Realizing in situ simultaneous measurement of full vibrational modes in tip-enhanced Raman spectroscopy (TERS) is of immense importance for improving the ability of quantitative analysis and measurement accuracy of the molecular spectrum. In this study, the electric field gradient effect in the TERS system was theoretically investigated by using the finite element method. The respective contributions of electric field and electric field gradient to dipole Raman and gradient Raman modes in the TERS spectrum were quantitatively distinguished. By comparing the relative intensity ratio between electric field gradient and electric field, the TERS configuration could be optimized to achieve a maximum of gradient Raman modes. Theoretical results indicate that molecular symmetries strongly influence the molecular hyperpolarizability, which significantly influences the plasmonic electric field gradient effects. Our theoretical results could reveal the contributions of electric field and electric field gradient to molecular vibration modes, useful for the design of the TERS platform with more spectrum information.
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