Abstract-Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase

1. Jian Lua, 
2. Yaqing Zhanga, 
3. Harold Y. Hwanga, 
4. Benjamin K. Ofori-Okaia, 
5. Sharly Fleischerb, and 
6. Keith A. Nelsona,1

Author Affiliations

1. aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
2. bDepartment of Chemical Physics, Tel-Aviv University, Tel Aviv 69978, Israel

1. Edited by Michael D. Fayer, Stanford University, Stanford, CA, and approved September 2, 2016 (received for review June 15, 2016)

Molecular rotations of small molecules provide a useful testbed for examining light−matter interactions with quantum mechanical systems, but the methods of modern spectroscopy have been largely unavailable in the terahertz frequency range where most of the rotational states that are thermally populated at ordinary temperatures absorb light. Applying a pair of strong terahertz pulses, we excite molecular rotations coherently, interrogate thermally populated rotational states, manipulate the rotational motions nonlinearly, and observe connections between different rotational states spectroscopically. The method is applicable to polar molecules in flames and other reactive conditions, and it enables enhanced control over molecular motion with light.

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, Roland Wester

(Submitted on 26 Feb 2016)

http://arxiv.org/abs/1602.08303

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 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 598596.08(19) MHz for J=1<-0 and 1196791.57(27) MHz for J=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.