Excited States Of Polyatomic Molecules Research Articles

Линейный дихроизм и двулучепреломление зондирующего излучения в спектроскопии накачка-зондирование в многоатомных молекулах

Probe beam dichroism and birefringency occurring in the excited states of polyatomic molecules under excitation with two femtosecond laser pulses have been theoreticaly studied as a function of delay between the pulses. General expressions describing the change of intensity and polarization of the probe laser pulse passed through the solution of arbitrary polyatomic molecules at any initital polarization of the laser pulses have been derived using the spherical tensor approach. The expressions take into account the coherence in excited molecule vibrational states and decay of these states due to vibrational relaxation, rotational diffusion, and radiative transitions. The expressions describe the effects of probe beam linear dichroism and birefringency occurring in molecular excited states. As shown, under certain conditions both effects can be observed simultaneously. It has been concluded that within the geometry of almost collinear propagation of the pump and probe laser pulses through the molecular sample the contributions from linear dichroism and birefringency can be completely separated from each other by an appropriate choice of the polarization analyzer placed in front of the photodetector. The obtained expressions were applied for description of the signals that can be recorded experimentally by means of the polarization-modulation scheme developed in the recent author's publication (Gorbunova et al,Phys. Chem. Chem. Phys. 2020, Vol. 22, 18155−18168). The theory predicts that the modulated signals of dichroism and birefringency appear in quadrature with respect to the double frequency reference modulation signal.

Observation of Anisotropic Relaxation in Biological Molecules with Subpicosecond Temporal Resolution

A new pump–probe technique has been developed for investigation of anisotropic relaxation and energy transfer processes in excited states of polyatomic molecules excited by femtosecond laser pulses. The proposed method was applied for studying the rotational diffusion of NADH with a temporal resolution of about 0.6 ps. Linear dichroism in excited states of biological molecules induced by laser pulses with energy below 1 nJ has been detected for the first time.

Достижение субпикосекундного временного разрешения при исследовании процессов анизотропной релаксации биологических молекул

A novel pump-probe method has been developed to study anisotropic relaxation and energy transfer in excited states of polyatomic molecules excited by femtosecond laser pulses. The method was used to study the rotational diffusion of NADH with a temporal resolution of about 0.6 ps. For the first time, absorption from the excited state of biological molecules pumped by laser pulses with energies of 1 nJ was detected

Excited state dynamics of molecules studied with femtosecond time-resolved mass spectrometry and photoelectron imaging

Study of quantum states of molecules, especially the evolution of excited states can help to understand their basic features and the interactions among different states. Furthermore, the information about the chemical reaction process and the interactions among several reaction channels can be obtained. Femtosecond time-resolved mass spectrometry (TRMS) and time-resolved photoelectron imaging (TRPEI), which combine pump-probe technique with time of flight mass spectrometry and photoelectron imaging, are powerful tools for detecting the molecular quantum state and for studying the molecular quantum state interaction and molecular ultrafast dynamics. With these methods, the photochemistry and photophysics mechanism of isolated molecule reaction process can be investigated on a femtosecond time scale. The principles of TRMS and TRPEI are introduced here in detail. On the basis of substantial research achievements in our group, the applications of TRMS and TRPEI are presented in the study of ultrafast internal conversion and intersystem crossing, wavepacket evolution dynamics at excited states of polyatomic molecules, energy transfer process of polyatomic molecules, ultrafast photodissociation dynamics and structural evolution dynamics of molecular excited states. In the study of ultrafast internal conversion and intersystem crossing, the methyl substituted benzene derivatives and benzene halides are discussed as typical molecular systems. In the study of wavepacket evolution dynamics at excited states of polyatomic molecules, the real-time visualization of the dynamic evolution of CS2 4d and 6s Rydberg wave packet components, the vibrational wave packet dynamics in electronically excited pyrimidine, the rotational wave packet revivals and field-free alignment in excited o-dichlorobenzene are reported. In order to discuss the energy transfer process of polyatomic molecules, the intramolecular vibrational energy redisctribution between different vibrational states in p-difluorobenzene in the S1 low-energy regime and the intramolecular energy transfer between different electronic states in excited cyclopentanone are presented. For the study of ultrafast photodissociation dynamics, the dissociation constants and dynamics of the A band and even higher Rydberg states are investigated for the iodine alkanes and iodine cycloalkanes. Structural evolution dynamics of molecular excited states is the main focus of our recent research. The structural evolution dynamics can be extracted from the coherent superposition preparation of quantum states and the observation of quantum beat phenomenon, by taking 2, 4-difluorophenol and o-fluorophenol as examples. Time-dependent photoelectron peaks originating from the planar and nonplanar geometries in the first excited state in 2, 4-difluorophenol exhibit the clear beats with similar periodicities but a phase shift of π rad, offering an unambiguous picture of the oscillating nuclear motion between the planar geometry and the nonplanar minimum. Also, the structural evolution dynamics in o-fluorophenol via the butterfly vibration between planar geometry and nonplanar minimum is mapped directly. Finally, the potential developments and further possible research work and future directions of these techniques and researches are prospected.

Optical control of nonradiative decay in polyatomic molecules

We propose an approach for elimination of radiationless transitions in the excited states of polyatomic molecules. Our method is based on the concept of electronically localized eigenstates of coupled vibronic Hamiltonians---the occurrence of stationary eigenfunctions of essentially pure electronic character in the (otherwise strongly mixed) spectra of excited manifold Hamiltonians of systems undergoing nonradiative decay. The origin of this phenomenon and its generality are explained within the framework of the theory of scars of unstable periodic orbits. Optimal control theory is combined with the notion of electronically localized states to prepare the system in an excited superposition that is immune to nonradiative decay. The approach is applied to suppress ultrafast radiationless transitions in two model systems that differ widely in coupling strength and electronic structure.

The Construction of Semi-diabatic Potential Energy Surfaces of Excited States for Use in Excited State AIMD Studies by the Equation-of-Motion Coupled-Cluster Method

The semi-diabatic potential energy surfaces (PESs) of the excited states of polyatomic molecules can be constructed for use in ab initio molecular dynamics (AIMD) studies by relying on the continuity of the electronic energy, oscillator strength, and spherical extent of an excited state along with first derivatives of these quantities as computed by using the equation-of-motion coupled-cluster (EOM-CC) method. The semidiabatic PESs of both the π → <TEX>$π^*$</TEX> valence excited state and the 3s-type Rydberg state of ethylene are presented and discussed in this paper, in conjunction with some of the AIMD results we obtained for these states.

分子の内殻励起と分子イオンのダイナミクス

Inner-shell excitation and ionization of linear molecules using synchrotron radiation are discussed and explained on a basis of recent outcome by several groups. Symmetry of excited states has been precisely investigated using angular distributions of produced fragment ions as well as highly energy-resolved photoabsorption spectra in consideration of theoretical prediction. It has been found that angular distributions of ion-ion coincidence signals are powerful for clarifying structural change in the inner-shell excited states of polyatomic molecules. These findings originate from fast decomposition processes following inner-shell excitation. The inner-shell hole is filled through Auger electron emission, and this emission is closely examined in these years because the Auger initial state can be clearly specified using intense monochromatic soft X-rays. Interference phenomenon between vibrational motion and core-hole lifetime has been observed in de-excitation spectra of diatomic molecules. Dissociation mechanism has been discussed using branching ratios and kinetic energies of fragment ions, and an Auger-electron fragment-ion coincidence technique is highly efficient for identifying excited molecular ion states before decomposition. A mapping technique of flight time of two fragment ions is utilized for clarifying correlation in kinetic energies and angular distributions among dissociation products in polyatomic molecules. Effect of vibrational excitation on dissociation is being studied through a progress in a brilliant and highly monochromatic soft X-ray instrument.

Mass identification of negative ions in excimer-laser-irradiated triethylamine: atomic rearrangements in electron attachment to highly excited states

Efficient negative-ion formation via electron attachment to triethylamine (TEA) laser-excited to energies above its ionization potential had been reported; however, the evidence for negative-ion formation was indirect, and the identities of the negative ions were not known. In this Letter, we describe an apparatus where the negative (and positive) ions produced in a gas cell via laser irradiation can be extracted to a time-of-flight mass spectrometer for mass identification and present the results obtained for excimer-laser-irradiated TEA. It is shown that extensive atomic rearrangements can occur in dissociative electron attachment to highly excited states of polyatomic molecules.

Theory of resonance Raman scattering and fluorescence from strongly vibronically coupled excited states of polyatomic molecules

A theoretical description of secondary emission from complex absorption bands of isolated polyatomic molecules is developed. The strong non-Born–Oppenheimer coupling associated with conical intersections of the multidimensional excited-state potential-energy surfaces is included in a fully microscopic manner by solving the time-dependent Schrödinger equation for appropriate model systems incorporating the most relevant electronic states and vibrational modes. The effect of the large number of remaining vibrational modes and of the weaker coupling with additional electronic states is modeled by phenomenological relaxation terms (lifetime broadening and pure dephasing) in the framework of the density-matrix formalism. Explicit eigenstate-free expressions for absorption, resonance Raman, and fluorescence spectra are derived via density-matrix perturbation theory. The computational feasibility of the resulting mixed microscopic/phenomenological theory is demonstrated for a simple three-mode model of the vibronic coupling of the S1(nπ*) and S2(ππ*) states of pyrazine. The effect of excited-state vibronic coupling and ultrafast S2→S1 internal conversion on resonance Raman and fluorescence spectra is analyzed on the basis of these model calculations.

A b i n i t i o studies of excited states of polyatomic molecules including spin-orbit and multiplet effects: The electronic states of UF6

The excited electronic states of UF6 between 3–10 eV are investigated by ab initio configuration interaction (CI) calculations using a relativistic effective core potential (RECP) on the uranium and nonrelativistic ECP’s on the fluorines. Spin-orbit coupling effects are incorporated by diagonalizing an effective one-electron operator over the CI wave functions. This represents one of the first studies of polyatomic molecules to treat both spin-orbit effects and multiplet effects arising from electron–electron interactions. A detailed analysis of the electronic states of UF6 is given both with and without spin-orbit coupling effects. Assignments for the major peaks in the 3–10 eV energy range are given, as aided by recent electron impact studies. Comparisons are made between the present many-electron states and previous one-electron theoretical treatments.

On the Temperature Dependence of Radiationless Electron Transitions. I. The Morse Oscillator Model in case of a Small Value of the Parameter Z

Abstract A number of experimental results have been recently made available on the temperature dependence of lifetimes of excited states of polyatomic molecules in condenced1–6 and gaseous phases. The dependence was shown to be quite substantial in experiments with both fluorescence5,6 and phosphorescence1–4 temperature quenching. The decay-rate constant of electronic excitation is 1/τ′ = β + 1/τ, where b is determined by radiative processes and is but slightly temperature-dependent5,6. As for the rate constant 1/τ of radiationless electronic transitions, it might be the very parameter that governs the temperature dependence of 1/τ.

Configuration Interaction Studies of Ground and Excited States of Polyatomic Molecules II. The Electronic States and Spectrum of Pyrazine

Ab initio SCF and CI treatments of the ground state and n → π* and π → π* excited states of pyrazine, C4H4N2, are reported for the molecule in its ground state equilibrium geometry. In the case of n → π* transitions, the two symmetry combinations of nitrogen lone pair orbitals n1 and n2 are found to correspond to the description n+ ≈ n1 + n2 + λσ and n− ≈ n1 − n2 in which the n+ orbital has the higher orbital energy and is significantly more delocalized than n− by interaction with the pyrazine ring sigma system. This result leads to a significant splitting at the CI level of description of 1.4 eV between the B3u and B2g excited states which are derived from the orbital promotions n+ → π* and n− → π*, respectively. Calculated transition energies to the lowest n → π* excited states B3u3 and B3u1 of 3.56 and 4.22 eV, respectively, are in good agreement with experimental values. A low-lying B1u3(π → π*) state for which there exists indirect experimental evidence is found to occur between the B3u3 and B3u1 states. Also, an allowed transition to the B2u1(π → π*) state is calculated at 5.29 eV, slightly above the observed range of absorption. Numerous other excited states are calculated, but of these only the B1u1(π → π*) and B2u1(π → π*) states can be identified with observed transitions and in these cases the calculated transition energies are too high by approximately 2.5 eV.

Calculation of Wavefunctions for the Excited States of Polyatomic Molecules

A new method for the calculation of the wavefunctions of the excited states of polyatomic molecules is presented. The method is independent of the symmetry of the state under consideration. Sample calculations on a number of molecules have been carried out in the INDO approximation, and the results, which represent a considerable improvement over comparable configuration interaction calculations, are presented.

Configuration Interaction Studies of Ground and Excited States of Polyatomic Molecules. I. The CI Formulation and Studies of Formaldehyde

A configuration interaction (CI) procedure which is designed to produce energies and wavefunctions from relatively large scale computations on ground and excited molecular states is presented based on an orthonormal set of molecular orbitals. The method of generating configurations is referenced to the particular state of interest and involves a two-pass procedure: First, the generation of configurations by single and double excitations from selected important configurations (parents), subject to a threshold criterion, followed by diagonalization of the energy to obtain an initial approximation to the wavefunctions of interest; second, the use of these wavefunctions as parents and a repeat of the generation and diagonalization steps to produce the final CI wavefunction. CI studies of the A21,3 (n→π*), 1,3A1 (π→π*, n→3p), 1B1 (σ→π*, π→3s), and B21 (n→3s) states of formaldehyde are reported for several large Gaussian basis sets of near atomic Hartree–Fock quality based on molecular orbitals determined from ground and excited state SCF treatments and from certain elementary virtual orbital transformations. Calculated Franck–Condon transition energies are in excellent agreement with experimental results for those transitions which have been observed; of particular interest is the occurrence of a n→3p transition on oxygen in the region of the spectrum atound 8 eV.