Higher-order nonlinear effects in a gas laser: Creation and detection of an hexadecapole moment in the neon 2p4 level

A theoretical and experimental analysis of the optical pumping of neon atoms by the 6328 Å laser line (transition 3s₂-2p₄; J = 1-J = 2) shows clear evidence for fourth-order and sixth-order nonlinear effects in gas lasers. These high-order effects involve the generation of an electric hexadecapole moment in the 2p₄ level. In a magnetic field, this moment evolves at four times the 2p₄ Larmor frequency. Due to the laser-induced coupling between 3s₂ and 2p₄, the 2p₄ hexadecapole is observed through a narrow resonance on the 3s₂ Hanle effect.     

Level inversion by excitation with a monoenergetic electron beam

The efficiency of level excitation with a monoenergetic electronic beam has been studied and compared with plasma excitation. The ratio of the 3s₂ and 2p₄ level populations of neon excited by a monokinetic beam was found to be three times higher than for plasma excitation. This result is in satisfactory agreement with theory.     

Disalignment collision cross sections of the neon levels 2p 4 and 2s 2 obtained by mode crossing

Alignment destroying collision cross sections of the Ne levels 2p ₄ and 2s ₂ were measured by investigation of the zero-field mode crossing resonances of a multimode He/Ne-laser operating on the 3s ₂→2p ₄ (λ=633nm) and 2s ₂→2p ₁ (λ=1.52μm) Ne transitions respectively. The measurements were carried out with the aid of a small sample cell which was placed inside the laser cavity and filled with pure Neon or a mixture of the He and Ne. Comparison of the measured cross sections concerning the 2p ₄ level with corresponding Hanle effect measurements shows that in multimode operation velocity changing collisions do not produce an additional broadening of the mode crossing resonances. The alignment destroying cross section is almost temperature independent in the case of He collisions; it decreases, however, with increasing temperature in the case of Ne. Due to the allowed dipol transition between the 2s ₂ and the ground state of Ne, radiation trapping and resonance interaction takes place if the gas filling contains Ne. The effect on the mode crossing resonance width is investigated.     

High resolution spectroscopy of stable xenon isotopes

In this paper we investigate the hyperfine structure and isotope shift of four near infrared xenon transitions by means of a semiconductor diode laser. Doppler free spectra were obtained by using either a saturation spectroscopy technique in cell or a laser induced fluorescence technique on a collimated atomic metastable beam. The hyperfine coupling constants of ¹²⁹Xe and ¹³¹Xe were determined for the levels 1s₅, 1s₄, 1s₄2, 2p₂, 2p₄ and 2p₆ (Paschen notation). Moreover, a systematic study of the isotope shift was performed for the 1s₃ → 2p₄ and 1s₅ → 2p₆ transitions for which all the stable isotopes ^128–131Xe , ¹³²Xe, ¹³⁴Xe, and ¹³⁶Xe were fully resolved. This revised version was published online in August 2006 with corrections to the Cover Date.     

Lifetime and absolute transition probabilities of the 2p 10(3 S 1) level of NeI by beam-gas-dye laser spectroscopy

The lifetime of the 2p ₁₀(³ S ₁) level in the NeI 2p ⁵3p configuration has been measured by means of the cascade-free two-step excitation technique of beam-gas-dye laser spectroscopy. The lifetime determined is τ=(25.43±0.09) ns, where the uncertainty quoted corresponds to one standard deviation. By using the same excitation technique the absolute transition probabilities of all fine-structure transitions from 2p ₁₀ to the four levels 1s _i(i=2?5) in the 2p ⁵3s configuration have been measured, too. The high level of experimental accuracy attained enables a comparative evaluation of the theoretical methods used in the literature to calculate the radial and angular parts of the transition integrals.     

Determination of the atomic-state population in A Kr-Br2 mixture

The atomic-state populations of the 5s[1$\text{1}\text{2}$]° metastable level of Kr and of the 5s²P_{$\text{3}\text{2}$} resonant level of Br in a Kr-Br₂ mixture have been determined as functions of current strength. Occurrence of energy transfer from the Kr-level to the Br-level has been verified.     

Excitation of bromine by means of krypton in Kr-Br2 mixtures

The atomic-state populations of the 5s⁴P_{$\text{3}\text{2}$} and 5s⁴P_{$\text{1}\text{2}$} resonant levels of Br in a Kr-Br₂ mixture have been determined as functions of current strength. An increase in flux of Br emission lines passing into 5s levels has been observed.     

Absolute measurement of an atomic cascade rate using a two photon coincidence technique. Application to the 4p21S0 -4s4p 1P1 -4s21S0 cascade of calcium excited by a two photon absorption

The two photon coincidence technique yields an absolute measurement of a cascade decay rate without knowing the detection efficiency of each detector. This method is applied to calcium atoms in an atomic beam excited to the 4p²¹S₀ state, and decaying via the resonant ¹P₁ state; it yields the excitation rate of the upper 4p²¹S₀ level. Since the excitation process (two photon absorption) is controlled, one can compute the excitation rate of the upper level as a function of the transition probabilities in the cascade. The lower transition is well known and we can therefore deduce the transition probability for the 4p²¹S₀ -4s4p ¹P₁, transition which previously had not been accurately determined. The preliminary measurements yield a line strength s = 0.98 ± 0.3 au in good agreement with a recent measurement of the lifetime of the 4p²¹S₀ state.     

Laser induced line narrowing in three-level systems: Application to the determination of the wavenumber of the 3s2-2p4 neon line (λ = 6328 Å)

The Doppler-free absorption line narrowing technique (A.L.N.) is applied to the ²⁰Ne (3s₂-2p₄-1s₅) three-level system, using a ¹²⁷I₂ stabilized He-Ne laser (λ = 6328 Å) and a tunable single-frequency cw dye laser (λ = 5945 Å). The relative frequency positions of the ²⁰Ne 3s₂-2p₄ line and of some iodine hyperfine components are measured with an uncertainty of 1 MHz. The wavenumber of the ²⁰Ne line at low pressure (0.2 Torr) is deduced from these measurements and is found to be σ(²⁰Ne, 3s₂-2p₄) = 1579800.060 ± 0.005 m^-1. With regard to the previous results, the accuracy is increased by two orders of magnitude.     

Electronic band structure of In2S3 and CdIn2S4 semiconductor spinels from the data of x-ray spectroscopy and theoretical calculations

The electronic band structure of the chalcogenide spinels In₂S₃ and CdIn₂S₄ has been studied using the FEFF8 program. It is shown that the valence band top is formed by the S p states mixed with the In s and In p states for In₂S₃ or with the Cd s, Cd p, In s, and In p states for CdIn₂S₄. Compared to In₂S₃, the presence of Cd atoms in the nearest environment of S atoms in CdIn₂S₄ does not considerably affect the electronic band structure. In CdIn₂S₄ the Cd 4d states, as well as the In 4d states, form a narrow localized band shifted deep into the valence band. The theoretical results are in good agreement with the experimental x-ray photoelectron and x-ray spectra.     

Relativistic R-matrix close-coupling calculations for photoionization of Ne-like Al IV

We present relativistic close-coupling photoionization calculations of Al IV using the Breit-Pauli R-matrix method to obtain photoionization cross-section of Al IV from the ground state and the lowest two J=0 (even) excited states. A multi-configuration eigenfunctions expansion of the core Al V is employed with spectroscopic configurations 2s²2p⁵, 2s2p⁶, 2s²2p⁴3s, 2s²2p⁴3p, 2s²2p⁴3d and 2s²2p⁴4s. We have included, for the first time, the lowest 68 level target states of Al V in the photoionization calculations of Al IV. Extensive configuration interaction wavefunctions are used to describe both the initial Al IV states and the final Al V states. Cross-sections are compared from three level calculations including only 2s² 2p^{5 2}P^o _3/2, _1/2 and 2s 2p^{6 2}S_1/2 levels of Al V. The present calculation using the lowest 68 target levels of Al V are presented for the first time and should provide reasonably complete database for practical application for photoionization cross-section for Al IV, where high-energy cross-sections along with near-threshold photoionization cross-section is required.     

Population of 2p 55s levels of neon in He-Ne plasma: I. The evolution of mechanisms in the discharge and the afterglow

The spectroscopic study of population processes of neon 2p ⁵5s states was carried out in helium afterglow with small admixture of neon at PHe = 38.1 torr, [He]/[Ne]=10^?5 with pulsed discharge afterglow in helium with small admixture of neon (pressure equal to 38.1 mm Hg; ). It is established that the main mechanism of population of 3s ₂ level (in Paschen’s notation) in the discharge and the initial after-glow is the excitation transfer from metastable atoms of He(2¹ S ₀). The other three levels—3s ₃, 3s ₄, and 3s ₅—corresponding to 2p ⁵5s configuration are populated in the afterglow as a result of the dissociative recombination HeNe⁺ of ions with electrons. The same process is also the main channel of population of 3s ₂ level in the late afterglow phase, when the concentration of He(2¹ S ₀) atoms is small. The hypothesis of recombination mechanism is confirmed by observation of the response of line intensities to pulsed electron heating. The partial coefficients of dissociative recombination into 2p ⁵5s states are estimated.     

The hyperfine structure of they 8 p-multiplet of 4f 7(8 S) 6s 6p in EuI

The hyperfine splitting constants of the 4f ⁷ 6s 6p y ⁸ P _7/2 level of Eul were determined using the level crossing method. These values and those for the hyperfine splittings of the other 4f ⁷ 6s 6p levels are compared to theoretical values calculated with eigenfunctions of Smith and Collins. For the quadrupole coupling constants a better agreement between theory and experiment is obtained if different 〈r ^?3〉 _p -parameters are assumed for 4f(⁸ S) 6s 6p(³ P) and for 4f ⁷(⁸ S) 6s 6p(¹ P).     

Determination of the rate coefficients for the collisional excitation and de-excitation of the upper laser levels of Ar

The absolute concentrations of the argon-ion states of the configurations 3d, 4s, 4p, 4p′, 4d and 5s have been measured for a pulsed laser discharge whose plasma parameters have been previously determined. For the resonant levels, the measurements were made by a self-absorption technique and for other levels by absolute spectroscopic observations. Variations of the populations of these levels as functions of discharge current and filling pressure (i.e. electron temperature and density) permitted calculations of the total excitation and de-excitation rate coefficients due to electrons as functions of temperature for each laser level. For excitation, collisions with the fundamental and long-lived states were taken into account while, for de-excitation, superelastic, excitation and ionization collisions were considered.     

Tensor polarizabilities of levels of the configurations 4f 13 6s6p and 4f 12 5d6s 2 in thulium I

The level-crossing technique with parallel electric and magnetic fields was used to measure the tensor polarizabilities of six levels of the configuration 4f ¹³ 6s6p + 4f ¹² 5d6s ² in the Tm I-spectrum. Using intermediate coupling wave functions given by Camus and the experimental values of the tensor polarizabilities, the radial integrals for electric dipole transitions from levels of the configuration 4f ¹³6s6p + 4f ¹²5d6s ² to levels of the configuration 4f ¹³ 5d6s + 4f ¹² 6s ²6p have been determined. The results areI(4f ¹³6s6p,4f ¹³5d6s)=1.98(45)ea ₀ andI(4f ¹²5d6s ²,4f ¹²6s ²6p) = 0.88(25)ea ₀. A comparison between the experimental and the calculated values of the tensor polarizabilities shows an excellent agreement, provided that these radial integrals and the radial integrals for the electric dipole transitions to the ground state configuration 4f ¹³6s ² as determined by Wallenstein from lifetime measurements are used in the calculation, instead of radial integrals computed by Camus with Hartree-Fock wave functions.     

Study of cross sections for the loss of outer 1s, 2s, and 2p electrons by fast ions and atoms of light elements

In the case of light-element ions propagating with velocities V = 1.83 and 3.65 au in H₂, He, N₂, Ne, and Ar, the loss cross sections σ_{i, i+m} for m electrons (m = 1, 2, 3) are considered. The partial loss cross sections σ_i(nl) for one of the outer 1s, 2s, or 2p electrons are determined using the obtained data. It is shown that the experimental cross sections for the loss of the 1s and 2s electrons by positive ions qualitatively agree with the theoretical values calculated in the Born approximation. In the case of the ion velocity V = 1.83 au, the cross sections σ_i for 2p electrons are greater than the cross sections σ_i (1s) and σ_i (2s) by a factor of 1.2–3 for the same binding energies of electrons in the ion (I _nl > 20 eV). It is found experimentally that, at V = 1.83 au, the cross sections σ_i (2p) for I _nl ～ 10–20 eV are less than the cross sections σ_i (1s) by a factor of 2–3, which is probably caused by a decrease in the screening parameter (θ_2p < 1) of the outer shell of atoms.     

Bestimmung der Lebensdauer des 3d 9 4s 4p 4 P 3/2-Terms im Cu I-Spektrum mit optischer Doppelresonanz

The lifetime of the 3d ⁹ 4s 4p ⁴ P _3/2 state of Cu has been determined with optical double resonance by observation of radiofrequency transitions ΔF=0, Δm _F =±1 between hfs-Zeeman levels of the stable odd isotopes. The result is τ(3d ⁹ 4s 4p ⁴ P _3/2, Cu)=3.18(16)·10^?7 sec.     

Magnetic coherence transfer induced by discharge radiation and the effect of radiation trapping on the lifetime of the hidden alignment of Ne terms

The dichroism of a neon gas discharge plasma in a weak magnetic field is investigated by scanning the 3s ₂-2p ₄ transition by monochromatic laser radiation. Magneto-optical resonances of the intrinsic macroscopic alignment and hidden alignment of the 2p ₄ level are separated. Against the background of these resonances, resonances attributed to coherence transfer from the 1s _{2, 4, 5} levels and the birefringence of the wings of the nearest absorption lines from the 1s _{4, 5} levels are observed. A new type of alignment is revealed—integral hidden alignment, whose lifetime, in contrast to the case with macroscopic alignment, increases without bound with increasing radiation trapping.     

ABO3型晶体中氧的电子自发极化

计算O^-2在晶体场中出现的Jahn-Teller效应,得到具有电矩的2p_π3s3d₀杂化原子轨道。BO₆八面体共价性质决定了ABO₃晶体的各种极化相。理论给出P_s比值、钛位移大小和热力学参数ξ′,ζ′均接近于实验结果。统一解释了钛酸钡的三个相转变。相变条件给出七种晶体中氧的屏蔽常数平均值与Slater的值一致。     

Role of helium in a He-Xe plasma

The populations of the 5p⁵6s and 5p⁵6p levels of Xe are reported as functions of current and partial pressure of He for the positive column of a glow discharge at medium pressures. The populations of the first ones (especially the metastable 5p⁵6s³P₂ one) decrease as the He pressure is raised, whereas those of the upper 5p⁵6p levels increase. An explanation is proposed for the observed changes in the level populations.We are indebted to Professor A. N. Tekuchev for direction and comments, and to Z. P. Gasko for help with the measurements.