SPADcam Velocity Map Imaging of single electrons and ions

We report on the novel application of a highly sensitive photon imaging sensor — a Single Photon Avalanche Diode (SPAD) camera imager — for Velocity Map Imaging (VMI) detection of single electrons or ions, coined SPADcamVMI. The exposure of the SPAD sensor (with 512 × 512 pixels) can be synchronised in time with sub-nanosecond precision to an external trigger, and can operate in time-gated mode with exposures as short as 6 ns. The synchronised exposure of the SPAD512 sensor can operate at full-frame readout rates up to 0.1 MHz. We demonstrate the performance of SPADcamVMI on electrons and ions produced in atomic xenon and molecular fenchone photoionization by a low rep-rate (30 Hz) tunable nanosecond laser, and by femtosecond lasers at variable rep-rates ranging from 100 Hz all the way up to 150 kHz. Single frame data can be read out in a 1-bit mode indicating whether or not each pixel registered a photon. Alternatively, a varying dynamic range from 4 to 12 bit depth per SPAD pixel can be obtained by integrating trigger events on-camera prior to data download. We show furthermore that a more accurate location of the particle position on the VMI-detector can be obtained by event-centroiding the SPADcamVMI position of single ions and electrons. The in-built time-gating capabilities of the SPAD512 camera permit the monitoring of specific time windows by selecting gate delay and width without requiring additional hardware. In the case of ion detection such gating may be used to select specific ion mass ranges using ion time-of-flight techniques. By simply adjusting the trigger delay and gate width we show four such fragment mass-selected ion VMI images obtained in the dissociative three-photon ionization of fenchone. The SPADcamVMI technology opens up many applications in fundamental research and commercial (OEM) instrumentation of single electron and ion momentum detection, with short time-gated synchronized detection at high particle data rates. Applications ranging from direct 3D-electron momentum tomography to mass-selected Imaging Mass Spectrometry, including electron-ion coincidence and covariance imaging are now feasible at very fast data rates in a simple experimental implementation at affordable costs. Furthermore, the rapid innovation cycle of SPAD-based sensors with many new sensors and imaging cameras expected in the next few years offers a promising potential for expanding this application range.