Photoemission spectroscopy

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Photoemission Spectroscopy refers to two separate techniques/

Photoemission Spectroscopy (PES) also known as Ultra-violet Photoelectron spectroscopy (UPS), developed originally for gas-phase molecules by David W. Turner, J.H.D. Eland and K. Kimura. Richard Smalley modified the technique to use a UV laser is used to excite the sample. This method is designed primarily to find the binding energy of electrons in gaseous molecular clusters.

[edit] Physical principle

The physics behind the PES (UPS) technique is really an application of Einstein's photoelectric effect. The material (usually gases or liquids) to be analyzed is exposed to a beam of UV or XUV light inducing photoelectric ionization of the sample atoms. The UV light penetrate several micrometers (1–3 µm) into the sample producing photoelectrons throughout the penetration depth of the X-rays. The energies of the emitted photoelectrons are characteristic of their original electronic states which include vibrational studies, rotational studies, molecular orbital studies, bonding state information, configuration interactions and the polarization effects of the adjacent atoms, which is one of the main reasons PES (UPS) is useful.

Typical PES (UPS) instruments use helium gas sources of UV light, with up to 52 eV of kinetic energy (23.7 nm). The photoelectrons that actually escaped into the vacuum are collected, energy resolved, slightly retarded and counted, which results in a spectrum of electron intensity as a function of the measured kinetic energy. Because binding energy values are more readily applied and understood, the kinetic energy values, which are source dependent, are converted into binding energy values, which are source independent. This is achieved by applying Einstein's relation $E k = h ν - E B$ . The $h ν$ term of this equation is due to the energy (frequency) of the UV light that bombards the sample.

The binding energies of the measured electrons are characteristic of the chemical structure and molecular bonding of the material. By adding a source monochromator and increasing the energy resolution of the electron analyzer, peaks appear with FWHM <5-8 meV.

An article on X-ray photoelectron spectroscopy that complements this article is located elsewhere.