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22-pole Ion Trap

22-pole ion trap Producing cold molecules becomes more difficult as the size of the species increases. This is due to the fact that larger molecules have more internal vibrations where the energy can be stored. Therefore very big molecules, like polycyclic aromatic hydrocarbons (PAHs) and fullerenes, cannot be cooled efficiently during the short-time scales involved in a supersonic expansion. An alternative approach is to collect the ions in an ion trap and cool them down through collisions with a cold buffer gas, like helium or argon.

In the experiment an electron impact source is used to ionize molecules coming from gas-phase molecules or from the evaporation of solid precursors produced by resistive heating. The ions produced are deflected 90o by a magnetic field, transported into a hexapole ion guide and then selected by a quadrupole mass filter. The chosen mass is bent another 90o by an electrostatic deflector and injected into a 22-pole trap, confining the cations in an inhomogeneous field. Inside the ion trap the positive molecular species undergo many collisions with the helium buffer gas, cooling them to the cryogenic temperature of the closed-cycle refrigerator on which the trap is mounted. A laser beam is then directed through the center of the ion trap to probe the sample. Photofragmentation processes can result upon the absorption of several photons. The photodissociation products induced from the absorption of light are mass selected by a second quadrupole and counted using a Daly detector. By scanning the laser frequency, resonant absorption can be searched for by monitoring the fragmentation yield. In this way an electronic spectrum is collected.

ion trap setup

Selected publications:

  • C.A. Rice, V. Rudnev, R. Dietsche, and J.P. Maier
    Astron. J., 140(1), 203–205, 2010.
    Gas-Phase Electronic Spectra of Polyacetylene Cations: Relevance of Higher Excited States
  • C.A. Rice, V. Rudnev, S. Chakrabarty, and J.P. Maier
    J. Phys. Chem. A, 114(4), 1684–1687, 2010.
    D2Πu, C2Πu ← X2Πg electronic transitions of NCCN+
  • V. Rudnev, C.A. Rice, and J.P. Maier
    J. Chem. Phys., 129(13), 134315/1–4, 2008.
    B2Σu+←X2Πg electronic spectrum of NCCN+ in the gas phase
  • A. Dhzonson and J.P. Maier
    Int. J. Mass Spectrom., 255–256, 139–143, 2006.
    Electronic absorption spectra of cold organic cations: 2,4-Hexadiyne
  • Anatoly Dzhonson, Dieter Gerlich, Evan J. Bieske, and John P. Maier
    J. Mol. Struct., 795(1–3), 93–97, 2006.
    Apparatus for the study of electronic spectra of collisionally cooled cations: para-dichlorobenzene