Role of metastable atoms and molecules of helium in excitation transfer to hydrogen atoms

The afterglow of a discharge in helium with a small admixture of hydrogen is studied spectroscopically (p=40 Torr, [e]≤10¹¹ cm^?3). The time-resolved measurements of intensities of the first four lines of the Balmer series are performed. The concentrations of metastable helium atoms and molecules are evaluated from the relative intensity of the absorption lines. The ratios of excitation transfer rates from atoms He(2 ³ S ₁) k ₁(n) and molecules of helium He₂(a 2sσ ³Σ _u ⁺ ) k ₂(n) to atomic hydrogen H*(n) are measured to be k ₁(n=3)/k ₂(n=3)=0.04±0.02 and k ₁(n=4)/k ₂(n=4)=0.01±0.02. The ratios of excitation rate constants k ₂(n) corresponding to different states H(n) are measured to be k ₁(n=4)/k ₂(n=3)=0.023±0.01; k ₁(n=5)/k ₂(n=3)≤0.013; and k ₁(n=6)/k ₂(n=3)≤0.007.     

Quenching of metastable antiprotonic helium atoms in collisions with deuterium molecules

Quenching of metastable antiprotonic helium atoms in collisions with deuterium molecules has been studied using laser spectroscopy at CERN's new Antiproton Decelerator facility. The quenching cross-sections of the states (n, l )= (39, 36), (39, 37), and (39, 38) were determined from the decay rates of the states which were observed using the “deuterium-assisted inverse resonance” (DAIR) method. The results revealed a similar (n, l )-dependence of the quenching cross-sections as in the case of hydrogen but the values were smaller by a factor of ∼1.5. Received 6 July 2001     

Ultracold metastable helium: from atoms to exotic molecules

We have written this article in the honor of our friend Vladilen Lethokov, who has produced so many brilliant ideas and results in the field of atomic and molecular physics and with whom we have had frequent unlightening discussions. We hope that the work described here will contribute to illustrate the richness of the field.     

He原子与HI分子碰撞截面的密耦计算

基于发展的分子间相互作用势, 采用密耦方法计算了入射能量从1到140?meV范围内He原子与HI分子碰撞的微分截面、分波截面和积分截面.通过与He-HX（X=F,Cl,Br）体系分波截面的比较, 印证了He-HI体系相互作用势以及密耦计算结果的可靠性.结果表明：小角散射的概率大于大角散射的概率;碰撞能量越高,散射概率就越小, 尾部效应也越弱.总积分截面主要来自弹性碰撞的贡献;非弹性积分截面以00→01和00→02跃迁的贡献为主,其中00→02跃迁的贡献最大. 关键词： 碰撞截面 密耦计算 HI-He体系     

氦原子与卤族氢化物分子相互作用势的研究

根据ab initio 计算构造的拟合势研究了He-HF, HCl, HBr三个系统的微分散射截面, 并与实验测量值比较. 结果表明: 该计算方法可以得到较为准确的相互作用势; 随着卤族原子半径的增大, 在0° 时, 相互作用势最小值变浅, 势阱位置向远处移动, 更多的是各向异性性质; 180°时, 相互作用势最小值变深, 势阱位置也同时向远处移动, 表现出更多的各向同性; 每个系统在T型结构下的势阱深度比线型结构下均要浅; 卤族原子半径变大, 反而表现出更多的球对称性, 并且从各向异性势的径向系数, 可以明显看出V₁, V₂, V₃, ··· 相对于V₀的贡献是在逐渐减小的, 这对理解激发态动力学的研究有着很重要的参考价值. 关键词： 各向异性势 密耦近似 微分散射截面     

The spectroscopic study of excitation and ionization in the collision of two metastable atoms

Collisions between two excited atoms leading to an increase in the excitation energy of the particles have been under investigation. All measurements were made in the afterglow of gas-discharge plasma. The cross sections of the following reactions have been determined: $\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) +}\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) →}\text{Hg}{}^7\text{+}\text{Hg}\text{(6}{}^1\text{S}{}_0\text{),}\text{He}{}_{\mathrm{m}}\text{(2}{}^{\mathrm{1,3}}\text{S}\text{) +}\text{Xe}{}_{\mathrm{m}}\text{(}{}^3\text{P}{}_{\mathrm{0,2}}\text{) → (}\text{Xe}{}^{\mathrm{+}}\text{)}{}^1\text{+}\text{He}{}_0\text{+}\text{e}$. The cross section of the first reaction for different transitions lies in the region (2?35) × 10^?15 cm² and the cross section of the second, in (0.2?2.4) × 10^?16 cm². Possible systematic errors and the role of cascade transitions are discussed. Cross sections of the Penning reaction He_m + Xe₀ → He₀ + Xe⁺ + e have also been measured. The result is σ (2³S) = (1.4 ± 0.2) × 10^?15 cm², σ (2¹S) = (1.2 ± 0.3) × 10^?15 cm².     

Magnetic trapping of metastable helium atoms

Metastable helium atoms were efficiently transferred from a magneto-optical trap (MOT) to a magnetic quadrupole trap, producing samples of up to 3᎒⁷ magnetically trapped atoms at a temperature of about 1 mK. We observe purely exponential decay of the samples with time constants of 9-10 s and derive an upper bound for the rate coefficient of inelastic Penning collisions.     

Ultracold collision properties of metastable alkaline-earth atoms

Ultracold collisions of spin-polarized 24Mg, 40Ca, and 88Sr in the metastable 3P2 excited state are investigated based on molecular potentials obtained from ab initio calculations. We calculate the long-range interaction potentials and estimate the scattering length and the collisional loss rate as a function of magnetic field. The scattering lengths show resonance behavior due to the appearance of a molecular bound state in a purely long-range interaction potential and are positive for magnetic fields below 50 mT. A loss-rate model shows that losses should be smallest near zero magnetic field and for fields slightly larger than the resonance field, where the scattering length is also positive.     

Lifetime of metastable magnesium atoms in superfluid helium

The metastable 3s3p³P_{0, 1, 2} states of the magnesium atom immersed into superfluid helium have been investigated. Absorption-fluorescence measurements were carried out to monitor the population of the³P₀,³P₁ and³P₂ level as a function of time. The population of these levels was found to decrease exponentially with a constant of =15±2 ms. This is about three times as long as the vacuum lifetime of the³P₁ level. In the resonant excitation band of the 3s3p³P states to the 3s4s³S state a blueshift of 70 nm compared to the emission and a large broadening were detected. The³P₂ and³P₀ states are not at all metastable any more. Additionally the weak intercombination transition of the³P₁ state to the¹S₀ ground state was investigated by monitoring this emission line as a function of time and of wavelength. The experiments resulted in the same exponential decay time as the excitation measurement. This outcome indicates a rather effective fine structure mixing of the considered Mg states in superfluid helium. Moreover, this raises the question whether common atomic quantum numbers are conserved and the selection rules are still valid.     

Penning collisions of laser-cooled metastable helium atoms

We present experimental results on the two-body loss rates in a magneto-optical trap of metastable helium atoms. Absolute rates are measured in a systematic way for several laser detunings ranging from -5 to -30 MHz and at different intensities, by monitoring the decay of the trap fluorescence. The dependence of the two-body loss rate coefficient β on the excited state ( 2³ P ₂) and metastable state ( 2³ S ₁) populations is also investigated. From these results we infer a rather uniform rate constant K _sp = (1±0.4)×10^-7 cm³/s. Received 8 September 2000 and Received in final form 19 December 2000     

Accurate determination of the scattering length of metastable helium atoms using dark resonances between atoms and exotic molecules

We present a new measurement of the s-wave scattering length a of spin-polarized helium atoms in the 2(3)S1 metastable state. Using two-photon photoassociation spectroscopy and dark resonances, we measure the energy E(nu)=14= -91.35+/- 0.06 MHz of the least-bound state nu = 14 in the interaction potential of the two atoms. We deduce a value of a=7.512+/-0.005 nm, which is at least 100 times more precise than the best previous determinations and is in disagreement with some of them. This experiment also demonstrates the possibility to create exotic molecules binding two metastable atoms with a lifetime of the order of 1 micros.     

Symmetric scattering in electron and positron impact ionization of metastable 2s-state hydrogen atoms

Triple differential cross sections for ionization of hydrogen atoms in the metastable 2s-state by the impact of electrons and positrons have been calculated for coplanar symmetric geometry. In this calculation a multiple scattering theory due to Das [10] and Das and Seal [11] has been used. An analysis of the results reveals that unlike scattering from the ground state, scattering from 2s-state is essentially a higher order process except for the binary collision direction. Moreover, here, the cross section results for 2s-state are much larger compared to those for scattering from the ground state. It is also found that the ionization mechanism at large scattering angles for ionization from the 2s-state is different from that for ionization from the ground state.     

Collisional quenching of metastable states of antiprotonic helium by hydrogen and deuterium molecules

The quenching of metastable states of antiprotonic helium by collisions with hydrogen and deuterium molecules was studied. A systematic investigation of the delayed annihilation time spectra at various H₂ and D₂ admixture ratios at the ppm level revealed characteristic changes of their shape, which indicated a strong principal and orbital quantum number dependent quenching of levels in both cases. Applying a laser spectroscopy technique to measure the lifetimes of individual states and cascades we deduced H₂ and D₂ quenching cross-sections for the states (n, l )= (39, 35) and (37, 34). These cross-sections establish for D₂ molecules the strong increase of the quenching efficiency with increasing principal quantum number n of the state under investigation previously reported for the case of H₂ admixtures. Our experiments indicate that the low-n state (37, 34) is somewhat less affected by D₂ than by H₂, while the high-n state (39, 35) is equally quenched by both isotopes. Received 2 October 2000     

Nanolithography with metastable helium atoms in a high-power standing-wave light field

We have created periodic nanoscale structures in a gold substrate with a lithography process using metastable triplet helium atoms that damage a hydrophobic resist layer on top of the substrate. A beam of metastable helium atoms is transversely collimated and guided through an intense standing-wave light field. Compared to commonly used low-power optical masks, a high-power light field (saturation parameter of 10⁷) increases the confinement of the atoms in the standing wave considerably, and makes the alignment of the experimental setup less critical. Due to the high internal energy of the metastable helium atoms (20 eV), a dose of only one atom per resist molecule is required. With an exposure time of only eight minutes, parallel lines with a separation of 542 nm and a width of 100 nm (one-eleventh of the wavelength used for the optical mask) are created.PACS 32.80.Lg; 39.25.+k; 81.16.Nd     

Trace detection of metastable helium molecules in superfluid helium by laser-induced fluorescence

We describe an approach to detecting ionizing radiation that combines the special properties of superfluid helium with the sensitivity of quantum optics techniques. Ionization in liquid helium results in the copious production of metastable He2 molecules, which can be detected by laser-induced fluorescence. Each molecule can be probed many times using a cycling transition, resulting in the detection of individual molecules with high signal to noise. This technique could be used to detect neutrinos, weakly interacting massive particles, and ultracold neutrons, and to image superfluid flow in liquid 4He.     

An investigation of high fractions of metastable helium atoms

Penning type discharge was adopted to excite helium atoms. It is suitable for generating high density metastables at a range from 0.1 mTorr to 0.5 Torr. The highest metastable density of 3.5 x 1010 cm-3 was observed at a static gas pressure of 0.5 Torr. The highest fraction of metastables (N12s/Nhe) of 10-3 in a low gas pressure was obtained. The variation of the magnetic field strength on the discharge does not result in a significant density change of the metastable helium atoms. When no magnetic field was applied, no discharge took place.