Electron spectroscopy for chemical analysis (ESCA, also known as X-ray photoelectron spectroscopy or XPS) exploits the photoelectric effect to obtain information about the chemical composition and structure of a surface. When a photon source (e.g., X-rays) is directed at a sample, the photons interact with the electrons present in the sample material. If the photon has sufficient energy, it causes an electron to be emitted from its orbital. The simple theoretical relationship that describes this process is

KE = hv - BE

where KE is the kinetic energy of the emitted photoelectron, hv is the energy of the photon, and BE is the binding energy for the emitted photoelectron. KE is measured in the ESCA experiment, hv is known, and BE can be calculated, yielding the energy with which the electron was held in its atomic or molecular environment.

For photoemission from solids, the work function term must be added to this equation. A unique work function is established for each ESCA instrument. This term expresses the additional energy required after the ionization process to get the emitted electron away from the surface and into the surrounding gas or vacuum space. Thus, the measured kinetic energy of the electron will be indicative of the element from which it came and the chemical environment of that element.

NESAC/BIO has two monochromatized Electron Spectroscopy for Chemical Analysis (ESCA, also known as X-ray Photoelectron Spectroscopy or XPS) systems. One is a Surface Sciences Instruments (SSI) M-Probe (also known as the “S-Probe”) and the other is the Kratos AXIS Ultra DLD. Both systems are capable of providing high-resolution, high-signal-to-noise surface analysis data for a wide variety of specimens. The features and capabilities of our systems allow the user to perform a wide range of surface analysis experiments on specimens of biomedical interest.

Since the major components of the ESCA systems are under computer control, automated data acquisition can be used for variable angle studies, multisample analysis, line scans, and area maps. This automation permits the ESCA instruments to be used 24 hours per day, seven days per week. The sample preparation and loading chambers permit samples to be transferred from atmospheric pressure into the UHV environment of the analysis system within 30 minutes. The ESCA systems are optimized to permit users to obtain data on a wide range of biomedical samples in an efficient manner.

ESCA/XPS Capabilities:
-Identification of all elements (except H and He) present in the outermost 100Å of a surface in concentrations > 0.1 atomic %-
-Determination of elemental surface composition (±10% or better)
-Information on lateral variation in composition (resolution ~25µm)
-Fast parallel Imaging (AXIS Ultra only)
-Information about the molecular environment (oxidation state, bonding atoms, etc.)
-Identification of organic groups using derivatization
-Elemental depth profiles up to several thousand Å

Special NESAC/BIO ESCA/XPS capabilities:
-Frozen-hydrated analysis ("cold stage")
-Multiple sample handling
-Non-destructive depth profiling using angular dependent ESCA in outer 100 Å
-Variable temperature analysis

ESCA Instruments at NESAC/BIO:
Surface Science Instruments S-Probe
Kratos AXIS Ultra