3.5 Imaging photoelectrons
3.5.2 Detection and processing
The ANU spectrometer employs two detectors, a retractable 18mm diameter microchan- nel plate (MCP) that is used for ion beam optimisation, and a 75mm diameter imaging MCP detector used to record the velocity-mapped photoelectron events. The main 75mm detector, shown in Fig. 3.17, consists of two matched 75mm Burle microchannel plates and a P47 phosphor screen. When a photoelectron strikes the first MCP this initiates a cascade of electrons which propagates through the plate. These electrons then strike the second MCP, which further amplifies the signal. However, as the signal propagates through the plate it will become slightly displaced, so to account for this the second MCP is anti-aligned to the first to ensure that the final signal exiting the second MCP will be at the original location. The amplified electron signal then strikes the P47 phosphor screen, causing the phosphor to fluoresce which is recorded by a CCD PCO 2000 2048×2048 pixel monochrome camera. Each camera frame is transferred to a computer at a 10 Hz repeti- tion rate and is processed in real time to identify electron events, centroiding position to a sub-pixel accuracy, to eliminate camera image noise from the accumulated velocity-map image.
Front MCP Front plate grid
Macor spacer Back MCP and
Phosphor screen
Phosphor HV
MCP HV
(a) Main imaging detector assembly. Two MCP’s are stacked between Macor spacers on top of a phosphor screen.
P47 Phosphor screen
Fluorescence from ion beam signal
(b) Back side of the imaging detector stack, showing the phosphor screen. The blue fluo- rescence visible is due to an ion beam signal striking the MCP stack, causing a cascade of electrons to hit the phosphor screen. In- dividual fluorescent spots represent a single ion event on the detector.
Figure 3.17: Components of the detection scheme used in the spectrometer. Electrons are velocity-mapped to the MCP stack, causing a cascade of electrons to strike a phosphor screen. The resulting fluorescence is recorded using a CCD camera, with electron positions centroided and sent to the control computer for analysis.
An image of the velocity-mapped photodetached electrons at the detector is then ob- tained through binning the centroided electron-event positions into a rectangular pixel-grid image, which may be of arbitrary pixel number. A large image pixel count provides more detail at the expense of greater statistical uncertainty in individual pixel intensities. The
§3.5 Imaging photoelectrons 47
velocity-map image is centred and then circularised by an angular dependent radial scaling determined by comparing adjacent radial slice intensity profiles. This correction is applied to the raw (x, y)-centroid coordinates before forming the velocity-map image, to eliminate any requirement for image pixel intensity interpolation. The inverse Abel transformation described above (Eq. 3.8) is then applied to return a slice image of the 3D photoelectron distribution for analysis.
Absolute energy calibration of photoelectron spectra may then be verified using mea- surements of published species, typically including O−[24]and O−
2 [65], that cover a similar radial area of the detector. The radial position of an electron on the detector is given by
r=√eKE× s 1000 |Vr| 100 R2E, (3.9)
where r is the radial position on the detector, eKE is the kinetic energy of the electron, Vr is the VMI repeller voltage, and R2E is a calibration factor (scaled by 100) dependent
on the spectrometer and operating conditions, with values typically between∼1.15−1.25 depending on how the image is circularised. By comparing circularised photoelectron spectra against known spectroscopic species, the correct R2E calibration factor may be applied. Known second order effects of the velocity map imaging lens, cause a small deviation in the relationship between detector radial position and electron kinetic energy. This effect has been characterised, using calibration measurements of O−2 as shown in Fig. 3.18. In most experiments this is a negligible effect, however for high precision spectra, a radial dependent R2E factor may be used.
300 400 500 600 700
Detector radial position (pixels) 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30
R2E Conversion Factor
O2 Detector Callibration
518 nm 520 nm 548 nm
Figure 3.18: Demonstration of the small variations that occur to the R2E factor at different regions of the detector.
During a measurement, we wish to only measure the detached electrons, and not the ionic or neutral molecules that are also present. This is achieved by switching the detector on and off, as the photoelectrons will reach the detector before the ions and other frag- ments. However MCP’s have an inherently high capacitance due to their structure, which makes rapidly switching from high to low voltages difficult, especially as the switching requirement of the experiment require a very short on time of∼15µs, to be continuously switching at 10 Hz. This problem is overcome by applying a base and a boost voltage to the plates. A base voltage of 1350 V, just below the minimum voltage required to achieve gain, is continuously supplied to the plates. When a trigger is received from the exper- iment control a fast Behlke switch quickly boosts the voltage up to 2000 V, essentially
switching the gain of the plates on so that incident electrons will be detected. After a 15µs window the plates switch back down to 1350 V, effectively turning the gain off. This reduces the voltage swing required from 2000 V to the much more achievable 650 V, while still ensuring that only the photoelectrons are detected.
Every aspect of the experimental set-up has been carefully designed to obtain the highest possible precision in the final image, which means even very small electric/magnetic fields can have a relatively large effect on the final resolution. To account for this, three orthogonal 2.4×2.4 ×2.4 m pairs of Helmholtz coils surround the imaging section of the spectrometer, with the currents carefully selected so as to cancel out static magnetic fields present in the laboratory. The entire imaging section is also enclosed in a shell of Mu-metal to shield from any DC magnetic fields that may arise.