Vibrational Relaxation of the Aqueous Protons in Acetonitrile : Ultrafast Cluster Cooling and Vibrational Predissociation

We study the ultrafast O-H stretch vibrational relaxation dynamics of protonated water clusters embedded in a matrix of deuterated acetonitrile, using polarization-resolved mid-IR fs spectroscopy. The clusters are produced by mixing triflic (trifluo-romethanesulfonic) acid and H₂O in molar ratios of 1:1, 1:2 and 1:3, thus varying the degree of hydration of the proton. At all hydration levels the excited O-H stretch vibration shows an ultrafast vibrational relaxation with a time constant T1<100 fs, leading to an ultrafast local heating of the protonated water cluster. This excess thermal energy, initially highly localized to the region of the excited proton, first redistributes over the aqueous cluster and then dissipates into the surrounding acetonitrile matrix. For clusters with a triflic acid to H₂O ratio of 1:3 these processes occur with time-constants of 320 ± 20 fs and 1.4 ± 0.1 ps, respectively. The cooling of the clusters reveals a long-living, underlying transient absorption change with high anisotropy. We argue that this feature stems from the vibrational predissociation of a small fraction of the proton hydration structures, directly following the ultrafast infrared excitation.