Ge Research Group University of California, Irvine, Deparment of Chemistry

Detailed knowledge of molecular structures and dynamics in condense phases is essential to a complete and predictive understanding of chemical, biological, and materials processes. Our research program is directed to determine how molecular structures change in time, at equilibrium and during reactions. Topics of current interest include: structure and dynamics of peptides, proteins, liquids, and interfaces; carrier dynamics in solar cell materials; and nanoplasmonics. These endeavors lead us to devise new techniques in nonlinear spectroscopy and microscopy that can provide detailed information on the time dependence of nuclear and electronic motions.

Ultrafast two-dimensional infrared (2D IR) spectroscopy and its applications in biomolecular structure determination and condensed matter dynamics

    We are exploiting new experimental techniques that are vibrational analogues of multidimensional NMR. The extension beyond a single dimension gives these techniques the ability to disentangle structural information from complex spectra where the couplings, correlations, and relative angular orientations between structural units are revealed as "cross peaks". Moreover, the picosecond time resolution of 2D IR makes it an ideal structural probe for short-lived intermediates in chemical or biological processes once initiated by external triggers. The necessary science of multidimensional multicolor IR spectroscopy is being developed in our laboratory. Experimental results are compared to computer simulations to bring out atomic level understanding of the processes in question.
   We are applying 2D IR to the study of complex systems such as peptides, proteins, liquids, membranes, and their composite interfaces. Structural distributions, evolutions, and their environmental dependence are investigated. Dynamics of vibrational relaxation, molecular reorientation, intermode vibrational energy transfer, and chemical exchange are studied. Strategic incorporation of isotope labels and vibrational markers are used to zoom into atomic moieties of particular interest. Current topics include peptide-membrane interactions, amyloid peptide aggregation, and enzyme catalysis.

Sum-frequency generation (SFG) spectroscopy and microscopy and their applications in surface chemistry, polymer physics, and nanoplasmonics

   We are developing SFG spectroscopic and microscopic techniques that can reveal structure and dynamics in noncentrosymmetric systems, such as surfaces, interfaces, and biological tissues.  By combining these techniques with surface plasmon enhancement of novel nanostructures, we will push the detection limit towards study of trace molecules at interfaces. We are building a femtosecond SFG microscope that is capable of time-resolving nonlinear response as well as multiplex detection in the frequency domain with submicron spatial resolution. Applications include catalysis, photon-driven devices, and biological systems.

Nonlinear scattering-type scanning near-field optical microscopy (s-SNOM) and its applications in nanoplasmonics and carrier dynamics

    We are pushing our temporal and spatial resolution to the femtosecond-nanometer range with a new  s-SNOM instrument that is capable of near-field imaging, nano FTIR, and pump-probe nanoscopy. We are applying s-SNOM to near-field mapping of plasmonic nanostructures and carrier dynamics in solar cell materials.

Highlight of some recent projects

Nonlinear response of gold nanocrescent array
J. Phys. Chem. C, ASAP (2020)
New noise reduction algorithm	Polarization sensitive SFG microscopy of collagen I fibers
Opt. Exp., 25 26262 (2017) Opt. Exp., 27 20323 (2019)	J. Phys. Chem. B., 119, 3356 (2015)
Conformational analysis of penta-alanine	The amide-I/II couplings in fully extended conformation
J. Phys. Chem. B., 120, 5325 (2016)	J. Phys. Chem. C., 118, 29448 (2014)
Mapping molecular orientation of collagen I fibers	Frequency correlation between different modes
J. Phys. Chem. B., 117, 6149 (2013)	Chem. Phys., 422, 22 (2013)
Rotational isomerization of a ketone molecule	The amide-I and II modes in fully extended peptide chains
J. Phys. Chem. B, 116, 11292 (2012)	J. Phys. Chem. B, 115, 5168 (2011)
Sum-frequency generation microscopy	Probe peptide-membrane interaction
Opt. Lett., 36, 3891 (2011)	J. Phys. Chem. B, 114, 1180 (2010)
Local interaction between the amide-I and II modes	Onset of the 310-helical secondary structure
J. Phys. Chem. B, 113, 11775 (2009) J. Am. Chem. Soc., 131, 2042 (2009)	J. Am. Chem. Soc. 130, 6556 (2008)
Comparative studies of simulated 2D IR spectra	Interplay between 310- and α-helices
J. Phys. Chem. B 114, 1434 (2010) J. Phys. Chem. B 113, 12037 (2009)	J. Phys. Chem. B 111, 3222 (2007) J. Phys. Chem. B 110, 5834 (2006)
Conformation of a model terminally-blocked amino acid
J. Phys. Chem. B 110, 19891 (2006)

Funding supports from the following agencies are gratefully acknowledged:

    National Science Foundation CHE-1414466, CHE-1310693, CHE-1013071