Studies of surfaces using optical sum-frequency generation

Technical Report · Thu Dec 31 23:00:00 EST 1987

OSTI ID:5417007

Hunt, J H

Sum-frequency generation (infrared-visible) was used as probe of molecular species at interfaces. The process being a second order non-linear optical process causes the technique to be surface-specific when used at an interface between two centrosymmetric media. The resonant enhancement of chi that occurs when the infrared input is tuned through a vibrational resonance allows one to determine the vibrational spectrum of the species adsorbed to the surface. The polarization dependence of the spectrum can be used to deduce the molecular orientations of the species at the interface. The laser system uses an amplified single pulse selected from the output train of a active/passive mode-locked Nd:YAG oscillator. The pulse contains sufficient energy to generate both an intense visible pulse via second-harmonic generation and an intense infrared pulse via optical parametric amplification. The first experiments were performed on series of alcohols at an air-glass interface. These experiments demonstrated that the basic technique produced spectra several orders of magnitude above the bare glass background. Although the spectra did not extend far enough into the infrared to identify the adsorbed species, the spectra were correlated with spectra taken in bulk samples. The technique was used to examine the spectra of a Langmuir film (air-water interface) of pentadecanoic acid at several surface densities. It was found that in the high density phase, the molecular hydrocarbon tails were fully extended and aligned nearly normal to the surface. At lower densities, the tail was free to bend and evidence of trans-gauche conformations was observed. The experiment makes use of the ability of sum-frequency generation to do in-situ spectral measurements not possible with other techniques. The use of difference-frequency generation was discussed.

• Research Organization: Lawrence Berkeley Lab., CA (USA)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 5417007
• Report Number(s): LBL-24692; ON: DE88006334
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ADSORPTION
AIR
ALCOHOLS
CARBOXYLIC ACIDS
ELECTROMAGNETIC RADIATION
ELECTRONIC EQUIPMENT
ENERGY LEVELS
EQUIPMENT
EXCITED STATES
FILMS
FLUIDS
GASES
GLASS
HARMONIC OSCILLATORS
HARMONICS
HYDROGEN COMPOUNDS
HYDROXY COMPOUNDS
INFRARED RADIATION
INTERFACES
LASERS
LAYERS
ORGANIC ACIDS
ORGANIC COMPOUNDS
OSCILLATIONS
OSCILLATORS
OXYGEN COMPOUNDS
RADIATIONS
SORPTION
SPECTROSCOPY
STRUCTURAL CHEMICAL ANALYSIS
SURFACES
VIBRATIONAL STATES
WATER