Photoelectron Coincidence Research Articles

Autoionization selectivity of Ne+ Rydberg states converging to Ne2+(1Se)

Indirect double photoionization processes of Ne have been studied in the photon energy range from 65.5 to 68.3 eV by using a threshold-photoelectron−photoelectron coincidence method, where intermediately formed Ne+ states have been defined by the detection of threshold photoelectrons and fast electrons from their autoionizing decay have been analysed in coincidence. The present investigation revealed that each Ne+ Rydberg series converging to Ne2+(1Se) shows notable selectivity in the autoionizing decay to the final Ne2+ states.

Photo-double ionization of hydrogen iodide: experiment and theory

Abstract Photo-double ionization of hydrogen iodide has been investigated over the photon energy range 29.0–32.5 eV by threshold photoelectrons coincidence (TPEsCO) spectroscopy and by ab initio calculations that included spin–orbit interaction. Good agreement is found between experiment and theory for the adiabatic ionization potentials for the formation of the X 3 Σ − , a 1 Δ and b 1 Σ + states of HI2+ and for the vibrational separations and relative vibrational intensities within the states. The spin–orbit splitting of the X 3 Σ − state of HI2+ is calculated to be 0.17924 eV for ν2+=0. Because this splitting is close to the calculated vibrational separations within the spin–orbit components of this state [e.g., for ν2+=0–1, 0.22382 eV in 3 Σ 0 − and 0.22163 eV in 3 Σ 1 − ], there is an overlapping of vibrational structure within the TPEsCO spectrum at the resolution used that is confirmed by the simulation spectrum.

Monte-Carlo simulation of Bremsstrahlung interference due to K-shell photoelectrons in Compton scattering experiments

As a part of a larger study of Compton scattering from the K-shell electrons of Ta and Pb, Monte-Carlo simulation has been used to study X-rays emitted from 0.05 mm thick Pb and Ta foils due to K-shell photoelectric absorption of 320 and 662 keV incident γ-rays. The interference in such studies due to Bremsstrahlung from photoelectrons in coincidence with the K X-ray photons mimics the characteristics of the Compton scattering of interest and cannot be eliminated experimentally. In this work, the detector was calibrated at eight energies by experimental determination of the full energy peak and a Monte-Carlo modelling of the physical processes, which determine the detector efficiency and response function was undertaken. This physical model was then used to calculate the Bremsstrahlung emitted from 0.05 mm thick Pb and Ta target foils due to K-shell photoelectrons originating from photoelectric absorption of 320 and 662 keV incident γ-rays on the target.

Experimental evidence for extreme surface sensitivity in Auger-Photoelectron Coincidence Spectroscopy (APECS) from solids

Core hole creation and subsequent Auger decay processes are studied with unprecedented discrimination by Auger-Photoelectron Coincidence Spectroscopy (APECS). Early works in this field have already pointed out the intrinsic surface sensitivity of these experiments. However, it was not until recently that a model calculation was developed to quantitatively evaluate it. Here we present the first attempt to experimentally establish an effective target thickness for such experiments. The angular distribution of 3p 3/2 photoelectron with kinetic energy of 160 eV is measured in coincidence with the M 3VV Auger electron with kinetic energy of 55 eV on a Cu (1 1 1) surface. Coincidence and non-coincidence photoelectron angular distributions display differences that, to large extent, are explained by confining the source of the coincident signal within the first two layers of Cu target, thus establishing an experimental upper limit for the effective target thickness of the APECS experiment.

Experimental evidence for extreme surface sensitivity in Auger-Photoelectron Coincidence Spectroscopy (APECS) from solids

Core hole creation and subsequent Auger decay processes are studied with unprecedented discrimination by Auger-Photoelectron Coincidence Spectroscopy (APECS). Early works in this field have already pointed out the intrinsic surface sensitivity of these experiments. However, it was not until recently that a model calculation was developed to quantitatively evaluate it. Here we present the first attempt to experimentally establish an effective target thickness for such experiments. The angular distribution of 3p3/2 photoelectron with kinetic energy of 160 eV is measured in coincidence with the M3VV Auger electron with kinetic energy of 55 eV on a Cu (1 1 1) surface. Coincidence and non-coincidence photoelectron angular distributions display differences that, to large extent, are explained by confining the source of the coincident signal within the first two layers of Cu target, thus establishing an experimental upper limit for the effective target thickness of the APECS experiment.

Dissociative double photoionisation of CO below the CO++ threshold

Dissociative double photoionisation of CO below the adiabatic double ionisation threshold has been studied by the time-of-flight photoelectron–photoelectron coincidence (TOF-PEPECO) and the velocity imaging photoionisation coincidence (VIPCO) methods. By combining the results from the two methods which give complementary information, the double photoionisation process is identified to CO + hv → CO +* + e − → C + ( 2 P) + [O + ( 2 D/ 2 P)] nl + e − → C + ( 2 P) + O + ( 4 S) + 2e − . The high energy resolution of the TOF-PEPECO spectrometer enables us to resolve individual fragment [O + ( 2 D/ 2 P)] nl states and also intermediate CO +* states. The angular correlations between electrons and ions, deduced from the VIPCO data, imply that the nuclear dissociation is faster or at least comparable in the time scale with molecular rotation, and that the second electron emission occurs after the C + and O * atoms are far apart. A possible generic identification of the intermediate states is suggested.

Non-adiabatic intramolecular and photodissociation dynamics studied by femtosecond time-resolved photoelectron and coincidence imaging spectroscopy

Time-resolved photoelectron spectroscopy (TRPES) is emerging as a useful tool for the study of non-adiabatic dynamics in isolated polyatomic molecules and clusters due to its sensitivity to both electronic and vibrational dynamics. A powerful extension of TRPES, coincidence imaging spectroscopy (CIS), based upon femtosecond time-resolved 3D momentum vector imaging of both photoions and photoelectrons in coincidence, is a new technique for the study of complex dissociative processes. Here we show how these spectroscopies can be used to study both non-adiabatic intramolecular and photodissociation dynamics in polyatomic molecules. Intramolecular dynamics in the alpha, beta-enones acrolein, crotonaldehyde and methyl vinyl ketone are studied using both TRPES and laser-induced fluorescence of HCO(X) product yields. The location of the methyl group is seen to have very dramatic effects on the relative electronic relaxation rates and the HCO(X) yield. Applying both TRPES and CIS to the 200 nm and 209 nm photodissociation of the nitric oxide dimer, (NO)2, we observe the fs time-scale evolution of the excited parent neutral via its photoelectron spectrum and the emergence of the NO(A) photofragment including its energy and angular distributions.

ミニチュア円筒鏡型電子エネルギー分析器（ＣＭＡ）の製作と評価，オージェ―光電子コインシデンス分光法への応用

We have developed a miniature cylindrical mirror electron energy analyzer (CMA) with a diameter of 26 mm. The CMA consists of a shield for electric field, inner and outer cylinders, a pinhole, and an electron multiplier. By assembling the CMA in a coaxially symmetric mirror electron energy analyzer (coASMA) coaxially and confocally we developed an analyzer for Auger-photoelectron coincidence spectroscopy (APECS). The performance was tested by measuring Si LVV Auger-Si 2p photoelectron coincidence spectra of a Si (111) surface. Features of the APECS analyzer are as follow. 1) The coincidence signal detection efficiency of the analyzer is improved by one order of magnitude from previous ones because of the large solid angle of the coASMA and the CMA. 2) The positioning is quite easy, because the coASMA and the CMA are assembled confocally on a rod with a mechanism for xyz positioning and tilt adjustment. 3) It can be installed to a general purpose ultrahigh vacuum chamber because it is constructed on a 203-mm-outer-diameter conflat flange with a 50-mm-retractable mechanism. 4) The production cost is low because the structure is simple and the number of the parts is relatively small.

Photo-double ionization of HBr and DBr studied by threshold photoelectrons coincidence spectroscopy

Photo-double ionization of HBr and DBr has been investigated in the 32–37 eV energy range using synchrotron radiation and the threshold photoelectrons coincidence (TPEsCO) method. The TPEsCO spectra of HBr and DBr, encompassing the formation of the X 3Σ −, a 1Δ and b 1Σ + states of HBr 2+ and DBr 2+, were recorded using a pair of penetrating-field electron spectrometers. All electronic band systems exhibited vibrational structure. In addition, the threshold photoelectron spectra of HBr and DBr for the formation of singly charged HBr and DBr were recorded simultaneously.

The Ag M 5N 45N 45 Auger photoelectron coincidence spectra of disordered Ag 0.5Pd 0.5 alloy

An effect of disorder broadening (DB) on the Ag M 5N 45N 45 Auger spectra in the random substituted Ag 0.5Pd 0.5 has been investigated by Auger photoelectron coincidence spectroscopy (APECS). Data were collected for the Ag M 5N 45N 45 Auger line coincident with the Ag 3d 5/2 photoelectron line (and its higher and lower binding energy sides). It is shown that the broadening of the Ag M 5N 45N 45 line is directly associated with the presence of disorder broadening of the Ag 3d 5/2 photoelectron line. The APECS experiment is used to demonstrate the broadening in a novel way.

Complete double photoionisation spectrum of NO

Abstract A new time-of-flight photoelectron–photoelectron coincidence technique, which gives complete two-dimensional e−–e− spectra in single-photon double photoionisation, is applied to nitric oxide. Direct double photoionisation is the major process at photon energies of 40.8, 48.4 and 50 eV, but dissociative ionisation with autoionisation of atomic fragments is also significant at these wavelengths and is dominant at 38.7 eV photon energy. The energy of the v=0 level of the A2Π state of NO2+ is corrected, leading to a new prediction of the wavelength of optical emission. The electron energy distributions in formation of the bound states of NO2+ are smooth, lacking sharp autoionisation features. Broad peaks found in the photoelectron distributions coincident with autoionisations of atomic oxygen are at different energies for each different O* state, indicating the involvement of several distinct dissociative superexcited states of NO+.

One, two and three-body channels of the core–valence–valence Auger photoelectron coincidence spectra of early transition metals

We propose a simple model of core–valence–valence Auger and APECS intensities from open-band solids to account for the alleged negative- U behavior of the spectra of early transition metals. In these systems the maximum of the line shape is shifted by the interaction to lower binding energy, which is the contrary of what happens in closed band materials where the two-hole Green’s function G ω (2) allows to understand the phenomenology of the spectra in terms of the U/ W ratio of the on-site repulsion U to the band width W. Actually, for open-band solids, only part of the intensity comes from the decay of unscreened core-holes and is obtained by the two-body Green’s function G ω (2), as in the case of filled bands. The rest of the intensity which arises from screened core-holes here is derived using a variational description of the relaxed ground state; this involves the two-holes-one-electron propagator G ω , that also contains one-hole contributions. We propose a practical scheme to calculate the three-body Green function by a summation of the perturbation series to all orders. We achieve that by formally rewriting the problem in terms of a fictitious three-body interaction. Our method grants non-negative densities of states, explains the apparent negative- U behavior and interpolates well between weak and strong coupling, as we demonstrate by test model calculations.

Ion desorption induced by core-level excitation of H2O/Si(100): Evidence of desorption due to the multielectron excitation/decay

This work is an investigation of the desorption by O 1s excitation of ions from Si(100) reacted with water. Photoelectron, photostimulated desorption, and electron-ion coincidence spectroscopy are used to observe the process. When the incident photons have energy levels which are near the 1s threshold of O, they induce Auger decay that is accompanied by shakeup/off excitation and cascade Auger decay, and they are shown to be the main factor responsible for desorption in this case. When the photons have energy levels which are above the shakeup threshold, most of the desorption that occurs is a result of the shakeup excitation that accompanies the core excitation. In both cases, the desorption is induced by the respective multihole final states. The ion desorption yield for the two-hole final states of the normal process of Auger decay is small. The results are discussed, with the help of the Auger electron spectra, mainly in terms of the lifetime of the final state of Auger decay.

On the L 3 level photoelectron spectrum of Cu metal measured in coincidence with the L 3–VV Auger-electron spectrum

We provide an answer to the question why the L 3 photoelectron line of Cu metal measured in coincidence with the L 3–VV ( 3F or 1G) Auger-electron line, does not line up with the L 3 single photoelectron line. We provide also an answer to the question why the L 3 coincidence photoelectron line is unshifted when the Auger-electron analyzer is moved away from the Auger-electron line. We show that it is the initial core–hole self-energy by the monopole excitation (screening) and the density of final states which play an important role in the shift and narrowing of Auger-photoelectron coincidence spectroscopy (APECS) spectral line. To explain the shifted APECS spectral line, Thurgate and Jiang (Surf. Sci. 466 (2000) L807) recently proposed the presence of two different core–hole states in the main-line state upon the L 3 level ionization in Cu metal. However, their explanation appears to be incorrect.

Investigating the structure of the Tin M 45N 45N 45 auger spectrum using auger photoelectron coincidence spectroscopy

Auger photoelectron coincidence spectroscopy (APECS) data were collected for the M 45N 45N 45 Auger peak in coincidence with the 3p 3/2, 3d 3/2 and 3d 5/2 photoelectron lines of Tin. Model spectra were created to fit the APECS data from sets of Gaussian curves defined by Parry-Jones et al., J. Phys. C: Solid State Physics, 12 (1979) 1587. These models were then combined using information about the relative intensities of the peaks from the aforementioned paper to produce a model of the Auger peak which proved a good comparison to high resolution AES spectra. The APECS data revealed satelite structure in the M 5N 45N 45 peak in coincidence with the 3d 5/2 photoelectron line (M 5N 45N 45:3d 5/2) due to the Mg Kα 3 line of the X-ray source. There was evidence of a small Coster–Kronig component in the M 4N 45N 45:3d 3/2 data and the M 45N 45N 45:3p 3/2 data showed intensity in the M 4N 45N 45 and M 5N 45N 45 regions also arising from Coster–Kronig processes. The contribution of the M 4N 45N 45 plasmon was included in each of the APECS models and was reflected in the high resolution AES spectra. Slight oxidation of the surface of the sample during each 24-h period produced a 0.7 eV shift of the singles Auger peak to lower kinetic energies. The shift was not reflected in the coincidence peak which produced a spectrum of a clean surface due to the nature of the coincidence experiment.

TPEsCO spectra of HCl 2+ and DCl 2+: experiment and theory

Vibrationally resolved threshold photoelectrons in coincidence (TPEsCO) spectra of the X 3Σ − and a 1Δ states of HCl 2+ and DCl 2+ are compared with ab initio simulations. The four resonances observed in the TPEsCO spectra of both the X 3Σ − and a 1Δ states of HCl 2+ are assigned as three quasi-bound vibrational levels and one continuum resonance with an energy greater than the potential barrier maximum. The four clearly identified resonances observed in the TPEsCO spectra of both the X 3Σ − and a 1Δ states of DCl 2+ are assigned as quasi-bound vibrational levels.

The ALOISA end station at Elettra:: a novel multicoincidence spectrometer for angle resolved APECS

Coincidence measurements have been extensively performed in atomic and molecular physics since early 1970s. To apply this methodology to solids and surfaces has been a major target since early days, but the long average time needed to complete a coincidence experiment has hampered its attainment. In particular the coincidence technique has not been yet applied in an angle resolved way such for studying the momentum correlation in the ejection of electron pairs from solid surfaces. The experimental chamber at the ALOISA beamline at Elettra, by means of a set of seven homemade electron analyzers, is the first apparatus able to perform Angle Resolved – Auger Photoelectron Coincidence Spectroscopy (AR-APECS) from solid surfaces. In the typical setup ten different pairs of coincident electrons can be measured simultaneously, so reducing the acquisition time by one order of magnitude.

Control of the fragmentation of excited ammonia clusters by femtosecond infrared laser pulses

Applying photoion and coincident photoelectron detection in femtosecond pump–probe experiments, we have studied the change of the fragmentation behavior of ammonia clusters excited by femtosecond (fs) laser pulses at 200 nm to the electronic à state which absorb an additional fs control photon 1–2 ps after the pump photon. Only a few 100 fs after the primary excitation, the (NH3)n clusters are partially transferred to the vibrationally highly excited H-transfer state (NH3)n−2NH4NH2 with a lifetime of a few ps. By irradiating the clusters in this state with control photons of a wavelength in the range of 1200–1400 nm, we were able to excite the clusters resonantly to the next higher electronic state in the H-transfer configuration with a strongly reduced vibrational energy. The excited H-transfer state corresponds to the 3s→3p transition in the NH4 component of the internally hydrogenated clusters. Due to the strong reduction of the vibrational energy after the control photon absorption, the fragmentation probability in the excited H-transfer state and correspondingly in the ionic proton transfer state is drastically reduced. For example, for the ammonia dimer the signal ratio of [(NH3)2+] to [NH4+] has been enlarged by nearly one order of magnitude by the resonant control photon absorption. Whereas the lifetime of the ammonia clusters in the nonexcited H-transfer state is nearly identical for all cluster sizes (2–4 ps) we found distinct lifetimes τ6 for the excited H-transfer state of the dimer and the trimer. For the dimer a lifetime τ6=130±50 fs has been obtained for undeuterated as well as for deuterated ammonia molecules. In contrast, for the trimer the lifetime τ6 is significantly larger and depends on the control wavelength as well as on the isotope composition.

Doubly Charged Molecular Ions Studied by Double Charge Transfer Spectroscopy

AbstractWe have developed a double charge transfer (DCT) spectroscope to study doubly charged molecular ions (dications). In the course of the development, DCT spectra of N2, CO, and NO has been ob served with energy resolution of about 0.5 eV. The spectral profiles observed have shown the Franck‐Condon (FC) behavior. We have made an effort to in crease the resolution up to about 140 meV, which is much better than those (1–1.5 eV) reported in previous works. In the present measurement, the vibrationally resolved DCT spectrum for N2++ could be obtained for the first time. The observed peak structures agree well with those calculated using FC factors, differing meaningfully from the spectra taken with Auger electron spectroscopy (AES) and thresh old photoelectrons coincidence (TPEsCO) spectroscopy. The DCT spectroscopy might be a powerful method to study the potential curves of short‐lived dications.

Line structure in photoelectron and Auger electron spectra of CuOx/Cu and Cu by Auger photoelectron coincidence spectroscopy (APECS)

AbstractIn this study, we present Cu L3M45M45 Auger spectra in coincidence with Cu 2p3/2 from the oxidized surface of Cu (i.e. CuOx/Cu) and Cu 2p3/2 lines in coincidence with Auger 1G and 3F terms of Cu L3VV from Cu. Comparison of the L3VV line structures of CuOx/Cu with that of Cu reveals that the Auger line has changed to band‐like in the CuOx/Cu material. The reduced mean free path of the Auger electron in coincidence with the photoelectron displays the Auger line of the CuOx surface. The asymmetric broadening of the Cu 2p3/2 lines in coincidence with 1G and 3F terms from Cu illustrates that a single‐step model is essential for describing the Auger lineshape of copper. Copyright © 2001 John Wiley & Sons, Ltd.