Ubiquitous nature of multimode vibrational resonances in positron-molecule annihilation

Positron annihilation on many molecules occurs via positron capture into vibrational Feshbach resonances, with annihilation rates often further enhanced by energy transfer to vibrational excitations weakly coupled to the positron continuum. Data presented here uncover another scenario in which the positron couples directly to a quasicontinuum of multimode vibrational states. A model that assumes excitation and escape from a statistically complete ensemble of multimode vibrations is presented that reproduces key features of the data.     

The role of bound states in positronium annihilation

The temperature dependence of the rates of positronium reaction with diamagnetic organic molecules in liquids, and the observed complicated conversive-chemical interaction of positronium with many paramagnetic molecules in the liquid and gas phases lead to the formulation of some general concepts about positronium interaction with quenchers. The positronium and acceptor (Ps-M) complexing is a very essential step of the interaction mechanism. In the case of unstable complexes the contribution of annihilation from the bound state can change depending on the physical conditions of the reaction (pressure in the gas phase, transition from the gas to the liquid phase, heating or replacement of the liquid solvent). The dependence on temperature and on the solvent nature is accounted for by formation of a “bubble” around the positronium in many liquids. At the moment of complexing the “bubble” shrinks with a swiftness depending upon properties of the liquid (surface tension and viscosity). With unstable complexes this leads to complex decomposition and, correspondingly, either to elimination of interaction with the diamagnetic acceptor or, in the case of paramagnetic acceptors, to conversion on the latter, rather than to annihilation from the bound state. With stable complexes further positronium annihilation will occur only from the bound state.     

强红外场作用下BCl3振动态传能动力学研究

基于文献[1],本工作在较高泵浦激光能通量范围,测量并研究了BCl₃分子振动激发v_3吸收谱及时间演变,观察了v₃激发弛豫的几种能量转移过程,以及对泵浦激光能量BCl₃气压等参数的依赖关系。表明泵浦光脉冲产生一个振动态非热分布的系综,转动能量转移对引起这种非热分布有重要作用。用简化碰撞动力学模型讨论了BCl₃振动激发吸收谱的演变过程,得到振动态再分布的简单关系;Pτ_v-v≌c/K′(T_v,T₀,q)和等效振动温度、平均吸收光子数的分析表达式,与实验结果定性地符合。 关键词：     

Suppressing the weakly bound states in the photoassociation dynamics by a frequency cut-off laser pulse

The photoassociation dynamics of ultracold lithium atoms controlled by a cut-off pulse has been investigated theoretically by solving numerically the time-dependent Schrödinger equation using the mapped Fourier grid method. The frequency components of the laser pulse close to the atomic resonance are partly cut off. Compared with the typical Gauss-type pulses, the cut-off pulse is helpful to suppress efficiently the weakly bound states and prepare the associated molecules in the lower vibrational states. Especially, the dependence of photoassociation probability on the cut-off position of the laser pulse is explored.     

Suppressing the weakly bound states in the photoassociation dynamics by using a frequency cut-off laser pulse

The photoassociation dynamics of ultracold lithium atoms controlled by a cut-off pulse has been investigated theoretically by solving numerically the time-dependent Schro¨dinger equation using the mapped Fourier grid method. The frequency components of the laser pulse close to the atomic resonance are partly cut off. Compared with the typical Gauss-type pulses, the cut-off pulse is helpful to suppress efficiently the weakly bound states and prepare the associated molecules in the lower vibrational states. Especially, the dependence of photoassociation probability on the cut-off position of the laser pulse is explored.     

Multistability of states of a molecular gas with the V-T relaxation time dependent on the vibrational energy

The possibility of realizing multistability of states under the action of IR radiation on a molecule under conditions of nonlinear absorption of light energy and dependence of the V-T relaxation time on its vibrational energy is shown. A two-wave optically bistable arrangement potentially having a highly contrasting range of states is proposed. The nonlinear dependence of the V-T relaxation time on the vibrational energy is shown to allow the realization of optical bistability based on the absorption of light energy at the frequency of the fundamental vibrational transition.     

Quantum ergodicity and energy flow in molecules

We review a theory for coupled many-nonlinear oscillator systems that describes quantum ergodicity and energy flow in molecules. The theory exploits the isomorphism between quantum energy flow in Fock space, that is, vibrational state space, and single-particle quantum transport in disordered solid-state systems. The quantum ergodicity transition in molecules is thereby analogous to the Anderson transition in disordered solids. The theory reviewed here, local random matrix theory (LRMT), describes the nature of the quantum ergodicity transition, statistical properties of vibrational eigenstates, and quantum energy flow through the vibrational states of molecules. Predictions of LRMT have been observed in computational studies of coupled nonlinear oscillator systems, which are summarized here. We also review applications of LRMT to molecular spectroscopy and chemical reaction rate theory, including adoption of LRMT in theories that predict rates of conformational change of molecules taking place at energies corresponding to those below and above the quantum ergodicity transition. A number of specific examples are reviewed, including the application of LRMT to predict (1) dilution factors of IR spectra of organic molecules, (2) rates of conformational change in chemical and photochemical reactions, (3) conformational dynamics of biological molecules in molecular beams, (4) rates of hydrogen bond breaking and rearrangement in clusters of biological molecules and water, and (5) excited state proton transfer reactions in proteins.     

Rovibrational state distribution of deuterium molecules resulting from D-atom interaction with Ni(110)

Using the REMPI technique we have studied the internal state distribution of deuterium molecules produced by the interaction of atomic deuterium with chemisorbed deuterium on Ni(110) at 180 K. We observed molecules in vibrational states up to v = 3 with a mean vibrational energy of 220 meV. The mean rotational energies of the molecules in the vibrational states v = 0 to 3 are 185 meV, 133 meV, 75 meV and 37 meV, respectively. The overall mean rotational energy amounts to 150 meV, again far in excess of the mean rotational energy of molecules accommodated to the surface. The data are consistent with direct interactions of the impinging particles with the adsorbed particles (Eley-Rideal reaction and collision induced desorption), for which it is assumed that a considerable amount of the potential and kinetic energy of the impinging atoms is channeled into translational and internal energy of the reaction products.     

Efficient non-resonant intermolecular vibrational energy transfer

Molecular excited vibrational states are metastable states and we incorporate their finite lifetimes into the theory of vibrational energy transfer between weakly interacting molecules, i.e., at internuclear distances at which they do not have a chemical bond. Expressions for the effective lifetime of an initially vibrationally excited molecule in the presence of a neighbouring molecule are derived in closed form. These expressions allow one to analyse the physics behind the energy transfer. It is shown that due to different finite lifetimes of the isolated excited molecules, a very efficient vibrational energy transfer can take place between them even if their energies are rather off-resonance. Examples are discussed.     

Positron annihilation in large polyatomic molecules. The role of vibrational Feshbach resonances and binding

We analyse the process of rapid positron annihilation in large polyatomic molecules due to positron capture into vibrational Feshbach resonances. Resonant annihilation occurs in molecules which can bind positrons, and we analyse positron binding to alkanes using zero-range potentials. Related questions of spectra of annihilation gamma quanta and molecular fragmentation following annihilation, are discussed briefly.     

A simulation study of vibrational relaxation of I− 3 in liquids

The temperature dependence of the vibrational relaxation of a flexible model of triiodide in a Lennard-Jones solvent (xenon) has been studied using equilibrium molecular dynamics simulations. The internal dynamics of the ion is calculated from a previously published semi-empirical valence bond model with a limited number of basis states. Vibrational decorrelation rates of the symmetric and antisymmetric stretching modes were found from the time correlation functions of the normal coordinate velocities and the vibrational energy relaxation rates from the time correlation functions of the kinetic energy in each mode. The vibrational dephasing rates and the energy relaxation rates decrease slowly as the temperature is lowered and do not show a discontinuity when the fluid solidifies, although the reorientational diffusion rates change rapidly at low temperatures. In order to interpret the results, perturbation theory expressions for the relaxation rates were evaluated for simulations of a rigid model of the ion and found to agree well with the direct observations. These showed that, unusually, both the solvent force and its derivative, the solvent potential curvature, contribute to the dephasing of the symmetric mode. The relevant fluctuation correlation times are very short, which may explain the insensitivity of the vibrational relaxation to the state of the solvent.     

Storage of Entangled States with Multiple Trapped Ions in Thermal Motion

This paper presents an alternative scheme to realize the storage of entangled states for multiple trapped ions including W state, Bell states, and GHZ states even with ions which exchange vibrational energy with a heating surrounding. Our scheme requires that the ions be simultaneously excited by two laser beams with different frequencies.In this scheme the vibrational degrees of freedom are only regarded as intermediate states and the ions exchange energy via the mediation of the vibration of the vibrational mode in coupling processes. The scheme is insensitive to both the initial vibrational state and heating if the system remains in the Lamb-Dicke regime. Since the effective Rabi frequency has a small dependence on the vibrational quantum number the heating will have no direct effect on the internal state evolution.     

Temperature Dependence of the Efficiency of Collisional Transfer of Vibrational Energy in Mixtures of Vibrationally Excited Triplet Molecules with Foreign Gases

By the pressure dependences of the decay rates of delayed fluorescence activated by vibrational excitation of triplet molecules of benzophenone and anthraquinone, the efficiencies of collisional transfer of vibrational energy (V–V-transfer) in the vibrational quasi-continuum of the triplet state have been estimated. It is shown that the efficiencies of the process in mixtures with foreign gases increase with increasing dipole moment and polarizability of colliding molecules. In the temperature range 433–513 K, we obtained an inverse temperature dependence of the V–V-transfer efficiency, which is satisfactorily described by empirical relations taking into account long-range attractive forces. The results obtained point to the determining role of long-range attractive forces in quasi-resonance V–V-transfer of vibrational energy by molecules excited in vibrational quasi-continuum.     

Photophysical Processes in the Gas Phase at High Levels of Vibrational Excitation of Triplet Molecules of Benzophenone and Fluorenone

By the delayed fluorescence activated by direct multiphoton excitation of triplet molecules by CO₂–laser radiation we have studied the prevailing deactivation pathways of triplet molecules with a high store of vibrational energy E _vib. The dependences of the kinetic characteristics of delayed fluorescence on the presence of vapors and foreign gases have been used to estimate the rates and efficiencies of intermolecular vibrational relaxation in the vibrational quasi–continuum of the triplet state T ₁. By the changes in the intensities and decay rates over a wide range of vibrational energies we have established the E _vib dependences of reversible intercombination conversion between the states T ₁ and S ₁ and interconversion from T ₁ to the ground electronic state S ₀ for both the case of isolated excited molecules and at a steady vibrational temperature. It is shown that at high vibrational temperatures the radiationless transition from the T ₁ state to S ₀ has an activation character and is accomplished through the energy barrier. The conditions for going to an exponential dependence have been determined. It has been found that the obtained dependences are in good agreement with the known experimental results. The influence of molecular and environmental characteristics on the decay rate of triplet molecules is compared.     

Accurate studies on the full vibrational energy spectra and molecular dissociation energies for some electronic states of N_2 molecule

1 IntroductionAccurate results of high-lying vibrational energies and molecular dissociation energy De of diatomic electronic states are very important for thermodynamics, molecular spec- troscopy, reactive scatterings, and the collision physics of ultracold atoms. For example, the binding energy of the highest bound vibrational state to the ground vibrational state determines the s-wave scattering length. This in turn determines the low- energy (pre- dominantly s-wave) atomic elastic-scatteri…     

Na2分子部分电子态的完全振动能谱和离解能的精确研究

对于大多数双原子分子的电子态，用现代实验方法或精确的量子理论方法往往可以获得含m个振动能级的能谱子集合［Ev］，而不易得到包含最高振动能级在内的所有高振动量子态能级的完全振动能谱{Ev}.鉴于Na2分子电子态的振动能谱和分子离解能De在实际研究和应用中的重要性，使用基于微扰理论的代数方法（AM），获得了Na2分子一些电子态的振动光谱常数和完全振动能谱；使用基于AM的代数能量方法（AEM）获得了这些电子态的正确离解能.研究结果表明：AM方法能从少数精确的实验能级获得精确的分子振动光谱常数集合和正确的完全振动能谱{Ev}，AEM方法获得的分子离解能比由文献发表的振动光谱常数计算得到的近似离解能值更准确，对于难以获得分子离解能的那些电子激发态，AEM方法能给出合理的离解能数据. 关键词： Na2分子 代数方法 振动能级 离解能 电子激发态     

Sum rules for radiative quarkonium decays from short-range interactions

Sum rules previously derived for proton decay are extended and applied to treat effects of bound states spectroscopy on radiative quarkonium decays. The transition involves boson (photon or pion) emission followed by quark annihilation. Sum rules for the contributions from different intermediate bound states are derived by using closure and the assumptions (1) that the boson emission is described by a plane wave or multipole operator which satisfies a wave equation, and (2) that the annihilation depends on the bound state wave function or its derivative at the origin.     

Formation of ultracold RbCs molecules by photoassociation

The formation of ultracold metastable RbCs molecules is observed in a double species magneto-optical trap through photoassociation below the ⁸⁵Rb(5S_1/2) + ¹³³Cs(6P_3/2) dissociation limit followed by spontaneous emission. The molecules are detected by resonance enhanced two-photon ionization. Using accurate quantum chemistry calculations of the potential energy curves and transition dipole moment, we interpret the observed photoassociation process as occurring at short internuclear distance, in contrast with most previous cold atom photoassociation studies. The vibrational levels excited by photoassociation belong to the 5th 0⁺ or the 4th 0^? electronic states correlated to the Rb(5P_{1/2, 3/2}) + Cs(6S_1/2) dissociation limit. The computed vibrational distribution of the produced molecules shows that they are stabilized in deeply bound vibrational states of the lowest triplet state. We also predict that a noticeable fraction of molecules is produced in the lowest level of the electronic ground state.     

Impact of the Vibrationally Excited States on the Macroscopic Behaviour of the Band-like Electron Beam Discharge Plasma in H/H2-Mixture

The investigation of the impact of the vibrationally excited molecules in the electronic ground state was performed by simultaneously solving a balance equation system for the main charge carriers, the H atoms, the metastable H atoms, the H₂ molecules in the different vibrational states and for the power transfer of the electrons in the beam discharge mixture plasma. The balance equations for the vibrational states include in particular one-quantum step excitation and deexcitation, electronic excitation, dissociation and ionization from each vibrational level in electron collisions as well as the finite life time of these states because of the gas transfer through the band-like plasma. A main finding is that due to the additional impact of vibrationally excited molecules there is a marked enhancement of the resulting dissociation and ionization degree in the beam discharge plasma at medium power input from the turbulent electric field. For discharge parameters of practical interest the ionization and dissociation budget, the population of the vibrational states, the different energy dissipation processes and the energy pumping into the ladder of the vibrational states were calculated and discussed in detail.     

Kinetics of CO Molecules Taking into Account Resonant VE Exchanges in a Nonequilibrium Nozzle Flow

The kinetics of CO molecules with excited electronic levels for a nonequilibrium flow in nozzles is studied. Three electronic energy levels are taken into account; the VV exchanges of the vibrational energy within each electronic energy level, VT transitions from the vibrational energy to the translational energy, and VE exchanges between the vibrational energy and the electronic terms occur in the system. The system of equations of vibrational level kinetics and gasdynamics, which is written in the zeroth approximation of the Chapman–Enskog method, is solved numerically. Special attention is paid to estimating the effect of the VE exchanges of the vibrational energy on the formation of nonequilibrium populations of the vibrational levels. It is shown that resonant VE transitions considerably affect the relaxation process and noticeably change the population of the upper vibrational levels even in the absence of recombination and dissociation processes; VE exchanges also affect the time dependence of the vibrational temperature.