Process Of Single-photon Ionization Research Articles

Decomposition of the transition phase in multi-sideband schemes for reconstruction of attosecond beating by interference of two-photon transitions

Reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) is a technique that can be used to determine the phases of atomic transition elements in photoionization processes. In the traditional RABBITT scheme, the so-called asymptotic approximation considers the measured phase as a sum of the Wigner phase linked to a single-photon ionization process and the continuum-continuum phase associated with further single-photon transitions in the continuum. In this paper, we extend the asymptotic approximation to multi-sideband RABBITT schemes. The predictions from this approximation are then compared with results obtained by an ab initio calculation based on solving the time-dependent Schr\"odinger equation for atomic hydrogen.

Atmospheric Pressure Single Photon Laser Ionization (APSPLI) Mass Spectrometry Using a 157 nm Fluorine Excimer Laser for Sensitive and Selective Detection of Non- to Semipolar Hydrocarbons

Atmospheric pressure single photon ionization (APSPLI) mass spectrometry utilizing a fluorine excimer laser (157 nm, 7.9 eV) is presented for the first time. For evaluation and optimization, polycyclic aromatic hydrocarbon (PAH) standard mixtures were used. The presented technique allowed for the selective ionization of semi- to nonpolar compounds in a single photon ionization process using VUV photons. Molecular radical cations were found as a base peak, whereas protonated species were almost absent. Although the ionization chamber is flushed by pure nitrogen, remaining oxygen and water traces caused unwanted oxidized ionization artifacts. Installation of water and oxygen filter cartridges significantly reduced the abundance of artifacts. For evaluating complex mixture analysis, APSPLI was applied to characterize a light crude oil. In addition to aromatic hydrocarbons, APSPLI allowed for the sensitive ionization of sulfur-containing aromatic constituents (PASH). A comparison of APSPLI to atmospheric pressure laser ionization (266 nm, 4.7 eV) revealed the additional compositional space accessible by the single photon process. APLI, conducted with UV radiation, is mainly restricted to PAH analysis. APSPLI overcomes this limitation, and PAH and PASH, which often occur simultaneously in complex mixtures, can be detected. This novel ionization concept is envisioned to have a high analytical potential further explored in the future.

Determination of the photoemission position in single-photon ionization with attosecond streaking spectroscopy

Attosecond metrology can directly measure attosecond emission time of photoexcited electrons from matter, providing unprecedented understanding of transition processes of electrons from bound to continuum states. However, some fundamental details of the electron dynamics in the entire emission process upon photoexcitation still remain debatable or unknown. The photoemission time delays deduced from attosecond streaking spectroscopy originate from photoelectron propagation in the coupled Coulomb-laser fields, encoding the spatial and spectral information of electrons upon photoexcitation. Here we demonstrate that attosecond photoemission delays can be used to image picometer-resolved photoemission position via a classical model. The electronic dynamics in the laser-assisted single-photon ionization process is fully captured by a quantum path-integral model. We trace the imaged photoemission position to the average position of spatially coherent superposition of electron waves upon photoexcitation and, in particular, predict emission position coinciding with the orbital radius of the ground state of hydrogen-like atoms, in contrast with previous predictions.

Photoionization of a strongly polarizable target

We demonstrate that the angular distribution of photoelectrons from a strongly polarizable target exposed to a laser field can deviate noticeably from the prediction of conventional theory. Even within the dipole–photon approximation the profile of distribution is modified due to the action of the field of alternating dipole moment induced at the residue by the laser field. This effect, being quite sensitive to the dynamic polarizability of the residue and to its geometry, depends also on the intensity and frequency of the laser field. Numerical results, presented for sodium cluster anions, demonstrate that dramatic changes to the profile occur for the photon energies in vicinities of the plasmon resonances, where the effect is enhanced due to the increase in the residue polarizability. Strong modifications of the characteristics of a single-photon ionization process can be achieved by applying laser fields of comparatively low intensities I0 ∼ 1010–1011 W cm−2.

Attosecond electron wave packet interferometry

Acomplete quantum-mechanical description of matter and its interaction with the environment requires detailed knowledge of a number of complex parameters. In particular, information about the phase of wavefunctions is important for predicting the behaviour of atoms, molecules or larger systems. In optics, information about the evolution of the phase of light in time1 and space2 is obtained by interferometry. To obtain similar information for atoms and molecules, it is vital to develop analogous techniques. Here we present an interferometric method for determining the phase variation of electronic wave packets in momentum space, and demonstrate its applicability to the fundamental process of single-photon ionization. We use a sequence of extreme-ultraviolet attosecond pulses3,4 to ionize argon atoms and an infrared laser field, which induces a momentum shear5 between consecutive electron wave packets. The interferograms that result from the interaction of these wave packets provide useful information about their phase. This technique opens a promising new avenue for reconstructing the wavefunctions6,7 of atoms and molecules and for following the ultrafast dynamics of electronic wave packets.

Combustion study with synchrotron radiation single-photon ionization technique

Here we report a combustion endstation at National Synchrotron Radiation Laboratory (NSRL) and some primary experimental results. Synchrotron radiation can provide the tunable vacuum ultraviolet (VUV) photon with the high intensity and the good collimation. VUV photoionization is a single-photon ionization process. Combined with molecular-beam mass spectrometry (MBMS), the VUV single-photon ionization can be applied to detect the combustion products, especially the intermediates and free radicals produced from combustion process. This method is proved to be a powerful tool for combustion study, which could be helpful for developing combustion kinetic models and understanding the mechanism of combustion reactions.

Photoelectron Imaging Spectroscopy for (2+1) Resonance-Enhanced Multiphoton Ionization of Atomic Bromine

Two-photon resonant third photon ionization of atomic bromine <TEX>$(4p^5\;^2P_{3/2}\;and\;^2P_{1/2})$</TEX> has been studied using a photoelectron imaging spectroscopy in the wavelength region 250 - 278 nm. The technique has yielded simultaneously both relative branching ratios to the three levels of <TEX>$Br^+(^3P_2,\;^3P_{0.1}\;and^1D_2)$</TEX> with <TEX>$4p^4$</TEX> configuration and the angular distributions of outgoing photoelectrons. The product branching ratios reveal a strong propensity to populate particular levels in many cases. Several pathways have been documented for selective formation of <TEX>$Br^+(^3P_2)$</TEX> and <TEX>$Br^+(^3P_{0.1})$</TEX> ions. In general, the final ion level distributions are dominated by the preservation of the ion core configuration of a resonant excited state. Some deviations from this simple picture are discussed in terms of the configuration interaction of resonant states and the autoionization in the continuum. The photoelectron angular distributions are qualitatively similar for all transitions, with a positive <TEX>$A_2$</TEX> anisotropy coefficient of 1.0-2.0 and negligible <TEX>$A_4$</TEX> in most cases, which suggests that the angular distribution is mainly determined by the single-photon ionization process of a resonant excited state induced from the third photon absorption.

Vacuum ultraviolet single photon versus femtosecond multiphoton ionization of sputtered germanium clusters.

Neutral atoms and clusters desorbed from a solid germanium surface by ion bombardment are detected by laser postionization and time-of-flight mass spectrometry. Two different photoionization schemes are compared which are generally believed to be candidates for the 'soft' ionization of polyatomic species without significant photon induced fragmentation. First, a single photon ionization process is employed using an F2 laser as an intense VUV source with a photon energy in excess of all relevant ionization potentials. It is shown that the available laser pulse energy is sufficient to saturate the ionization of Ge atoms and all detected Ge(n) clusters. The resulting mass spectra are compared to those obtained with a non-resonant multiphoton ionization process using a high intensity laser delivering pulses of 250 femtoseconds duration at a wavelength of 267 nm. Also in this case, the ionization process can apparently be driven into saturation. The mass spectra measured under these conditions are found to be almost identical to those obtained using single photon ionization. We take this as an indication that the results obtained with both postionization techniques closely reflect the true cluster sputtering yields and, in particular, are not dominated by photon induced fragmentation.

A contribution to the understanding of the plasma ignition mechanism above a metal target under UV laser irradiation

In this paper, the plasma ignition process above a metallic surface submitted to UV laser irradiation is studied. An easy model based on the hypothesis of thermal equilibrium between ejected vapour and heated surface, and of a local thermodynamic equilibrium state of the vapour, is used to characterize the metallic vapour at the end of the laser pulse. Then the efficiency of the different elementary mechanisms liable to sustain or to prevent the ionization process in this medium is discussed depending on the laser power density. In this work, the calculations are applied to the case of the interaction between an excimer XeCl laser beam ( nm, ns) and titanium target. It is shown that the thermal heated metallic vapour is partially ionized and contains excited and singly ionized species at high densities ( atoms ). The electron temperature in this medium is found to be around 1 eV. The study of the ionization rise in the vapour evidences the important role played by the single-photon ionization process and the electron/ion inverse bremsstrahlung effect. For laser power densities above 100 MW (laser fluence of 2 J cm), the ionization level is found to increase before the laser pulse end, and a thermal evaporation regime is reached. As the laser power density exceeds 500 MW (fluence of 10 J ), an avalanche breakdown is liable to occur in the vapour before the pulse end and the plasma governs the evaporation mode. The results presented here are in good agreement with experimental observations and with results from more complex models reported in the literature.

Measured densities and rotational temperatures of metastable H2 in a multicusp ion source.

We report the first successful application of resonant multiphoton ionization to the diagnosis of a discharge. This technique is used to measure rotational temperatures of ${\mathrm{H}}_{2}$ in the metastable c ${\mathrm{}}^{3}$${\mathrm{\ensuremath{\Pi}}}_{\mathrm{u}}^{\mathrm{\ensuremath{-}}}$ state, which are in the range of 400 to 600 K. The density of these molecules is determined by a single-photon ionization process, yielding a value of 3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ at a discharge current of 30 A. The relevance of these metastable molecules to ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ production in hydrogen discharges is discussed.