Fritz-Haber-Institut der Max-Planck-Gesellschaft

Department of Molecular Physics

Infrared excitation of gas-phase molecules and clusters


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Ion mobility spectrometry (IMS):

In IMS, a short pulse of gas-phase ions is injected into a low pressure (typically 1-100 mbar helium) drift region. A weak electric field gently pulls the ions through that region. The ions will drift with a constant velocity which is proportional to the electric field and the proportionality constant is the ion mobility. The ion mobility in turn is inversely proportional to the average collision cross section (the size) of the ions.

The experimentally obtained value for the ion mobility gives thus direct structural information and can be compared to theoretical predictions. When a molecule occurs in a mixture of structures (conformers) with different sizes, they can be separated in space and time and experiments on particular conformers can be performed.

The experimental setup

Ions are brought into the gas phase by nano electrospray ionization. After transfer into the low pressure region of the machine, they are collected and trapped in a watchglass type ringelectrode ion funnel. Short pulses of ions are then released into a drift region consisting of conductive glass tubes. At the end of the funnel, ions are collected in a second ringelectrode ion funnel and passed via differential pumping stages to a quadrupole mass spectrometer.

Recently we modified the instrument for spectroscopic experiments. The instrument can for example be coupled to the FHI free electron laser (FEL), which enables the measurement of IR spectra of conformer-selected molecules.


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