Secondary Electron Emission Yields from a CsI Surface Under Impacts of Large Molecules at Low Velocities (5 × 103−7 × 104 ms−1)

Abstract

The yields of electron emission from a CsI surface bombarded with very heavy molecular ions have been measured by coincidence counting in time-of-flight mass spectrometry. These ions were produced by the matrix-assisted laser desorption/ionization technique. The masses ranged from 700 to 66 000 Da with velocities V between 5 × 10 3 and 7 × 10 4 ms For a few years, the interaction of heavy clusters and molecules with solids has been the subject of welldeveloped research. In these investigations the projectiles have had energies from a few eV/atom to a few MeV/atom. Over this large domain of velocities, the projectiles deposit a large amount of energy near the surface of the target. For masses in the range of 10 000 u to 100 000 u, energies of 10 keV to 2 MeV are deposited in a small volume. The transfer of very high densities of transitory and out-of-equilibrium energy into the solid leads to important modifications of the material and/or to the formation of craters independent of the energy range, the mass of the projectiles and the nature of the bombarded surface.1-5 Crater formation is accompanied by a very high sputter yield, as was first observed by Andersen and Bay. 6 The other de-excitation channels, such as electron and ion emission, have also been studied. The ion emission yield exhibits a non-linear increase as a function of the number of constituents or the mass of the projectile over the velocity range studied.7-10 For electron emission, results are scarce. Fundamentally, the electron emission induced by ions is connected to the ion/electron interaction in the medium. The electron emission processes are not simple, and the experimental results show both linear and non-linear relationships of the electron emission yield as a function of the velocity of the projectiles. 11-13

Using a superconducting tunnel junction detector to measure the secondary electron emission efficiency for a microchannel plate detector bombarded by large molecular ions

“…The factor of 2 does not disqualify the data from being used to determine e e , because e det can still be calculated by normalizing the data using substance P. It is well established that for low-mass ($1000 Da) ions at 30 keV, the probability of emitting secondary electrons from various surfaces is close to unity. 5,6 For substance P (1384 Da) at 30 keV, on average, C MCP /C STJ = 2.2. Thus, the probability of one or more ions impacting on the masked-MCP detector is 2.2-times greater than the STJ detector.…”

“…(6). The secondary electron yield, g e , depends both on the mass, m, of the molecular ion and its incident velocity, v. In order to separate out the dependence of g e on these two variables, the data for g e (shown in Fig.…”

“…1-3 However, more recent experiments reveal a non-vanishing secondary electron yield for ions below 10 km/s and show that it is possible to detect ions traveling as slow as about 4 km/s. 5,6 In light of these more recent measurements, a velocity 'threshold' is better interpreted as the velocity of the molecular ion where the secondary electron yield drops below unity.It remains possible to detect ions when the secondary electron yield is less than unity. Brunelle et al showed that the detection probability for large, slow moving molecular ions (66 kDa ion at 26 keV) is less than about 0.2.…”

“…[1][2][3][4][5][6] A result of this decrease is that ion detection schemes based on secondary electron detection have limitations in detection efficiency (and thus, sensitivity) for large, slow moving molecular ions. An ion detection velocity 'threshold' around 10 to 18 km/s, below which it is claimed that no secondary electrons are emitted, has been discussed.…”

“…Brunelle et al showed that the detection probability for large, slow moving molecular ions (66 kDa ion at 26 keV) is less than about 0.2. 6 Diminished detection efficiency for high-mass ions necessitates longer data acquisition times (or more sample) to achieve reasonable statistics and introduces limitations on quantitative assessments of the relative concentrations of analyte ions with varying masses. Overall, this decrease in the efficiency of producing secondary electrons limits the performance of mass spectrometry, particularly MALDI-TOFMS, as an analytical tool.…”