C Ortéga

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.

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.

Dissociation rate, fluorescence and Infrared radiative cooling rates of Naphthalene studied in electrostatic storage Miniring

We report studies on fluorescence and Infra Red (IR) radiative cooling rates of naphtalene cation for a large time range (40ms). The dissociation rates versus internal energy have been estimated in a large energy range to reproduce the measured emitted neutral yield of the hot naphthalene.

PAH radiative cooling and fragmentation kinematics studied within an electrostatic ring

Radiative cooling of polycyclic aromatic hydrocarbon (PAH) cations has been studied using a compact electrostatic ion storage ring, the Mini-Ring, in a time range up to 8 ms. The time evolution of the internal energy distribution of the ensemble of stored ions shows evidences of fast cooling which is attributed to the fluorescence from thermally excited electronic states. The internal energy distribution was probed by inducing unimolecular dissociation with single-photon absorption at given storage times. Information on the fragmentation kinematics and the dissociation channels were obtained by analyzing the image of the emitted neutrals detected with a time and position sensitive detector.

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.

Excitation energy distributions and statistical dissociation of C702+ prepared in collisions with F+ ions at 3 keV

The statistical dissociation of C702+ has been studied as a function of the internal energy using collision induced dissociation under energy control (CIDEC). Doubly charged ions C702+ were prepared in F+ (3 keV) + C70 → F− + C702+ collisions. Up to seven successive evaporation of C2 have been observed in a time range of 1.7 μs. The dissociation energies of C70-2m2+ (m=1-7) were 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.