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 investigated 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 tentatively 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 transient signal were coincident with the same rise and fall times of less than similar to 10 ps. The irradiation of visible pulses also induced a weakening of the v = 1 <-- 0 bands and transient peaks, but the decay time on the metal surface was longer than 100 ps, The temporal and spectral features imply that the persistent v = 1 <-- 0 band and the transient band originated 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 pulses on to the NiO(111) surface suggest that the photogenerated hot electrons induced the observed spectral changes. The long decay time observed on irradiating visible pulses on to the Ni(111) surface suggests the occurrence of a thermally driven process. Participation of the negative ion state was considered to be one of the possible mechanisms for the rapidly decaying features. (C) 1999 Elsevier Science B.V. All rights reserved.