Trois types de spectrometrie de masse sont etudies: la spectrometrie NIBS («Neutralized ion beam spectroscopy»), la spectrometrie NRMS («Neutralization-reionization mass spectrometry») et la spectrometrie de masse CIDI («Collision-induced dissociative ionization»)

The neutralization of organic cations

During the epoch of first star formation, molecular hydrogen (H-2) generated via associative detachment (AD) of H- and H is believed to have been the main coolant of primordial gas for temperatures below 10(4) kelvin. The uncertainty in the cross section for this reaction has limited our understanding of protogalaxy formation during this epoch and of the characteristic masses and cooling times for the first stars. We report precise energy-resolved measurements of the AD reaction, made with the use of a specially constructed merged-beams apparatus. Our results agreed well with the most recent theoretically calculated cross section, which we then used in cosmological simulations to demonstrate how the reduced AD uncertainty improves constraints of the predicted masses for Population III stars.

https://science.sciencemag.org/content/sci/329/5987/69.full.pdf

Experimental Results for H2 Formation from H- and H and Implications for First Star Formation

Fragmentation energies for radicals of the type RH2 (RH=CH4, NH3, and H2O) produced by electron capture interactions of 5 keV RH2+ ion with Na or K atoms are reported. The experimental technique involves measurement of spatial beam profiles resulting from dissociation of neutral radicals following their formation in a near resonant electron transfer process. Cross sections for RH2+–Na capture reactions are typically 1x10−14 cm2. Fragmentation energies from measurements with Na target atoms are −2.65±0.14, −0.22±0.03, and −1.12±0.07 eV for CH5, NH4, and H3O, respectively. From our results with Na and K targets and published values for proton affinities, the vertical electron affinities of CH5+ and H3O+ are calculated to be 5.3±0.2 eV and 5.1±0.3 eV, respectively. Beam profiles for ND4 show this species to be metastable with a lifetime of about 1 μs. From this we estimate a potential barrier to dissociation in NH4(ND4) between 0.36 and 0.48 eV, indicating this species should be stable at low temperatures. C...

Energetics of fragmentation of CH5, H3O, and NH4 from neutralized ion‐beam experiments

The scattering of hydrogen from a cesiated tungsten surface

Secondary electron emission induced by impact of low-velocity molecular ions on a microchannel plate

The counting efficiency of a channel electron multipier is measured for H+ and H− impact with the input of the Channeltron grounded and for impact energies between 0.3 and 5 keV. The counting efficiency is found to be about 30% greater for H− than for H+ at energies below 1 keV. This difference and an observed difference between H− and H+ in the variation of counting efficiency across the Channeltron opening are attributed to the surface potential of the Channeltron near the opening, which can accelerate or decelerate the incident particles.

Channeltron efficiency for counting of H+ and H− at low energy

Absolute differential cross sections for the production of ${\mathrm{D}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{0}$ + ${\mathrm{D}}_{2}^{0}$ have been determined for single and double electron-capture collisions into dissociative states by 10- to 50-keV ${\mathrm{D}}_{2}^{+}$ in Cs vapor Integrations of the differential cross sections over the angles yield the total cross sections for the production of ${\mathrm{D}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{0}$ + $\mathrm{D}_{2}^{}{}_{}{}^{0}$ A theory has been developed of the angular distributions of dissociation fragments resulting from electron capture into dissociative states of diatomic molecular systems Predictions are made that the angular distribution $\frac{d{\ensuremath{\sigma}}_{f}}{d\ensuremath{\omega}}$ of any fragment $f$ should scale as $E_{i}^{}{}_{}{}^{\ensuremath{-}1}\frac{d{\ensuremath{\sigma}}_{f}}{d\ensuremath{\omega}}=f({E}_{i}{\ensuremath{\theta}}^{2})$, where ${E}_{i}$ is the incident energy of the diatomic molecular ion The observed distributions fit this scaling law An interesting structure is found at 235 keV ${\mathrm{deg}}^{2}$, which yields information about the dependence of the electron-capture cross section on the internuclear separation of the incident molecular ion during the interaction time

Angular distributions and total cross sections for D - formation from interaction of D + and D 0 with cesium

HIGHLY EXCITED STATES OF HYDROGEN MOLECULES. I. H$sub 2$, HD, AND D .

Differential cross sections for the production of D/sup -/ and D/sup 0/ + D/sub 2//sup ts0/ + D/sub 3//sup ts0/ are given for electron-capture collisions into D/sub 3//sup ts-/ and D/sub 3//sup ts0/ repulsive states by bombardment of Cs with D/sub 3//sup ts+/ ions in the energy range 1.5--7.5 keV. A theory is presented for the angular distributions of dissociation fragments in the special cases in which the dissociation (i) produces two fragments, or (ii) produces three fragments, one of which has a small energy in the molecular center-of-mass frame. In both cases a scaling law is predicted: (..gamma../sub n/E/sub i/)/sup -1/ dsigma/sub n//d..omega.. = f (..gamma../sub n/E/sub i/theta/sub n//sup 2/), where dsigma/sub n//d..omega.. is the differential cross section for finding fragment n at angle theta/sub n/, E/sub i/ is the incident energy, and ..gamma../sub n/ is a mass ratio. The D/sup -/ distributions are observed to fit this scaling law. Peaks are found in both the D/sup -/ and D/sup 0/ + D/sub 2//sup ts0/ + D/sub 3//sup ts0/ distributions for which 1/2E/sub i/theta/sup 2/ approx. = 15 keV deg/sup 2/ and 14 keV deg/sup 2/, respectively. A corresponding peak is also found in the neutral distribution for whichmore » 2E/sub i/theta/sup 2/ approx. = 14 keV deg/sup 2/. The peaks indicate a repulsive state in D/sub 3//sup ts-/ which decays into D/sup -/ + D/sub 2//sup ts0/, and a repulsive state in D/sub 3//sup ts0/ which decays into D/sup 0/ + D/sub 2//sup ts0/ and which has an energy surface close to that of the D/sub 3//sup ts-/ repulsive state. An energy surface of a repulsive singlet state of D/sub 3//sup ts-/ is presented which does decay into D/sup -/ and D/sub 2//sup ts0/.« less

Electron capture to D/sub 3//sup ts-/ and D/sub 3//sup ts0/ repulsive states by D/sub 3//sup ts+/ in Cs

Absolute differential cross sections for the production of D/sup -/ and D/sup 0/ + D/sub 2//sup 0/ have been determined for single and double electron-capture collisions into dissociative states by 1.0- to 5.0-keV D/sub 2/ /sup +/ in Cs vapor. Integrations of the differential cross sections over the angles yield the total cross sections for the production of D/sup -/ and D/sup 0/ + D/sub 2//sup 0/. A theory has been developed of the angular distributions of dissociation fragments resulting from electron capture into dissociative states of diatomic molecular systems. Predictions are made that the angular distribution dsigma/subf//d..omega.. of any fragment f should scale as E/subi//sup -1/ dsigma/subf//d..omega.. = f (E/subi/theta/sup 2/), where E/subi/ is the incident energy of the diatomic molecular ion. The observed distributions fit this scaling law. An interesting structure is found at 23.5 keV deg/sup 2/, which yields information about the dependence of the electron-capture cross section on the internuclear separation of the incident molecular ion during the interaction time. (AIP)

J. A. Ray

Papers

Channeltron efficiency for counting of H+ and H− at low energy

Angular distributions and total cross sections for D - formation from interaction of D + and D 0 with cesium

HIGHLY EXCITED STATES OF HYDROGEN MOLECULES. I. H$sub 2$, HD, AND D .

Electron capture to D/sub 3//sup ts-/ and D/sub 3//sup ts0/ repulsive states by D/sub 3//sup ts+/ in Cs

Angular distributions and total cross sections for D − formation from interaction of D 2 + with cesium