Hydrogen Bond Dissociation and Reformation in Methanol Oligomers Following Hydroxyl Stretch Relaxation

Vibrational relaxation and hydrogen bond dynamics in methanol-d dissolved in CCl4 have been measured with ultrafast infrared pump−probe spectroscopy. We excited the subensemble of methanol-d molecules both accepting and donating hydrogen bonds at ∼2500 cm-1. Following vibrational relaxation with a ∼500 fs lifetime, the signal does not decay to zero. Rather, the signal increases to a second maximum at ∼4 ps. The decay from the second maximum occurs on two time scales. We propose a model in which hydrogen bond dissociation, following vibrational relaxation, decreases the concentration of methanol-d molecules that accept and donate hydrogen bonds and produce the observed long-lived bleach of the absorption signal. Using a set of coupled kinetic equations, the time constants for hydrogen bond dissociation and reformation have been determined. Hydrogen bond breaking occurs with ∼200 fs and ∼2 ps time constants. We attribute the fast rate to a direct breaking mechanism wherein the excited hydroxyl stretch decay...