Validation of velocity map imaging conditions over larger areas

Abstract

We have established through simulations and experiments the volume in which Velocity Map Imaging (VMI) conditions prevail. We designed a VMI setup in which we can vary the ionization position perpendicular to the center axis of the time-of-flight spectrometer. We show that weak extraction conditions are far superior over standard three-plate setups if the aim is to increase the ionization volume without distorting VMI conditions. This is important for a number of crossed molecular beam experiments that already utilize weak extraction conditions, but to a greater extent for surface studies where fragments are desorbed or scattered off a surface in all directions. Our results on the dissociation of NO 2 at 226 nm show that ionization of the fragments can occur up to ± 5.5 mm away from the center axis of the time-of-flight spectrometer without affecting resolution or arrival position. a) sven.koehler@manchester.ac.uk 2

Adsorption site, orientation and alignment of NO adsorbed on Au(100) using 3D-velocity map imaging, X-ray photoelectron spectroscopy and density functional theory

“…The 3D-VMI apparatus has been described elsewhere; 25 here only brief description will be given. The spectrometer consists of two chambers, a molecular beam chamber and a UHV chamber that contains the Au(100) single crystal and the twelve plates VMI optics.…”

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.…”

Translational and Rotational Energy Distributions of NO Photodesorbed from Au(100)

“…The experimental setup was designed to measure the velocity distribution of fragments photodesorbed from a surface in three dimensions independently using a combination of TOF and VMI methods as described in detail elsewhere. 31 In brief, the spectrometer consists of two chambers, namely a molecular beam chamber which is only in operation when gas-phase calibration experiments are performed, and a UHV chamber that contains the Au(100) single crystal and the VMI optics. This chamber is evacuated by a 1000 L s -1 turbomolecular pump backed up by a mechanical pump achieving a base pressure of 5 × 10 -10 Torr.…”

“…Twelve plates were employed in order to increase the volume in which VMI conditions prevail. 31 Prior to experiments, the gold single crystal is cleaned by heating to 700 K and subsequently cooled to ∼178 K. Pure NO gas is introduced into the chamber through a molecular beam valve at a background pressure of ∼5 × 10 -6 Torr for a few minutes to establish a sub-monolayer NO coverage. The valve was then adjusted to yield a background pressure of NO of 5 × 10 -8 Torr; this suppressed the background signal sufficiently, while allowing the surface to be re-dosed continuously during the experiment.…”

“…The elaborate design of the ion optics (by using twelve plates, resulting in a shallow electric field gradient and weak extraction conditions) allows all those NO molecules ionized by the laser to be accelerated under true VMI conditions. 31 We recorded TOF profiles by varying the time delay between the desorption and the probe laser; these profiles were converted into velocity distributions that deliver the velocity component along the surface normal (vz) exclusively, i.e. not the overall speed of the desorbed molecules.…”