Probing the valence electronic structures of aerosols
Aerosol VUV Photoelectron Spectroscopy
The surface and sub-surface region of aerosols plays a critical role in governing their chemical activities and fates, as i) aerosol often exhibit a relatively large surface-to-volume ratio, and ii) most physical and chemical activities of aerosols begin to take place from their surfaces. Photoelectron spectroscopy is a surface sensitive technique. Due to the finite photon penetration depth and the nano-scaled escape length of photoelectrons, the resulting photoelectron spectra often display a strong size dependent effect, with a distinctive surface effect. Such a surface-sensitivity makes the photoelectron spectroscopy an extremely suitable for aerosol characterization.
CCW Aerosol Lab has successfully developed the aerosol photoelectron spectroscopy, equipped with a high resolution hemispherical energy analyzer. The key components of the system includes:
Adjustable aerodynamics lens (AADL) system
Hemi-spherical electronic energy analyzer (VG Scienta, R3000)
Photoionization source:
• 9 cm period undulator (U9)
• 6 m cylindrical grating monochromator
• 1016 ~1017 photons/sec/mm2/mrad2 between 5-30 eV with 0.1 % bandwidth
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We have investigated several representative types of organic and biological relevant aqueous aerosols by using the aerosol photoelectron spectroscopy, from which we were able to extract valuable information regarding the valence electronic structures of aqueous aerosols and their evolution with their chemical surrounding. For example, our research team has recently studied the valence electronic properties, interfacial solvation structures of phenolic-containing aqueous nanoaerosols, from which we found that the amphiphilic organic solutes are only partially solvated at the interfacial region of aqueous aerosols, with the hydrophilic –OH functional group more deeply immersed into the aqueous nanoaerosols, while the hydrophobic aromatic ring remain above the aqueous interface. Also, the chemical compositional distribution of organic-containing aqueous aerosols are significantly different from the bulk aqueous solution. Moreover, we have also identified from the aerosol photoelectron spectroscopy that the valence electronic structures of aqueous aerosols can vary dramatically with (1) increasing degrees of hydration, (2) increasing extents of deprotonation, (3) increasing pH, and (4) increasing numbers of −OH group.
Valence electronic structures play a decisive role in governing the chemical activities of aerosols.