Orientation and vibrational relaxation of acetonitrile at a liquid:solid interface, observed by sum-frequency spectroscopy

The vibrational spectrum of acetonitrile at the liquid:solid interface with yttrium aluminium garnet is investigated in the CN- and CH-stretching region, using two-color pump–probe sum-frequency spectroscopy in internal total reflexion geometry. An average tilt angle of the molecular symmetry axis with the surface normal of ≅53° is determined suggesting weak interaction with the surface. Measuring the coherent excitation of the symmetric CH3-transition yields a dephasing time of T2=420±100 fs. An effective population lifetime of 5.9±1.8 ps and a faster energy redistribution with a time constant of 1.8±1.0 ps are found in the CH-stretching region. Ground state recovery of the molecules at the interface occurs slowly with 150±50 ps, quite different to the bulk liquid.