Submitted to: Journal of Molecular Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2005
Publication Date: 6/1/2005
Citation: Stephenson, S.K., Saykally, R.J. 2005. Teerahertz laser velocity modulation spectroscopy of ions. Journal of Molecular Spectroscopy, Issue 2, 231:145-153.
Interpretive Summary: Small ions are essential for life processes as well as for the chemistry occurring in outer space. Ions of particular interest are ones including a positively charged hydrogen atom (also known as simply a proton). The interaction and transport of protons between various molecules is the basis of photosynthesis itself. At the fundamental level, there are large voids in our understanding of how and why the protons behave as they do. This study made modifications to an existing spectroscopic technique for studying small ions by using it with lower energy light. Using lower energy light to probe the molecule simplifies results and can allow for easier interpretation of the fundamental motions that are occurring. Two molecules, protonated argon (ArH+) and protonated water (H3O+), were studied to demonstrate the effectiveness of the technique. It was shown that this method can indeed be used to gather basic information on the interactions and motions of small ions. This could allow for easier observation of such ions in interstellar space. It also lays the foundation for experimentally observing the quintessential proton transfer reaction, the transfer of a proton from one water molecule to another.
Technical Abstract: Velocity modulation spectroscopy has been investigated in the terahertz region, employing pure rotational transitions of ArH+ and rotation-tunneling transitions of H3O+ to study the competition between pressure broadening and Doppler broadening on the lineshapes, neutral suppression and modulation efficiency. Velocity modulation is demonstrated to be effective to frequencies as low as 60 cm-1, yielding S/N values for ArH+ (770/1) that surpass published terahertz FM results by an order of magnitude.