New Methods in Molecular Spectroscopy:
Cologne Center for THz Spectroscopy
O. Asvany1, S. Brünken1, Pavol Jusko1, S. Thorwirth1,C. Endres1,2, F. Lewen1 and S. Schlemmer1
12I. Physikalisches Institut, Universität zu Köln, Germany
Max-Planck Institute for Extraterrestrial Physics, Garching, Germany

The Cologne Center for THz Spectroscopy invites guests to use a wide variety of instruments
to obtain spectra from the far-infrared to the mid-infrared region. Spectra of complex organic
molecules (COMs) are recorded in several conventional absorption spectrometers in order toidentify them in observations of the ALMA, Herschel and SOFIA telescopes. Driving
questions in this area of research are, e.g., up to which molecular complexity do these species
grow in the interstellar environment and how can they be formed under the prevailing low
pressure and temperature conditions.
Apart from these traditional spectroscopy tools, new instruments are built in the Cologne
laboratories to improve the sensitivity, frequency coverage and spectral resolution, inparticular to allow the study of radicals and ions which shall be found in future observations.
We present a 75-110 GHz emission spectrometer which is operated at room temperature. An
immediate bandwidth of 2.5 GHz is recorded with this instrument at a noise temperature of
only several hundred Kelvin. Apart from its high sensitivity new features will be discussed. A
chirped pulse spectrometer has been constructed not only to record broad band spectra but inparticular to determine rate coefficients for inelastic and reactive collisions.
The method of light induced reactions (LIR) has been developed over the last 20 years in ourgroup in order to record spectra of mass selected, cold molecular ions which are stored in ion
traps. Compared to traditional absorption spectroscopy in discharge cells the sensitivity of ourdevices is increased by many orders of magnitude. The spectral resolution is high enough topredict THz spectra at sub-MHz accuracy which is sufficient to identify those ions inastronomical observations [1]. Changing the population of the rotational states andinterrogating them by LIR in double resonance experiments leads to predictions of THz
transition frequencies at the 10-9 level [2], which challenge observations [3]. The ion trap
instrument FELion allows to record spectra of cold ions over a range of 250 – 3200 cm-1. Thisapproach allows the identification of isomers and conformers of the molecular species
selected by mass spectrometry. Moreover, identifying the vibrational bands of more complex
molecular ions challenges quantum chemical predictions [4] and is used as a basis forhigh-resolution studies in order to find those molecules in space [5].
References[1]
S. Brünken, O. Sipilä, E.T. Chambers, J. Harju, P. Caselli, O. Asvany, C.E. Honingh, T. Kamiński, K.M.
Menten, J. Stutzki, and S. Schlemmer, An age of at least 1 Myr for a dense cloud core forming Sun-like stars,Nature, 516, 219–221 (2014).
[2]
P. Jusko, O. Asvany, A.-C. Wallerstein, S. Brünken, S. Schlemmer, Two photon rotational action
spectroscopy of cold OH- at 1 ppb accuracy, Phys.Rev.Lett. 112 (2014) 253005.
[3]
S. Brünken, L. Kluge, A. Stoffels, O. Asvany, and S. Schlemmer, Laboratory Rotational Spectrum ofl-C3H+ and Conformation of its Astronomical Detection, ApJ 783 (2014) L4.
[4]
O. Asvany, P. Kumar, I. Hegemann, B. Redlich, S. Schlemmer and D. Marx, Understanding the LIR
Infrared Spectrum of Bare CH 5+, Science 309, 1219-1222 (2005)[5]
O. Asvany, K.M.T. Yamada, S. Brünken, A. Potapov, S. Schlemmer, Experimental Ground State
Combination Differences of CH5+, Science 347 (2015) 1346-1349