Picosecond pump-probe experiments on the C-O stretch vibration of acet
ic acid in solution with infrared pulses from a free-electron laser ar
e presented. We show that under typical experimental conditions of pul
ses that are short compared to the inverse absorption linewidth and op
tically thick samples (necessary to observe significant transmission c
hanges), the experimental pump-probe data are strongly influenced by p
ulse propagation effects (vibron polaritons) and coherent coupling bet
ween pump and probe vibron polaritons. To extract the vibrational life
time fi om the data, it is therefore necessary to perform a full simul
ation of the propagation and coupling effects. (C) 1998 Elsevier Scien
ce B.V.