Hybrid BEEQ tandem mass spectrometer for the study of ion/surface collision processes

Abstract

A hybrid mass spectrometer consisting of a magnetic sector, two electric sectors, and a quadrupole mass filter (BEEQ) has been built for the study of polyatomic ion/surface collision phenomena over the energy range of a few electron volts to several keV. Primary ions are generated by electron ionization or by chemical ionization, and the first two sectors are used to deliver a monoenergetic beam of ions, of a selected mass-to-charge m/z ratio, to a decelerator which sets the desired collision energy. The target, which can be introduced into the system without breaking vacuum, is mounted on a goniometer and situated in an electrically shielded region in the center of a large scattering chamber which contains an electric sector and a quadrupole mass analyzer used for kinetic energy and mass measurements on the ejected ions. These analyzers rotate around the scattering center to allow selection of the scattering angle of ions leaving the surface. Ultimate pressures attainable in the main scattering chamber are below 10v9 Torr allowing molecular targets, such as self-assembled monolayers of alkyl thiols on gold, to be examined without surface contamination. Low-energy (20-100 eV) collisions of polyatomic ions are reported, and examples are given of the effects of collision energy and scattering angle on surface induced dissociation mass spectra, The kinetic energy of the inelastically scattered ions is also measured, and in some cases, the internal energy can be estimated, the two measurements together providing information on energy partitioning associated with surface collisions. For example, it is shown that n-butylbenzene molecular ions of 25 eV colliding with ferrocenyl-terminated self-assembled monolayer surfaces, rebound with 10 eV of recoil energy and 3 eV of internal energy. The remainder of the energy goes into the surface. The capability of the BEEQ instrument to provide data on ion/surface reactive collisions is also illustrated with reactions such as alkyl group transfer at self-assembled monolayer surfaces. In addition, data are given showing the ability of the system to provide information on the kinetic energy distributions of ions generated in the course of high-energy collisions at the surface. Mass analysis of the sputtered products provides the instrument with secondary-ion mass spectrometry capabilities.

Collisions of ions with surfaces at chemically relevant energies: Instrumentation and phenomena

“…The reported values for kinetic energies of fragment ions range up to about 15 eV, although in most cases they are just a few eV, 79,91,95,96 demonstrating the high inelasticity of collisions that lead to SID. In a recent study on biological ions at normal impact 97 very small (ϳ0.1 eV) energies have been measured.…”

“…͑Note the nomenclature B ϭmagnetic sector, Eϭelectrostatic analyzer, Qϭquadrupole mass filter, and TOFϭtime-of-flight.͒ Since the ion beam leaving the double focusing sector field mass spectrometer is spatially focused onto the exit slit of the mass spectrometer, a special lens system must be used to refocus the ion beam onto the surface, which is typically some distance away. Suitable lens systems for this purpose include a static quadrupole doublet 95 and associated deceleration lens system, as in the instrument at Purdue, or an einzel lens 121 and associated deceleration lens system, as in the instrument at Innsbruck. 108 There are also instruments that use only a single magnetic sector for mass separation of the projectile ions, as in the B-TOF instrument at the University of Osnabrück 122 and the Purdue BQ instrument.…”

Covalent chemical modification of self- assembled fluorocarbon monolayers by low- energy CH2Br2+· ions: a combined ion/surface scattering and X-ray photoelectron spectroscopic investigation

“…45 Briefly, dibromomethane was introduced into the ion source (10 À5 Torr nominal sample pressure) and ionized by electron impact (70 eV). The resulting ions were accelerated to 2 keV translational energy, and mass and energy selected, respectively, by the magnetic and electrostatic analyzers of a double focusing mass spectrometer.…”

Differentiation of isomeric oxygenated adsorbates using low-energy ion/surface collisions

“…The kinetic energy analyzer was set in a low-resolution mode so as to pass ions of a broad range of energies in the range of a few eV. These conditions are known to efficiently transfer the products of inelastic, reactive, and chemical sputtering processes [36]. The selected conditions give maximum scattered ion transmission without significant angular or velocity discrimination, although the latter types of experiments have been performed earlier, using the same instrumentation.…”

“…All experiments were performed in a custom-built, hybrid mass spectrometer with geometry BEEQ (B = magnetic sector, E = electric sector, Q = quadrupole mass analyzer), a detailed description of which has been provided [36]. Briefly, volatile samples were independently introduced into the ion source (10 −5 Torr nominal sample pressure) and ionized by electron impact (70 eV).…”