol direct radiative forcing on the sea surface temperature, O–A fluxes and the hydrological cycle over the Mediterranean.French National Research Agency (ANR) ANR-11-BS56-0006ADEMEFrench Atomic Energy CommissionCNRS-INSU and Meteo-France through the multidisciplinary programme MISTRALS (Mediterranean Integrated Studies aT Regional And Local Scales)CORSiCA project - Collectivite Territoriale de Corse through Fonds Europeen de Developpement Regional of the European Operational ProgramContrat de Plan Etat-RegionEuropean Union's Horizon 2020 research and innovation program 654169Spanish Ministry of Economy and Competitivity TEC2012-34575Science and Innovation UNPC10-4E-442European Union (EU)Department of Economy and Knowledge of the Catalan Autonomous Government SGR 583Andalusian Regional Government P12-RNM-2409Spanish Government CGL2013-45410-R 26225

Inner-shell processes in two-photon ionisation of ions by vuv laser light

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Two-photon emission of a 2p electron from Ne+ and consequences for sequential double ionization of Ne

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Large-scale advection of continental aerosols during INDOEX

International audienceIn this paper, we present passive and active remote sensing measurements of atmospheric aerosols over the North Indian Ocean (NIO) during the Intensive Field Phase (IFP, January to March 1999) of the Indian Ocean Experiment. The variability of the aerosol load over NIO is discussed based on three‐dimentional numerical simulations made at a local scale by use of Regional Atmospheric Modeling System (RAMS) and at a regional scale using the zoomed Laboratoire de Météorologie Dynamique global circulation model (LMD‐Z version 3.3). Ground‐based measurements of the columnar aerosol optical thickness (AOT) and of surface black carbon (BC) concentration were carried out at two different sites in India: Goa University on the NIO coast and Dharwar 150 km inland. Local‐scale investigations point out that the trend in BC concentration at the ground is not correlated with AOT. Lidar profiles obtained both from the surface at Goa and in the NIO from the Mystere‐20 research aircraft indicate that a significant contribution to the total AOT (more than 50%) is due to a turbid monsoon layer located between 1 and 3 km height. RAMS simulation shows that the advection of aerosols in the monsoon layer is due to the conjunction of land‐sea breeze and topography. We present the regional‐scale extent of the aerosol plume in terms of AOT derived from the visible channel of Meteosat‐5. During March, most of the Bay of Bengal is overcast by a haze with a monthly average AOT of 0.61±0.18, and a spatially well‐defined aerosol plume is spreading from the Indian west coast to the Intertropical Convergence Zone with an average AOT of 0.49±0.08. Those values are bigger than in February with AOT at 0.35±0.18 and 0.37±0.09 for the Bay of Bengal and the Arabian Sea, respectively. One of the principal findings of this paper is that a significant contribution to the aerosol load over the NIO is due to the advection of continental aerosols from India in a well‐identified monsoon layer above the marine boundary layer. Moreover, it is suggested that the increase in biomass burning plays a significant role in the increasing trend in AOT during the winter dry monsoon season