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					Technical aspects of a VMI experiment



                  ICONIC kickoff meeting
                  Berg en Dal
                  16-18 Oct 2009
    Central idea

       Start a technical discussion on the ICONIC
        website
       Collate ideas on a FAQ list
       Use these as input for Training Schools




2                                    Berg en Dal, 16-18 October 2009
    Main questions

       How to do a VMI experiment?
       How to analyze the results?
        → What do you want to know?
              Angular – β, γ parameters
              Speed – branching b/w different channels




3                                               Berg en Dal, 16-18 October 2009
    Molecular beam preparation

       Pulsed valve / skimmers / chopper wheel
       Cooling / heating nozzle
       Hexapole state selector
       IR excitation, Raman excitation
       Ablation laser (MALDI)
       Discharge AC/DC


4                                   Berg en Dal, 16-18 October 2009
    Reactions / (in)elastic scattering

       Crossed beams: on- / off-axis; fixed /
        differential angle
       Single beam: co-expansion; single collision
        conditions (Theo)
       Steric effects: oriented molecules
       Ion-molecule / molecule-molecule / …



5                                    Berg en Dal, 16-18 October 2009
    Photodissociation exps

       Lasers for dissociation, REMPI detection
       Single- / multiple colour
       Laser polarization: linear ↔↕/ circular LR
       Precursor preparations, e.g. OH from H2O




6                                    Berg en Dal, 16-18 October 2009
    Detection

       REMPI (1+1’)/(2+1’)
       REMPI vs non-resonant MPI: all states the same
        sensitivity?
       Electron recoil
       Rydberg tagging + PFI
       TOF: pulsed extraction?
       Slice Imaging
        –   Pulsing MCP’s, slow extraction
        –   Laser sheet slicing
        –   Cylindrical / spherical lens?


7                                            Berg en Dal, 16-18 October 2009
    Data analysis
       Without inversion method
         –   Raw image analysis
         –   Slice images analysis
         –   Forward convolution (simulation)
       With inversion method
         –   Algorithms: BASEX, Iterative inversion, onion, Hankel, …
         –   Programs: e.g. Jean-Paul Visticot (JPV)
       Extraction angular & speed distributions: differences in
        programs.
       From velocity to TKER distribution
       Apparatus function: how to correct for image distortions,
        centroiding, sharpening / smoothing / deconvolution ideas.


8                                                     Berg en Dal, 16-18 October 2009
    How to set up:
       Spectroscopy: laser wavelength scan
       Doppler scanning (not for bound-free transitions)
       Use broad / narrowband laser?
       Autotracker; KDP, BBO X-tals
       Zoom lens for VMI (to be developed, MarcV did it for electrons)
       Electron imaging (avoid magnetic fields)
       Coincidence exps: ion/electron; ion/ion
       Oriented molecules / steric effects (MauriceJ)
       Laser polarizations + data analysis ank, bnk (PeterR, ClaireV)
       Proper calibration of TKER
       Deconvolution 2-step dissociation
       Flux-to-density transformation (George McBane)

9                                                Berg en Dal, 16-18 October 2009
     How to deal with:

        Laser instability?
        Pulse-to-pulse fluctuations →multiphoton branching changes
        Background subtraction
        Detector inhomogeneities
        Event counting / sequence summing?
         –   error propagation in analysis
        CCD Peltier cooling: dark counts reduction
        Choice of CCD camera + MCP detector + PS/scintillator
         –   Alternative: MediPix CCD for ions/electrons directly → pressures
             up to 1 torr.



10                                                    Berg en Dal, 16-18 October 2009
     What is the status of:
        3D imaging
          –   2 cameras; w/wo image intensifier (Zajfman)
          –   Double exposure camera (WimvdZ, MarcV)
          –   Delay line detectors (Gericke, RoentDek)
          –   Other x,y,t cameras? (multi-anode, MediPix: NIKHEF)
        FEL exps (THz region) Wim vdZ
          –   Hi rep rate (FELIX)
          –   μs macro pulse + pulsed extraction → space focusing (Joost Bakker)
        Fs pump-probe exps (Soep, BenW, ValerieB, MarcV)
          –   Study wavepacket evolution
          –   Pulseshape optimization via genetic algorithms
        CW exps
        Spatial mode imaging
        Multimass imaging (MarkB)
        UV power measurement



11                                                                  Berg en Dal, 16-18 October 2009
     Electrical discharge issues

        AC/DC?
        Electrode configuration?
        Ground loops → camera failure
        Current limitation
        Background signals from plasma: ions, electrons,
         excited atoms, X-rays
         –   Use deflection E-field?
         –   Different plasma source? Microwave?
        Beam stop for large center dot?


12                                                 Berg en Dal, 16-18 October 2009
     Electrode design

        Flat plates, thick/thin
        Rings
        Shapes
         –   Pulsed extraction pulsing a single electrode (Theo)
        Simion, use version with time-varying extraction
         (Chris)
        Status of automated design with genetic algorithms
         (M. Vrakking)


13                                                 Berg en Dal, 16-18 October 2009
     Example

     Photodissociation of CH3OH → CH3(v2) + OH(v=1,2,3,..)
     We’d like to know the OH(v) branching v=0:1:2:…
     1. We can detect CH3(v=0) REMPI
     2. Or CH3 non-resonant MPI
     3. Or detect OH(v=0,1,2,…) REMPI
     4. Or non-laser based ionzation? X-ray? e-impact?
     5. VUV 1 photon ionization, synchrotron (Nahon,Orsay)




14                                     Berg en Dal, 16-18 October 2009

				
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