How long does it take an electron to tunnel?

How long does it take an atom to absorb a photon and loose an electron? And what if not one but many photons are needed for ionization? How much time would absorption of many photons take? These questions lie at the core of attosecond spectroscopy, which aims to resolve electronic motion at its natural time scale. Ionization in strong infrared fields is often viewed as electron tunnelling through a potential barrier, created by the combination of the atomic potential that binds the electron and the electric field of the laser pulse that pulls the electron away. Thus, unexpectedly, attosecond spectroscopy finds itself facing an almost age-old and controversial question: how long does it take an electron to tunnel through a barrier? Ionization times Ionization times (left axis) reconstructed using the ARM theory from offset angles (right axis) obtained numerically using TDSE calculations. Red circles are the numerically calculated offset angles, divided by the laser frequency, θ/ω. Blue diamonds show the offset angles with the correction due to the substraction of the pulse envelope effect, ti0 = θ/ω-|Δtienv (θ,ppeak)| . Green inverted triangles show the Coulomb correction to the ionization time evaluated at the peak of the photoelectron distribution, |ΔtiC (θ,ppeak|. Orange triangles show the ionization times we obtain by applying the reconstruction procedure defined by equation (4) in the paper. In terms of the figure, this is simply the result of subtracting the green curve from the blue curve. (Image: MBI) In the paper by Torlina et al ("Interpreting attoclock measurements of tunnelling times"), this question is studied by using the so-called atto-clock setup. The attoclock uses the rotating electric field of a circularly polarized laser pulse as a hand of the clock. One full revolution of this hand takes one laser cycle, about 2.6 fs for experiments with 800 nm pulse of a Ti-sapph laser. As the electric field rotates, so does the tunnelling barrier. Thus, electrons tunnelling at different times will tunnel in different directions. This link between time and direction of electron motion is what allows the attoclock to measure times. In every clock, a time zero must be established. In the attoclock, this is done by using a very short laser pulse, which lasts only one-two cycles. Tunnelling occurs in a small window where the rotating electric field passes through its maximum. Next, like any other clock, the attoclock must be calibrated. One has to know how the time of electron emission – its exit from the tunnelling barrier – maps onto the angle at which the electron is detected. This calibration of the attoclock has now been accomplished by Torlina et al, with no ad-hoc assumptions about the nature of the ionization process or the underlying physical picture. Combining analytical theory with accurate numerical experiments, and having calibrated the attoclock, the authors could finally carefully look at delays in electron tunnelling. They arrive to the surprising answer: this time delay may be equal to zero. At least within the realm of non-relativistic quantum mechanics, the electron tunnelling out of the ground state of a Hydrogen atom spends zero time under the tunnelling barrier. The situation may change, however, if this electron encounters other electrons on the way, which may become important in other atoms or molecules. The interaction between the electrons may lead to delays. Thus, the attoclock provides a unique window not only into the tunnelling dynamics, but also into the interplay of different electrons that participate in the ionization process, and how the electrons staying behind readjust to the loss of their comrade.
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How to flow ultrathin water layers - a liquid flatjet for X-ray spectroscopy

Element-specific x-ray methods play a key role in determining the atomic structure and composition of matter and functional materials. X-ray spectroscopy is sensitive to the oxidation state, the distances, coordination number and species of the atoms immediately surrounding the selected element. A large variety of x-ray spectroscopic techniques have been applied to gas-phase, bulk liquid or solid-state samples, or have been used to probe molecular systems at interfaces. X-ray spectroscopy is predominantly done at large-scale synchrotron facilities, or in more recent years with x-ray free electron lasers, probing steady-state and time-resolved material properties. Solution phase soft-x-ray absorption spectroscopy (XAS, energy range approximately from 0.2 - 1.5 keV) is not an easy method: experiments need to be done under vacuum conditions, an environment obviously incompatible with the high vapor pressure of water. Furthermore, if measured in transmission, absorption cross sections demand sample thicknesses in the micrometer and submicrometer range (1 micrometer = 10-6 m = one millionth of a meter). Alternatively, if secondary signals such as x-ray fluorescence are measured, the experiment is limited to comparably large solute concentrations. Using sample cells with thin membrane windows enables control of appropriate sample thicknesses, but sample degradation upon x-ray illumination (or upon pump laser illumination in time-resolved experiments) makes this approach disadvantageous for photolabile molecular systems. Sample refreshment is possible with a liquid jet, generated by pumping a solution through a nozzle with a small orifice, into the vacuum chamber. Single liquid jets have, however, difficulties to implement the required (sub)micron thicknesses. Liquid flatjet system Liquid flatjet system, showing the two nozzles from which two impinging single jets form a 1 mm wide and 5 mm long liquid water sheet with a thickness of 1 - 2 µm as determined by measuring the transmission at the oxygen K absorption edge (left), with which XAS measurements in transmission can be made on aqueous solutions, as exemplified with the nitrogen K absorption edge spectrum of ammoniumchloride (right). (Image: MBI) (click on image to enlarge) A collaboration between scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), the Helmholtz-Zentrum Berlin (HZB) and the Max Planck Institute for Dynamics and Self-Organization (MPIDS) have now demonstrated the successful implementation of a liquid flatjet with a thickness in the Å-µm range, allowing for XAS transmission measurements in the soft-x-ray regime ("A liquid flatjet system for solution phase soft-x-ray spectroscopy"). Here a phenomenon well known in the field of fluid dynamics has been applied: by obliquely colliding two identical laminar jets, the liquid expands radially, generating a sheet in the form of a leaf, bounded by a thicker rim, orthogonal to the plane of the impinging jets. The novel aspect here is that a liquid water flatjet has been demonstrated with thicknesses in the few micrometer range, stable for tens to hundreds of minutes, fully operational under vacuum conditions (‹10-3 mbar). For the first time, soft x-ray absorption spectra of a liquid sample could be measured in transmission without any membrane. The x-ray measurements were performed at the soft x-ray synchrotron facility BESSYII of the Helmholtz-Zentrum Berlin. This technological breakthrough opens up new frontiers in steady-state and time-resolved soft-x-ray spectroscopy of solution phase systems.
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Registration of chemicals: the Belgian register of nanomaterials

Contrary to the general opinion, companies established in Belgium have much more extensive obligations than mere compliance with REACH – the European legislation on registration of chemical substances. Indeed, in Belgium a national register for nanomaterials will enter into force on 1st January 2016. This is of significant importance as companies will be required to inform their Committee for Protection and Prevention at Work (CPPW) of the registration of nanomaterials. The companies involved must ensure that they have the relevant mechanisms in place before the deadline of 1st January 2016. Which companies are involved? All companies importing, producing or distributing chemical substances and products fall within the remit of the Belgian register. Although not all the chemical substances are considered to be nanomaterials by the Belgian legislation, is it important to know and to understand this legislation in order to determine the obligations of those involved in the purchase or sale of chemical substances. Any individual or private entity that is active in R&D is also concerned, regardless of whether they sell their own substances. Why is 1st January 2016 so important? As of this date, the relevant companies must declare any chemical substances which, under the Belgian law, are considered nanomaterials and which they have previously produced, imported or distributed for professional purposes. From this date, the relevant companies will also need to register their materials before placing them on the market. Furthermore, as of 1st January 2017, compulsory registration will be extended to mixtures containing nanomaterials and the registration of products containing any form of nanomaterial could also become compulsory from 2018. How to register in Belgium? The Belgian registration requires companies to submit relevant scientific and commercial information to the National Public Health Instances. Furthermore, the registration requires a juridical justification of confidentiality in respect of any data that contains trade secrets or information which may otherwise be regarded as confidential. Which penalties can be imposed in the absence of registration? Penalties for failure to register include prison sentences of up to 3 years and/or fines of up to 720.00 euros. Is the registration of nanomaterials relevant for suppliers and customers of the company? This registration aims the complete chain of the establishment of nanomaterials on the market, but excludes sales to consumers. Each company must also develop relevant coordination and the follow-up mechanisms between its suppliers and its own professional customers, which will include a full review of any contracts currently in place. Should you have any queries regarding the registration of nanomaterials in Belgium, please feel free to contact Anthony Bochon.
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