Enhanced spectroscopy and photochemistry of molecules adsorbed on metal surfaces

Technical Report · Sat Sep 01 04:00:00 EDT 1984

OSTI ID:6480184

Goncher, G M

Raman enhancement on silver single crystals roughened by ion bombardment in ultrahigh vacuum is correlated with the appearance of particular types of surface topography on the crystals. The surfaces that produce maximum enhancement contain surface roughness features on two size scales, one of 400 A dia and the other of 2000 A dia. Raman enhancement is greatest for incident wavelengths near 500 nm, in good agreement with the resonance wavelength predictions of electrodynamic theory. The distance dependance of the enhancement indicates that enhancement of surface fields accounts for a factor of 100 of the total observed enhancement of 1000 for an adsorbed monolayer; the remaining enhancement is due to a mechanism involving surface interactions that occur for only the first layer of molecules. Resonant photodecomposition was observed of a variety of aromatic molecules on these rough surfaces. A continuous ion laser source was used to produce graphitic carbon on the surface, monitored by Raman spectroscopy at the 1580 cm/sup -1/ band of surface carbon. Laser power dependence studies of the fragmentation rate for several molecules at 406.7 nm indicate that the initial absorption step is a two-photon procss, and energetic considerations imply that this is the case for other molecules observed to undergo photofragmentation. The only exception to this is benzaldehyde at 350.7 nm excitation. Distance dependence studies of the fragmentation rate using an inert spacer layer of molecules show a maximum decomposition rate for pyridine 15 to 20 A from the surface, indicating that energy transfer from molecules to the metal surface competes with enhanced excitation near the surface. Pulsed laser studies indicate that surface ionization at the low intensities used for photodecomposition is unlikely.

• Research Organization: Lawrence Berkeley Lab., CA (USA)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 6480184
• Report Number(s): LBL-18452; ON: DE85000618
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400500* -- Photochemistry
ACETOPHENONE
ADSORPTION
ALDEHYDES
AZINES
BENZALDEHYDE
CHEMICAL REACTIONS
CHEMISTRY
CORRELATIONS
CRYSTALS
DECOMPOSITION
DISSOCIATION
ELECTROMAGNETIC RADIATION
ELEMENTS
ENERGY TRANSFER
HETEROCYCLIC COMPOUNDS
IONIZATION
KETONES
LASER RADIATION
METALS
MONOCRYSTALS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOCHEMISTRY
PHOTOIONIZATION
PHOTOLYSIS
PYRAZINES
PYRIDINE
PYRIDINES
RADIATIONS
RAMAN EFFECT
RESONANCE
ROUGHNESS
SILVER
SORPTION
SURFACE PROPERTIES
SURFACES
TRANSITION ELEMENTS