Surface Plasmon Polaritons Enable Reaction‐Specific Energy Transduction for Molecular Ionization and Isomer Discrimination

ABSTRACT Efficient and controllable conversion of optical energy into chemical energy at interfaces remains a fundamental challenge in photochemistry and catalysis. Here we show that surface plasmon polaritons (SPPs) enable reaction‐specific energy transduction for molecular ionization by regulating energy partitioning between non‐equilibrium hot carriers and electron–phonon coupling. Using propagating SPPs excited in a back‐side plasmon excitation geometry, hot carriers drive molecular ionization while phonon‐mediated energy governs desorption, decoupling ionization from thermal dissociation. Consequently, both nonpolar molecules undergoing electron‐removal ionization and polar molecules undergoing proton‐coupled ionization are efficiently ionized under ambient conditions, enabling the analysis of analytes ranging from p ‐terphenyl to cytochrome C with an absolute limit of detection of 400 amol. By tuning the incident optical energy, selective ionization enables direct discrimination of structural, positional, and stereochemical epimeric isomers with adsorption energy differences as small as 0.12 eV, establishing an SPP laser desorption ionization mass spectrometry (SPP‐LDI MS) platform for isomer discrimination. More broadly, this work transforms molecular ionization from stochastic energy deposition into a programmable interfacial energy transduction process, opening new opportunities for precision energy control and isomer‐resolved chemical analysis at interfaces.