Precision Infrared Spectroscopy on Small Molecules
Optical frequency comb technology has greatly simplified the task of comparing unknown optical frequencies to radio frequency references, such as atomic clocks, and also to other optical frequency references. This has enabled many high-precision measurements of infrared, visible, and UV transitions to be performed with absolute accuracy. Most precision optical measurements thus far have focused on gas-phase atoms and atomic ions, particularly systems which are either very simple or systems which can be laser cooled. Simple systems such as atomic hydrogen can be modeled with high-level theory to test fundamental physics, while slow, laser-cooled atoms can be coherently interrogated for several seconds, enabling very precise measurements of their transition frequencies.
Molecules have vibrational and rotational degrees of freedom, which are not present in atoms, and are much more sensitive to certain fundamental physical effects than atoms. For example, several experiments are currently underway to determine whether the electron has an electric dipole moment using heavy diatomic molecules, and other experiments are being carried out to measure energy differences between left- and right-handed enantiomers of chiral molecules due to the weak nuclear force. Vibrational, rotational, and tunneling transitions in molecules are also directly sensitive to a possible time-variation of the proton-electron mass ratio.
Despite these interesting properties, molecules are often more difficult to study than atoms. For one, the complex structure of molecules makes it more difficult to prepare a cold sample, and as a result, most precision measurements on molecules thus far have made use of relatively fast molecular beams, limiting the interaction time. Secondly, many of the interesting transitions in molecules are in the mid-infrared wavelength range where there has been less development on narrow-linewidth laser sources.
The research of this group is focused on extending precision spectroscopy to vibrational transitions in light diatomic molecules, with particular emphasis on the gas-phase OH radical. To overcome the two obstacles mentioned above, we are developing new narrow-linewidth (< 1 kHz) laser sources in the 2-5 µm wavelength range which will be linked to ultrastable lasers in the near-infrared using an optical frequency comb. We will also use the Stark decelerator technology developed at the Fritz Haber Institute in Berlin to slow the OH molecules and thereby increase the interaction times with the spectroscopy laser.