[1] The thermal evolution of magma oceans produced by collision with giant impactors late in accretion is expected to depend on the composition and structure of the atmosphere through the greenhouse effect of CO 2 and H 2 O released from the magma during its crystallization. In order to constrain the various cooling timescales of the system, we developed a 1-D parameterized convection model of a magma ocean coupled with a 1-D radiative-convective model of the atmosphere. We conducted a parametric study and described the influences of the initial volatile inventories, the initial depth of the magma ocean, and the Sun-planet distance. Our results suggest that a steam atmosphere delays the end of the magma ocean phase by typically 1 Myr. Water vapor condenses to an ocean after 0.1, 1.5, and 10 Myr for, respectively, Mars, Earth, and Venus. This time would be virtually infinite for an Earth-sized planet located at less than 0.66 AU from the Sun. Using a more accurate calculation of opacities, we show that Venus is much closer to this threshold distance than in previous models. So there are conditions such as no water ocean is formed on Venus. Moreover, for Mars and Earth, water ocean formation timescales are shorter than typical time gaps between major impacts. This implies that successive water oceans may have developed during accretion, making easier the loss of their atmospheres by impact erosion. On the other hand, Venus could have remained in the magma ocean stage for most of its accretion.
Photoelectron-photoion-photoion coincidence (PEPIPICO) mass spectrometry is applied to Si 2p core ionization.The ion yield spectrum is compared to the spectrum of the tetramethylsilane molecule in order to point out resonances due to the Si-Si chemical bond. Simple coincidence mass spectra are dominated by the SiC3HgC fragment ion and do not show a strong dependence on photon wavelength. PEPIPICO spectra demonstrate that dissociation dynamics is dominated by stepwise fragmentation of SiC3H9+ and that double ionization always involves S i S i chemical bond rupture, shown to be faster than the Si-C rupture. We discuss the results in term of a fast decay of Si-Si into singly and doubly charged molecules followed by a cascade of slow fragmentation and isomerization of SiC3Hg+. IntroductionIn the past few years, considerable interest has been developed in the study of the dissociation processes of core excited moleculesId because of possible site-selective fragmentation pathways. Our recent work on tetrahedral silicon compound molecules such as SiH4,' Si(CH3)4,8 and SiF49 photoexcited near the Si 2p ionization edge shows that the nature of and the intensity ratio between single-and double-ionization decay channels vary strongly with the photon energy in the region of resonances, especially when the comparison is made below and above the core ionization limit, leading thus to different dissociation channels. For a discrete core-excited state, the excited electron in the valence electron cloud, which acts as a spectator or a nonspectator during the electronic decay channels, controls the nature of the final electronic states of the ion mostly with a single positive charge ion and its subsequent fragmentation. In contrast, in the core ionization continuum, normal Auger (including cascade Auger) processes explain the enhancement of double-(or triple-) ionization channels at the expense of single-ionization ones, giving rise to the observation of lighter fragments.Mass spectrometry of polymethylsilanes and siloxanes has been the subject of many studies because of the very high stability of the trimethylsilyl, Si(CH3)3+,In the present work, we report new mass spectrometry measurements with the multicoincidence technique known as PEPIPICO mass spectrometry or charge separation mass spectrometry)I3 (CSMS) applied to hexamethyldisilane, Si2(CH3)6 (HMDS), photoexcited near the Si 2p edge (i.e., from 100-to 130-eV photon energy). The interest of this molecule compared to the previously studied monosilane molecules is the presence of S i S i and Si-C bonds with different strengths. The different bonding pattern of the silicon atoms in HMDS is shown to affect the resonance pattern near the Si 2p edge compared to those of tetramethylsilane8 (Me&), for which the silicon atom is bound only to carbon atoms. The main purpose of the present work is to study the dissociation dynamics of such a core-excited molecule after single, double, and triple ionization, though this technique also allows analysis of metastable states. The problem of...
We have measured the nondipolar contribution to the Ar ls photoelectron angular distribution over the 30 -2000 eV electron-energy range. The nondipolar interaction results in a forward or backward asymmetry with respect to the photon beam. The asymmetry is directed backward near threshold, is symmetric near 230 eV, and becomes increasingly forward directed at higher energies. The measured asymmetries are in excellent agreement with theoretical calculations, which include interference between the electric-dipole and electric-quadrupole photoionization amplitudes.PACS numbers: 32.80.Fb Current understanding of atomic photoionization phenomena is largely based on the dipole approximation [1][2][3][4]. Within this approximation, the standard transition matrix element used to describe photoionization between initial and final states, M;f = (f~e xp(ik r)c p~i), is simplified. In this expression, exp(ik . r)E describes the photon field (k is the photon propagation vector, r is the electron position vector, and c the photon polarization vector), and p is the electron momentum operator. In the dipole approximation, only the first term of the expan-0031-9007/95/75(26)/4736(4) $06.00
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