ABSTRACT
Photoelectron spectroscopy is based on the photoelectric effect1 where the energy of an incident photon (hν) is transferred to a valence or core electron of an atom, leading to the emission of an electron if the incident photon energy is larger than the binding energy (BE) of the electron in the atom. Using energy-dispersive detectors, the kinetic energy (KE) of the emitted electron is measured. From the difference of the incident photon energy and the measured KE, the BE of the electron within the atom can be determined. The BE for a core-level electron is sensitive to the chemical bonds to neighboring atoms (“chemical shifts”) and can be used to probe the oxidation state of a given atom or different functional groups. The inelastic mean free path (IMFP) of photoelectrons in a condensed sample is on the order of a few Angstroms to many nanometers and depends both on the KE and the chemical composition of the sample. Figure 15.1a shows a general functional form for electron escape lengths versus KE based on an empirical formula from Seah and Dench.2 It is the short IMFP of electrons that makes photoemission spectroscopy a surface-sensitive probe of chemical composition. On the other hand, electrons are also scattered by gas molecules. Since gases densities are a small fraction (10−6) of the condensed phase, the IMPF in a gas is about 106 times longer in gases than in solids. The electron IMPF depends on KE and pressure as shown in Figure 15.1b for nitrogen (a proxy for air) and water vapor.3-6 Figure 15.1b shows that the IMPF of electrons at kinetic energies above 30 eV is much shorter than that below 30 eV. Spectroscopic techniques that measure electrons at higher kinetic energies are therefore restricted to low-pressure environments or have to utilize differential pumping schemes that limit the path length of the electrons in the
15.1 Introduction .......................................................................................................................... 367 15.2 Photoelectric Charging of Aerosols ...................................................................................... 370
15.2.1 Aerosol Adsorption and Desorption Kinetics........................................................... 371 15.2.2 Circular Dichroism in Aerosol Photoemission ......................................................... 373 15.2.3 Probe Molecule Aerosol Photoemission ................................................................... 375 15.2.4 Summary .................................................................................................................. 377
15.3 Synchrotron-Based Aerosol Photoemission ......................................................................... 378 15.3.1 Vacuum UV Photoemission of Aerosols using Velocity Map Imaging ................... 378 15.3.2 Photoemission from Biological Nanoparticles ......................................................... 379 15.3.3 Angle-Resolved Photoemission from Nanoparticles ................................................ 382 15.3.4 Nanoparticle Photoemission in Interstellar Dust Clouds.......................................... 387 15.3.5 Threshold Photoemission from Multicomponent Aerosol ........................................ 387 15.3.6 Summary: Threshold Aerosol Photoemission and VMI .......................................... 389 15.3.7 XPS of Submicron Particles ..................................................................................... 389 15.3.8 XPS of Liquid Aerosols ............................................................................................ 391
15.4 Summary and Outlook ......................................................................................................... 394 Acknowledgments .......................................................................................................................... 395 References ...................................................................................................................................... 395
high-pressure region. Techniques that measure low KE electrons, on the other hand, can operate even at atmospheric pressure.
