Doubly-charged ions in the planetary ionospheres: a review.

This paper presents a review of the current knowledge on the doubly-charged atomic and molecular positive ions in the planetary atmospheres of the Solar System. It is focused on the terrestrial planets which have a dense atmosphere of N(2) or CO(2), i.e. Venus, the Earth and Mars, but also includes Titan, the largest satellite of Saturn, which has a dense atmosphere composed mainly of N(2) and a few percent of methane. Given the composition of these neutral atmospheres, the following species are considered: C(++), N(++), O(++), CH(4)(++), CO(++), N(2)(++), NO(++), O(2)(++), Ar(++) and CO(2)(++). We first discuss the status of their detection in the atmospheres of planets. Then, we provide a comprehensive review of their complex and original photochemistry, production and loss processes. Synthesis tables are provided for those ions, while a discussion on individual species is also provided. Methods for detecting doubly-charged ions in planetary atmospheres are presented, namely with mass-spectrometry, remote sensing and fine plasma density measurements. A section covers some original applications, like the possible effect of the presence of doubly-charged ions on the escape of an atmosphere, which is a key topic of ongoing planetary exploration, related to the evolution of a planet. The results of models, displayed in a comparative way for Venus, Earth, Mars and Titan, are discussed, as they can predict the presence of doubly-charged ions and will certainly trigger new investigations. Finally we give our view concerning next steps, challenges and needs for future studies, hoping that new scientific results will be achieved in the coming years and feed the necessary interdisciplinary exchanges amongst different scientific communities.

Crossed-beam scattering studies of electron-transfer processes between the dication CO2(2+) and neutral CO2: electronic states of reactants and products involved.

Crossed-beam scattering experiments were carried out at collision energies of 4.51 and 2.71 eV to elucidate the electronic states involved in the nondissociative and dissociative electron-transfer reactions observed following CO(2)(2+)/CO(2) collisions. Specifically, we focus on the observation that, in the dissociative electron-transfer reaction, forming CO(+), the majority of the CO(+) product ions are formed via electron capture by the CO(2)(2+) rather than via ejection of an electron from the neutral CO(2) reaction partner. The main channels resulting in nondissociative electron transfer are reactions of the ground (X(3)Sigma(g)(-)) and excited states of CO(2)(2+) to give different combinations of the ground and excited states of the product pair of CO(2)(+) ions in which the combination AA appears to be significant. The CO(+) ions appear mainly to arise from slow dissociation of CO(2)(+)(b(4)Pi(u)) formed following electron capture by the ground state of the dication reactant (X(3)Sigma(g)(-)), with possible contributions from electron capture by higher triplet excited states of the dication.

Fast ion-molecule reactions in planetary atmospheres: a semiempirical capture approach.

The description of planetary and interstellar chemistry relies strongly on ion-molecule reaction rate data collected at room temperature or above. However, the temperature in the ionospheres of planets and in the interstellar medium can decrease down to 100 K and 10 K, respectively. We present here a simple semiempirical method to extend available measurements towards those temperatures. Our approach is based on the long-range capture theory combined with room temperature data. Results are presented for cation-molecule and anion-molecule reactions. An overall good agreement is observed between our model and various experimental data in the temperature range 20-295 K. Deviations larger than a factor of 2 are found, however, with ion trap measurements below approximately 50 K. Predictions are also made for reactions of carbon chain and hydrocarbon ions with atomic hydrogen, of particular importance in Titan's atmosphere and in interstellar clouds.

Very high resolution mass spectrometry of HCN polymers and tholins.

HCN polymers are complex organic solids resulting from the polymerization of hydrogen cyanide (HCN) molecules. They have been suspected to contribute to the refractory carbonaceous component of comets as well as the distributed CN sources in cometary atmospheres. Titan's tholins are also organic compounds produced in a laboratory setting but result from the complex chemistry between N2 and CH4 induced by UV radiation or electric discharges. Some of these compounds have optical properties in the visible range fairly similar to those of Titan's aerosols or those of the reddish surfaces of many icy satellites and small bodies. It has been proposed that HCN polymers are constituents of tholins but this statement has never received any clear demonstration. We report here on the comparative analysis of tholins and HCN polymers in order to definitely establish if the molecules identified in the HCN polymers are present in the tholins as well. First, we present a global comparison of HCN polymers with three kinds of tholins, using elemental analysis measurements, infrared spectroscopy and very high resolution mass spectrometry of their soluble fraction. We show that the chemical composition of the HCN polymers is definitely simpler than that of any of the tholins studied. Second, we focus on six ions representative of the composition of HCN polymers and using mass spectrometry (HRMS and MS/HRMS), we determine that these tholins contain at best a minor fraction of this kind of HCN polymers.

Chemical characterization of Titan's tholins: solubility, morphology and molecular structure revisited.

In this work Titan's atmospheric chemistry is simulated using a capacitively coupled plasma radio frequency discharge in a N(2)-CH(4) stationnary flux. Samples of Titan's tholins are produced in gaseous mixtures containing either 2 or 10% methane before the plasma discharge, covering the methane concentration range measured in Titan's atmosphere. We study their solubility and associated morphology, their infrared spectroscopy signature and the mass distribution of the soluble fraction by mass spectrometry. An important result is to highlight that the previous Titan's tholin solubility studies are inappropriate to fully characterize such a heterogeneous organic matter and we develop a new protocol to evaluate quantitatively tholins solubility. We find that tholins contain up to 35% in mass of molecules soluble in methanol, attached to a hardly insoluble fraction. Methanol is then chosen as a discriminating solvent to characterize the differences between soluble and insoluble species constituting the bulk tholins. No significant morphological change of shape or surface feature is derived from scanning electron microscopy after the extraction of the soluble fraction. This observation suggests a solid structure despite an important porosity of the grains. Infrared spectroscopy is recorded for both fractions. The IR spectra of the bulk, soluble, and insoluble tholins fractions are found to be very similar and reveal identical chemical signatures of nitrogen bearing functions and aliphatic groups. This result confirms that the chemical information collected when analyzing only the soluble fraction provides a valuable insight representative of the bulk material. The soluble fraction is ionized with an atmospheric pressure photoionization source and analyzed by a hybrid mass spectrometer. The congested mass spectra with one peak at every mass unit between 50 and 800 u confirm that the soluble fraction contains a complex mixture of organic molecules. The broad distribution, however, exhibits a regular pattern of mass clusters. Tandem collision induced dissociation analysis is performed in the negative ion mode to retrieve structural information. It reveals that (i) the molecules are ended by methyl, amine and cyanide groups, (ii) a 27 u neutral moiety (most probably HCN) is often released in the fragmentation of tholin anions, and (iii) an ubiquitous ionic fragment at m/z 66 is found in all tandem spectra. A tentative structure is proposed for this negative ion.

Laboratory studies of molecular growth in the Titan ionosphere.

Experimental simulations of the initial steps of the ion-molecule reactions occurring in the ionosphere of Titan were performed at the synchrotron source Elettra in Italy. The measurements consisted of irradiating gas mixtures with a monochromatic photon beam, from the methane ionization threshold at 12.6 eV, up to and beyond the molecular nitrogen dissociative ionization threshold at 24.3 eV. Three gas mixtures of increasing complexity were used: N(2)/CH(4) (0.96/0.04), N(2)/CH(4)/C(2)H(2) (0.96/0.04/0.001), and N(2)/CH(4)/C(2)H(2)/C(2)H(4) (0.96/0.04/0.001/0.001). The resulting ions were detected with a high-resolution (1 T) FT-ICR mass spectrometer as a function of time and VUV photon energy. In order to interpret the experimental results, a Titan ionospheric model was adapted to the laboratory conditions. This model had previously allowed the identification of the ions detected in the Titan upper atmosphere by the ion neutral mass spectrometer (INMS) onboard the Cassini spacecraft. Comparison between observed and modeled ion densities validates the kinetic model (reactions, rate constants, product branching ratios) for the primary steps of molecular growth. It also reveals differences that we attribute to an intense surface chemistry. This result implies that heterogeneous chemistry on aerosols might efficiently produce HCN and NH(3) in the Titan upper atmosphere.

Competition of electron transfer, dissociation, and bond-forming processes in the reaction of the CO(2)(2+) dication with neutral CO(2).

The bimolecular reactivity of the CO(2)(2+) dication with neutral CO(2) is investigated using triple quadrupole and ion-ion coincidence mass spectrometry. Crucial for product analysis is the use of appropriate isotope labelling in the quadrupole experiments in order to distinguish the different reactive pathways. The main reaction corresponds to single-electron transfer from the neutral reagent to the dication, i.e. CO(2)(2+) + CO(2) --> 2CO(2)(+); this process is exothermic by almost 10 eV, if ground state monocations are formed. Interestingly, the results indicate that the CO(2)(+) ion formed when the dication accepts an electron dissociates far more readily than the CO(2)(+) ion formed from the neutral CO(2) molecule. This differentiation of the two CO(2)(+) products is rationalized by showing that the population of the key dissociative states of the CO(2)(+) monocation will be favoured from the CO(2)(2+) dication rather than from neutral CO(2). In addition, two bond-forming reactions are observed as minor channels, one of which leads to CO(+) and O(2)(+) as ionic products and the other affords a long-lived C(2)O(3)(2+) dication.

Revision of the second ionization energy of toluene.

Charge stripping (CS) of the molecular ion of toluene, C(7)H(8) (+)-->C(7)H(8) (2+)+e, is often used as a reference for the determination of second ionization energies in energy-resolved CS experiments. For calibration of the kinetic energy scale, a value of IE(C(7)H(8) (+))=(15.7+/-0.2) eV derived from the appearance energy of the toluene dication upon electron ionization has been accepted generally. Triggered by some recent discrepancies between CS measurements on the one hand and different experimental methods as well as theoretical predictions on the other, we have reinvestigated the photon-induced double ionization of toluene using synchrotron radiation. These photoionization measurements yield phenomenological appearance energies of AE(C(7)H(8) (+))=(8.81+/-0.03) eV for the monocation and AE(C(7)H(8) (2+))=(23.81+/-0.06) eV for the dication. The former is in good agreement with a much more precise spectroscopic value, IE(C(7)H(8))=(8.8276+/-0.0006) eV. Explicit consideration of the Franck-Condon envelopes associated with photoionization to the dication in conjunction with the application of the Wannier law leads to an adiabatic ionization energy IE(a)(C(7)H(8) (+))=(14.8+/-0.1) eV, which is as much as 0.9 eV lower than the previous value derived from electron ionization. Because in many previous CS measurements the transition C(7)H(8) (+)-->C(7)H(8) (2+)+e was used as a reference, the energetics of several gaseous dications might need some readjustment.

Ion chemistry of OV(OCH3)3 in the gas phase: molecular cations and anions and their primary fragmentations.

Trimethyl vanadate(V), OV(OCH(3))(3) (1), is examined by various mass spectrometric means. Photoionization experiments yield an ionization energy of IE(OV(OCH(3))(3)) = 9.54 +/- 0.05 eV for the neutral molecule. The primary fragmentation of the molecular cation 1(+), i.e., loss of neutral formaldehyde, can occur via two independent routes of hydrogen migrations to afford the formal V(IV) compounds HOV(OCH(3))(2)(+) and OV(OCH(3))(CH(3)OH)(+), respectively. These two pathways are associated with low-lying activation barriers of almost identical height. At elevated energies, direct V-O bond cleavage of 1(+) allows for expulsion of a methoxy radical concomitant with the generation of the cationic fragment OV(OCH(3))(2)(+), a formal V(V) compound. Trimethyl vanadate can also form a molecular anion, 1(-), whose most abundant dissociation channel involves loss of a methyl radical, thereby leading to the formal V(V) compound OV(OCH(3))(2)O(-). Various mass spectrometric experiments and extensive theoretical studies provide detailed insight into the ion structures and the relative energetics of the primary dissociation reactions of the molecular cations and anions of 1.