Delayed Electron Emission Research Articles

DELAYED IONIZATION OF FULLERENES AND FULLERENE DERIVATIVES UPON LASER DESORPTION AND SURFACE COLLISION

Delayed electron emission has been observed upon laser desorption for several fullerenes, endohedral metallofullerenes, and large carbon clusters formed by coalescence reactions. Odd-numbered carbon clusters and fullerene derivatives with exohedral functional groups do not show delayed ionization under similar conditions, presumably due to lower dissociation energies. Thus, delayed electron emission is suggested to be a characteristic indicator for strongly bound fullerene structures. Measurements of the rate of delayed electron emission are used together with structural information from other sources to calculate ionization potentials for these species. Selective observation of delayed electron emission from scattered [Formula: see text] after surface impact and neutralization of [Formula: see text] on graphite confirms an earlier report of this process by Whetten et al.6

DELAYED ELECTRON EMISSION OF NEGATIVELY CHARGED TUNGSTEN CLUSTERS

The delayed electron emission of negatively charged tungsten clusters has been investigated on a time scale from 1 to 500 ms. After being stored in a Penning trap clusters ions [Formula: see text] were heated via multiphoton absorption (hν=1.81 eV). In contrast to alkali and coinage metals no photofragmentation could be detected. Instead, for all cluster sizes studied so far only a decrease in the initial ion intensity as a function of time after excitation was observed. This decrease is not caused by ion loss from the trap, but has to be attributed to neutralization via delayed electron emission. The presented results strongly suggest that this process can be viewed as “thermionic emission” known from bulk-metal surfaces.

Thermionic emission from small clusters: direct observation of the kinetic energy distribution of the electrons

Photoelectron spectra of small mass-separated W n − clusters cooled cooled in a seeding gas are recorded with a low laser flux that ensures single-photon events. The spectra exhibit (i) sharp features due to direct photoemission and (ii) a broad emission signal whose intensity decreases with increasing kinetic energy. The latter is caused by delayed electron emission, a process similar to bulk ‘thermionic emission’. The kinetic energy distribution of these electrons can be approximated by a Boltzmann function with a temperature corresponding to the photon energy divided by the number of vibrational degrees of freedom of the cluster.

Plasma desorption mass spectrometry in studies of formation and sputtering of fullerenes by MeV atomic ions

An account is presented on plasma desorption mass spectrometry (PDMS) studies of different carbon-containing organic solids utilizing megaelectronvolt (MeV) atomic ions from the Uppsvala EN-tandem accelerator. Positive ions of even-numbered carbon clusters (C + n , n = 40 to > 200) are ejected as a result of the interaction of the fast MeV ions with the target. The distribution of cluster sizes suggests that stable, closed carbon-cage structures — fullerenes - are formed. Among the investigated materials that produce carbon clusters are poly(vinylidenefluoride) and fluorinated fullerenes — C 60F 2 m . For comparison purposes data from C 60 targets have been also collected and analyzed. PDMS has been used for the in situ assessment of the damaging of C 60 films by MeV heavy ions. Results on delayed electron emission from C − 60 sputtered by MeV ions from C 60 fullerene targets are also presented. A model of fullerene formation as a result of MeV ion interactions with the organic solid, including the yield dependence on primary ion charge state, is summarized. Both the data and the model suggest that fullerenes are formed as a result of a single primary ion impact and that they are ejected from an axially expanding infratrack plasma region. Results on different types of coalescence reactions in synthetic C 60 fullerene targets and in blends of pure (synthetic) C 60 with polystyrene leading to ejection of higher mass positive fullerene ions (C + k , k from 60 to more than 200) are also reported. The coalescence reactions are induced by the interaction of a single MeV ion with the solid. We argue that our data contribute to elucidating some general patterns of the fullerene formation mechanism.

Delayed electron emission from electronically sputtered C60- ions.

Rates of electron detachment form secondary ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$ ions sputtered by swift atomic projectiles (19.7-MeV S, 48.6-MeV Br, and 78.2-MeV I) from ${\mathrm{C}}_{60}$ films are determined and employed to estimate the internal excitation energy of the sputtered ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$ ions. While the ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$-ion yields vary as the third power of the incident ion energy loss in the solid, no dependence has been found in the internal energy content of sputtered ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$ ions on ion parameters. These results illustrate the use of ${\mathrm{C}}_{60}$ fullerenes as a ``molecular thermometer,'' and contribute to the elucidation of the ion-sputtering process.

Thermionic emission from free, photoexcited tungsten clusters

We report on delayed electron emission from free tungsten clusters, excited by light from a Q-switched YAG laser. Using a novel ion extraction lens, electron emission can be analyzed over a time range of 50 ns–5 μs after the laser pulse without interference from prompt ions. All clusters of size 5≤n≤40 exhibit delayed emission on this time scale, while delayed emission from smaller clusters does not occur. We analyze the time dependence and size dependence of the emission rate for different wavelengths and fluences. The yield of delayed ions may exceed the yield of prompt ions for intermediate laser fluences. A statistical model is proposed which is based on the assumption that energy randomization in the electronically excited clusters proceeds much faster than in 50 ns, i.e., that the observed phenomenon is the (cluster) analog of thermionic emission. Good agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other delayed deexcitation channels, namely, emission of atoms or photons, are not significant under our experimental conditions.

Delayed electron emission from higher fullerenes Cn, n .ltoreq. 96, following excitation at 1064 nm

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDelayed electron emission from higher fullerenes Cn, n .ltoreq. 96, following excitation at 1064 nmKeith W. Kennedy and Olof EchtCite this: J. Phys. Chem. 1993, 97, 27, 7088–7091Publication Date (Print):July 1, 1993Publication History Published online1 May 2002Published inissue 1 July 1993https://doi.org/10.1021/j100129a027RIGHTS & PERMISSIONSArticle Views60Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (504 KB) Get e-Alerts

Rates of Electron Emission from Negatively Charged, Impact-Heated Fullerenes

Thermal emission of electrons is ordinarily considered to be exclusively a property of macroscopic condensed matter. Slow electron emission occurs for certain small metal clusters as well as for silicon and carbon clusters, but the nature of this process has not been established. Electron emission rates have been obtained and analyzed from extensive real-time measurements on negatively charged fullerenes for several sizes and over a wide, continuous range of energies. These results confirm that delayed electron emission is a simple activated process that depends strongly on the internal energy and size of the cluster and that it has a common underlying mechanism, independent of size. However, the Arrhenius form deduced is inconsistent with the emission rate theory used for bulk surfaces. These results allow the question of the correct microscopic description of this newly observed electron emission process to be assessed.

Delayed emission of electrons from C60 following energetic impact against graphite

The yield of C60+ ions reflected from the surface of graphite is found to be a bimodal function of the energy of impact. Below 140 eV (6 km/s), the yield decreases with energy and the scattering time-of-flight peak is sharp. Above 170 eV, the yield increases, but the peak is broad and asymmetric, corresponding to delayed electron emission from an impact-heated, neutralized cluster. There is also evidence for fragmentation to C58 and C56. The emission process may be analogous to that reported for scattered C60−. Application of statistical rate theory to either observation (C60±) gives an estimated 25–30 percent transfer of impact energy into cluster internal modes. The decreasing yield at highest energies (>400 eV) may result from adhesion processes (C60-surface bonding) that could also account for the absence of delayed electron emission in C60-silicon collisions.

Delayed electron emission from photoexcited C60

Delayed emission of electrons is observed after absorption of several photons in the wavelength range from 212.8 to 532 nm. This phenomenon is explained by thermionic emission of electrons from hot C60 molecules. Photoelectrons and photoions are recorded under the same experimental conditions for further evidence.