Ultrafast vibrational excitation transfer and vibrational cooling of propionic acid dimers investigated with IR-pump IR-probe spectroscopy

Ultrafast IR-pump IR-probe spectroscopy is used to study the relaxation dynamics of propionic acid dimers. The spectral changes in the regions of CO-stretching at ∼1700 cm−1, OH and CH-bending modes around 1400 cm−1 are investigated after excitation in the range of the CH- and CO-stretching modes. A strong coupling of the OH-stretch vibration to CO-stretching and OH/CH-bending modes is found. The intramolecular redistribution of vibrational energy takes place on a 1 ps time-scale. A further cooling process and energy transfer to the surrounding occur with ∼12 ps. The dynamics depend on the wavelength of the IR-excitation and point to a transient breaking of one of the two H-bonds of the propionic acid dimers.