Abstract

Many fundamental and technologically relevant surface processes take place over Angstrom to nanometre length scales and picosecond to nanosecond timescales. However, the combination is extremely challenging to study experimentally; microscopy cannot achieve the necessary time-resolution, whereas spectroscopic experiments generally have poor spatial resolution. In this talk, I will introduce the Helium Spin-Echo technique, a method pioneered at the Cavendish, which has enabled the regime to be accessed experimentally for the first time. The technique involves scattering helium atoms while simultaneously using nuclear spin-polarisation of the atoms to split and recombine the helium wavepackets, giving a reciprocal-space surface-correlation measurement, with sensitivity over the picosecond range. Helium Spin-Echo measurements can be applied to a wide range of surface processes and in many cases have revolutionised our understanding of underlying physical phenomena. I will discuss a series of examples from recent experiments including the rates and mechanisms of nanoscale diffusion, the behaviour arising from complex extended molecules compared to point particles, determination of interaction potentials and rate limiting energy barriers, the transition to quantum transport, and energy exchange rates / vibrational lifetimes giving subsequent insights into friction on the nanoscale.