Chapter 3. UED Experimental Methodology 39

Transcription

1 Chapter 3. UED Experimental Methodology 39 Interaction region with molecular beam Detector Diffracted electrons Laser baffle L M Electron pulses Electron source L Translation Stages Shutter NDQ I I Femtosecond pulses; nm Initiation pulses M BS Translation Stage M M Fig A sche mat i c of t he U E D e xpe ri m enal set up. Fem t os e cond UV pulses a r e u s ed to bot h g enera te el ect rons i n the e l ect ron gun a s w e l l as i ni ti a te t he r e act i on in the mol e cul ar bea m. E xci ta t ion puls es are t i m e- del ay ed to obtain t he ti m e-resolve d di f frac ti on pa tt erns.

2 Chapter 3. UED Experimental Methodology 4 SSA M Amplifier (Spitfire) Pump (Empower/ Merlins) Tripler M Pump (Millenia) Oscillator M (Tsunami) M Spectrum analyzer Fi g Schem at i c of am pl i f i ed fem t osecond l a s er sys t e m used i n U E D expe ri ment s. Pul s e s fr om the osci l la t or are a m pli fi ed by two stage s of T i :Sapphi re ga in. A m pl i f i e d pulses are frequency t ri ple d t o yiel d t he ul t ravio l et pho tons neede d dow nst r e am. Ban dw idt h is m oni tored aft er the os c i ll at or and t he pulsewidth a fter t he am pli fi er.

3 Chapter 3. UED Experimental Methodology 41 seed utp t o u pmp u pump Fig Schem a tic o f the chirped-pulse amplification system used to produce the high-energy infra r ed pulses. The pulses from the oscillator (seed) enter the amplifier and are temporally stretched before being amplified t h roug h two Ti:Sapphire stages regenerative and multi-pass, each pumped by ~1 W of green light. Amplified p ulses are then temporally compressed to about 12 fs before exiti n g the module. This figure is ada p ted from the Spect r aphy s ics Spit f ire manual.

4 Chapter 3. UED Experimental Methodology 42 (a) (b) (c) Fig The electron gun. (a) shows a cross-section of the entire gun. Laser pulses enter through the window at the right and strike the photocathode. Ejected electrons are accelerated toward the anode before passing through the magnetic lens and into the scattering chamber where the path of the beam may be manipulated by three pairs of charged plates. (b) The removable part of the electron gun containing the photocathode and anode. (c) Simulation showing the electric field lines between the cathode (left) and anode (right) at 3 kv potential.

5 Chapter 3. UED Experimental Methodology 43 (a) (b) (c) (d) (e) (f) Fig C r i tical functional uni t s of the electron gun. (a) T he photocathode and anode separated (3 mm ) by ridged Macor spac e r s. (b) A coil of wire comprising the magnet i c le n s. (c) A 2 mm hole separa t ing the ele c tron gun chamber from the main sca t tering c ha mber. (d) Vertical electrosta t ic defl e cti on plates. (e) Horizontal electrostatic deflection plates. (f) Ve r tical ele c trostatic strea ki ng pla t es.

6 Chapter 3. UED Experimental Methodology 44 Fig Cross-section of the electron gun chamber (upper) and main scattering chamber (lower) showing a view from the top. The electrons are generated 447 mm from the interaction region at the center of the scattering chamber where electrons, laser, and molecular beam intersect in a mutually perpendicular alignment.

7 Chapter 3. UED Experimental Methodology 45 (a) cooled CCD camera fiber optic reducing taper MCP image intensifier fiber optic reducing taper phosphor scintillator (b) front back Fig (a) Schematic of the detection system showing the arrangement of component parts. Electrons strike the phosphor scintillator, resultant photons travel through a fiber-optic taper and are amplified by a microchannel plate (MCP) image intensifier. The amplified photons are detected by a cooled CCD chip. (b) Schematic of the phosphor scintillator showing the aluminum strip and Faraday cup on the front and the variable neutral-density filter on the reverse.

8 Chapter 3. UED Experimental Methodology 46 Fig The efficiency of P-47 phosphor (the type used in UED). Extrapolated to 3 kev, one impinging electron should spawn about 5 photons. Data from El-Mul Technologies, Ltd.

9 Chapter 3. UED Experimental Methodology 47 (a) 1 Transmission Radius, pixel (b) 1 Intensity, arb.un. 1 1 (c) Radius, pixel Intensity, arb.un Radius, pixel Fig (a) The effect of the radial symmetric filter on the transmission through the fiber-optic face plate. (b) The unfiltered one-dimensional scattering signal of trifluoromethyl iodide. (c) The filtered one-dimensional scattering signal of trifluoromethyl iodide showing the increase in dynamic range of detection.

10 Chapter 3. UED Experimental Methodology 48 (a) 9 8 of counts y 2 [ln( x / )] 2 2 ( x) y A e Number y = ±.96 A = 6.86 ± 1.37 = ±.49 =.42 ± ADU/ electron (b) 9 8 Mean intensity (ADU/pulse) Mean intensity/pulse = 734 ± 4 ADU electron pulse Fig (a) The intensities of single electrons on the detector fit with a lognormal distribution. The mean value provides the response of the detector to a single electron and is needed in the calibration of the experiment. (b) Intensities of pulses of electrons showing the shot-to-shot stability in the number of electrons over 1 pulses.

11 Chapter 3. UED Experimental Methodology 49 y( x) A 2 x x 1 4 w c (a) 4 35 Intensity (ADU) A = 3886 ± 77 ADU x = 1173 ± 3 m w = 357 ± 13 m c = 175 ± 32 ADU nozzle position (mm) (b) 4 35 Intensity (ADU) A = 3417 ± 63 ADU x = 1184 ± 4 m w = 421 ± 15 m c = 188 ± 31 ADU nozzle position (mm) Fig (a) Vertical and (b) horizontal intensity profiles of an electron pulse. The data shown is the average of 1 pulses and each profile is fit with the Lorentzian distribution shown at the top.

12 Chapter 3. UED Experimental Methodology 5 1 e a b c e pulsewidth (ps) 1 a =.88 ±.47 b = 8.4E-6 ± 5.9E-6 c = 1.2 ±.47 Temporal Electron density (e /mm ) Fig The correlation between the electron density of UED electron pulses and those pulses temporal widths. Electron density is calculated by dividing the number of electrons per pulse by the FWHM area on the detector.

13 Chapter 3. UED Experimental Methodology 51 8 y( x) A 2 x x 1 4 w c scattering intensity (ADU) 6 4 A = 6551 ± 85 ADU x = 1597 ± 4 m w = 656 ± 14 m c = 1231 ± 27 ADU needle position, m Fig Molecular beam size measurement. Scattering intensity as a function of molecular beam position along the axis perpendicular to the electron beam (parallel with the excitation laser). The data is fit with a Lorentzian distribution from which the width of the molecular beam may be deconvoluted. The distribution also supplies the position of maximum overlap between electron and molecular beams.

14 Chapter 3. UED Experimental Methodology 52 (a) Time zero (b) 6. Vertical profile FWHM (pixels) translation stage position (ps) Fig Beam alignment and zero-of-time determination. (a) The shadow of the needle as seen on the detector with the defocused electron beam. Rough alignment of the laser beam to the needle is done in a similar way by viewing the needle shadow after the Brewster outlet window. (b) Lensing measurements showing the change in vertical profile of the electron beam as a function of the excitation laser delay time. Time zero is marked as the point at which the electron beam vertically contracts. The inset is a coarser scan showing the vertical profile change over a wider range of time points.

15 Chapter 3. UED Experimental Methodology 53 (a) velocity mismatch broadening, vm (ps) w e w m 35m (b) velocity mismatch broadening, vm (ps) w l w m 35m 1E-6 1E-5 1E-4 1E-3.1 laser beam FWHM, w l (m) 2.5 1E-6 1E-5 1E-4 1E-3.1 electron beam FWHM, w e (m) (c) velocity mismatch broadening, vm (ps) w e w l 35m (d) velocity mismatch broadening, vm (ps) E-6 1E-5 1E-4 1E-3.1 molecular beam FWHM, w m (m) 1E-3 1E-6 1E-5 1E-4 1E-3.1 interaction region FWHM, w e = w l = w m, (m) (e) velocity mismatch broadening, vm (ps) / 2 (f) Overall experimental time resolution (s) 1E-9 1E-1 1E-11 1E-12 w e w l w m 35m 1 1E7 1E8 electron pulse velocity, v e (m/s) 1E-13 1E-12 1E-11 1E-1 1E-9 electron pulsewidth, e (s) Fig The dependency of the temporal broadening due to velocity mismatch when (a) the spatial width of the laser is varied, (b) the spatial width of the electron beam is varied, (c) the spatial width of the molecular beam is varied, (d) the spatial widths of all three beams are varied while remaining equal to one another, (e) the velocity of the electron pulses are varied while the laser beam and electron beam intersect the molecular beam (all w = 35 mm) in a mutually perpendicular arrangement. (f) The overall experimental time resolution as a function of the electron pulsewidth.