Fritz-Haber-Institut der Max-Planck-Gesellschaft

Department of Molecular Physics

Infrared excitation of gas-phase molecules and clusters


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Helium droplets

Liquid helium droplets are a near perfect matrix for molecules in spectroscopic experiments because:

  • they are very cold (0.38 Kelvin)
  • they are superfluid and interact only weakly with dopant molecules
  • they are transparent from the deep UV to the far IR

Helium droplets can be generated by expanding cold helium gas at high pressure into vacuum. Depending on experimental parameters, the droplets can vary in size between a few thousand atoms up to macroscopic droplets containing > 1010 atoms.
Foreign species can be incorporated by passing a beam of droplets through a volume which contains those species in the gas phase. The droplets can then incorporate atoms or molecules by mechanical impact. The result is a beam of droplets of which some contain the species of interest at the low temperature of the droplet. Such an approach has been successfully applied in many laboratories to various small molecules, which can be thermally evaporated.

Mass/charge selected ions in helium droplets

Many interesting species, such as for example most biomolecules, will decompose when trying to bring them into the gas phase via evaporation. To investigate those in helium droplets, we developed a technique in which molecules are brought as charged species into the gas phase, stored in a linear ion trap and then picked up by helium droplets. The approach is conceptual similar to pickup of evaporated species from a gas cell. In our experiment, a linear hexapole ion trap is used. Perpendicular to the hexapole rods, the ions are confined by a radio-frequency potential. In longitudinal direction, two electrodes provide a confining potential of 1-5 eV.

The helium droplets that traverse the trap have a kinetic energy of Ekin=1/2 m v2. Their velocity is not very high, however they have a large mass. Doped droplets will thus have a kinetic energy that readily exceeds the longitudinal trapping potential and they can leave the trap. For the stored ions, leaving the trap inside a droplet is the only way out of the trap which makes the method very efficient.

The experimental setup

A scheme of the experiment is shown in the following figure:


Ions are brought into the gas phase via electrospray ionization. After traveling through two ions guides, they enter the high vacuum part of the apparatus. Then, mass selection in a quadrupole mass spectrometer takes place. The mass selected ion beam is then bend 90 degrees and injected into the linear hexapole ion trap. After having filled the trap (1-10 seconds), the ion beam is turned off and the droplet beam turned on. The doped droplets leave the trap and can be exposed to laser radiation and detected on a detector.


Using this setup, we have been able to incorporate small ions as well as large species such as proteins in helium droplets. The first goal is to perform optical spectroscopy. In initial experiments, we investigated the iron-porphyrin complex Hemin in the UV spectral range. Since very recently, we are able to study peptides and proteins in helium droplets using the FHI free electron laser.


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Address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany