Abstract-Terahertz-visible two-photon rotational spectroscopy of cold OD−

Seunghyun Lee, Daniel Hauser, Olga Lakhmanskaya, Steffen Spieler, Eric S. Endres, Katharina Geistlinger, Sunil S. Kumar, and Roland Wester

Phys. Rev. A 93, 032513 – Published 21 March 2016

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.93.032513

We present a method to measure rotational transitions of molecular anions in the terahertz domain by sequential two-photon absorption. Ion excitation by bound-bound terahertz absorption is probed by absorption in the visible on a bound-free transition. The visible frequency is tuned to a state-selective photodetachment transition of the excited anions. This provides a terahertz action spectrum for just a few hundred molecular ions. To demonstrate this we measure the two lowest rotational transitions, J=1←0 and J=2←1 of OD− anions in a cryogenic 22-pole trap. We obtain rotational transition frequencies of 598 596.08(19) MHz for J=1←0 and 1 196 791.57(27) MHz forJ=2←1 of OD−, in good agreement with their only previous measurement. This two-photon scheme opens up terahertz rovibrational spectroscopy for a range of molecular anions, in particular for polyatomic and cluster anions.

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