Editorial pubs.acs.org/JPCL
Photons and Physical Chemistry
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macroscopic domainan example of the power of a photonbased theory.
ight is often used to probe molecular structure. The two Perspectives in this issue give insights into frontiers of this endeavor. Chen and Zhang describe how transient X-ray spectroscopy can be used to follow changes in molecular structure. Andrews describes the nature of the photon and its importance in physical chemistry. It has long been a dream of chemists to watch how structure changes with time during a chemical reaction. Researchers have recently made great progress toward this aimwhere challenges of resolving atomic positions in space and time at high resolution are considerable. Two primary approaches have been pursued. X-ray or electron diffraction is carried out as a function of time after initiating dynamics with a short laser pulse. In this issue of the Journal of Physical Chemistry Letters, Chen and Zhang focus on advances in X-ray methods, highlighting how structural as well as spectroscopic information is resolved. Spectroscopic probes include XANES (X-ray absorption near-edge Structure) and XAFS (X-ray absorption fine structure). We all know what a photon represents, but do we view the photon as an essential element in our descriptions and formalisms for spectroscopy of molecules? Indeed, is there a difference between light waves and photons for our intents and purposes? Thinking about these questions is becoming more salient now that experiments with entangled photons have been proposed and carried out. In such work, a quantum description of light is essential. Andrews provides some compelling examples of why photons matter in this very readable account. One theme of Andrews’s work has been to explore theories for intermolecular interactions where the Coulomb interaction is explicitly mediated by photons (real or virtual). The formalism for this “molecular quantum electrodynamics” approach comes largely from the work of Edwin Power. Recognizing that the charges comprising molecules produce fields in the vicinity of each molecule, Power and Thirunamachandran explored how the weak Coulomb interactions that couple molecules can be considered to be mediated entirely by the radiation field. The idea is that the effect on one molecule of the displacement vector field on another can be calculated to provide the electronic coupling, for example, dipole−dipole coupling that mediates transfer of electronic excitation. Because the interaction is explicitly mediated by the field in this formalism, it can be discovered how there is a natural transition between essentially instantaneous Coulomb interactions between closely separated molecules and interactions that propagate with the speed of light between remote molecules. The electronic coupling is transmitted by virtual and real photons, respectively. Andrews has previously examined these limits in detail to show the connection between Förster energy transfer and the process known as the inner filter effect in fluorescence spectroscopy. The inner filter effect comes about when fluorescence is emitted by one molecule and the emitted photon is reabsorbed by another molecule. Nice physical insights emerge from this theory that extrapolates from the molecular domain to the © 2013 American Chemical Society
Gregory D. Scholes*
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Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
Views expressed in this Editorial are those of the author and not necessarily the views of the ACS.
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RELATED READINGS
(1) Chen, L. X.; Zhang, X. Photochemical Processes Revealed by Xray Transient Absorption Spectroscopy. J. Phys. Chem. Lett. 2013, 4, 4000−4013. (2) Andrews, D. L. Physicality of the Photon. J. Phys. Chem. Lett. 2013, 4, 3878−3884. (3) Rischel, C.; Rousse, A.; Uschmann, I.; Albouy, P. A.; Geindre, J. P.; Audebert, P.; Gauthier, J. C.; Forster, E.; Martin, J. L.; Antonetti, A. Femtosecond Time-resolved X-ray Diffraction from Laser-Heated Organic Films. Nature 1997, 390, 490−492. (4) Baum, P.; Yang, D.-S.; Zewail, A. H. 4D Visualization of Transitional Structures in Phase Transformations by Electron Diffraction. Science 2007, 318, 788−792. (5) Dwyer, J. R.; Hebeisen, C. T.; Ernstorfer, R.; Harb, M.; Deyirmenjian, V. B.; Jordan, R. E.; Miller, R. J. D. Femtosecond Electron Diffraction: ‘Making the Molecular Movie’. Philos. Trans. R. Soc. London, Ser. A 2006, 364, 741−778. (6) Schlawin, F.; Dorfman, K. E.; Fingerhut, B. P.; Mukamel, S. Suppression of Population Transport and Control of Exciton Distributions by Entangled Photons. Nat. Commun. 2013, 4, 1782. (7) Upton, L.; Harpham, M.; Suzer, O.; Richter, M.; Mukamel, S.; Goodson, T. Optically Excited Entangled States in Organic Molecules Illuminate the Dark. J. Phys. Chem. Lett. 2013, 4, 2046−2052. (8) Power, E. A.; Thirunamachandran, T. Quantum Electrodynamics with Nonrelativistic Sources. II. Maxwell Fields in the Vicinity of a Molecule. Phys. Rev. A 1983, 28, 2663−2670. (9) Power, E. A.; Thirunamachandran, T. Quantum Electrodynamics with Nonrelativistic Sources. III. Intermolecular Interactions. Phys. Rev. A 1983, 28, 2671−2675. (10) Andrews, D. L. A Unified Theory of Radiative and Radiationless Molecular-Energy Transfer. Chem. Phys. 1989, 135, 195−201.
Published: November 21, 2013 4019
dx.doi.org/10.1021/jz402108d | J. Phys. Chem. Lett. 2013, 4, 4019−4019
