Single Molecules under High Pressure

Single-molecule Raman spectra were studied at high pressure (1–4 GPa) in a diamond-anvil cell (DAC) with an Ar hydrostatic pressure medium, with the intent of resolving the different pressure-induced vibrational blueshifts of individual molecules. The molecules were two isotopologues of the dye rhodamine 6G (R6G and d4-R6G), adsorbed on colloidal Ag particles immobilized in poly(vinyl alcohol) (PVA). Surface-enhanced Raman (SERS) ensemble measurements were compared to single-molecule surface-enhanced Raman (SMSERS) measurements made in a confocal Raman microscope. Spectra of mixed isotopologues in the 610 cm–1 region (the “isotope-sensitive” transition) allowed us to identify when the majority of spectra came from single-isotope sites, and were thereby statistically likely to arise from single molecules. There was a dramatic drop in SERS intensity when samples were pressurized in the DAC. SMSERS measurements revealed the intensity drop was caused by a pressure-induced destruction of SMSERS-active hot spot...