Electronic spectroscopy of carbon chains L Carbon chains are of interest in astrophysics and in terrestrial processes, such as fullerene formation. The electronic spectra of a variety of such chains have now been identified in the laboratory using a technique which enables absorption spectra of mass-selected species to be measured in neon matrices. The understanding of their transitions and the trends apparent for these homologous series point out which type and size of carbon chains are relevant for consideration as carriers of the diffuse interstellar bands. 1 Introduction Spectroscopic characteristics of carbon species are desirable not only as basic knowledge, but are necessary in the interpretation of astronomical observations. The availability of laboratory spectra led in the past to the identification of the simple carbon molecules C2and C3 in comets via their electronic transitions as well as to long polycyanoacetylene chains in dark clouds by microwave spectroscopy.' Also in terrestrial chemical proc- esses, such as the formation of fullerenes, carbon chains are postulated as precursors.2 Thus, an understanding of their spectroscopic properties is prerequisite to the study of their reaction mechanisms in the laboratory.This goal to observe and assign the spectra of carbon chains, and their isoelectronic species involving nitrogen and hydrogen, has only recently been realized. It has come about as result of the development of a technique which enables the electronic absorption spectra of mass-selected species in neon matrices to be measured.3 Prior to this, the electronic transitions of only C2,C3, C2- and C2+ in the gas phase had been On the other hand, rotationally resolved infrared spectra of the carbon chains C, in the range n = 3-13 have been obtained in recent years.5 Several studies of carbon vapour condensed in rare gas matrices have John P.Maier received a BSc in chemistry at Nottingham University, and a DPhil in physical chemistry from Oxford University in 1972 with D. W. Turner. He then moved to the Institute of Physical Chemistry, University of Basel, as a Royal Society Postdoctoral Fellow with E. Heilbronner, He remained there as a member of the academic staf and in I992 was appointed to the chair ofphysical chemistry at Basel University.He was awarded the Werner prize of the Swiss Chemical Society (1979), the Marlow medal of the Royal Society of Chemistry (1990),the Chemistry prize of the Gottingen Academy, Germany (1986)and the Latsis prize of the Swiss National Science Founda- tion (I 987). His research inter- ests lie in the development and application of methods for the spectroscopic characterization of ions, ionic clusters and radicals. been reported with some suggested assignments to specific species.6.7 This has been plagued with uncertainty due to the concomitant presence of a number of species. We could circumvent this handicap by conducting such measurements on mass-selected species and this has resulted in the identification of the characteristic x--JIelectronic transitions of a variety of carbon chains, neutral and ionic.These observations allow the relevance of specific species to astrophysical measurements to be discussed and indicate the directions to be pursued in trying to identify the molecules likely to be responsible for the diffuse interstellar bands.8 These are spectral features due to starlight absorption by molecules present in interstellar space, but their identification has remained a mystery for over half a century. It is one of the aims of spectroscopic studies on the carbon chains to establish whether these could be the carriers responsible. Because the electronic transitions in the neon matrix have been observed, measurement of the spectra in the gas phase has become a realistic proposition.2 Approach The difficulties in trying to measure the spectra of transient species in the gas-phase, the ultimate goal, are threefold -their generation in sufficient concentration, availability of a sensitive approach to measure their spectrum and knowledge of the energy region of the transitions are required. We have now developed a technique which combines the virtures of mass- and matrix isolation-spectroscopies.3 Mass-selection alleviates the need to identify the absorbing species, whereas the neon matrix enables sufficient concentrations of ions and radicals to be attained so that direct absorption measurements can be made. The philosophy is then to identify the electronic transitions of a chosen species in the inert neon environment, and with this knowledge in hand to attempt a gas-phase characterization.The studies of the carbon species in the gas phase C2+9and recently C2n-n = 2,3,4,10 were only realised once the electronic transitions were identified in the neon matrix. 11 3 Experimental considerations Matrix isolation spectroscopy is a well-established technique, and neon matrices show the smallest energy shifts relative to the gas-phase.l2 An experimental advantage compared to tradi- tional matrix spectroscopy where relatively thick matrices tend to be grown on sapphire substrates, and measurements follow as transmission, is the implementation of a waveguide technique. l3 This enables matrices 150 ym thick to be grown in approx- imately two hours and the probing light is propagated through the thin side along the 2 cm length of the matrix.For this purpose the matrices are grown on copper substrates coated with rhodium, providing good reflectivity from the UV to IR. The other half of the instrument provides a mass-selected ion beam. Experience has shown that ion currents of more than 1nA are necessary for the recording of the absorption spectra. Thus, the ion source is a crucial element -hot cathode discharge and a caesium sputter source have been used.14 Electron lenses and a high transmission quadrupole mass-spectrometer steer the ion beam onto the matrix. In Fig. 1 the overall arrangement of the instrument is depicted.Chemical Society Reviews, 1997 21 Codeposition of the mass-selected ions, usually with nominal kinetic energies 50-200 eV, with excess of neon leads to matrix formation with ion densities in the 1015-16 cm-3 range. This is sufficient for the absorption spectra to be discernible. The waveguide approach is used in the 220-1000 nm region, and a reflection method using a Fourier transform instrument, though with an order of magnitude less sensitivity, covers the near IR. 4 Electronic structure considerations The lowest energy, strong electronic transitions encountered in the carbon chains involve the excitation of n electrons. The open-shell species, polyacetylene cations, HC,H+, and the isolectronic chains, C,H, C,-, have the ground state configu-ration n3 and thus X2I-I symmetry.The excited states of relevance are the ones corresponding to n-n electron promo-tion, resulting in 2ILX2I-I electronic transitions. In the case of the bare carbon chains, C,, the species are paramagnetic for n = even but not for n = odd? The former have triplet ground states, X32,-arising from the configuration Matrix Deflector Fig. 1Schematic arrangement of the instrument combining mass and matrix isolation spectroscopies. Mass-selected ions produced in a hot-cathode or caesium sputter source are codeposited with excess of neon to form a matrix at 5 K, and the absorption spectrum is measured by a waveguide technique. HCfjH+ n2.The characteristic transition has 3&--X3Xc,-symmetry, to which configurations arising from both x*-~tand ~t-nexcita-tions contribute.The ones with an odd number of carbon atoms have the electron configurationn4in the ground state and XIZ + symmetry. The n*-n excitation corresponds to the 12,+-X1Z> electronic transition. 5 Polyacetylene chains Polyacetylene cations can be readily produced in a hot cathode source with acetylene or diacetylene as precursor. It is then merely a matter of mass-selecting the appropriate HC,H+ species and codepositioning them with neon. The electronic spectrum of the cation kept under isolated conditions can subsequently be measured, This has so far been carried out for the ion chains with n = 4-16.16 The spectra of the species with an even number of carbon atoms are shown in Fig.2. The polyacetylene cations are open-shell species with 2ll ground state. The transitions observed are of 211-X211 sym-metry and shift monotonically to the red with increasing number of carbon atoms. This is evident from Fig. 3; the energy of the transition is inversely proportional to the length of the species. This trend can be easily modelled by a particle in a ID box where the n electrons are excited. Such an approach also predicts that the oscillator strength of the transition correlates linearly with the number of carbon atoms. This feature is a factor of why the spectra of the longer chains can be detected, even though the attainable current for the mass-selected ion is decreasing. These electronic transitions of the polyacetylene cations HC,H+ with n = 4, 6, 8 have in fact been detected in the gas-phase as their emission spectra.17 They are shifted to higher energies in the gas phase relative to the neon matrix by 79, 135 and 143cm-l, respectively.On the basis of these trends, and the observations in the neon environment, the region for the search of these transitions in the gas-phase for the larger species is predicted sufficiently well to make this feasible. The electronic absorption spectra of the neutral poly-acetylenes, HC,H can be obtained by codeposition of their cationic counterparts with neon, but under conditions such that neutralization is enhanced. This can be achieved by irradiation HC~*H+ 0.1 -0.07-* ' 850 1 0.051 0.00 10.0051 900 950 1000 1050 0.0 800 850 hl nm hl nm Fig.2 The *n-XTI electronic transitions of the polyacetylene cations detected as absorption spectra in neon matrices after mass-selection 22 Chemical Society Reviews, 1997 of the matrix after deposition with broadband UV radiation. specifically the transitions observed are of 3Zu--X32g-This photodetaches electrons from the anions present in the symmetry. The same trend as for the cations manifests itself -matrix which then recombine with the cations. By this means an approximate linear dependence of the wavelength of the spectroscopy on mass-selected neutral species can be carried transition on the number of carbon atoms. This is shown as one out.of the plots in Fig. 3. In Fig. 4 are shown some of the recorded absorption spectra The polyacetylenes with an even number of carbon atoms, for HC,H n = odd.l8 The electron excitation is n-n type again; HC2,H, show corresponding absorption in the UV part of the spectrum. These have been measured previously in the gas phase, for n = 2-5, using a standard absorption approach,]g and also for the ones with n up to 10 in solution.20 6 Carbon anion chains To apply the mass-selected approach to characterize the electronic transitions of the bare carbon chains and their ions, it 800 proved necessary to implement a different ion source. This is a caesium sputter source, which produces copious amounts of 700 carbon anions. Caesium cations are accelerated towards a 600 graphite rod and the resulting carbon anions are extracted.In Fig. 5 is shown the observed mass pattern; species up to C12-500 are produced with sufficient current for the measurement of the absorption spectrum to be possible. 400 Consider Cg-as an example. After the matrix is formed the I,! IIIIIIII I I 4 5 6 7 8 9 10 11 12 13 14 15 16 spectrum included in Fig. 6, with origin near 600 nm, is Number of carbon atoms 0b~erved.l~It is readily confirmed that this spectrum is of an Fig. 3 The wavelength of the origin band of the JC-JC electronic transition of anion by photobleaching experiments, and that it is a member of the polyynes shows an approximate linear dependence on the number of an homologous series, because similar spectra, but red-shifted carbon atoms in the chain by regular increments, are observed for the successively larger 0.2 0.1 0.0350 400 450 5000.3 0.2 0.1 0.0 hl nm hl nm Fig.4 Absorption spectra of the n-JCelectronic transition (3C,--X3C,-symmetry) of the HC2,+ 1Hmolecules in neon matrices. These were grown using a mass-selection of the corresponding cations and subsequent UV irradiation. 1 O-”A csc; I II I 1 20 40 60 80 100 120 140 mlz Fig. 5 Mass spectrum of the carbon anions produced using a caesium sputter source. A particular species is mass-selected and codeposited with excess of neon for the spectral measurement. The neutral chains are observed when the matrix is irradiated during or after deposition with UV light, photodetaching the electrons from the anions.Chemical Society Reviews, 1997 23 Afwu~lJl.JL. oh ' ' ' ' ' ' ' ' 400"450" "800' ' 900 1000 01600 1700 2i2 1 .,...,,dM,J. 10 550 600 8 1100 1200 1300 0 2000 2100vi,l 31,,;4, 81 0 0 700 800 8 1400 1500 2300 2400 A/nm A/nm A/nm Fig. 6 The *H-X2JJ electronic transitions of carbon anions measured in absorption in neon matrices Om2 -c4 0.1 -I Q);0.0 300a 350 400 00,0.02 ClO rl El 0.00 650 700 750 hl nm hl nm Fig. 7 The 3Xu--X3X,-absorption spectra of mass-selected carbon chains C, (n = even) in neon matrices species.ll The nature of the transition is also clear; Cg-is have been able to identify the strong transitions, 3Z,--X3Zg-isoelectronic with HC6H+, and thus it is of 211-X211 symmetry.(n = even)" and lZU+-X1Zg+(n = odd)*1 for the C, chains. The Franck-Condon profile and vibrational pattern of these two The spectrum of a specificneutral carbon chain is obtainedby ions is similar (cf. spectrum of HC6H+ in Fig. 2). Also the codeposition of the corresponding C,-anion with excess of vibrational frequencies of the corresponding modes are compa-neon to grow the matrix at 5 K, with concomitant irradiation rable: HC6H+:212 = 2053, 213 = 1880, 214 = 617; with broad band UV light. The photobleaching can also bec6-:v1 = 2064, v2 = 1817, v3 = 607 cm-l. The 2n-x2n achieved after deposition; in both cases the identified electronic transitions of the even-numbered carbon anions up to C20-have transition of the carbon anion disappears while a new band been observed.system appears. By this means, the 3Zu--X3Z,-band systems for C, n = 4, 6, 8, 10 have been observed as can be seen in Fig. 7. The assignment is based on (1) mass-selection, (2)7 Neutral carbon chains photobleaching behaviour, (3) trend within an homologous The C, chains with n = even have 3C,-ground state symmetry, series (Fig. 8), and (4) in the case of the smaller species, C4 and whereas those with n = odd have a lZg+ closed-shell c6,by comparison with high level a6 initio calculations. It is configuration. The IR spectra of most of the n = 4-13 linear interesting to note, that it has not proved possible to detect the species have been obtained in the gas phase.5 However, their corresponding transitions for species larger than Clo.The electronic transitions have not been identified, though the few initially reported spectral1 of Clo, C12and C14 have turned out articles reporting the absorption spectra of carbon vapour to be the transitions to higher electronic states of their anions; condensed in rare gas matrices have made suggestions on the the Clo system has now been detected and is the one included in assignment of bands to specifically sized carbon species.4.6 The Fig. 7. This may be taken as an indication that the cyclic forms main support for this has come from correlation of intensities of dominate for the neutral species of these sizes, in accord with electronic and IR bands.7 Using the mass-selected approach, we the conclusions drawn from ion mobility measures.21 Pre-24 Chemical Society Reviews, 1997 I I I 0 C2n A C2n+1 700 600 E5 500 400 300 I I I 4 6 Fig.8 Dependence of the origin band wavelength of the JC-JC IZu+--X1Xg+)on the number of carbon atoms 1+c, tx IcS' I I I I I I I I I I I I I I I 7 8 91011 I I I 13 15 Number of carbon atoms electronic transition of the bare carbon chains C, (n = even: TZc,--X3X,-; n = odd: lXu+-XIXg+ transition of C3 should be located near 170 nm. The well-known Comet band system of C3, A1IIU-X1Xg+,lies near 300 nm, and the corresponding transitions of C5 and C7 are also discernible in the matrix near 510 and 542 nm, respectively.22 Om2 cg 8 Monohydrogenated carbon chains 0.0 The electronic spectra of the C2,H chains, with n = 3-8, have also been detected by mass-selection of C2,H- anions, and photodetachment during growth of the matrix." The hot- 0 Oml cathode anion source with diacetylene as input proved expe- a 1 A Cll dient for the C2,H- production.The C2,H chains are isoelectronic with the C2,- and HC2,H' s 0.0ions, and accordingly show the strong 213-X211 transition in a similar wavelength region. Furthermore, the discernible vibra- 5: tional frequencies for such isoelectronic species are similar as is c13 the overall Franck-Condon shape of the band systems. The ' k-0.0 A c15 LL 300 350 400 450 hl nrn Fig. 9 Absorption spectra of the carbon chains, lX,+-XICg+ electronic transition, in neon matrices using mass-selection.The weaker structure above 300 nm in the spectrum of C9 is due to a forbidden transition. sumably, the sensitivity of the mass-selected approach is not yet sufficient to detect the weaker electronic transitions of the cyclic isomers. On the other hand, the lzI,+-XIZg+ transition of the odd- numbered carbon chains could be detected up to n = 15.22 These band systems for the longer species are shown in Fig. 9. A plot of wavelength vs. length of chain dependence is shown in Fig. 8, an extrapolation of which yields a prediction for the hitherto unmeasured longer species. Furthermore, the trend on the high energy size predicts that the yet to be observed corresponding transitions of the chains with odd-number of carbon atoms, C2, + 1H have not yet been obtained.9 Nitrogen containing chains It proved possible to measure and assign the 2II-X2Il electronic transitions of cyanopolyacetylene cations HC,CN' (n = 4-12) and NCC,CN+ (n = 2-10).23 This was carried out using a hot- cathode discharge source for the cation production with cyanoacetylene or dicyanoacetylene as precursor gas. The same pattern as for the carbon chains is manifested for these homologous series. Because the HC,CN+, NC,N+, C, + 2H and HC,+2H+ species are isoelectronic, the series can be directly compared and they indeed show the 211-X211 band systems in a similar region. This can be seen by comparison of the spectra of C12-, C12H, HC12H+, HCIIN' and NCloN+ in Fig. 10; the origin bands lie within a 0.6 eV span.Table 1 shows the wavelengths of the observed origin bands of the electronic transitions of the carbon chains identified hitherto in neon matrices using the mass-selected approach. 10 Gas-phase studies Because it has always been the aim to use the matrix observations as a basis for gas-phase studies of the carbon chains, it is useful to consider the data for the species where this Chemical Society Reviews, 1997 25 Table 1 Wavelength (h/nm) of the origin band of the characteristic n-n transition of carbon chains observed in neon matrices C2, + 1: I&+ cx 1Xg+ c2,-: 2rl tx 2n C2,: 32"- tx 32,-C7 253 c4-457 C4 380 Cg 295 c6-608 c6 511 CI~336 c*-773 C8 640 c13 380 Clop 967 ClO 735 C15 420 C12-1249 C14-1460 C16-1729 C18-2069 (220-2440 HC2,+ 1H: 3Cu- tX 3Cg-C2,H: 211 tX 2l-I HC2,N+: 2ll tX 2ll HCSH 434 CGH 530 HC6N+ 570 HC7H 505 CgH 631 HCsN+ 657 HCgH 582 CloH 722 HC ION+ 747 HCllH 655 C12H 801 HC12N+ 832 HC13H 721 C14H 866 HC15H 781 C16H 924 HC2,+ IN+: 2ntX 211 NC2,N': 211tX 2ll NC2,+ 1 N+: 211 tX 2ll HC5N+ 584 NC4N+ 598 NC7N+ 629 HC7N+ 674 NC6N+ 659 NCgN+ 7 13 HCgN+ 77 1 NCSN+ 74 1 NCI IN+ 795 HCIIN+ 871 NCloN+ 831 HC 13N+ 973 NC12N+ 923 HC2,H': *nucX TIg HC2, + 1H+:TI tX 2lI HC4H+ 509 HC5H+ 499 HC6H+ 605 HC7H+ 600 HCsH+ 7 13 HCgH+ 659 HCloH+ 823 HC1 IH+ 789 HC12H+ 934 HC 13H+ 873 HC14H+ 1047 H&H+ 959 Hc16H+ 1160 7 21-ICX211 0.002.nJ,L,,, 0.000J 1200 1250 dL, , , , , , , 700 750 800 850 900 h/nm Fig. 10 The 2n-X2ll electronic absorption spectra of isoelectronic carbon chains in neon matrices has already proved possible. Also relevant is a comparison of Recently, the 21T-X21T transitions could be detected in the gas- the gas-neon matrix shifts and apparent trends. phase for the three carbon anions C,-n = 4,6,8 using a two- Prior to the study of the electronic spectra of mass-selected colour photodetachment approach.10 The latter experiments carbon chains and their ions in neon matrices, the 21T-X211 could be carried out because the location of the transitions was transitions of cations of three polyacetylenes, HC,H+ n = 4,6, known from the matrix studies." The 2rI-X211 band system of 8, and of three cyanopolyacetylenes, HCsN+,NC,N+ n = 4,6, C6H could for the same reason be observed in a discharge by a were observed in the gas-phase by emission spectro~copy.~~ sensitive laser absorption technique ('cavity ring-down').25 Chemical Society Reviews, 1997 Table 2 Gas-neon matrix shifts of the origin bands of the 2Il tX 2Il electronic transitions of carbon chains.All values in cm-1. Species Gas Neon Shift Ref. 19724 19645 79 16, 17 16670 16535 135 16, 17 14160 14017 143 16, 17 17190 17130 60 23,24 16781 16719 62 23, 24 15260 15173 87 23,24 18996 18854 142 11,25 2 1872 21896 -24 10,ll 16476 16458 18 10, 11 12963 12933 30 10,ll In Table 2 is given a comparison of the gas and matrix frequencies of the origin bands of the electronic transitions of these carbon chains.It can be seen that the shift increases with size of the molecule; in the gas-phase the transitions are found to higher energies (with the exception of C,-). The anions show rather modest shifts, and it appears that cations and neutrals which have about the same lengths show similar displacements: HC6H+ (135 cm-'), C6H (142 cm-l). On the other hand, the isoelectronic cyanopolyacetylene cations show smaller shifts: HC5N+ (60 cm-'), NC4N+ (62 cm-1). 11 Predictions for other carbon chains As is evident from Fig. 10, carbon chains which are isoelec- tronic have the corresponding electronic transitions in a similar spectral region. Therefore this information can be used to predict the absorption characteristics of the yet unstudied species.For example, the HC,+, and CnP1Nf, ions are isoelectronic with the bare carbon chains, and will follow their pattern (Fig. 8). Similarly, the neutral C, -IN, and HC,, chains, will show strong n--JI transitions in the same region as the isoelectronic HC,H+ species, whose 3C,--X3C,-band systems are located in the plots of Fig. 3 and can be extrapolated accordingly. 12 Relevance to the diffuse interstellar bands Carbon chains are among the particularly attractive candidates as carriers of the diffuse interstellar bands (DIBs),26 especially in view of the detection of carbon chains in dark clouds and carbon stars.' Thus it is worthwhile to relate the understanding of their electronic spectra to this problem.In fact, on the basis of the observations made on the absorption spectra of mass- selected species in neon matrices in the initial experiments, the number of coincidences (i.e.within a certain uncertainty taken to be representative of the gas-matrix shift) between the absorption bands and DIBs, indicated that the carbon chains and their monohydro derivatives are good candidates for the carriers .27 One can now consider the species in more detail by combining the spectroscopic results with the astronomical observations as well as physical and chemical restrictions on the molecules. The stellar observations are that nearly two hundred absorption features are attributed to DIBs.~~ These are found in the 400-900 nm region, though a few absorptions below and in the near IR have also been so interpreted.Thus from the observed trends among the carbon chains studied, whereby the strong n-n transition shifts by a particular increment to the red as the number of carbon atoms increases (Figs. 3,8), the length of the chain necessary for the molecule to absorb in the DIB window can be determined from the plots. Analogously, this can be predicted for the hitherto unmeasured, but isoelectronic, or structurally similar chains, also those including nitrogen or oxygen atoms. The series of carbon chains likely to be easily formed in the diffuse clouds are: C,, C,H, HC,H (or their H2C, isomers) and their ions, as well as the related N-and O-containing species.The astrophysical conditions dictate that the species have to be large enough to be stable with respect to photodissociation to starlight penetrating the diffuse clouds, or that efficient syntheses are operative.29 Thus a sufficient size has been considered to be molecules with say > 10-15 atoms.30 If this criterion is applied, then a number of the smaller carbon chains that would absorb in the 400-900 nm spectral window can be eliminated. Examples of this are the bare carbon chains with even number of atoms. As is seen in Fig. 8, C, n = 6, 8, 10 would absorb in the DIB region but are deemed as too small to be photostable. On the other hand the odd-numbered carbon chains, C, n = 15-37 will show their strong n--JItransition in the 400-900 nm gap and are large enough.However, it has been shown both experimentally and theoretically, that carbon species compris- ing more than 20 atoms prefer a ring structure,*l which may be the chemical restriction on the length of the chains to be considered. These arguments lead to a natural restriction on the number of possible species, which is attractive because there appear to be a limited number of stronger DIBs. The chains which would satisfy the above discussed criteria are then the odd carbon chains C2, + 1, and their isoelectronic analogues HC2, + If and C2,Nf, with n > 9 but perhaps terminating in the mid-twenties. Not specifically discussed, but chains which will have similar spectral characteristics and need to be considered are the oxygen-containing ones, C,O, and the cumulene type structures H2C,, H2C,O.As an example, the C,,O species with n = odd have lECg+ground state, whereas those with n = even YZc,-, and will behave spectroscopically like the odd- or even-numbered C, chains, respectively (cJ Fig. 8). It is relevant to note that H2Cn, C,H and C,O type chains have been detected in dark clouds by radioastronomy.31 13 Outlook The observation of the electronic spectra of a number of homologous series of the carbon chains in neon matrices by utilising mass-selection, has given the breakthrough in provid- ing the location of the transitions so that gas-phase studies are a realistic proposition, as well as for astrophysical considerations. The interesting chains still to be characterized by this technique are the species containing oxygen and the cumulenes.In connection with the DIBs assignment, the data define on which type of carbon chains interest should be focused. The next stage is then a concerted effort at obtaining the electronic spectra of the relevant species in the gas-phase. To this end a variety of laser-based approaches are envisaged. In the case of the anions the two-colour laser photodetachment approach has already proven to be successful for the smaller species; lo for the neutrals laser absorption methods, such as used for C6H detecti01-1,~~and resonant photon ionization approaches are likely to succeed because the wavelength region of the electronic transitions is known in neon matrices.14 Acknowledgements The studies in Base1 have only been possible due to the motivation and ability of a number of PhD students and postdoctoral fellows, who are authors in the cited articles. 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