Raman scattering mediated by neighboring molecules

Abstract

Raman scattering is most commonly associated with a change in vibrational state within individual molecules, the corresponding frequency shift in the scattered light affording a key way of identifying material structures. In theories where both matter and light are treated quantum mechanically, the fundamental scattering process is represented as the concurrent annihilation of a photon from one radiation mode and creation of another in a different mode. Developing this quantum electrodynamical formulation, the focus of the present work is on the spectroscopic consequences of electrodynamic coupling between neighboring molecules or other kinds of optical center. To encompass these nanoscale interactions, through which the molecular states evolve under the dual influence of the input light and local fields, this work identifies and determines two major mechanisms for each of which different selection rules apply. The constituent optical centers are considered to be chemically different and held in a fixed orientation with respect to each other, either as two components of a larger molecule or a molecular assembly that can undergo free rotation in a fluid medium or as parts of a larger, solid material. The two centers are considered to be separated beyond wavefunction overlap but close enough together to fall within an optical near-field limit, which leads to high inverse power dependences on their local separation. In this investigation, individual centers undergo a Stokes transition, whilst each neighbor of a different species remains in its original electronic and vibrational state. Analogous principles are applicable for the anti-Stokes case. The analysis concludes by considering the experimental consequences of applying this spectroscopic interpretation to fluid media; explicitly, the selection rules and the impact of pressure on the radiant intensity of this process

Quantum electrodynamics in modern optics and photonics: tutorial

“…Hence, for example, many Tutorial studies of the molecular Raman effect have been pursued using QED methods; see, for example, Refs. [5,[33][34][35][36][37][38].…”

Symmetry analysis of Raman scattering mediated by neighboring molecules

“…It is appropriate to begin with an expression, whose detailed derivation has been presented previously, 5 for the radiant intensity, I', for Raman emission into a solid angle Ω', from a molecule A, incorporating all of the fundamentally permissible contributory mechanisms associated with the electronic influence of a neighboring molecule B: …”

“…5 They are long and complicated expressions, whose detail would be required in any calculations to quantify their relative contributions for particular molecules. As we shall see, it suffices here to summarize their form of participation.…”

“…For the symmetry analysis that is to be our focus, a substantial simplification can now be effected. Although the full rate equations entail the detailed form of each of the molecular response tensors introduced in our previous paper, 5 all of the symmetry information necessary to determine the modified Raman selection rules can be captured in simple icons as displayed in Table II -essentially the 'left-hand sides' of the Feynman diagrams in Fig. 1.…”

“…Such differences usually correspond to transitions between vibrational levels, while the electronic states stay the same. In recent work 5 we have developed a framework of quantum electrodynamics (QED) 6,7 to address Raman scattering influenced by a second, neighboring molecule; this is one of many situations in which such a theory of fundamental coupling is applied. [8][9][10][11][12][13][14][15][16] Our interest is not only in the modification of line intensities (beyond the familiar heterogeneous spectral broadening): we have shown that it is possible to engender additional spectra lines that are not conventionally Raman active, identifying the constraints that apply.…”