Positive and negative ionic desorption from condensed formic acid photoexcited around the O 1s-edge: Relevance to cometary and planetary surfaces

Photon stimulated ion desorption (PSID) studies have been performed in condensed formic acid using oxygen 1s-edge synchrotron radiation from the Brazilian synchrotron light source (LNLS), operating in a single-bunch mode. Ion formation was discussed in terms of the Auger stimulated ion desorption (ASID) and X-ray induced electron stimulated desorption (XESD) mechanisms. It is found that O 1s(C-OH) → π*(CO) and O 1s(CO) → 3s/σ*(HCO) transitions favored the production of C⁺, CH⁺, O⁺, O⁻ and H⁻ ions. The hydroxyl anion has not been observed while the hydroxyl cation showed low intensity or was absent. Some anion formation routes from dissociative reactions are suggested taking into account the positive ion yields.     

紫外光子在星周天体化学中的作用

介于1到8个太阳质量之间的恒星在演化后期的渐近巨星分支(AGB)阶段,会以气体和尘埃的形式损失大量物质. AGB星提供了星际介质中高达35%的尘埃,其中包含形成太阳系所需要的物质. 此外,AGB星周包层是复杂有机化学所发生的重要场所之一,在其中已探测到的有机分子已超过80多个. 这里展示了AGB星自身或者其近距离的伴星所辐射的紫外光子会显著地改变星周包层的化学特性,尤其是具有团块结构的星周包层. 研究发现,在恒星存在伴星(例如白矮星)的情况下,高通量的紫外光子会破坏富碳AGB星周包层内部的H₂O分子,使其含量低于观测到的水平,并且在星周包层内部产生C⁺等物质,这与以往的星周化学模型的研究结果是不同的.     

Diffusion-Limited Reactions and Mortal Random Walkers in Confined Geometries

Motivated by the diffusion-reaction kinetics on interstellar dust grains, we study a first-passage problem of mortal random walkers in a confined two-dimensional geometry. We provide an exact expression for the encounter probability of two walkers, which is evaluated in limiting cases and checked against extensive kinetic Monte Carlo simulations. We analyze the continuum limit which is approached very slowly, with corrections that vanish logarithmically with the lattice size. We then examine the influence of the shape of the lattice on the first-passage probability, where we focus on the aspect ratio dependence: Distorting the lattice always reduces the encounter probability of two walkers and can exhibit a crossover to the behavior of a genuinely one-dimensional random walk. The nature of this transition is also explained qualitatively.     

The millimeter-wave spectrum of 2-cyanoethanol

In order to better characterize the chemistry involving CN groups in regions of star-formation, we have recorded the millimeter-wave rotational spectrum of 2-cyanoethanol. Almost 6000 new transitions were assigned to two separate conformers and three torsionally excited states. The lowest energy conformer, gauche-2-cyanoethanol, its 1st excited torsion, and the ground state for the anti-I-conformer are best characterized with basic asymmetric rigid-rotor Hamiltonians including distortion. All fit close to experimental uncertainty. Fits for the 2nd and 3rd excited torsional states for the gauche-conformer are of somewhat lower quality, which might be explained by a potential interaction between these states. The results of the various fits are discussed in this paper. In all, the present characterization of the spectrum and extensions through prediction should be more than sufficient to guide future millimeter-wave observational studies of this species in the interstellar medium.     

Reactions of CO2 with H, H2 and deuterated analogues

Combining a temperature variable 22-pole ion trap with a cold effusive beam of neutrals, rate coefficients k(T) have been measured for reactions of CO₂⁺ ions with H, H₂ and deuterated analogues. The neutral beam which is cooled in an accommodator to T_ACC, penetrates the trapped ion cloud with a well-characterized velocity distribution. The temperature of the ions, T_22PT, has been set to values between 15 and 300 K. Thermalization is accelerated by using helium buffer gas. For reference, some experiments have been performed with thermal target gas. For this purpose hydrogen is leaked directly into the box surrounding the trap. While collisions of CO₂⁺ with H₂ lead exclusively to the protonated product HCO₂⁺, collisions with H atoms form mainly HCO⁺. The electron transfer channel H⁺ + CO₂ could not be detected (<20%). Equivalent studies have been performed for deuterium. The rate coefficients for reactions with atoms are rather small. Within our relative errors of less than 15%, they do not depend on the temperature of the CO₂⁺ ions nor on the velocity of the atoms (k(T) lays between 4.5 and 4.7 × 10⁻¹⁰ cm³ s⁻¹ with H as target, and 2.2 × 10⁻¹⁰ cm³ s⁻¹ with D). For collisions with molecules, the reactivity increases significantly with falling temperature, reaching the Langevin values at 15 K. These results are reported as k = α (T/300 K)^β with α = 9.5 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.15 for H₂ and α = 4.9 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.30 for D₂.     

星际氧气分子的气相-尘埃模型研究

本文采用气相-尘埃模型Nautilus研究了星际氧气的演化过程,使用了两种典型初始丰度值下的恒温模型和变化物理条件下的加热模型进行模拟计算. 结果表明,在冷致密云的条件下,CO、O₂和H₂O达到峰值的时间依赖于氢核密度的多少,其随氢核密度的增大而减小. 在加热模型中,在温度升高后的10⁵年后,氧气的丰度值与观测结果符合较好. 在温度大于30 K后,氧气的稳态丰度值将不再随氢核密度而变化,且大于此温度可以阻止氧气大量的沉降到尘埃表面. 此外,无论在恒温模型还是在加热模型中,低氢核密度更有利于O₂的生成.     

Volatile Depletion in Planet-Forming Disks

Newly born stars are surrounded by gas and dust with a flattened axisymmetric distribution termed protoplanetary disk, in which planets are formed. Observations of these objects are necessary for understanding the formation and early evolution of stars and planets, and for revealing the composition of the raw material from which planets are made. Numerical models can extract important parameters from the observational data, including the gas and dust mass of the disk. These parameters are used as input for further modeling, e.g., to calculate the chemical composition of the disk. A consistent thermochemical model should be able to reproduce the abundances of different species in the disk. However, this good wish has been challenged for many disks: models over-predict the emission line intensity of some species; namely, they are depleted (with respect to expectations from canonical models). In this review we show how this disparity indicates that dust evolution has significant effects on gas chemistry, and may indicate the earliest stages of planet formation.     

原子分子物理—研究宇宙物质的基础科学

艹勾清泉教授所从事、发展和传授的原子、分子结构理论在天体物理、天体化学、固体物理、材料科学以及极端条件物理的发展中起到基础理论的重要作用。他的研究群体活跃在各个相关的基础与应用领域里