【正文】 1. Introduction The simple ionization collision of a hydrogenic atom by the impact of a structureless particle, the “threebody problem”, is one of the oldest unsolved problems in physics. The twobody problem was analyzed by Johannes Kepler in 1609 and solved by Isaac Newton in 1687. The threebody problem, on the other hand, is much more plicated and cannot be solved analytically, except in some particular cases. In 1765, for instance, Leonhard Euler discovered a “collinear” solution in which three masses start in a line and remain linedup. Some years later, Lagrange discovered the existence of five equilibrium points, known as the Lagrange points. Even the most recent quests for solutions of the threebody scattering problem use similar mathematical tools and follow similar paths than those travelled by astronomers and mathematicians in the past three centuries. For instance, in the centerofmass reference system, we describe the threebody problem by any of the three possible sets of the spatial coordinates already introduced by Jacobi in 1836. All these pairs are related by lineal point canonical transformations, as described in [1]. In momentum space, the system is described by the associated pairs (kT,KT), (kP,KP) and (kN,KN). Switching to the Laboratory reference frame, the final momenta of the electron of mass m, the (recoil) target fragment of mass MT and the projectile of mass MP can be written in terms of the Jacobi impulses Kj by means of Galilean transformations [1] 41 For decades, the theoretical description of ionization processes has assumed simplifications of the threebody kinematics in the final state, based on the fact that ? in an ion–atom collision, one particle (the electron) is much lighter than the other two, ? in an electron–atom or positron–atom collision, one particle (the target nucleus) is much heavier than the other two. For instance, based on what is known as Wick’s argument, the overwhelming majority of the theoretical descriptions of ion–atom ionization collisions uses an impactparameter approximation, where the projectile follows an undisturbed straight line trajectory throughout the collision process, and the target nucleus remains at rest [2]. It is clear that to assume that the projectile follows a straight line trajectory makes no sense in the theoretical description of electron or positron–atom collisions. However, it is usually assumed that the target nucleus remains motionless. These simplifications of the problem were introduced in the eighteenth century. The unsolvable threebody problem was simplified, to the socalled restricted threebody problem, where one particle is assumed to have a mass small enough not to influence the motion of the other two particles. Though introduced as a means to provide approximate solutions to systems such as Sun–pla–et within a Classical Mechanics framework, it has been widely used in atomic physics in the socalled impactparameter approximation to ion–atom ionization collisions. Another simplification of the threebody problem widely employed in the nieenth century assumes that one of the particles is much more massive than the other two and remains in the center of mass unperturbed by the other two. This approximation has been widely used in electron–atom or positron–atom ionization collisions. 2. The multiple differential cross section A kinematically plete description of a threebody continuum finalstate in any atomic collision would require, in principle, the knowledge of nine variables, such as the ponents of the momenta associated to each of the three particles in the final state. However, the condition of momentum and energy conservation reduces this number to five. Furthermore, whenever the initial targets are not prepared in any preferential direction, the multiple differential cross section has to be symmetric by a rotation of the threebody system around the initial direction of motion of the projectile. Thus, lea。 Wannier。 Positron impact。 Electron spectra。 Collision dynamics。橫剖面也許會(huì)被很多巨大的困難所阻礙 , 但值得高興的是 , 我們一直沒有錯(cuò)過對問題許多不同的全方位的觀察 , 唯一的遺憾就是對總橫剖面的研究。 終于 , 有被解釋對象由于所謂的 備鞍點(diǎn) 電離機(jī)制的極小值。 你是知名的電子捕獲對連續(xù)流峰頂。 39 圖 5 8. 結(jié)論 總結(jié)結(jié)果提出了在這通 信 , 我們由正子的沖擊調(diào)查了分子氫的電離。 圖 5 表示 , 結(jié)構(gòu)完全出現(xiàn)從 tp 期限。 圖 3 和圖 4 精確地設(shè)置早先條件在任何能量和角度三個(gè)微粒符合的那些點(diǎn)。 這個(gè)機(jī)制被描述在圖 4. 因而 , 檢查備鞍點(diǎn)的提案是正確的 , 我們看是否我們的演算顯示與備鞍點(diǎn)電子生產(chǎn)的這個(gè)描述是一致的結(jié)構(gòu)。 能量和動(dòng)量保護(hù)原則的應(yīng)用表示 , 正子偏離在角度 終于 , 為電子涌現(xiàn)在方向和正子一樣 , 它必須遭受隨后碰撞以殘余中堅(jiān)力量在 a 托馬斯象過程。 在 離子原子碰撞案件 , 查尋這個(gè)機(jī)制的理論和實(shí)驗(yàn)性證據(jù)是陰暗由生動(dòng)的爭論 [ 1418 ] 。 想法是 , 電子能從離子原子碰撞涌現(xiàn)由在在子彈頭和殘余的目標(biāo)離子潛力的備鞍點(diǎn)。 7. 備鞍點(diǎn)機(jī)制 結(jié)構(gòu)的起源在大約 一定更難辨認(rèn)。 但有其它結(jié)構(gòu) , 在大約 。 在這種情況下 , 從電子和正子大量是相等的 , 這兩個(gè)過程干涉在 45 。 每個(gè)這 些過程包括正子電子二進(jìn)制碰撞 , 被偏折跟隨被 90 輕的微粒的當(dāng)中一個(gè)被重的中堅(jiān)力量。 6. 托馬斯機(jī)制 現(xiàn)在讓我們走回到 H2 的電離由 1 keV 正子沖擊。 他們第一次測量了四倍有差別 37 的電離橫剖面在 collinear 幾何為離子原子碰撞 , 并且發(fā)現(xiàn) ECC 尖頂和在正子沖擊在大角度。 從目標(biāo)反沖不充當(dāng)在這個(gè)實(shí)驗(yàn)性情況的重大角色 , 當(dāng)前一般理論給結(jié)果相似與那些由Berakdar [ 11 ] 獲得 , 并且兩個(gè)跟隨嚴(yán)密實(shí)驗(yàn)性 價(jià)值。 Kover 和 Laricchia 測量了在 1998 dr/dEedXkdXK 橫剖面在一個(gè) collinear 情況在零的程度 , 為 H2 的電離分子由 100 keV 正子沖擊 [ 10 ] 。 因而 , 觀察這結(jié)構(gòu)它是必要增 加橫剖面的維度。 并且這一定是如此。 這爭執(zhí)的原因是那 , 與離子對比盒 , 正子外出的速度與那不是相似沖擊 , 但主要傳播在角度和巨大。 因?yàn)?ECC 尖頂 是一個(gè)推測橫跨捕獲電離極限入高度激動(dòng)的一定的狀態(tài) , 這個(gè)同樣作用必須是存在在正子原子碰撞。 第一理論解釋 [ 9 ] 表示 , 它分流以與 1 相似的方式 k 。 它對應(yīng)于電子捕獲于連續(xù)流 (ECC) 尖頂被發(fā)現(xiàn)在離子原子碰撞三十年前由Crooks 和 Rudd [ 8 ] 。圖 2, 我們觀察三個(gè)不同結(jié)構(gòu) : 二個(gè)極小值和土坎。 我們選擇作為二個(gè) 獨(dú)立參量散發(fā)的電子動(dòng)量組分 , 平行和垂線對正子子彈頭的行動(dòng)的最初的方向。 (1992), 雖然他們保留許多制約在他們的離子沖擊電離分析。 另外 , Garibotti 和 Miraglia 忽略了互作用潛力的矩陣元素在接踵而來的子彈頭和目標(biāo)離子之間 , 并且做銳化的略計(jì)評估轉(zhuǎn)折矩陣元素。 在連續(xù)流波浪作用這個(gè)選擇的最后渠道擾動(dòng)潛力是 [ 5 ] 在純凈的庫侖潛力情況下 , 畸變被給 關(guān)于 這個(gè)模型由佳瑞波帝和馬瑞吉拉 [ 6 ] 提議為離子原子碰撞 , 并且由 Brauner 和布里格斯六年后為正子原子和電子碰撞 [ 7 ] 。 為了是一致的與動(dòng)力學(xué)的我們充分的治療 , 它是必要描述最終狀態(tài) W f 通過考慮所有互作用在同 35 樣立足處的 wavefunction 。 橫剖面利益在這范圍內(nèi)是 34 轉(zhuǎn)折矩陣可能供選擇地被寫在崗位或預(yù)先的形式 那里擾動(dòng)潛力被定義 為出生類型初始狀態(tài) 哪些包括子彈頭的自由行動(dòng)和最初的一定的狀態(tài) Ui 目標(biāo) , 并且擾動(dòng)潛力 vi 簡單地是正子電子和正子中堅(jiān)力量互作用的