The microwave spectrum and molecular conformation of peroxynitric acid (HOONO2)

The rotational spectrum of peroxynitric acid has been investigated in the 40- to 120-GHz region. The spectrum of the ground state is complicated by tunneling of the OH group, which causes a doubling of the asymmetric rotor spectrum. The magnitude of the tunneling splitting is such that it causes Coriolis interactions between the energy levels of the two tunneling states which lead to perturbations in the rotational spectrum. A combined analysis of the a- and b-type pure rotational transitions with the c-type tunneling transitions allows a perturbation-free determination of the rotational constants for the ground state. A similar analysis of the low-lying NO₂ torsional vibration at 145(6) cm⁻¹ has also been carried out. The dipole moments for each state have been determined by analysis of the second-order Stark effect. The molecular structure analysis indicates that all the heavy atoms are planar and only the hydrogen atom is out of the heavy atom plane. The preferred orientation of the hydrogen atom with respect to the plane of the heavy atoms is at an angle ∼106° with respect to the cis conformation.     

Features of the bremsstrahlung accompanying collisions with a hydrogen atom in an excited state

The features of the bremsstrahlung appearing during a collision of a fast charged particle with a hydrogen atom (or hydrogenic ion) in an excited state are investigated. It is shown that the emission spectrum of photons with energies greater than the ionization potential of a given excited state (except the 2s state) displays narrow lines, which are caused by de-excitation of the atom in an intermediate state. It is demonstrated that the scattering of a charged particle on an excited hydrogen atom produces a feature which is not observed in the case of scattering on a ground-state hydrogen atom. Expressions are obtained for the generalized dynamic polarizability of the hydrogen atom and hydrogenic ions in the 1s, 2s, and 3s states. A method is developed for deriving expressions for the generalized dynamic polarizabilities of other excited states through the use of the Coulomb Green’s function and representation of the electronic wave function in terms of the differentiation of the generating functions of Laguerre polynomials. The bremsstrahlung cross sections for electrons and positrons colliding with hydrogen atoms in the 1s, 2s, and 3s states are calculated. Zh. Tekh. Fiz. 69, 7–13 (October 1999)     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Pure multi-photon ionization of the ground 1S and metastable 2S state of the hydrogen atom with plane-polarized light of threshold frequency

Formulas for N-photon ionization of the hydrogen atom states 1S and 2S with plane-polarized light at the threshold frequency $\text{ω=}\text{I}\text{N}\text{h}\text{?}$ (I ionization energy) are proposed and shown to reduce in the one-photon case to the well-known Stobbe expressions.     

Multiphoton and tunneling ionization of atoms in an intense laser field

We study the ionization probabilities of atoms by a short laser pulse with three different theoretical methods,i.e.,the numerical solution of the time-dependent Schro¨dinger equation(TDSE),the Perelomov-Popov-Terent’ev(PPT) theory,and the Ammosov-Delone-Krainov(ADK) theory.Our results show that laser intensity dependent ionization probabilities of several atoms(i.e.,H,He,and Ne) obtained from the PPT theory accord quite well with the TDSE results both in the multiphoton and tunneling ionization regimes,while the ADK results fit well to the TDSE data only in the tunneling ionization regime.Our calculations also show that laser intensity dependent ionization probabilities of a H atom at three different laser wavelengths of 600 nm,800 nm,and 1200 nm obtained from the PPT theory are also in good agreement with those from the TDSE,while the ADK theory fails to give the wavelength dependence of ionization probability.Only when the laser wavelength is long enough,will the results of ADK be close to those of TDSE.     

Single attosecond burst generation during ionization of excited atoms by intense ultrashort laser pulses

We develop an analytical approach to describing the generation of a single attosecond burst during barrier-suppression ionization of a hydrogen atom by an intense laser pulse. We derive analytical expressions that describe the evolution of the electron wave packet in the time interval between the detachment from the atom and the collision with the parent ion for an arbitrary initial atomic state by assuming the atom to be fully ionized in one laser-field half-period. For various s-states, we derive expressions for the profile of the attosecond burst generated when the electron packet collides with the ion and analyze the dependence of its generation efficiency on the principal quantum number n of the initial atomic state. The results obtained are compared with the results of three-dimensional numerical calculations. We show that the attosecond pulse generation efficiency can be several orders of magnitude higher than that in the case of ionization from the ground state when pre-excited atomic states are used.     

Tunneling and above-barrier ionization of atoms in a laser radiation field

Calculations are made of the energy and angular distributions of photoelectrons during tunneling ionization of an atom or an ion under the action of high-power laser radiation (for all values of the Keldysh parameter γ). Cases of linear, circular, and elliptic polarizations of the electromagnetic wave are considered. The probability of above-barrier ionization of hydrogen atoms in a low-frequency laser field is calculated. Formulas are given for the momentum spectrum of the electrons when an atomic level is ionized by a general type of alternating electric field (for the case of linear polarization). An analysis is made of tunneling interference in the energy spectrum of the photoelectrons. Analytic approximations are discussed for the asymptotic coefficient C _κ of the atomic wave function at infinity (for s-wave electrons).     

Ionization of a molecular hydrogen ion by a strong low-frequency electromagnetic field of laser radiation

Simple analytical expressions are obtained for the energy and angular distributions of outgoing electrons in ionization of a molecular hydrogen ion by a strong low-frequency electromagnetic field as well as for the ionization probabilities per unit time. The cases of linear and circular polarization of the laser radiation are studied. It is shown that in contrast to the case of the ionization of atoms oscillations appear in the energy spectra of the photoelectrons as a function of their kinetic energy. The well-known limits for the tunneling ionization probabilities for the hydrogen atom by a strong low-frequency alternating field are obtained in the case of large internuclear separations. Zh. é ksp. Teor. Fiz. 113, 583–592 (February 1998)     

Correlation of momenta of electrons and the nucleus in atoms

The process of resonant multiphoton ionization of a hydrogen atom in the ground 1s state is studied by direct numerical integration of the nonstationary Schrödinger equation for a quantum system in an electromagnetic field. The dependence of photoionization probability on the radiation intensity is found to be nonmonotonic. It is established that the minima of ionization correspond to multiphoton resonances between the ground state and one of the excited (Rydberg) atomic states perturbed by the laser field. It is shown that ionization is suppressed due to rearrangement of Rydberg states in a strong electromagnetic field and is accompanied by efficient Raman Λ transitions, which connect a set of closely lying Rydberg states via the continuum.     

Modeling pressure-ionization of hydrogen in the context of astrophysics

Abstract

The recent development of techniques for laser-driven shock compression of hydrogen has opened the door to the experimental determination of its behavior under conditions characteristic of stellar and planetary interiors. The new data probe the equation of state (EOS) of dense hydrogen in the complex regime of pressure ionization. The structure and evolution of dense astrophysical bodies depend on whether the pressure ionization of hydrogen occurs continuously or through a “plasma phase transition” (PPT) between a molecular state and a plasma state. For the first time, the new experiments constrain predictions for the PPT. We show here that the EOS model developed by Saumon and Chabrier can successfully account for the data, and we propose an experiment that should provide a definitive test of the predicted PPT of hydrogen. The usefulness of the chemical picture for computing astrophysical EOS and in modeling pressure ionization is discussed.     

Effect of localized charge states on the low-frequency part of the tunneling current spectrum (1/<Emphasis Type="Italic">f</Emphasis><Superscript><Emphasis Type="Italic">α</Emphasis></Superscript>)

The dependence of the low-frequency part of the tunneling current spectra (1/f ^α ) above a clean surface and above isolated impurity atoms on the InAs(110) cleaved surface has been investigated by scanning tunneling microscopy/spectroscopy in high vacuum. A theoretical model is proposed to explain the experimental results, which takes into account the many-body interaction of conduction electrons (with suddenly switched on and off Coulomb interaction on an impurity atom and on the scanning tunneling microscope tip) with the continuous-spectrum states in the tunneling contact leads.     

Low-frequency strong-field ionization and excitation of hydrogen atom

The dynamics of hydrogen atom in the presence of a strong radiation field of the titanium-sapphire laser is studied for the Keldysh parameters γ ≥ 1 and γ ≤ 1. It is demonstrated that the ionization is supplemented with the effective population of the excited states with the principal quantum numbers n = 5–10 in the entire range of variation in the Keldysh parameter. The population of the excited Rydberg states can be interpreted as a consequence of the multiphoton resonance involving the initial 1s state and a group of excited states in the vicinity of the continuum boundary with the simultaneous repopulation of these states by Λ-type Raman transitions under the action of the laser field. The resulting coherent Rydberg packet appears to be stable with respect to ionization, so that the ionization of the atomic system in the presence of strong electromagnetic field is suppressed. Physical reasons for the stabilization are discussed. An interpretation of the effective population of the Rydberg states in the recent experiments on the ionization of atomic helium by the titanium-sapphire laser is proposed.     

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.     

强激光场中模型氢原子和真实氢原子的高次谐波与电离特性研究

利用数值方法求解含时薛定谔方程，研究了一维、二维模型氢原子和真实的三维氢原子在强激光场中产生的高次谐波和电离特性.结果表明，在多光子电离区域和过垒电离区域，模型氢原子与真实的氢原子产生的高次谐波和电离概率差别很小；在隧道电离区域，它们产生的高次谐波的平台特征和截止位置相似，电离概率随时间变化的趋势相近，但其数值有明显的差异.对产生这种差异的原因进行了分析. 关键词： 强激光场 高次谐波 电离概率     

Quantum molecular dynamics of partially ionized plasmas

We study a partially ionized hydrogen plasma by means of quantum molecular dynamics, which is based on wave packets. We introduce a new model which distinguishes between free and bound electrons. The free electrons are modelled as Gaussian wave packets with fixed width. For the bound states the 1s-wave function of the hydrogen atom is assumed. In our simulations we obtain thermodynamic properties in the equilibrium such as the internal energy and the degree of ionization. The degree of ionization is in good agreement with theoretical predictions. The thermodynamic functions agree well with results from quantum statistics for 10000 K ≲ T ≲ 40000 K.     

Surface dependence of field ion energy distributions,He on W

Kinetic energy distributions of helium ions produced by field ionization above the (100), (110), and (111) planes of tungsten field emitters are reported. The energy resolution of these measurements if 0.3 eV fwhm. The main peak of these distributions is found to shift by as much as 0.7 eV at $\text{F = 5.3 V/}\text{A}\text{?}$, T = 21 K, when the origin of the ions collected is changed slightly. Several possible explanations for this shift are discussed. The most plausible of these involves electron tunneling through a field adsorbed helium atom. These results have important implications for the use of field ion energy distributions as a probe of the density of electronic states of the emitter.     

Electron transport in granular amorphous silicon dioxide films with ferromagnetic nanoparticles placed in a magnetic field

Electron transport in amorphous silicon dioxide films with embedded nanoparticles (Co, Nb, Ta) was studied. The mean number of localized states in the interparticle tunneling channel was derived from the temperature dependence of conductivity for various grain concentrations under the assumption of the electron transport being governed by resonance tunneling in a chain of localized states between grains. To confirm the assumption of the inelastic character of tunneling, the dependences of the magnetoresistance on grain concentration, temperature, and magnetic field were studied. Accepting the single-orbital model, where the intergrain tunneling magnetoresistance is determined by s-s tunneling, it was found that the existence of weakly split localized states in the tunneling channel results in a lack of magnetoresistance saturation in strong magnetic fields. The combined effect of a decrease in the s-s tunneling coefficient and of growth in the probability of inelastic electron spin scattering with increasing length of the chain of localized states between particles in which the electron is tunneling accounts for the characteristic temperature-concentration dependences of the magnetoresistance. The experimental observation of these features provides an argument for the electron transport in a-SiO₂(Co,Nb,Ta) structures being governed by inelastic resonance tunneling through intergrain localized states.     

Separable State Decompositions for a Class of Mixed States

We study certain quantum states for which the PPT criterion is both sufficient and necessary for separability. A class of n×n bipartite mixed states is presented and the conditions of PPT for these states are derived. The separable pure state decompositions of these states are explicitly constructed when they are PPT.     

Collision-energy-resolved Penning ionization electron spectroscopy of styrene, 2-vinylpyridine, and 4-vinylpyridine with He*(23S) metastable atoms

Collisional ionization of styrene (phenylethylene), 2-vinylpyridine, and 4-vinylpyridine with metastable He*(2³S) atoms were studied by means of collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy. Collision energy dependence of partial ionization cross-sections, which reflects the anisotropic interactions between a He*(2³S) atom and the target molecules, indicates that attractive interaction for the out-of-plane access of a He*(2³S) atom to phenyl group is stronger than that for the out-of-plane access to vinyl group. Moreover, it was found for vinylpyridines that the attractive interaction around π electrons became weaker than that for styrene, and that the attractive interaction for the in-plane access to the nitrogen atom is stronger than that for out-of-plane π-directions. However, in 2-vinylpyridine, the hydrogen atom of vinyl group prevents a He*(2³S) atom from approaching to the nitrogen atom along in-plane directions, and thus the attractive interactions around the nitrogen atom were shielded by the vinyl group. The experimentally observed anisotropic interactions were qualitatively supported with ab initio model interaction potential calculations between a Li (He*(2³S)) atom and the target molecule. Concerning with electronic structures of investigated molecules, the assignment of Penning ionization electron spectrum for 4-vinylpyridine was discussed on the basis of different behavior of collision-energy dependence of partial ionization cross-sections, and the satellite ionization band in Penning ionization electron spectra was also reported for styrene.