Time-resolved SFG study of the vibrational excitation of adsorbed CO on Ni(111) and NiO(111) surfaces under the irradiation of UV and visible photons

The effects of irradiation by picosecond ultraviolet (266 nm) and visible ( 532 nm) pulses of GO-covered Ni(111) and NiO(111)/Ni(111) surfaces were inv estigated by infrared-visible sum-frequency generation (SFG) spectroscopy. As a result of the irradiation of UV pulses, the SFG signal by the v = 1 <- - 0 bands of the CO stretching mode weakened and transient peaks tentativel y assigned to the v = 2 <-- 1 hot band transition appeared with a downshift by 18 and 14 cm(-1) from v = 1 <-- 0 bands on the metal and oxide-layered surfaces, respectively. The time profile of the weakening and that of the t ransient signal were coincident with the same rise and fall times of less t han similar to 10 ps. The irradiation of visible pulses also induced a weak ening of the v = 1 <-- 0 bands and transient peaks, but the decay time on t he metal surface was longer than 100 ps, The temporal and spectral features imply that the persistent v = 1 <-- 0 band and the transient band originat ed from the same CO molecule. Short decay times observed on irradiating UV pulses on to the N(111) and NiO(111) surfaces and that of the visible pulse s on to the NiO(111) surface suggest that the photogenerated hot electrons induced the observed spectral changes. The long decay time observed on irra diating visible pulses on to the Ni(111) surface suggests the occurrence of a thermally driven process. Participation of the negative ion state was co nsidered to be one of the possible mechanisms for the rapidly decaying feat ures. (C) 1999 Elsevier Science B.V. All rights reserved.