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.

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.








[1] D. Pentlehner, R. Riechers, B. Dick, A. Slenczka, U. Even, N. Lavie, R. Brown, and K. Luria. Rapidly pulsed helium droplet source. Rev. Sci. Instr. 80, 043302 (2009).
[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).
[3] C. Bostedt, M. Adolph, E. Eremina, M. Hoener, D. Rupp, S. Schorb, H. Thomas, A. R. B. de Castro, and T. Möller. Clusters in intense FLASH pulses: ultrafast ionization dynamics and electron emission studied with spectroscopic and scattering techniques. J. Phys. B 43, 194011 (2010).
[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 (2000).

Laser emission source for LDM experimental station  

The seed pulse starts from a femtosecond Ti:Sapphire oscillator (Vitara, Coherent) which is repetition rate locked to the reference timing signal distributed over the facility. The main part of the oscillator output beam is amplified in a chirped-pulse regenerative amplifier-single pass amplifier tandem, which delivers pulses with an energy of up to 7 mJ and duration about 100 fs at wavelength 784nm (14 nm FWHM).
after leaving the Ti:Sapphire amplifier the IR pulse is split and about 70% of its energy is used to pump an OPA (optical parametric amplifier OPerA-Solo, Coherent) and for THG (third harmonic generation).  UV emission from OPA or THG is used as seed for FEL operation. In the other arm, the remaining IR pulse energy (typically about 1.5 mJ) propagates to the experimental stations along a total distance of about 150 m. The beam propagation to the beamlines preserves a very good beam quality, and introduces a slight positive chirp and pulse lengthening to about 120 fs due to material dispersion. The duration of the transported pulse can be varied in the range 80-250 fs range by a compact transmission grating pulse compressor installed at the end of the beam transport system.
 

LDM beamline optical setup description 

The optical setup used to prepare and deliver the external laser pulse for pump-probe experiments at the LDM end-station includes two optical breadboards which are used for laser beam manipulation, insertion into the chamber and diagnostics. The breadboards are covered with protective boxes and filled with nitrogen to prevent dust contamination if optical components. CCD1 controls the position and spatial parameters of the incoming beam after the vacuum window W1 and is incorporated in the beam transport system. The motorized mirror mount M1 and CCD2 allow to control the laser beam direction of propagation on the optical breadboard. An optical attenuator, made of half waveplate (l/2 @ 784 nm)WP1 and two thin film polarizers (PL1, PL2), allow adjustment of the exact pulse energy delivered to the region of interaction.  A translation stage TS1 (PI M403.8PD) provides a variable pulse delay relative to the FEL pulse in the range of +-570ps with minimum step delay of 1.6 fs. The setup includes a high-bandwidth copper coax cable based antenna. A directly at crossing position of laser and FEL beam which allows to estimate time delay between laser and FEL pulses with fast oscilloscope (typical resolution of pulse position less than 50 ps). The fast photodiode (PD) signal is used as a trigger source for the oscilloscope in this case.   A compact single-shot auto-correlator AC, build from Fresnel prism, thin BBO SHG crystal and CCD3 camera, is installed for input pulse duration diagnostics. It is possible to change laser wavelength to 392 nm or to 261 nm by generating second harmonic (SH) or third harmonic (TH)  in BBO crystals,. In the TH case there are additional calcite time delay plate TDP and double wavelength waveplate (l/2@780 nm,l@390 nm) WP2. Interference filter F1 (HT@390 nm, HR@780nm) is used to select for operation only second harmonic emission when required. Set of four mirror with HR@260 nm, HT@390 nm HT@780nm is used as filter F2 to select only the TH UV emission. Polarization state control of the laser light arriving in the chamber is performed by using rotation stage RS2 and flipper FL2, where there are installed waveplates. Half wave waveplates WP l/2 are installed on rotation stage RS2 andquarter wave waveplates WP l/4 are installed on flipper FL2. This design provides possibility to provide horizontal/vertical linear polarization or left/right circular polarization of the emission for any operating wavelength (changing set of l/2 and l/4 waveplates). Energy meter heads EM1 and EM2 allow to control pulse energy delivered in region of interaction. Optical system including three lenses L3, L4, L5 provide focusing laser beam to a spot diameter down to 80 μm (at level 1/e²). The lens L3 is mounted on motorized translation stage TS2 (Standa 8MT173-25), that allows to keep focus position at switching between the above mentioned of operation wavelengths or to increase beam diameter up to 500 μm (at level 1/e²). CCD cameras CCD4 and CCD5 are used as virtual focal points and allow controlling beam size and position in the interaction region. CCD4 is used for operation at fundamental (784 nm) and second harmonic (392 nm) wavelength. CCD5 and UV-visible converter UVC are used for operation at third harmonic (261 nm) wavelength. Motorized mount M2 with incorporated piezo Tip-Tilt mirror allow to precisely scan the beam position or used as pointing stabilization system with beam position stability (rms) less than 5μm.  IR pilot laser IRPS is installed on the breadboard to assist the preparation of the experiment when the SLU is used by other beam lines at Fermi. 
 

Main parameters (see also here)

Parameter	Value
Laser emission wavelength (nm)	784 nm 392 nm 261 nm
Maximum pulse energy	750 μJ @ 784 nm 100-200 μJ @ 392 nm 20-50 μJ @ 261 nm
Pulse energy stability (rms)	<0.5 % @ 784 nm <1.0 % @ 392 nm <1.0 % @ 261 nm
Pulse duration, fs ( FWHM)	80-250 @ 784 nm 80-120 @ 392 nm 120-170 @ 261 nm
Pulse repetition rate	1-10 Hz, 50 Hz
Timing jitter relative to FEL (rms)	<10 fs
Scan range pulse delay relative to FEL	-570…+570 ps
Minimal step delay	1.6 fs
Beam diameter at region of interaction (at level 1/e2)	80-500μm
Beam position stability (rms)	<3μm