Spectroscopy

Microsolvatation & Microaggregation


In this research, we study the microsolvation and microaggregation of molecular clusters in helium nanodroplets via high-resolution infrared spectroscopy. Helium droplets with an average size of several thousand helium atoms are formed in a supersonic expansion of precooled gaseous helium (~16 K) through a 5 µm diameter nozzle. In a second chamber, the suprafluid helium droplets are doped with one molecule at a time at 0.37 K. Each molecule is cooled to 0.37 K by evaporative cooling before aggregation.

After crossing the interaction zone with an IR laser, the nanodroplets are detected in a quadrupole mass spectrometer. Absorption of an infrared photon and subsequent cooling to the ground state results in the evaporation of several hundred helium atoms yielding a depletion of the mass spectrometer signal due to a reduced droplet ionization cross-section.

We have two experimental setups: the Bochum He-nanodroplet apparatus in combination with a cw-Optical Parametric Oscillator (OPO) with frequency coverage in the range from 2600-3400 cm-1, high output power (several Watt), and high resolution (0.0001 cm-1) or with quantum cascade lasers with medium output power (several 100mW) in the range of 1000-1400 cm-1. A second apparatus is currently being used in conjunction with the Free-Electron-Laser FELIX at Radboud University in Nijmegen, covering a frequency range of 80-3000 cm-1 with a high average power, which is unable to be reached by other radiation sources.

Helium droplets provide an ideal environment to study ultracold chemistry. They act like a vacuum cleaner as they pick up molecules from the vacuum chamber and cool them down to 0.37 K. The typical droplet size of a few nm allows investigation of molecules with low vapour pressure (e.g. amino acids). The associated cooling times are shorter than the timescales for migration of the dopants within the droplet, as well as the timescale between single pickup processes. Thus, pre-cooled monomers aggregate sequentially which can yield non-equilibrium structures not found at higher temperatures. Helium nanodroplets are also found to be a “gentle” matrix environment, as, due to their suprafluid nature, they induce only very small shifts (less than 1 cm-1) compared to the gas phase. Hence, entirely new reaction- and aggregation-mechanisms can be uncovered at ultracold temperatures.


Most relevant publications

 

Gruenerpfeil"High resolution spectroscopy of NO in helium droplets: A prototype for open shell molecular interactions in a quantum solvent", K. von Haeften, A. Metzelthin, S. Rudolph, V. Staemmler, M. Havenith, Phys. Rev. Lett. 95, 21531 (2005).

Gruenerpfeil
"Probing phonon-rotation coupling in Helium nanodroplets: IR spectroscopy of CO and its isotopomers", K. von Haeften, S. Rudolph, I. Simanowski, M. Havenith, R.E. Zillich, K.B. Whaley, Phys. Rev. B 73, 054502-1 (2006).

Gruenerpfeil
"Observation of ro-vibrational transitions of HCl, (HCl)2 and H2O-HCl in liquid He nanodroplets", M. Ortlieb, Ö. Birer, M. Letzner, G.W. Schwaab, M. Havenith, J. Phys. Chem. A 111 (49), 12192 (2007).

Gruenerpfeil"Aggregation induced dissociation of HCl (H2O)4 below 1 K: The smallest droplet of acid", A. Gutberlet, G. Schwaab, Ö. Birer, M. Masia, A. Kaczmarek, H. Forbert, M. Havenith, D. Marx, Science, 324, 1545-1548 (2009).

Gruenerpfeil"High resolution spectroscopy of HCl-water clusters: IR bands of undissociated and dissociated clusters", M. Letzner, S. Grün, D. Habig, K. Hanke, T. Endres, P. Nieto, G. Schwaab, Ł. Walewski, M. Wollenhaupt, H. Forbert, D. Marx, M. Havenith, J. Chem. Phys. 139, 154304/1 (2013).

Gruenerpfeil"Understanding the microsolvation of radicals: Infrared spectroscopy of benzyl radical water clusters", D. Leicht, M. Kaufmann, R. Schwan, J. Schäfer, G. Schwaab, M. Havenith, J. Chem. Phys. 145, 204305/1-8 (2016).

Gruenerpfeil"Infrared spectroscopy of the v2 band of the water monomer and small water clusters (H2O)n = 2, 3, 4 in helium droplets", R. Schwan, M. Kaufmann, D. Leicht, G. Schwaab, M. Havenith, Phys. Chem. Chem. Phys. 18, 24063 (2016).

Gruenerpfeil"Understanding the ionic liquid [NC 4111][NTf 2] from individual building blocks: An IR-spectroscopic study", K. Hanke, M. Kaufmann, G. Schwaab, M. Havenith, C.T. Wolke, O. Gorlova, M.A. Johnson, K. Bishnu, W. Sander, E. Sanchez-Garcia, Phys. Chem. Chem. Phys. 17, 8518 (2015).