Research overview

Conformer selection of neutral molecules

Even at the low temperatures in cold supersonic jets, large molecules typically exhibit multiple conformers (structural isomers). In some cases, this presence of many conformers does not pose a problem. A variety of (spectroscopic) experiments can still be conducted, because most often the conformers can be detected individually. For some experiments, e. g., scattering experiments and X-ray or electron diffraction imaging, however, it is highly desirable to have only one single conformer present in the beam.
While the different conformers of large molecules (i. e., 3-aminophenol, tryptophan) have the same mass and similar rotational constants, they often have widely varying dipole moments. Exploiting dynamic focusing in strong inhomogeneous ac electric fields, these dipole moment differences can be used to separate the individual conformers in an m/µ-selector, similar to an m/q-selector for charged particles.

• Frank Filsinger, Undine Erlekam, Gert von Helden, Jochen Küpper, Gerard Meijer
  Selector for structural isomers of neutral molecules
  Phys. Rev. Lett. 100 (2008), 133003

• Frank Filsinger, Jochen Küpper, Gerard Meijer, Jonas L. Hansen, Jochen Maurer, Jens H. Nielsen, Lotte Holmegaard, Henrik Stapelfeldt
  Pure Samples of Individual Conformers: the Separation of Stereo-Isomers of Complex Molecules using Electric Fields
  Angew. Chem. Int. Ed. 48 (2009), 6900

Alignment and orientation with laser fields

The state selection achieved using strong inhomogeneous electric fields can be exploited for improved laser alignment and mixed field orientation experiments.

• Lotte Holmegaard, Jens H. Nielsen, Iftach Nevo, Henrik Stapelfeldt, Frank Filsinger, Jochen Küpper, and Gerard Meijer
  Laser-Induced Alignment and Orientation of Quantum-State-Selected Large Molecules
  Phys. Rev. Lett. 102 (2009), 023001

Molecular Frame photoelectron angular distributions

We have exploited strongly 3D oriented samples of the prototypical asymmetric top molecule benzonitrile to obtain molecular frame photoelectron angular distributions of molecule fixed in space.

• Lotte Holmegaard, Jonas L. Hansen, Line Kalhøj, Sofie Louise Kragh, Henrik Stapelfeldt, Frank Filsinger, Jochen Küpper, Gerard Meijer, Darko Dimitrovski, Mahmoud Abu-samha, Christian P. J. Martiny, and Lars Bojer Madsen
  Photoelectron angular distributions from strong-field ionization of oriented molecules
  Nat. Phys. 6 (2010), 428

Deceleration of large molecules

Structure precedes functionality, so examining the geometry of molecular systems helps understanding the chemistry they are involved in. From specialized knowledge obtained from high resolution spectroscopy new insight into fundamentals of chemistry and nature can be derived. The preparation of clean and isolated samples will aid high-resolution spectroscopy of large and complicated molecules, such as modular molecules or bio-molecules. The deceleration of such molecules allows the separation and slowing of the species present in the molecular beam, their trapping, and increased measurement times for successive experiments.
Moreover, slow or trapped molecules will be extremely useful for precision spectroscopy, as ultimately the resolution in any spectroscopic experiment is limited by the interaction time between the molecules and the radiation. This time can be increased by several orders of magnitude by storing the molecules in a trap. Therefore fundamental physics studies such as the search for the electric dipole moment of fundamental particles, i. e., the electron, or the manifestation of weak interactions in chiral molecules will profit from decelerated and trapped molecules.

• Kirstin Wohlfart, Fabian Grätz, Frank Filsinger, Henrik Haak, Gerard Meijer, and Jochen Küpper
  “Alternating gradient focusing and deceleration of large molecules”
  Phys. Rev. A 77 (2008), 031404

• Hendrick L. Bethlem, Michael Tarbutt, Jochen Küpper, David Carty, Kirstin Wohlfart, Ed Hinds, and Gerard Meijer
  “Alternating Gradient Focusing and Deceleration of Polar Molecules”
  J. Phys. B 39 (2006), R263

We have performed several prototype experiments using decelerated molecules. For example, we have demonstrated the improved resolution using slow molecules in the high-resolution spectroscopic investigation of the ammonia inversion mode:

• Jacqueline van Veldhoven, Jochen Küpper, Hendrick L. Bethlem, Boris Sartakov, Andre J. van Roij, Gerard Meijer
  “Decelerated molecular beams for high-resolution spectroscopy: The hyperfine structure of 15ND3”
  Eur. Phys. J. D 31 (2004), 337

Diffractive imaging of aligned or oriented gas-phase molecules

Using our experimental techniques to control the motion of large molecules, pure, oriented samples of large molecules can be created. These samples are ideal targets for coherent diffractive imaging experiments using upcoming XFEL sources. We have performed first benchmark measurements of such experiments at the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC) in collaboration with the Max-Planck Advanced Study Group, the CFEL Coherent Imaging Division, Henrik Stapelfeldt's group in Aarhus, Marc Vrakking’s group at the Max-Born-Institute in Berlin, and others.

Radicals in superfluid helium droplets

In a project with the group of Roger E. Miller, who untimely deceased at the end of 2005, at the University of North Carolina in Chapel Hill, NC, USA I studied small clusters in superfluid helium droplets using high resolution IR laser spectroscopy. I developed a continuous pyrolysis source operating at low pressures that has been used as a pick-up source of radicals for helium droplet experiments. These allowed us to perform the first experiments on transient radical species in helium droplets. The main goals of the project were the understanding of intermolecular interactions with special emphasis on the interactions between free radicals. Moreover the applicabilities for chemical energy storage were considered.

• J. Küpper and J. M. Merritt
  Spectroscopy of free radicals and radical containing entrance-channel complexes in superfluid helium nano-droplets
  Int. Rev. Phys. Chem. 26 (2007), 249

High-resolution spectroscopy

Rotationally resolved electronic spectroscopy allows to study the structure and (solvent) interactions of large molecules in different electronic states, giving detailed insight into the structural and electronic properties of important chemical species. These experiments were all performed at very high resolution (relative resolution of 1 : 10⁹). We have, for example, studied the electronic coupling between the ¹L_a and ¹L_b states of indole, which are very relevant for the photochemistry and -stability of tryptophan and proteins. Supporting the experimental results with quantum chemistry calculations we could disentangle the complex intramolecular dynamics in these excited electronic states:

• Christian Brand, Jochen Küpper, David W. Pratt, W. Leo Meerts, Daniel Krügler, Jörg Tatchen, and Michael Schmitt
  Vibronic coupling in indole: I. Theoretical description of the ¹L_a-¹L_b interaction and the electronic spectrum
  Phys. Chem. Chem. Phys. 12 (2010), 4968-4979

• Jochen Küpper, David W. Pratt, W. Leo Meerts, Christian Brand, Jörg Tatchen, Michael Schmitt
  Vibronic coupling in indole: II. Investigation of the ¹L_a-¹L_b interaction using rotationally resolved electronic spectroscopy
  Phys. Chem. Chem. Phys. 12 (2010), 4980-4988

Furthermore, we have also investigated the interactions of prototypical large molecules with solvents, such as argon, water, or methanol. The high resolution spectra yield especially detailed information on the intermolecular interaction through interferences of the internal rotations of the solvents in the rotation-spectrum:

• Timothy M. Korter, David W. Pratt, and Jochen Küpper
  Indole-H₂O in the Gas Phase. Structures, Barriers to internal Motion, and S₁←S₀ Transition Moment Orientation. Solvent Reorganization in the Electronically Excited State
  J. Phys. Chem. A 102 (1998), 7211–7216