Monitoring Vibrational Evolution in Jahn-Teller Effect by Raman Imaging

The Jahn-Teller effect (JTE) reduces the geometrical symmetry of a system with degenerate electronic states via vibronic coupling, playing a pivotal role in molecular and condensed systems. In this Letter, we propose that vibrational resolved tip-enhanced Raman scattering images can visualize the vibrational evolutions in JTE in real space. Taking an experimentally viable single zinc phthalocyanine (ZnPc) molecule as a proof-of-principle example, not only the degenerate vibrational splitting but also the overlooked vibration mixing caused by the JTE in its anionic form can be straightforwardly characterized by Raman images. Leveraging Raman images, the controllable configuration of JTE distortion with partial isotopic substitution could be further identified. These findings establish a practical protocol to monitor the detailed vibrational evolutions when a single molecule experiences JTE, opening a door for visualization of spontaneous symmetry breaking in molecular and solid-state systems.

💡 Research Summary

In this work the authors present a comprehensive theoretical study demonstrating that tip‑enhanced Raman scattering (TERS) imaging can directly visualize the vibrational evolution associated with the Jahn‑Teller effect (JTE) at the single‑molecule level. Using a zinc‑phthalocyanine (ZnPc) molecule as a realistic test case, they first show that first‑principles calculations of both ordinary Raman and TERS spectra for the neutral D₄h‑symmetric ZnPc and its anionic D₂h‑symmetric counterpart (ZnPc⁻) reproduce the experimentally observed frequency shifts (≈ 39 cm⁻¹ red‑shift and ≈ 26 cm⁻¹ blue‑shift) when a 532 nm laser and a highly localized 3 Å plasmonic field are employed. This agreement validates the electronic and vibrational structures used in the subsequent analysis.

The authors then compute spatially resolved Raman images for the dominant vibrational modes. For the neutral molecule the E_u modes (ν₁₀₈, ν₁₀₅) generate four‑fold symmetric intensity patterns, reflecting the D₄h point‑group symmetry. After electron injection, the JTE lowers the symmetry to D₂h, splitting the degenerate E_u mode into two non‑degenerate B₂u and B₃u components (ν′₁₀₈a, ν′₁₀₈b). The corresponding TERS images display two‑fold symmetry with distinct intensity distributions, directly visualizing the spatial localization of the split vibrations. By positioning the plasmonic hotspot over different nitrogen atoms, the authors demonstrate that one component can be selectively enhanced while the other is suppressed, providing a practical route to identify each split mode experimentally.

Beyond degenerate mode splitting, the study uncovers a previously overlooked phenomenon: mixing of originally non‑degenerate vibrations after symmetry reduction. Using the A₂g mode ν₂₃ and the B₂g mode ν₆₅ as an example, they show that JTE‑induced electronic coupling mixes these vibrations into two B₁g modes (ν′₂₃, ν′₆₅). A simple analytical expression, T = λ S₁₂² / ΔE, is derived, where S₁₂ quantifies the spatial overlap of the two vibrational displacement patterns, ΔE is their energy separation, and λ is a fitting parameter (found to be ≈ 2.8 cm⁻¹). The model accurately reproduces the mixing coefficients extracted from the TERS images, confirming that strong overlap (S₁₂ ≈ 1) leads to pronounced mixing inversely proportional to ΔE, whereas zero overlap (S₁₂ = 0) suppresses mixing entirely. This quantitative framework provides a clear criterion for predicting when vibrational mixing will be observable in TERS.

Finally, the authors explore controllable JTE distortions via partial isotopic substitution. By replacing selected carbon atoms with ¹³C and peripheral hydrogen atoms with deuterium along one diagonal of ZnPc⁻, they generate two isomers (iso‑1 and iso‑2) that differ only in the direction of the elongation caused by JTE. Free‑energy calculations at 7 K and ultra‑low pressure predict a ΔG of 0.38 kJ mol⁻¹ favoring iso‑1, corresponding to a 99.84 % Boltzmann population. Although the electronic structures of the two isomers are virtually identical, their vibrational modes exhibit subtle mass‑dependent shifts. TERS images of selected modes (e.g., ν′₁₂₁a, ν′₁₂₁b) reveal additional moderate intensity lobes along the isotopically modified axis in one isomer but not the other, enabling unambiguous discrimination of the two configurations.

In summary, the paper establishes that vibrationally resolved TERS imaging can (i) resolve the splitting of degenerate vibrations caused by JTE, (ii) detect and quantify mixing between different irreducible representations, and (iii) identify isotopically engineered JTE distortion directions. By providing a practical, optical method to monitor symmetry breaking at the single‑molecule level, this work opens new avenues for studying and controlling electron‑phonon interactions in molecular electronics, catalysis, and solid‑state materials.