Detecting Collective Molecular Vibrations with Surface-Enhanced Raman Spectroscopy

Niclas Mueller Fritz-Haber Institute

In surface-enhanced Raman scattering (SERS), vibrations of molecules couple with optical modes of a plasmonic nanocavity via a molecular optomechanical interaction. This eLect is typically assumed to occur at individual molecules neglecting intermolecular vibrational coupling. Here, we show instead how collective vibrations are observed in SERS through two diLerent mechanisms: 1. Molecules can couple into collective vibrations through direct infrared (IR) dipole coupling, which is usually observed with IR spectroscopy. We show that cooperative frequency shifts from these collective IR vibrations can be also detected with SERS [1]. These collective states can be tuned with mixed self-assembled monolayers and allow us to monitor photochemical reactions. 2. Molecules can also form collective vibrations through the coupling of induced Raman dipoles when driven with intense laser pulses [2]. We show how such collective Raman vibrations enhance the optomechanical coupling with plasmonic nanocavities and reduce the onset of vibrational nonlinearities.

Finally, I will show recent experiments where we probe the lifetime of molecular vibrations in plasmonic nanocavities with ultrafast Raman spectroscopy [3]. Using time-resolved coherent and incoherent anti-Stokes Raman spectroscopy we separate the contributions of vibrational dephasing and population decay, and isolate molecular signals from four-wave mixing.

[1] Mueller et al. Collective Mid-Infrared Vibrations in Surface-Enhanced Raman Scattering, Nano Lett. 22, 7254 (2022)

[2] Jakob, Juan-Delgado, Mueller et al. Optomechanical Pumping of Collective Molecular Vibrations in Plasmonic Nanocavities, submitted (2024)

[3] Jakob, Deacon et al. Accelerated molecular vibrational decay and suppressed electronic nonlinearities in plasmonic cavities through coherent Raman scattering, Phys. Rev. B 109, 195404 (2024)