IN-SITU SFG SPECTROSCOPY OF FILM GROWTH - II - DEPOSITION OF FORMIC-ACID ON NI(110) SURFACE

Infrared-visible sum-frequency generation (SFG) spectroscopy was used to monitor in situ the growth of multiple layers of formic acid on a N i(110) surface. The signal by the CH stretching band displayed a chara cteristic interference pattern as the deposition proceeded and the fea ture was analyzed by the formula presented in the preceding article. T he effect of substrate structure was examined to reveal that the depos ition of a SFG active layer on alien surfaces requires the substrate t emperature higher than 138 K with the optimum at 158 K, but the same g rowth as under the optimum condition was sustained at 143 K once the a ctive layer had been formed beforehand. No SFG-wise change was observe d when the sample temperature was changed in vacuum either from the SF G undetectable 143-153 K or to the opposite direction to indicate that the layers formed at the two temperatures are not related with phase transition. However, the deposition of the SFG-active layer took place at 143 K, when the sample was precovered by the active layer or the S FG-inactive layer was heated to 153 K. Covering by a DCOOD layer, whic h has no SFG signal at the examined frequency, gave a significant supp ression of the interference feature. An interpretation is given on the crystal structure and molecular orientation of the SFG-active and SFG -inactive layers. (C) 1998 American Institute of Physics.