Correlation-Enabled Beatings in Two-Dimensional Electronic Spectroscopy

Abstract

Long-lived beatings in two-dimensional electronic spectroscopy (2DES) remain difficult to interpret within standard excitonic open-system models, which typically assume factorized initialization and predict rapid coherence decay. We show that persistent beatings can arise from a correlation-driven mechanism that requires both slow bath memory and ultrafast pulse sequences that propagate system-bath correlations across optical interactions. In this regime, the pulse sequence unitarily dresses the bath-memory contribution and activates nonsecular population-coherence transfer during field-free evolution, sustaining coherence signatures far beyond factorized or weak-memory descriptions. Rather than addressing what is oscillating (excitonic versus vibronic) or quantum-versus-classical semantics, this work reframes long-lived beatings as a protocol-level dynamical effect: correlation-mediated retrieval under ultrafast control.

Figures (5)

• Figure 1: Schematic: factorized single-map dynamics yields only population--population and coherence--coherence channels ($\mathcal{D}_S$), while pulse-dressed bath memory adds $\mathcal{D}_{\mathrm{mem}}$ and activates nonsecular population--coherence transfer. In either the weak-memory or factorized limit, $\mathcal{D}_{\mathrm{mem}}\!\approx\!0$.
• Figure 2: Sub-Ohmic slow bath ($s=0.9$) with correlation-aware dynamics. (a) Absorptive 2D spectrum at $T=10$ fs; the marked cross peak defines the amplitude observable $A_{\mathrm{CP}}$. (b) Waiting-time cross-peak amplitude $A_{\mathrm{CP}}(T)$ exhibiting persistent oscillations. (c) Fourier spectrum of $A_{\mathrm{CP}}(T)$ showing a dominant, spectrally concentrated beating component.
• Figure 3: Ohmic bath ($s=1$) with correlation-aware dynamics (weak-memory case). (a) Absorptive 2D spectrum at $T=10$ fs. (b) $A_{\mathrm{CP}}(T)$ showing strongly reduced oscillatory behavior relative to Fig. \ref{['fig:slowbath_CA']}. (c) Fourier spectrum of $A_{\mathrm{CP}}(T)$ without a comparably sharp peak indicating a persistent beating component.
• Figure 4: Sub-Ohmic slow bath ($s=0.9$) without correlation-aware propagation (factorized reset at pulse boundaries). (a) Absorptive 2D spectrum at $T=10$ fs. (b) $A_{\mathrm{CP}}(T)$ shows strongly reduced oscillatory structure relative to the correlation-aware case, indicating that slow bath memory alone is insufficient without pulse-induced dressing of pre-existing system--bath correlations. (c) Fourier spectrum of $A_{\mathrm{CP}}(T)$ without a comparably sharp beating component.
• Figure 5: Structured low-frequency bath. Representative three-panel summary for the structured-bath model defined in Eqs. \ref{['eq:J_structured_def']}--\ref{['eq:J_lin_tail']}, shown in the same format as the canonical spectral-density cases. (a) Absorptive 2D spectrum at $T=10$ fs; the marked cross peak defines the amplitude observable $A_{\mathrm{CP}}$. (b) Waiting-time cross-peak amplitude $A_{\mathrm{CP}}(T)$ exhibiting pronounced oscillatory structure. (c) Fourier spectrum of $A_{\mathrm{CP}}(T)$ showing a dominant peak near $200~\mathrm{cm}^{-1}$, consistent with the excitonic energy splitting. The re-emergence of beating structure reflects the restoration of slow bath memory via low-frequency spectral structure, rather than resonant vibronic matching.