Nonadiabatic Multielectron Dynamics in Strong Field Molecular Ionization

We report the observation of a general strong field ionization mechanism due to highly nonadiabatic multielectron excitation dynamics in polyatomic molecules. We observe that such excitation mechanisms greatly affect molecular ionization, fragmentation, and energetics. We characterized this phenomenon as a function of optical frequency, intensity, and molecular properties.     

Sequential decay involving multiple continua

We have utilized the Green's function method to derive an explicit solution for the problem of sequential decay involving multiple continua with constant coupling between adjacent continua. This model system is applicable for theoretical studies of dynamics of photodissociation, predissociation and electronic quenching of polyatomic molecules.     

Controlling electronic spin relaxation of cold molecules with electric fields

We present a theoretical study of atom-molecule collisions in superimposed electric and magnetic fields and show that dynamics of electronic spin relaxation in molecules at temperatures below 0.5 K can be manipulated by varying the strength and the relative orientation of the applied fields. The mechanism of electric field control of Zeeman transitions is based on an intricate interplay between intramolecular spin-rotation couplings and molecule-field interactions. We suggest that electric fields may affect chemical reactions through inducing nonadiabatic spin transitions and facilitate evaporative cooling of molecules in a magnetic trap.     

Elementary sequential spectral line and statistical properties of the thermal reservoir of free polyatomic molecules

The formation of vibronic spectra of free polyatomic molecules is studied by taking the concept of sequences as constituent elements of vibronic bands. Statistical properties of the thermal reservoir of oscillators, which randomly perturbs optically active transitions in a polyatomic molecule, are considered. The spectral shape, position, and width of the elementary sequential lines (ESL) are determined. It is shown that the ESLs have a Lorentzian shape if the perturbations are short-term and the pulse shape is symmetric about its origin. The position of the ESLs depends on the total vibrational energy of the thermal reservoir of oscillators in the initial electronic state of a molecule. The analysis of the statistical properties of the thermal reservoir of polyatomic molecules shows that one should distinguish between the ESL linewidth of individual molecules and the ESL linewidth averaged over the entire thermal ensemble. It is shown that individual molecules of the thermal ensemble that have different total vibrational energies are characterized by different widths of the ESLs. Consequently, the exchange and redistribution dynamics of the vibrational energy stored in the initial electronic state should manifest itself in the difference between the single-molecule ESL widths measured at different instants in time.     

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S ₀ the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO₂ laser energy density and vapor P _v and foreign gas pressure P _FG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy E _vib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.     

Roles of classical dynamics and quantum dynamics on activated processes occurring in liquids

This paper reviews the reactive flux correlation function approach to studying the classical dynamics of activated processes in liquids. The possibilities and consequences of nonadiabatic electronic transitions in affecting this dynamics is also considered. We emphasize the feasibility of quantitative trajectory studies and the fact that these studies have yet to be fully exploited in the development of approximate theories of activated processes.     

Light-assisted ion-neutral reactive processes in the cold regime: radiative molecule formation versus charge exchange

We present a combined experimental and theoretical study of cold reactive collisions between laser-cooled Ca+ ions and Rb atoms in an ion-atom hybrid trap. We observe rich chemical dynamics which are interpreted in terms of nonadiabatic and radiative charge exchange as well as radiative molecule formation using high-level electronic structure calculations. We study the role of light-assisted processes and show that the efficiency of the dominant chemical pathways is considerably enhanced in excited reaction channels. Our results illustrate the importance of radiative and nonradiative processes for the cold chemistry occurring in ion-atom hybrid traps.     

用飞秒离子产率谱和光电子影像解析振动波包动力学

时间分辨光电离是揭示多原子分子激发态动力学的一种强有力的实验方法.根据收集信号的种类,可以采用不同的测量方法:时间分辨离子产率谱(TR-IYS)和时间分辨光电子成像(TR-PEI).本文综述介绍了光电离测量与电子结构的基本概念,以及在实验上区分不同几何结构之间的振荡波包运动的几个重要研究工作,并举例说明飞秒TR-IYS和TR-PEI是如何被用来探测激发态势能面上相干振动波包的演化过程.     

Nonadiabatic quasiparticle dynamics in time resolved electron spectroscopies of surface bands

Surface localized electronic states constitute electronic environment for a variety of physical and chemical phenomena taking place on surfaces. Various processes of model catalytic reactions may be influenced or mediated by hot electrons and holes excited in quasi-two-dimensional bands occurring on a large number of metal surfaces. Here we discuss several important aspects of nonadiabatic dynamics of these excitations which may affect the measurements of surface electronic properties by ultrafast electron spectroscopies.     

Coulomb explosion of large polyatomic molecules assisted by nonadiabatic charge localization

The electron-nuclear dynamics of the Coulomb explosion of a large polyatomic molecule, anthracene, is probed using kinetic energy distributions of produced H+ ions. The kinetic energy release of ejected protons exceeds 30 eV for anthracene exposed to 10(14) W cm(-2), 800 nm pulses of 60 fs duration. We propose a strong-field charge localization model, based on nonadiabatic dynamics of charge distribution in a (multiply) ionized molecule; the charge localization lasts many laser periods and is sustained through successive ionizations of the molecular ion. The model explains quantitatively the dependence of the H+ kinetic energy on the laser intensity. Dissociative ionization of a polyatomic molecule enabled by long-lived charge localization is a new type of electron-nuclear dynamics and is essential for understanding the pathways of molecular or ionic fragmentation in strong fields.     

A combined theoretical and experimental study on molecular photonics

A detailed account of quantum chemical procedures for estimating the rate constants of the photophysical processes at work in polyatomic organic molecules is given. The results obtained from combined experimental and theoretical research into the spectral-luminescent properties of acridine, 9-aminoacridine, 2,7-dimethyl-9-diphenylaminoacridine, and of their protonated forms are reported. The electronic absorption and fluorescence spectra of acridine have been investigated at room temperature in ethanol solution of varying pH and in other solvents of different chemical nature and polarity. The energy of excited states, the rate constants of the deactivation of the excited states, and the dipole moments obtained by quantum chemical methods for the examined compounds are presented. The findings of the investigations are discussed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 93–107, October, 2008.     

Quantum-mechanical description of the intensity distribution in two-photon absorption spectra of dihalogenated benzenes

Using molecules of dihalogenated benzenes as an example, the verification of an earlier proposed method of direct quantum-mechanical calculation of the relative intensities of lines in the two-photon absorption spectra of polyatomic molecules is continued. The method of calculation is based on the adiabatic model in the context of the Herzberg-Teller approximation. A comparative analysis of the intensity distribution in the two-photon absorption spectra of dihalogenated and monosubstituted benzenes is carried out, and the vibrational characteristics of these molecules in the lowest singlet excited electronic states are determined. A satisfactory agreement with experimental values, along with the analysis of the possibilities of the method, confirms the expediency of its use for gaining information about the intensity distribution in two-photon absorption spectra of polyatomic molecules.     

Voronoi-cell finite difference method for accurate electronic structure calculation of polyatomic molecules on unstructured grids

We introduce a new numerical grid-based method on unstructured grids in the three-dimensional real-space to investigate the electronic structure of polyatomic molecules. The Voronoi-cell finite difference (VFD) method realizes a discrete Laplacian operator based on Voronoi cells and their natural neighbors, featuring high adaptivity and simplicity. To resolve multicenter Coulomb singularity in all-electron calculations of polyatomic molecules, this method utilizes highly adaptive molecular grids which consist of spherical atomic grids. It provides accurate and efficient solutions for the Schrödinger equation and the Poisson equation with the all-electron Coulomb potentials regardless of the coordinate system and the molecular symmetry. For numerical examples, we assess accuracy of the VFD method for electronic structures of one-electron polyatomic systems, and apply the method to the density-functional theory for many-electron polyatomic molecules.     

Higher Electric Multipole Moments for Some Polyatomic Molecules from Accurate SCF Calculations

Higher electric multipole moments for the ground-state electronic configuration of some polyatomic molecules, i.e. CH₄, NH₃, H₂O, were calculated from SCF-HFR wavefunctions using Slater-type orbital basis sets. The calculated results for electric multipole moments of these molecules are in good agreement with the theoretical and experimental ones.     

飞秒时间分辨质谱和光电子影像对分子激发态动力学的研究

分子量子态的研究,特别是分子激发态演化过程的研究不仅可以了解分子量子态的基本特性和量子态之间的相互作用,而且可以了解化学反应过程和反应通道间的相互作用.飞秒时间分辨质谱和光电子影像是将飞秒抽运-探测分别与飞行时间质谱和光电子影像相结合的超快谱学方法,为实现分子内部量子态探测,研究分子量子态相互作用及超快动力学过程提供了强有力的工具,可以在飞秒时间尺度下研究单分子反应过程中的光物理或光化学机理.本文详细介绍了飞秒时间分辨质谱和光电子影像的技术原理,并结合本课题组的工作,展示了这两种方法在量子态探测及相互作用研究领域,特别是激发态电子退相、波包演化、能量转移、分子光解动力学以及分子激发态结构动力学研究中的广泛应用.最后,对该技术的发展前景以及进一步的研究工作和方向进行了展望.     

在ADC(2)水平上通过从头算非绝热动力学方法模拟硝酸甲酯的光解

在ADC(2)水平上通过轨线面跳跃方法模拟了硝酸甲酯的非绝热动力学. 结果证实该体系存在快速的非绝热动力学过程,导致了体系回到电子基态. 当动力学从S₁和S₂电子态开始时,光解产物是CH₃O+NO₂,这个发现与实验研究的结果以及更高精度的XMS-CASPT2水平上模拟出的结果一致. 在ADC(2)水平上,当动力学从S₃态开始时,光解产物依然是CH₃O+NO₂. 该研究表明：ADC(2)方法可用于研究硝酸甲酯在长波下的光解机理,然而无法用于理解其在短波段下的光解动力学. 本文为在ADC(2)水平上处理类似化合物的光诱导过程提供了有价值的信息.     

Intense interactions of molecules with a short-wavelength electromagnetic radiation field: II. Resonance scattering of radiation and fluorescence

Within the framework of the nonadiabatic approach developed in the preceding paper, the resonance scattering, resonance Raman scattering, and resonance fluorescence are studied in detail for diatomic and triatomic molecules, and polyatomic symmetric and antisymmetric top molecules, which interact with the field of short-wavelength radiation with a wavelength λ ≥ Å and an intensity up to 10¹⁴ W/cm². The coherent excitations of high-lying Rydberg and autoionizing states are taken into account. Analytical expressions for calculating the tensors and cross sections of the above processes are derived.     

Characteristic features of electronic and vibrational spectra of free polyatomic molecules

The functional relationships between the basic characteristics of individual bands of electronic and vibrational spectra of free polyatomic molecules are ascertained. These basic characteristics are the position of the maximum of an individual electronic-vibrational band, the rate of the temperature shift of the maximum of a band, the half-width of a homogeneously or heterogeneously broadened electronic-vibrational band, the rate of the temperature variation of the half-width of a band, the asymmetry parameter of a heterogeneously broadened band, and the conditions for symmetric or asymmetric distribution of the intensity over the contour of a spectral band. The interrelations between the basic spectral characteristics result from the sequential mechanism of formation and broadening of individual bands of electronic and vibrational spectra of free polyatomic molecules. It is shown that the investigation of the basic spectral characteristics of electronic-vibrational bands and their interrelations allows one to elucidate the nature of the internal motion determining the optical properties of polyatomic molecules.