采用二维坐标和变分方法计算磷烷阳离子的伞型振动

提出一个新的二维变分方法计算PH₃⁺(X²A₂")的对称伸缩振动(v1)和伞形振动(v2). 因为采用了对称化的笛卡尔坐标,所以动能项变得简单,同时伞形振动模式也能得到很好的反映. 相比采用经常使用的一维模型计算伞形振动,这个二维模型不需要约化质量的假设,同时也考虑了v1和v2振动模式之间的相互作用. 用二维模型对PH₃⁺首次进行了计算, 前七个能级的理论值和实验值的平均相对误差小于3 cm^{-1. 用相同的方法也计算了NH₃,结果没有PH₃+理想,说明这个方法有一定的局限性.}     

Overshoot inversion of a Nd laser system

An important parameter of pulsed solid state lasers is the maximum inversion n_max which has to exceed the threshold inversion n_th to start the laser. The overshoot Δn=n_max-n_th is determined experimentally for a Nd-glass laser, operating in the TEM₀₀ mode and compared with theoretical predictions. Both investigations indicate a relation between normalized pump rate and inversion overshoot of the type Δn ∝ (W/W_th-1)^{$\text{1}\text{2}$}.     

Experimental observation on anomalous emission in Mg vapor

When the 3s3p ¹P level of an Mg atom is resonantly excited by a laser, an anomalous emission with a wavelength of 5184 Å can be observed, which corresponds to the transition for 3s4s ³S to 3s3p ³P₂. According to the temporal behavior of this emission and its intensity dependence on atom density, it is believed that the experiment seems to show the transition from a spontaneous process to a cooperative process. The population inversion between 3s4s ³S and 3s3p ³P₂ is created by inelastic collisions of the lower excited 3s3p ¹P atoms.     

Infrared laser-emission study in a resonantly excited sodium vapor

We report on a study of time dependence of ir laser emission at 3.41 μm in a dense Na vapor that is resonantly excited to the 3² P levels by a flashlamp, pumped dye laser. Energy-pooling collisions (excitation transfer) between excited atoms create a large population inversion among the 5² S _1/2 and 4² P _3/2 states which is destroyed by electron collisions as a result of subsequent plasma formation. The ir laser emission may serve as a new diagnostic tool to follow the time evolution of such media.     

Spontaneous emission of a two-level system and the influence of the rotating-wave approximation on the final state. I

By using a modified Robertson projection technique an exact equation of motion for the expectation value of the population inversion operatorS ^z of a single two-level atom in the case of spontaneous emission is derived. Afterwards, by making the Markov approximation, it is shown that the ground state expectation value〈S ^z 〉 _t =? 1/2 fort→∞ will be reached only if the rotating-wave approximation or the Born approximation is made additionally.     

Rearrangement of Rare Earth Defects Under Domain Inversion in LiNbO3

Using high resolution excitation-emission spectroscopy we investigated the changes occurring in the optical transition of Er^3? ions in LiNbO₃ during inversion of the ferroelectric axis. In stoichiometric LiNbO₃ we find that a drastic reconfiguration among the different defect sites takes places favoring those centers which have already been dominant in the as-grown sample. The reconfiguration is attributed to changes in the arrangement of the local charge compensators. Furthermore, we find a small shift of the emission transition energy, which is consistent with an increase of the intrinsic electric field. These findings make the Er^3? ions very suitable probes for 3D imaging of domain structures and for in-situ studies of the dynamics of the domain inversion processes and the defect/domain wall interaction.     

Luminescence and EPR spectroscopy of neutron-irradiated single crystals of magnesium aluminium spinel

Neutron irradiated single crystals of stoichiometric MgAl₂O₄ and MgAl₂O₄:Mn²⁺ (0.15 wt. %) were studied using the methods of luminescence spectroscopy and versions of electron paramagnetic resonance. In stoichiometric MgAl₂O₄:Mn²⁺ single crystals, the transition of a part of manganese impurity ions from tetrahedral to octahedral coordination, caused by irradiation with fast fission neutrons, is detected using photoluminescence spectra. This fact confirms the partial inversion of a normal spinel due to neutron-irradiation. Using pulse EPR technique it is shown that an electron in the field of oxygen vacancy (i.e. from neutron-irradiation induced F⁺ centres, which are spatially separated from each other) is in hyperfine interaction only with neighbouring ²⁷Al nuclei with nuclear spin 5/2.     

Direct observation of population inversion between Al+11 levels in a laser-produced plasma

Measured X-ray intensities of the resonance line series of Al⁺¹¹ in a laser-produced plasma shows population inversion between the n = 4,5 levels and the n =3 level at a plasma density N_e～10²⁰ cm^-3. The cooling of the expanding plasma leading to inversion is enhanced by a special target configuration. The gain coefficient in the 4 → 3 transition at 129.7 Å is estimated to be ～ 10 cm^-1, using both measured line intensities and numerical simulation.     

The effect of deuterium substitution in the amino group on the internal-rotation barrier of acetamide

Peptide molecules XCO-NYY′ are characterized by low potential barrier V₃ to internal rotation of a methyl group substituted for X and/or Y. A most conspicuous example is acetamide, for which V₃ was previously reported to be 25.043857(19) cm⁻¹[8]. The present study intended to clarify why V₃ is so low in acetamide, by examining the effect of the out-of-plane bending or inversion of the amino group on the molecular structure through deuterium substitution for amino hydrogens. The potential barrier V₃ in acetamide was found to decrease by 2.630, 2.986, and 5.532 cm⁻¹, when H′s at cis, trans, and both positions in the amino group were replaced by deuterium atoms, respectively. The reduction was proportional to the effective mass of the out-of-plane bending mode of the amino group (hereafter referred to as the amino inversion), which was in turn ascribed to the change in electronic resonance character of the peptide linkage. The amino inversion is coupled with the CH₃ internal rotation, producing an interaction term proportional to τ sin 3α, where τ and α denote the amino inversion and methyl internal rotation angles, respectively. This coupling term, when the inversion is treated by second-order perturbation, yields a V₆ term in the internal-rotation potential function of the methyl group, in agreement with the finding of Ilyushin et al. [8], who derived an unusually large V₆ term of −10.044874(73) cm⁻¹. It is quite interesting that even a small perturbation such as deuterium substitution causes a substantial change in electronic structure of the peptide linkage.     

Deuterated ammonia in krypton and nitrogen matrices: High-resolution spectroscopy of the ν2 mode

FTIR spectra of ND₃ in krypton and nitrogen matrices are recorded at low temperatures (7.5 K). The analysis of the ν₂ region shows that in krypton the rotation is slightly perturbed, with an inversion splitting of the ν₂ state equal to 2 cm⁻¹. In nitrogen, the inversion splitting has been searched for without success under a resolution of 0.06 cm⁻¹; in this matrix, a low-resolution measurement yields a librational frequency of 133 cm⁻¹. The present experimental results are compared with previous ones on NH₃ and with a recent theoretical model.     

Optically active Er3+ centers in Si/Si1 ? x Ge x structures under optical pumping

The Si/Si_{1 − x} Ge _x :Er/Si heterostructures, which are of interest for laser applications, have been investigated. The types of optically active Er³⁺ centers making a dominant contribution to the photoluminescence response of the structures studied are analyzed and their relationship with the parameters of the Si_{1 − x} Ge _x :Er heterolayer and the post-growth annealing conditions is shown. On the basis of the PL kinetic analysis of the structures with an isolated type of optically active Er³⁺ centers, it is concluded that population inversion of Er³⁺ ion states can be obtained under optical pumping and the effect of nonradiative recombination channels in Si_{1 − x} Ge _x :Er heterolayers on the excitation efficiency of Er centers and the conditions for population inversion is shown. Original Russian Text ? L.V. Krasilnikova, N.A. Baidakova, M.V. Stepikhova, Z.F. Krasilnik, V.Yu. Chalkov, V.G. Shengurov, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 103–108.     

Collision-induced amplification of short-wave radiation on transitions to the ground state of atoms

A new version of obtaining short-wave lasing on transition between highly excited states and the ground state of active atoms in the buffer gas atmosphere is studied theoretically. The mechanism of obtaining population inversion on such a transition is associated with the establishment of the local Boltzmann distribution of populations in a group of highly excited levels due to frequent collisions. If the excitation of the upper-lying level is performed by two laser radiation sources with frequencies ω₁ and ω₂, short-wave lasing can be obtained at a frequency close to the total frequency ω₁ + ω₂. The conditions for the emergence of population inversion are analyzed and simple analytic formulas are derived. It is shown that collision-induced enhancement of short-wave radiation can occur for pumping intensities on the order of 100 W/cm². For pumping intensities on the order of 1000 W/cm², the amplification factor for short-wave radiation may attain values of 3 cm^?1 (for an active atom concentration of N ～ 10¹⁵ cm^?3), which is sufficient for the development of lasing per path through an active medium (superradiance condition) for a length of the active medium on the order of 10 cm.     

Vibration-inversion-rotation spectra of ammonia: Centrifugal distortion, Coriolis interactions, and force field in NH3, NH3, ND3, and NT3

An effective inversion-rotation Hamiltonian has been developed for NH₃ which avoids the necessity of having to include high powers of the inversion motion coordinate in the Taylor expansions of the potential energy and the inverse moment of inertia tensor. This nonrigid bender Hamiltonian describes the centrifugal distortion and the Coriolis interactions in the ground and excited inversion states. It also describes the inversion doublings in the ground and excited vibration-inversion states of ammonia. A least-squares procedure that includes the numerical integration of the Schrödinger wave equation has been used to determine the harmonic force field and the double-minimum inversion potential function for (¹⁴NH₃, ¹⁵NH₃) and for (¹⁴ND₃ and ¹⁴NT₃).The anomalous rotational dependence of the inversion doublings in the (±l) components of the v₄ = 1 state of ¹⁴NH₃ has been explained by the Coriolis interactions between v₂=1, v₄ = 1, v₂ = 2, v₂ = 1, v₄ = 1, and v₂ = 3 vibration-inversion states.     

Oxidation of cleaved InAs(1 1 0) surfaces at room temperature: Surface band-bending and ionization energy

The interaction of unexcited, molecular oxygen with cleaved InAs(1 1 0) surfaces was investigated at room temperature by using a Kelvin probe and photoemission spectroscopy excited by HeI radiation. Exposures up to 10⁵ L of O₂, which result in an oxygen uptake up to only a few percent of a monolayer, cause the formation of an inversion layer on the specimen doped p-type and of an accumulation layer on those doped n-type. Between 10⁵ and 10⁸ L of O₂ the Fermi level is found to be pinned at 0.13 eV above the bottom of the conduction band on samples doped p- as well as n-type. This energy position of the Fermi level agrees with the charge- neutrality level of the virtual gap states as calculated by Tersoff. Since the ionization energy remains unchanged up to exposures of 10⁸ L of O₂ it is concluded that the oxygen is incorporated into rather than adsorbed on cleaved InAs surfaces.     

Determination of the g-factor of electrons in N-type silicon surface inversion layers

The g-factor of conduction electrons in the surface inversion layer on a silicon (100) surface has been determined using the tilted magnetic field method developed by Fang and Stiles.The value of (m¹/m₀)·g at the fixed magnetic field was independent of surface carrier density n_s, whereas it had a sharp peak at about 97 koe. At strong magnetic field limit the value was constant and 0.4. If we take the effective mass of conduction electrons in the inversion layer on the (100) surface as 0.2m₀, the g-factor is about two which is the same as that for conduction electrons in bulk silicon.     

Density functional and spectroscopic studies of nitrogen inversion in substituted dizocilpines

While developing a synthesis towards tagged dizocilpine (MK‐801) analogues, we observed highly restricted inversion of a nitrogen centre in a number hydroxylamines obtained as key intermediates. These compounds are shown to possess some of the structural elements which are expected to significantly hinder the nitrogen inversion, potentially leading to hydroxylamines with a chiral nitrogen centre. Free energy barriers (ΔG^≠) of the nitrogen inversion were estimated to be ca. 22 kcal mol^?1 at temperatures near 420 K using variable temperature NMR measurements in DMSO‐d₆. Further density functional studies of a number model systems were undertaken in order to better rationalize the measured inversion barriers and follow the role of various structural factors in raising the barrier height of the nitrogen inversion process. Copyright © 2008 John Wiley & Sons, Ltd.     

Laser induced collisional energy transfer processes in a rubidium-potassium mixture

Two laser induced collisional energy transfer processes in alkali vapours are investigated. For process K(4²S_1/2)+ Rb(5²P_3/2)+?ω_T→K(6²S_1/2)+Rb(5²S_1/2), a population inversion between the final state K(6²S_1/2) and lower states, leading to a superradiant emission has been observed. Comparison between experiment and theory shows a qualitative agreement in the limit of low atomic densities and laser power. However the transfer profile width is about two times larger than the calculated one for both processes.     

Many-body effects in the first excited subband of the n-inversion layer of Si

We have calculated the self-energies of electrons in the lowest and first excited sub-bands of Si inversion layers. The self-consistent wavefunctions calculated in the Hartree approximation were used, and dynamic screening was approximated by the Lundqvist-Overhauser model. The correlation energy of an electron in the excited band is quite large: about ?10 meV at an inversion layer density of 10¹¹ cm^?2 to about ?16 meV at 3 × 10¹² cm^?2. The calculated separation between subbands is in very good agreement with available experimental measurements. An exciton is predicted with a binding energy of 0.9 meV at N_inv = 10¹² cm^?2 calculated in the static approximation.     

Reduction of the effective rotational Hamiltonian for pyramidal XY3 molecules with a large-amplitude inversion motion

The previously described reduction of the effective rotational Hamiltonian for semirigid molecules of C_3v or D₃ point group symmetry [M. R. Aliev and V. T. Aleksanyan, Opt. Spectrosc.24, 201–206 (1968)] has been extended to nonrigid molecules with a significant inversion splitting of the energy levels (e.g., NH₃, H₃O⁺, CH₃^?, or SiH₃ molecules). Although for semirigid molecules like PH₃ or AsH₃, the parameters α and η′₃ which appear in the terms α[J₊³ + J_?³, J_z]₊ and η′₃(J₊⁶ + J_?⁶) are almost completely correlated, the effects of the inversion splitting and the accidental resonance which can occur between the interacting rotational levels in nonrigid molecules make it possible to determine α and η′₃ separately. The results of fitting the experimental data for ¹⁴NH₃ and ¹⁵NH₃ [?. Urban, Romola D'Cunha, K. Narahari Rao, and D. Papou?ek, Canad. J. Phys.62, 1775–1791 (1984); Romola D'Cunha, ?. Urban, K. Narahari Rao, L. Henry, and A. Valentin, J. Mol. Spectrosc.111, 352–360 (1985)] are in agreement with this conclusion. The possibility of the determination of the sign of η₃ from a simultaneous analysis of the allowed and Δk = ±3 forbidden transitions in semirigid XY₃ molecules has been discussed.     

Measurement of azimuthal magnetic fields in a Z-pinch gas discharge using the faraday rotation of a probe beam

The Faraday rotation of the plane of polarisation of a probe beam by azimuthal magnetic fields in the presence of beam deflection caused by refractive index gradients is discussed for a plasma carrying an axial current. A method for calculation of the magnetic field profile from experimental data is described. Bθ can be found by Abel inversion if the electron density is known and if deflected rays can be collected by an optical system and focussed onto a detector. Typical Faraday rotations calculated for the Bennett pinch assuming small beam deflection are found to be proportional to the plasma current and to the angle of deflection. If the probe beam wavelength is chosen to satisfy the approximate relation N₀λ² ≈ 3.5 × 10¹³ m^-1, where N₀ is the electron density on the axis, measurement of Bθ with beam deflections less than 2 × 10^-2 radians should be possible in cases where small rotations can be detected in the mid-to-far infra-red part of the spectrum.