Raman optical activity

Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on the difference in intensity of Raman scattered right and left circularly polarised light due to molecular chirality. History of Raman optical activity The field began with the doctoral work of Laurence D. Barron with Peter Atkins at the University of Oxford and was later further developed by Barron with David Buckingham at the University of Cambridge. More developments, including important contributions to the development of practical Raman optical activity instruments, were made by Werner Hug of the University of Fribourg, and Lutz Hecht with Laurence Barron at the University of Glasgow. Theory of Raman optical activity The basic principle of Raman optical activity is that there is interference between light waves scattered by the polarizability and optical activity tensors of a chiral molecule, which leads to a difference between the intensities of the right- and left-h

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Chiral control of spin-crossover dynamics in Fe (II) complexes

Majed Chergui, Jérôme Lacour, Malte Oppermann

Iron-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin state to be kinetically stable-a feature that has been achieved only at cryogenic temperatures. Here we demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4'-dimethyl-2,2'-bipyridine) in solution, associated for stereocontrol with the enantiopure Delta- or Lambda-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-(KO1)-O-2,O-2)phosphorus(V) (P(O2C6Cl4)(3)(-) or TRISPHAT) anions. We characterize the high-spin-state relaxation using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption and anisotropy measurements. We find that the high-spin-state decay is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, our results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics.

NATURE PORTFOLIO2022

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Molecular cavity optomechanics as a theory of plasmon-enhanced Raman scattering

Christophe Marcel Georges Galland, Tobias Kippenberg, Nicolas Piro Mastracchio

The exceptional enhancement of Raman scattering by localized plasmonic resonances in the near field of metallic nanoparticles, surfaces or tips (SERS, TERS) has enabled spectroscopic fingerprinting down to the single molecule level. The conventional explanation attributes the enhancement to the subwavelength confinement of the electromagnetic field near nanoantennas. Here, we introduce a new model that also accounts for the dynamical nature of the plasmon-molecule interaction. We thereby reveal an enhancement mechanism not considered before: dynamical backaction amplification of molecular vibrations. We first map the system onto the canonical Hamiltonian of cavity optomechanics, in which the molecular vibration and the plasmon are parametrically coupled. We express the vacuum optomechanical coupling rate for individual molecules in plasmonic 'hot -spots' in terms of the vibrational mode's Raman activity and find it to be orders of magnitude larger than for microfabricated optomechanical systems. Remarkably, the frequency of commonly studied molecular vibrations can be comparable to or larger than the plasmon's decay rate. Together, these considerations predict that an excitation laser blue-detuned from the plasmon resonance can parametrically amplify the molecular vibration, leading to a nonlinear enhancement of Raman emission that is not predicted by the conventional theory. Our optomechanical approach recovers known results, provides a quantitative framework for the calculation of cross -sections, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode -selective enhancements. It also provides a quantum mechanical framework to analyse plasmon-vibrational interactions in terms of molecular quantum optomechanics.

Nature Publishing Group2015

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Raman scattering intensities in alpha-quartz: A first-principles investigation

Alfredo Pasquarello

Using a first-principles density functional approach, we calculate the first-order Raman intensities of alpha -quartz. The dynamical charge tensors, vibrational frequencies and eigenmodes, and polarizability tensors are obtained within a perturbational approach. We calculate Raman intensities by evaluating the variation of the polarizability tensors for finite displacements of the atoms. Calculated intensities agree well with experimental data, showing an average error of 13% for relative intensities. Using our first-principles results as reference, we critically examine simple models for the Raman activity. We first consider a bond polarizability model, for which the parameters are derived from our first-principles results for alpha -quartz. This model reproduces the first-principles intensities with an average error of 15%. In the attempt of reducing this error, we then introduce a model in which the symmetry of the first neighbor shell is accounted for in the most general way. For alpha -quartz, this model extends the bond polarizability model, which is recovered as a special case. The model, which fully accounts for the local symmetry, describes the first-principles results within an average error of 12%, marginally improving upon the bond polarizability model (15%). However, when these models with parameters derived for alpha -quartz are applied to a cristobalite polymorph, only the bond polarizability model shows good transferability properties. These results support the use of the bond polarizability model as a simple scheme for calculating Raman intensities in tetrahedrally bonded SiO2 systems.

2001

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Dichroïsme circulaire

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Vibrational circular dichroism

Vibrational circular dichroism (VCD) is a spectroscopic technique which detects differences in attenuation of left and right circularly polarized light passing through a sample. It is the extension of

PHYS-207(c): General physics : quanta

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PHYS-470: Nonlinear optics for quantum technologies

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