Electronic spectroscopy of cold ions in a radio-frequency trap

The low density of ions in mass spectrometers generally precludes direct absorption
measurements. The spectrum of ions can nonetheless be obtained by
using resonance two-color photofragmentation or multiphoton photofragmentation techniques. In
Basel an ion-trap instrument has been built to measure the electronic spectra of cations at low
vibrational and rotational temperatures (20-40 K), as of relevance to astronomical observations.
Ions are produced in an electron-impact ion source and are mass selected with a quadrupole filter
before being fed into a 22-pole radio-frequency trap. Typically 3000 ions are held for
approximately 70 ms, during which the ions are equilibrated to low temperatures through
collisions with cryogenically cooled helium gas. Subsequently an electronic transition is induced
through excitation with a tunable nanosecond laser. A second UV photon then initiates
photofragmentation of the excited ions. Afterward the trap is opened and the fragment ions are
mass selected by a second quadrupole and detected. The recently obtained results of the excited
states of some polyacetylene cations, caynogen cation, C4H3Cl+ cation in the gas phase are
presented.