Atomically terminated and nanoscopically smooth silver tips effectively focus light on the angstrom scale, allowing tip-enhanced Raman spectromicroscopy (TER-sm) with single molecule sensitivity and submolecular spatial resolution. Through measurements carried out on cobalt-tetraphenylporphyrin (CoTPP) adsorbed on Au(111), we highlight peculiarities of vibrational spectromicroscopy with light confined on the angstrom scale. Field-gradient-driven spectra, orientational fingerprinting, and sculpting of local fields by atomic morphology of the junction are elucidated through measurements that range from 2D arrays at room temperature to single molecule manipulations at 5 K. Notably, vibrational Stark tuning within molecules, reflecting intramolecular charge distributions, becomes accessible when light is more localized than the interrogated normal modes. The Stark images of CoTPP reveal that it is saddled, and the distortion is accompanied by charge transfer to gold through the two anchoring pyrroles.Keywords: confined light; field-gradient-driven Raman; quadrupolar scattering; scanning tunneling microscopy; spectromicroscopy; Stark shift; tip-enhanced Raman spectroscopy;

Optical activity, which is used as a discriminator of chiral enantiomers, is demonstrated to be orientation dependent on individual, and nominally achiral, plasmonic nanosphere dimers. Through measurements of their giant Raman optical activity, we demonstrate that L/R-handed enantiomers can be continuously turned into their R/L-handed mirror images without passing through an achiral state. The primitive uniaxial multipolar response, with demonstrable broken parity and time reversal symmetry, reproduces the observations as resonant Raman scattering on plasmons that carry angular momentum. The analysis underscores that chirality does not have a quantitative continuous measure and recognizes the manipulation of superpositions of multipolar plasmons as a paradigm for novel optical materials with artificial magnetism.Keywords: chiral connectedness; chiroptical activity; multipolar Raman; nonreciprocal; PT invariance; Raman optical activity;

We present first-principles analysis of the Stark effect of CO adsorbed on an atomically sharp silver asperity, and current versus potential (I–V) characteristics of the Ag-CO-Ag junction. The analysis supports the suggestion that CO-bridged plasmonic junctions represent rectifying nanoantennas at optical frequencies and that the CO vibrational spectrum serves as a molecular voltmeter [M. Banik et al. ACS Nano 2012, 6, 10343]. The Stark effect is principally controlled by the field-induced charge redistribution between the antibonding 2π*-orbitals of CO and the s-electrons of Ag. The Stark tuning rate of the CO stretch, 1.5 × 10–6 cm–1/V cm–1, is ∼25% larger on atomically sharp asperities than on flat Ag, and remains constant over a large window of applied fields (±0.8 V/Å). As such, both sign and strength of local electric field can be quantitatively determined by the vibrational shift of CO. The I–V curve of the Ag-CO-Ag junction is nonlinear, rendering it an effective rectifier with responsivity S = (∂2I/∂V2)/(∂I/∂V) = −2.8 μA/V at zero bias. A more explicit treatment of rectification at optical frequencies is presented through time-dependent density functional simulations of the coupled electronic and nuclear degrees of freedom of the junction, to include dynamical impedance in the confirmation of the optical rectenna. The computed impedance correctly predicts the experimentally observed sign and magnitude of the rectified optical field, as measured by the Stark effect.

Surface-enhanced coherent anti-Stokes Raman scattering (SECARS) measurements carried out on individual nanosphere dimer nantennas are presented. The ν-domain and t-domain CARS measurements in the few-molecule limit are contrasted as vibrational autocorrelation and cross-correlation, respectively. We show that in coherent Raman spectroscopies carried out with ultrashort pulses, the effect of surface enhancement is to saturate stimulated steps at very low incident intensities (100 fJ in 100 fs pulses), and the principal consideration in sensitivity is the effective quadratic enhancement of spontaneous emission cross sections, σ* = (EL/Eo)2σ. Through multicolor femtosecond SECARS measurements we show that beside enhancement factors, an effective plasmon mode matching consideration controls the interplay between coherent electronic Raman scattering on the nantenna and vibrational Raman scattering on its molecular load. Through extensive measurements on individual nantennas, we establish the tolerable average and peak intensities that can be used in ultrafast measurements at nanojunctions, and we highlight a variety of plasmon-driven chemical and physical channels of signal and sample degradation.

Through STM images, we show that azobenzene-terminated alkanethiols hover and twirl when confined between the Ag tip and Au(111) substrate of an STM junction. In contrast with mechanisms of activation used to drive molecular rotors, twirling is induced by the effective elimination of lateral corrugation in the energy landscape when molecules hover by their van der Waals attraction to the approaching tip. While in the stationary state the benzenes of the head group lie flat with an inter-ring separation of 7.5 Å, they stand on-edge as the molecule twirls and their separation contracts to 5.2 Å, close to the value of the free molecule. The captured images of motion allow the characterization of physisorption potentials.

While exploring photoisomerization of azobenzyl thiols (ABT) adsorbed on Au(111), through joint scanning tunneling microscopy (STM) and tip-enhanced Raman scattering (TERS) studies, the reversible photoisomerization of one molecule is captured in TERS trajectories. The unique signature of single molecule isomerization is observed in the form of anticorrelated flip-flops between two distinct spectra with two discrete, on- and off-levels. The apparently heterogeneously photocatalyzed reaction is assigned to cis–trans isomerization of an outlier, which is chemisorbed on the silver tip of the STM. Otherwise, the ensemble of ABT molecules that lie flat on Au(111) remain strongly coupled to the surface, excluding the possibility of photoisomerization or detection through TERS.

A combined experimental and theoretical study of the facile polymerization of biphenyl-4,4′-dithiol (BPDT) to form intrinsically conductive linear chain [−Ag–S–BP–S−]n polymers is described. BPDT readily polymerizes and extrudes on roughened surfaces of elemental silver under ambient conditions. The self-assembled polymers can be sharply imaged through scanning electron microscopy because of their silver content and conductivity. Cyclic current versus voltage measurements (I/V curves) using a scanning tunneling microscope establish that the conductivity is intrinsic, consistent with the metallic conductivity of the linear polymer predicted through density functional theory. Systematic calculations identify that the roughness-catalyzed polymerization is driven by mobile Ag adatoms and adatom-mobilized monomers.

We demonstrate a conductance switch controlled by the spin-vibronic density of an odd electron on a single molecule. The junction current is modulated by the spin-flip bistability of the electron. Functional images are provided as wiring diagrams for control of the switch’s frequency, amplitude, polarity, and duty-cycle. The principle of operation relies on the quantum mechanical phase associated with the adiabatic circulation of a spin-aligned electron around a conical intersection. The functional images quantify the governing vibronic Hamiltonian.Keywords: Jahn−Teller effect; molecular Aharonov−Bohm effect; spin-vibronic coupling

Electroluminescence (EL) in scanning tunneling microscopy (STM), which enables spectroscopy with submolecular spatial resolution, is shown to be due to radiative ionization with vibronic shape resonances that carry Fano line profiles. Since Fano progressions retain phase information, the spectra can be transformed to the time domain to reconstruct the vibronic motion. In effect, measurements within a molecule are accessible with joint space–time resolution at the Å–fs limit. We demonstrate this through EL-STM on the Jahn–Teller-active Zn-etioporphyrin radical anion and visualize the orbiting motion of scattered electrons upon sudden reduction and oxidation. We discuss the elements that enable spectroscopy with submolecular spatial resolution through EL-STM and the closely related STM-Raman process.Keywords: electroluminescence; Fano line shape; Jahn−Teller effect; scanning tunneling microscopy; submolecular spectroscopy

Intensity spikes in Raman scattering, accompanied by switching between line spectra and band spectra, can be assigned to shorting the junction plasmon through molecular conductive bridges. This is demonstrated through Raman trajectories recorded at a plasmonic junction formed by a gold AFM tip in contact with a silver surface coated either with biphenyl-4,4′-dithiol or biphenyl-4-thiol. The fluctuations are absent in the monothiol. In effect, the making and breaking of chemical bonds is tracked.

The Jahn–Teller (JT) active unpaired electron of single metalloporphyrin radical anions is imaged through scanning tunneling microscopy. It is demonstrated that the electron is delocalized over the porphyrin macrocycle and its topographic image is determined by vibronic motion: the orbital of the electron adiabatically follows the zero-point pseudorotation of skeletal deformations. Transformation of the polar graphs of the observed images allows visualization of the adiabatic vibrational density to which the electron is coupled. The vibronic potential at the conical intersection is visualized and the half-integer angular momentum characteristic of the Berry phase is revealed in the radial function of the electron. The measurements underscore the economy of Jahn–Teller dynamics: small atomic displacements (∼10–1 Å) determined by weak interactions (∼10 meV) control the motion of the electron on a 10 Å scale and determine the molecular shape and function.

We describe combined AFM/Raman measurements on single nanodumbbells, consisting of silver nanospheres linked with dibenzyl-4,4′-dithiol (DBDT). The measured surface-enhanced Raman scattering (SERS) enhancement factor, EFexp = 3 × 107 at 532 nm, corresponds to the observed signal strength of a single DBDT molecule, the Raman cross section of which was determined to be dσ/dΩ = 6 × 10–28 cm2/sr. We show that the product of the local field enhancement, EFP = (Ei/E0)2(Es/E0)2= 3 × 106, and the chemical contribution due to reduced detuning, EFC = (Δ0/Δ)2 = 12, account for the observed effect. The chemical contribution is assessed by exploring model structures Agn–S–DB–S–Agm (n, m = 0, 3, 7, 20). The π–π* transition at 287 nm, which determines the polarizability of the bare molecule, acquires a DBDT-to-silver charge-transfer character upon binding to silver. The CT transition near 400 nm reduces the detuning but remains nonresonant at 532 nm. We observe a soft polarization dependence, which suggests optical activity, which in part is ascribed to coupling between plasmons and conjugated electrons of DBDT. Modest enhancement factors are sufficient to detect single molecules through nonresonant SERS.

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 1012. 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.Keywords: antenna; biphenyl; break junction; carbon monoxide; charge transfer plasmon; dibenzyldithiol; dumbbell; nanosphere; plasmon; rectenna; SEROA; SERS

We describe Raman spectroscopy measurements of distyrylbenzene (DSB) molecules equipped with plasmonic antennae in the form of silver dumbbells in aqueous solution under ambient conditions. A synthetic strategy in which the dithiolated molecule is used as the linker between silver nanospheres ensures that the molecules are attached at the intersphere gap where local fields are maximally enhanced. The measured and calculated enhancement factors are in excellent agreement. The reported method has sufficient sensitivity to also allow for the detection of molecules tethered to single spheres, with 100–1000-fold weaker enhancement. Spectral analysis allows assignment of structures and reveals that in addition to the normal Raman active modes IR active transitions appear in the Raman spectra where field gradients dominate.

We introduce an experimental platform designed around a thermomechanical helium fountain, which is aimed at investigating spectroscopy and dynamics of atoms and molecules in the superfluid and at its vapor interface. Laser ablation of copper, efficient cooling and transport of Cu and Cu2 through helium vapor (1.5 K < T < 20 K), formation of linear and T-shaped Cu2−He complexes, and their continuous evolution into large Cu2−Hen clusters and droplets are among the processes that are illustrated. Reflection is the dominant quantum scattering channel of translationally cold copper atoms (T = 1.7 K) at the fountain interface. Cu2 dimers mainly travel through the fountain unimpeded. However, the conditions of fountain flow and transport of molecules can be controlled to demonstrate injection and, in particular, injection into a nondivergent columnar fountain with a plug velocity of about 1 m/s. The experimental observables are interpreted with the aid of bosonic density functional theory calculations and ab initio interaction potentials.

We report transient grating measurements carried out on single crystals of bromine clathrate hydrates and on bromine dissolved in water. In all cases, excitation into the B-state of Br2 leads to prompt predissociation, followed by cage-induced recombination on the A/A′ electronic surfaces. In liquid water, the vibrationally incoherent recombinant population peaks at t = 1 ps and decays with a time constant of 1.8 ps. In the hydrate crystals, the recombination is sufficiently impulsive to manifest coherent oscillations of the reformed bond. In tetragonal TS-I crystals, with the smaller cages, the recombination is fast, t = 360 fs, and the bond oscillation period is 240 fs. In cubic CS-II crystals, the recombination is slower, t = 490 fs, and the visibility of the vibrational coherence, which shows a period of 290 fs, is significantly reduced due to the larger cages and the looser fit around bromine. The mechanical cage effect is quantified in terms of the recombination time-distribution, the first three moments of which are associated with size, structural rigidity, and anelasticity of the cage. In the crystalline cages, the distribution is symmetric about the mean: mean time tm = 300 fs, 400 fs and standard deviation σ = 70 fs, 100 fs, in TS-I and CS-II, respectively. The finding is consistent with the assignment of occupied cages: principally 51262 polyhedra in TS-I and 51264 polyhedra in CS-II. In liquid water, with diffuse cages, the distribution characterized by tm = 555 fs and σ = 400 fs, is strongly skewed (γ1 = 1.88) toward delayed recombination—the effective liquid phase hydration shell is larger than that in a hydrate phase, structurally disordered, and anelastic. Information about dipolar disorder, comparable in all three media, is extracted from electronic predissociation rates of the B-state, which is sensitive to the symmetry in the guest–host interaction.

We report transient grating measurements carried out on single crystals of bromine clathrate hydrates and on bromine dissolved in water. In all cases, excitation into the B-state of Br2 leads to prompt predissociation, followed by cage-induced recombination on the A/A′ electronic surfaces. In liquid water, the vibrationally incoherent recombinant population peaks at t = 1 ps and decays with a time constant of 1.8 ps. In the hydrate crystals, the recombination is sufficiently impulsive to manifest coherent oscillations of the reformed bond. In tetragonal TS-I crystals, with the smaller cages, the recombination is fast, t = 360 fs, and the bond oscillation period is 240 fs. In cubic CS-II crystals, the recombination is slower, t = 490 fs, and the visibility of the vibrational coherence, which shows a period of 290 fs, is significantly reduced due to the larger cages and the looser fit around bromine. The mechanical cage effect is quantified in terms of the recombination time-distribution, the first three moments of which are associated with size, structural rigidity, and anelasticity of the cage. In the crystalline cages, the distribution is symmetric about the mean: mean time tm = 300 fs, 400 fs and standard deviation σ = 70 fs, 100 fs, in TS-I and CS-II, respectively. The finding is consistent with the assignment of occupied cages: principally 51262 polyhedra in TS-I and 51264 polyhedra in CS-II. In liquid water, with diffuse cages, the distribution characterized by tm = 555 fs and σ = 400 fs, is strongly skewed (γ1 = 1.88) toward delayed recombination—the effective liquid phase hydration shell is larger than that in a hydrate phase, structurally disordered, and anelastic. Information about dipolar disorder, comparable in all three media, is extracted from electronic predissociation rates of the B-state, which is sensitive to the symmetry in the guest–host interaction.