Ultrafast relaxation and anharmonic coupling of O–H stretching and bending excitations in cyclic acetic acid dimers

The relaxation and anharmonic coupling of vibrational O–H stretching and O–H bending excitations of hydrogen-bonded acetic acid dimers in solution is studied by femtosecond two-color pump–probe spectroscopy in the mid-infrared. The O–H stretching and O–H bending population lifetimes are 200 and 250 fs. After excitation of the stretching mode, anharmonic coupling leads to pronounced absorption changes of the bending mode, without involving relaxation induced excess populations of the bending mode. On a longer picosecond time scale, a similar reshaping of the vibrational spectra is induced by anharmonically coupled modes which are populated through vibrational relaxation.