Bacchus-Montabonel, M.C.

Barbatti, Mario

Buchet, J-P.

Carre, M.

De Castro Faria, N.V.

Farizon, B.

Farizon, M.

Gaillard, M.J.

Gobet, F.

Goncalves, N.

Jalbert, G.

Louc, S.

Luna, H.

Maerk, T.D.

de Castro Faria, N.V.

HAL-IN2P3

Event by event analysis of collision induced cluster ionfragmentation: sequential monomer evaporation versusfission reactionsF. Gobet, B. Farizon, M. Farizon, M.J. Gaillard, S. Louc, N. Goncalves, M.Barbatti, H. Luna, G. Jalbert, N.V. De Castro Faria, et al.To cite this version:F. Gobet, B. Farizon, M. Farizon, M.J. Gaillard, S. Louc, et al.. Event by event analysis ofcollision induced cluster ion fragmentation: sequential monomer evaporation versus fission re-actions. Physical Review Letters, American Physical Society, 2001, 86, pp.4263-4266. <in2p3-00009876>HAL Id: in2p3-00009876http://hal.in2p3.fr/in2p3-00009876Submitted on 30 Aug 2001HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.VOLUME 86, NUMBER 19 P H Y S I C A L R E V I E W L E T T E R S 7 MAY 2001Event-by-Event Analysis of Collision-Induced Cluster-Ion Fragmentation:Sequential Monomer Evaporation versus Fission ReactionsF. Gobet, B. Farizon, M. Farizon, M. J. Gaillard, and S. LoucInstitut de Physique Nucléaire de Lyon, IN2P3-CNRS et Université Claude Bernard, 43 boulevard du 11 Novembre 1918,F-69622 Villeurbanne Cedex, FranceN. Gonçalves, M. Barbatti, H. Luna, G. Jalbert, and N. V. de Castro FariaInstituto de Fisica, Universidade Federal do Rio de Janeiro, CP 68528, Rio de Janeiro, 21945-970 RJ, BrazilM. C. Bacchus-Montabonel, J. P. Buchet, and M. CarréLaboratoire de Spectrométrie Ionique et Moléculaire, CNRS UMR 5579 et Université Claude Bernard,43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne Cedex, FranceT. D. MärkInstitut für Ionenphysik, Leopold Franzens Universität, Technikerstrasse 25, A-6020 Innsbruck, Austria(Received 9 March 2000)The most abundant decay channels have been studied quantitatively for high-energy 60 keVamucluster ions H31H2m1 14 colliding with He atoms employing a recently developed multicoincidencetechnique for the simultaneous detection of the correlated fragments on an event-by-event basis. Thisallows us to identify decay reactions and their underlying decay mechanisms responsible for the occur-rence of the U-shaped fragmentation pattern.DOI: 10.1103/PhysRevLett.86.4263 PACS numbers: 36.40.QvSpontaneous fragmentation of finite systems, e.g., nu-clei, molecules, clusters, has recently attracted increasedinterest, in particular, because of the recognition thatsome of the general features in the decay of these systemsare rather independent of the actual system studied [1].Therefore, one important field in cluster science [2] is thestudy of the characteristics of the fragmentation of excitedcluster ions produced by the excitation of neutral orionized clusters by photons, electrons, ion impact, orsurface collisions.Fragmentation after low-energy deposition has beensuccessfully interpreted in the frame of the evaporativeensemble model [3–6] invoking the sequential evapora-tion of neutral monomer units for singly charged clusterions [7] or by sequential fissionlike reactions of multiplycharged cluster ions [8]. In contrast, high-energy depo-sition, e.g., by collisions of the clusters with high-energyheavy particles or vice versa [9–12] and by collisionsof clusters with multiply charged ions [13,14], has ledto bimodal fragment ion distributions which have beeninterpreted (i) by the presence of sequential (monomer)evaporations (see above) leading to the production offragment ions with large masses and (ii) by the presenceof multifragmentation processes leading to the formationof the fragment ions with the small masses. Thesebimodal distributions, sometimes exhibiting a U shape[10,12], sometimes consisting of two peaks separated bya strong minimum [9,11,13,14] have also been observedfor primary mass spectra after electron impact or photonimpact ionization of C60 and after C60 ions impacting asurface (see [15] and references therein). Moreover, these0031-90070186(19)4263(4)$15.00distribution patterns obtained are similar to what havebeen seen in nuclear fragmentation [16].Although today several experiments have measuredthese “total (inclusive) fragment ion mass distributions,”mostly for fullerene targets, and although the origin ofthis bimodal distribution is generally thought to be due tothese two differing fragmentation mechanisms, knowledgeon the details of multifragmentation reaction mechanisms(and its contribution to the bimodal distribution) is veryscarce and mostly due to molecular simulations [17,18]. Inaddition, some coincidence experiments have been carriedout (using either highly charged ion projectiles or chargedC60 ion projectiles) where two fragmentation productsfrom C60 have been identified, thereby providing moreinformation on the fragmentation dynamics [11,19–23].The fragmentation behavior is observed to be stronglydependent on the charge state of the fullerene after thecollision [24] and on the nature of the projectile ionused [25].Here we have extended these studies in several aspects.On the one side, we are using hydrogen cluster ionswith different sizes Hn#311 instead of C601 as in theearlier studies, thus allowing us to study fragmentationas a function of cluster size n. Moreover, the protonatedhydrogen clusters represent a completely different class ofsystems where a quantum solute is solvated by a quan-tum solvent; the added proton becomes trapped and atightly localized H31 core is surrounded by solvatingH2 molecules, H31H2m#14 [26]. On the other side,we are employing here a recently developed multi-coincidence technique for the simultaneous detection (on© 2001 The American Physical Society 4263VOLUME 86, NUMBER 19 P H Y S I C A L R E V I E W L E T T E R S 7 MAY 2001an event-by-event basis) of the correlated, ionized, andneutral fragments from collision of 60 keVamu hydrogencluster ions with a He target. This allows us to investigatefor the first time in a quantitative manner (absolute crosssections) the occurrence of the various possible decaychannels and their relative importance as a function ofm. Focusing here on some of the most abundant decayreactions (involving the production of neutral fragmentsand one charged fragment), we are able (i) to distinguishexperimentally the two different decay channels mostimportant for occurrence of the U-shaped total fragmen-tation distribution, and (ii) to obtain novel insights aboutthe excitation and subsequent decay mechanisms in thesemolecular cluster ions.Mass selected hydrogen cluster ions with an energyof 60 keVamu are prepared in a high-energy cluster-ionbeam facility consisting of a cryogenic cluster jet expan-sion source (stagnation temperature of 34 K) combinedwith a high performance electron ionizer and a two stepion accelerator (consisting of an electrostatic field and aradio frequency quadrupole postaccelerator). After mo-mentum analysis by a magnetic sector field, the mass se-lected high-energy projectile pulse (pulse length 100 ms,repetition frequency 1 Hz), consisting in the present studyof H31H2m#14 cluster ions, is crossed perpendicularly bya helium target beam effusing from a cylindrical capillarytube. Prior to this, the ion beam is collimated by two aper-tures ensuring an angular dispersion of about 0.16 mrad.One meter behind this collision region the high-energy hy-drogen collision products (neutral and ionized) are passinga magnetic sector field analyzer. The undissociated pri-mary H31H2m#14 cluster projectile ion, or the neutraland charged fragments resulting from reactive collisions,are then detected approximately 0.3 ms after the collisionevent with a multidetector device consisting of an array ofpassivated implanted planar silicon surface barrier detec-tors located at different positions at the exit of the magneticanalyzer. This allows us to record for each event simulta-neously the number (multiplicity) of each mass-identifiedfragment ion resulting from the interaction (for more ex-perimental details, see Ref. [10] and references therein).Moreover, for each event we can also monitor in coin-cidence with the detected ions the sum of the masses ofall the neutral fragments. From additional measurements,the identity of neutrals, either H atoms or H2 molecules,can be inferred [27]. Thus, we are able to analyze on anevent-by-event basis the identity of all fragments producedin a collision between the H31H2m#14 cluster ion and theHe target atom, except for the distinction between H2 andtwo H. The validity of single collision conditions has beenascertained by measurements at different He target pres-sures and allows also to derive absolute cross sections forthe occurrence of specific reaction channels (partial crosssections) (for details, see [28]).Figure 1 shows the absolute cross section as a functionof cluster size m of the projectile ion H31H2m for one ofthe most abundant reaction channels, i.e., the production of4264FIG. 1. Collision cross section as a function of clustersize m for the dissociation channel: H31H2m 1 He !H31H2m21 1 H2 (or H 1 H), designated by soneloss, and forthe dissociation channel: H31H2m 1 He ! H31 1 neutralfragments, designated by sallloss.only one charged reaction product (fragment ion) by eitherthe loss of one H2 monomer leading to a H31H2m21fragment ion,H31H2m 1 He ! H31H2m21 1 H2 or H 1 H ,(1)or by a reaction involving the production of the centralH31 core ion of these clusters via loss of all the neutralligands:H31H2m 1 He ! H311 neutral fragments H2 and H . (2)The cross section values increase in both cases monoto-nously with cluster size n starting at 0.5 3 10216 cm2 atthe smallest cluster size H31H2 and reaching for m  14the value 1.4 3 10216 cm2 for reaction (1) and 2.2 310216 cm2 for reaction (2). This result is remarkable intwo ways, firstly reaction (2) shows an unexpected strongsize dependence and, secondly, reaction (2) is more effec-tive than reaction (1). In contrast, the cross section for theloss of two H2 monomers units via reaction (3),H31H2m 1 He ! H31H2m22 1 H2 or H 1 H1 H2 or H 1 H , (3)stays in the 0.5 3 10216 cm2 range or below for all clustersizes studied; see also Fig. 2 which shows a biparametricm, k representation of the cross sections for the fragmen-tation channel:H31H2m 1 He ! H31H2k1 neutral fragments H2 and H , (4)with 0 # k , m. It can be seen that, for each cluster sizem, all possible reactions (4) (with 1 # k , m 2 1) haveVOLUME 86, NUMBER 19 P H Y S I C A L R E V I E W L E T T E R S 7 MAY 2001FIG. 2. Biparametric m, k representation of the collision cross sections for the fragmentation channel: H31H2m 1 He !H31H2k 1 neutral fragments.4265smaller cross sections than reaction (1) (with k  m 2 1)and (2) (with k  0), a fact which is responsible for theoccurrence of the U-shaped total fragmentation patternsas other reactions involving more charged fragments inthe exit channel are much less probable and thus have noleading influence on this pattern.The cross section dependence observed here for reac-tion (1) is similar to the one observed by Reuss and co-workers [29] for collision-induced dissociation ofprotonated hydrogen clusters in the 250–900 eV energyrange. This type of decay can be interpreted by theevaporative ensemble model, where a primary excitationevent initiated by the interaction of the He atom withone of the cluster constituents [see the excitation ofspecific vibrational modes of one solvent molecule in theH31H2m ion by infrared techniques [30] ], is followed byenergy transfer and partitioning between all of the clusterconstituents (mostly in intracluster vibrational modes) andsubsequent statistical (sequential) predissociation event(s)[3–8] viaH31H2m 1 He ! H31H2m , (5a)H31H2m ! H31H2m21 1 H2 , (5b)H31H2m21 ! H31H2m22 1 H2 , (5c)and so on. The observed increase with cluster size m atthe same collision velocity simply reflects the increasinginteraction volume between the reaction partners withincreasing cluster size. It is interesting to note that we areoperating the experiment at collision energies where directvibrational excitation (or momentum transfer) is neg-ligible as compared to electronic excitation (see, forinstance, [31]), and thus we have to conclude that, inthis case, excitation from the n  0 of the H2 electronicground state to a electronically excited level into n . 0providing the necessary (initial) vibrational motion for thestatistical evaporation events.In contrast, the size dependence for reaction (2)— re-ported here for the first time—cannot be explained inthis way, as reaction (2) is not proceeding via sequentialevaporation. The latter has been inferred indirectlyfrom the U-shaped nature of the fragmentation patternand other properties of the statistical decay mechanism[9,13,17,18,32], and it was concluded that reactions oftype (2) are due to prompt multifragmentation events. Thepresent results shed new light on the nature of this event.The cross section dependence immediately excludesreaction channels proceeding via a direct interaction of theHe with the H31 core (followed by a subsequent direct ex-pulsion of all neutral solvent molecules by an excited H31core) as the probability for these reactions would notVOLUME 86, NUMBER 19 P H Y S I C A L R E V I E W L E T T E R S 7 MAY 2001depend on cluster size. Moreover, the much larger crosssection for reaction (2), as compared to the one forreaction (1), excludes also the possibility of successivecollisions of the He within the cluster as the probabilityfor scattering on two solvent molecules can be estimatedto be less than about 10% of the probability for singlecollision. In addition, we can conclude that reaction (2)does not necessarily involve the interaction of the Heatom with an inner solvent molecule, but proceeds alsoin the case where the He interacts with a molecule of theoutermost solvent shell. This can be seen by comparingthe cross section for H31H24 [which is, according toab initio calculations [26], a closed shell H31H2m ionwith an additional weakly bound H2 molecule] with thecross section for the closed shell H31H23 ion. The crosssection for the former one 0.72 3 10216 cm2 is largerthan for the latter one 0.92 3 10216 cm2 indicating thatinteraction of the He with the outer shell molecule is alsocontributing to the overall cross section for reaction (2)leading to the complete destruction of the cluster.Turning now to the mechanism of reaction (2), the onlyplausible explanation is excitation of one of the moleculesin a high enough electronic energy level to allow the sub-sequent destruction of the entire cluster by direct couplingof at least part of this energy into intracluster modes. Inanalogy to the reactions observed in argon clusters, wherethe excitation of a metastable excimer dimer state (withinthe ionized cluster) and the subsequent radiative decay ofthis excimer into the dimer ground state led to the suddenrelease of about 1 eV into two fast moving Ar atoms fol-lowed by the direct expulsion and evaporation of up to halfof the monomers [33], we are proposing that reaction (2)proceeds via the excitation of a dissociative electronic statein one of the solvent molecules. It has to be noticed thatthe technique successfully achieved for the mass analysisof the neutral fragments (distinction between an H2 mole-cule and two H atoms) in the H31 fragmentation studies[34] is not practicable in the cluster case, due to the in-crease of the number of dissociation channels.The dissociation of an excited H2 leads to the formationof two fast moving (possibly excited) hydrogen atoms inthe vicinity of the H31 core. Whereas part of the destruc-tion of the cluster ion will proceed via a direct impulsiveinteraction (in a similar manner as observed for excimerdecay reactions in rare gas cluster ions [33]), part of thedestruction may also proceed via interaction of the hydro-gen atom with the H31 core. This is more likely, as it iswell known that the reactionH2 1 H21 ! H31 1 H (6)is very efficient [35]. A hydrogen atom in the cluster couldtherefore induce a large change in the H31 polarization andthereby weaken the interaction potential with the remain-ing neutral ligands leading to the evaporation of all thesolvent molecules.4266[1] Fragmentation Phenomena, edited by D. Beysens,X. Campi, and E. Pfefferkorn, Les Houches Series (WorldScientific, Singapore, 1995).[2] H. Haberland, Clusters of Atoms and Molecules I and II(Springer-Verlag, Berlin, 1994).[3] C. E. Klots, J. Phys. Chem. 92, 5864 (1988).[4] R. Wörgötter et al., J. Chem. Phys. 104, 1225 (1996).[5] C. Lifshitz, Mass Spectr. 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Event by event analysis of collision induced cluster ion fragmentation: sequential monomer evaporation versus fission reactions

Event by event analysis of collision induced cluster ion fragmentation: sequential monomer evaporation versus fission reactions

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