silver island films were prepared by vacuum deposition of 7  nm silver onto glass coverslips .
directly before use , films were annealed for 1  min at 300  c under argon .
the sers substrates were immediately immersed in fresh 2  mm ethanolic thiol solutions for 1518  h. to alter the pntp ( fluka , technical grade ) surface concentration , the ratio between pntp and thiophenol ( merck , for synthesis ) was adjusted to keep the total thiol concentration constant .
xray photoelectron spectroscopy measurements indicated no large deviation in thiol ratio once adsorbed onto the silver island film .
sers spectra were measured through a 5 objective ( na 0.12 ) at 532  nm excitation with a power density of 610  w  cm by using a renishaw invia raman microscope .
data processing was done with matlab  2013a and the baselinecorrected peak integration performed by using trapezoidal numerical integration .
normalization of the total sers intensity was performed on the basis of the total sers intensity between 1020 and 1625  cm .
a zeroorder savitzky  golay filter of 7 pixels was used to smooth the timedependent peak areas . the nonlinearmodelfitting function in mathematica  9.0
the sum of residuals and squares was calculated separately for both reactant and product , then added , to calculate r 
 .
 abstractheterogeneous catalysis is a surface phenomenon . yet , though the catalysis itself takes place on surfaces , the reactants and products rapidly take the form of another physical state , as either a liquid or a gas . 
 catalytic reactions within a selfassembled monolayer are confined within two dimensions , as the molecules involved do not leave the surface . 
 surfaceenhanced raman spectroscopy is an ideal technique to probe these selfassembled monolayers as it gives molecular information in a measured volume limited to the surface . 
 we show how surfaceenhanced raman spectroscopy can be used to determine the reaction kinetics of a twodimensional reaction . as a proof of principle , we study the photocatalytic reduction of pnitrothiophenol . 
 a study of the reaction rate and dilution effects leads to the conclusion that a dimerization must take place as one of the reaction steps . 
Experimental Section