Vibrational-rotational behavior of diatomic heteronuclear molecular systems in intense laser pulses

The dynamics of diatomic heteronuclear molecules in the low-frequency intense laser pulses is studied by numerical solution of the time-dependent Schr?dinger equation and both rotational and vibrational degrees of freedom are taken into account. The interference stabilization against the dissociation process is found to take place in a strong field and is shown to result in trapping of population in different rotational-vibrational bound states due to efficient Raman V- and ??-type transitions. The interplay between the vibrational and rotational dynamics of a molecule is investigated. The dissociation suppression due to the interference mechanism and all its attributes are established in the case of multiphoton coupling between the initial state and dissociation continuum.     

Rotational and vibrational excitation of sputtered diatomic molecules

The rotational and vibrational level population distributions of ground and electronically excited states of several homonuclear and heteronuclear molecules are calculated for direct ejection, association, and associative ionization mechanisms of molecule sputtering. The cascade properties, formation mechanism, initial atom separation, and axis orientation to the surface are clearly reflected in the sputtered molecule distributions. In general, extensive non-Boltzmann rotational and vibrational distributions are predicted. The probability that an atom pair survives the ejection, for typical sputtering parameters, is high; approximately 50% for homonuclear molecules to near 100% for hydrides. The predictions are compared with experimental optical emission spectra in the accompanying paper.     

Dissipation of vortex disturbances in a vibrationally nonequilibrium diatomic gas

The influence of the vibrational relaxation on suppression of the Kelvin??Helmholtz instability in an evolving shear layer of a vibrationally nonequilibrium diatomic gas is studied numerically on the basis of equations of two-temperature aerohydrodynamics. Planar waves with the maximum growth rates, which were computed within the framework of a linearized system of equations of inviscid two-temperature gas dynamics, are used as the initial disturbances. It is shown that relaxation of the nonequilibrium vibrational mode at excitation levels, which can be obtained in diatomic gases in nozzle flows, in underexpanded jets, or in flows with moderate laser pumping, is accompanied by noticeable suppression of vortex disturbances. The associated relative enhancement of dissipation of kinetic energy of a large vortex structure averaged over its ??lifetime?? reaches approximately 13 %.     

Dissociation of adsorbed diatomic molecules: Combined effects of the surface magnetic field,a laser and the phonon continuum

Effects of the surface magnetic field (SMF), a laser and admolecule-phonon coupling on the dissociation of adsorbed diatomic molecules are studied theoretically. It is observed that the laser photon-dressing and the SMF-splitting of the molecular states give rise to a multiwell configuration, and the dissociation rate of a particular vibrational state in this configuration is altered as compared to the gas-phase laser-induced process. The presence of the phonon continuum also affects dissociation by dampening local transition probabilities at avoided crossings. Dissociation rates as function of laser power densities for a few specific systems are derived with a phonon-free situation and also with the presence of the phonon continuum, and the extent of phonon effects are analyzed.     

Coherent artifact as a tool for probing vibrational distribution in molecules excited by femtosecond IR laser pulse

We suggest a new method of probing vibrational distribution in a mode of a molecule pumped by an intense resonant infrared laser pulse, and its change in course of intramolecular vibrational redistribution. The method is based on the so-called coherent artifact attending to the pump-probe experiments. This effect is treated analytically for the anharmonic oscillator, and also numerically for a realistic spectroscopic model of a molecular mode. It is shown that the artifact signal is very sensitive to the distribution properties, in particular, the mean energy in the mode. Possible applications of the suggested method are discussed, namely for studies of intramolecular vibrational redistribution that are important for the femtosecond laser chemistry, and also as a part of feedback in laser control schemes for optimization of the laser pulse shape.     

Electric flux distribution in photodetachment of heteronuclear diatomic molecular negative ion

The photodetachment of a hetero-nuclear diatomic molecular negative ion is studied by using a two-centre model. An analytic formula is presented for the electron flux distribution of a heteronuclear diatomic molecular negative ion. Taking HF^- as an example, we calculated the electron flux distributions of this ion for various detached electron energies. The results show that the electron flux distributions exhibit oscillatory structures, which are caused by the interference effect between the two nuclei. Besides, the laser light polarization also has a great influence on the electron flux distribution. The oscillation amplitude is the largest when the laser polarization is parallel to the z-axis; when the laser polarization is perpendicular to the z-axis, the oscillation almost vanishes. This study provides a new understanding of the photodetachment of a heteronuclear diatomic molecular negative ion.     

Algebraic approach to thermodynamic properties of diatomic molecules

A simple model extending Lie algebraic techniques is applied to the analysis of thermodynamic vibrational properties of diatomic molecules. Local anharmonic effects are described by means of a Morse-like potential and the corresponding anharmonic bosons are associated with the SU(2) algebra. The total number of anharmonic bosons, fixed by the potential shape, is determined for a large number of diatomic molecules. A vibrational high-temperature partition function and the related thermodynamic functions are derived and studied in terms of the parameters of the model. The idea of a critical temperature is introduced in relation to the specific heat. A physical interpretation in terms of a quantum deformation associated with the model is given.     

On the C-X system of diatomic mercury chloride

The C-X system of diatomic mercury chloride has been re-investigated in emission. The previous vibrational assignments of this system by Horne and others have been confirmed by measurement of the chlorine isotopic shift and improved vibrational constants for the ‘C’ state determined.     

Theoretical simulation of the photoassociation process for NaCs

We investigate the two-step association process of NaCs using the time-dependent wave packet method.Ground state atoms can be photoassociated to the low vibrational levels of the ground state for an NaCs molecule by the two-step association.The time-dependent Schro¨dinger equation of the association process is solved within a three-state model and the wave packet is propagated with the "split operator-Fourier transform" scheme and the rotating-wave approximation(RWA).The vibrational population distribution of the ground state can be obtained by projecting the wave packet to every vibrational level of the ground state.The results not only show that for NaCs achievement of photoassociation production is accompanied by the photodissociation of the higher vibrational molecules,but also show that the vibrational distribution in lower vibrational levels of the ground state changes with the laser parameters.     

Spatial and time resolved analysis of CN bands in the laser induced plasma from graphite

Analysis of the emission bands of the CN molecules in the plasma generated from a graphite target irradiated with 1.06 μm radiation pulses from a Q-switched Nd:YAG laser has been done. Depending on the position of the sampled volume of the plasma plume, the intensity distribution in the emission spectra is found to change drastically. The vibrational temperature and population distribution in the different vibrational levels have been studied as function of distance from the target for different time delays with respect to the incidence of the laser pulse. The translational temperature calculated from time of flight is found to be higher than the observed vibrational temperature for CN molecules and the reason for this is explained.     

Coulomb asymmetry in strong field multielectron ionization of diatomic molecules

We measure the angular distribution of an electron emitted by a strong elliptically polarized two-color laser field from exploding doubly charged molecular nitrogen. This angular distribution is vastly different for emission of the electron from the up-field core of the molecule as compared to that from the down-field core. The emission from the down-field core leads to a slight rotation with respect to the internuclear axis in the direction expected by the Coulomb effect of the remaining ion, while, for the emission from the up-field core, this direction is inversed. Our semiclassical simulations suggest that this unexpected angular distribution is caused by an initial longitudinal momentum of the electron freed by over-the-barrier ionization above the inner barrier in the molecule. The initial kinetic energy is in the range of the potential energy of the Stark-shifted orbital above the barrier.     

Reflection and interference structure in diatomic Franck-Condon distributions

The Franck-Condon distributions for diatomic radiative transitions from a single vibrational level of a given electronic state to all possible levels (bound and free) of a second electronic state exhibit either “reflection” or “interference” structure. In reflection structure there is a one-to-one mapping of peaks in the initial state probability distribution into peaks in the spectrum. No such simple relationship is known for interference structure, originally termed “internal diffraction” by Condon [Phys. Rev.32, 858–872 (1928)]. A semiclassical treatment of the quantum mechanical overlap integrals shows that the condition for reflection structure is a monotonic difference potential in the range of internuclear distance sampled by the initial wavefunction, whereas interference structure occurs when a “polytonic” difference potential is sampled. The qualitative validity of the semiclassical treatment is illustrated through quantum calculations which show that the Franck-Condon integrals accumulate near the points of stationary phase.     

A minimum covariance basis for diatomic vibrational constants

An alternative semianalytical representation of diatomic vibrational energy levels to the Dunham expansion, is presented in the form of an expansion over Newton polynomials. This latter representation produces smaller variances and correlation coefficients than Dunham's representation, especially for light molecules. The method is illustrated by application to the vibrational levels of the ground state of the H₂ molecule. Extension of the method to vibrational-rotational levels is discussed briefly.     

Photoassociation of NaRb with an asymmetric laser pulse

We investigate the photoassociation dynamics of cold NaRb molecule controlled by an asymmetric laser pulse called slowly-turned-on and rapidly-turned-off (STRT) laser pulse. This new shaped laser pulse has a remarkable merit, compared with the typical Gauss-type pulses, so that we can efficiently associate molecules with the state expected instead of going back to the continuum state. Using the three-state model, we solve the quantum mechanical equation with the “split operator-Fourier transform” method under the rotating-wave approximation (RWA) in propagation of the wave packet. By the projection of the obtained wave function onto each vibrational state, we can get the vibrational population of the ground electronic state. The results reveal that, with the STRT laser pulse, an efficient photoassociation process can be achieved and the vibrational distribution in the ground state can be controlled by the laser parameters.     

Analytical solution of the inverse spectroscopic problem: Determination of electron moments of vibrational transitions of diatomic molecules by their probabilities

The inverse problem of spectroscopy of vibrational transitions of diatomic molecules was analytically solved on the basis of wave functions of the Morse oscillator. An example of the determination of the transition dipole-moment function by intensities of several lines of molecule vibrational-rotational spectra measured experimentally was considered. A similar approach based on simple expressions for matrix elements can be applied to consider optimal lasing conditions of the CO electronic transition laser.     

用可变阶数的代数方法研究双原子体系的完全振动能谱和离解能

不同双原子分子电子态的势能(或振动能谱)的展开性质可能不同,文章将固定阶数的代数方法(AM)改进为可变阶数的代数方法,使得该方法可以研究各种不同性质(不同能量展开阶数)的双原子分子电子态,也可以解决光谱计算中可能出现的"蝴蝶效应"问题。利用阶数可变的AM方法研究了原来固定阶数的AM方法难以给出正确结果的N2-a′1Σu-,Li2+-2 2Σg+,4 He D+-X1Σ+和39 K85 Rb-(2)3Σ+等不同双核体系的完全振动能谱与离解能,不但得到了与实验数据精确相符的理论结果,还正确地预言了许多由于实验条件与技术原因而未能测得的物理数据。研究表明阶数可变的AM方法能够更广泛地用于研究各类双核电子态体系的完全振动能谱和体系离解能。     

Normal modes of oscillations of one-dimensional monoatomic and diatomic lattices

A polynomial equation is obtained for the solutions of the vibrational frequencies of one-dimensional monoatomic and diatomic lattices with particles connected by identical springs, but with arbitrary springs connecting the end particles to rigid walls. The exact expressions of the different normal modes of oscillations of the linear chain of particles for monoatomic, diatomic and defective lattices are derived in a straightforward way. As special cases of our problem we have considered the effects of different end springs on the vibrational frequencies. One interesting result is that very high frequencies are allowed when the ends of the diatomic lattice are rigidly fixed with the boundary walls.     

Highest rotational and vibrational excitation of swift diatomic molecules scattered by solid surface

It is shown that at nondissociative scattering of swift molecules by solid surface and inverse population of the highest rotational and some vibrational states unattainable at thermal excitation could be achieved. For single crystal surface a specific polarization and orientational effects in rotational and vibrational excitation of the scattered molecules could be observed.     

双原子分子振动能谱与离解能的精确研究

代数方法(AM)的建立解决了实验方法和精确量子力学理论方法难以获得双原子分子的包含最高振动能级在内的所有高阶振动能级的精确数值这一问题.基于LeRoy与Bernstein的工作,孙卫国等又建立了精确计算双原子分子离解能的新解析表达式.应用新公式和代数方法(AM),研究了一些双原子分子部分电子态的振动能谱和分子离解能,获得了与实验值符合非常好的理论结果.该方法在理论上提供了获得双原子分子完全振动能谱和精确分子离解能的物理新方法.     

双原子离子振动光谱与离解能的研究

结合代数方法(AM)和最近建立的计算精确的分子离解能的新解析表达式,进一步研究了部分双原子离子电子态的完全振动能谱和离解能,获得了与实验值符合得非常好的理论结果.研究结果表明,AM方法和新解析式相结合的理论方法同样也适用于双原子离子体系,该方法在理论上提供了获得双原子离子精确的振动光谱和离解能的物理新方法.