Modern ESCAThe Principles and Practice of X-Ray Photoelectron Spectroscopy

Tery L. Barr

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000142402
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Modern ESCA: The Principles and Practice of X-Ray Photoelectron Spectroscopy is a unique text/reference that focuses on the branch of electron spectroscopy generally labeled as either Electron Spectroscopy for Chemical Analysis (ESCA) or X-ray Photoelectron Spectroscopy (XPS). The book emphasizes the use of core level and valence band binding energies, their shifts, and line widths. It describes the background, present status, and possible future uses of a number of recently developed branches of ESCA, including:

Chapter 1

INTRODUCTION

I. INTRODUCTORY STATEMENT

This book is intended as a bridge to the recent advances in the science of photoelectron spectroscopy, particularly the branch now commonly labeled as X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA). It is primarily aimed at the analytical user (or potential user) of the spectroscopy, and not the ESCA expert in either the instrumental or theoretical areas, although the latter should also find it a useful source of information. Although the presentation is primarily focused on descriptions of the more recent advances, it was felt that the proper perspective for the nonexpert would be aided by also reviewing the history, methods of practice, basic equipment, and fundamental theory involved. Thus, in the present, the latter topics are used to set the stage for the principal themes to follow. Some emphasis is placed on several of the problems inherent in the methodology, particularly as raison d’être for some of the recent developments. This discussion of problems should not be construed as criticisms of the spectroscopy. The nearly 1000 ESCA units in operation throughout the world are obvious testimony to the viable utility of the spectroscopy as the premier method for chemical analyses of surfaces.

II. HISTORICAL PERSPECTIVE

The photoelectron effect had its birth in controversy. Discovered by Heinrich Hertz¹ in 1887 as part of his monumental attempt to experimentally verify the implications of Maxwell’s relations, the production of photoelectrons did the opposite, demonstrating that the (then) theory of radiation was incomplete. Numerous scientists tried diligently to mend this crack in the foundation of classical physics, but all of the good experiments merely reinforced Hertz’s initial observation.² The theoretical explanation of the photoelectron effect awaited “the dawn of a new light” as Hoffmann³ has described the year 1905, and the contributions therein by Albert Einstein. In one of Einstein’s major achievements,⁴ he brilliantly employed the new quantum of energy concept of Planck⁵ to explain how radiation of low intensity, but high frequency, could eject electrons from metal foils, whereas the converse might fail to produce any electrons. As a result Einstein logically evolved a consistent, single relationship that (in slightly modified form) is still today the basis of photoelectron spectroscopy, i.e.,

$\text{hv}\text{=}{\text{1/2mv}}_{\text{e}}^{\text{2}}{\text{+E}}_{\text{b}}\text{+qΦ}$

(1-1)

where v is the frequency of the incoming quanta (later dubbed as “photons” by Lewis⁶), h Planck’s constant, $1/2{\text{mv}}_{\text{e}}^2$ the kinetic energy of the outgoing “photoelectron” of mass m and velocity v_c, q a reference charge, Φ the work function of the emitting material, and E_b the so-called “binding energy”, expressed here against the Fermi level of the material. It is the latter feature, of course, that is the backbone of the spectroscopy we describe in this book. Had Einstein left the argument at this point, his paper would have still been very famous, and perhaps not nearly so controversial. Unable, however, to accept the political view in place of the correct view (as he saw it), Einstein added a section to his paper challenging the totality of the Maxwellian wave theory of radiation, and suggested the possibility of a wave-particle duality.⁴ Even Planck hated these “alterations” of his original concepts, and the disputes over the latter half of Einstein’s paper (that ushered in the Quantum Theory of Radiation) were not completely resolved until the advent of quantum mechanics in the mid 1920s.² In fact, although Einstein’s 1921 Nobel Prize was granted primarily for this paper, it specifically mentions only the first half — the laws of the photoelectron effect. To the ever-honest Einstein and his growing band of followers, the dispute was illogical, as you “can’t have one half of this paper without the other.” In any case, all attempts (pro and con) to challenge the validity of Einstein’s interpretation of the photoelectron effect succeeded instead in verifying it, culminating in the beautiful confirmation studies of Millikan.⁷ Following these studies, scientists turned to attempts to use this new phenomenon, particularly as a form of spectroscopy. Preeminent in these early studies were the investigations of Robinson,⁸ who easily verified that an elemental spectrum could be realized, but found that so many difficulties were inherent to these measurements that the studies did as much to discourage the evolution of this spectroscopy as to promote it. The lack of availability of a continuous, high vacuum capability, the inability to control stray interfering fields, and the unrealized need for integrated, stable electronics caused most scientists of the 1930s and 1940s to ignore the prospect of a “useful” photoelectron spectroscopy.

These problems were eventually rectified in the laboratories of Kai Siegbahn in Uppsala, Sweden.⁹ Professor Siegbahn and colleagues were initially attempting to develop a spectroscopy to examine β decay, but they soon realized that a similar version could be employed to examine photoelectrons. Using giant Helmholtz coils to “deflect” all outside fields, they achieved their first spectrum in 1955. Soon thereafter they made a dramatic discovery. The binding energy, E_B, detected for the C(1s) lines in several different carbon containing systems was (reproducibly) found to exhibit different values. The same feature was found for the two chemically different sulfurs in a thiosulfate. Thus, accurately measured photoelectron spectroscopy was found to produce chemical shiftsl Professor Siegbahn and his group were able to produce similar shifting effects for many other compounds, both in the gas state¹⁰ and as solids.¹¹ They noted, as we now expect, that the photoelectrons from atoms in a more electropositive state are emitted with less kinetic energy (greater E_B) and vice versa for atoms that are made more negative. Thus, the use of photoemission to map changes in oxidation state and other chemical details seemed to be a viable possibility. Realizing the potential impact of these observations, Siegbahn dubbed his new technique electron spectroscopy for chemical analysis, or ESCA.* Some other scientists have preferred to identify the method with both the nature of the excitation source and the emitted particles, thus calling it various types of photoelectron spectroscopy (e.g., XPS, when X-rays are employed as the emitted photons, and ultraviolet photoelectron spectroscopy, or UPS, when UV radiation is used). The latter process was developed as a companion technique soon after the Swedish development in a number of labs, particularly those of Turner et al.¹² and Price and Turner.¹³

Obviously the choice of radiation frequency, v, is a matter that depends upon the experimental parameters (as long as hv exceeds the so-called threshold energy, E_b + qΦ). In point of fact, it is often advantageous to have available the ability to employ variable energies, such as provided by many synchrotron radiation centers. In the following, however, we will concentrate upon the area often reserved for the term ESCA (i.e., that employing X-ray emission, XPS). We do this for brevity, and to emphasize that area that has had, by far, the largest impact in materials analysis, particularly in the applied (industrial) setting. We will, however, where warranted, refer to interconnections with other radiation sources.

REFERENCES

1. Hertz, H., Uber einen Einfluss des Utravioletten Lichtes auf die Electrische Entladung, Wiedemannsche Ann. Phys., 31, 982, 1...