Non-linear surface spectroscopies to probe intermolecular interactions
Mischa Bonn, Christian Hess, Martin Wolf, and Minhaeng Cho
Leiden Institute of Chemistry, P.O.Box 9502, 2300 RA, Leiden, the Netherlands.
The nonlinear optical technique of infrared-visible surface sum frequency
generation (IV-SFG) has proven itself to be a very versatile tool in studying the
structure and dynamics of molecules absorbed on surfaces and at interfaces.[1] This
technique relies on the finite second-order nonlinear optical susceptibility (    ) at the
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surface or interface.
We demonstrate that higher-order (  
3
and    ) surface
4
nonlinear spectroscopic techniques can be used to obtain information about
intermolecular coupling of molecules on surfaces.
These novel spectroscopic
techniques require high-intensity (~10  J in ~100 fs) infrared laser pulses, and are
demonstrated for the C-O stretch vibration of carbonmonoxide adsorbed on Ruthenium
Ru(001) surface.
As for the third-order technique, this is an extension of IV-SFG to
infrared-infrared-visible sum-frequency-generation spectroscopy (IIV-SFG).[2] We
demonstrate that this novel doubly vibrationally resonant    -spectroscopy provides
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useful information on the intermolecular coupling of adsorbed molecules and
intermolecular distances, in analogy to this type of four-wave mixing spectroscopies in
bulk.
With this doubly vibrationally resonant technique, information on intermolecular
coupling can be obtained that is inaccessible otherwise and two-dimensional vibrational
spectroscopy of molecules on surfaces becomes possible. The fourth-order technique
relies on the transfer of a significant fraction (~10%) of the CO molecules to their first
vibrationally excited state. Hence, after excitation, SFG from the red-shifted excited
state (1→2) can be observed, in addition to the SFG from the ground state (0→1).[3]
With increasing CO coverage, the onset of inter-molecular coupling leads to the
disappearance of the discrete vibrational transitions due to delocalization of the
vibrational energy within the adlayer: the energy can ‘hop around’ between Co
molecules, to eventually give rise to a 2-D phonon.[4] Monitoring the transition form a
localized transition to a delocalized phonon, we find resonant vibrational energy transfer
times within the adlayer in the ps range, in very good agreement with calculations of
Förster energy transfer rates for such a dipole-coupled system.
References:
1. Y.R. Shen, Nature 337 (1989) 519 and references therein.
2. M Bonn, C. Hess, J.H. Miners, H.J. Bakker, T.F. Heinz and M. Cho, Phys. Rev.
Lett. 86, 1566 (2001); M. Cho, C. Hess and M. Bonn, Pys. Rev B 65, 205423
(2002).
3.Ch. Hess, M. Bonn, S. Funk and M. Wolf, Chem. Phys. Lett. 325, 139 (2000).
4. Ch. Hess, M. Wolf, M. Bonn, Phys. Rev. Lett. 85, 4341 (2000).