Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy with\nPicosecond Excitation

Abstract

Tip-enhanced Raman spectroscopy (TERS) provides chemical information\nabout adsorbates with nanoscale spatial resolution, but developments\nare still required in order to incorporate ultrafast temporal resolution.\nIn this Letter, we demonstrate that a reliable TER signal of rhodamine\n6G (R6G) using picosecond (ps)-pulsed excitation can be obtained in\nultrahigh vacuum (UHV). In contrast to our previous observation of\nirreversible signal loss in ambient TERS (Klingsporn, J. M.; Sonntag, M. D.; Seideman, T.; Van Duyne, R. P. J. Phys. Chem. Lett. 2014, 5, 106−110), we demonstrate that the UHV environment\ndecreases irreversible signal degradation. As a complement to the\nTERS experiments, we examined the rate of surface-enhanced Raman (SER)\nsignal decay under picosecond irradiation and found that it is also\nslowed in UHV compared to that in ambient. Signal decay kinetics suggest\nthat the predominant mechanism responsible for signal loss in ps SERS\nof R6G is surface diffusion. Both diffusive and reactive phenomena\ncan lead to pulsed excitation TER signal loss, and a UHV environment\nis advantageous in either scenario.