Seeing the vibrational breathing of a single molecule through time-resolved coherent anti-Stokes Raman scattering

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

Spectroscopic signatures as a probe of structure and dynamics in condensed-phase systems : studies of iodine and gold ranging from isolated molecules to nanoclusters

This thesis focuses on spectroscopic studies of several different iodine- and gold-containing molecules, complexes and nanoscopic clusters. The systems have been studied in the condensed phase with a wide range of experimental and theoretical techniques. New spectroscopic signatures have been established for molecular h in solid crystalline Xe and for isolated I2-Xe-complex in solid Kr by using spectroscopic techniques. The vibrational signatures of several neutral (I2)n aggregate complexes in solid Kr have been identified using Resonance Raman spectroscopy. Electronic and vibrational absorption signatures of thiolate-protected gold nanocluster Au102(p-MBA)44 have been characterized experimentally from UV-region to far-IR region in solid and liquid phases. Implications of the spectroscopic findings for the dynamical and structural properties of the studied systems have been argued based on density functional calculations and molecular dynamics simulations. The new experimental results confirm the predicted electronic state structure for the Au102(p-MBA)44, and identify the elementary steps of h aggregation toward the crystalline phase. The results establish a novel electronic transition for the weak van der Waals complex I2-Xe in vacuum-UV absorption and suggest a plausible explanation for the origin. The results also reveal a novel spectroscopic finding of sharp vibronic B↔X structures for condensed-phase I2. The results open several interesting possibilities for additional studies of these systems

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Electronic spectroscopy of I2-Xe complexes in solid Krypton

In the present work, we have studied ion-pair states of matrix-isolated I2 with vacuum-UV absorption and UV-vis-NIR emission, where the matrix environment is systematically changed by mixing Kr with Xe, from pure Kr to a more polarizable Xe host. Particular emphasis is put on low doping levels of Xe that yield a binary complex I2–Xe, as verified by coherent anti-Stokes Raman scattering (CARS) measurements. Associated with interaction of I2 with Xe we can observe strong new absorption in vacuum-UV, redshifted 2400 cm−1 from the X → D transition of I2. Observed redshift can be explained by symmetry breaking of ion-pair states within the I2–Xe complex. Systematic Xe doping of Kr matrices shows that at low doping levels, positions of I2 ion-pair emissions are not significantly affected by complexation with Xe, but simultaneous increase of emissions from doubly spin-excited states indicates non-radiative relaxation to valence states. At intermediate doping levels ion-pair emissions shift systematically to red due to change in the average polarizability of the environment. We have conducted spectrally resolved ultrafast pump-probe ion-pair emission studies with pure and Xe doped Kr matrices, in order to reveal the influence of Xe to I2 dynamics in solid Kr. Strikingly, relaxed emission from the ion-pair states shows no indication of complex presence. It further indicates that the complex escapes detection due to a non-radiative relaxation.peerReviewe

Rotational coherence imaging and control for CN molecules through time-frequency resolved coherent anti-Stokes Raman scattering

Numerical wave packet simulations are performed for studying coherent anti-Stokes Raman scattering (CARS) for CN radicals. Electronic coherence is created by femtosecond laser pulses between the X²Σ and B²Σ states. Due to the large energy separation of vibrational states, the wave packets are superpositions of rotational states only. This allows for a specially detailed inspection of the second- and third-order coherences by a two-dimensional imaging approach. We present the time-frequency domain images to illustrate the intra- and intermolecular interferences, and discuss the procedure to rationally control and experimentally detect the interferograms in solid Xe environment.peerReviewe

Orientation-Dependent Handedness of Chiral Plasmons on Nanosphere Dimers: How to Turn a Right Hand into a Left Hand

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

Long-Lived Electronic Coherence of Iodine in the Condensed Phase: Sharp Zero-Phonon Lines in the B↔X Absorption and Emission of I₂ in Solid Xe

Our study of B←X absorption of molecular iodine (I₂) isolated in a low-temperature crystalline xenon has revealed an exceptionally long-lived electronic coherence in condensed phase conditions. The visible absorption spectrum shows prominent vibronic structure in the form of zero-phonon lines (ZPLs) and phonon side bands (PSBs). The resolved spectrum implies weak interaction of the chromophore to the lattice degrees of freedom. The coherence extends past the vibrational period of the excited state molecule, unlike that observed in any condensed phase environment for I₂ so far. The ZP transitions from the relaxing B-state populations were resolved in the hot luminescence when the 532 nm laser was used for excitation

Effect of molecular Stokes shift on polariton dynamics

When the enhanced electromagnetic field of a confined light mode interacts with photoactive molecules, the system can be driven into the regime of strong coupling, where new hybrid light–matter states, polaritons, are formed. Polaritons, manifested by the Rabi split in the dispersion, have shown potential for controlling the chemistry of the coupled molecules. Here, we show by angle-resolved steady-state experiments accompanied by multi-scale molecular dynamics simulations that the molecular Stokes shift plays a significant role in the relaxation of polaritons formed by organic molecules embedded in a polymer matrix within metallic Fabry–Pérot cavities. Our results suggest that in the case of Rhodamine 6G, a dye with a significant Stokes shift, excitation of the upper polariton leads to a rapid localization of the energy into the fluorescing state of one of the molecules, from where the energy scatters into the lower polariton (radiative pumping), which then emits. In contrast, for excitonic J-aggregates with a negligible Stokes shift, the fluorescing state does not provide an efficient relaxation gateway. Instead, the relaxation is mediated by exchanging energy quanta matching the energy gap between the dark states and lower polariton into vibrational modes (vibrationally assisted scattering). To understand better how the fluorescing state of a molecule that is not strongly coupled to the cavity can transfer its excitation energy to the lower polariton in the radiative pumping mechanism, we performed multi-scale molecular dynamics simulations. The results of these simulations suggest that non-adiabatic couplings between uncoupled molecules and the polaritons are the driving force for this energy transfer process.peerReviewe