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John J. McMahon.
Associate Professor of Chemistry .
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Spectroscopic and Electrochemical Studies of Structure and Processes at Electrode Surfaces |
Topics in Physical Chemistry
(Thermodynamics, Quantum Mechanics, Kinetics)
Thermodynamics (Carnot Cycle) |
Pressure-volume expansions and compressions between a hot and a cold temperature permit extraction of work from the cycle (- Wmax is the red area under the curve.) . |
Quantum Mechanics
(Fourier series)
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A particle defined by P(x) existing between -1 and 1 is fit with a superposition of Fourier component waves, f(x), of different wavelengths. The more waves used, the better the fit, the more confident we are of the particle's position. However, the more waves we use, the larger the range of wavelengths and, according to deBrogile, the larger the range of momenta (mv). Thus, the more certain we are of a particle's position the less certain we become of its momentum. - Heisenberg uncertainty principle. |
(molecular vibrations of water)
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Because each atom of a molecule may move along x, y, or z, a molecule with N atoms has 3(N) degrees of freedom of motion. Water, with three atoms, has 3(3) or 9 degrees of freedom. Translation of the entire molecule along the x, y, and z axes constitutes 3 of the 9 degrees of freedom. Rotation of the entire molecule about the x, y, and z axes constitutes 3 more of the 9 degrees of freedom. The remaining degrees of freedom are internal vibrational motions. For non-linear molecules, like water, we therefore expect 3(N)-6 vibrational motions. The 3 vibrations in water are shown at the left. |
(interactive Demo: hard sphere collision theory)
| The likelihood of collisions between molecules, a prerequisite to chemical reactions called the "law of mass action", depends upon the size of the collisional cross section, S. Molecules that approach close enough so that the separation of their centers is contained within S will collide. The larger the collisional cross section, the more likely a collision, the faster the rate of reaction between the molecules. The size of S naturally depends upon the diameter (d12) of the approaching molecules, but also will depend on the relative velocity (Vr) of the two molecules and on whether the two molecules have attractive (att) or repulsive (rep) potential energies of interaction (Ur). The above interactive demonstration allows adjustment of each of these parameters, along with an impact parameter, b, defined in the figure above. The impact parameter, b, has a maximum value, bmax, above which the molecules will not collide which is defined by the size of the collisional cross section, S. |
Research Specialties
Spectroscopic and Electrochemical Studies of Structure and Processes at Electrode Surfaces
surface and normal Raman spectroscopy(SERS) , fuel cells, mass spectrometry.
SERS
Surface-Enhanced Raman Spectrum
of Bis-(4-pyridyl)-acetylene (BPA)
Adsorbed at a Silver Electrode
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| The surface-enhanced Raman spectrum for BPA adsorbed at a silver electrode is tracked as a function of applied voltage at the silver. Beginning around -0.5 V and accompanying the passage of reduction current (insert) the SERS indicates that BPA reduces to an ethylenic product. The reduction is evidenced by loss of intensity at 2230 cm-1 (the symmetric stretch of the acetylenic bridge) and replacement with a peak at 1635 cm-1 (the symmetric stretch in the ethylenic reduction product), one of the new red peaks that appear at more negative applied voltages. |
Fuel Cells
Photocatalysis of the Oxygen Reduction Reaction in Fuel Cells
| The BPA-covered silver working electrode was irradiated with light from an argon ion (Ar+) laser at 514 nm (green) in a timed experiment... 4 sec off / 4 sec on / 4 sec off. |
The resultant current observed during the irradiation period accompanies the reduction of dissolved oxygen. CONCLUSION... The efficiency of fuel cells, limited by the slow kinetics of oxygen reduction, can be improved by irradiating the cathode. |
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Secondary Ion Mass Spectrometry (SIMS)
SIMS of Stilbazole (MW 181) Adsorbed at a Silver Electrode
SIMS at m/z = 181 |
video capture of the surface |
mass spectrum of stilbazole |
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SIMS image was gathered by rastering a beam of cesium ions (Cs+) across the electrode surface and collecting secondary ions of mass m/z = 181, the molecular weight of stilbazole. |
Photograph of the same surface. |
Secondary Ion mass spectrum of stilbazole adsorbed at a silver electrode surface. |
Recent Publications
"MS Analysis of Surface Photoproducts - Stilbazole at Silver" Proceedings of the 47th ASMS Conference on Mass Spectrometry and Allied Topics, June 13 - 17, 1999, Dallas, Texas, published on searchable CD-ROM. (view abstract), (full text in pdf format)
"Photochemical Charge Transfer Excitation of Trans-4-Stilbazole at a Silver Electrode", J. J. McMahon, T. J. Gergel, D. M. Otterson, C. R. McMahon, R. M. Kabbani, Surface Science 440(3), 357-374 (1999). (view abstract), (full text in pdf format)
"Secondary Ion Mapping of Ca in Bone", P. J. Todd, C. A. McCandlish, J. J. McMahon, J. M. McMahon, Proceedings of the 48th ASMS Conference on Mass Spectrometry and Related Topics, published on searchable CD-ROM, (2000). (full text document in pdf format)
"Synthesis of 2-Acetamido-5,6-dihalo-phenyl Acetates", D. D. Clarke, H. Gershon, J. J. McMahon, Monatshefte fur Chemie Chemical Monthly, 131(7) 795 - 798 (2000). (view abstract), (full text in pdf format).
"SIMS Imaging of trans-4-Stilbazole Adsorbed at a Silver Electrode"J. J. McMahon, J. M. McMahon , T. G. Schaff , Peter J. Todd, Proceedings of the 49th ASMS Conference on Mass Spectrometry and Related Topics, published on searchable CD-ROM, (2001). (view abstract),(full text document in pdf format)
"Preparation and fungitoxicity of some dibromo-8-quinolinols?h Gershon, Herman; Clarke, Donald D.; McMahon, John J.; Gershon, Muriel. Monatshefte fuer Chemie 132(7), 833-838 (2001). (view abstract), (full text in pdf format).
Imaging Charge Transfer Induced Surface Photochemistry with Ion Trap SIMS , J. J. McMahon*; T. G. Schaaff; J. M. McMahon; P. J. Todd, Proceedings of the 50th ASMS Conference on Mass Spectrometry and Related Topics, published on searchable CD-ROM, (2002). (full text document in pdf format)
"Preparation and Antifungal Activity of 3-Iodo- and 6-Iodo-8-quinolinols", H. Hershon, D. D. Clarke, J. J. McMahon, Monatshefte fur Chemie Chemical Monthly, 133(10) 1325-1330 (2002). (view abstract), (full text in pdf format)
"Effect of Dimethyl Sulfoxide and Dimethylformamide on the Stability of 4-Halo-8-quinolinols" Gershon, Herman; Clarke, Donald D.; McMahon, John J.. Monatshefte fuer Chemie 134(8), 1099-1104 (2003). (view abstract), (full text in pdf format).
"Photocatalytic Electrode and Fuel Cell", J. J. McMahon, Patent pending: U. S. Patent Application Serial No. 11/393,630, filed March 30, 2006, published October 4, 2007 (full text in pdf format).
"SERS and Photocatalysis of Oxygen Reduction at Bis-(4-Pyridyl)Acetylene-Covered Silver", J.J. McMahon, K. Breen, M.S. Barry, Proceedings of the 20th International Conference on Raman Spectroscopy, Yokohama, Japan, 2006 (view abstract) , (view poster).
"Photocatalytic Electrode and Fuel Cell", J. J. McMahon, U. S. Provisional Patent Application Serial No. 60/921,107, filed March 30, 2007.
"Photocatalytic Electrode and Fuel Cell", J. J. McMahon, U. S. Patent Application Serial No. 11/823,760, filed June 28, 2007, published December 20, 2007 (full text in pdf format).
"Photocatalysis of the Oxygen Reduction Reaction at Adsorbate-Covered Silver", John J. McMahon, Michael Barry (FC '06), Kristin J. Breen (FC '07), Anna K. Radziwon (FC '05), Leanne D. Brooks (FC '05), Michael R. Blair (FC '04), J. Phys. Chem. C 112(4), 1158 -1166 (2008) (view abstract), (full text in pdf format).
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