High-resolution Study of Molecular Adsorbates in the Near-infrared by Difference-frequency Generation

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Abstract
A near-infrared laser has been developed, the source being the difference-frequency component of dye-laser and argon-ion-laser radiation nonlinearly mixed in a lithium niobate crystal. Phase-matching necessary for difference-frequency generation is achieved via temperature tuning of the refractive indices of the crystal. The resultant infrared source is tunable from approximately 2700 to 4000 cm−1, and is continuously electronically scannable over 1 cm−1. The source is used in an infrared reflection absorption spectroscopy (IRAS) experiment to conduct the first high-resolution study of the stretching vibrations of two molecular adsorbate surface systems. These are the C—H stretch of the formate ion and the O—H stretch of the hydroxyl ion adsorbed on to aluminium-supported alumina, HCOO−/Al2O3/Al and OH−/Al2O3/Al respectively.