Analytical Sciences

Laser Response of a Quartz Crystal Microbalance: Frequency Changes Induced by Light Irradiation in the Air Phase

Takayoshi KAWASAKI, Tetsuhiro MOCHIDA, Jun-ichi KATADA, and Yoshio OKAHATA

Department of Biomolecular Engineering, Tokyo Institute of Technology, B-53 4259 Nagatsuta, Midori, Yokohama 226-8501, Japan

A weak laser irradiation (523 − 785 nm, 5 − 60 mW) onto an Au electrode surface of a 27-MHz quartz crystal microbalance (QCM) caused a frequency increase (a mass decrease) in the air phase. These frequency changes depended on the wavelength of the irradiated laser in the order of 523 nm > 636 nm > 785 nm, which corresponds to the light absorbance of the Au electrode of the QCM. The laser response increased linearly with increasing laser power (5 − 60 mW). In addition, the laser response showed a maximum at the incidence angle of 72 degrees when the P-polarized 636 nm laser was irradiated on the Au surface, due to the evanescent effect. These laser responses were also observed in the humid air of H₂O, D₂O, and in the vapors of various alcohols. Based on these findings, the observed frequency increase (mass decrease) can be explained by the photo-induced reversible desorption of water molecules from the Au electrode surface of the QCM due to the interfacial property changes.

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