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We report on attachment of free electrons to fullerenes C(n) (n = 60, 70, 76, 78, 80, 82, 84, 86) and to Sc(3)N@C(80). The attachment cross sections exhibit a strong resonance at 0 eV for all species. The overall shape of the anion yield versus electron energy is quite similar for the higher fullerenes, with a minimum around 1 eV and a maximum which gradually shifts from 6 eV for C(60) to approximately 4 eV for large n. The endohedral Sc(3)N@C(80) exhibits a particularly shallow minimum and a maximum below 4 eV. We model autoionization of the anions with due consideration of the internal energy distributions. The relatively low electron affinity of Sc(3)N@C(80) is reflected in its reduced ion yield at higher attachment energies.

Mass spectrometric analysis of metastable decay reactions is devoted to the measurements of the kinetic energy release distribution (KERD) for the decay of singly charged rare gas dimer ions and , the doubly charged acetylene parent ion and the singly and doubly charged SF6 fragment ions, like for example , and . The KERDs are obtained either from high-resolution mass analysed ion kinetic energy spectra or the measurement of ion beam profiles using a specially improved mass spectrometric system. Due to the high energy resolution measurements and theoretical studies based on ab initio calculations of potential energy curves it is possible to assign the reaction products of the rare gas dimer decays to electronic transitions in the excited parent ion. The and also the ions are investigated because of obscurities in the production of their fragment ions. The unusual shape of the ionization cross section indicates that at sufficiently high electron energies the fragmentation channel of doubly charged contributes significantly to the ion yield. Additional measurements of the corresponding appearance energies confirm the existence of this second production channel.

Electron ionization cross sections for hydrogen and deuterium clusters have been measured near the threshold energy with a high-resolution electron impact ionization mass spectrometer. Appearance energies are reported for odd-sized H+n and D+n cluster ions for sizes n ≤ 11.

We report absolute partial cross sections for the formation of selected positive and negative ions resulting from electron interactions with uracil. Absolute calibration of the measured partial cross sections for the formation of the three most intense positive ions, the parent C4H4N2O+2 ion and the C3H3NO+ and OCN+ fragment ions, was achieved by normalization of the total single uracil ionization cross section (obtained as the sum of all measured partial single ionization cross sections) to a calculated cross section based on the semi-classical Deutsch–Mark formalism at 100 eV. Subsequently, we used the OCN+ cross section in conjunction with the known sensitivity ratio for positive and negative ion detection in our apparatus (obtained from the well-known cross sections for SF+4 and SF−4 formation from SF6) to determine the dissociative attachment cross section for OCN− formation from uracil. This cross section was found to be roughly an order of magnitude smaller, about 5 × 10−22 m2 at 6.5 eV, compared to our previously reported preliminary value. We attribute this discrepancy to the difficult determination of the uracil target density in the earlier work. Using a reliably calculated cross section for normalization purposes avoids this complication.

The recent addition of (i) a third sector field to our two sector field mass spectrometer (resulting in a BE1E2 field configuration) and of (ii) a high performance electron gun enables us now to study in detail the time dependence of the kinetic energy release distribution (KERD) over a relatively wide range of cluster ion lifetimes. Using this newly constructed device we have studied here for the first time KERDs and deduced binding energies BEs (using finite heat bath theory) of large rare gas cluster ions (an upper size limit in earlier studies arose from the fact that different naturally occurring isotopes will contribute to a chosen metastable peak when the size exceeds a certain value) and in addition of fullerene ions smaller and larger than \( {\rm{C}}_{{\rm{60}}}^{\rm{ + }} \) (here again contaminating coincidences did not allow such studies earlier). Moreover, high precision KERD measurements for the decay of rare gas dimer ions in conjunction with model calculations (using recently calculated potential energy curves for the rare gas dimer ions) also enable us to obtain information on the dynamics and the mechanisms of the underlying spontaneous decay reactions . In addition, we are also reporting here a novel method (unified breakdown graph method) to determine cluster ion binding energies using a recently constructed tandem mass spectrometer BESTOF allowing us to measure fragmentation patterns arising from the unimolecular decay of molecular cluster ions induced by surface collisions. The fragmentation and reaction patterns of protonated ethanol cluster ions investigated here clearly demonstrate in contrast to some of the earlier cluster ion studies that unimolecular dissociation kinetics determines the formation of product ions in the surface-induced decomposition.

We determined appearance energy (AE) values AE(Xn+/X) for the formation of singly (He+) and doubly charged (He2+) He ions and multiply charged Kr ions Krn+ up to n = 6 following electron impact on He and Kr atoms using a high-resolution electron impact ionization mass spectrometer. The data analysis employs an iterative, non-linear least-squares fitting routine, the Marquart–Levenberg algorithm, in conjunction with either a 2-function or a 3-function fit based on a power threshold law. This allows us to extract the relevant AEs and also the corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and with spectroscopic AE values and the extracted exponents p are compared with other available experimental data and with the predictions of the various Wannier-type power law models. One observation is particularly noteworthy, namely the fact that none of the available experimental data seem to support the large values of 'p' predicted by the Wannier–Geltman and the generalized Wannier law for n > 3.

We analyze metastable fragmentation of mass selected neon cluster ions, produced by electron impact ionization of a neutral cluster beam under conditions that lead to isolated electronically excited species (excitons) within the cluster. We have determined metastable fractions for parent ions up to Ne 20 + . Moreover, the average kinetic energy of the fragment ions has been measured. The kinetic energy obtained for the exciton driven metastable decay exceeds the value obtained for statistically driven monomer evaporation reactions by at least an order of magnitude.

Using a high resolution two sector field mass spectrometer of reversed geometry we have measured metastable fractions and mass-analyzed ion kinetic energy peaks for metastable decay reactions involving monomer evaporation via (O2)n+*→(O2)n−1++O2. Both, average kinetic energy release 〈KER〉 data derived from the peak shapes and the time dependence of the metastable fractions show a characteristic dependence on cluster size indicating a change in the metastable fragmentation mechanism when going from the dimer to the dekamer ion. Moreover, the 〈KER〉 data contain information about the transition state temperature and thus one can use finite heat bath theory to calculate the binding energies of the decaying cluster ions. The results obtained are in fair agreement with (i) previous results based on gas phase ion equilibria measurements and with (ii) the corresponding bulk value.

Electron impact ionization cross sections measured close to threshold are reported for both the monomers and dimers of ozone and OClO using a new high resolution electron impact apparatus. The present appearance energies AE(O3+/O3)=12.70±0.02 eV, AE (OClO+/OClO)=10.55±0.02 and AE(ClO+/OClO=13.37±0.03 eV derived from the measured ionization cross sections are in excellent agreement with the vertical threshold values determined for these ions by high resolution PES and PIMS photoionization studies. The corresponding appearance energies determined for the dimer ions, 10.10±0.3 eV for (O3)2+ and 9.87±0.2 eV for (OClO)2+, are both red shifted with respect to the monomer case. The bond energy (0.70–0.3+0.5) eV of (OClO)2+ estimated from these data is similar to that of other dimer ions, whereas the bond energy of (O3–O3+) with (2.55−0.4+0.6) eV is rather large suggesting an unusual structure for the cationic ozone dimer ion. Based on quantum chemical calculations on various levels we are led to the conclusion t...

We have carried out measurements on metastable fragmentation of mass selected argon cluster ions which are produced by electron impact ionization of a neutral argon cluster beam. From the shape of the fragment ion peaks (MIKE scan technique) one can deduce information about the distribution of kinetic energy that is released in the decay reaction. In this study, for Ar 5 + to Ar 15 +, it is Gaussian and thus we can calculate from the peak width the mean kinetic energy release 〈KER〉 of the corresponding decay reactions. Using finite heat bath theory we calculate from these data the binding energies of the decaying cluster ions.

In this review experimental techniques developed in the past years in our laboratory concerning (i) the measurement of partial electron impact ionization cross sections, (ii) the determination of kinetic energy release distributions for fragment ions produced by spontaneous and electron-induced dissociations of molecular ions and (iii) the derivation of appearance energies for parent and fragment ions will be summarized and results will be given for the propane molecule as an example relevant for plasma applications.

Using a recently constructed high resolution crossed beams apparatus involving a hemispherical electron monochromator we have studied from threshold up to 25 eV electron attachment to N2O clusters in the size range up to about 10. Attachment spectra obtained for the production of (N2O)nO− exhibit with increasing cluster size a gradual redshift of the maximum peak position of the major rather broad resonance located at around 2 eV, i.e. values varying from 2.58 ± 0.1 eV for O− to 1.70 ± 0.1 eV for (N2O)7O−. In addition, with increasing cluster size the anion signal is more fully developed near 0 eV, i.e., whereas for n = 0 the O− ion signal is zero at about zero energy (in accordance with expectations from thermochemical data), there exists an appreciable ion signal at zero energy for n = 7. Above the 2 eV resonance there exists a shallow minimum in the cross section at about 4 eV with a slightly increasing, probably structured, broad feature toward higher energies. The present results are compared to previous attachment studies concerning the monomer and clusters of N2O and to O− desorption studies from condensed N2O thereby elucidating the origin and evolution (under different phase conditions) of the various features of the attachment spectra.

Negative ion formation following low energy (0–10 eV) electron attachment to free and bound CF2Cl2 molecules is studied in (1) a molecular beam experiment (single molecules, homogeneous clusters, and mixed CF2Cl2/NH3 clusters) and (2) a UHV surface experiment where desorption of negative ions from condensed CF2Cl2 is observed. From single gas phase CF2Cl2 molecules we observe Cl− and F− generated via dissociative electron attachment from a resonance near 0 eV and 3 eV, respectively, as the most abundant ions. From homogeneous clusters (CF2Cl2)n, we additionally detect undissociated complexes of the form (M)n−(M=CF2Cl2) including the stabilized monomer CF2Cl2− and also “solvated fragment ions” of the form Mn⋅X−(X=Cl, F). Their relative abundance vs size (n) of the final product varies in a significant different way between (M)n− and Mn⋅X− reflecting the different relaxation probabilities in the initial cluster. In the desorption spectra, the dominant low energy Cl− gas phase resonance is strongly suppressed in favor of a significant resonant feature appearing near 8 eV. These last results are discussed in light of previously reported giant enhancements of electron induced desorption of Cl− and F− from CF2Cl2 on Ru coadsorbed with water or ammonia ices under 250 eV electron impact [Q. B. Lu and T. E. Madey, Phys. Rev. Lett. 82, 4122 (1999); J. Chem. Phys. 111, 2861 (1999)].

We have measured the kinetic energy release (KER) distributions for electron-induced dissociation of mass-selected molecular parent and fragment ions of propane. They are compared with distributions determined for spontaneous (metastable) dissociation. The average KER for induced dissociation of C3H8+ into C3H7+ is 13.2±1.2 meV, about 42% larger than for the spontaneous reaction. This large difference is attributed to the dramatically reduced time at which the induced reaction can be sampled. In contrast, the KER for dissociation of C3H7+ into C3H5+, which is characterized by a large reverse activation energy, is hardly affected by the experimental time scale.

Using newly developed experimental techniques we have studied spontaneous (metastable) dissociation, electron-induced dissociation, and surface-induced dissociation reactions of singly and multiply charged fullerene ions. In particular, we will discuss results concerning (1) competition in spontaneous decay reactions between evaporation of neutral and ionized fragments, thereby extending our own previous studies on the spontaneous decay of multiply charged fullerene ions, (2) electron induced dissociation for singly and doubly charged C60 ions with special emphasis on the time dependence of kinetic energy release distribution thereby extending the recent cross section measurements of Salzborn and co-workers [Phys. Rev. Lett. 71 (1993) 3439], and (3) surface induced dissociation of singly and doubly charged C581 ions thereby extending previous studies by Kappes and co-workers [J. Chem. Phys. 104 (1996) 3629, 3638] on singly charged C60 ions. (Int J Mass Spectrom 192 (1999) 267‐280) © 1999 Elsevier Science B.V.

Highly monochromatized electrons (with energy distributions of less than 30 meV FWHM) are used in a crossed beam experiments to investigate electron attachment to oxygen clusters (O2)n at electron energies from approximately zero eV up to several eV. At energies close to zero the attachment cross section for the reaction (O2)n+e→(O2)m− (for m=1, 2, and 3) rises strongly with decreasing electron energy compatible with s-wave electron capture to (O2)n. Peaks in the oxygen attachment cross sections present at higher energies (≈80 meV, 193 meV, 302 meV) can be ascribed to vibrational levels of the anion populated by attachment of an electron to a single oxygen molecule within the target cluster via a direct Franck–Condon transition from the ground vibrational state v=0 to a vibrational excited state v′=7,8,9,… of the anion produced. The vibrational structures observed here for the first time can be quantitatively accounted for by model calculations using a microscopic model to examine the attachment of an ele...

Highly monochromatized electrons (with 30 meV FWHM) are used in a crossed beams experiment to investigate electron attachment to oxygen clusters (O2)n at electron energies from approximately zero eV up to 2 eV. At energies close to zero the attachment cross section for the reaction (O2)n +e → O2− varies inversely with the electron energy, indicative of s-wave electron capture to (O2)n. Peaks in the attachment cross section present at higher energies can be ascribed to vibrational levels of the oxygen anion. The vibrational spacings observed can be quantitatively accounted for. In addition electron attachment to mixed oxygen/ozone clusters has been studied in the energy range up to 4 eV. Despite the initially large excess of oxygen molecules in the neutral clusters the dominant attachment products are undissociated cluster ions (O3)m− including the O3− monomer while oxygen cluster ions (O2)n− appear with comparatively low intensity.

Electron impact ionization of C60 and C70 was studied using a molecular/electron beam ion source in combination with a two sector field mass spectrometer operated in the ion beam deflection mode. Relative partial ionization cross sections for the production of singly and multiply charged parent ions (up to charge state z=4) and fragment ions (down to C+44 in the case of C60 and down to C2+50 in the case of C70) were determined from threshold up to 1000 eV electron energy. Absolute partial and total ionization cross sections are obtained using a novel approach for the absolute calibration involving an intercomparison of the cation with the anion yield. The results obtained reveal not only an anomalous large parent ion cross section as compared to other ionization channels [e.g., σ(C+60/C60) is more than a factor of 30 larger than σ(C+58/C60)] but also anomalies for the production of multiply charged parent and fragment ions. For instance, the maximum cross section for the formation of C2+60 amounts to 30% ...

Electron attachment to BrCN and ClCN is studied in a crossed beam experiment. Relative cross sections for the formation of negative ions in the energy range 0−15 eV are reported. The kinetic energy...

Based on the results of recent swarm experiments, it has been proposed that the increase in the cross section for SF 5 − formation observed at an electron energy, E e of about 0.3 eV in electron beam studies of electron attachment to SF 6 is due to the combined (opposing) effects of the vibrational heating of the molecule by the attached electron, which enhances the dissociation of the nascent (SF 6 − ) ∗ ion, and the reduction of the cross section for capture (s-wave) of the electron by SF 6 with increasing E e . Further, it has been shown that the dissociation reaction is endothermic by 0.12 eV, and that, contrary to previous suggestions, there is no potential barrier to this dissociation reaction. Now we have carried out electron beam studies of the SF 6 attachment reaction in Berlin at gas temperatures, T g , over the range 300 to 920 K and in Innsbruck at T g below 300 K. These studies have provided support for the above proposals concerning the appearance of the SF 5 − peak and for a reaction endothermicity of 0.12 eV. Thus these studies have clarified the doubts about the products of the SF 6 attachment reaction at low electron energy.

Metastable fragmentation of neon dimer ions has been investigated by measuring and analyzing high resolution kinetic energy release distributions. Data were obtained by a modified MIKE (mass-analyzed ion kinetic energy) scan technique. Due to the high energy resolution it was possible to distinguish two different reaction mechanisms in the micros time regime which produce Ne+ ions with different kinetic energy distributions. Theoretical studies based on ab initio calculations of potential energy curves allowed the assignment of the reactions to specific electronic transitions in the excited Ne2+ ion. The unusual bimodal kinetic energy release distribution arises from competition between radiative and non radiative decay of the long-lived Ne2+ II(1/2)u state.

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.

Total and partial cross-sections for electron impact ionization of CH 4 (and other hydrocarbon molecules) were determined from threshold to 1000 eV. Ion kinetic energy distributions were measured applying a deflection and retarding field method. The extraction of ions from the ion source was simulated fully three dimensionally including the presence of a guiding magnetic field for the electrons. Thereby discrimination factors were determined as a function of the initial ion kinetic energy. Multiplication of these factors with the ion signal leads to accurate relative partial cross-sections. By normalizing the sum of these partial cross-sections to an absolute value for the total cross-section taken from the literature absolute partial cross-sections were obtained that agree well with previous measurements where a complete collection of the product ions has been demonstrated. In addition, with the present method it is possible to determine cross-sections that are differential with respect to the initial kinetic energy of the ion.

The production of O− and O2− by dissociative electron attachment to ozone has been studied between about 0 and 10 eV with a high resolution crossed beam apparatus recently developed in the Innsbruck laboratory. A previously unobserved sharp structure is observed in the formation of O− ions at zero incident energy. This large additional cross section peak has important consequences for the role of ozone in anion formation processes in the terrestrial ionosphere.

Much effort has been recently devoted to the experimental determination of absolute partial and total electron impact ionization cross sections of molecules and radicals due to the ever increasing importance of these cross sections in many applications such as low- and high-temperature plasma physics and chemistry, atmospheric physics and mass spectrometry. In addition supporting calculations have been recently developed in order to allow the analytic quantitative description of these cross section functions for modeling codes. This review summarizes recent experimental and theoretical progress in this area of electron impact ionization including also a short description of (i) recent advances in the field of electron/ion collisions (i.e., kinetic energy release distributions in dissociative excitation reactions of molecular ions) and (ii) a novel approach to the determination of electron induced appearance energies.

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (n up to 70) have been produced in a crossed neutral beam/electron beam ion source. The abundance and stability of the ions formed have been analyzed with a double focussing sector field mass spectrometer in a series of experiments. Positive and negative ion mass spectra observed exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate with correspondingly small and large metastable fractions of (O2) + and (O2) − ions, respectively. (O2) + ions may also lose up top=(n−1) monomers by collision induced dissociation with monotonously decreasing probability with increasingp. Metastable fractions determined for (O2) − ions produced with appr. zero eV electrons are in general larger than those for ions produced with appr. 7 eV electrons. (O2) − ions are also observed to decay via autodetachment, with lifetimes increasing with increasing cluster size. Finally, here we were able to prove that an apparent loss of the monomer fragment O (and higher homologues) observed in the metastable time regime is due to ordinary metastable monomer evaporation in the acceleration region. Moreover, we will also present here some new data and interpretation concerning the electron attachment cross section function for O2 clusters.

z+ ions with z = 2-4 and were invariably found to be higher than for the corresponding spontaneous eliminations. The results are interpreted as being due to a time-dependent KER, since the non-fixed energy in the transition state is increased for higher internal energies and shorter lifetimes of the reactant ions. These results are compared with time-resolved KER values obtained previously, using an ion trap/reflectron instrument, for C 2 elimination from C60 + . The two sets of data are modeled by finite heat bath theory (FHBT) using calculated rate energy dependencies. Calculations are performed using currently available informa- tion on the activation parameters. The dissociation is modeled with an Arrhenius pre-exponential factor, A = 5 × 10 19 s -1 (at a temper- ature kBT = 0.2 eV) and a vaporization (activation) energy ∆ Evap = 10.3 eV. Modeling the data necessitates inclusion of radiative decay in parallel with the dissociation. Radiative decay is dominant at times longer than a few microseconds.

The interaction of an electron beam of variable energy (2–180 eV) with a beam of neutral carbon dioxide clusters was studied using high resolution mass spectrometry. The ions (CO 2 ) x + , (CO 2 ) x CO + , (CO 2 ) x O + , (CO 2 ) x C + , (CO 2 ) x O 2 + , (CO 2 ) x O − and (CO 2 ) x O − were observed with x up to 10 and 15 for the negative and positive ions, respectively. The occurrence of (CO 2 ) x O 2 + is especially exciting, because O 2 + cannot be produced by direct electron ionization of CO 2 and thus this cluster ion has to be created via an internal cluster ion/molecule reaction. Moreover, despite the fact that the neutral clusters are generated under the same expansion conditions, the size distribution of the negative duster ions, in particular of (CO 2 ) x − , is completely different from that of the positive duster ions. The (CO 2 ) x − distribution shows pronounced structure, reflecting the intrinsic stability of the CO 2 − solvated by the remaining ligands. In addition, measurements of the stagnation pressure dependence of observed positive and negative ion intensities yields information about the neutral precursor of a specific cluster ion. Furthermore, the electron energy dependences of the negative cluster ions have been measured between threshold and 25 eV, showing a significant difference between the two negative duster ion series observed.

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 + .

Clusters of (O 2 ) n molecules produced by supersonic nozzle expansion have been investigated using electron impact ionization mass spectrometry. Mass-resolved spectra (with n up to 110) show three homologous series consisting of (O 2 ) n + , ((O 2 ) n ·O) + and ((O 2 ) n ·O) 2+ ions. The critical appearance size found for doubly charged, stoichiometric and fragment, O 2 cluster ions is n 2 =92. This confirms a recent prediction about n 2 in van der Waals clusters. The appearance energy AE of ((O 2 ) n · O) 2+ is shifted well below the appearance energy of O 2+ from O 2 . This is in accordance with a recently suggested sequential ionization mechanism for the production of stable doubly charged cluster ions. Moreover, the shape of the ionization cross section curve of singly charged cluster ions indicates that above the appearance energy of doubly charged ions an appreciable number of singly charged ions are produced via Coulomb explosion of unstable doubly charged ions.

Electron attachment to CO2 clusters formed by nozzle expansion was investigated in a crossed molecular‐beam–electron‐impact–mass spectrometer system. In addition to cluster ions previously observed at 3–4 eV electron energy we observe presently cluster ions produced at around zero electron energy. Some of these ions are likely produced by a less dissociative production mechanism allowing the probing of cluster beams with better reliability than previously.

In previous experiments on doubly charged Xe clusters, no Xe2+n cluster ions were observed for n less than 53 atoms and no Xe3+n for n less than 114 atoms. From recent computer simulations it was inferred that Xe2+51 ions have lifetimes ∼100 ps while Xe2+55 ions have lifetimes ∼10 μs in agreement with these previous experimental results. Using a high performance mass spectrometer we have repeated these experiments. In contrast to these earlier findings, we are here able to observe doubly charged Xe cluster ions with sizes down to n2=47 (at lifetimes of ∼400 μs). The critical appearance size for triply charged Xe clusters is n3=108. In accordance with a recently suggested sequential ionization mechanism for the production of stable doubly charged van der Waals cluster ions, the appearance energy of Xe2+n was found to lie well below the appearance energy of Xe2+ from Xe.

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.

SO − 2 , SO 2 ·O − and SO 2 ·SO − are produced by electron attachment to SO 2 clusters under single collision conditions. SO − 2 , which is not produced at all in low-pressure ion sources, is more abundant than SO 2 ·O − and SO 2 ·SO − . Measured relative attachment cross sections for these ions show significant differences when compared to O − and SO − produced by attachment to SO 2 , i.e. the first resonance is very efficiently quenched in favor of SO − 2 production. This is in contrast to previous findings in O 2 , CO 2 and N 2 O.

Kr clusters produced in a supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry. Mass resolved spectra (with n up to 180) show two homologous series consisting of Kr+n and Kr2+n ions. The distribution of Kr+n ions shows distinct magic number effects, the observed abundance anomalies being very similar to the ones observed in Ar and Xe. This confirms the superior stability of closed‐shell and ‐subshell icosahedral structures. Moreover, we have found evidence for the occurrence of Kr3+n and Kr4+n ions. It was possible to determine appearance sizes of these multiply charged cluster ions (yielding n2=69, n3=156, and n4=264), and to study the electron energy dependence of singly and doubly charged cluster ions (yielding a linear threshold law). These results are discussed in view of various theoretical considerations and previous results where available.

Electron attachment to SO2 clusters formed by nozzle expansion was investigated (n up to 8) in a molecular-beam electron-ionization mass-spectrometer system. Electron ionization of SO2 clusters was also studied (n up to 18) showing no pronounced structure in the mass spectra and no strong dependence on electron energy, the dominant positive ion being the (SO2) + series. Also present but less abundant are the fragment ion series (SO2) n SO+, (SO2) n S+ and (SO2) n O+ with decreasing intensities in that order. The dominant negative ion is (SO2) − . The homologous series (SO2) n O− and (SO2) n SO− are less abundant, the series (SO2) n S− has not been observed at all (except SO2·S− produced in the background gas via secondary processes). The negative ion mass spectra show a strong dependence on electron energy due to a rich resonance like structure of the attachment cross sections involved. These attachment cross sections have been determined (up to 40 eV) and they show significant differences for the different homologous series. The most striking feature is that for instance (SO2) n SO− ions do not show any signal at the first main resonance of SO− from SO2. Likewise (SO2) n O− ions only show a strongly diminished signal at the first main resonance of O− from SO2. This is in contrast to results in O2, CO2 and N2O. Conversely, (SO2) − ions show — besides peaks at the position of the first and second O− resonance — additional resonances below and above these peaks. In addition, (SO2) 5 − and larger ones show a zero energy peak consisting of stoichiometric SO2 cluster ions similar to observations in O2, CO2 and H2O. The attachment cross section of S− from SO2 has been found to show an additional previously undetected peak at ∼1 eV. Moreover, the present study revealed the existence of a S − (n up to 8) series being produced with nearly zero energy electrons via volume and surface processes in and around the ion source.

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 yiel...

Unimolecular and collision-induced dissociation of Ar/sub 2/ /sup +/ produced by electron-impact ionization of Ar/sub 2/ were studied quantitatively with a double-focusing mass spectrometer. The occurrence of the metastable dissociation process Ar/sub 2/ /sup +asterisk/..-->..Ar/sup +/+Ar is interpreted qualitatively in terms of potential-energy curves calculated recently using quantum-mechanical methods.

Clusters of N2 molecules formed in a supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry. Mass resolved spectra (with n up to 190) show three homologous series, i.e., consisting of (N2)+n, (N2)nN+, and [(N2)n N]2+ ions. The distribution of (N2)+n ions shows distinct magic number effects. Moreover, there is evidence for the existence of (N2)2+n and (N2)3+n ions. The critical appearance sizes of observed multiply charged cluster ions are (N2)2+99, (N2)99N2+, and (N2)3+215. These results are compared with theoretical predictions. Moreover, a study of the electron energy dependence of singly and doubly charged N2 cluster ions close to threshold (appearance energies) gives new insight into the ionization mechanism.

In this note we present the first account of a study of metastable (unimolecular decay) and collision-induced dissociation of Ar3+ cluster ions using an experimental setup consisting of a molecular beam-electron impact ion source and a double focussing (reversed Nier Johnson geometry) mass spectrometer. The existence of the following metastable decay processes (accessible by our sampling time window) could be demonstrated: Ar3+* Ar2+ and Ar3+* Ar+. The processes were studied as a function of electron impact energy. The present results are of importance in order to provide some guidance for the development of appropriate theoretical models for the dissociation of cluster ions.

Electron attachment and electron impact ionization of SF6 and SF6/Ar clusters have been investigated in a molecular beam/electron impact ion source/mass spectrometer system. Positive ion mass spectra of neat SF6 clusters contain [(SF6)n⋅SFx]+ fragment ion series with 0≤x≤5 and in addition [(SF6)nS2F5]+ ions which have to be formed via international ion molecule reactions. Positive ion mass spectra of mixed SF6/Ar clusters include ion series of nonstoichiometric fragment ions (SF5⋅Arn)+, (SF6⋅SF5⋅Arn)+, [(SF6)2⋅SF5⋅Arn]+, and, in addition, stoichiometric ions comprised of (SF6⋅Arn)+ and [(SF6)2Arn]+. These stoichiometric ions are only present with n≥3. Electron attachment to neat SF6 clusters gives rise only to stoichiometric (SF6)−n ions, whereas attachment to mixed SF6/Ar clusters yields ion series consisting of (ArnSF6)−, [Arn(SF6)2]−, and [Arn⋅(SF6)3]−. This indicates that the dissociative channel into SF−5 present in the monomer is efficiently quenched in case of clusters. The negative ion mass spectr...

Electron attachment to O 2 clusters formed by nozzle expansion was investigated in a molecular-beam electron-impact mass spectrometer system. Two homologous series, (O 2 ) n − and (O 2 ) n O − (with n up to 15), were observed. Their relative attachment cross-section functions show significant differences. (O 2 ) n O − ions have a similar overall cross-section shape to O − produced by dissociative attachment to O 2 (i.e. a single peak at ∼ 7 eV), whereas (O 2 ) n − ions have an additional peak at ∼ 0 eV electron energy. The latter result explains the large thermal electron attachment rates observed in high-pressure and low-temperature swarm experiments. Moreover, measured stagnation pressure characteristics for different ions at different electron energies show that (O 2 ) n − formed with electrons close to zero energy are probably produced by a less dissociative production mechanism than other ions. Electron ionization of O 2 clusters was also studied. Measured mass spectra at 70 eV show a characteristic odd/even alternation for (O) n + ions.

The negative ions O− and C− produced by dissociative attachment of electrons in CO are measured using a quadrupole mass spectrometer. A trochoidal monochromator is used to give an electron energy distribution of about 0.07 eV. Within this resolution, the O− ion current exhibits a vertical onset at threshold and structure is observed at higher energies. The cross section of the production of C− shows an onset at 10.20 ± 0.04 eV, a peak at 10.4 eV, and a second peat at 10.9 eV. The cross section at the first C− peak is found to be smaller than the peak cross section for O− production by a factor of 3300 ± 20%. Using the previously determined value for the O− cross section (2 × 10−19 cm2) the peak cross section for C− formation is about 6 × 10−23 cm2.

Stace suggests that the cluster dissociation effects observed recently by the present authors are due to photoexcitation of ion rather than neutral clusters. Experimental proof is given here that the observed phenomena are due to the coaxial interaction of visible laser light with the neutral cluster beam and cannot be due to the interaction of the laser light with the cluster ions, which are produced in the ion source to monitor changes in the neutral cluster beam.

Total and partial cross sections for electron impact ionization of CH4 were measured from threshold to 1000 eV. Ion kinetic energy distributions were measured applying a deflection and retarding field method. The extraction of ions from the ion source was simulated fully three-dimensionally including the presence of a magnetic field for electron guidance. Thereby, discrimination factors were determined as a function of the initial ion kinetic energy. Multiplication of these factors with the ion signal leads to relative partial cross sections. By normalizing the sum of these partial cross sections to an absolute value taken from the literature, absolute partial cross sections were obtained that agree well with previous measurements where a complete collection of the product ions has been demonstrated. Moreover, with the present method, it is possible to determine cross sections that are differential with respect to the initial kinetic energy of the ion.

We have measured the kinetic energy released in the metastable fragmentation of mass selected neon and krypton cluster ions, produced by electron impact ionization of a neutral cluster beam. Finite heat bath theory is applied to determine the transition state temperatures and binding energies of the metastable cluster ions.

Dissociative electron attachment (DA) to single NO molecules and NO clusters is studied in two independent crossed electron--molecular-beam experiments with high-energy resolution including a kinetic-energy analysis of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ fragment ions. It is shown that the DA cross section exhibits a vertical onset near 7.45 eV that corresponds to the energetic threshold of the DA channel ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}{(}^{2}P)+{\mathrm{N}}^{*}{(}^{2}D).$ No indication of the ground-state channel ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}{(}^{2}P)+\mathrm{N}{(}^{4}S)$ [O. J. Orient and A. Chutjian, Phys. Rev. Lett. 74, 501 (1995)] and no indication of long-lived ${\mathrm{N}}^{\mathrm{\ensuremath{-}}}$ [H. Hiraoka, R. K. Nesbet, and L. W. Welsh, Jr., Phys. Rev. Lett. 29, 130 (1977)], as recently proposed, could be found. ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ formation from NO clusters shows the same features as from single NO molecules, indicating the same reaction mechanism, thus also excluding coupling to the ground-state DA channel clusters.

C3H52+, formed by electron impact ionization of propane, undergoes metastable decay into C2H2++CH3+. We have monitored this reaction in a magnetic mass spectrometer of reversed geometry that is equipped with two electric sectors (BEE geometry). Three different techniques were applied to identify the fragment ions and determine the kinetic-energy release (KER) of spontaneous Coulomb explosion of C3H52+ in the second and third field free regions of the mass spectrometer. The KER distribution is very narrow, with a width of about 3% [root-mean square standard deviation]. An average KER of 4.58±0.15 eV is derived from the distribution. High level ab initio quantum-chemical calculations of the structure and energetics of C3H52+ are reported. The activation barrier of the reverse reaction, CH3++C2H2+ (vinylidene), is computed. The value closely agrees with the experimental average KER, thus indicating that essentially all energy available in the reaction is partitioned into kinetic energy.

Metastable fractions and MIKE-peaks for metastable decay reactions involving O2 and O3 evaporation from odd numbered oxygen cluster ions have been measured using a high resolution two sector field mass spectrometer of reversed geometry. Whereas O5+∗ exclusively decays via O3 loss, the larger non-stoichiometric oxygen clusters evaporate O2 molecules. The average kinetic energy release 〈KER〉 data derived from the peak shapes is independent from the emitted neutral particle between 1 and 2 meV; however, the time dependence of the metastable fractions shows a characteristic dependence on cluster size indicating a change in the metastable fragmentation mechanism when going from the O5+∗ to the larger odd numbered oxygen cluster ions. The present results are in excellent agreement with binding energies of oxygen and ozone molecules to oxygen cluster ions published in the literature.