G Montagne

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.

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.

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.

The cooling of naphthalene cations studied within an electrostatic storage ring

Fast population decay of naphthalene cations (C10H8+) has been observed in a compact electrostatic storage ring, the Mini-Ring, up to 5 ms. Laser induced dissociation due to single-photon absorption was used to probe the internal energy distribution (IED) of the stored molecular ions as a function of the storage time. To determine the energy distribution of naphthalene cations, the experimental neutral decay curve was analysed with a model including the competition between dissociation and radiative cooling. Fast cooling rates from about 70 s−1 at the internal energy 5.6 eV to 140 s−1 at 6.2 eV were measured and compared with the data in the literature. This fast cooling mechanism is attributed to the fluorescence from thermally excited electrons and may have important implications in astrophysics for the lifetime and the critical size of Polycyclic Aromatic Hydrocarbons (PAH) in the interstellar medium.

Fragmentation of anthracene induced by collisions with 40 keV Ar8+ ions

We report on the fragmentation of anthracene molecular ions C14H10r+ as a function of the parent ion initial charge r (= 1–4). Neutral anthracene molecules in the gas phase were ionized and excited in collisions with Ar8+ ions at 40 keV and the mass-to-charge spectra of the parent ions C14H10r+ (1 ≤ r ≤ 4) were obtained. Stable molecular ions C14H10r+ (1 ≤ r ≤ 3) are observed. Branching ratios for the competitive evaporation (loss of neutral fragments) and fragmentation (charge separation) processes were measured for C14H102+ parent ions. For C14H103+ parent ions, the results indicate that fragmentation is the only dominant process and quasi-symmetric fission is observed.

Fragmentation of multicharged C70q+ prepared in collisions with F+ ions at 3 keV

Ionization and successive evaporation of C70r+ is studied as a function of the internal energy using collision induced dissociation under energy control. Multicharged ions C70r+ are prepared in F+ (3 keV) + C70 → F− + C70r+ + ne collisions. Up to seven successive evaporation of C2 are observed in an excitation energy range from 40 to 100eV. The dissociation energies of C70-2m2+ (m=1-7) are determined using a statistical cascade model to reproduce the excitation energy distribution of C702+ parent ions for each dissociation channel. Results are in good agreement with previous theoretical calculations.

Time Evolution of the internal energy distribution of molecules studied in an electrostatic storage ring, the Mini-Ring

We report on studies about time evolution of internal energy of polycyclic aromatic hydrocarbon PAH and fullerene in a small electrostatic storage device. Laser heating at different time delays was used as the probe to determine the energy distributions.