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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. Main questions. How to do a VMI experiment?
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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 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 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 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 / … 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 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? 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. 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) 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. 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 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? 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) 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:… • We can detect CH3(v=0) REMPI • Or CH3 non-resonant MPI • Or detect OH(v=0,1,2,…) REMPI • Or non-laser based ionzation? X-ray? e-impact? • VUV 1 photon ionization, synchrotron (Nahon,Orsay) Berg en Dal, 16-18 October 2009