Momentum mapping spectrometer for probing the fragmentation dynamics of molecules induced by keV electrons

Abstract

We describe a new experimental setup for studying the fragmentation dynamics of molecules induced by the impact of keV electrons using the well-known technique of recoil ion momentum spectroscopy. The apparatus consists of mainly a time- and position-sensitive multi-hit particle detector for ion analysis and a channel electron multiplier detector for detecting the ejected electrons. Different components of the setup and the relevant electronics for data acquisition are described in detail with their working principles. In order to verify the reliable performance of the setup, we have recorded the collision-induced ionic spectra of the CO2 molecule by the impact of keV electrons. Information about the ion pairs of CO+:O+, C+:O+ and O+:O+ resulting from dissociative ionizing collisions of 20 and 26 keV electrons with a dilute gaseous target of CO2 molecules has been obtained. Under conditions of the present experiment, the momentum resolutions of the spectrometer for the combined momenta of CO+ and O+ ions in the direction of the time-of-flight axis and perpendicular to the direction of an electron beam are found to be 10.0 ± 0.2 and 15.0 ± 0.3 au, respectively.

Dissociative-ionization cross sections for 12-keV-electron impact on CO2

“…The detailed description of the experimental setup is given in our recent publications [28,29]. However, some further modifications in the earlier setup have been made which are given as follows: Firstly, to get rid of the possible oil vapor contamination in the system, the oil diffusion pump is replaced by a 400 liter s −1 turbo molecular pump (Model 8/450, M/s Elettrorava, Italy) which gives the same ultimate vacuum in the system as before; secondly, we have modified our spectrometer by incorporating three new high-transmission (95%) nickel meshes in place of M 1 , R 1 , and M 2 in the spectrometer shown in Fig.…”

“…However, some further modifications in the earlier setup have been made which are given as follows: Firstly, to get rid of the possible oil vapor contamination in the system, the oil diffusion pump is replaced by a 400 liter s −1 turbo molecular pump (Model 8/450, M/s Elettrorava, Italy) which gives the same ultimate vacuum in the system as before; secondly, we have modified our spectrometer by incorporating three new high-transmission (95%) nickel meshes in place of M 1 , R 1 , and M 2 in the spectrometer shown in Fig. 1 of [29]; the present overall transmission efficiency T of the spectrometer is estimated to be ∼43%. This is done to increase the ion collection efficiency and to get better uniformity of the applied extraction field inside the spectrometer.…”

“…After these modifications we have observed ∼20% increase in the mass resolution of the spectrometer and, on average, a 10% increase in ion collection efficiency in comparisons to our previous values of these parameters. All other experimental parameters are the same as used in [29]. Briefly, 12-keV electrons extracted from an electron gun collide with a dilute CO 2 gas beam emanating from a hypodermic needle.…”

Ionic fragmentation of the CO molecule by impact of 10-keV electrons: Kinetic-energy-release distributions

“…The experimental steup and data analysis procedures are described in detail in our previous publications [33,34]. Briefly, a monoenergetic beam of 10-keV electrons extracted from an electron gun is made to collide with CO molecules (99.99%) effusing from a hypodermic needle (length = 1.2 cm; diameter = 0.01 cm).…”

“…The errors for RIFs and precursor-specific relative PICSs are calculated using the above given uncertainties for each ion single. The momentum vectors of fragment ions detected in coincidence are calculated from their respective TOFs (t) and positions (x,y) on the MCP [33]. The kinetic energy of individual fragment ions is determined from their momentum vectors and the total KER is obtained from the sum of the kinetic energies of individual fragments involved in that particular dissociation channel.…”