Craig S. Slater

Multimass velocity-map imaging with the Pixel Imaging Mass Spectrometry (PImMS) sensor: an ultra-fast event-triggered camera for particle imaging.

We present the first multimass velocity-map imaging data acquired using a new ultrafast camera designed for time-resolved particle imaging. The PImMS (Pixel Imaging Mass Spectrometry) sensor allows particle events to be imaged with time resolution as high as 25 ns over data acquisition times of more than 100 μs. In photofragment imaging studies, this allows velocity-map images to be acquired for multiple fragment masses on each time-of-flight cycle. We describe the sensor architecture and present bench-testing data and multimass velocity-map images for photofragments formed in the UV photolysis of two test molecules: Br(2) and N,N-dimethylformamide.

PImMS, a fast event-triggered monolithic pixel detector with storage of multiple timestamps

PImMS, or Pixel Imaging Mass Spectrometry, is a novel high-speed monolithic CMOS imaging sensor tailored to mass spectrometry requirements, also suitable for other dark-field applications. In its application to time-of-flight mass spectrometry, the sensor permits ion arrival time distributions to be combined with 2D imaging, providing additional information about the initial position or velocity of ions under study. PImMS1, the first generation sensor in this family, comprises an array of 72 by 72 pixels on a 70 μm by 70 μm pitch. Pixels independently record digital timestamps when events occur over an adjustable threshold. Each pixel contains 4 memories to record timestamps at a resolution of 25 ns. The sensor was designed and manufactured in the INMAPS 0.18 μm process. This allows the inclusion of significant amounts of circuitry (over 600 transistors) within each pixel while maintaining good detection efficiency. We present an overview of the pixel and sensor architecture, explain its functioning and present test results, ranging from characterisation of the analogue front end of the pixel, to verification of its digital functions, to some first images captured on mass spectrometers. We conclude with an overview of the upcoming second generation of PImMS sensors.

The application of the fast, multi-hit, pixel imaging mass spectrometry sensor to spatial imaging mass spectrometry

Imaging mass spectrometry is a powerful technique that allows chemical information to be correlated to a spatial coordinate on a sample. By using stigmatic ion microscopy, in conjunction with fast cameras, multiple ion masses can be imaged within a single experimental cycle. This means that fewer laser shots and acquisition cycles are required to obtain a full data set, and samples suffer less degradation as overall collection time is reduced. We present the first spatial imaging mass spectrometry results obtained with a new time-stamping detector, named the pixel imaging mass spectrometry (PImMS) sensor. The sensor is capable of storing multiple time stamps in each pixel for each time-of-flight cycle, which gives it multi-mass imaging capabilities within each pixel. A standard velocity-map ion imaging apparatus was modified to allow for microscope mode spatial imaging of a large sample area (approximately 5 × 5 mm(2)). A variety of samples were imaged using PImMS and a conventional camera to determine the specifications and possible applications of the spectrometer and the PImMS camera.

Three-dimensional imaging of carbonyl sulfide and ethyl iodide photodissociation using the pixel imaging mass spectrometry camera

The Pixel Imaging Mass Spectrometry (PImMS) camera is used in proof-of-principle three-dimensional imaging experiments on the photodissociation of carbonyl sulfide and ethyl iodide at wavelengths around 230 nm and 245 nm, respectively. Coupling the PImMS camera with DC-sliced velocity-map imaging allows the complete three-dimensional Newton sphere of photofragment ions to be recorded on each laser pump-probe cycle with a timing precision of 12.5 ns, yielding velocity resolutions along the time-of-flight axis of around 6%-9% in the applications presented.

Application of fast sensors to microscope mode spatial imaging mass spectrometry

The application of fast detectors to microscope mode spatial imaging mass spectrometry, in which the two-dimensional distributions of particular ion masses over a sample are projected onto a two-dimensional detector, allows the imaging of all mass peaks desorbed from a sample on each time-of-flight cycle. Detecting all fragments per duty cycle means that fewer ionisation and acquisition cycles are required for a full set of data, leading to a reduction in overall detection time, amount of sample required, and sample degradation. Results from a proof-of-concept experiment, in which a fast frame transfer charge-coupled device camera (CCD) with 10 ns time resolution was coupled to a modified velocity-mapped imaging apparatus, are presented.

PImMS: A self-triggered, 25ns resolution monolithic CMOS sensor for Time-of-Flight and Imaging Mass Spectrometry

In this paper, we present the Pixel Imaging Mass Spectrometry (PImMS) sensor, a pixelated Time-of-Flight (TOF) sensor for use in mass spectrometry. The device detects any event which produces a signal above a programmable threshold with a timing resolution of 25ns. Both analogue and digital readout modes are available and all pixels can be individually trimmed to improve noise performance. The pixels themselves contain analogue signal conditioning circuitry as well as complex logic totalling more than 600 transistors. This large number can be achieved without any loss of quantum efficiency thanks to the use of the patented Isolated N-well Monolithic Active Pixels (INMAPS) process. In this paper, we examine the design of the PImMS 1.0 device and its successor PImMS 2.0, a significantly enlarged sensor with several added features. We will also present some initial results from mass spectrometry experiments performed with PImMS 1.0.

Ion charge-resolved branching in decay of inner shell holes in Xe up to 1200 eV

Using a new multi-electron multi-ion coincidence apparatus and soft x-ray synchrotron radiation we have determined branching ratios to final Xe n+ states with 2 < n < 9 from the 4d−1, 4p−1, 4s−1, 3d−1 and 3p−1 Xe+ hole states. The coincident electron spectra give information on the Auger cascade pathways. We show that by judicious choice of coincident electrons, almost pure single charge states of the final ions can be selected.

Time-Resolved Studies of Induced Torsional Motion

This chapter reports the application of the PImMS camera to the imaging of laser-induced torsional motion of axially chiral biphenyl molecules through femtosecond Coulomb explosion imaging (CEI).

Pulsed-Field Electron-Ion Imaging

This chapter presents a new method of extracting and velocity-mapping both the ions and electrons resulting from photoionisation onto a single detector in each acquisition cycle. It is demonstrated that it is possible to maintain a high velocity resolution using this approach through the simultaneous imaging of the photoelectrons and photoions resulting from the (\(3+2\)) resonantly enhanced multi-photon ionisation of Br atoms produced following the photodissociation of Br\(_{2}\) at 446.41 nm. Pulsed ion extraction represents a substantial simplification in experimental design over conventional photoelectron-photoion coincidence (PEPICO) imaging spectrometers and is an important step towards performing coincidence experiments using a conventional ion imaging apparatus coupled with a fast imaging detector. The performance of the PImMS camera in this application is investigated, and a new method for the determination of the photofragment detection efficiencies based on a statistical fitting of the coincident photoelectron and photoion data is presented.

Principles of Coulomb Explosion Imaging

This chapter introduces the concept of Coulomb explosion imaging (CEI), which is central to the work presented in Chaps. 6 and 7. The important principles upon which the technique is based are described, and the current capabilities and limitations of such experiments are discussed in the context of recent work.