Author: צ'ונטונוב אולגה

Surface-Enhanced Dual-Frequency Two-Dimensional Vibrational Spectroscopy of Thin Layers at an Interface

The development of spectroscopic approaches to study molecules at interfaces is important as the molecular properties often differ from those in the bulk. Implementation of surface-enhanced two-dimensional infrared (SE 2DIR) spectroscopy using lithographically fabricated plasmonic nanoarrays is demonstrated for nanometer thick films. The sample, 4-azidobutyrate-N-hydroxysuccinimide ester (azNHS), dispersed in polystyrene was deposited onto the nanoarray. Raw enhancements in the SE 2DIR spectra exceeding 5 × 104 and 1.3 × 103 fold were achieved for the CO and NN peaks, respectively. The field enhancement provided by the nanoarray was sufficient to record cross-peaks in 1 nm thick samples under dilute conditions for azNHS (∼0.1 M). Note that the cross-peaks were recorded for vibrational modes frequency separated by ∼350 cm–1 with the enhancement factor of 4.1 × 104. The effective electric field enhancement factors, measured for NN and CO modes via linear and two nonlinear IR techniques, have similar sample-thickness dependences, which permit using linear spectroscopy for enhancement evaluation. High-quality cross-peak waiting-time dependences were recorded for samples as thin as 1 nm involving several IR reporters demonstrating the applicability of an arsenal of 2DIR approaches, including spectral diffusion, chemical exchange, and relaxation-assisted 2DIR, to interrogate samples in nanometer thick films. The study opens new opportunities in analyzing structures and dynamics of molecules at interfaces.
Authors: Robert T Mackin, Bar Cohn, Andrey Gandman, Joel D Leger, Lev Chuntonov, Igor V Rubtsov

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Communication: Probing the interaction of infrared antenna arrays and molecular films with ultrafast quantum dynamics

Narrowband vibrational molecular transitions interacting with the broadband resonance of infrared plasmonic antennas lead to Fano lineshapes observed in linear (FTIR) and third-order (transient absorption and 2DIR) spectroscopic experiments. Both molecular and plasmonic components are inherently dissipative, and the effects associated with their coupling can be observed, in principle, when measuring the corresponding ultrafast quantum dynamics. We used 2DIR spectroscopy to study the waiting time evolution of quantum coherence excited in the carbonyl stretching modes of rhodium (acetylacetonato) dicarbonyl molecules, which were embedded in an 80 nm-thick polymer film spin-coated on an array of infrared half-wavelength gold antennas. Despite the pronounced Fano lineshapes obtained for the molecular transitions, and up to a four order of magnitude enhancement of the third-order signals, which taken together, indicate the coupling between the plasmonic and molecular transitions, the dynamics of the quantum coherence were identical to that obtained with 3 μm-thick film without the interaction with the plamson mode. This suggests that the coupling rate between the molecular and plasmonic excitations is significantly smaller than the relaxation rates of the molecular excitations monitored in the experiment. Here, the Fano lineshape, observed at the frequency of the molecular transition, can result from the mutual radiation damping of the molecular and plasmon modes.
Authors: Bar Cohn, Amit K Prasad, Lev Chuntonov

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Two-dimensional Fano lineshapes in ultrafast vibrational spectroscopy of thin molecular layers on plasmonic arrays

Two-dimensional femtosecond infrared (2DIR) spectroscopy routinely provides insights into molecular structure and ultrafast dynamics in 1–100 μm thick bulk samples. Confinement of molecules to surfaces, gaps, crevices, and other topographic features, frequently encountered on the nanometer length scale, significantly alters their structure and dynamics, affecting physical and chemical properties. Amplification of 2DIR signals by the plasmon-enhanced fields around metal nanostructures can permit structural and dynamics measurements of the confined molecules. Fano resonances, induced by the interaction between laser pulses, plasmon, and vibrational modes significantly distort 2D lineshapes. For different detuning from plasmon resonance, the interference between multiple signal components leads to different line shape asymmetry, which we demonstrate on a set of linear absorption, transient absorption, and 2DIR spectra. An intuitive model used to describe experimental data points to the interference’s origin. Our results will facilitate the application of surface-enhanced 2DIR spectroscopy for studies of molecular structure and dynamics in a nanoconfined environment.
Authors: Andrey Gandman, Robert Mackin, Bar Cohn, Igor V Rubtsov, Lev Chuntonov

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A Spin-Boson Screening approach for unraveling dominant vibrational energy transfer pathways in molecular materials

Vibrational energy transfer driven by anharmonicity is the major mechanism of energy dissipation in polyatomic molecules and in molecules embedded in condensed phase environment. Energy transfer pathways are sensitive to the particular intra-molecular structure as well as to specific interactions between the molecule and its environment, and their identification is a challenging many-body problem. This work introduces a theoretical approach which enables to identify the dominant pathways for specified initial excitations, by screening the different possible relaxation channels. For each channel, the many-body Hamiltonian is mapped onto a respective all-vibrational Spin-Boson Hamiltonian, expressed in terms of the harmonic frequencies and the anharmonic coupling parameters obtained from the electronic structure of the molecule in its environment. A focus is given on the formulation of the relaxation rates when different limits of perturbation theory apply. In these cases the proposed Spin-Boson Screening approach becomes especially powerful.
Authors: Lev Chuntonov, Uri Peskin

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Vacuum Rabi splitting in a plasmonic cavity at the single quantum emitter limit

The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light–matter interactions. Plasmonic cavities hold the promise of attaining the strong coupling regime even under ambient conditions and within subdiffraction volumes. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules; however, strong coupling at the limit of a single quantum emitter has not been reported so far. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). A transparency dip is observed in the scattering spectra of individual bowties with one to a few QDs, which are directly counted in their gaps. A coupling rate as high as 120 meV is registered even with a single QD, placing the bowtie-QD constructs close to the strong coupling regime. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations.
Authors: Kotni Santhosh, Ora Bitton, Lev Chuntonov & Gilad Haran

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2D-IR spectroscopy of hydrogen-bond-mediated vibrational excitation transfer

Vibrational excitation transfer along the hydrogen-bond-mediated pathways in the complex of methyl acetate (MA) and 4-cyanophenol (4CP) was studied by dual-frequency femtosecond two-dimensional infrared spectroscopy. We excited the energy-donating ester carbonyl stretching vibrational mode and followed the transfer to the energy-accepting benzene ring and cyano stretching vibrations. The complexes with no, one, and two hydrogen-bonded 4CP molecules were studied. Vibrational relaxation of the carbonyl mode is more efficient in both hydrogen-bonded complexes as compared with free MA molecules. The inter-molecular transport in a hydrogen-bonded complex involving a single 4CP molecule is slower than that in a complex with two 4CP molecules. In the former, vibrational relaxation leads to local heating, as shown by the spectroscopy of the carbonyl mode, whereas the local heating is suppressed in the latter because the excitation redistribution is more efficient. At early times, the transfer to the benzene ring is governed by its direct coupling with the energy-donating carbonyl mode, whereas at later times intermediate states are involved. The transfer to a more distant site of the cyano group in 4CP involves intermediate states at all times, since no direct coupling between the energy-donating and accepting modes was observed. We anticipate that our findings will be of importance for spectroscopic studies of bio-molecular structures and dynamics, and inter- and intra-molecular signaling pathways, and for developing molecular networking applications.
Authors: Lev Chuntonov

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Kinetics of Exchange between Zero-, One-, and Two-Hydrogen-Bonded States of Methyl and Ethyl Acetate in Methanol

It has recently been shown that the ester carbonyl stretching vibration can be used as a sensitive probe of local electrostatic field in molecular systems. To further characterize this vibrational probe and extend its potential applications, we studied the kinetics of chemical exchange between differently hydrogen-bonded (H-bonded) ester carbonyl groups of methyl acetate (MA) and ethyl acetate (EA) in methanol. We found that, while both MA and EA can form zero, one, or two H-bonds with the solvent, the population of the 2hb state in MA is significantly smaller than that in EA. Using a combination of linear and nonlinear infrared measurements and numerical simulations, we further determined the rate constants for the exchange between these differently H-bonded states. We found that for MA the chemical exchange reaction between the two dominant states (i.e., 0hb and 1hb states) has a relaxation rate constant of 0.14 ps–1, whereas for EA the three-state chemical exchange reaction occurs in a predominantly sequential manner with the following relaxation rate constants: 0.11 ps–1 for exchange between 0hb and 1hb states and 0.12 ps–1 for exchange between 1hb and 2hb states.
Authors:Lev Chuntonov, Ileana M. Pazos, Jianqiang Ma, Feng Gai

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The simplest plasmonic molecules: Metal nanoparticle dimers and trimers

This review discusses research on the plasmonic properties of small clusters of metal nanoparticles, with two and three particles. These are the simplest examples of ‘plasmonic molecules’. Coupling between two particles leads to new surface plasmon resonances and to the creation of a hot spot of a strong electric field in the gap between the particles. Such a hot spot can be used to enhance Raman scattering or fluorescence, making plasmonic dimers useful for applications in spectroscopy and sensing. Trimers offer a richer spectrum of options for coupling between particles, which can be analyzed using group theory to obtain the plasmonic modes, in analogy to vibrational modes. Symmetry plays an important role in our understanding of the physics of plasmonic dimers and trimers, and new physical phenomena may appear when the symmetry of a dimer or a trimer is broken, including directional emission, Fano resonances, plane chirality and more. The review introduces some of these concepts, the basic physics behind them and their possible applications. Focus Point sections describing selected outstanding recent developments accompany the review.

Authors:Nir Zohar, Lev Chuntonov, Gilad Haran

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Non-linear infrared spectroscopy of the water bending mode: direct experimental evidence of hydration shell reorganization?

The structure and dynamics of liquid water are further studied by investigating the bend vibrational mode of HDO/D2O and pure H2O via two-dimensional infrared spectroscopy (2D-IR) and linear absorption. The experimental findings and theoretical calculations support a picture in which the HDO bend is localized and the H2O bend is delocalized. The HDO and H2O bends present a loss of the frequency–frequency correlation in subpicosecond time scale. While the loss of correlation for the H2O bend is likely to be associated with the vibrational dynamics of a delocalized transition, the loss of the correlation in the localized HDO bend appears to arise from the fluctuations/rearrangements of the local environment. Interestingly, analysis of the HDO 2D-IR spectra shows the presence of multiple overlapping inhomogeneous distributions of frequencies that interchange in a few picoseconds. Theoretical calculations allow us to propose an atomistic model of the observed vibrational dynamics in which the different inhomogeneous distributions and their interchange are assigned to water molecules with different hydrogen-bond states undergoing chemical exchange. The frequency shifts as well as the concentration of the water molecules with single and double hydrogen-bonds as donors derived from the theory are in good agreement with our experimental findings.

Authors:Lev Chuntonov, Revati Kumar and Daniel G. Kuroda

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Effect of Nanoparticle Symmetry on Plasmonic Fields: Implications for Single‐Molecule Raman Scattering

The strong electromagnetic field localization required for the observation of single‐molecule surface‐enhanced Raman scattering (SMSERS) is readily obtained when the studied molecule is adsorbed inside the gap between closely adjacent nanoparticles. The interactions of adsorbed molecules with the electric field of light are influenced by the nanoparticle arrangement. This chapter discusses a series of spectroscopic and theoretical studies to better understand the mechanisms behind such interactions. It presents a systematic summary of the plasmon mode picture of the smallest nanoparticle clusters that are relevant to the discussion: dimers and trimers of spherical silver nanoparticles. The chapter shows how the shape of the plasmon spectrum of a cluster can be correlated with the shape of the Raman spectrum of an adsorbed molecule. It examines the relation between the polarization of the Raman‐scattered light and the plasmon mode structure of the cluster.

Authors: Lev Chuntonov, Gilad Haran

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