Electron impact ionization of sodium Rydberg atoms below 2 eV

We have observed electron impact ionization of highly excited sodium Rydberg atoms in ns and nd states, n=35-51, below E=2 eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from sigma(35d) approximately 7 x 10(-10) to sigma(50d) approximately 4 x 10(-9) cm(2). The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and sigma(n) follows a power law in n. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.     

Cs2分子的光解光谱

利用He-Cd激光器的441.6 nm线光解Cs2分子, 使Cs(n2L)(nL=7P, 6D)态得到布居, 在Cs密度1到9×1015 cm-3范围内测量原子荧光对分子荧光强度比, 得到碰撞转移率系数对解离率之比分别为2.9×10-17和7.4×10-18 cm3. 测定Cs nLJ对nLJ'荧光分支比, 得到72P, 62D态精细结构解离率之比分别为0.53和0.43. 从远翼激发得到的精细结构转移截面与从其他激发过程得到的截面结果相符, 给出了碰撞转移到6D态外(即Cs(6D)+Cs(6S)→Cs(6D)以外的态)的截面为1.9×10-14 cm2.     

低能电荷转移截面和高能转移电离研究(英文)

介绍了对于Na+ 离子和Rb(5s ,5p)原子碰撞中态选择单电子俘获微分截面的理论计算,并与实验数据进行比较,实验结果是离子与在磁光学阱中用激光冷却的碱金属靶原子碰撞而测量到的;还对高能量质子引起的He原子的转移电离进行了理论研究,在Shakeoff模型的基础上 ,分析解释了转移电离截面与单电子俘获截面的比值 ,并与最新实验结果作了比较. We report a theoretical study of state-selective differential single-electron capture cross sections between Na + and Rb(5s, 5p) atoms. The experimental data have been obtained with laser cooled target in a magnetic optical trap. We also report a theoretical study of transfer ionization of He by protons at high collision energies and analyze the transfer ionization cross section with respect to single electron capture cross section in terms of a shakeoff model.     

Inner-shell ionization of atoms by electron,positron and photon impacts

Plane wave Born approximation with Coulomb, relativistic and exchange corrections is employed to obtain L1-, L2- and L3-subshell ionization cross sections of several atoms due to electron and positron impacts for projectile energy varying from the threshold of ionization to 60 times the threshold energy. Photoionization cross sections for all the three L-subshells of the atoms are also calculated using the hydrogenic approximation for the atomic wave functions. For L3-subshell the present cross sections due to electron impact are in good agreement with a number of experimental data for different atoms over the entire energy range investigated. For L1- and L2-subshells the present calculations yield qualitative agreement with the experimental data. The agreement between the present results and the limited experimental data for positron impact is also satisfactory. The hydrogenic approximation for the L-subshell photoionization is found to be good at small photon energies but it underestimates the cross sections at large photon energies.     

Excited-state absorption in a terpyridyl platinum(II) pentynyl complex

The singlet excited-state lifetime of a terpyridyl platinum(II) pentynyl complex was determined to be 268+/-87 ps by fitting femtosecond transient absorption data, the triplet excited-state lifetime was found to be 62 ns by fitting nanosecond transient absorption decay data, and the triplet quantum yield was measured to be 0.16. A ground-state absorption cross section of 2.5 x 10(-19) cm(2) at 532 nm was deduced from UV-vis absorption data. Excited-state absorption cross sections of 3.5 x 10(-17) cm(2) (singlet) and 4.5 x 10(-17) cm(2) (triplet) were obtained by using a five-level dynamic model to fit open-aperture Z scans at picosecond and nanosecond pulse widths and a variety of pulse energies.     

He-HI碰撞的转动激发分波截面

基于作者所发展的分子间相互作用势,采用密耦方法计算了入射He原子能量分别为40,75和100　meV时与HI分子碰撞的弹性和非弹性分波截面.研究表明：激发分波截面比弹性分波截面收敛得快;激发态越高,激发分波截面收敛得越快;能量越高,激发分波截面收敛得越慢. 关键词： 分波截面 密耦方法 He-HI复合物     

Few-photon multiple ionization of ne and ar by strong free-electron-laser pulses

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of Ne2+ production reveals the dominance of nonsequential two-photon double ionization at intensities of I<6x10(12) W/cm2 and significant contributions of three-photon ionization as I increases. Ne2+ recoil-ion-momentum distributions suggest that two electrons absorbing "instantaneously" two photons are ejected most likely into opposite hemispheres with similar energies.     

Crossed beam photodissociation imaging of HeH+ with vacuum ultraviolet free-electron laser pulses

Molecular photofragmentation has been studied by event imaging on HeH+ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into He(1snl,n > or = 2)+H+ was observed to yield significant photodissociation at 32 nm with an absolute cross section of (1.4+/-0.7) x 10(-18) cm2, releasing energies of 10-20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.     

Electron Transitions during the Capture of an Electron by a He<Superscript>2+</Superscript> Ion at the Argon Atom

We have measured the absolute values of the total cross section of the one-electron capture by He²⁺ ions in the kinetic energy range 2–30 keV at the Ar atoms. The absolute values of the differential scattering cross sections of He⁺ ions formed during the one-electron capture and the electron capture with ionization at energies of 2.2, 5.4, and 30 keV have been determined. The electronic states of the formed ions have been determined using collision spectroscopy based on analysis of the change in the kinetic energy of He⁺ after the interaction. We have measured doubly differential (with respect to the kinetic energy and the scattering angle) cross sections of the formation of free electrons. The free electron formation channels (direct ionization and electron capture with ionization) have been analyzed by calculating the electron terms of the (HeAr)²⁺ system. The calculated cross section of capture with ionization is in conformity with the cross section measured using collision spectroscopy.     

Periodic structures in the Franck-Hertz experiment with neon: Boltzmann equation and Monte-Carlo analysis

A new experimental technique for the measurements of the total cross section for electron scattering from atoms and molecules at very low energy is described. Momentum transfer cross sections for scattering from Ar, Kr and Xe at very low energies were carefully derived using the modified effective range theory from the recently measured total cross sections, which were obtained with a new experimental technique utilizing the threshold photoelectron source. A significant discrepancy between the momentum cross sections derived from the present analysis and those determined in the previous electron swarm studies was found at energies below 100 meV. The findings emphasize the need of further high precision experiments in the very low energy region as well as re-analysis of the previous swarm data.     

Measured total cross sections of slow neutrons scattered by gaseous and liquid 2H(2)

The total scattering cross sections for slow neutrons with energies E in the range 300 neV to 3 meV for gaseous and liquid ortho-2H2 have been measured. The cross sections for 2H2 gas are found to be in excellent agreement with both the Hamermesh and Schwinger and the Young and Koppel models. For liquid 2H(2), we confirm the existing experimental data in the cold neutron range and the discrepancy with the gas models. We find a clear 1 / square root[E'] dependence at low energies for both states. A simple explanation for the liquid 2H2 cross section is offered.     

Continuous-wave laser operation of Yb:LuVO4

We report on the room-temperature spectroscopic properties and continuous-wave laser operation of a new Yb:LuVO4 crystal. The peak absorption cross section for the pi-polarization is 8.42 x 10(-20) cm2 at 985 nm, and the stimulated emission cross section at 1020 nm is 1.03 x 10(-20) cm2. An output power of 0.36 W at 1041 nm was obtained with a slope efficiency of 47% by use of Ti:sapphire laser pumping. With diode pumping the output power reached 1.05 W at the highest available incident pump power of 6.8 W.     

Luminescence decay and the absorption cross section of individual single-walled carbon nanotubes

The absorption cross section of highly luminescent individual single-walled carbon nanotubes is determined using time-resolved and cw luminescence spectroscopy. A mean value of approximately 1 x 10(-17) cm2 per carbon atom is obtained for (6,5) tubes excited at their second optical transition, and corroborated by single tube photothermal absorption measurements. Biexponential luminescence decays are systematically observed, with short and long lifetimes around 45 and 250 ps. This behavior is attributed to the band edge exciton fine structure with a dark level lying a few meV below a bright one.     

电子入射双电离He原子的理论研究

完成了高入射能(E05.6keV),低敲出能(E1=E2=10eV)情形下,电子入射双电离He原子五重微分截面(FDCS)的理论计算.计算中使用了三个电子在He+2离子场中的四体末态波函数.计算结果与其他理论结果进行比较发现:所得结果与最新的(e,3e)实验数据较好地一致.     

Many-body ionization in a frozen Rydberg gas

In a dense gas of 300 microK 85Rb atoms of n approximately 50 ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 K blackbody radiation. Rapid ionization is accompanied by spectral broadening, with the spectrum becoming continuous at n=88 at a density of 5x10(10)cm(-3). The atomic transitions broaden both smoothly and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.     

Investigations on molecular constants of the CD（X^2∏） radical and elastic collisions between ground-state C and D atoms at low temperatures

The potential energy curve of the CD(X~2Π) radical is obtained using the coupled-cluster singles-doublesapproximate-triples [CCSD(T)] theory in combination with the correlation-consistent quintuple basis set augmented with diffuse functions,aug-cc-pV5Z.The potential energy curve is fitted to the Murrell-Sorbie function,which is used to determine the spectroscopic parameters.The obtained D0,De,Re,ωe,ωeχe,αe and Be values are 3.4971 eV,3.6261 eV,0.11197 nm,2097.661 cm 1,34.6963 cm 1,0.2083 cm 1 and 7.7962 cm 1,respectively,which conform almost perfectly to the available measurements.With the potential obtained at the UCCSD(T)/aug-cc-pV5Z level of theory,a total of 24 vibrational states have been predicted for the first time when J = 0 by solving the radial Schr¨odinger equation of nuclear motion.The complete vibrational levels,the classical turning points,the inertial rotation constants and centrifugal distortion constants are reproduced from the CD(X~2Π) potential when J = 0,and are in excellent agreement with the available measurements.The total and the various partial-wave cross sections are calculated for the elastic collisions between the ground-state C and D atoms at energies from 1.0×10 11 to 1.0×10 4 a.u.when the two atoms approach each other along the CD(X~2Π) potential energy curve.Only one shape resonance is found in the total elastic cross sections,and the resonant energy is 8.36×10 6 a.u.The results show that the shape of the total elastic cross section is mainly dominated by the s partial wave at very low temperatures.Because of the weak shape resonances coming from higher partial waves,most of them are passed into oblivion by the strong total elastic cross sections.     

Collisional properties of cold spin-polarized metastable neon atoms

We measure the rates of elastic and inelastic two-body collisions of cold spin-polarized neon atoms in the metastable 3P2 state for 20Ne and 22Ne in a magnetic trap. From particle loss, we determine the loss parameter of inelastic collisions beta=6.5(18) x 10(-12) cm(3) s(-1) for 20Ne and beta=1.2(3) x 10(-11) cm(3) s(-1) for 22Ne. These losses are caused by ionizing (i.e., Penning) collisions and occur less frequently than for unpolarized atoms. This proves the suppression of Penning ionization due to spin polarization. From cross-dimensional relaxation measurements, we obtain elastic scattering lengths of a=-180(40)a(0) for 20Ne and a = +150(+80)(-50)a(0) for 22Ne, where a(0)=0.0529 nm.     

Sensitive detection of cold cesium molecules formed on Feshbach resonances

We observe the dynamic formation of quasibound Cs2 molecules near Feshbach resonances in a cold sample of atomic cesium. Using an external probe beam, more than 15 weakly coupled molecular states are detected with high sensitivity, whose collisional formation cross sections are as small as sigma=2 x 10(-16) cm(2). By modeling the molecule formation and dissociation processes with rate equations, we conclude that at an atomic density of 10(13) cm(-3) and temperature of 5 microK, more than 5(1)x10(5) Cs2 molecules in a single rovibrational state coexist with 10(8) Cs atoms in our trap.     

Nonlinear atomic response to intense ultrashort x rays

The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne(8+) ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.     

Electron impact ionization cross sections of beryllium and beryllium hydrides

We report calculated electron impact ionization cross sections (EICSs) for beryllium (Be) and some of its hydrides from the ionization threshold to 1 keV using the Deutsch-Märk (DM) and the Binary-Encounter-Bethe (BEB) formalisms. The positions of the maxima of the DM and BEB cross sections are very close in each case while the DM cross section values at the maxima are consistently higher. Our calculations for Be are in qualitative agreement with results from earlier calculations (convergent close-coupling, R matrix, distorted-wave and plane-wave Born approximation) in the low energy region. For the various beryllium hydrides, we know of no other available data. The maximum cross section values for the various compounds range from 4.0 × 10^?16 to 9.4 × 10^?16 cm² at energies of 44 to 56 eV for the DM cross sections and 3.0 × 10^?16 to 5.4 × 10^?16 cm² at energies of 40.5 to 60 eV for the BEB cross sections.