External ion sources

The laser vaporization process (LVAP) is suitable for generating metal clusters. The metal to be examined is vaporized by a laser beam with a wavelength of 532 nm (Nd-YAG laser, frequency-doubled) in a high vacuum (pressure approx. ^10-6 mbar) (see figure). This results in plasma temperatures in the range of several thousand Kelvin, whereby the metals are largely present in the form of individual atoms and some ions. The formation of a molecular beam is now achieved by pulsing helium gas at a pressure range of 5 to 15 bar (gas pulse in the device axis). This causes the helium atoms to expand into a vacuum, which cools the helium gas jet down to a few Kelvin. Collisions with the hot metal atoms cause them to cool down, leading to condensation of the metal atoms and thus to cluster formation. Subsequent ionization is not necessary, as the reaction produces a sufficient number of mostly singly charged particles. There is no addition of helium to the metal clusters. However, if pure argon or at least an argon/helium mixture is used as the expansion gas, argon atoms also attach to the metal clusters. However, these atoms are so weakly bound to the cluster that they can evaporate very easily and cause further cooling of the cluster.

Electrospray ionization sources are used to transfer molecular ions, in particular transition metal complexes, into the gas phase. We are also building our own source for variable use on our existing equipment.

LaserInducedLiquidBeadIonDesorption(LILBID) is a very gentle method for transferring molecular ions from the solution into the gas phase. A microdosing head (or droplet launcher) is used to generate individual, free-flying microdroplets from an analyte solution in a 10 Hz cycle. Aliphatic alcohols or water are used as solvents. The droplets are transferred to a high vacuum where they are bombarded with intense infrared laser radiation. The frequency of the laser radiation is set to an absorption band of the solvent. A self-built Optical Parametric Oscillator (OPO) unit converts the fundamental radiation of a Nd:YAG laser (1064 nm) into the desired frequency. The solvent absorbs the radiation, which causes a sharp rise in temperature and the formation of a supercritical state. In the subsequent explosion, the dissolved ions are released from the droplet into the gas phase. The ions can be fed into a mass spectrometer for further analysis.

The aggregation source is currently under construction.