Chemical lasers

This paper reviews recent research on chemical lasers, considering separately lasers which oscillate on transitions between rotational, vibrational, and electronic energy levels. Emphasis is placed on the close interplay between development of chemical lasers and fundamental research into the detailed kinetics of chemical reaction and energy transfer.     

Vibrational-kinetics methods and their applications to molecular lasers and laser chemistry

The vibrational kinetics of molecules simulated by anharmonic oscillators is developed under essentially nonequilibrium conditions. Analytic expressions are obtained for the vibrational distribution function, the relaxation time, and the dependence of the vibrational energy on the pump power; the limiting capacity of the energy reservoir of the anharmonic molecules is estimated. Vibrational kinetics is investigated in mixtures and a redistribution of the vibrational energy among the modes is observed as a function of the component concentration. The singularities of vibrational relaxation in liquids and molecular crystals, due to collective interactions, are studied. A number of concrete applications are considered: the rate of nonequilibrium dissociation is calculated, an analytic model is developed for the CO laser, isotope separation in chemical reactions of vibrationally excited molecules is investigated, and a new type of lasers based on intramolecular transitions in liquids and molecular crystals is proposed.The article represents the contents of a dissertation submitted for the degree of Candidate of Physicomathematical Sciences. Guidance chairmen-Doctor of physicomathematical sciences A. L. Shelepin and Candidate of physicomathematical sciences B. F. Gordiets. Defended 27 October 1975 at the P. N. Lebedev Physics Institute of the Academy of Sciences of the USSR.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Akademii Nauk SSSR, Vol. 107, pp. 3–67, 1979.     

Spectroscopic transitions supporting chemiluminescence in the reaction of calcium with hydrogen chloride

The main aim of this study was to show that chemiluminescence transitions of free radicals and molecules formed during the reaction are important for stability. After detection of certain geometrical structures that are valuable for the reaction, the most likely electrical and vibrational transitions in density functional theory were determined. Another factor that suppresses or promotes electronic transitions, as well as geometric position, is the vibrational energy levels of the molecular system. In experimental studies, vibrational energy distributions are worth examining while studying electron density or population. In these geometric regions, the quantum states where the infrared transitions take place were calculated through the energy eigenvalues equation. Thus, the potential energy surface obtained by density functional theory method was compared with a realistic potential energy surface and experimental values in the literature. The stability of the nascent calcium chloride molecule is determined by the position of the hydrogen atom separated from the chlorine atom rather than the energy of the calcium atom.     

Efficiency of stimulation of gaseous chemical reactions by a diffracted light beam whose energy is absorbed by vibrational transitions

Laser-induced reversible chemical reaction kinetics in a gas mixture with the absorption of light energy by molecular vibrational transitions is investigated by mathematical modeling. The influence of beam diffraction, the relaxation time of vibrational energy into heat, and the order of vibrational transition on the structure of the region of high absorption and on the propagation of switching waves is analyzed. The discussion applies to pulses having a duration much longer than the time of energy transfer between vibrational levels. Zh. Tekh. Fiz. 69, 65–73 (April 1999)     

Non Equilibrium Vibrational Population and Dissociation Rates of Oxygen in Electrical Discharges: The Role of Atoms and of the Recombination Process

The influence of oxygen atoms on the population densities of vibrational levels of molecular oxygen has been studied for discharge conditions in which the atoms come 1) from electronic transitions to Herzberg and Schumann systems 2) from the pure vibrational mechanism (PVM) discussed in Chem. Phys. 30 (1978) 95. In the first case the atoms deactivate the first vibrational levels of O₂, thus reducing the vibrational temperature and the propagation of the introduced vibrational quanta by V — V exchanges. Under these conditions PVM disappears. An assisted recombination dissociation mechanism, however, holds in these conditions, due to the recombination process which induces a strong disequilibrium on the levels near to the dissociation continuum. These levels can redissociate by collisions with heavy particles. The results of pure vibrational mechanism in the presence of atoms are very similar to those discussed in our previous work. The production of oxygen atoms self adjusts to small values due to the increased importance of V — T deactivation by oxygen atoms.     

Radiation-stimulated diffusion in solids

Irradiation of solids produces a microscopic nonequilibrium state in which the vibrational energy distribution function of the atoms deviates from the thermodynamically equilibrium function. Expressions are obtained for the nonequilibrium distribution function and for the frequencies of activational transitions of atoms out of a potential well. It is shown that the radiation stimulation of diffusion processes involves a deviation of the temperature dependences of the frequencies of transitions of the atoms out of positions of equilibrium from the Arrhenius law. Under subthreshold irradiation conditions the rate of diffusion processes is higher for atoms whose vibrations thermalize over long times and depends linearly on the irradiation intensity. Under above-threshold irradiation conditions the characteristics of cascade regions in solids — their sizes and the vibrational excitation energy of the atoms — can be determined by comparing the computed and experimental temperature dependences of the diffusion coefficient. Zh. Tekh. Fiz. 68, 67–72 (August 1998)     

Coherent Raman spectroscopy: From statics to dynamics and kinetics,progress in nonlinear methods

In spite of its 60-year history Raman spectroscopy is still progressing nowadays. Highly stable lasers and short pulse oscillators, perfect electronic data acquisition systems, new nonlinear optical approaches created new exciting perspectives for Raman spectroscopy. One of the most important tendencies is Raman spectroscopy application for studying nonequilibrium states, fast dynamics and kinetics of atoms, molecules and condensed matter. All these problems were until recently regarded as inaccessible for optical spectroscopy. Nonlinear optical techniques of Coherent Anti-Stokes Raman Scattering (CARS) and modulation spectroscopy appeared to be most effective and provided important real-time information on molecular excitation and dissociation dynamics, deep cooling of molecules in a supersonic jet, short laser pulse induced phase transitions at semiconductor interface and so on. Problems yet to be solved include direct measurement of intramolecular vibrational relaxation, conformations in biomolecules, optical “oscilloscopy” of molecular vibrations.     

Collisional transfer of rotational energy in the 2B1 electronic state of nitrogen dioxide

Collisional satellite lines have been observed in fluorescence from nitrogen dioxide excited by the 4545-Å line of the argon laser. The 13_0,13 level of the (0, 8, 0) vibrational state is populated by the laser and undergoes collisionally induced transitions to the 11_0,11, 15_0,15, and 17_0,17 states. These collisionally populated states are identified by their fluorescence to the well-studied (0, 0, 0) and (0, 1, 0) levels of the ground electronic state. These satellite lines are also observed in fluorescence to the (0, 2, 0) and (0, 3, 0) vibrational levels of the ground electronic state. The wavenumbers of those lines, together with those from unrelaxed fluorescence and previously published microwave transitions, allow vibrational and rotational constants for the higher vibrational states to be determined more accurately than was previously possible. Several much weaker forbidden transitions have also been observed, including ΔK_a = 0 through ?6 transitions in the (0, 8, 0)-(0, 0, 1) band.     

Femtosecond infrared spectroscopy of semiconductors and semiconductor nanostructures

Infrared spectroscopy on ultrafast time scales represents a powerful technique to investigate the nonequilibrium dynamics of elementary excitations in bulk and nanostructured semiconductors. In this article, recent progress in this field is reviewed. After a brief introduction into electronic excitations below the fundamental bandgap and ultrafast processes in semiconductors, infrared pulse generation and the methodology of time-resolved infrared spectroscopy are reviewed. The main part of this paper is devoted to coherent optical polarizations and nonequilibrium excitations of the electronic system in the spectral range below the fundamental band gap. The focus is on the physics of single component plasmas, i.e. electrons or holes. In particular, intraband, inter-valence and intersubband transitions are considered. Processes of phase relaxation, carrier and energy redistribution are analyzed. The potential of ultrafast infrared technology and spectroscopy for future applications is discussed in the final part.     

Intermolecular Processes in Excited Electronic States in the Gas Phase (Review)

A brief review of works on the temperature dependences of the rate constants k_q of the intermolecular processes proceeding in the excited electronic states in the gas phase is given. The dependences k_q(T) for such biomolecular processes as intermolecular vibrational energy transfer in the triplet state vibrational quasi-continuum, triplet-triplet electron excitation energy transfer, and intermolecular photoinduced electron transfer have been compared. The experimental data have shown that in the gas phase for all analyzed intermolecular processes both an increase and a decrease in k_q with increasing temperature (T) is observed, which is not associated with the specific intermolecular interactions leading to the formation of long-lived components. The change in the type of temperature dependence is due to the change in the mechanisms of the radiationless transitions with increasing density of vibrational levels in the final electronic state. The applicability of the known models based on the theory of radiationless transitions for predicting the temperature dependences k_q(T) is discussed. __________ Translated from Zhurnal Prikladnoi Spektros-kopii, Vol. 72, No. 4, pp. 429–439, July–August, 2005.     

Generation Kinetics of Nonequilibrium Charge Carriers in Crystals with Deep Impurities Involving Two-Center Transitions between Band and Impurity States

A new and efficient mechanism of nonlinear photoexcitation of a transparent crystal with deep impurity centers is proposed. It is hypothesized that the energy of a quantum of light is smaller than the energy gap between the bottom of the conduction band and the impurity level, but is larger than the gap between the impurity level and the top of the valence band. The kinetics of nonlinear cascade generation of nonequilibrium charge carriers is considered taking into account two-center processes in which the energy transfer and the photon absorption occur in one and the same elementary event. The dependences of the quasi-equilibrium concentrations of nonequilibrium charge carriers in the bands and of the occupancy of impurity states on the laser-radiation intensity are obtained. It is shown that the generation process of nonequilibrium electron–hole pairs is of a threshold nature. Depending on the concentration of impurity centers, the threshold intensities can be ~10⁵–10⁷ W/cm², while the setting time of the quasi-equilibrium occupancies of electronic states is ~10–0.1 ns.     

Chiroptical spectra of molecules with nearly degenerate electronic states

The influence of vibronic interactions on the chiroptical spectra associated with pairs of nearly degenerate electronic transitions in chiral systems is examined on a formal theoretical model. We consider the special case in which two nearly degenerate electronic states are coupled by a single non-totally symmetric vibrational mode. Formal expressions are developed for the rotatory strengths of individual vibronic transitions in this coupled system. Calculations based on these expressions are carried out for a large number of parameter sets appropriate for various energy spacings between the unperturbed electronic states, vibronic coupling strengths, oscillator (vibrational mode) frequencies, and electronic rotatory strengths. The calculated results demonstrate the profound influence of vibronic interactions on the sign patterns and intensity distributions within the rotatory strength spectrum associated with the two coupled electronic states. The implications of these results for interpretations of circular dichroism spectra are discussed.     

Generation of VUV radiation in Lyman and Werner bands of pulsed space discharge in hydrogen mixtures with helium

A self-consistent numerical model for the self-sustained high-current pulsed discharge is constructed based on the solution of the equations of the population of H₂ electronic and vibrational states. The model accounts for electronic, ion-molecular, and vibrational kinetics, electron attachment to and detachment from the H₂ molecule, Lyman and Werner band emission, and their radiation trapping. The equations of electron-vibrational kinetics are solved simultaneously with the Boltzmann equation for the electron energy distribution function and the external electrical circuit equations.     

A phase method of measuring the vibrational relaxation time for CO2 molecules

Conclusions The phase method can be applied to other compounds, but the choice of molecules and vibrational levels is restricted by the need for the level to have two allowed IR transitions. Also, the use of the method is restricted by the availability of strong IR sources.The phase method is analogous in concept to the spectrophone method [I] The advantage of the phase method is that it allows measurement of the lifetime for a single level no matter where it lies in the system of vibrational levels. Cascade transitions in deactivation (stepwise transformation of vibration energy to heat) greatly complicates the interpretation in the speetrophone method and essentially restricts its use to the first excited levels. Establishment of the phase zero is the main source of error in the spectrophone method and requires special calibration [5]. This problem has a simple solution in the phase method.In some cases the vibrational relaxation time can be determined from the amplitude dependence as affected by pressure, but this requires correction for the pressure dependence of the absorption coefficient for the exciting transition as well as that of the conditions for escape of the spontaneous emission through-out the vibration-rotation band.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 10, No. 2, pp. 252–255, February, 1969.     

What is so mysterious about the electronic states of SCI?

The 15 lowest lying doublet electronic states of the molecule SCI have been investigated theoretically at a high level of correlation treatment (MRCI). For the ground state (X ²II), spectroscopic constants were obtained from a set of eight vibrational intervals. This result extends our knowledge about this state beyond the experimentally known data that presently were derived from only two bands. Spin-orbit constants, transition probabilities and radiative lifetimes complement its spectroscopic characterization. For the excited electronic states, a global view of the doublet states is presented that can help us understand the scarcity of experimental data on electronic transitions for this system and also the difficulty of assigning the only two transitions so far recorded. Most of these states are repulsive, and for the few high lying bound ones, of Rydberg character, avoided crossings restrict the number of accessible vibrational states. Crossing by repulsive states and predissociation is also a factor that can prevent further emissions. Two new bound excited states, ²δ and ²Σ, predicted in this study, are of direct relevance to an interpretation of the limited experimental data available on electronic transitions.     

Numerical simulation of nonequilibrium flows by using the state-to-state approach in commercial software

Nonequilibrium flows of a two-component oxygen mixture O₂/O behind a shock wave are studied with due allowance for the state-to-state vibrational and chemical kinetics. The system of gas-dynamic equations is supplemented with kinetic equations including contributions of VT (TV)-exchange and dissociation processes. A method of the numerical solution of this system with the use of the ANSYS Fluent commercial software package is proposed, which is used in a combination with the authors’ code that takes into account nonequilibrium kinetics. The computed results are compared with parameters obtained by solving the problem in the shock-fitting formulation. The vibrational temperature is compared with experimental data. The numerical tool proposed in the present paper is applied to study the flow around a cylinder.     

Low energy electron spectroscopy of CO in the 10.600–14.400 eV energy loss range

Excitation spectra of CO have been obtained at low electron impact energy in the 10.600–13.400 eV energy loss range for scattering angles from 10 to 120°, with a 35 meV experimental resolution. The angular behaviour of the observed peaks is used to discriminate singlet-singlet and singlet-triplet transitions. Previously calculated Rydberg states are observed, in particular the triplet analogue of the F¹Σ⁺ state. A new high energy valence triplet state is identified; the first observed vibrational level is at 11.595 eV and the vibrational spacing is 90 meV. Upper levels are strongly affected by predissociation.     

Intramolecular non-radiative transitions

Non-radiative electronic transitions are discussed. The probability W_i of the electron staying in the initial state i is obtained as a function of the vibrational relaxation time, in both large and small molecules. The result at low temperature is

where ε is the vibrational relaxation rate, ?ωm' is the energy difference of the levels which are involved in the transitions. An expression which depends on the phonon temperature is also obtained.     

可激发气体分子声弛豫过程振动模式能量转移速率计算

研究可激发气体中振动模式能量转移速率和声弛豫过程形成的关系,将单一气体Tanczos弛豫方程理论[J.Chem.Phys.25,439(1956)]扩展应用于混合气体中振动模式的振动-振动(V-V)和振动-平动(V-T)能量转移速率的计算。在室温下CO₂,CH₄,CL₂,N₂和O₂组成的多种混合气体中,振动模式能量转移速率的计算结果表明:对于多个振动模式所形成的声复合弛豫过程,各振动模式的声激发能可由V-V能量转移相互耦合后传递给具有最快V-T转移速率的最低振动频率振动模式,再通过该振动模式的V-T转移退激发形成主弛豫过程。这种选择最快转移路径的声激发量弛豫方式,造成了大多数可激发气体中声弛豫吸收谱的实测数据只存在一个吸收峰的现象。从而提供了一个可通过计算微观振动能量转移速率分析混合气体声弛豫过程形成机理的理论模型。