Quasielastic Dynamic Light Scattering Photon Correlation Spectroscopy

10 ° and 10 cm s per photon, or 1-100 in the unofficial but frequently used units of Goppert-Mayers (1 G.M. = 10 erne s). Psoralen has a two-photon absorption cross section of 20 x 10 ° erne s [92], and quantum dots with cross sections as high as 47,000 x 10 ° cm s have been described [93], To put these values in perspective, with the instrumentation described by Xu and Webb [91], a 1 mW continuous laser beam gave a pulse intensity of about 10 photons/(cm -s), or (/) 10 photon /(cm s ). A molecule with a two-photon absorption cross section of 10 ° cm s per photon would be excited about 10 times per second. 

Two-photon excitation has been used to visualize cell-cell interactions [107] and linear dichroism in cell membranes [108]. It has been combined with resonance energy transfer to image protein-protein interactions during T cell activation [109]. 

9 Quasielastic (Dynamic) Light Scattering (Photon Correlation Spectroscopy) 

Our discussion of light scattering in Sects. 12.1 and 12.2 focused on scattering by an individual electron. We now consider how interference between light scattered from different regions of a macromolecule can provide information on molecular 

12 Raman Scattering and Other Multi-photon Processes 

---

In 1953, Stockmayer and Fixman observed that the ratio, h, of hydrodynamic or Stokes radii (/ h) of the branched and linear macromolecules could also be used as a measure of branching. The relatively recent introduction of online quasielastic light scattering (QELS, also known as dynamic light scattering and as photon correlation spectroscopy) detectors for SEC permits direct determination of h by... 

QELS stands for quasielastic light scattering. It was the name first given to the technique because, when photons are scattered by mobile particles, the process is quasielastic. QELS measurements yield information on the dynamics of the scatterer this gave rise to the acronym DLS, which stands for dynamic light scattering. Finally, since most measurements are made with a digital correlator, the acronym PCS, which stands for photon correlation spectroscopy, is widely used. 

---