Femtosecond Mid-Infrared Pump–Probe Study of Wave Packet Motion in a Medium-Strong Intramolecular Hydrogen Bond

We inspect the O–H stretching vibration of the medium-strong hydrogen bond in phthalic acid monomethyl ester in nonpolar solution by use of single-colour spectrally integrated femtosecond mid-infrared spectroscopy. We derive amplitude and phase characteristics of the coherent modulations of the pump-probe signals as a function of excitation wavelength. We exclude the role of Fermi resonances in the oscillatory contribution to the pump–probe signals. The observed periodic feature is due to wave packet motion of an underdamped out-of-plane 100 cm-1 mode that modulates the hydrogen bond distance. This low-frequency mode anharmonically coupled to the O–H stretching vibration is impulsively excited by the ultrashort mid-infrared pump pulse.