The microwave spectrum of difluoroacetic acid

Transitions due to two conformations have been observed in the microwave spectrum of difluoroacetic acid. In both rotamers the carboxylic hydrogen atom is cis to the CO bond, whereas the other hydrogen atom is either trans or gauche with respect to the CO bond. By means of isotopic substitution of the hydrogen and oxygen atoms a serious deviation of local symmetry in the CHF₂ group is deduced for the gauche form. This is thought to be an artifact of the substitution method, although no satisfactory explanation can be given. In torsionally excited states of the gauche conformation, doublet splittings were observed due to tunneling through a barrier of 190 ± 10 cm^?1. For the second excited state a satisfactory treatment could be given, but for the third excited state large differences between observed and calculated transition frequencies remained. A computer program to account for the internal rotation in asymmetric molecules was written, and it was shown qualitatively that interactions with excited states of the trans conformation could very well be responsible for the discrepancies. From intensity measurements the energy difference E(gauche) - E(trans) was found to be 370 ± 180 cm^?1. The large uncertainty is due to the failure of attempts to determine the dipole moments experimentally; they were estimated from an improved set of bond moments which was derived from related compounds. Both energy difference and dihedral angles are at variance with the results of an earlier electron diffraction investigation.     

Casimir-Polder interaction of excited media

The potential of long-range interaction between two dissimilar atoms, one of which is excited, drops as 1/R ₂ with the distance for the Casimir-Polder limit of large distances in comparison with the wave-length of atom transitions (E.A. Power and T. Thirunamachandran, Phys. Rev. A 51, 3660 (1995)). It is shown that such a dependence, obtained with the help of perturbation technique, results in a divergence for the interaction potential between an excited atom and a medium of dilute gas. We develop a nonperturbative method based upon quantum Green’s functions (Yu. Sherkunov, Phys. Rev. A 72, 052703 (2005)) to calculate the interaction potential for an excited atom and a ground-state atom embedded in a dielectric medium, taking into account the absorption of photons in the dielectric medium. The exponential suppression of the interaction between the atoms is demonstrated. The force acting on an excited atom near the interface of dilute gas medium is calculated. The result is no more divergent. The force between gas media in Casimir-Polder regime is calculated as well. The text was submitted by the author in English.     

Variational Calculation of 4He Tetramer Ground and Excited States Using Realistic 4He–4He Potentials

We calculated binding energies and wave functions of the ⁴He tetramer ground and excited states employing various realistic ⁴He?⁴He potentials which includes the currently most accurate one with the adiabatic, relativistic, QED and residual retardation corrections. We used our Gaussian expansion method (GEM) for ab initio variational calculations of few-body systems. We found that precisely the same shape of the short-range correlation (r _ij < 4Å) in the dimer appear in the ground and excited states of trimer and tetramer. The four kinds of the binding energies of the trimer and tetramer ground and excited states, ${B_3^{(0)}, B_3^{(1)}, B_4^{(0)}}$ and ${B_4^{(1)}}$ , for the different potentials exhibit perfect linear correlations over the range of binding energies relevant for ⁴He atoms; namely, six types of the generalized atomic Tjon lines were observed.     

On the mechanism of formation of helium molecules in a low-temperature plasma

The glow of He₂ molecules in a low-pressure (P ≤ 2 Torr) helium plasma excited by a monochromatic electron beam is detected and studied. The experimental data point to the existence of two basic mechanisms of formation of an excited He₂ molecule, namely, the collision of an excited helium atom with two unexcited atoms and the recombination of a molecular ion.     

The microwave spectrum of α-fluoropropionic acid

The rotational absorption lines in the microwave spectrum of α-fluoropropionic acid were shown to originate from two different molecular conformations. The rough geometries of the two conformations could be determined from the observed dipole moments and the substitution coordinates of the carboxylic hydrogen atom. In both conformations the fluorine atom is near the plane of the carboxyl group; in the conformation with the fluorine atom trans with respect to the hydroxyl group the carboxyl group has the usual geometry, while in the cis conformation the molecule is stabilized by an internal OH?F hydrogen bond. By measuring the relative intensities of the absorption lines it was found that the cis conformer is 0.5 ± 0.2 kJ mole^?1 lower in energy than the trans conformer. The barrier to internal rotation of the methyl group in the trans conformation was determined from A-E line splittings in the second excited vibrational state V₃ = 13.5 ± 0.3 kJ mole^?1.     

Excited atoms in argon gas discharge plasma

General principles are discussed for a gas discharge plasma involving excited atoms where electron-atom collision processes dominate. It is shown that an optimal kinetic model of this plasma at not large electric field strengths can be based on the rate constants of quenching excited atom states by electron impact. The self-consistent character of atom excitation in gas discharge plasma is important and results in the tail of the energy distribution function of electrons being affected by the excitation process, which in turn influences the excitation rate. These principles are applied to an argon gas discharge plasma where excitation and ionization processes have a stepwise character and proceed via formation of argon atom states with the electron shell 3p ⁵4s.     

Retardation of the relaxation over quantum-well energy levels in CdSe/ZnS quantum dots with increasing number of excited carriers

The differential transmission spectra of CdSe/ZnS quantum dots are investigated. It is revealed that the differential transmission spectra measured upon resonant excitation of electrons into the first excited state 1P(e) exhibit a number of specific features, such as a decrease in transmission at the pump frequency, bleaching in the course of the pump pulse at frequencies corresponding to the fundamental optical transition 1S _3/2(h)-1S(e) and transitions between excited hole states and the 1S(e) electron ground state, and retardation of this process with an increase in the energy of the pump pulse. The observed specific features can be explained by the following factors: (i) the absence of a “phonon bottleneck” for electrons due to the energy transfer from hot electrons to rapidly relaxing holes, (ii) relaxation through intermediate quantum-well energy levels of holes, and (iii) retardation of relaxation with increasing number of excited charge carriers in a quantum dot.     

M4,5N4,5N4,5 auger electron spectrum of Ba metal

The Al Kα excited M_4,5N_4,5N_4,5 Auger spectrum of Ba has been measured from the metallic sample evaporated on a Ag substrate. The spectrum has been decomposed into individual line components after the background subtraction. The decomposed spectrum has been compared with the theoretical spectrum calculated for the 4d^?2 final state configuration in the mixed coupling scheme applying jj-coupling for the initial state and intermediate coupling for the final state. The most prominent structure of the spectrum shows the two 4d-hole coupling, but the structure which is caused by the Auger transitions M_,45N_2,3V has also been observed. The screening of the core holes in Ba metal seems to be produced by (5d6s) electrons. The simple excited atom model HF-calculations give an Auger kinetic energy shift (metal-free atom) of 16.7 eV, which is comparable to the experimental value 14–18 eV.     

Spherically compressed helium atom described by perturbative and variational methods

We analyze the energy spectrum of the three lowest-lying S symmetry states for the spherically confined helium atom as a function of the box radius by using an approach based on a time independent perturbation theory and two variational methods. The first treatment depends on exact solutions for confined one-electron atoms, whereas in the latter two methods exponents and linear coefficients are variationally optimized via {s,t,u}-Hylleraas functions and Generalized {r₁,r₂,r₁₂}-Hylleraas basis sets that fulfill appropriate boundary conditions. Although it is found that throughout most of the box radii here analyzed the variational energies for the three states lie below those perturbatively obtained, an opposite trend occurs toward the weak and strong confinement regions for the singlet excited and triplet states, respectively.     

Neutrinoless double electron capture as a tool to measure the electron neutrino mass

A nucleus (Z, A) may capture two atomic electrons to become (Z ? 2, A). For Majorana neutrinos this may occur with no neutrino emission, the process is a virtual mixing of the parent atom and the daughter atom with two electron holes. The process becomes real as the daughter atom de-excites. In some cases where the daughter nucleus is excited, the neutrinoless decay may be enhanced by its proximity to a virtual resonance. We identify the ${}^{112}\text{Sn}\text{→}{}^{112}\text{Cd}$ transition as a good case. The no-neutrino lifetime for m_ν = 30 eV ranges from 10²² to 10²⁷ years as a function of the insufficiently well determined distance to resonance. The signatures of the two- or no-neutrino modes are very different.     

Transfer of a weakly bound electron in collisions of Rydberg atoms with neutral particles. II. Ion-pair formation and resonant quenching of the Rb(nl) and Ne(nl) States by Ca, Sr, and Ba atoms

Electron-transfer processes are studied in thermal collisions of Rydberg atoms with alkaline-earth Ca(4s ²), Sr(5s ²), and Ba(6s ²) atoms capable of forming negative ions with a weakly bound outermost p-electron. We consider the ion-pair formation and resonant quenching of highly excited atomic states caused by transitions between Rydberg covalent and ionic terms of a quasi-molecule produced in collisions of particles. The contributions of these reaction channels to the total depopulation cross section of Rydberg states of Rb(nl) and Ne(nl) atoms as functions of the principal quantum number n are compared for selectively excited nl-levels with l ? n and for states with large orbital quantum numbers l = n ? 1, n ? 2. It is shown that the contribution from resonant quenching dominates at small values of n, and the ion-pair formation process begins to dominate with increasing n. The values and positions of the maxima of cross sections for both processes strongly depend on the electron affinity of an alkaline-earth atom and on the orbital angular momentum l of a highly excited atom. It is shown that in the case of Rydberg atoms in states with large l ～ n ? 1, the rate constants of ion-pair formation and collisional quenching are considerably lower than those for nl-levels with l ? n.     

Structured emission of relaxing systems: molecular excitons in rare-gas crystals

We have computed the emission spectra of relaxing excitated-state systems in solids as a function of the rates of radiative decay (γ) from excited to ground state and non-radiative decay (γ) within the excited state. When these are comparable a pronounced dip is obtained in the central region of the spectrum, which then may have the appearance of two distinct, unrelated emission bands. The computations well reproduce the observed W and M₂ bands in Ar crystals. Accordingly, these arise from the initially excited and near repulsive regions of the A¹Σ⁺ emitting state, respectively.     

Control of dissipation of energy via reservoirs of coherent states

Dynamic profiles of various chemical reactivity indices like chemical potential, chemical hardness, electrophilicity, susceptibility, etc., within a confined environment during the interaction of atoms with strong oscillating time dependent magnetic fields have been studied. In the present study hydrogen and helium atoms in ground state (n = 1), as well as in excited state (n = 20) are considered. Time-dependent Schrödinger equations are solved for the ground and excited states of hydrogen atom and the Rydberg state of the helium atom while a generalized nonlinear Schrödinger equation is solved for the ground state of the helium atom. Dirichlet type boundary condition has been used to implement confinement to the systems. With an increase in the degree of confinement the system gets harder and hence becomes more stable. Keeping the confinement radius fixed, systems get more stabilized in strong field compared to weak field.     

On the Choice of the Wavefunction of the Ground State of He for Precision Calculations of Autoionization State Parameters above the Excited Ion Formation Threshold

The necessity of choosing multiparametric wavefunctions for describing the ground state of an atom in the problems of ionization of atoms by photons and electrons has been substantiated for the He atom as an example. Comparative analysis of application of different ground-state wavefunctions for this atom has been performed. The energies, widths, and partial widths of the lower autoionization state ¹P of the He atom above the excited ion formation threshold has been performed. It is shown that in contrast to total widths of quasi-stationary states, which differ insignificantly for different wavefunctions of the ground state, the partial widths are substantially different.     

The effect of field strength on the resonance structure of three-photon ionization spectra of the samarium atom

Amplitude ratios of resonance maxima caused by two-photon transitions from different levels of the ground term of the samarium atom are studied experimentally and theoretically as functions of the laser-radiation field strength. The results of these studies are shown to be promising for identifying the resonance structure of three-photon ionization spectra of the samarium atom. A not yet observed excited even-parity state with energy E ≈ 36789.1 cm^?1 and hypothetical values of total angular momentum J = 5, 6 has been discovered.     

Electronic excitations of the rare-gases in the solid phase

We present the results of a simple non-structural approach to the study of the exciton levels in solid rare-gases. We start from the excited states of the free atom and we introduce the solid state effects only through the dielectric constant ?₀ and the effective mass μ. The agreement with the most recent and accurate experimental data is complete for the observed exciton levels in all solid rare-gases. We also provide accurate values for the energy gaps E_g and the effective masses μ.     

Mesures de durees de vie dans le cas de faibles intensites lumineuses

A simple method concerning the possibility of lifetime's measurement in the case of low light level, 20 photons/s, is described. Application to some n¹S₀ and ¹D₂ excited states of the Hg atom is given.     

Sm原子的偶宇称高激发态的光谱研究

采用双色三步激发和光电离过程,对Sm原子的偶宇称高激发态的光谱进行了研究.先采用两条激发路线分别将Sm原子两步共振激发至待测的高激发态,然后利用光电离技术对其进行探测.分别将第一束激光的波长固定在627.50nm和624.41nm上,以便将Sm原子从亚稳态共振激发到由4f⁶6s6p电子组态所构成的两个原子状态上.第二束激光在440-700nm的波段范围内扫描,不仅使Sm原子在30040-38065cm^-1能域内的偶宇称高激发态上布居,将其进一步光电离,测量了其光谱.通过光谱定标和选择定则等分析手段,本工作不仅精确获得了136个态的能级位置,而且也唯一确定了其总角动量,并且给出了相关跃迁的相对谱线强度.