2p Ionization Research Articles

Two-color photoionization of atoms in low and high radiation fields

Atomic photoionization was studied by means of electron spectroscopy using two photon sources of different wavelengths for the excitation. In one part of the experiments, alkaline atoms were excited by an optical laser of high spectral resolution prior to their photoionization by short wavelength radiation from the synchrotron radiation source BESSY. These low field studies concentrated on measurements of the linear alignment dichroism in the angular distribution (LADAD) and of the linear magnetic dichroism (LMDAD) in the 2p ionization of laser-polarized sodium atoms. In another type of experiments, the high intensity of a femtosecond optical laser was used for the investigation of dressed atoms, i.e. of the photoionization dynamics in strong external fields. In particular, the above threshold ionization (ATI) of rare gases was studied at the free electron laser facility FLASH in Hamburg, Germany. Measurements of the linear dichroism, here in the direct two-photon ionization of the 1s-shell in He, were performed in order to determine the partial photoionization cross sections related to the emission of electrons with s- and d- symmetry, respectively.

Partial ion yield spectroscopy around the Cl 2p and C 1s ionization thresholds in CF3Cl

We present a partial ion yield experiment on freon 13, CF(3)Cl, excited in the vicinity of the C 1s and Cl 2p ionization thresholds. We have collected a large amount of cationic fragments and a few anionic fragments at both edges. We have observed a strong intensity dependence of Rydberg transitions with ion fragment size for the CF(n)Cl(+) and CF(n)(+)/F(+) (n=0-3) series at both the Cl 2p and C 1s ionization edges. Selectivity in the fragmentation processes involving the C-Cl and C-F bonds are highlighted by the intensities of the C 1s to lowest unoccupied molecular orbital (LUMO) and LUMO+1 transitions measured on the CF(n)Cl(+) and CF(n)(+) yields. Equally, by comparison with their cation counterpart, we discuss possible bond-length dependence for the anion formation at the carbon 1s edge.

Linear dichroism in the innershell photoionization of alkaline atoms

The partial photoionization cross section of the 2p ionization of laser-excited aligned as well as ground-state oriented Na atoms were determined by high-resolution photoelectron spectroscopy and compared to multi-configuration Dirac-Fock and Hartree-Fock calculations. The measurements of the linear alignment dichroism (LAD) and the linear magnetic dichroism (LMDAD) are in very good agreement with the theoretical results. Similar studies were undertaken for the 3p photoionization of atomic K.

A versatile electron-ion coincidence spectrometer for photoelectron momentum imaging and threshold spectroscopy on mass selected ions using synchrotron radiation

We present a photoelectron-photoion coincidence (PEPICO) spectrometer named DELICIOUS II which combines a velocity map imaging apparatus with a Wiley-McLaren time of flight analyzer for the study of gas phase samples in interaction with the synchrotron radiation (SR). This versatile system is capable of providing photoelectron images on mass-selected compounds with kinetic energy resolutions of DeltaE/E=5% and a 17 eV bandwidth, as well as threshold photoelectron spectra with a measured resolution of 0.8 meV, as demonstrated on the 3p(-1) ionization of argon. This instrument is also employed for threshold PEPICO experiments, allowing the selection of the parent ion's internal state with sub-meV resolution for light masses (<40 amu) and with typically 2 meV resolution for a mass of 100 amu and with a mass resolving power above 200. The continuous operation of the extraction fields and the independence from the electron's time of flight are well adapted to the quasicontinuous multibunch mode of the SR. This, together with the high transmission of both the electron and ion detection, allows a high coincidence counting rate and facilitates the subtraction of false coincidences. We illustrate the spectrometer's coincidence principle of operation with examples from the valence photoionization of an Ar+Xe mixture and of CF(4).

Formation of Metastable Fragments around the Cl 2p Ionization Thresholds of HCl

Formation of Metastable Fragments around the Cl 2p Ionization Thresholds of HCl

Site-specific fragmentation caused by core-level photoionization in F3SiCH2CH2Si(CH3)3 vapor: Comparison between Si:1s and 2p photoionizations by means of photoelectron-photoion-photoion triple-coincidence spectroscopy

Site-specific fragmentation caused by Si:1s and 2p core-level photoionizations in F(3)SiCH(2)CH(2)Si(CH(3))(3) vapor was studied by energy-selected-photoelectron photoion-photoion triple-coincidence spectroscopy. The difference between the chemical shifts of the two Si sites is larger for the 1s ionization than for the 2p (2s) ionization. The fragmentation caused by the Si:1s ionization is more violent than that caused by the Si:2p ionization. The ions and ion pairs showing high site specificity for the Si:1s ionization belong to small fragments compared to those in the Si:2p ionization. Criteria for high site-specificity in fragmentation are discussed in conjunction with the present results.

Multi-coincidence in cascade Auger decay processes

Two-electron emission following inner shell ionization of rare-gas atoms by synchrotron radiation has been investigated experimentally with a magnetic bottle electron time of flight spectrometer. The detection in coincidence of a photoelectron with all Auger electrons allows a complete understanding of inner shell hole decay. This method has been applied to the study of single and double Auger decays following Xe 4d, Ar 2p and Kr 3d inner shell ionization. The double Auger decay scheme reveals direct double Auger process but is dominated by cascade processes involving inner-valence electrons in a first step followed by autoionization. Post collision interaction (PCI) associated to distorted Auger spectra and double photoionization paths in the vicinity of Xe 4d threshold, is also discussed.

Molecular frame and recoil frame angular distributions in dissociative photoionization of small molecules

Photoelectron angular distributions in the dipole approximation can be written with respect to several different reference frames. A brief review of the molecular frame and recoil frame are given. Experimentally, one approach for obtaining such angular distributions is through angle-resolved coincidence measurements of dissociative ionization. If the system dissociates into two heavy fragments, then the recoil frame angular distribution can be measured. Computed molecular frame and recoil frame photoelectron angular distributions are compared to experimental data for the Cl 2p ionization of CH3Cl.

Dissociation dynamics of anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations

The state-selective dissociation dynamics for anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations were characterized by combining measurements of the photon-induced anionic dissociation, x-ray absorption and UV/visible dispersed fluorescence. The transitions of core electrons to high Rydberg states/doubly excited states in the vicinity of both Si 2p and Cl 2p ionization thresholds of gaseous SiCl4 lead to a remarkably enhanced production of anionic, Si− and Cl−, fragments and excited neutral atomic, Si*, fragments. This enhancement via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of the contributions from the Auger decay of doubly excited states, shake-modified resonant Auger decay, or/and post-collision interaction. These complementary results provide insight into the state-selective anionic and excited neutral fragmentation of gaseous molecules via core-level excitation.

Photofragmentation of HCl near the chlorine L2,3 ionization threshold: new evidence of a strong ultrafast dissociation channel

We report a study using partial-ion-yield spectroscopy around the Cl 2p ionization threshold of HCl. All cation channels have been measured, while no evidence for anion production was obtained. The absence of any detectable H− can be related to the electronegativity difference between the two atoms and the observed low probability of directly creating Cl++ and Cl+++. In the photon-energy region around the Cl 2p3/2,1/2 → 6σ* excitation process, production of H+ is almost completely suppressed, which indicates a dominant ultrafast dissociation process leading to neutral H.

The ionization of Mg by electron impact at 1000 eV studied by (e, 2e) experiments

The ionization of Mg 3s and 2p and He 1s has been studied in (e, 2e) experiments at about 1000 eV incident energy and 20 eV ejected electron energy for a momentum transfer between 0.5 and 2.1 au. The comparison with the predictions of the distorted wave Born approximation model shows a generally good agreement between experiment and theory. The differences observed between the He and Mg angular distributions can be explained as an initial state effect and are attributed to the differences between the He 1s and Mg 3s wavefunctions in the momentum space.

The M4,5N–NNN Auger transitions in Kr

The M4,5N–NNN Auger transitions appearing as a satellite structure in the M4,5–NN Auger spectrum of krypton have been studied in detail. The M4,5N–NNN satellite Auger spectra were measured with photon energies below the 3p ionization threshold at 208 eV and above the 3p ionization threshold at 217.6 and 280 eV. The kinetic energies and intensities of the M2,3–M4,5N and M4,5N–NNN Auger transitions were calculated by using the Hartree–Fock (HF) wavefunctions with Cowan's code. The population of the 3d−14p−1 initial states produced by direct double ionization was estimated semiempirically. The most intense M4,5N–NNN Auger transitions were assigned with the aid of the calculations.

Many-electron effects in 2p photoionization and Auger decay of atomic aluminium

High-resolution 2p photoionization and subsequent Auger decay of atomic Al have been investigated both experimentally and theoretically. Fine-structure-resolved experimental main and satellite photoelectron spectra were reproduced using extensive relativistic multiconfiguration Dirac–Fock calculations. Emphasis was placed on the electron correlations and how they affect the coupling of the valence and semi-valence electrons. The Auger electron spectrum was used to probe the electron correlation between the three electrons above the ionized 2p orbital and a breakdown of the two-electron picture describing the Auger emission was observed. Continuum wave selection was used to expound the intensity behaviour seen in the Auger electron spectra. Direct double photoionization thresholds and behaviour of the relative partial cross section as a function of photon energy during 2p ionization were experimentally obtained from the Auger spectra.

Absolute photoionization cross sections with ultra-high energy resolution for Ar, Kr, Xe and N 2 in inner-shell ionization regions

The high-resolution absolute photoionization cross sections for Ar, Kr, Xe and N 2 in the inner-shell ionization region have been measured using a multi-electrode ion chamber and monochromatized synchrotron radiation. The energy ranges of the incident photons for the target gases were as follows: Ar: 242–252 eV (2p Rydberg excitation), Kr: 1650–1770 eV (near the 2p ionization thresholds), Xe: 665–720 eV (near the 3d ionization thresholds) and 880–1010 eV (near the 3p ionization thresholds), N 2: 400–425 eV (N 1s excitation and ionization). It is the first time to measure the absolute ionization cross sections of Ar, Kr, Xe and N 2 over the present energy ranges with the energy resolution of over 10,000. The natural lifetime widths of 2 p 3 / 2 − 1 4 s , 2 p 3 / 2 − 1 3 d , 2 p 3 / 2 − 1 4 d and 2 p 1 / 2 − 1 4 s resonances for Ar, 3 d 5 / 2 − 1 6 p resonance for Xe, and 1 s − 1 π g * ( v ′ = 0 ) resonance for N 2 have been obtained based on the cross sections determined. The ionization energies into the Ar + ( 2 p 3 / 2 − 1 ), Ar + ( 2 p 1 / 2 − 1 ) and Xe + ( 3 d 5 / 2 − 1 ) ionic states are also determined using the Rydberg formula.

Energy loss in polycarbonate and polyethylene terephthalate by 2.0–6.5 MeV 14N, 31P and 75As ions

Energy loss for 2.0–6.0MeV 14N ions, 3.0–6.5MeV 31P ions and 3.0–6.5MeV 75As ions in thin foils of polycarbonate and polyethylene terephthalate were measured by means of a transmission technique using a half-covered silicon surface barrier detector. During the measurement, the charged particle count rates were kept below 100cps in order to minimize dead time and radiation damage in the system. The obtained data are compared with the SRIM 2003 calculation and the ICRU Report 73. Most of the measurement results compare quite favorably with the ICRU Report 73.

(e,2e) measurements using a magnetic angle changer

While measurements of triple differential cross sections (TDCS) for electron impact ionization have been performed for almost 40 years, the angular range of available data has generally been restricted due to physical constraints of the apparatus. We have overcome these constraints by incorporating a magnetic angle changer (MAC) into a conventional (e,2e) spectrometer. Use of a MAC in an (e,2e) experiment allows access to regions where as yet unobserved physical effects occur. One such example is where the scattered and ejected electrons emerge at the same angle, a region where the Coulomb repulsion between the two should be largest. This ‘post-collision interaction’ (PCI) is thought to play an important role in low energy electron impact ionization. We discuss recently published data for TDCS of the inner valence shell 3s ionization of argon measured over the backward scattering direction. We also present new data for outer shell 3p ionization of argon over the full angular range in the scattering plane, including the region where scattered/ejected electrons emerge at the same angle. To our knowledge, this is the first reported TDCS over the complete 360° angular range using a conventional (e,2e) spectrometer.

Distribution of the oscillator strengths in the 2p absorption spectra of 3d transition metal films

The absorption cross-sections in the region of the resonance structure of the 2p ionization threshold of metal films are originally measured for the total series of 3d metals from Sc to Cu. The partial 2p absorption cross-sections are found, and the integral oscillator strengths are determined. The linear dependence of the sum of oscillator strengths in the range from 0 to 80 eV above the L3 absorption edge on the number of unfilled 3d states is established. The dependence breaks down for Cr, Fe, and Ni.

Fluorescence emission from photo-fragments after resonant S 2p excitations in H2S

Visible-UV fluorescence emission of gas-phase hydrogen sulfide, H(2)S, has been studied at the S 2p edge with synchrotron radiation excitation. Dispersed fluorescence measurements in the wavelength range 300-900 nm were taken at several photon energies corresponding to the excitations of the S 2p electrons to the unoccupied molecular and Rydberg orbitals. The spectra reveal fluorescence from the H, S, S(+), HS and HS(+) photo-fragments. H is found to be the strongest emitter at Rydberg excitations, while the emission from S(+) is dominant at the molecular resonances and above the S 2p ionization thresholds. The intensities of hydrogen Lyman-alpha (122 nm), Balmer-alpha (656 nm), Balmer-beta (486 nm) transitions as well as the visible-UV total fluorescence yield (300-900 nm) and the total ion yield were measured by scanning the photon energy in small steps across the S 2p edge. The different Balmer lines show some sensitivity to the specific core excitations, which is, however, not so strong as that recently observed in the water molecule [E. Melero García, A. Kivimäki, L. G. M. Pettersson, J. Alvarez Ruiz, M. Coreno, M. de Simone, R. Richter and K. C. Prince, Phys. Rev. Lett., 2006, 96, 063003].

Radiative relaxation and fragmentation dynamics of S 2p-excited hydrogen sulfide

Radiative relaxation of S 2p-excited hydrogen sulfide (H(2)S) is investigated by dispersed ultraviolet and visible fluorescence spectroscopies. We observe distinct changes in the fluorescence spectra as a function of excitation energy. Excitation to Rydberg states below the S 2p ionization threshold yields intense fluorescence from neutral and ionic atomic fragments (H, S(+), and S(2+)). In addition to the atomic emission, fluorescence of the molecular fragment ion HS(+) is preferably found after excitation of the S 2p electron into the unoccupied 6a(1) and 3b(2) orbitals with sigma(*) character. This is interpreted as evidence for ultrafast dissociation of the core-excited molecule prior to electronic relaxation. The rotationally resolved fluorescence spectra of the A (3)Pi-->X (3)Sigma(-) transition are analyzed in terms of the fragmentation dynamics leading to the formation of the excited molecular fragment ion, where changes in bond angle are discussed in terms of the rotational population.

Design and performance of an (e, e+ion) spectrometer for studies of fragmentation of dipole and non-dipole states of gaseous molecules

An (e, e+ion) spectrometer has been constructed that has the capability to measure time-of-flight (TOF) mass spectra of gases in coincidence with energy selected inelastically scattered electrons, as a function of impact energy, electron energy loss and electron scattering angle. Relative to earlier dipole-regime implementations of the (e, e+ion) technique, this spectrometer can be used to study molecular fragmentation under both dipole and non-dipole electronic excitation and ionization conditions. The spectrometer uses a position-sensitive electron detector and a TOF tube positioned at 90° with respect to the electron impact and scattering plane. The TOF design makes it possible either to extract all ions from the interaction region, or to discriminate preferentially for ions that have kinetic energy along the axis of the TOF tube, thus allowing one to collect spectra with maximum efficiency, or to study the dynamics of the production of the molecular and fragment ions. The design and construction of the spectrometer is described, along with preliminary results for ionization in the S 2p excitation and ionization region of SF 6.