Multimass Velocity-Map Imaging with the Pixel Imaging Mass Spectrometry (PImMS) Sensor: An Ultra-Fast Event-Triggered Camera for Particle Imaging Andrew T. Clark, † Jamie P. Crooks, † Iain Sedgwick, † Renato Turchetta, † Jason W. L. Lee, ‡ Jaya John John, ∥ Edward S. Wilman, ‡ Laura Hill, ∥ Edward Halford, § Craig S. Slater, § Benjamin Winter, § Wei Hao Yuen, § Sara H. Gardiner, ‡ M. Laura Lipciuc, ‡ Mark Brouard, § Andrei Nomerotski, ∥ and Claire Vallance* ,‡ † Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K. ‡ Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, U.K. § Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K. ∥ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, U.K. * S Supporting Information ABSTRACT: 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. 1. INTRODUCTION Velocity-map imaging 1,2 has been used with great success in the field of small-molecule reaction dynamics to study molecular photofragmentation events and other processes. The velocity distributions of fragment ions are highly sensitive to the detailed dynamics of the dissociation and yield information on the identity of the potential energy surface(s) involved, transition state geometries, bond strengths, and any product internal excitation. In recent years, the size of the molecular systems studied by velocity-map imaging (VMI) has steadily increased, and it has been shown that even relatively large molecules often yield structured, and therefore information- rich, images. 3 VMI and related techniques are now routinely used to study the fragmentation of small to medium sized organic molecules in the gas phase, and in the longer term, such an approach has potential applications in mass spectrometric fragmentation studies. Tandem mass spectrometry (or MS/ MS) is becoming increasingly important in the study of biological molecules in the gas phase, 4 and the ability to image the fragments as they fly apart from each other has the potential to add a new dimension to such studies. The velocity distributions of the fragments contain information on the energetics of the fragmentation process and could provide a useful probe of bond strengths and internal excitation as well as a rapid means to distinguish between parent and daughter ions. Imaging studies on larger molecules present a different set of challenges to those on small molecules. The denser energy level structure of larger fragments means that resolving individual quantum states in the radial structure of the images is rare, and thus achieving the ultimate in velocity resolution is less important. However, such molecules often have multiple fragmentation pathways and understanding the competition between these pathways is an important aspect of probing the fragmentation dynamics. In a small-molecule experiment, it is usually sufficient to image a single fragment in order to obtain a complete picture of the fragmentation dynamics. However, for larger molecules, it becomes highly desirable to image multiple fragments on each time-of-flight cycle. The detectors used in most VMI experiments consist of a pair of microchannel plates (MCPs), which convert incoming ions into electron bursts, followed by a fast phosphor screen that creates an optical image of the electrons. The image on the phosphor is captured using a charged-coupled device (CCD) Received: October 5, 2012 Revised: October 22, 2012 Published: October 26, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 10897 dx.doi.org/10.1021/jp309860t | J. Phys. Chem. A 2012, 116, 10897−10903