Surface three-dimensional velocity map imaging: A new technique for the study of photodesorption dynamics

Abstract

Validation of velocity map imaging conditions over larger areas Rev. Sci. Instrum. 84, 044101 (2013) The authors describe the application of a combination of velocity map imaging and time-of-flight ͑TOF͒ techniques to obtain three-dimensional velocity distributions for surface photodesorption. They have established a systematic alignment procedure to achieve correct and reproducible experimental conditions. It includes four steps: ͑1͒ optimization of the velocity map imaging ion optics' voltages to achieve optimum velocity map imaging conditions; ͑2͒ alignment of the surface normal with the symmetry axis ͑ion flight axis͒ of the ion optics; ͑3͒ determination of TOF distance between the surface and the ionizing laser beam; ͑4͒ alignment of the position of the ionizing laser beam with respect to the ion optics. They applied this set of alignment procedures and then measured Br͑ 2 P 3/2 ͒ ͑Br͒ and Br͑ 2 P 1/2 ͒ ͑Br ‫ء‬ ͒ atoms photodesorbing from a single crystal of KBr after exposure to 193 nm light. They analyzed the velocity flux and energy flux distributions for motion normal to the surface. The Br ‫ء‬ normal energy distribution shows two clearly resolved peaks at approximately 0.017 and 0.39 eV, respectively. The former is slightly faster than expected for thermal desorption at the surface temperature and the latter is hyperthermal. The Br normal energy distribution shows a single broad peak that is likely composed of two hyperthermal components. The capability that surface three-dimensional velocity map imaging provides for measuring state-specific velocity distributions in all three dimensions separately and simultaneously for the products of surface photodesorption or surface reactions holds great promise to contribute to our understanding of these processes.

Ion and velocity map imaging for surface dynamics and kinetics

“…Others have previously reported using position sensitive detection methods for studying surface scattering, 30 based on ion imaging [31][32][33] or velocity map imaging techniques, which were developed at around the same time as those for the gasphase imaging experiments. 34,35 These have predominantly used a geometry in which the surface and the detector are parallel, [36][37][38][39][40][41] which is convenient for the production of the imaging electric fields but prevents the projection of the full velocity distribution in the scattering plane onto the detector. Our new imaging setup (shown in Fig.…”

Exploring surface photoreaction dynamics using pixel imaging mass spectrometry (PImMS)

“…3 Although the technique was initially used to study gas-phase reactions, several groups have used this approach to study reaction dynamics at surfaces. [4][5][6][7][8][9][10][11][12][13][14] A common feature to all of these studies is the use of a time-of-flight mass spectrometer (TOF-MS) that is capable of spatially separating ions based upon differences in kinetic energy and trajectory. By using ion optics capable of velocity-vector focusing, ions with a common velocity and trajectory are focused to the same spot on the detector, typically a microchannel plate (MCP) coupled to a phosphor screen.…”

Validation of velocity map imaging conditions over larger areas

“…18,19,20,22 The solid angle of the cone that is reliably covered by a REMPI laser sheet is given by 2 × tan -1 (d ℓ / 2 × z) where d ℓ is the diameter of the laser sheet and z is the surface-laser distance. In other words, we could easily choose to record the whole velocity distribution within a 120° cone by placing the laser sheet 3.2 mm in front of the surface.…”

“…Some early reports that use VMI for surface desorption and scattering studies have emerged from various laboratories over the last three years. 18,19,20,21,22 It is evident that neutral fragments desorbed from or scattered off a surface can fly in all directions of the hemisphere above the surface. In contrast to surface studies using rotatable mass-spectrometers, surface VMI can record these slicing methods applied in typical gas-phase imaging experiments is helpful here.…”