Dianion diagnostics in DESIREE: High-sensitivity detection of C 2− n from a sputter ion source
K. C. Chartkunchand, M. H. Stockett, E. K. Anderson, G. Eklund, M. K. Kristiansson, M. Kamińska, N. de Ruette, M. Blom, M. Björkhage, A. Källberg, P. Löfgren, P. Reinhed, S. Rosén, A. Simonsson, H. Zettergren, H. T. Schmidt, H. Cederquist
aa r X i v : . [ phy s i c s . a t m - c l u s ] A p r Dianion Diagnostics in DESIREE: High-Sensitivity Detection of C − n from a SputterIon Source K. C. Chartkunchand, a) M. H. Stockett, E. K. Anderson, G. Eklund, M. K.Kristiansson, M. Kami´nska,
1, 2
N. de Ruette, M. Blom, M. Bj¨orkhage, A. K¨allberg, P. L¨ofgren, P. Reinhed, S. Ros´en, A. Simonsson, H. Zettergren, H. T. Schmidt, b) and H. Cederquist Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm,Sweden Institute of Physics, Jan Kochanowski University, 25-369 Kielce,Poland (Dated: 9 November 2018)
A sputter ion source with a solid graphite target has been used to produce dian-ions with a focus on carbon cluster dianions, C − n , with n = 7–24. Singly and doublycharged anions from the source were accelerated together to kinetic energies of 10 keVper atomic unit of charge and injected into one of the cryogenic (13 K) ion-beam stor-age rings of the DESIREE (Double ElectroStatic Ion Ring Experiment) facility atStockholm University. Spontaneous decay of internally hot C − n dianions injected intothe ring yielded C − n anions with kinetic energies of 20 keV, which were counted with amicrochannel plate detector. Mass spectra produced by scanning the magnetic fieldof a 90 ◦ analyzing magnet on the ion injection line reflects the production of internallyhot C − –C − dianions with lifetimes in the range of tens of microseconds to millisec-onds. In spite of the high sensitivity of this method, no conclusive evidence of C − was found while there was a clear C − signal with the expected isotopic distribution.This is consistent with earlier experimental studies and with theoretical predictions.An upper limit is deduced for a C − signal that is two orders-of-magnitude smallerthan that for C − . In addition, C n O − and C n Cu − dianions were detected. a) Electronic mail: [email protected] b) Electronic mail: [email protected] . INTRODUCTION In this paper, we present a highly sensitive method to probe the production of multiplycharged anions utilizing the cryogenic electrostatic ion storage ring DESIREE. A numberof outstanding issues relating to the inherent stabilities and reactivities of such ions exists,which is of clear interest for astrophysical applications. Examples include the C − n carboncluster dianions, and in particular the thermodynamic stability of C − , as well as thestabilities of C − and other fullerene dianions .Small and large carbon-bearing molecules are likely to be important for various processesin space. There, they are often ionized and may also carry extra electrons, i.e., they maybe positively or negatively charged. Examples of the latter are C H − , C H − , C H − , CN − ,C N − , and C N − , which were discovered through detailed comparisons between laboratoryrotational spectra and astronomical observations . Similarly sized pure carbon clusteranions, C − n , are also believed to play important roles in space but have so far not beendetected. Astrophysical carbon cluster anions are likely to be formed “hot,” i.e., with highinternal excitation energies, through the attachment of free electrons . Recently, largedifferences between cooling rates of internally hot molecular ions have been reported anddiscussed in terms of so-called recurrent fluorescence processes , which should be animportant consideration when modeling astrochemical reaction networks . Other carbon-bearing molecules, such as neutral C and C fullerenes, have been observed in planetarynebulae , and a number of C +60 absorption bands have been identified with some of the so-called diffuse interstellar bands (DIBs) . In addition, polycyclic aromatic hydrocarbons(PAHs) or other fullerenes, as well as their ions, have been suggested as carriers of otherDIB features . For astrophysical applications, there is thus a clear need to find efficientways to produce and study carbon-based molecular anions. Multiply charged anions may be present in dark interstellar clouds and could also play arole in chemical reaction networks. However, up until now the inherent stabilities of suchsystems have been very difficult to study. This is due to difficulties in producing sufficientquantities of such ions and to low electron emission rates, as even thermodynamically unsta-ble systems may have high-energy barriers against electron emission. Many measurementsare also complicated by the influence of the blackbody radiation field, which may be sub-stantial in typical room-temperature experiments. This situation is now rapidly improving2s techniques to cryogenically store and analyze singly and multiply charged anions andcations in electrostatic ion-beam storage rings and traps have become available .Cryogenic electrostatic ion-beam storage devices allow for the storage of ions of any massat high velocities under superb vacuum conditions, with only 10 –10 rest gas molecules per cm . This opens the possibility for action spectroscopy studies (see for examplesRefs. for descriptions of this method) and the counting of individual reaction prod-ucts . So far, only three cryogenic electrostatic ion-beam storage rings are in operationworldwide. These are the 35-meter circumference Cryogenic Storage Ring (CSR) at theMax Planck Institute in Heidelberg, Germany , the three-meter circumference RIKENCryogenic Electrostatic (RICE) ring at the RIKEN laboratory in Tokyo, Japan , and theDESIREE rings at Stockholm University in Sweden . The latter has a unique double-ringconfiguration, with two 8.6-meter circumference rings sharing a common straight section formerged anion-cation reaction studies . A few cryogenic electrostatic ion traps are also inoperation where similar studies may also be performed.In the present study, one of the DESIREE storage rings is used to diagnose the productionof beams of carbon cluster dianions from a sputter ion source. While very short-lived( ∼ − dianions have been observed , C − is the smallest carbon cluster dianion forwhich thermodynamical stability is predicted . This has so far not been investigated indetail experimentally, although metastable and/or stable C − ions have previously beenobserved . The lack of evidence for C − and C − in previous experiments alsoseems to support C − as the smallest thermodynamically stable carbon cluster dianion. Arelated open issue concerns the thermodynamic stability of small fullerene dianions suchas C − and C − , which so far have only been investigated using room-temperature storagedevices and a radio-frequency trap cooled to 70 K. Cryogenic storage rings havea clear advantage over the room-temperature devices as they can be used to follow decayprocesses on much longer timescales .In Section II, the detection technique developed for DESIREE, which can be applied toany dianion production method, will be described. In Section III, we will show the resultingpure dianion mass spectra and compare them with the much higher-intensity mass spectraof anions also produced in the source. A weak signal at the position expected for C − inthe dianion spectrum is present, but we show that this signal is primarily due to secondaryparticles from the C − beam. 3 I. EXPERIMENTAL APPARATUS
20 keV C n - (cid:1) ectors10° De (cid:0) ectorsFocusingQuadrupolesPosition-SensitivePickupsDetectorsOverlap ControlDe (cid:2) ectors Symmetric RingAsymmetric Ring q keV C nq -
20 keV C n (cid:3)
20 keV C n - + e - C n FIG. 1: Schematic of the DESIREE storage rings. Carbon cluster anions are injected intothe Symmetric Ring, which is configured to only pass 20 keV C − n anions resulting fromspontaneous decay of 20 keV C − n dianions within the first straight section of the ring (seetext for details). Only the Vertical Correction Element (VCE) used in this experiment isshown; other such elements are present in the ring but omitted in the figure for clarity.Beams of carbon-containing anions were produced using a SNICS cesium-sputter ionsource with a solid graphite target mounted in a copper cathode. The ions were acceleratedto 10 keV per atomic unit of charge and their mass-to-charge ratios selected by means of a 90 ◦ double-focusing dipole magnet. The field in this analyzing magnet was measured by meansof a LakeShore Model 460 3-Channel Hall-effect Gaussmeter and this reading was used tocalibrate the mass-to-charge ratio scales in the anion and dianion mass spectra as describedbelow. After the magnet, ions with a well-defined mass-to-charge ratio were transportedto the injection port of the so-called Symmetric Ring of DESIREE, shown in Fig. 1. Atthis point the ions have traveled a distance of 10 . m/q , have the same velocity but different kinetic energies of 10 and 20 keV,respectively. As an example, a 20 keV C − ion travels 1 m in about 5 µ s. Taking into accountthe time for transport at lower energy between the source and the 10 keV acceleration stage,the C − n ions typically reach the injection stage of the ring in hundreds of microseconds4fter leaving the sputter source. This means that dianions will only reach the ring if theydecay on comparable or longer timescales, or if they are stable. The deflection plates inthe injection stage were set for injection of ions with 10 q keV kinetic energy, q being thecharge state of the ion. Thus, 10 keV anions ( q = 1) and 20 keV dianions ( q = 2) with thesame mass-to-charge ratios enter the ring as parts of the same ion beam. For measurementsof dianion mass spectra, the voltages on the deflectors of the first 180 ◦ -bend, consisting oftwo 10 ◦ -bends and one cylindrical 160 ◦ -bend as shown in Fig. 1, were set to only admitions with a kinetic energy of 20 keV per atomic unit of charge. That is, only those C − n ions that lose one of their extra electrons in the first straight section of the Symmetric Ringbetween the injection stage and the entrance of the first 180 ◦ -bend can continue as 20 keVC − n ions through this bend. After this they continue straight to the detector positioned afterthe second straight section of the ring (see Fig. 1) and are counted by means of a position-sensitive microchannel plate (MCP) detector operated at cryogenic temperatures . Thevoltages of the second 180 ◦ -bend in the Symmetric Ring were set to zero. Mass spectra werealso recorded by measuring the ion current in a Faraday cup (FC) after the exit slit of theanalyzing magnet. This current was dominated by 10 keV C − n ions, but also contained smallamounts of 20 keV C − n ions. Separate mass spectra of dianions and anions were measured byrecording the count rate of dianions on the MCP detector and by recording the FC currentas a function of the 90 ◦ analyzing magnet field settings, with an overall mass resolution of m/ ∆ m ∼ III. RESULTS AND DISCUSSION
Mass spectra for C − n and C − n are shown in Figs. 2 and 3, respectively. Fig. 2 showsC − n dianions in the range m/q = 32–150 u, corresponding to the expected positions fordianions with n = 6–24, while Fig. 3 shows C − n dianion peaks overlaid with C − n anion peaksin the range m/q = 40–85 u. The pure C − n mass spectrum clearly exhibits an odd-evenalternation in intensities, which has been observed in previous measurements . Masspeaks are clearly defined for carbon cluster dianions up to n = 17. Peaks above this areless well-defined, making it more difficult to determine whether they are due to pure C − n clusters or some mixture of other dianionic species. In particular, the peak in betweenthe C − and C − peaks shown in Fig. 2 occurs at m/q = 138 . m/q [u] I o n C o un t s [ A r b . U n i t s ]
2x data collection time C C C C C * C C C C C C C C C C C C C ? FIG. 2: The C − n mass spectrum recorded according to the procedures in the text. Twicethe counting time was used for the m/q = 106–150 u segment of the mass scan. Thedashed arrow labeled “ C − ?” indicates the expected location of this ion in the dianionspectrum. Note that the observed signal is from those dianions that have lost one of theiradditional electrons, and that the relative intensities of the observed peaks are not simplyproportional to the probability of dianion formation. The peak labeled with an asteriskindicates the presence of an unknown dianionic species close to the expected location ofthe C − peak (see discussions in the main text).as would be expected for C − . This can be explained by the presence of an unknown,doubly-charged impurity that dominates over any C − that may have been injected intothe ring. In Fig. 3, the dianion mass spectrum was recorded from particle counts on theMCP detector (left vertical scale, blue spectrum), while the anion spectrum was recordedfrom FC current readings (right vertical scale, red spectrum); note the logarithmic verticalscale of the anion spectrum (in red). While the anion spectrum reflects the relation betweenthe different anion production efficiencies in the sputter source under the present operatingconditions, the interpretation of the dianion mass spectrum is less straightforward. The6 m/q [u] I o n C o un t s [ A r b . U n i t s ] Dianion Mass Spectrum (Left Linear Axis) I o n B e a m C u rr e n t [ n A ] Anion Mass Spectrum (Right Logarithmic Axis) Cu -65 Cu -12 C
4- 12 C
5- 12 C
6- 12 C C C - 12 C C -12 C C
82- 12 C
92- 12 C C C C C C C - FIG. 3: The C − n mass spectrum (in blue) overlaid with the C − n mass spectrum in therange m/q = 40–87 u. Note the logarithmic scale for the ion beam current correspondingto the C − n mass scan (in red).intensities of the dianion peaks depend on the efficiencies with which dianions are producedin the source but also on the C − n → C − n + e − decay rate in a rather involved way. If thisrate is high, the dianions decay before reaching the ring; if it is low, it is unlikely thatspontaneous decay occurs in the first straight section of the Symmetric Ring and the signalbecomes very weak. For sufficiently cold, thermodynamically stable C − n dianions, thereis no signal apart from possible very minor contributions due to electron stripping in theextremely dilute residual gas density of the ring (10 –10 H molecules per cm ). Suchdianions could be detected through photodetachment with a laser introduced co-linearly tothe ion beam along the straight section on the ion-injection-side of the ring. In this case, C − n produced through single-electron detachment would pass through the first 180 ◦ -bend and bedetected, indicating the presence of C − n injected into the ring. However, this has not beenimplemented for the present pilot study.Anions and dianions are produced hot and with broad internal energy distributions in7he sputter ion source. Typically vibrational and rotational temperatures are several thou-sand Kelvin, as shown in previous studies of anion relaxation processes in ion-beam storagerings and traps . In many cases, these properties are manifested as non-exponentialdecays of neutralization signals due to spontaneous decay of the ions as functions of thestorage time t . This type of decay instead often follows a t − δ trend, i.e., a power lawbehavior in which δ is often, but not always, small . Depending on the vibronic structureof the anion, recurrent florescence can also become a significant relaxation process with anear-exponential decay of the signal strength over time . In the present experiment, onlythose dianions that are produced in reasonable amounts in the source and which decay ontimescales that are neither too short nor too long can contribute significantly to the signal.In Fig. 4, a zoom-in is shown of the C − group of peaks and the mass region expectedfor a possible C − contribution. The intensity distribution of the C − peaks is consistentwith the one expected from the natural C abundance of 1.1%. The appearance of peaks athalf-integer mass numbers, in this case at 42 . C C − and thus also C − . In terms ofC − , there is a very weak peak which consistently appeared at the expected C − position.However, this peak seems to be due to other particles produced through the interaction of therather strong C − beam with electrode surfaces in the Symmetric Ring. This was concludedafter applying small voltages to the Vertical Correction Element (VCE) located before theMCP detector (see Fig. 1). The position of the beam spot on the detector for the m/q = 42signal (C − ) could be moved vertically with these voltages, while the corresponding spotdue to the m/q = 36 signal (C − ) was merely reduced in intensity by the same deflectionfields (see Fig. 5). While the exact nature of the signal recorded at m/q = 36 is unknown, itclearly does not behave like a proper ion beam traveling through the ring and hence is mostlikely not due to any 20 keV C − ions that would indicate the presence of C − injected intothe ring. This, however, does not prove that C − does not exist as a stable or metastableion, only that our sputter ion source operating under the present conditions was unable toproduce it in sufficient amounts and in states with suitable lifetimes.Finally, we remark on the sensitivity of our technique for dianion detection. In Fig. 6, bothdianion and anion mass spectra in the regions around the C − , C − , and C − peaks are shown.In these mass spectra, we first note the presence of C n O − dianions with n = 10 ,
12. Dianionsof the type C n O − have been observed previously when oxygen was actively introduced into8 m/q [u] I o n C o un t s [ A r b . U n i t s ] Dianion Mass Spectrum (Left Linear Axis) I o n B e a m C u rr e n t [ n A ] Anion Mass Spectrum (Right Logarithmic Axis) C C
3- 12 C C - 12 C C C - InducedScatter FIG. 4: C − n and C − n mass spectra in the range m/q = 33–45 u, including C − and anypotential signal due to C − .the sputter ion source . No such enhancement of C n O − production was done in the presentexperiment, demonstrating the sensitivity of our technique. In regards to the C − , , peaks,we also note that the peak intensities for substitution of one C atom are greater thanexpected based on the natural abundance of C. Since there are no indications in theanionic mass spectra of anionic species at the appropriate m/q and as these peaks occur athalf-integer mass numbers, we conclude that the additional contribution to these peaks comefrom other dianionic species. Possible such dianions could include C , , H − carbon clusterhydrides or mixed cesium-carbon clusters CsC − , ; the latter is consistent with the ease withwhich CsC − n clusters are produced in cesium sputter sources . A mixed cesium-carboncluster dianion, in particular CsC − , may in fact be responsible for the unknown dianionpeak at m/q = 138 . − , , peaks. For instance,we note that the mass peaks at 79 . . m/q as C Cu − and C Cu − , respectively. Since a graphite target and copper cathode are used in the cesium9 P o s i t i o n X Position-100 V -50 V 0 V +50 V +100 V0 V +50 V +100 Vm/q = 42m/q = 36 Y P o s i t i o n X Position
FIG. 5: Ion beam positions on the MCP detector for m/q = 42 u and m/q = 36 u(corresponding to C − and C − , respectively) for different voltages applied to the VerticalCorrection Element (VCE). The dashed line overlaid on both sets of images corresponds tothe vertical center of the beam position with 0 V applied to the VCE.sputter source only carbon-, cesium-, oxygen-, and copper-containing anions are produced,making this a reasonable assignment for these two mass peaks. These dianions have, to ourknowledge, not been detected previously. IV. CONCLUSIONS AND OUTLOOK
We have demonstrated a highly sensitive method to investigate dianion production meth-ods and their efficiencies. By using the Symmetric Ring of the DESIREE facility as akinetic-energy-per-charge analyzer, carbon cluster dianion and anion mass spectra could bemeasured separately, opening new ways to detect small dianion contributions in DESIREE.This method was used to investigate if metastable C − could be produced in a cesium-sputter ion source. We determined an upper limit for the corresponding signal rate to betwo orders-of-magnitude smaller than that for C − with the present ion source conditionsand diagnostic method. Other dianionic species such as C n Cu − were also detected without10 I o n C o un t s [ A r b . U n i t s ]
73 74 75 76 77 78 79 80 81 m/q [u]
87 88 89 90 91 92 93
Dianion Mass Spectrum (Left Linear Axis) I o n B e a m C u rr e n t [ n A ] Anion Mass Spectrum (Right Logarithmic Axis) Cu C Cu C
2- 12 C C C -12 C C C C
3- 12 C C O Cu -65 Cu - 12 C C O * * * C Cu C Cu FIG. 6: Dianion and anion mass spectra in the C − , C − , and C − regions of interest.Peaks marked with an asterisk (*) indicate those resulting from substitution of one Catom into C − , , , along with other dianionic species of the same m/q (see discussion inmain text).any special enhancement for their production. Plans are under way to store carbon clusterand fullerene dianion beams in DESIREE and to monitor C − n → C − n + e − decays as functionsof time after injection in order to investigate the inherent stabilities of these C − n dianions,as well as to measure their decay lifetimes and binding energies. Alternative and colder di-anion production methods will be tested using the diagnostic method presented here. Theseinclude electrospray ionization techniques, charge exchange in a cesium-vapor cell, and/orpre-cooling in radio frequency buffer-gas pre-traps operating at low temperatures. ACKNOWLEDGMENTS
This work was performed at the Swedish National Infrastructure, DESIREE (SwedishResearch Council Contract No. 2017-00621). Further support was provided by the SwedishResearch Council (Contract No. 821-2013-1642, No. 621-2015- 04990, No. 621-2014-4501,11o. 621-2016-06625, No. 621-2016-04181, and No. 2016-03675) and the Knut and AliceWallenberg Foundation through an earlier DESIREE-investment grant. M. K. acknowledgesfinancial support from the Mobility Plus Program (Project No. 1302/MOB/IV/2015/0)funded by the Polish Ministry of Science and Higher Education.
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