Hadjar, O

University of Groningen

   University of GroningenHidden sides of C60Hadjar, OIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hadjar, O. (2001). Hidden sides of C60. Groningen: s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-20198 SummaryIn this thesis time-of-flight fragment rnass spectrometry has been em-ployed to study interactions with the buckminsterfullerene Coo in the gasphase. Projectile ions of different species have been used, ranging from He-liunr to Xenon (2:2-54) covering a velocity range of v:0.1-1 a.u. Chargestates har'e been varied between g:1 and q:28. Collisional processes suchas ionization (Coo *Clii 'e+). e.raporation (C[]- -Clil..,nz) and multifrag-mentation (Cln,..r) of the fullerene were investigated. The study performedin this thesis shows that the stored energy in the C6s can be tuned bl.threequantities of the ionic projectile: charge state q, velocity v and nuclearcharge Z.Highly charged ions carry very high potential energies and induce mul-tiple electron removal from C6s already at large distances. This results inthe production of intact vibrationally and electronically colcl C[j-. In fact,the TOF spectra are dominated by multiply charged C[]- ions, similar tospectra using heavy atomic targets. In this sense C66 behaves as an atomicentity. StaUility of the fullerene against nine electron removal was observedin our experiments. Relatively close capture distances are accompanied byenergy transfer to the fullerenes' vibrational and electronic degrees of free-dom. A closer look into these processes is achieved by switching from highlycharged to singly or doubly charged ions like Heq+.Varl.ing the Heq+ velocity, shows a dramatic change in the fullerene re-sponse. At low projectile velocities, C66 suffers mainly from evaporativeprocesses leading to loss of one, two, or many C2, units. In tliis regime theatomic projectile interacts with the carbon atoms (C66 constituents) for arelatively long time. This leads to an energy transfer to the carbon atoms(elastic process) and as a consequence, the vibrational modes of the C661091 1 0 Summaryare excited. The fullerene then reduces the excess of energy by eraporatingunits of C2.. The ionger the He-C interactiori time, the higher the arnountof energy transferred to the carbon atoms, which in turn leads to strongerc66 evapolation. The increase of the Heq+ velocity. or the decrease of theinteraction time, retluces the strength of the eraporation processes dramat-ically. Tleating the different collisional partners as atoms and describhgthe interactions by analytical interatornic potentials, our classical moleculard1'namics simulations predict a proportionalitl. between the elastic energ)'loss of the atomic projectile and the interaction time. The observed 1/udecrease of the measured relative evaporation cross section, in the case ofHeq+ projectile. is a strong irrdication that the eraporation processes in c6eare mainly due to direct excitation of vibrational modes (nuclear stopping).The second aspect observed in the same scan of Hes+ verocities, is thelinear increase of the measured C6s direct ionization and rnultifragrnentationcross sections as a function of the projectile velocitl. l. Tliis opposite be-havior (as compared to tlie evaporation), is attributed to inelastic pr.cesses(electronic stopping). Treating the c6s valence electrons as an electrongas rvith a certain electron density distribution, and considering projectileenergy loss described bv the electronic friction models:#:ut (z :2,n. )implies that the Heq+ inelastic energy loss or the energy stored in the c6eelectronic excitation modes is proportional to the Heq+ velocity. The ol>served linear increase of the measured direct ionization and multifragmen-tation yields is the first clear indication that those quantities stem from11gc+ projectile ind.ced excitation of electronic modes of c6s (electronicstopping).A closer look at electronic stopping related processes lecl us to an investi-gation of the effect of the projectile nuclear charge Z on the direct ionizationand fragmentation of Coo. In experimental studies on electronic stopping insolids and in theoretical calculations within the frame work of the densityfunctional theory, an oscillatory behavior of the electronic stopping as afunction of the projectile nuclear charge has been obserr.ed. However botlimethods r'"'ere dealing with an infinitely large reservoir of electrons formingthe stopping mediurn. our experiments done'r'ith projectile nuclear chargeranging from He (Z:2) fo Ar (Z:18) on C6s showed for the first time thatsimilar oscillations can be observed in clusters rvhere they manifest them-selves as oscillations in direct ionization and fragmentation cross sections ofC6s as a function of Z.It has to be kept in mind that the interaction times involved in o'r exper-iments are very short (-5 fs), while the coo fragmentation and evaporationoccur on much longer time scales (around a few pico and micro seconds(8 1)-41 1 1respectivel)'). Despite the very brief interaction time, the C66 seems toremember in n'hich channel the energy u'as deposited and responds accord-ingly.4{D +nn/rFig. 8.1: Time-of-fl,ight spectra showing som,e similarities betweert thehonto-nuclear C 66 rnolecule and, the hetero-nuclear Pentaphengl-ethermolecule.Outside the scope of C6s experiments, a first attempt towards hetero-nuclear molecules was performed using diffusion pump oil as a target. Fig.8.1 shows tirne-of-flight spectra of the C6s and the Pentaphenyl-ether. Thesimilarities lvhich can be seen are the obser'"ation of singly and doublycharged intact molecules (ionization process) and the small fragmcnts sep-arated by one carbon mass (by analogy to the C66 multifragmentation).The knowledge acquired in this thesis may be used for much largermolecules like biological molecules, to study qualitatively and quantitativeiythe effect of the three tunable ionic projectile parameters. Indeed the ad-vantage of ions oppositell' to electrons or photons, is the fact that a veryt@Ea)tn 350t50t@n( . , n \ / \ / { , /,,!""'-td't l-"ttt, f=::r::\ lt'--'\* ' i .  , ' 0 9 , .  , , f o ' ,  l o t r  , ;. c - c .  , ,  - ,  l - \  l - \t l t l t t [ { f i i l t |\ /l-=\'-\r ,rt-^l-\rl llr t2 Summarylarge amount of energy can be transferred to the bio-molecule, enough tobreak the stronsest bonds.1l

Hidden sides of C60

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