Short-lived reactive formate species on NiO(111) observed by picosecond temperature jump

The combination of an instantaneous temperature jump induced by the irradiation of picosecond laser pulses and the subsequent observation by sum-frequency generation spectroscopy enabled the identification of a short-lived reactive intermediate in the decomposition of formate on NiO(111) surface. The irradiation of 1064 nm laser pulses caused the vibrational peak at 2160 cm(-1), which is the C-D stretching mode nu(CD) of bidentate formate, to weaken and the nu(CD) of monodentate formate at 2190 cm(-1) to appear. The spectral changes recovered in 100 ps time scale but not fully above 400 K. The observations suggested that the formate in bidentate configuration transformed to monodentate formate prior to decomposition. Temperature- and time-dependent features indicated that the two types of formate were in equilibrium and the equilibrium shifted towards the monodentate form by a rapid laser-induced temperature jump of about 250-300 K. The experiment enabled us to identify short-lived species and to estimate kinetic parameters. The enthalpy difference between bidentate and monodentate formates and the activation energy for the decomposition of monodentate formate were 19+/-5 and 30+/-10 kJ/mol, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.