Alkali Metal Targets Research Articles

Charge Inversion Mass Spectrometry: Dissociation of Resonantly Neutralized Molecules

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Charge inversion mass spectrometry: dissociation of resonantly neutralized molecules.

Charge inversion mass spectrometry is an MS/MS method in which the electric charge of the precursor ions is opposite to that of the secondary product ions. Charge inversion mass spectrometry is classified into four types depending on the electric charge and time scale of collisions. Charge inversion mass spectrometry using collisions with gaseous targets in the keV energy collision range has provided insights into the structures and reactions of ions and neutral molecules. The characteristics of charge inversion experiments are presented in terms of the reaction endothermicities and the cross sections and their dependence on the target species. In the case of rare-gas or simple molecular targets, double-electron transfer in one collision is effective to form positive ions from negative ions, while, in the case of alkali metal targets, successive single-electron transfers in two collisions is effective to form negative ions from positive ions. On the basis of the observed target-density dependence of the product ion intensity and thermochemical considerations for internal energy distribution using thermometer molecules, the charge inversion processes using alkali metal targets have been confirmed to occur by electron transfers in successive collisions and the dissociation processes are found to occur in energy-selected neutral species formed from near-resonant neutralization with alkali metal targets. While collisionally activated dissociation (CAD) is due to dissociation of activated ions with broad internal energy distributions, the charge inversion process using alkali metal targets is due to dissociation of energy-selected neutral species with narrow internal energy distributions. The charge inversion/alkali metal spectra provide clear differentiation of the isomeric cations of C(2)H(2), C(3)H(4) and dichlorobenzenes. The CAD spectra of these isomeric cations are similar.

Internal energy distribution in charge inversion mass spectrometry using alkali metal targets

A charge inversion mass spectrometer (PMS/NMS) by using alkali metal targets has been developed which produces clearer differentiation of hydrocarbon isomers than collision induced dissociation (CID). In PMS/NMS, mass-selected positive ions are made to collide with an alkali metal target, and the resulting negative ions formed upon two-electron transfer are mass analyzed. On the basis of the observed target-density dependence of product ion intensity and thermochemical considerations, we propose that the process of dissociative negative ion formation in PMS/NMS is by way of near-resonant neutralization, followed by spontaneous dissociation of the neutrals and then endothermic negative ion formation. CID spectra and PMS/NMS spectra have been measured for two types of so-called thermometer molecules, namely partially deuterated methanol and W(CO)6. The differences between the CID spectra and the PMS/NMS spectra of partially deuterated methanol demonstrate that the major process in PMS/NMS involves dissociation of the excited neutral species. The internal energy deposition of the charge inversion measured for W(CO)n+ (n = 4–6) ions indicates that dissociation occurs in the energy-selected neutrals formed by way of near-resonant neutralization. The relative peak intensities in the PMS/NMS spectra for some hydrocarbons depend strongly on the alkali metal target used, indicating the importance of internal energy in the dissociation of the excited neutral intermediates. These results demonstrate the utility of PMS/NMS as a technique for the investigation of the dissociation of energy-selected neutral intermediates and for isomeric differentiation.

Study of the dissociation of neutral intermediates using charge inversion mass spectrometry

In charge inversion experiments using a tandem mass spectrometer, mass-selected positive ions are made to collide with an alkali metal target, and the resulting negative ions formed upon two-electron transfer are mass-analyzed. The internal energy depositions are measured for the so-called thermometer molecule W(CO)6. The difference between the centered value of the internal energy of W(CO)6 and the energy level of the precursor ion is in good agreement with the ionization energy of the Cs target. This correlation indicates that dissociation occurs from energy-selected neutrals formed via near-resonant neutralization.

Dissociation mechanism of electronically excited C 3H 4 isomers by charge inversion mass spectrometry

The mechanism for dissociation of C 3H 4 isomers to form neutral radicals was investigated by charge inversion mass spectrometry using alkali metal targets. Electronically excited allene and propyne were formed from the corresponding positive ions by neutralization with alkali metal, and the neutral fragments resulting from dissociation of the excited molecules were converted into negative ions which were mass-analyzed and detected. From the comparison of peak intensities and peak widths among allene, propyne and 3,3,3-trideutero-propyne (CD 3CCH), the dissociation mechanism of electronically excited C 3H 4 was elucidated. The principal feature of the dissociation process involves the loss of two H atoms from the electronically excited hydrocarbons, which is attributed not to two independent CH bond cleavages, but rather to loss of a H 2 molecule. The loss of a H 2 molecule has a higher activation energy than loss of a H atom, despite the final state of the former having a lower heat of formation than that of the latter. In a CH bond cleavage, the weaker the bond, the more easily it is cleaved. Cleavage of a CC bond is substantially less favorable than the loss of hydrogen atoms, even though the former has a lower energy than the later. This may be attributed to the lower frequency of the CC bond vibration compared with that of the CH bond.

Discrimination of C 3H 4+ isomeric ions by charge inversion mass spectrometry using an alkali metal target

Charge inversion mass spectrometry was performed using an MS/MS instrument in which mass-selected positive ions were made to collide with alkali metal targets, and the resulting negative ions formed upon two-electron transfer were mass analyzed. Charge inversion spectra using Cs targets were measured for C 3H 4 + ions produced from allene (CH 2CCH 2) and propyne (CH 3CCH) by electron impact. The peak associated with C 3H 4 - in the charge inversion spectra is twice as intense for propyne as for allene, whereas the profile of the peaks associated with C 3H n − ( n = 0–2) is similar for both isomeric precursors. The kinetic energy release values at FWHM of the peak associated with C 3H 3 − formed from allene ions and propyne ions are 0.28 ± 0.04 eV and 0.64 ± 0.06 eV, respectively. A clear differentiation between the isomeric precursors can be made on the basis of these differences in the charge inversion spectra. The differences observed are attributed to the formation of excited C 3H 4 states by near-resonant neutralizations of allene and propyne ions. Collisionally activated dissociation (CAD) spectra using He targets were measured for the same C 3H 4 + isomeric precursor ions. The CAD spectra are similar for both isomeric precursor ions, although slight differences were detected in the weak peaks associated with the species formed upon CC bond cleavage. Charge inversion mass spectrometry using alkali metal targets was found to provide a much clearer differentiation between the isomeric ions of unsaturated hydrocarbons than CAD.

Relative cross-sections and kinetic energy release values in dissociative negative ion formation by successive single-electron transfer from alkali metal targets

Positive ions formed from C 2H 2 were collided with Cs, K and Na targets in the kiloelectronvolt energy range. The dissociative negative ion formations of C − 2 and C 2H − ions from C 2H + 2 ions and C − 2 ions from C 2H + ions with an alkali metal target were determined to be double-collision processes using the target density dependence. By considering the reaction energies and comparing with reionization to positive ions, main processes of the dissociative negative ion formations were assigned to successive single-electron transfers with the dissociation of excited neutrals. The kinetic energy release values at FWHM of C 2H − ions from C 2H + 2 ions were 0.55eV for Cs, 0.28 eV for K and 0.17eV for Na. The relative cross-sections of C 2H − ions from C 2H + 2 ions to those of C 2H − ions from C 2H + 2 ions were 5.35 for Cs, 1.89 for K and 0.32 for Na. These dependences on target species are discussed in relation to the energy difference for the exothermic neutralization.

Positronium formation in positron-lithium collisions

Differential and total cross sections have been studied for ground-state positronium (Ps) formation using the Glauber-eikonal approximation in positron-lithium atom collisions. The valence electron of the alkali-metal target and the effect of the core have been treated in the framework of a pseudopotential formalism. Results are reported for intermediate and high energies along with the first-order Born-approximation results. Even at very high energies the eikonal cross sections are found to be different from the corresponding first-order Born approximation showing the importance of higher-order effects.

Predicted phase differences in proton stopping by alkali metal targets

In this paper we report stopping powers calculated for alkali metal vapors (monatomic gases) and solid targets. The calculations are done using the Kinetic Theory of Stopping. The required electron velocity distributions are taken from Hartree-Fock calculations for the atoms, and from experimental Compton profiles for the solids. The calculations show that the stopping of the alkali metals is much greater in the vapor phase, about a factor 3–4 at the peak value of the stopping power where the difference is largest, and that this difference results primarily from phase dependent differences in the mean excitation energy. We present the stopping power curves for the alkali metal solids and vapors for projectile (p +) energies 50 keV < E p < 40 MeV. The phase difference is somewhat smaller than the value we predicted earlier from comparison of atomic stopping calculations with experimental solid state data.

Observation and characterization of the CH5 radical by neutralized ion beam techniques

The CH5(CD5) radical has been produced by charge transfer neutralization of fast ion beams using Na and Zn targets. The results obtained with Na targets are in excellent agreement with previous similar studies using alkali metal targets. In particular, no evidence for metastability was observed. Alternatively, results obtained with Zn targets show clear evidence for production of metastable states of the radical with lifetimes in the 1–10 μs range. Measurement of the kinetic energy released upon dissociation indicates that some neutral radicals produced in CH+5(CD+5)/Zn reactions have energies very close to the ionization limit. It is proposed that the observed metastability may be explained by the production of high Rydberg levels of the radical having sufficiently long radiative lifetimes.

H−andD−production by backscattering from alkali-metal targets

Measurements have been made of the total backscattered ${\mathrm{D}}^{\ensuremath{-}}$ and ${\mathrm{H}}^{\ensuremath{-}}$ yields from thick, clean targets of Cs, Rb, K, Na, and Li, bombarded with ${\mathrm{H}}_{2}^{+}$, ${\mathrm{H}}_{3}^{+}$, ${\mathrm{D}}_{2}^{+}$, ${\mathrm{D}}_{3}^{+}$ with incident energies from 0.15 to 4.0 keV/nucleus. All of the measurements were made at background pressures less than ${10}^{\ensuremath{-}9}$ Torr, and the alkali-metal targets were evaporated onto a cold substrate ($T\ensuremath{\sim}77$ K) in situ to assure thick, uncontaminated targets. For each target, the ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{\ensuremath{-}}$ yields exhibited maxima (as high as 0.08 per incident proton and deuteron) at incident energies between 0.3 and 1.4 keV/nucleus. For both hydrogen and deuterium incident at any energy, the negative-ion yield decreases in going from Cs to Li in the order given above. Also, a definite isotope effect was observed for every target used, with the ${\mathrm{H}}^{\ensuremath{-}}$ yield peaking at a lower incident energy than the ${\mathrm{D}}^{\ensuremath{-}}$ yield and in most cases, the maximum ${\mathrm{H}}^{\ensuremath{-}}$ yield was higher than the maximum ${\mathrm{D}}^{\ensuremath{-}}$ yield. Measurements of the ${\mathrm{D}}^{\ensuremath{-}}$ yield as a function of Cs coverage were also made for ${\mathrm{D}}_{3}^{+}$ bombarding a Ni substrate. The ${\mathrm{D}}^{\ensuremath{-}}$ yield maximized at or near the coverage at which the surface work function reached a minimum.

Sputtering of Alkali Atoms by Inert Gas Ions of Low Energy

Direct measurements of sputtering have been made using alkali metal targets bombarded by inert gas ions in high vacuum, the sputtered atoms being counted by a surface ionization detector. Measurements were made in the range 0-1800 ev ion energy for the ions ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, ${\mathrm{A}}^{+}$, and ${\mathrm{Xe}}^{+}$ incident upon Na and K target surfaces. Secondary electron coefficients $\ensuremath{\gamma}$ were measured simultaneously. It was found that over most of the energy range covered, the measured sputtering coefficient ${\ensuremath{\theta}}^{\ensuremath{'}}$ (number of sputtered atoms reaching the detector per positive ion incident on the target) was linear in $\mathrm{ln}E$ for ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{+}$ and in $\ensuremath{\surd}E$ for ${\mathrm{A}}^{+}$ and ${\mathrm{Xe}}^{+}$, where $E$ is the ion energy. At energies less than about 150 volts the sputtering rate appeared to be controlled by the amount of kinetic energy which could, on the average, be transferred to a single surface atom in a direct two-body collision. Thus at low energies ${\mathrm{Ne}}^{+}$ produced the most sputtering from a Na surface, whereas ${\mathrm{A}}^{+}$ produced the most from a K surface. At high energies, the sputtering rate was in the same order as (but not proportional to) the mass of the ion. The effect of surface contamination was also studied. An old surface showed a lower sputtering coefficient than a fresh surface and a higher and more erratic secondary electron coefficient, the effect being most pronounced for the light ions. The absolute sputtering coefficient $\ensuremath{\theta}$ (total number of sputtered atoms per incident ion) could not be measured directly, but it was estimated from the geometry that for 1000-volt ions the value of the coefficient ranged from about 1.0 for ${\mathrm{Xe}}^{+}$ on K to 0.05 for ${\mathrm{He}}^{+}$ on Na.