Double Focusing Sector Field Mass Research Articles

Unimolecular metastable decay of molecular clusters ions via monomer-fragment ejection: [(O 2) n] −*→[(O 2) n−1 O] −+O

The stability of (O 2) n − ions produced by electron attachment to a neutral cluster beam has been studied with a double-focusing sector field mass spectrometer. Besides monomer evaporation via metastable dissociation reactions (O 2) n −*→(O 2) n−1 −*+O 2 it was possible to observe for the first time metastable cluster ion dissociations via ejection of a monomer-fragment, i.e. [(O 2) n ] −*→[(O 2) n−1 O] −+O. Mechanism, dynamics and energetics of this reaction are presented and discussed.

Metastable decay of oxygen cluster cations and anions

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (up to n≈ 70) have been produced in a crossed neutral cluster/electron beam ion source. The abundance and stability of these ions have been studied with a double focusing sector field mass spectrometer (reversed geometry) at two different time windows after ion formation (in the metastable time region). Positive and negative mass spectra exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate in both cases with corresponding small and large metastable fractions (for the loss of O2) determined in the first field-free region, respectively. In accordance with the predictions of Klots, the probability of losing one monomer from (O2)+n exhibits a non-exponential dependence on time since ion formation. Metastable fractions determined for (O2)–n ions produced with ca. 0 eV electrons are generally larger than those for ions produced with ca. 7 eV electrons. This observation is due to a two-step dissociative production process of the latter ions via the 2πu state leading to less energetic ions than in case of direct attachment of zero-energy electrons. Furthermore, non-stoichiometric ions (O2)nO+ and (O2)nO– not only decay by O2 evaporation but also by O evaporation (and higher homologues in case of the positive ions). Branching ratios for the different decay channels observed may be rationalized in terms of energy considerations. Finally, strong autodetachment has been observed in case of small O2 cluster anions. Autodetachment lifetimes appear to increase with increasing cluster size.

Transfer of vibrational energy in nitrogen clusters doped with O 2: New evidence for the origin of quantum effects in the metastable decay series of (N 2) n+*

The decomposition of metastable (N 2) n +*, ((N 2) nO 2) +* and (0 2) n +* ions ( n up to 10) has been studied quantit a double-focusing sector field mass spectrometer as a function of cluster size n, and the number of evaporated neutral monomers p (with p up to n-1 for each cluster ion series). Whereas (N 2) n +* cluster ions show the known oscillatory pattern in the evapor probability versus the number of evaporated monomers (attributed to radiationless relaxation of vibrationally excited N 2 molecules in clusters), nitrogen cluster ions doped with one O 2 molecule do not show this behavior. In accordance with observations in liquids, this finding is interpreted as being due to an increased relaxation rate in the presence of the O 2 dopant.

Determination of absolute partial and total electron impact ionization cross-sections for CF 2Cl 2 from threshold up to 180 eV: an improved experimental method

Electron impact ionization of CF 2Cl 2 has been studied as a function of electron energy from threshold up to 180 eV using a double focussing sector field mass spectrometer (reversed geometry) with an improved ion transmission characteristic. With this set-up complete and/or controlled collection of mass selected parent and fragment ions can be achieved, thus permitting the accurate measurement of quantitative electron impact ionization cross-sections. Absolute partial ionization cross-section functions for the production of CF 2Cl + 2, CFCl + 2, CF 2Cl +, CCl + 2, Cl + 2, CFCl +, FCl +, CF 2 +, CCl +, Cl +, CF +, F +, C +, CFCl 2+ 2, CF 2Cl 2+, CFCl 2+, CCl 2+ 2, CCL 2+ and Cl 2+ have been determined. In addition, the total ionization cross-section function of CF 2Cl 2 is obtained and compared with previous experimental and theoretical results.

Appearance size and appearance energy of doubly and triply charged ammonia clusters

Multiply charged ammonia clusters are produced by electron impact ionization of neutral NH 3 clusters formed by nozzle expansion. The properties of these ions are studied with a high-resolution double-focusing sector field mass spectrometer. The observed critical appearance sizes are n 2=51 and n 3=121 for doubly and triply charged clusters, respectively. The appearance energy of doubly and triply charged ammonia clusters is found to be shifted way below the respective appearance energy of the monomer. This is in accordance with a recently suggested sequential ionization mechanism for the production of stable multiply charged van der Waals cluster ions.

Dynamics and kinetics of the metastable decay series: Ar3+*→Ar2+*→Ar+

Metastable decay of Ar+*3 produced by electron impact ionization of a supersonic Ar cluster beam has been investigated qualitatively and quantitatively with a double focusing sector field mass spectrometer. We present direct experimental proof that the observed metastable decay of Ar+*3 into Ar+ in the microsecond time regime proceeds predominantly via a sequential decay series Ar+*3→k3,2Ar+*2→ k2,1Ar+ involving the evaporation of a single monomer in each of these successive decays. The metastable decay rates deduced are very different, i.e., k3,2 =45 s−1 and k2,1 =680 s−1 at an ion acceleration voltage of 3 kV. Moreover, the metastable decay rates of Ar+*3 exhibit a strong dependence on time elapsed since electron impact production of the Ar+3, and on ion production parameters (i.e., size and internal excitation energy of the neutral precursor). For instance photodissociation (with visible laser light) of the neutral cluster beam leads to a dramatic increase of the observed metastable reaction rules yielding values of up to 100 000 3−1 in the μs time regime.

Quantized sequential decay series of metastable N 2 cluster ions (N 2) n+* → (N 2) n-1 +* → … → N 2+

The first experimental observation of sequential decay of metastable molecular cluster ions is reported. N 2 cluster ions are produced by electron impact ionization of a supersonic N 2 cluster beam and analyzed by a double-focusing sector field mass spectrometer (reversed geometry). It is found that metastable N 2 cluster ions lose more than one N 2 molecule in the microsecond time regime and decay predominantly via sequential series (N 2) n +* → (N 2) n-1 +* → … → N 2 +, evaporating a single monomer in each of these successive decay steps. The metastable decay rates determined in detail for cluster sizes 2 ⩽ n ⩽ 6 and n = 20 lie between 1 and 10 6 s −1 and depend strongly on the time elapsed after ion formation. The decay rates exhibit a quasi-periodic pattern, consistent with the assumption that up to four vibrational quanta (stored in neutral N 2 molecules within the cluster) relax into the phonon bath of the cluster during the sampling time.

Isotope enrichment in Ne clusters

Clusters of Ne atoms produced by adiabatic nozzle expansion have been studied with a high resolution double focusing sector field mass spectrometer. Analysis of the isotopic composition of these clusters (up to n=22) showed instead of the expected statistical distribution a pronounced shift to the heavier neon isotope 22Ne. The shift was found to correspond to an increase of the isotope ratio of 22Ne from 8.7% in the monomer to ∼12.5% in the clusters with n>4 (enrichment factor of ∼1.5.)

Metastable dissociation of doubly charged carbon dioxide cluster ions

Multiply charged CO/sub 2/ cluster ions are produced by adiabatic nozzle expansion and subsequent ionization by electron impact. They are analyzed in a double focussing sector field mass spectrometer (reversed geometry). (CO/sub 2/)/sub n//sup 2 +/ cluster ions are only detected above an electron energy of about 30 eV and with sizes n greater than or equal to 45 in the direct mass spectrum. Metastable decay reactions of these ions occurring in the first field free region are investigated by decoupling the analyzer fields. It is shown that (CO/sub 2/)/sub n//sub 2//sup +/ with sizes of n greater than or equal to 44 lose one neutral monomer or, roughly ten times less probable, two neutral monomers in these dissociation reactions. The total effective dissociation rates for these dissociation processes are very large, e.g. approx.2 x 10/sup 4/ s/sup -1/ for (CO/sub 2/)/sub 47//sup 2 +/. Conversely, no singly charged fragment ions were observed to be produced by Coulomb explosion from doubly charged clusters in the first field free region. Moreover, besides doubly charged stoichiometric ions the existence of doubly charged fragment cluster ions with n greater than or equal to 44 in observed in the mass spectra.

Unimolecular decay of metastable Ar cluster ions. Evolution of magic numbers in Ar cluster mass spectra

Unimolecular dissociation processes Ar n + → Ar m + (with n up to 25) were investigated with a double focussing sector field mass spectrometer. Ar n + (with n up to 97) and Ar n 2+ (with n from 91 up to 191) were produced by electron ionization of Ar clusters formed in a supersonic nozzle expansion under various stagnation gas conditions. In addition to collision-induced dissociation, the existence of metastable dissociation processes in the μs time scale was confirmed. In particular, it was demonstrated that metastable dissociation rates are strongly dependent on the stagnation gas temperature for Ar 3 + and Ar 4 + (with Ar 4 +* → Ar 2 + reaching a record metastable decay rate of 26 000 s −1 at −10° stagnation temperature), whereas metastable decays of Ar 2 + and Ar 5 + are roughly independent of stagnation conditions. Furthermore, we describe the observation that metastable decay not only occurs via the loss of one monomer, but also, for Ar 3 +, Ar 4 +, and Ar 5 +, via the loss of two monomers. Conversely, larger clusters (Ar 10 + to Ar 25 +) only decay in a metastable dissociation via the loss of one monomer. For cluster sizes 10 ≤ n ≤ 25, the highest total dissociation rate is found for Ar 20 + (8850 s −1). Moreover, because anomalies in the dissociation rate correspond to an abundance anomaly (magic number) in the normal mass spectrum, it is possible to conclude that the occurrence of magic numbers is caused by the intrinsic stability of the cluster ions rather than being due to the neutral growth process. These results are shown to be in agreement with recent calculations. In addition, we have studied collision-induced dissociation as a function of stagnation temperature for Ar 2 +, Ar 3 +, Ar 4 +, and Ar 5 +. For Ar 21 +, we were able to deduce an estimate of the collision-induced dissociation cross-section as a function of reaction product all the way from Ar 21 + → Ar 20 + to Ar 21 + → Ar 2 +.

Decomposition channels for multiply charged ammonia clusters

Multiply charged ammonia cluster ions are produced by adiabatic nozzle expansion and subsequent ionization by electron impact. They are analyzed in a double focussing sector field mass spectrometer (reversed geometry). Doubly charged clusters are only detected above a critical size of 51 and triply charged clusters above 121. Some of these multiply charged ions decay via metastable dissociation processes in the experimental time window accessible. Doubly charged ammonia clusters with sizes ofn≧51 lose one neutral monomer or, roughly ten times less probable, two neutral monomers. Conversely, triply charged ammonia clusters with sizes 110≦n≦120 show an extremely asymmetric Coulomb dissociation resulting in doubly charged cluster ions of about 90% of the initial mass

Unimolecular dissociation of N+4

The unimolecular dissociation process N+4 → N+2+N2 was investigated with a double focusing sector field mass spectrometer. N+4 was produced by electron impact ionization of N4 present in a free jet expansion. Besides collision induced dissociation, the existence of a metastable dissociation process with a decomposition rate of about 102 s−1 was demonstrated. The occurrence of this metastable process is interpreted using the results of quantum mechanically calculated potential energy curves of N2–N+2.

Metastable Arg cluster ions: temperature dependence of the decay rate

Unimolecular dissociation of Ar+ into Ar+ was investigated with a double focusing sector field mass spectrometer. Ar 3 was produced by electron impact ionization of Ar3 formed by nozzle: expansion. It was found that the metastable dissociation rate is strongly dependent on the stagnation gas temperature.