Non-Condon Effects on the Doubly Resonant Sum Frequency Generation of Rhodamine 6G

Abstract

We report first-principles simulations of the doubly resonance sum-frequency generation (DR-SFG) spectrum for rhodamine 6G (R6G). The simulations are done using a time-dependent formalism that includes both Franck-Condon (FC) and Herzberg-Teller (HT) terms in combination with time-dependent density functional theory (TDDFT) calculations. The simulated spectrum matches experiments, allowing a detailed assignment of the DR-SFG spectrum. Our work also shows that non-Condon effects are important and the DR-SFG spectrum of R6G is highly dependent on both FC and HT modes. This is surprising as R6G is known to be a strong FC resonant Raman scatterer. The simulations predict an orientation where the xanthene plane of R6G is perpendicular to the surface with binding through one of the ethyl amine groups. Our results show the importance of first-principles simulations for providing a detailed assignment of DR-SFG experiments, especially for large molecules where such an assignment is complicated due close-lying vibrational modes.

Doubly resonant SFG and DFG spectroscopies: An analytic model for data analysis including distorted and rotated vibronic levels. I. Theory

“…As a consequence, all molecular approaches of DR-FG processes converge to a common formalism which can be introduced either through the conventional SOS method 69 or by a time correlator in a TT or TDM formulation. 70 This derivation forms the basis of conventional interpretation of DR-FG data in the literature. 20,22,23 To a lesser extent than for RRS, extensions have been introduced in DR-FG beyond the conventional approximations.…”

“…20,22,23 To a lesser extent than for RRS, extensions have been introduced in DR-FG beyond the conventional approximations. The main approaches focus on the inclusion of non-Condon 28,70 and non-adiabatic 71,72 terms, in particular to account for the specific properties of DR-SFG from chiral liquids. As for quadratic coupling, one formulation of the DR-SFG response including mode distortion and mode mixing in the TT frame has been published.…”

“…Introduction of non-Condon 83 and quadratic coupling terms 84 follows naturally, and a methodology for a complete vibronic structure calculation by first principle methods is now available. 48 Calculation of DR-SFG spectra by configuration interaction 85 , DFT/TD-DFT 70 or DFT/CCSD 27 have appeared more recently but start becoming a routine way to analyze experimental data.…”

All-experimental analysis of doubly resonant sum-frequency generation spectra: Application to aggregated rhodamine films

“…In addition to their interest in device development, rhodamines have often been used as probes or benchmark molecules for new resonant optical spectroscopies in the visible, or to compare theoretical predictions to experiments in surface-enhanced 18 (SERS) and tip-enhanced 19 (TERS) Raman scattering, hyper-Raman scattering, 20 surface enhanced hyper-Raman scattering, 21,22 twophoton fluorescence, 23 hyper-Rayleigh scattering, 24 stimulated Raman spectroscopy, 25 single molecule SERS 26 and TERS, 27 and plasmonic coupling. 28 As for second order nonlinear optics, rhodamines have been choice molecules for studying the structure, aggregation properties and optical response of adsorbed thin films using second harmonic generation 29 (SHG) and sum-frequency generation (SFG), [30][31][32] even before the measurement of the first SFG photon. 33 In order to optimize the conversion of solar energy into electricity, the interaction between the dyes and the surface of the photo-electrode must be optimized.…”

Fully Polarizable QM/Fluctuating Charge Approach to Two-Photon Absorption of Aqueous Solutions

“…In particular, we have selected the challenging case of rhodamine 6G (R6G) in aqueous solution, which has been studied extensively both theoretically and experimentally. 16,[91][92][93][94][95][96][97] The large interest in such a molecule -in particular to its TPA spectrum -is due to the transition between the ground and second excited state, which is dark in OPA due to symmetry selection rules.…”