基于第三代同步辐射光源,在20 ke V的入射X射线能量下测量了NO与C2H2分子的康普顿轮廓.考虑到本次实验结果在pz≈0附近的统计精度达到了0.2%,本文报道的NO和C2H2的康普顿轮廓可以作为严格检验理论的实验基准.除此之外,还分别采用HF方法及密度泛函方法选用不同的基组计算了NO与C2H2康普顿轮廓.通过对比实验结果与理论计算,发现对于NO分子,加入弥散函数基组理论计算结果与实验符合更好,说明NO分子基态的电子分布较为弥散.对于C2H2分子,HF方法理论计算的结果与实验符合较好.
The Compton profile of molecular hydrogen has been determined at an incident photon energy of 20 ke V based on the third generation synchrotron radiation, and the statistical accuracy of 0.2% is achieved at p z = 0. Different theoretical methods, i.e., the density functional method, and the Hartree–Fock method, were used to calculate the Compton profiles of hydrogen with different basis sets, and the theoretical calculations are in agreement with the experimental observation in the whole p z region. Compared with the HF calculation, the DFT-B3 LYP ones are in better agreement with the present experiment, which indicates the electron correlation effect is very important to describe the wavefunction in the ground state of hydrogen.
The decay pathways of the structured ionization region of oxygen at different momentum transfers,i.e.,0,0.23 a.u.（atomic unit）,and 0.91 a.u.,are studied by measuring the ion and the scattered electron coincidently.It is found that the dipole-forbidden superexcited states of（2σu）^-1（c^4Σu^-）npσu ^3Σg^-← X^3Σg^-decay into different channels according to the principal quantum number n.The broad ridge above 35 eV,which may be due to inner-valence excited states of（2σg）^-1nλ or multiply excited states,is observed both at small and large momentum transfers,and its decay channel of O＋＋ O is dominant.
The knowledge of the energy level structures of atoms and molecules is mainly obtained by spectroscopic experiments. Both photoabsorption and photoemission spectra are subject to the electric dipole selection rules （also known as optical selection rules）. However, the selection rules for atoms and molecules in the scattering experiments are not identical to those in the optical experiments. In this paper, based on the theory of the molecular point group, the selection rules are derived and summarized for the electric monopole, electric dipole, electric quadrupole, and electric octupole transitions of diatomic molecules under the first Born approximation in scattering experiments. Then based on the derived selection rules, the electron scattering spectra and x-ray scattering spectra of H2, N2, and CO at different momentum transfers are explained, and the discrepancies between the previous experimental results measured by different groups are elucidated.
The ionization and ionic dissociation of the superexcited state of N20 are studied by using electron energy loss spectroscopy and positive ion time-of-flight mass spectroscopy at different momentum transfers; that is, 0 and 0.23 a.u. （atomic unit） . The transitions at 13.8 eV and 14.0 eV are reassigned as 3pπ（000） and 3pσ（000） converging to A^2∑＋, respectively. The competition between the main decay pathways of superexcited states at different momentum transfers is revealed. It is found that 3dσ converging to C^2∑＋ mainly decays into N2O^＋ while 4dσ can decay into both N2^O＋ and NO^＋.
