Stimulated Raman Adiabatic Passage enables Core-Nonpenetrating Rydberg states to be populated

Every molecule has Rydberg states, highly electronically excited states where the Rydberg electron is generally far from the rest of the molecule, but still bound. Core-nonpenetrating (CNP) states are long-lived enough to be useful for spectroscopic purposes, allowing in-depth investigations into electric properties of molecular cations and the mechanism for light electron – heavy nucleus exchange of energy and angular momentum. A research team has demonstrated coherent two-photon population transfer to Rydberg states of barium atoms using a combination of a pulsed dye laser and a chirped-pulse millimeter-wave spectrometer with results presented in The Journal of Chemical Physics.

This proof of principle demonstrates the feasibility of observation and exploitation of optical-mm-wave Stimulated Raman Adiabatic Passage (STIRAP) for efficient coherent population transfer to CNP Rydberg states of predissociating molecules. David Grimes, lead author, describes two challenges the team faced pursuing this approach. The first was accounting for the motion of the atoms. Due to the Doppler effect, any atomic motion towards (or away) from the laser radiation source causes the atoms to see a slightly different wavelength, degrading the efficiency of the population transfer. Using a cryogenic buffer gas cooled beam as their source reduced the lab-frame velocity of the beam so that the Doppler effect would not degrade results. The second challenge was the relative weakness of the transition accessed with the laser. The weakness of this transition meant that imperfections in the laser became far more harmful to population transfer than they would otherwise be.

This paper outlines a system with stronger transitions that should be sufficient to demonstrate full STIRAP. According to Grimes, work on that system is in progress, in collaboration with Holger Herburger and Tim Barnum.

Source: “Coherent laser-millimeter-wave interactions en route to coherent population transfer,” by David D. Grimes, Timothy J. Barnum, Yan Zhou, Anthony P. Colombo, and Robert W. Field, The Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4997624 .