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By taking advantage of the tensor nature of surface-enhanced Raman scattering (SERS), we track trajectories of the linker molecule and a CO molecule chemisorbed at the hot spot of a nano-dumbbell consisting of dibenzyldithio-linked silver nanospheres. The linear Stark shift of CO serves as an absolute gauge of the local field, while the polyatomic spectra characterize the vector components of the local field. We identify surface-enhanced Raman optical activity due to a transient asperity in the nanojunction in an otherwise uneventful SERS trajectory. During fusion of the spheres, we observe sequential evolution of the enhanced spectra from dipole-coupled Raman to quadrupole- and magnetic dipole-coupled Raman, followed by a transition from line spectra to band spectra, and the full reversal of the sequence. From the spectrum of CO, the sequence can be understood to track the evolution of the junction plasmon resonance from dipolar to quadrupolar to charge transfer as a function of intersphere separation, which evolves at a speed of ∼1 Å/min. The crossover to the conduction limit is marked by the transition of line spectra to Stark-broadened and shifted band spectra. As the junction closes on CO, the local field reaches 1 V/Å, limited to a current of 1 electron per vibrational cycle passing through the molecule, with associated Raman enhancement factor via the charge transfer plasmon resonance of 10(12). The local field identifies that a sharp protrusion is responsible for room-temperature chemisorption of CO on silver. The asymmetric phototunneling junction, Ag-CO-Ag, driven by the frequency-tunable charge transfer plasmon of the dumbbell antenna, combines the design elements of an ideal rectifying photocollector.

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.

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The caging of I2 in solid Kr is followed in real-time following its dissociative excitation on the A (31'1 t .) surface. The experiments involve pump-probe measurements with a time resolution of 2 150 fs. The experimental signals are reproduced using classical molecular dynamics simulations, and the classical Franck approximation. The comparison between experiment and simulation allows an unambiguous interpretation of features in the observed signal as being due to the initial impulsive stretch of the I-I bond, collision of the atoms with the cage wall, recoil and recombination, and the subsequent coherent oscillations of the nascent I, molecule. These detailed observations are possible due to retention of coherence along the I-I coordinate throughout the caging process. The extent of coherence is dictated mainly by the initial impact parameters of the molecule-cage collision, which in turn is controlled by the thermal and zero-point amplitudes of lattice vibrations. The caging is well-described as a sudden process, involving a binary collision between I and Kr atoms with nearly complete energy loss of the I atom upon completion of the first collision. Vibrational relaxation of the bound molecule proceeds on the time scale of 12 ps. The nontrivial relation between this relaxation time and decay rates that may be extracted from experimental transients is discussed. Although the interplay between the nested A and A ' potentials is not detectable, it is clear that in the studied range of initial excess energies, 1000-1700 cm-', the recombination remains effectively adiabatic, and does not involve the ground state.