The motion of chemical bonds within molecules can be observed in real time,
in the form of vibrational wavepackets prepared and interrogated through
ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are
commonly performed on large ensembles of molecules, and as such, are limited to
the extent that ensemble coherence can be maintained. Here, we describe
vibrational wavepacket motion on single molecules, recorded through
time-resolved, surface-enhanced, coherent anti-Stokes Raman scattering. The
required sensitivity to detect the motion of a single molecule, under ambient
conditions, is achieved by equipping the molecule with a dipolar nano-antenna
(a gold dumbbell). In contrast with measurements in ensembles, the vibrational
coherence on a single molecule does not dephase. It develops phase fluctuations
with characteristic statistics. We present the time evolution of discretely
sampled statistical states, and highlight the unique information content in the
characteristic, early-time probability distribution function of the signal.Comment: 17 pages, 5 figure
