Photoelectron spectroscopy

The energy spectra of photoejected electrons provides a direct method for determining the electronic binding energies in atoms, molecules and solids. The binding energies may be used to identify a particular atom in a molecule or solid and, as well, to indicate from which orbital the electrons were removed. The relative intensities of peaks in the photoelectron spectrum reveal the nature of the chemical bond in a molecule and are a measure of the relative transition probabilities for ionization to a particular state. The effects of autoionization and angular distribution are discussed, and some applications are given.     

Photoelectron spectroscopy of 9,10-dihaloanthracenes

The photoelectron spectra of anthracene and its 9,10-dichloro- and dibromo-derivatives have been obtained. Analysis of the spectra is based mainly on the shifts in the anthracene molecular orbital ionization energies observed upon substitution. These shifts are interpreted on the basis of a perturbation model comprising short-range and long-range inductive terms and a mesomeric term. The electronic perturbation caused by the halogen atom is observed to depend largely on the unperturbed energy of the halogen np electrons and the charge distribution in the anthracene molecular orbitals. It has been possible to separate out Coulombic interactions and mesomeric interactions which depend on the energy gap between hydrocarbon and substituent orbitals. Although the observations are not conclusive, owing to the fact that only two orbitals have measurable mesomeric shifts, results indicate that α_Cl′, the Coulombic perturbation parameter, may well be negative, in contrast to previous work.     

Photoelectron spectroscopy of phosphites and phosphates

The ultraviolet photoelectron spectra (UPS) of trimethyl and triethyl phosphite, trimethyl and triethyl phosphate and four substituted phosphates are presented. Assignments are based on analogies to the UPS of phosphorus trichloride and phosphoryl trichloride and are substantiated by CNDO/2 computations. The mechanism of PO (axial) bond formation is discussed.     

Photoelectron spectroscopy of some biological molecules

The ultraviolet photoelectron spectra (UPS) of the following biologically-active compounds are reported and/or assigned: 2,4-dinitrophenol, nicotinic acid, nicotinamide, barbituric acid, xanthine, hypoxanthine, uric acid, uracil, thymine, cytosine, adenine, guanine, β-carotene, menadione, purine and pyrimidine. The importance of UPS data is exemplified in two ways: First, by investigating the validity of the Pullman k-index approach ¹⁴ to ionization energies; and, second, by generating an experimental scale of electron-donating ability.     

Photoelectron spectroscopy of doped helium nanodroplets

The photoionization dynamics of aniline doped helium droplets has been investigated by photoelectron spectroscopy. The photoelectron spectra resemble closely that of gas phase aniline, except for a droplet-size-dependent shift. This shift is caused by lowering of the ionization threshold upon solvation and can be readily estimated. The individual peaks in the photoelectron spectrum are broadened towards lower kinetic energy which is attributed to the relaxation of the photoelectrons as they pass through the helium droplet.     

Photoelectron spectroscopy of solids using gas-phase spectrometers

By using continuous deposition of metal surfaces or a continuous scrape technique the ultraviolet photoelectron spectra of clean polycrystalline Zn, Cd, Sn and Pb have been obtained using spectrometers originally designed for gas-phase work. Spectra are calibrated with respect to an internal gas-phase standard. A study of the oxidation of lead is presented to illustrate the applicability of these techniques to adsorption studies. A second oxide in addition to the established orthorhombic form of PbO is observed for low O₂ doses.     

Photoelectron spectroscopy of Se2 and Te2

The He(I) photoelectron spectra of Se₂ and Te₂ are presented. In both these molecules, ground-state Ω splitting, described by Hund's case (c)     

Photoelectron spectroscopy of liquids: External ionization energies

A method is reported here for the determination of ionization energies of liquids or solutions by means of photoelectron spectroscopy (PES). It is, to our knowledge, the first determination of its kind in liquid-phase ultraviolet PES. A number of applications is foreseen.     

Photoelectron spectroscopy of C(84) dianions

We report the laser photoelectron spectra of doubly negatively charged C84 (D2 and D(2d)) using 532 nm and 355 nm radiation. From these spectra, values for the second electron affinity and vertical detachment energy, as well as upper and lower limits for the repulsive Coulomb barrier, are obtained. These values are discussed in the context of classical electrostatic models. The experimental spectra are compared with the accessible excited states of the C-84 product ion calculated in the framework of time dependent density functional theory.     

Photoelectron spectroscopy investigation of defects on LiNbO3 surfaces

Results of UPS and XPS measurements performed on LiNbO₃ single crystals reduced by Ar⁺-ion bombardment are presented. The influence of reduction dose on the type and concentration of induced defects is studied. It is shown that Nb ions change their valency state from 5+to 4+for low reduction doses and to 3+and 2+for high reduction doses. Further it is shown that Nb⁴⁺ ions can be considered as bulk defects which manifest themselves in a broad emission band in the gap centred at 1.6 eV below the Fermi level and that defect ions Nb³⁺ and Nb²⁺ manifesting themselves in a narrow emission band at 3.9 eV below the Fermi level are present only in the outermost layers of the surface. An attempt is made to compare photoelectron spectra with optical spectra of LiNbO₃ and correctness of this procedure is discussed.     

Photoelectron spectroscopy of size-selected cluster ions using synchrotron radiation

A new experimental setup for photoelectron spectroscopy of size-selected cluster ions using synchrotron VUV radiation as generated by the Swiss Light Source is presented. An intense positively charged cluster ion beam is produced in a high-intensity magnetron sputter source. The clusters are subsequently mass selected in a sector magnet. To maximize the residence time of the cluster ions in the ionization region of the velocity map imaging spectrometer, the cluster ion beam is decelerated where it crosses the light beam. First experiments on (MoO₃) _n ⁺ (n = 69 and 59) cluster cations show that the approach is capable of delivering photoelectron spectra of size-selected transition metal cluster ions.     

Photoelectron spectroscopy of the valence electronic structure of polymers

Photoelectron spectroscopy has been highly developed as an investigative tool of the bulk and surface chemical and electronic structure of condensed matter during the past 2 decades. The use of soft X-ray sources led to the development of electroq spectroscopy for chemical applications (ESCA),^1,2 which is also known to the physicst as X-ray photoelectron spectroscopy (XPS). These developments were followed by the use of ultraviolet sources for the study of gases³ and the bulk and surface electronic structure of solids.^4,5 Recently, the use of the continuous spectral distribution of synchrotron radiation has had a major impact upon the study of solid surfaces^6,7 and has made the largely historical terminology of XPS (or ESCA) and ultraviolet photoelectron spectroscopy (UPS) somewhat less meaningful. Angle-resolved UPS and XPS (ARUPS and ARXPS) can provide additional information about electronic band structure, surface states, and adsorbates, but are typically applied to the study of the surfaces of ideal single crystals, where crystallographic high-symmetry directions as well as the polarization of the light source can be used to high advantage.⁷ Since polymers are almost never single crystals, the angle-resolved issue will not be addressed here.     

Photoelectron spectroscopy of some thiocyanates,isocyanates and isothiocyanates

Photoelectron spectra of some thiocyanates (RSCN, R = CH₃, C₂H₅, n-C₄H₉), isocyanates (RNCO, R = C₂H₅, n-C₄H₉) and isothiocyanates (RNCS, R = C₂H₅, n-C₄H₉) have been measured, to study interactions between nonbonding and π orbitals, mainly localized on the SCN, NCO or NCS fragments. The spectral interpretation of CH₃SCN is based on semiempirical CNDO/S calculations, sum-rule considerations, and intensity differences between He(I) and HE(II) spectra. For the larger molecules, comparison of the spectra is used as an aid in the interpretation. In a number of aromatic isocyanates (o?, m?, p-tolylisocyanate and m?, p-chlorophenylisocyanate), interactions between the isocyanate group and the highest occupied π and σ orbitals of the phenyl ring are studied. Spectra are assigned on the basis of semiempirical INDO/S calculations.     

Photoelectron spectroscopy without vacuum: nanoparticles in gas suspension

The photoelectric yield Y of nanoparticles in gas suspension delivers a fingerprint of the particle surface as it interacts with the surrounding gas, but bulk properties of the particle can also be probed. We show in three new experiments how this can be applied to acquire remarkable insight into processes of considerable general interest. In the first example, soot reduction by heterogeneous catalysis is examined. We show that fuel additives lead to the formation of transition metal oxide nuclei in the combustion zone. The carbonaceous matter preferentially condenses on the surface of the nuclei where it is burnt in the last stage of the combustion. In the second example, we present a method for the investigation of the desorption dynamics from particle surfaces using excimer laser pulses for photoemission. The kinetics of thermal perylene desorption from various types of particles is measured with a time resolution of a few milliseconds. The desorption follows a first order rate law and can be described by the Arrhenius model. In the third example, we show that there exists circular dichroism (CD) in photoemission from particles built with chiral molecules. The dependence of CD on the size of the nanoparticles shows that the crystalline order in the particles is important.     

Photoelectron spectroscopy studies of the band structure of silver halides

X-ray photoelectron spectroscopy has been used to probe the valence bands of the silver halides. Previous ultraviolet photoemission and optical absorption experiments together with the theoretical band structure calculations form the basis for interpretation of the spectra. Density of states maxima from the halogen p levels are clearly resolved from those of the silver 4d states. Additional splittings due to k space symmetry are observable and in the case of AgCl and AgBr, give excellent conformation of existing band structure calculations. The role of spin-orbit coupling is shown to be unimportant in determining the primary shape of the photoelectron spectrum. The spectrum of AgF shows the inversion of silver 4d and halogen 2p levels suggested by the optical absorption spectra and predicted by several recent band calculations. A new interpretation is proposed for the AgF optical spectrum in which the observed excitons are due to the forbidden Γ₁₂→ Γ₁ and Γ_25′ → Γ₁ tr the high temperature body centered cubic structure. These spectra clearly show the broadening of the I 5p levels which causes the band gap to decrease above the transition temperature.     

Photoelectron spectroscopy of brominated derivative of pyrimidine: 2-bromopyrimidine

In this study the brominated derivative of pyrimidine, 2-bromopyrimidine, was investigated by photoelectron spectroscopy. Outer valence photoelectron spectra recorded at 21.22, 45 and 100 eV photon energy for this compound are presented. The recorded spectra have a higher resolution than that previously reported in the literature. The bromine 3d and 3p edge photoelectron spectra have also been recorded in a photon impact experiment at 100 and 225 eV. All measurements were performed using Double Toroidal Coincidence Spectrometer showing its potential as a versatile apparatus for spectroscopic studies.     

Photoelectron spectroscopy of atomic oxygen using the Elettra synchrotron source

Atomic oxygen has been studied using angle resolved photoelectron spectroscopy (PES) and constant-ionic-state (CIS) measurements using radiation from the Elettra synchrotron as the photon source. Relative partial photoionization cross-sections and angular distributions for the O⁺(⁴S) ← O(³P) and O⁺(²D) ← O(³P) ionizations have been measured as a function of photon energy from threshold (13.6 eV) to 19.0 eV. Comparison of the results obtained with recent experimental work performed at lower resolution reveals a number of differences and comparison with results of recent calculations shows the need for the inclusion of coupling intermediate between the j-j and L-S limits in future calculations of photoionization cross-sections and angular distributions. This work has demonstrated the feasibility of and results to be expected from angle resolved PES and CIS measurements on reactive intermediates at Elettra, a third-generation synchrotron source, and further studies on small molecular radicals are proposed.     

Photoelectron spectroscopy of nanocrystalline anatase TiO2 films

Nanocrystalline TiO₂ (anatase) films were prepared using either colloidal suspensions or a sol-gel route. The electronic structure of these films was analyzed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Apart from pristine films, films containing defects introduced by annealing under ultra-high vacuum conditions or by ion bombardment were investigated. Generally, annealing in the temperature range up to 720 K results in no significant changes in the XPS and UPS spectra as compared to the pristine state, indicating that the amount of defect formation is too low to be observable by these techniques. On the other hand, ion irradiation causes the appearance of distinct defect states; these could be identified in agreement with previous data from photoemission studies on rutile and anatase single crystals. From UPS, a valence-band width of ∼4.6 eV was determined for the nanocrystalline anatase films.     

Photoelectron spectroscopy of Ar/Cu(100) interface states

Buried interface states in Ar/Cu(100) were studied by means of one- and two-photon photoemission experiments. With increasing Ar overlayer thickness, a transition from broad electron scattering resonances in the Ar conduction band into a hydrogen-like series of quasi-bound states at the Ar/Cu interface was observed. The thickness dependence of energies and lifetimes is compared to theoretical resonance positions and linewidths derived from a parameterized one-dimensional potential. PACS 73.20.-r; 73.40.Ns; 79.60.-i; 78.47.+p