Direct observation of proton transfer in ice Ih using femtosecond spectroscopy

We studied proton transfer in ice samples containing the photoacid 8-hydroxypyrene-1,3,6- trisulfonic acid and the base sodium formate using femtosecond pump-probe spectroscopy. Pump pulses, centered at a wavelength of 400 nm, electronically excited the photoacid molecules which released their protons. These protons subsequently traveled from the photoacid through the ice lattice to the base and were observed as they arrived at the base using the transient absorption of an infrared probe pulse. Both the temperature and concentration dependence of the proton transfer dynamics were modeled using a discrete set of two intrinsic transfer rates, associated with short and long-range proton transfer, respectively. Proton transfer in configurations where the acid and base were separated by up to about two water molecules, was found to occur on a ∼1 ps time scale for all temperatures (240-270 K). Long range direct proton transfer through water wires of about four water molecules in length was found to occur on a ∼300 ps time scale at 270 K. This latter process was observed to slow down significantly with decreasing temperature, with an activation energy of ∼80 kJ/mol.