SFG study of unstable surface species by picosecond pump-probe method

We succeeded in identifying the intermediates in thermal decomposition reac tions by utilizing the combination of an instantaneous temperature jump ind uced by the irradiation of picosecond laser pulses and the subsequent obser vation by time-resolved sum-frequency generation (TR-SFG) spectroscopy. The short-lived reactive intermediates in the decomposition of formats on NiO( 111) and Ni(111) surfaces were identifie. The irradiation of 1064 nm laser pulses caused the vibrational peak of the C-D stretching mode v(CD) of bide ntate formate on NiO(111) to weaken and the v(CD) band of monodentate forma te to appear. The result on Ni(111) showed the weakening of the v(CD) band of bridging formate and the appearance of the C=O stretching mode v(C=O) of monodentate formate. The spectral changes recovered on a 100 ps time scale but not fully above 400 K for the NiO(111) system and 320 K for the Ni(111 ) system, indicating the onset of thermal decomposition in the high-tempera ture period. The observations suggested that the formate in the stable bide ntate/bridging configurations transformed to unstable monodentate formats p rior to decomposition. Temperature- and time-dependent features indicated t hat the two types of formate were in equilibrium and the equilibrium shifte d towards the monodentate form by the rapid laser-induced temperature jump of about 250-300 K. (C) 1999 Elsevier Science B.V. All rights reserved.