P2 OAM-enabled transitions in CS-molecules
The scientific goal of subproject P2 is to experimentally study the interaction of diatomic molecules with twisted light and verify or disprove the theoretical predictions made by M. Lemeshko et al. in P3. To this end, two sets of experiments will be carried out. The team will start out by measuring dipole allowed transitions in the first overtone band of Carbon Monosulfide (CS). With that, the goal is to provide an updated set of CS transitions for the P- and R-branches of the first overtone band of CS. To the best of our knowledge, no modern precision spectroscopy on the n = 2-0 transitions of CS has been reported.
The measurements will further allow the research group to determine ideal experimental conditions for their second objective, the measurement of dipole forbidden transitions in the S-branch of the first overtone band of CS. Here, our scientists will target transitions that are enabled by vortex beams carrying orbital angular momentum (OAM).
By probing the heteronuclear diatomic molecule CS with a frequency comb carrying a topological charge, the researchers will be able to study transitions with a change of the magnetic quantum number m of |Dm| ≤ 2 in contrast to established spectroscopy where Dm is bound to values of 0, ±1. In this part of the SFB, the plan is to experimentally verify the existence of transitions with D m = ±2 when probing with light carrying OAM.
The team of P2 has chosen CS as model-gas for heteronuclear diatomic molecules due to its large dipole moment of 2 Δ and its large momentum of inertia. In fact, the dipole moment of CS is among the strongest of all accessible diatomic molecules in the gas-phase, which makes it an ideal molecule for the study of interaction between the molecule and the vortex beam.
Moreover, the strong dipole moment of CS has a few distinctive advantages when working with the molecule. CS features strong absorption lines even under low pressure and concentration. Working with strong lines allows us to do a significant part of our work in the low-pressure, Doppler-broadened regime where we can ignore pressure broadening (pCS ≤1 mbar), which allows our scientists to work with non-congested spectra. Working with a low partial pressure of CS will also mitigate the contamination of mirrors and windows with CS2, which is a known problem when working with CS.
Calculations from subproject P3 predict observable Dm = 2 transitions for an electric field of <10 kV/cm with a splitting from the Dm = 1 transitions of several tens of MHz. The strength of these Dm = 2 absorption lines scales with the amount of OAM.
Calculations from subproject P3 predict observable Dm = 2 transitions for an electric field of <10 kV/cm with a splitting from the Dm = 1 transitions of several tens of MHz. The strength of these Dm = 2 absorption lines scales with the amount of OAM.
A high-level outline of the planned experimental setup is given in the figure below. The beam from a commercial mid-IR frequency comb is converted into a vortex beam using a spiral phase plate. This beam further interacts with the CS-molecules in a buffer-gas cooled enhancement cavity. An aperture placed directly after the cavity allows us to sample only a part of the beam and hence to study the spatial dependence of OAM-transfer to quadrupole transitions. A comb-mode resolved spectrometer finalizes the setup.
Team of P2
The sub-project P2 “OAM-enabled transitions in CS-molecules” is led by Oliver H. Heckl at the University of Vienna. Monika Bahl, who has extensive experience with generating and modeling vortex-beams and has also gathered experience with supervision of students, adds her expertise to the project as a postdoctoral researcher. Together with Mirela Encheva, she is supervising two graduate students who are working on the experiment: Tom Jungnickel and Timo Gaßen. All involved researchers, the PI, the postdoctoral researchers, and the two graduate students will join forces to first measure high-resolution comb-mode resolved absorption spectrum of CS at room temperature, then cooled down in the buffer gas cell probed with a normal Hermite-Gaussian beam as well as a beam carrying OAM.