Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C$_{24}$H$_{12+m}^+$ $m=0-7$, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C$_{16}$H$_{10+m}^+$, $m=0$, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.Comment: 10 pages, 5 figure

Ultraslow radiative cooling of Cn-(n = 3–5)

Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, C−n (n = 3–5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of ≈10−14 mbar and temperature of ≈13 K, allowing storage of mass-selected ions for hours and providing conditions coined a “molecular cloud in a box.” Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute time scale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving 1e cooling lifetimes of 3.1 ± 0.1 s (C−3), 6.8 ± 0.5 s (C−4), and 24 ± 5 s (C−5). Fits of the photodetachment spectra for cold ions, i.e., those stored for at least 30 s, provide adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy on jet-cooled anions, confirming that radiative cooling has occurred in DESIREE. Ion cooling lifetimes are simulated using a simple harmonic cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of a wide range of molecular anions

Sélectivité rovibrationnelle de l'ionisation de H2 en champ laser intense. Analyse détaillée de la dissociation induite par électron ou photon d'ions H2+ préparés par laser

Les développements récents dans le domaine des technologies laser ont ouvert d'intéressantes perspectives en ce qui concerne l'étude de molécules en champ électromagnétique intense. Du point de vue de la théorie, seule la molécule la plus simple H2 et son ion H2+ ont été abondamment étudiés. Expérimentalement, il est cependant malaisé d'exposer un ion moléculaire à un champ laser intense et en particulier, d'en connaître et d'en contrôler l'état vibrationnel de départ. Un montage expérimental nous a permis de déterminer l'excitation vibrationnelle accompagnant l'ionisation de H2 par impulsions brèves et intenses. Les résultats montrent une excitation beaucoup plus faible que celle supposée pendant plus de dix ans. Cette étude a démontré qu'il est possible de produire en abondance des ions moléculaires dans un état d'excitation vibrationnelle et rotationnelle bien précis grâce à une résonance avec un état intermédiaire à fort caractère de Rydberg. Des expériences nécessitant la sélection du niveau vibrationnel initial de l'ion sont alors réalisables. Nous avons ainsi pu mesurer, en injectant dans l'anneau de stockage TSR de Heidelberg les paquets d'ions produits par notre laser nanoseconde accordable, la section efficace de recombinaison dissociative et d'excitation dissociative de H2+, dans les états quantiques v+=0 et 1, N+=1, 2 et 3, et d'en observer la désexcitation vibrationnelle par collision avec des électrons lents. Ces données sont du plus grand intérêt car elles peuvent être directement comparées à la théorie sans faire intervenir de moyenne thermodynamique. Nous pouvons aussi nous intéresser au comportement d'ions parfaitement caractérisés du point de vue de leur état quantique en présence d'un champ laser bref et intense, voire d'une séquence d'impulsions. Les expériences réalisées jusqu'ici ne concernent que des ions préparés par impact électronique dans une distribution vibrationnelle de type Franck-Condon. Seuls les effets dus aux niveaux élevés ont pu être mis en évidence, ce qui pourra être évité en produisant sélectivement H2+ dans les niveaux de vibration v+=0 ou 1. Des résultats préliminaires ont été obtenus, et une technique de piégeage a été élaborée pour pouvoir réaliser de telles expériences dans l'avenir, combinant des lasers dont le taux de répétition diffère de plusieurs ordres de grandeur.(PHYS 3) -- UCL, 200

Spontaneous dissociation and rovibrational structure of the metastableD2−anion

Long-lived rovibrational states of the metastable D2− molecular anion, with lifetimes of the order of microseconds, were studied by recording the time-of-flight difference between D and D− fragments produced by spontaneous dissociation of the D2− complex. The simulated time-of-flight spectrum was adjusted to the experimental results, allowing us to extract the resonance energy relative to the dissociation threshold. A single value was found, 22.8±0.3 meV, which is somewhat larger than resonance energies predicted by theory for long-lived D2− rovibrational states with (J,v) quantum numbers (37,0), (37,1), and (38,0) [Phys. Rev. A 75, 012507 (2007)]. This discrepancy seems due to the extreme sensitivity of these metastable states to minute features of the potential energy curve. The spectral feature is explained by the competition between autodetachment and spontaneous dissociation decay channels

Branching ratio for O + H+3 forming OH+ + H2 and H2O+ + H

The gas-phase reaction of O+H+3 has two exothermic product channels, OH+8 +H2 and H2O++H. In the present study, we analyze experimental data from a merged-beams measurement to derive thermal rate coe_cients resolved by product channel for the temperature range from 10 to 1000 K. Published astrochemical models either ignore the second product channel or apply a temperature-independent branching ratio of 70% vs. 30% for the formation of OH+ 12 + H2 vs. H2O+ + H, respectively, which originates from a single experimental data point measured at 295 K. Our results are consistent with this data point, but show a branching ratio that varies with temperature reaching 58% vs. 42% at 10 K. We provide recommended rate coe_cients for the two product channels for two cases, one where the initial _ne-structure population of the O(3PJ ) reactant is in its J = 2 ground state and the other one where it is in thermal equilibrium

Laser-intensity dependent vibrational excitation and alignment of molecular ions in the ultrafast multiphoton regime.

H2 molecules were ionized in the ultrafast ( approximately 150 fs) multiphoton regime (263 nm, approximately 10;{13} W cm;{-2}). Earlier experiments investigated the kinetic energies of electrons or ions only. Using a unique experiment, we show that the vibrational excitation of molecular ions contains essential information about the dynamics of the process. In addition, we prove some earlier interpretations wrong. A realistic model based on vibronically excited intermediates, Stark shifting into resonance, reproduces the measurements, demonstrating that resonances continue to be important in the femtosecond regime. This eventually enables ultrafast control of the vibrational excitation of molecular ions, which is relevant to the whole field of molecular physics and physical chemistry

Laboratory studies into the cosmic origins of organic chemistry

We have constructed a novel merged-beams apparatus to study the cosmic origins of organic chemistry. Here we report rate coefficients measurements for reactions of atomic C with H3+. These data are important for astrochemical models

Experimental study of the proton-transfer reaction C + H 2 + → CH + + H and its isotopic variant (D 2 + )

We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of E r ∼ 0.01–10 eV