Stationary vs. singular points in an accelerating FRW cosmology derived from six-dimensional Einstein-Gauss-Bonnet gravity

Six-dimensional Einstein-Gauss-Bonnet gravity (with a linear Gauss-Bonnet term) is investigated. This theory is inspired by basic features of results coming from string and M-theory. Dynamical compactification is carried out and it is seen that a four-dimensional accelerating FRW universe is recovered, when the two-dimensional internal space radius shrinks. A non-perturbative structure of the corresponding theory is identified which has either three or one stable fixed points, depending on the Gauss-Bonnet coupling being positive or negative. A much richer structure than in the case of the perturbative regime of the dynamical compactification recently studied by Andrew, Bolen, and Middleton is exhibited.Comment: 9 pages, 20 figures, accepted in PL

Physical Processes in Star Formation

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00693-8.Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms.Peer reviewedFinal Accepted Versio

Excited singlet and triplet states and of the new photoionization electron-vibrational mechanism in the laser-active molecules

Physical principles of the simulating of structure, spectral-luminescence, and lasing properties of complex N, O, S heteroaromatic molecules by means of the quantum-chemical one-electron approximation models LCAO-MO SCF CI CNDO/S and INDO/S (complete and intermediate neglect of differential overlap, sp-valence basis), PPP/S (Pariser-Parr-Pople σπ* - Approximation) approaches are considered. The principles of control of the molecular structure; the properties of the excited singlet (Si*) and triplet (Ti) states; and the parameters of the S1* → S0, S0 → Sn*, S 1* → Sn*, T1 → T n and T1 → S0 transitions aimed at the selection of organic luminophores and UV-laser-active molecules with preset properties for specific scientific and technical applications are demonstrated. In LCAO-MO CSF CI methods, the wave function of the quantum systems contains the information on each atom and spectral parameters of atoms. The ionization potential and the electron affinity are taken from the gas-phase experiments. A complex multiatomic molecule can be represented as a system of excited electronic states with different spin and orbital structures where the energy of a quantum evolves under the action of intramolecular mechanisms with determine photophysical and photochemical properties of organic molecules

Localization of spin-orbital coupling and transitions in spectra of pump-induced reabsorption in excited singlet and triplet states of laser-active molecules in LCAO MO SCFCIINDO/S models

\This paper considers the relation between the mechanisms behind nonoptical energy deactivation of electron-vibrational excitation associated with inner and spin-orbital conversion, governed by the nonadiabaticity operator (or the operator of spin-orbital coupling) in series of N, O, S azocyclic molecules, which are capable of fluorescing within the range of wavelengths lambda(fl)(max) approximate to 260-460 nm and lasing within the range of wavelengths lambda(osc)(max) approximate to 344-460 nm under different conditions. The semiempirical LCAO MO SCF CI INDO/S method was applied to calculate the energies of singlet and triplet quantum states; matrix elements [S-i* \(H) over cap(S0)\T-f(alpha)] Of spin-orbital coupling; rate constants of radiative decay, intercombination, and inner conversion; fluorescence quantum yields; and cross sections of absorption and stimulated emission. It is demonstrated that, as the number of subsystems in a molecular structure increases, i.e., we pass from mono- to bi- and tricyclic systems, the fluorescence wavelength displays a bathochromic shift from lambda(fl)(max) = 260 to 350 nm, which is accompanied by the increase in the energy of excited states of the n pi* type, the decrease in the energy of pi pi*-type states, the lowering of rate constants of nonoptical excitation deactivation, and the growth of rate constants of radiative decay. It is shown that the inversion of n pi*- and pi pi*-type levels within the range of wavelengths lambda(fl)(max) = 320-330 nm and the growth in the oscillator strength of a fluorescent transition (as well as the oscillator strength of 0-0 transitions) from f(fl)(e) = 0.2 up to f(gl)(e) = 1.0 are accompanied by a separation of selective bands corresponding to fluorescence (maximum gain) and reabsorption induced by optical pump (or a flux of particles) for S-1* --> S-n* and T-1 --> T-n transitions in the optical spectrum. Frequency separation of the bands of stimulated emission and induced active losses in an excited organic substance suggests the existence of molecules with a high gain (which implies, within the framework of the proposed model, that the limiting duration of the leading edge of the pumping pulse allowing the implementation of lasing can be increased). For laser-active molecules, all the excited states of the n pi* type lie above fluorescent states of the pi pi* type, and selective spin-orbital interaction mainly couples high-lying singlet and triplet states. Therefore, such systems are characterized by a high fluorescence quantum yield, gamma = 0.4-1.0, while low active losses in a medium allow one to minimize the threshold pumping energy density required for lasing, which improves the photostability of molecules

The physical principles of simulating the structure and photophysical properties of complex heteroaromatic compounds by means of the LCAO MO SCF methods

Physical principles of the simulation of structure, spectral-luminescence, and lasing properties of complex N, O, and S heteroaromatic molecules by means of the quantum-chemical LCAO MO SCF PPP (Parisier-Parr-Pople approximation) methods within the framework of the CNDO/S and INDO/S (complete and intermediate neglect of differential overlap, sp-valence basis) approaches are considered. The principles of applying quantum mechanics to the computation of spectral parameters of laser molecular systems, such as fluorescence and phosphorescence quantum yields and lifetimes (γfl, τfl, γPh, and τPh), stimulated emission cross sections (σ31osc), cross sections of reabsorption through the T1 → Tn (σ2T*) and S1* → Sn* (σ3s*) transitions at pumping and lasing wavelengths, and the ultimate duration of the pumping pulse when lasing is still possible in a laser active clement, are discussed. The principles of control of the molecular structure; the properties of the excited singlet (Si*) and triplet (Ti) states; and the parameters of the S0 → Sn*, S1* → Sn*, T1 → Tn, and T1 → S0 transitions aimed at the selection of organic luminophores with preset properties for specific scientific and technical applications are demonstrated. Specific examples are considered, and new UV laser-active molecules with improved optical parameters (compared with the catalogued molecules) are reported. Copyright © 1997 by MAIIK Hayka/Interperiodica Publishing

The structure of spin-orbital transitions in N, O, S azoaromatic molecules with allowance for high-lying singlet and triplet states populated with a pump in LCAO MO CSFCIINDO/S models

This paper investigates the structure of spin-orbital transitions and processes of inner conversion in laser-active organic compounds with allowance for the interaction of both low- and high-lying singlet and triplet states, efficiently populated by a high-power UV lamp or laser pump. Models of the LCAO MO CSF CI INDO/S quantum-chemical method are applied to calculate a broad spectrum of excited states of different spin and orbital nature for a 2-(n-aminophenyl)-5-phenyl-1,3,4-oxadiazole (n-NH2-PDP) molecule, which is capable of fluorescing with a high quantum yield, gamma approximate to 0.8-0.9, and lasing within the UV spectral range in various solvents. We provide a mathematical background and present the basic formulas for the calculation of matrix elements corresponding to spin-orbital coupling between pi pi*- and pi pi*-type states for polyatomic systems. The Robinson-Frosch formula modified for problems of laser physics is employed to calculate the constants of optical and nonoptical deactivation of electron-vibrational excitation energy and cross sections of stimulated emission and secondary reabsorption in spectra of pump-induced reabsorption through S-1* --> S-n* and T-1 --> T-n transitions. The calculated rate constants of radiative decay and intercombination conversion and fluorescence quantum yields of molecules in vapors and solutions are used to determine the characteristic limiting duration of the pumping pulse that allows the lasing of an n-NH2-PDP molecule. The results of these calculations are compared with characteristics measured for different experimental conditions

Excited electronic states of complex heteroatomic molecules in series and in different aggregation states of matter

We investigate the spectroscopic and photophysical properties of new series complex molecules, which is capable of fluorescence and some generating light in solvents of various kinds within the wavelength region 308 - 420 nm with a high fluorescence quantum yield γ = 0.01 - 0.97 and a low threshold pump density Elp(Plp). We worked from the measured lifetimes and to calculate the rate constants for radiative decay (Kfl) and intercombinational conversion, (KST), the cross section for a stimulated emission (σ31osc), and the characteristic time tlp in solvents of various types and in the vapor. The latter is the limiting rise time of the pump pulse (tlp) at which the generation of electromagnetic radiation is still possible. The cross sections for the induced singlet-singlet (σ3S*) and triplet-triplet (σ2T*) absorption in the generation band, required for calculating tlp, were taken from a variety of sources: experimental and for all other complex molecules, we used results calculated by some semiempirical methods SCF MO LKAO the Pariser-Parr-Pople (PPP/CI) and in complete neglect of differential overlap (INDO/S-CI). The results show that the decrease or increase of in the switch from solution to vapor, or as the properties of the solvent change, stems from dynamic separation or overlap of the lasing and induced absorption bands of the S1* → Sn* and T1 → Tn transitions. Lasing does not occur in concentrated acids because of the nearly complete overlap of the limiting gain and induced absorption T1 → Tn transitions (compare it with the behavior of dyes in their chemo absorbed state). Using the density matrix method, we show that the solvent affects the distribution of electron density among the individual atoms and fragments of the complex geteratomic molecule in the ground state, leading to systematic changes in geometry. As a result, there are changes in the distribution of bond lengths in the rings of the azo-cycles aromatic molecule. Photonation of the nitrogen atoms in the azocycles molecules changes the structure of the excited electronic singlet (Si*) and (Ti) triplet states and in the transitions SO → Sn*, S1* → S0, S1* → Sn*, T1 → Tn, T1 → S0, which determine the spectroscopic and generation characteristics of the complex compounds. The calculated geometry vapor complex molecules and [σπ]-anion and [σπ]-kation radicals compound AM-1/CI and PPP/CI methods in the ground state in their chemosorbtion state of the surface

Возбужденные состояния, фотофизические свойства и структура переходов сложной молекулы 1,4-фенилен-2,2'-бисоксазол

Photophysical properties of a new complex active molecule 1,4-phenylen-2,2'-bisoxazol (OPO) are considered. This molecule demonstrates fluorescence and light generation in the wavelength region lambda approximately 340 -420 nm with high quantum efficiency of fluorescence gamma approximately 0.45 - 0.93 and low threshold pump densities E(lp) in solvents of different nature. By means of the density matrix approach it has been shown that the solvent affects the localization of the- electron density over atoms and fragments of the complex OPO molecule in the ground state. This leads to a regular change of the geometry of the molecule. As a consequence of the change of the distribution of bond lengths in the cycles of the OPO molecule, when the hydrogen atom attacks the undivided pair of electrons of the nitrogen atoms, the structure of excited electron singlet (S(i)*) and triplet (T(i) states and S0 --> S(n)*, S1* --> S0, S1* --> S(n)*, T1 --> T(n)* T1 --> S0 transitions is changed too. These states and transitions define photophysical properties of complex compounds. Spectral-fluorescence and generation characteristics of the complex heteroatomic OPO molecule in solvents of different nature and in the gaseous phase have been measured and calculated. It has been established that an increase or decrease of E(lp) in vapors or when the properties of the solvent are changed are connected with dynamic separation or overlapping of the light generation bands and the bands of induced absorption on S1* --> S(n)* and T1 --> T(n) transitions. The absence of light generation in concentrated acids is connected with practically complete overlapping of the bands of limiting gain and of induced T1 --> T(n) absorption. The possibility to apply the method of a priori-structural modeling to the quantum mechanics control of the properties of excited states and transitions has been demonstrated.Рассмотрены фотофизические свойства новой активной сложной молекулы 1,4-фенилен-2,2'-бисоксазол (ОРО), способной флуоресцировать и генерировать излучение в области длин волн λ ≈ 340–420 нм с высоким квантовым выходом флуоресценции γ ≈ 0.45–0.93 и низкими пороговыми плотностями накачки Elp в растворителях различной природы. Методом матриц плотности показано, что растворитель влияет на характер локализации электронной плотности по атомам и фрагментам сложной молекулы ОРО в основном состоянии, что приводит к закономерному изменению ее геометрии. Следствием изменения распределения длин связей в циклах молекулы ОРО при протонировании атомов азота является изменение структуры возбужденных электронных синглетных (Si*) и триплетных (Ti) состояний и переходов S0 → Sn*, S1* → S0, S1* → Sn*, T1 → Tn, T1 → S0, определяющих фотофизические характеристики сложных соединений. Измерены и вычислены спектрально-флуоресцентные и генерационные характеристики сложной гетероатомной молекулы ОРО в растворителях различной природы и в газовой фазе. Установлено, что снижение или увеличение Elp в парáх или при изменении свойств растворителя связано с динамичным разнесением либо перекрытием полос генерации излучения и наведенного поглощения на переходах S1 → Sn* и T1 → Tn. Отсутствие генерации излучения в концентрированных кислотах связано с практически полным перекрытием полос предельного усиления и наведенного (T1 → Tn)-поглощения. Показана возможность практического применения метода априорно-структурного моделирования для квантово-механического управления свойствами возбужденных состояний и переходов

Возбужденные электронные состояния и генерация излучения в рядах сложных многоатомных молекул

Photophysical properties of the rows of novel and previously known complex organic molecules based on phenyl-, furil-, tienil-oxazoles and -oxadiazoles, which possess fluorescence and generate radiation in 340-410 nm wavelength region in numerous organic solvents, have been investigated. With the help of the density matrix method (PPP and INDO/S methods), the spectral properties of the excited singlet and triplet electron states have been studied. An interconnection between the structure and spectral-fluorescence and generation properties of complex molecules, forming a quasi-homologous row, has been found. The capabilities of the a priori-structural modeling of active complex molecules as applied to the control of the properties of excited states and transitions ir terms of quantum mechanics, have been demonstrated.Рассмотрены фотофизические свойства новых, а также некоторых известных органических сложных молекул на основе фенил-, фурил-, тиенил-оксазолов и -оксадиазолов, флуоресцирующих и генерирующих излучение в области длин волн 340–410 нм в широком наборе органических растворителей. Методами матрицы плотности изучены свойства широкого спектра возбужденных синглетных и триплетных электронных состояний. Установлена взаимосвязь строения, спектрально-флуоресцентных и генерационных свойств сложных молекул, образующих квазигомологический ряд. Показаны возможности использования метода априорно-структурного моделирования активных сложных молекул для квантово-механического управления свойствами возбужденных состояний и переходов

Excited state and photoprocesses in the series laser active molecules

[No abstract available