Phase-Delay Rabi-Flopping Spectroscopy:  A Method Sensitive to Rydberg Species at Surfaces

Spectra observed with grating spectrometers from surface layers on alkali-promoted catalysts are studied with a confocal microscope setup, focusing to a nominal resolution of a few micrometers. The spectra are quite similar to Raman spectra but are caused by phase-delayed light with no wavelength shift. Strong effects related to circularly polarized light are observed, supporting the phase-delay process. That information from the surface layer is obtained is proved since the spectra vary over the emitter surface and with the microscope distance to the surface. The theory for phase-delay spectroscopy is developed. In the anti-Stokes range, the spectral shifts correspond to energy differences between levels connected by forbidden transitions, for example, between a high Rydberg level and the ground state in the K atom. Transitions mediated by Rydberg species are instead observed in adsorbed molecules in the Stokes range.