Non-destructive depth profile analysis using synchrotron radiation excited XPS

We have used synchrotron radiation as excitation source in an X-ray photoelectron spectroscopy (XPS) experiment to analyse surface-near element depth profiles non-dectructively. By tuning the photon energy one can vary the kinetic energy of the photoelectrons and in turn the information depth of the measurement. To quantify the sample geometry (e.g. layer thicknesses) model calculations similar as for angle-resolved XPS (ARXPS) measurements are necessary. We have successfully applied this technique to several samples. We will show how to calculate the relative intensities of the peaks, using photoionization cross sections and an experimentally determined analyzer transmission function and the procedure to quantify the geometry for a model sample: natively oxidized Ta covered by carbon contamination. At Sn-doped indium oxide samples we found a sub-monolayer of segregated Sn at the surface which was expected from previous investigations.     

Photoionization mass spectrometry of molecular clusters using synchrotron radiation

The photoionization and dissociative ionization of molecular aggregates using synchrotron radiation is reported. The main objective of the review is to consider the intracluster relaxation processes after ionization. For hydrogen-bonded systems proton transfer is dominant. For small clusters (n<4) appearance potentials, ionization potentials, absolute proton affinities, proton solvation energies and intermolecular bond energies in the ionic clusters are deduced. For van der Waals aggregates proton transfer can also be used to characterize the intermolecular bond in the ionic cluster. Aggregates of CH₄, SiH₄, CH₃F show proton transfer in contrast to simple aromatic compounds, which reveal no proton transfer. From the fragmentation pattern and appearance potentials relaxation by intracluster ion molecule reactions is discussed. In heterogeneous clusters intracluster Penning ionization is observed. The shift of the charge transfer resonances depends on the π-electron density in the aromatic system. The width and spectral position of these resonances are influenced by the cluster size.     

Single-photon double ionization of mercury clusters using synchrotron radiation

Single-photon doublet-ionization experiments have been carried out on mercury resulting in the formation of doubly charged clusters. The efficiency is very high and similar to that obtained by electron impact. The mechanism of double ionization is discussed and our results favor a two-step process both in photon and electron impact ionization.     

Magic numbers of large rare gas clusters

The superior stability of closed-shell icosahedral structures is evident from size distributions of argon, krypton and xenon cluster ions in the size range 100?n?1000.     

Ab-initio calculations of excited rare gas alkali ions

The excimer-like ions formed by combining a rare gas ion with an alkali atom were investigated by an ab initio HF-CI calculation. The resulting four bound excited states¹Σ⁺,¹Π,³Σ⁺ and³Π were subjected to a semiempirical spin orbit (SO)-coupling yielding eight fine structure levels with angular momenta Ω=2, 1(3) and 0(4). For (NeLi)⁺, (NeNa)⁺, (ArLi)⁺, (ArNa)⁺, (ArK)⁺ and (KrLi)⁺ the wavelengths for the five allowed transitions to the ground state were calculated. The results are in close agreement with the experimental results. Also other spectroscopic properties as binding energies, transition moments and vibrational quanta are given.     

No fluorescence decay from low-lying electronic states excited into single vibronic levels with synchrotron radiation

Fluorescence decay of NO has been studied by single photon counting using the synchrotron radiation from the Orsay Electron Storage Ring (ACO) as a source of excitation. Emissions from A²Σ⁺(υ = 0,1,2,3), B²Π(υ = 5), C²Π(υ = 0, 1) and D²Σ⁺(υ = 0, 1, 2, 3) levels have been observed in function of NO gas pressure in the 0.02–4 torr range. Collision-free lifetimes and self-quenching rate constants have been derived from these measurements for all these levels and compared to previous data. Particular attention has been paid to the C²Π(υ = 0) level decay properties. By narrow-band (≈ 45 cm^?1) excitation inside the rotational envelope of this level we show that the decay is non-exponential with a short-living component (≈ 3 ns) a long-living one (≈ 20 ns). We develop a number of arguments in order to prove the short-living levels (J > 5 or 7) are weakly predissociated.     

Time and spectrally resolved fluorescence of Xe2 molecules excited with synchrotron radiation

Time and energy resolved fluorescence of Xe has been investigated using pulsed synchrotron radiation. The radiative lifetime of the relaxed Xe*₂ (I_u) and its collisional mixing with Xe*₂ (O⁺_u) are measured. Rate constants for molecular formation are deduced. Different steps of the reaction kinetics are isolated by using various excitation wavelengths.     

Comments on heteronuclear electronically excited rare gas diatomic molecules

Experimental evidence for the stability of the electronically excited (KrAr)^* molecule is presented from the study of the vacuum ultraviolet luminescence spectra of Kr/Ar gaseous, liquid and solid mixtures.     

Fragmentation of large ionized clusters of rare gas atoms

Molecular dynamics is used to examine the fragmentation of clusters of rare gas atoms after ionization. The cohesive energy is given by a quantum mechanical model with a delocalized hole. Very small clusters dissociate entirely into single atoms and a positively charged dimer. Larger clusters (e.g. Ne₁₃, Xe₁₃ and Ne₅₅) first eject rapid atoms, then thermalize and evaporate further atoms, which strongly decreases their size. Very large clusters (e.g. Xe₅₅) are only heated up after ionization and do not loose atoms. Thus the peaks in mass spectra do not show the atomic shell structure of neutral clusters up to rather large cluster sizes. Instead the stability of ionized clusters is reflected.     

Photoionization in rare gases with synchrotron radiation: Some basic aspects for critical tests with theory

Three different aspects of angle-resolved electron spectrometry using synchrotron radiation are discussed on the basis of three specific examples: the line resolution, the kinetic energy and the angular distribution parameter. It is shown that for rare gases data of high quality can be obtained such that the data provide the basis for critical tests with different theoretical treatments. The selected examples concern the 3s/3p satellite spectrum of argon, the influence of post-collision interaction in the electron spectrum related to 4d-photoionization in xenon and the angular distribution parameter for 2p-photoionization in neon.     

Theory for excess electrons in clusters of rare gas atoms

An exact theory for excess electrons in clusters of rare gas atoms is limited to small sizes,n < 20, because of many body polarization interactions which make it necessary to solve a large system of linear equations. We present a simple dielectric screening approximation, which avoids this difficulty and which is in very good agreement with the exact calculation. This approximation can be used to examine the excitation energies of excess electrons and exciton energies in large clusters. A new atomic structure is proposed for a cluster of 12 Xe atoms, resulting in an increased binding energy for the excess electron, which is larger than the electron affinity of a cluster of 13 atoms. This might explain the relatively large abundance of Xe ₁₂ ^? observed in the experiment.     

X-ray fluorescence spectrometry with synchrotron radiation

X-ray fluorescence spectrometry (x.r.f.) can be done through excitation with synchrotron radiation. This permits multi-element determinations in the trace region with improved detection limits compared to conventional x.r.f. Detection limits are evaluated and compared with theoretically calculated values. For a beam diameter of 0.5 mm and a sample of 1 mg cm^?2, absolute detection limits are between 0.1 and 0.4 pg. The dependence of the detection limit on the atomic number is reduced, when white synchrotron radiation is used for excitation instead of monochromatic radiation. The optimum of the limit of detection on the Z-scale can be shifted to higher atomic numbers and improved through filtration of the primary radiation by aluminium absorbers. Preparation of samples on different polymeric films is discussed in relation to blank values.     

Spectral and kinetic studies of the second continuum fluorescence of xenon excited by synchrotron radiation

Synchrotron radiation at LURE has been used to excite xenon in the pressure range of 50 to 200 torr. The subsequent second continuum fluorescence at 1720 Å has been spectrally and temporally analyzed. The observed two-component decay of this emission, as well as various spectral features, are shown to be in disagreement with some results previously reported in the literature.     

Electron spectroscopy study of the 27.5 eV satellite line in acetylene excited by synchrotron radiation

Valence electron spectra from acetylene using synchrotron radiation with 50 and 70 eV excitation energy are presented. The 27.5 eV satellite line is resolved into two components. The intensities of these satellite lines are compared to the previously measured intensities at 1487 eV excitation energy. β values for the satellite lines and for all the valence levels are determined at 70 eV excitation enery. A previously unknown satellite line is found at 31.0 eV binding energy. This line corresponds to an onset of the double ionization continuum.     

Velocity map imaging of HBr photodissociation in large rare gas clusters

We have implemented the velocity map imaging technique to study clustering in the pulsed supersonic expansions of hydrogen bromide in helium, argon, and xenon. The expansions are characterized by direct imaging of the beam velocity distributions. We have investigated the cluster generation by means of UV photodissociation and photoionization of HBr molecules. Two distinct features appear in the hydrogen atom photofragment images in the clustering regime: (i) photofragments with near zero kinetic energies and (ii) "hot" photofragments originating from vibrationally excited HBr molecules. The origin of both features is attributed to the fragment caging by the cluster. We discuss the nature of the formed clusters based on the change of the photofragment images with the expansion parameters and on the photoionization mass spectra and conclude that single HBr molecule encompassed with rare gas "snowball" is consistent with the experimental observations.     

Photophoresis of small-size particles in a gas excited by resonance radiation

The motion of small aerosol particles in uniform gas excited by a resonance radiation is caused by the specific mechanism. In the case of a small deviation of the laser radiation frequency from that of the electronic or the vibration-rotation transition, due to the Doppler effect, only the molecules whose velocity projection on the radiation propagation direction lies within a certain velocity range can be excited. For this reason, the relationship between the excited and the nonexcited molecules in the aforesaid velocity range differs from the corresponding relationship in the velocity range, which is symmetrical with regard to the center of the Maxwell distribution. A resultant force, which is applied by the gas to aerosol particles, arises due to the different scatterings of the excited and nonexcited molecules by the surface of aerosol particles.