Karthik Shastry

Surface properties measured under UHV conditions cannot be extended to surfaces interacting with gases under realistic pressures due to surface reconstruction and other strong perturbations of the surface. Surface probing techniques require UHV conditions to perform efficiently and avoid data loss due to scattering of outgoing particles. This poster describes the design of an Operando Positron Annihilation Gamma Spectrometer (OPAGS) currently under construction at the University of Texas at Arlington. The new system will be capable of obtaining surface and defect specific chemical and charge state information from surfaces under realistic pressures. Differential pumping will be used to maintain the sample in a gas environment while the rest of the beam is under UHV. Elemental content of the surface interacting with the gas environment will be determined from the Doppler broadened gamma spectra. This system will also include a time of flight (TOF) Auger spectrometer which correlates with the results of the Doppler measurements at lower pressures. By employing the unique capabilities of OPAGS together with those of the TOF PAES spectroscopy the charge transfer mechanisms at the surface in catalytic systems can be understood.

We present the Auger Photoelectron Coincidence Spectroscopy (APECS) measurements of Ag (100) and Cu (100) over a full range of emitted energies from 0 eV to 81eV. The coincidence measurements were successful in separating the low energy Auger lines from a large background, arising from loss processes unrelated to the Auger transition. The measurements have revealed a well formed Auger peak at 60 eV for Cu (100) and an Auger peak at 40 eV for Ag (100) accompanied by a low energy tail (LET) associated with M2,3VV transition. The Low Energy Tail (LET) extends to 0 eV and has a broad maximum at 6eV in the case of Cu (100) and 10 eV in the case of Ag (100). The integrated intensity of the Low Energy Tail (LET) in Cu and Ag were 6 and 2 times larger than that of the Auger peak itself respectively. The origin of the LET is discussed in terms of extrinsic mechanisms in which electrons from the peak lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi-electron Auger processes distribute the energy gained by the filling of the core-hole to multiple valence electrons.

Time of flight- positron annihilation induced auger electron spectroscopy (TOF-PAES) is extremely surface selective with close to 95% of the PAES signal stemming from the top-most atomic layer. In PAES, a beam of low energy (1eV -- 25eV) positrons is made incident on a surface where they become trapped in an image potential well. A fraction (up to several percent) of the positrons in the surface state annihilate with the core electrons of atoms at the surface resulting in core-holes. Electrons in higher levels can fill these core-hole via an Auger transition in which the energy associated with this filling the core hole is transferred to another electron which can leave the atom and the surface. The energy of the outgoing (Auger) electrons is characteristic of the energy levels of the atom and can be used to identify the specific element taking part in the transition. In this talk I will present a brief review of how the TOF PAES technique can be used to obtain Auger spectra that is completely free of secondary electron background.

Measurements of the energy distribution of electrons resulting from very low energy positron bombardment of a polycrystalline Au and Cu(100) surfaces provide evidence for a single step transition from an unbound scattering state to an image potential bound state. The primary positron energy threshold for secondary electron emission and cutoff in the secondary electron energy spectra are consistent with a process in which an incident positrons make a transition from a scattering state to a surface-image potential bound while transferring all of the energy difference to an outgoing secondary electron. Estimates of the probability of this process as a function of incident positron energy are also presented. Background free Auger spectra of the MVV transition in Cu and the OVV transition in Au were obtained by setting the incident positron beam energy below the secondary electron emission threshold. Auger electron emission resulted from the annihilation of surface state positrons with core electrons. The low energy tail associated with the low energy CVV Auger transitions in Cu and Au were found to have integrated intensity several times larger than Auger peak providing strong evidence for multi-electron Auger processes.

Surface properties measured under UHV conditions cannot be extended to surfaces interacting with gases under realistic pressures due to surface reconstruction and other strong perturbations of the surface. We present the design of an Operando Positron Annihilation Gamma Spectrometer (OPAGS) currently under construction at the University of Texas at Arlington. This new system will enable us to probe the surface and gather defect specific chemical and charge state information from surfaces under realistic pressures. Differential pumping will be used to maintain the sample in a gas environment while the rest of the beam is maintained under UHV. The Elemental content of the surface interacting with the gas environment will be determined from the Doppler broadened gamma spectra. This system will include a time of flight (TOF) positron annihilation induced Auger spectrometer (TOF-PAES) which correlates with the Doppler measurements at lower pressures. These new technique help to understand the charge transfer mechanisms at the surface.

Positron spectroscopy performed with low energy beams can provide highly surface specific information due to the trapping of positrons in an image potential surface state at the time of annihilation. Here we present design details of a new positron beam system for the analysis of surfaces gas environments. The new system will employ differential pumping to transport the positrons most of the way from the source to the sample under high vacuum. The positrons will then be transported through a thin gas layer surrounding the sample. The positrons will be implanted into the sample at energies less than ˜10 keV ensuring that a large fraction will diffuse back to the surface before annihilation. The Elemental content of the surface interacting with the gas environment will then be determined from the Doppler broadened gamma spectra.

Positron annihilation induced Auger electron spectroscopy was used to obtain Cu and Au Auger spectra that are free of primary beam induced background by impinging the positrons at energy below the secondary electron emission threshold. The removal of the core electron via annihilation in the PAES process resulted in the elimination of post-collision affects. The spectrum indicates that there is an intense low energy tail (LET) associated with the Auger peak that extends all the way to 0 eV. The LET has been interpreted as being due to processes in which the filling of the core by a valence electron results in the ejection of two or more valence electrons which share the energy of the otherwise Auger electron

In conventional spectroscopic measurements, low energy Auger lines are superimposed upon a large background due to secondary electrons that arise from loss processes that are unrelated to the Auger process. Here we present the results of measurements in which Auger-Photoelectron coincidence techniques were used to eliminate background unrelated to the Auger process and obtain the energy distribution of electrons emitted as a result of the M23VV transition in Cu (100) over the full range of emitted energies (0 eV -- 81 eV). The measurements revealed a well formed Auger peak at ˜60eV accompanied by a low energy tail (LET) associated with the MVV transition. The LET extends to 0 eV and has a broad maximum at ˜ 6eV. The integrated intensity of the LET was ˜ 6 times larger than that of the Auger peak itself. The origin of the LET will be discussed in terms of extrinsic mechanisms in which electrons from the peak lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi-electron Auger processes distribute the energy gained by the filling of the core-hole to multiple electrons.

Details of the design and construction of a new time of flight positron annihilation induced Auger electron (TOF-PAES) spectrometer are presented. The new spectrometer will be equipped with a 2 meter long ``TOF'' tube that can be biased at a potential different from that of the sample in order to increase or decrease the kinetic energy of the electrons traveling through the tube. The time of flight will be determined from timing signals obtained from the detection of the annihilation gamma (signaling the start of the flight) and detection of the annihilation induced Auger electron at the end of the 2 meter flight path (signaling the end of the flight). The 2 meter long flight path is a factor of two longer than used in previous TOF-PAES systems. The longer flight path can be expected to result in a fractional energy width: delta E/ E that is .5ex1 -.1em/ -.15em.25ex2 as large as the current UTA lab based TOF-PAES spectrometer.

Time of flight- positron annihilation induced Auger electron spectroscopy (TOF-PAES) is a surface analysis technique with high surface selectivity. Almost 95% of the TOF-PAES signal emerges from the topmost layer of the sample due to the trapping of positrons in an image-potential-well before annihilation. In this poster we will present new results that demonstrate how very low energy positron beams can be used together with the time of Flight (TOF) technique developed at The University of Texas at Arlington to obtain Auger spectra that are completely free of secondary electron background.

We present the Auger Photoelectron Coincidence Spectroscopy (APECS) measurements of Ag (100) and Cu (100) over a full range of emitted energies from 0 eV to 81eV. The measurements were successful in separating the low energy Auger lines from a large background, due to loss processes unrelated to the Auger transition. The measurements reveal a well formed Auger peak at 60 eV for Cu and an Auger peak at 40 eV for Ag accompanied by a low energy tail (LET). The LET extends to 0 eV with a broad maximum at 6eV and 10 eV in the case of Cu and Ag respectively. The integrated intensity of the LET in Cu (100) and Ag (100) were 6 and 2 times larger than that of the Auger peak itself. The origin of this LET is discussed in terms of extrinsic mechanisms in which electrons from the peak lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi-electron Auger processes distribute the energy gained by the filling of the core-hole to multiple valence electrons.

Low energy Auger lineshapes are difficult to measure because they sit on a large background due to secondary electrons arising from loss processes unrelated to the Auger mechanism. Auger photoelectron coincidence spectroscopy (APECS) was used to the spectrum of the MVV and NVV Auger peaks and associated low energy tails (LETs) in Cu and Ag (100) respectively. The backgrounds due to secondary electrons unrelated to the auger process were suppressed by measuring the Auger spectra in coincidence with the M and N core levels. The APECS measurements reveal a well formed Auger peak at 40 and 60 eV for Cu and Ag respectively accompanied by a significant Auger related intensity in the low energy region. Spectra obtained using APECS are compared with Positron Annihilation Induced Auger Electron Spectroscopy (PAES) measurements which also show a large LET. The LET is discussed in terms of extrinsic mechanisms in which the electrons from the peak lose energy as they propagate to the sample surface and intrinsic mechanisms in which multi- electron auger processes distribute the energy gained by filling of the core hole to multiple electrons.

We present the results of measurements in which Auger photoelectron coincidence spectroscopy (APECS) was used to obtain the energy distribution of electrons emitted as a result of the MVV transition in Cu over the range of 0eV-81eV and the NVV transition in Ag over the range 0eV - 100 eV. A novel differences technique was used together with APECS to eliminate all backgrounds from inelastic processes not directly related to the selected Auger transitions. The measurements reveal a well formed auger peak at 40 and 60 eV for Cu and Ag respectively accompanied by a significant spectral weight in a low energy tail of that extends to 0eV with a characteristic cascade like bump at low energies. We posit that the LET in the Cu and Ag spectrum is due to both extrinsic processes in which Auger electrons emitted with the full energy of the Auger transition lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi- electron Auger processes distribute the transition energy to multiple electrons

Recent measurements have provided evidence that low energy positrons incident upon a metal surface can make a single step transition from an unbound scattering state to an image potential bound state resulting in the creation of an electron-hole pair. Because the transition into the surface state results in the release of an additional ˜3 eV of energy as compared to a transition into a bulk state, the direct transition from scattering state to surface state can result in the creation of secondary electrons even at beam kinetic energies below the energy threshold necessary to generate secondary electrons in scattering processes in which the positron final state is a bulk state. In this poster we present a comparison of the experimental results with model calculations from which the rate of the direct process is estimated and the implications of these measurements in the understanding of quantum-sticking of positrons to surfaces are considered.

The low energy Auger peak sit on large background due to secondary electrons that arise from loss processes unrelated to the Auger process. Auger photoelectron coincidence spectroscopy (APECS) technique was used to probe the surfaces of Cu (100) and Ag (100) to suppress background unrelated to the auger process and obtain the energy distribution of the electrons emitted as a result of the MVV transition in Cu and NVV transition in Ag over the full range of emitted energies (0eV-81eV). The measurements reveal a well formed auger peak at 40 eV and 60 eV for Cu and Ag respectively accompanied by a significant back ground in the low energy region of the spectrum. The origins of the low energy tail (LET) are discussed in terms of extrinsic mechanisms in which the electrons from the peak lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi- electron auger processes distribute the energy gained by filling of the core hole to multiple electrons.