Water Adsorption, Solvation, and Deliquescence of Potassium Bromide Thin Films on SiO2 Studied by Ambient-Pressure X-ray Photoelectron Spectroscopy

The adsorption of water on KBr thin films evaporated onto SiO2 was investigated as a function of relative humidity (RH) by ambient pressure X-ray photoelectron spectroscopy. At 30% RH, adsorbed water reaches a coverage of approximately 1 ML (monolayer). As the humidity continues to increase, the coverage of water remains constant or increases very slowly until 60% RH, followed by a rapid increase. At low RHs, the Br/K ratio on a submonolayer-thick film drops to less than 0.1, as a result of loss of Br atoms in the film due to radiation damage. With increasing humidity, solvation increases ion mobility and gives rise to a partial recovery of the Br/K ratio. Above 60% RI-I, the increase of the Br/K ratio accelerates. Above the deliquescence point (85% RH), the thickness of the water layer continues to increase and reaches more than three layers near saturation. The enhancement of the Br/K ratio at this stage is roughly a factor of 2.3 on a 0.5 nm KBr film, indicating a strong preferential segregation of Br ions to the air-liquid interface of the thin saline solution on SiO2.