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

http://www.pnas.org/content/113/42/11800

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

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.