Dephasing-Induced Vibronic Resonances in Difference Frequency Generation Spectroscopy

Abstract

The difference frequency generation (DFG) signal from a two electronic level system with vibrational modes coupled to a Brownian oscillator bath is computed. Interference effects between two Liouville space pathways result in pure-dephasing-induced, excited-state resonances provided the two excitation pulses overlap and time ordering is not enforced. Numerical simulations of two-dimensional DFG signals illustrate how the ground and excited electronic state resonances may be distinguished.

Theoretical Modeling of Interface Specific Vibrational Spectroscopy: Methods and Applications to Aqueous Interfaces

“…16,22,[50][51][52][53] However, there is growing interest in using both time domain, mixed time, and frequency domain approaches, [46][47][48][49] as well as other second-order processes such as difference frequency generation (DFG) spectroscopy. [61][62][63][64] The theoretical methods, which will be discussed below, are capable of describing any of these second-order processes. Thus, before specializing the theoretical expressions to the typical monochromatic frequency domain experiment, it is helpful to examine the formal theoretical structure of second-order nonlinear processes.…”

“…61 This is accomplished by identifying terms in the response function that oscillate in time so as to phase cancel with those from the applied fields and by subsequently discarding the remaining terms. This is called the RWA.…”

“…Computationally, this approximation allows for inclusion of only fully resonant Louiville space pathways and depends implicitly on the reference or model system being considered. 55,61,65 The RWA is a computational convenience; it is possible to include the entire response function and perform the integration in eq 3 (or presented specifically for second-order processes such as SFG in eq 5 below) explicitly. 67 Note that a particular experiment measures either the modulus of the complex P (N) (k s , t) (homodyne detection) or its' real or imaginary parts (heterodyne detection).…”

Laser-Controlled Fluorescence in Two-Level Systems

“…a two-state model may be applied. 15,16,18,20,[22][23][24][25][26][27][28][43][44][45][46][47][48][49][50][51][52] To be clear, the assumption is that the character of the fluorescence emission process, including the effect of the probe radiation, is dominated by two electronic levels; it is not to be presumed that the state from which the fluorescence decay occurs is necessarily the same as the state initially populated by photoexcitation. …”

Partially-time-ordered Schwinger-Keldysh loop expansion of coherent nonlinear optical susceptibilities

“…Upon the substitution of these results in Eqs. (13) or (16) and (17) we can calculate χ (3) . Using the Brownian oscillator model for the correlations function we have [8]…”