Exchange Report by Marius Hermanns

Project affiliation: B4 – Chirped Pulse FT Spectroscopy for Astrophysics
Supervisor: Prof. Dr. Stephan Schlemmer
Hosting institution: Patterson Research Group, MIT- Harvard Center for Ultracold Atoms, Harvard University, Boston MA, USA
Duration of stay abroad: 03.10.2016 – 03.12.2016

Introduction

In my diploma thesis in the Lab Astrophysics Group of Prof. Schlemmer I developed and characterized a chirped pulse experiment measuring inversion transitions of ammonia as well as rotational transitions of methanol in the cm-wave regime. Chirped pulse experiments use a excitation source with a very high power, typically many watts, to change level population ratios in gas phase molecules and detect the emitted signals of the relaxation of these populated transitions.

While the source signal is typically a broadband chirped signal (the signal increases or decreases linear the frequency over time), the emitted signal, the free induction decay, contains only those frequencies specific to the probed transitions. After Fourier Transformation the resulting spectrum contains frequency and intensity information for the probed molecules and/or complexes.

For my doctoral project in the same group I planned to extend the use of the chirped pulse technique with different approaches to measure complex molecules of astrophysical interest. Maximum line intensities at room temperature of those larger and more complex molecules as methyl cyanide, propanediol and others with higher moments of inertia are shifted with the Boltzmann distribution to higher frequencies. So one main goal is to reduce the temperature of the probed gases and therefore reduce the number of populated states to those in lower frequency ranges.

One method in reducing the temperature of a gas is using cooled helium as a coolant gas. The probe gas is injected in a cryogenic buffer gas cell filled with helium, thus cooled by collisions with these He-atoms. This method, first developed by Frank DeLucia in the late 1980s and used by the Patterson Research Group and the Doyle Group at the MIT-Harvard Center for Ultracold Atoms (CUA), will reach the desired temperatures of 5-10 K and increase the line intensity with T-7/2 [1].

[1] Cooling molecules in a cell for FTMW spectroscopy, D. Patterson and J. M. Doyle, Molecular Physics, Vol. 110 #15-16, 1757-1766 (2012)

Experiences

During my stay with the Patterson Research Group in Cambridge, mainly Dr. David Patterson and Dr. Sandra Eibenberger, I became familiar with their helium buffer gas cell experiment. Their setup uses a cubic cell of 200m length inside a rectangular vacuum chamber, cooled down to 4.5 K with a two stage pulse cooler. A chirped pulse excitation signal of 2-20 GHz is coupled into the chamber via small, directly attached horn antennas and the resulting FID is detected with another antenna and digitized with a fast ADC pc card.

One of my first duties was to build and prepare a multilayer charcoal coated copper stack to use as a cryopump inside the vacuum chamber. During the cooling a steady flow of helium is needed to keep the buffer gas density at desired values. During measurements the gas cell is leaking helium to the outer perimeter inside the vacuum chamber and the copper stack is used as a cryogenic trap while cooled down to 4 K.

In the first few weeks of my visit we prepared the first of two setups to measure propanediol as a complex molecule - a measurement that was done before to compare different conformers of the same molecule and their different behavior in this buffer gas cooling environment. This time the focus was not only on the 7 known conformers, but also on different substituted isotopic species, such as 13C. In theory detecting different substitutions of isotopes in various places of a molecule lets one identify the structural appearance of a molecule by identifying their different moment of inertia through their spectrum. In this case I could benefit from my experience in detecting isotopologues with high amount of averaging and analyzing and separating the detected combined sidebands.

Since the second setup, a cell of similar size and configuration, was half way done my goal there was to finish this and learn on the fly what is needed to build and run a buffer gas cell experiment. Further more I was able to extend the experimental setup with newly acquired components to the higher frequency range of 18-26.5 GHz and beyond. On my last day in the lab we successfully measured lines of methylacetylene and acetone.

Life in Cambridge

Visiting places way outside of our used timezone can be challenging and very educational. Cutting ties to family and long learned behaviors makes it even more necessary to use the surrounding, mainly the City of Cambridge, Sommerville and Boston, and various activities to compensate. In this regard the east coast of the United States is rich in possibilities and supply. Together with my colleagues from Harvard and the MIT, as well as Oli from Cologne, we conquered many attractions and explored a wide variety of activities: we went whale watching, hiking on the Appalachian Trail in the White Mountains, visited local farms and cities in New Hampshire and Massachusetts.

Boston is roughly 4 hours driving distance from New York so a weekend trip to Brooklyn, WTC One, the East Side and Times Square was imperative. Just at that time the US held its presidential election with an unanticipated outcome and the demonstrations on fifth avenue were a stop worth. Later that month Thanksgiving, a holiday uncharted to most Europeans, was due and friends had invited me to Virginia, so I did a large road trip – these are somewhat unexceptional to Americans as distances are greatly larger than in Europe. We had the full, conventional turkey with stuffing and gravy, and a Black Friday hike in the Blue Ridge Mountains of the Shenandoah Valley.

The more direct surrounding, Cambridge and Boston, is rich in cultural and educational sites, which led to an extensive visit of the MIT, museums, the city of Boston as well as theater, cinema and night live. With bad luck I missed the last game of the season with the Red Socks of Boston, but Football was a ever recurring evening excuse to go to a local Cambridge bar – go Patriots!

Conclusion

In summary my two month visit was an outstanding experience, scientifically as well as personal. Having had the possibility to study in Harvard Yard or run on the bank of the Charles River with the Boston skyline was awesome – I am really grateful for the CRC 956 to have supported me on this trip.