The BerlinTrap is designed to measure electronic transitions of gas phase ions and clusters by a method of action spectroscopy referred to as electronic photodissociation (EPD) spectroscopy. In this technique, the intensity of daughter fragments generated by photodissociation of electronically excited parent ions is measured as a function of excitation energy and represents the lower limit for the optical absorption cross section. The laser setup allows measurements over a wavelength range of 205 to 2000 nm. BerlinTrap is comprised of five main chambers housing (i) either an electrospray or electron impact ionization source, (ii) a hexapole ion trap for accumulation and thermalization, (iii) a quadrupole mass filter (QMS) for ion selection, (iv) a 22-pole cryogenic ion trap for storing and cooling the ions via He buffer gas, and (v) a ReTOF mass spectrometer to monitor both parent and fragment ions.
In a typical experiment, ions are generated in the source chamber from a sample and filtered by the QMS to select the ion or cluster of interest. Various lenses guide and focus the ions into the 22-pole ion trap where they are stored for a sufficient amount of time to be cooled to temperatures near 5 K by a He buffer gas pulse. This virtually eliminates the presence of hot bands in the spectrum and is a great advantage to measuring vibronic features in high detail. The cold ions are then extracted from the trap and guided into the extraction region of an orthogonal reflectron time-of-flight (ReTOF) mass spectrometer equipped with a microchannel plate detector. Shortly before being deflected into the ReTOF, they are irradiated by photons emitted from a pulsed optical parametric oscillator (OPO) laser (GWU, VersaScan), which is pumped by a Nd:YAG laser(Innolas, Spitlight1000). We also employ a dye laser (Radiant Dyes, NarrowScan) for higher resolution measurements. Mass spectra reveal the appearance of photodissociated fragments at excitation energies resonant to an electronic transition. EPD spectra are generated for each fragment mass by simultaneously monitoring the integrated signal of both parent and photofragment ions with the ReTOF. At every wavelength, the mass spectra are linearly normalized for photon flux and parent ion intensity. The normalized signals of each photofragment are then summed and plotted as a function of photon energy to produce the total EPD spectrum.
diamondoids, flavines, biomolecules