ClusterTrap experiment

ClusterTrap has been designed to investigate properties of atomic clusters in the gas phase with particular emphasis on the dependence on the cluster size and charge state. The combination of cluster source, ion traps and time-of-flight mass spectrometry allows a variety of experimental schemes including collision-induced dissociation, photo-dissociation, further ionization by electron impact, and electron attachment.

 

Experimental setup

Positively or negatively singly-charged clusters are produced in a laser-ablation source (1) and accumulated in a radiofrequency ion trap (2a). Then, the cluster ion bunch is transferred via a quadrupole ion deflector into a Penning ion trap (3) and there prepared and subjected to one or several reaction steps (4). Subsequently, the product ions are analyzed by time-of-flight (ToF) mass spectrometry (5). Alternatively, the cluster ion bunch is transferred into a second radiofrequency ion trap (2b) for electron interaction studies. Again, product analysis is realized via storage in the Penning trap and ToF mass spectrometry.

 

Experimental cycles

The setup is operated with two or more different cycles, which are alternately (and therefore quasi-simultaneously) executed. In the “measurement cycle” the experiment parameter of interest is varied, while it remains at a constant value in the “reference cycle”. Thus, changes in the ion signal over time (due to causes different from the parameter varied for the particular experiments) are monitored and compensated in the data evaluation. Typical cases for such applications are drifts of the cluster ion source accompanied by changes in the cluster production.

 

Cluster ion source and cluster-preparation trap

The clusters are produced by laser vaporization of a metal wire in a helium gas pulse. The laser pulse (532nm, 10mJ, 5ns) and the helium gas pulse enter the cluster source through the same port. Material of the metal wire (element M) is vaporized and cooled in the gas pulse, causing formation of metal clusters Mnz, consisting of n atoms (cluster size), which are neutral or having the charge state z. Driven by expansion of the gas pulse through a nozzle into vacuum, the clusters leave the source through two skimmers, which a part of differential pumping stages. An electrical dc-field between the nozzle and the first skimmer additionally accelerates cluster ions of respective charge sign (i.e. either cations or anions).

After leaving the source, the cluster ions are captured in a linear radio-frequency quadrupole (RFQ) trap. A radio-frequency electric field applied between pairs of RF-electrode rods provides radial ion confinement (x-y plane). Potentials applied to endcap electrodes confines the ions in axial direction, i.e. in direction parallel to the RF-electrodes. By consecutive capture events, cluster ions are accumulated in the RFQ. Application of a dc-potential to axially mounted “plate electrodes”, whose distance to the trap axis varies along the axial dimension, causes an axial compression of the trapped cluster ensemble. Additionally, selection of single cluster species by removal of the unwanted ions might be realized by application of RF-excitation fields (e.g. SWIFT-excitation). For easy variation of the trapping parameters, the RFQ is operated as a “Digital Ion Trap” (DIT), i.e. rectangular RF-voltages are used, rather than sinusoidal ones.

 

Penning trap

Cluster ions are stored in a Penning ion trap for up to several seconds. A superconductiong magnet provides a magnetic field (B = 12 T), which confines the ions in the plane perpendicular to the magnetic field lines. A voltage potential applied to cylindrical endcap, correction and ring electrodes of the trap provides an harmonic trapping potential U(z) for axial ion confinement. The ring electrode is azimuthally split into 8 segments to allow for application of radial excitation fields (e.g. dipolar, quadrupolar and octupolar). Different excitation schemes provide motional excitation of trapped ions, used for e.g.

- increase of ion oscillation radii for collision experiments with neutral background gas,

- increase of ion oscillation radii for selective removal of cluster species from the trap,

- conversion of ion oscillation modes (cyclotron & magnetron) for buffer-gas assisted centering of ions.

In a complementary fashion to radial excitation, also axial excitations fields can be applied, e.g. used for axial excitation of trapped electrons. Furthermore, the split ring electrode allows for a non-destructive ion detection method, the Fourier Transform Ion Cyclotron Resonance (FTICR), where radial ion motion is detected by means ions' image currents in the ring segments after a broadband excitation. A Fourier analysis of the obtained time signal resolves the ion cyclotron frequencies, which depend on the mass-over-charge-ratio of the trapped ion species. FTICR-detection allows for very high mass accuracy and mass resolution. Another non-destructive method is the electronic ion detection, where the axial ion motion is monitored. By variation of the trapping potential U0 the axial motional frequencies of the trapped ions are successively brought into resonance with a LC circuit, causing an energy transfer into the trap which is detected. However, as both detection methods require a certain ion density in the trap, they are less suitable for the present cluster experiments, where only little ion numbers are involved.


Time-of-flight mass spectrometer and potential lift

For sensitive detection of reaction products, these are axially ejected from the Penning trap into the drift section of a Time-of-Flight (ToF) mass spectrometer and recorded on a single-ion-counting detector (MCP with conversion electrode). To obtain high kinetic energies, the ejected ions enter a drift tube, which is operated as a potential lift. The ions are strongly accelerated into the potential lift by a strong potential gradient, caused by a high potential at the tube. While the ions pass the tube, its potential is lowered again to match the potential of the remaining drift section. The ions leave the potential lift and continue to the detector at a high kinetic energy, without having the complete drift section to be floated at correspondingly high potentials. This is advantageous, as part of the drift section is also used for ion transfer into the Penning trap at much lower ion kinetic energies. However, the potential lift allows only the detection of a limited range of ion masses, because the length of the potential-lift tube relates to the time, the ions need to pass the tube. The lift potential must be varied after the heaviest (i.e. slowest) ions have entered the tube, but also before the lightest (i.e. fastest) ions pass the end of the tube.



Reviews of the ClusterTrap experiment: