R. Brédy

Fragmentation of adenine under energy control

We present results on the fragmentation of adenine dication as a function of the excitation energy. The adenine molecule is charged and excited in a single collision with Cl(+) ion at 3 keV and the excitation energy distribution is obtained for each fragmentation channel by measuring the kinetic energy loss of the projectile. This method named collision induced dissociation under energy control is based on the formation of a negative scattered projectile as a result of double electron capture from the target molecule. Comparison between the main dissociation channels of singly and doubly charged adenine shows that fragmentation patterns are very similar consisting mainly of the successive emission of neutral HCN or H(2)CN(+). The energy distributions of the parent adenine dication and the kinetic energy release of the fragments measured for the most abundant fragmentation channels confirms the assumption of successive emission dynamics. A specific fragmentation pathway of the adenine requiring less energy than the usual successive emission of neutral HCN could be identified. It consists of the emission of a charged H(2)CN(+) following on by the emission of a dimer of HCN (precisely HC(2)N(2)). This new channel, measured for a mean excitation energy of 8.4 eV for the adenine dication is very closed to the emission of HCN monomer measured at 7.9 eV. The implications of these results concern the formation of adenine in the sealed-tube reaction of HCN with liquid ammonia as well as the possible formation of the adenine molecule in the interstellar medium. This last point is briefly discussed in relation to astrobiology and exobiology interests.

Precise Determination of 2‐Deoxy‐D‐Ribose Internal Energies after keV Proton Collisions

The interaction of keV protons with building blocks of DNA is of particular biological relevance in view of the increasing number of facilities employing MeV proton irradiation for tumor treatment. When ions traverse tissue and are decelerated to MeV and sub-MeV energies, the Bragg peak is reached. At ion energies in the Bragg peak region, the induced damage is highest due to maximum linear energy transfer and relative biological effectiveness. The volume selectivity given by the existence of a well-localized Bragg peak region renders proton therapy such a promising tool for cancer treatment. Furthermore, biological consequences of irradiation with energetic protons from galactic cosmic rays and solar particle events are a limiting factor for human space exploration. This issue is of particular importance for future manned missions outside low earth orbit, for example, lunar or Mars missions. It is well-known that biological radiation damage is the ultimate result of ionization and fragmentation of cellular DNA. To explore the molecular mechanisms underlying radiation-induced DNA damage, numerous recent studies focused on ionization and fragmentation of DNA building blocks upon irradiation with slow electrons, photons and ions. In their pioneering studies, Sanche and co-workers showed that low-energy (secondary) electrons can cause singleand doublestrand breaks of plasmid DNA. Huels and coworkers investigated interactions of hyperthermal ions with DNA building blocks and concluded that heavy ions have the potential to induce particularly complex damage to DNA in the Bragg peak region. Ionization and fragmentation of 2-deoxy-d-ribose (dR, C5H10O4) molecules upon impact of keV and sub-keV ions has recently been investigated for isolated molecules as well as for the condensed phase. In both studies, almost complete disintegration of the molecule was observed and it was concluded that direct ion-induced DNA damage—for example in heavy ion or proton therapy of malignant tumors—may be dominated by deoxyribose fragmentation. Furthermore, Deng and coworkers have recently reported evidence for reactive scattering damage of 2-deoxy-d-ribose thin films by hyperthermal ions. 9] In the gas-phase studies, the mass spectrum of the fragment ions largely follows a power-law distribution with a characteristic exponent t of the order of 2, depending on the ion velocity, charge state and the atomic number of the impinging ion. From this, it was concluded that the fragmentation is to a large extent statistical. t was found to be an ideal parameter for the quantification of damage inflicted upon a molecule whose fragmentation yield is very close to 100%, but this quantification was purely phenomenological, since the fragmentation channels and the characteristic exponent could not be directly related to the amount of energy initially deposited into the deoxyribose molecules. In a study on MeV Si + collisions with C60, Itoh et al. have already concluded that t carries information about the energy deposition Eexc into the target molecule. In their work, the target excitation energies Eexc were calculated and a simple exponential dependence of Eexc on the characteristic exponent t was found. However, the value of Eexc was not experimentally obtained in their study. Very recently Chen et al. presented a new experimental approach to measure directly the amount of internal energy present in the transient C 60 * formed by double electron transfer to keV protons. Using this technique, different fragmentation channels could be assigned a corresponding excitation energy. We have now applied the same technique to double electron capture processes in keV proton collisions with 2-deoxy-d-ribose (H +deoxyribose!H +deoxyribose*). To our knowledge, this is the first report on the fragmentationchannel-specific experimental determination of the excitation energy present in a transient dicationic biomolecular complex. The concept of the applied experimental technique is based on the idea of double-charge transfer (DCT) spectroscopy (introduced by Durup and coworkers in the early 1970s) but in an event-by-event mode with coincident detection of all ionic molecular fragments. In a keV ion collision with an isolated molecule, the target can be subject to ionization, electron transfer and excitation. For an accurate determination of the internal energy of the transient excited molecular ion, a process is chosen for which the complete energy balance is experimentally accessible. The target internal energy is equal to the sum of projectile kinetic energy loss, the projectile excitation energy and the difference in binding energies of the electrons. Since only the projectile kinetic energy is easily accessible experimentally, an ideal outgoing projectile has no stable excited states. This is the case for H . Since it is experimentally very challenging to measure kinetic energies of emitted electrons while keeping the electron detection efficiency high, only H formation by double electron capture from deoxyribose is considered [Eq. (1)]: [a] Dr. F. Alvarado, Prof. Dr. R. Hoekstra, Dr. T. Schlathçlter KVI Atomic Physics University of Groningen, Zernikelaan 25 9747AA Groningen (The Netherlands) Fax: (+ 31) 50 363 4003 E-mail : tschlat@kvi.nl [b] Dr. J. Bernard, B. Li, Dr. R. Br:dy, Dr. L. Chen, Dr. S. Martin Universit: Lyon 1, CNRS, LASIM UMR 5579 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne (France)

Activation energies for fragmentation channels of anthracene dications—experiment and theory

We have studied the fragmentation of the polycyclic aromatic hydrocarbon anthracene (C14H10) after double electron transfer to a 5 keV proton. The excitation energies leading to the most relevant dissociation and fission channels of the resulting molecular dication were directly determined experimentally. Density functional theory calculations were performed to explore the potential energy surfaces on which the fragmentation dynamics proceed. There is clear experimental evidence for a dominance of fission into C11H7+-C3H3+ over C2H2+ loss. The energetic ordering of the dissociation and fission channels and the kinetic energy releases are in good agreement with the theoretical results. It can be concluded that the unique combination of experiment and theory presented here is an excellent tool to study the fragmentation of complex molecular ions in unprecedented detail.

An introduction to the trapping of clusters with ion traps and electrostatic storage devices

This paper presents an introduction to the application of ion traps and storage devices for cluster physics. Some experiments involving cluster ions in trapping devices such as Penning traps, Paul traps, quadrupole or multipole linear traps are briefly discussed. Electrostatic ion storage rings and traps which allow for the storage of fast ion beams without mass limitation are presented as well. We also report on the recently developed mini-ring, a compact electrostatic ion storage ring for cluster, molecular and biomolecular ion studies.

Fragmentation of singly charged adenine induced by neutral fluorine beam impact at 3 keV

The fragmentation scheme of singly charged adenine molecule (H(5)C(5)N(5)(+)) has been studied via neutral fluorine impact at 3 keV. By analyzing in correlation the kinetic energy loss of the scattered projectile F(-) produced in single charge transfer process and the mass of the charged fragments, the excitation energy distribution of the parent adenine molecular ions has been determined for each of the main dissociation channels. Several fragmentation pathways unrevealed in standard mass spectra or in appearance energy measurements are investigated. Regarding the well-known hydrogen cyanide (HCN) loss sequence, we demonstrate that although the loss of a HCN is the dominant decay channel for the parent H(5)C(5)N(5)(+) (m = 135), the decay of the first daughter ion H(4)C(4)N(4)(+) (m = 108) involves not only the HNC (m = 27) loss but also the symmetric breakdown into two dimers of HCN.

Excitation and dissociation of tungsten hexacarbonyl W(CO)6: statistical and nonstatistical dissociation processes.

We have studied the excitation and dissociation processes of the molecule W(CO)(6) in collisions with low kinetic energy (3 keV) protons, monocharged fluorine, and chlorine ions using double charge transfer spectroscopy. By analyzing the kinetic energy loss of the projectile anions, we measured the excitation energy distribution of the produced transient dications W(CO)(6)(2+). By coincidence measurements between the anions and the stable or fragments of W(CO)(6)(2+), we determined the energy distribution for each dissociation channel. Based on the experimental data, the emission of the first CO was tentatively attributed to a nonstatistical direct dissociation process and the emission of the second or more CO ligands was attributed to the statistical dissociation processes. The dissociation energies for the successive breaking of the W-CO bond were estimated using a cascade model. The ratio between charge separation and evaporation (by the loss of CO(+) and CO, respectively) channels was estimated to be 6% in the case of Cl(+) impact.

Time evolution of the internal energy distribution of singly charged anthracene in a storage ring

We have stored singly charged anthracene molecular ions C14H10+ in a compact electrostatic storage ring called the Miniring. The neutral yield curves due to dissociation of the ions induced by laser excitation at different storage times were analyzed and fitted with a t−α law. The evolution of the internal energy distribution of the stored molecular ion ensemble was obtained as a function of the storage time by using a simple model which related the experimental decay factor α to the high-energy edge of the modeled energy distribution.

Negative ions as a probe to study the relaxation of clusters and molecules as a function of the excitation energy

We have investigated the effect of the reaction window for the energy deposition in molecules or clusters with various projectiles (H+, F+, Cl+) in collision induced dissociation under energy control (CIDEC). The formation dynamics of scattered negative ions in such collision is discussed. We have measured the production rate of Cl- ions in Cl+ + C60 collisions at 18 keV to be 4%. This rate is compared to previous results obtained with Fq+ (q=1-3) projectile ions. The CIDEC method has been extended to neutral projectiles (F0) and we report on the choice of the projectile to control the amount of the energy deposited in the molecules or the clusters. Results on the excitation energy distributions of C60+ and C602+ are presented.

High negative ion production yield in 30 keV F2+ + adenine (C5H5N5) collisions

In collisions between slow F2+ ions (30 keV) and molecular targets, adenine, scattered particle production yields have been measured directly by simultaneous detection of neutrals, positive and negative ions. The relative cross-section for a negative ion formation channel was measured to be 1%. Despite a slight decrease compared to a larger target, the fullerene C60, the measured negative ion formation cross section is still at least one order of magnitude larger than the yield in ion–atom interactions.

Cooling of isolated anthracene cations probed with photons of different wavelengths in the Mini-Ring

We report on a direct measurement of the Internal Energy Distribution (IED) shift rate of an initially hot polycyclic aromatic hydrocarbon (PAH) molecular ensemble, anthracene cations (C14H10+). The ions were produced in an electron cyclotron resonance (ECR) ion source and stored in an electrostatic ion storage ring, the Mini-Ring. Laser pulses of two wavelengths were sent successively to merge the stored ion bunch at different storage times to enhance the neutral fragment yield due to fast laser induced dissociation. Using this technique, we have been able to determine directly the energy shift rate of the IED, without involving any theoretical simulation or any assumption on dissociation rates, cooling rates, or the initial IED. Theoretical energy shift rates have been estimated from the evolution of simulated IEDs by taking into account the effects of the unimolecular dissociation and two radiative decay mechanisms: the Poincaré fluorescence and the infrared vibrational emission. The comparison between the experimental results and the model provides new evidence of the important role of the Poincaré fluorescence in the overall cooling process of anthracene cations. Although in the short time range the commonly accepted intuition says that the cooling would result mostly from the dissociation of the hottest ions (depletion cooling), we demonstrate that the Poincaré fluorescence is the dominant contribution (about 85%) to the net cooling effect.