Beamline Description

The end-station consists of a differentially pumped vacuum system featuring interchangeable beam sources, three detectors that can be independently or simultaneously operated, all with single-shot capabilities, and several diagnostics units. Droplets and clusters can be further doped on-the-fly in a series of gas cells, ovens, and laser evaporation sources.

Chamber

The end-station itself is a modular design comprising six vacuum Chambers, attached to each other along the axis of the atomic/cluster beam in the following order: source chamber, doping chamber, differential pumping chamber, detector chamber, quadrupole mass spectrometer chamber, surface ionization detector chamber. This concept allows any chamber to be replaced to adapt to the needs of the experiments, which may require a non-standard configuration of the apparatus. Gate valves mounted between the chambers section the end-station off into four units providing additional flexibility and vacuum safety.





Schematic view of the experimental station: (1) Source chamber. (2) Doping chamber. (3) Differential pumping chamber. (4) Detector chamber. (5) Quadrupole mass spectrometer chamber. (6) Surface ionization detector chamber.

Beam sources

Beams of atoms, molecules, radicals and helium droplets as well as clusters of atoms, molecules and metals can be produced by different type of pulsed valves.

Pulsed Cryogenic Source An Even-Lavie (E-L) valve based is used  for producing a dense sample of atoms or clusters particularly those of rare gases [1]. Temperature range 6-300K Backing pressure range 10-100bar Opening time 18-30 μsec diameter of the nozzle 100 μm diameter of the sealing gasket 50 μm

Pulsed High Temperature Source This is a versatile source which can be used for the producing of atomic and molecular beams, water droplets beams, molecular clusters. In this source both the nozzle and an internal reservoir can be heated. Heating may be used to eliminate cluster formation of condensable gases, or to increase the vapour pressure of samples such as liquids. The construction of this E-L valve is similar to that of the cryogenic one. The main differences is a bigger gasket orifice (100 μm diameter) and the temperature range is 300-500K.

Pulsed Micro-Plasma Cluster Source

 

Flash Pyrolysis and Laser Photolysis Source to generate radicals.

Doping

 The atomic and molecular beams may be seeded, and the clusters and    droplets may be pure, or doped with other atoms and molecules. For  these purposes 2 gas cells as wells as 2 ovens have been installed into  the doping chamber. Each doping oven has a water cooled shield and  can be heated up to 900 ^oC. The construction of the gas pick-up cell  allows direct measurement of the pressure inside its active volume using  a capacitance vacuum gauge which is insensitive to the type of gas.

Ovens and gas cells installed into the doping chamber. View from the side (a) and top (b).

Laser evaporation

here the laser and the ablated material can be adjusted to optimize the doping to the droplet beam. The ablation laser currently available at LDM is an Edgewave pulsed Nd-YAG laser [http://www.edge-wave.de/web/en/produkte/short-pulse-systeme/is-serie/]

• Pulse energy: ~10 mJ
• Wavelength: 532nm (SHG)
• There is roughly 30% of the fundamental (1064nm) in the laser path.
• Pulse length: low ns range 

Detection

The electrons, ions and x-ray scattering patterns produced during the interaction of the samples with intense light of FERMI can be analysed by the available spectrometers, to give mass spectra and energy as well as angular distributions of charged particles. The design of the detector allows simultaneous detection of electrons and ions using velocity map imaging (VMI) and time-of-flight (TOF) as well as diffraction images using photon scattering detectortechniques, respectively. The instruments have a high energy/mass resolution and large solid-angle collection efficiency.

Velocity map imaging spectrometer The present VMI spectrometer consists of electrodes shaped as truncated cones, a flight tube and a detection system based on a Photonis MCP stack of 75 mm diameter, phosphor screen and the conventional CCD camera with a good spatial resolution (2560 ×2160 pixels, of size 6.5 microns) and the large dynamic range.[www.andor.com/scientific-cameras/neo-and-zyla-scmoscameras/neo-scmos].This custom-designed instrument has been built to detect charged particles in the kinetic energy range 1 to 100 eV, with calculated energy resolution of 2%, and experimental resolution of 4%. (left) Cut-away view of the main chamber, showing the VMI and the TOF mass spectrometer. (right) Photograph of the detectors: (1) VMI MCP, (2) extractor, (3) interaction region, (4) repeller, (5) flight tube. The repeller–extractor gap is 18 mm. The photograph is inverted with respect to the drawing and its orientation when mounted.

TOF mass spectrometer           TOF mass spectrum of singly and doubly ionized Xe and residual gas. Photon energy: 21.7 eV. The TOF spectrometer is located opposite to the VMI spectrometer, and the first accelerating region is defined by the VMI geometry. It may be operated at the same time as the VMI to obtain ion and electron spectra simultaneously, but the resolution of the TOF depends on whether it operates independently or simultaneously with the VMI. For example, the mass resolution is 350 when the VMI is set to detect electrons with maximum kinetic energy 30 eV (mode 1), while it is 1400 in the independent regime (mode 2).

Cut-away view of the main chamber, showing the VMI and the 2D scattering detectors.

2D scattering detector The 2D scattering detector is based on the gated MCP combining with the P-43 type phosphor screen and the 3 mm center hole for the FEL beam propagation through the detector [3]. The actual size of the MCP is 75 mm diameter with the 25 μm pore diameter. The MCP is covered by additional gold and magnesium fluoride coatings to decrease the total MCP resistance (usually used for the gating application) and increase the quantum efficiency of MCP (from 10% up to 50%) in the 40-100 nm wavelength range, correspondingly. The images from the phosphor screen are projected to a CCD camera outside vacuum by the plane mirror installed at the 45° angle with respect to the FEL beam. The LDM scattering detector has been recently commissioned and as a result on the figure 8 the images of the single-cluster scattering pattern from a He droplet of diameter 950 nm, taken with FERMI pulses at a wavelength of 64 nm are presented.

First single-cluster scattering pattern from a He droplet detected by 2D scattering detector at LDM.

B-TOF spectrometer The B-TOF mass spectrometer consists of two acceleration regions, an adjustable slit (or iris diaphragm), two permanent magnets (or Helmholtz coils), a time-off-light tube and a position sensitive detector based on the combination of the MCP with a position sensitive delay-line anodes, which allows unambiguous determination of arrival time and position of at least eight simultaneously detected particles. The spectrometer will operate in two regimes: without and with a magnetic field. The magnetic field with maximum 800 G will be applied during the second regime.

Supersonic beam diagnostics The instrumentation for the atomic and cluster beam diagnostics consists of an EXTREL MAX-1000 quadrupole mass spectrometer equipped with 90◦ion deflection optics. The instrument has a mass range of 0–1000 amu and resolving power of 1800.A Langmuir–Taylor surface ionization detector of custom design is also installed in a dedicated chamber with separate pumping system, and is suitable for detecting clusters doped with alkali or alkaline earth atoms [4].

‘Magnetic-bottle’ electron spectrometer

Recently this type of spectrometer designed and built by Raimund Feifel and co-workers [http://www.physics.gu.se/english/about-the-department/staff?languageId=100001&userId=xfeira] has been commissioned at LDM. The name ‘magnetic-bottle’ arises from the shape of the magnetic field (see figure 9). Magnetic-bottle spectrometer can be employed for electron detection both normal time-of-flight methods as well as zero-kinetic energy electron detection with almost laser-limited resolution  The electrons are detected at the end of the flight tube by two microchannel plates (MCPs) chevronned together The magnetic-bottle electron spectrometer has the advantage of high detection efficiency for the detached electrons. The intrinsic energy resolution of the magnetic-bottle spectrometer depends on various parameters like the length of the drift region, the length of the laser pulse, the details of the magnetic fields, and the size of the laser focus.








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[2] E Barborini, P Piseri and P Milani. A pulsed microplasma source of high intensity supersonic carbon cluster beams. J. Phys. D: Appl. Phys.32, L105–L109 (1999).
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[4] Stienkemeier F., Wewer M., Meier F. and Lutz H. O.,Langmuir–Taylor surface ionization of alkali (Li, Na, K) and alkaline earth (Ca, Sr, Ba) atoms attached to helium droplets. Rev.Sci. Instrum. 71, 3480 (2000).
[5] A.M. Rijs et al.,‘Magnetic bottle’ spectrometer as a versatile tool for laser photoelectron spectroscopy. Journal of Electron Spectroscopy and Related Phenomena 112, 151–162, (2000).