Rate Constants For Excitation Research Articles

Modeling of vibrational excitation dynamics in a nanosecond CO2 discharge

The temporal dynamics of excitation of the asymmetric vibrational mode of CO2 molecules in a nanosecond discharge is simulated. The values of the electric field andthe number density of electrons versus time, calculated for a known experimental dependence of the discharge current on time, are used to evaluate the number densities of the first and second vibrational levels of the asymmetric mode. It is shown that the dynamics of the densities of these levels, calculated using generally accepted values of the vibrational excitation rate constants, is in reasonable agreement with experimental data.

Participation of fractional charge transfer on the efficiency of singlet oxygen production: Heteroleptic ruthenium (II) bipyridine derivatives

The photophysical properties of some synthesized heteroleptic polypyridyl ruthenium(II) complexes of the form [Ru(bpy)2L](PF6)2 where L = 2,2′-bipyridine, 4,4′-dimethoxy-2,2′-bipyridine, 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine, 4,7-diphenyl-1,10-phenanthroline and 3,4,7,8-tetramethyl-1,10-phenanthroline, are investigated in acetonitrile. The excited state lifetime of the metal to ligand charge transfer (3MLCT) states of these complex ions are found to be ligand dependent and were found to be in the range 0.56 μs to 1.5 μs. Oxygen quenching of the excited states rate constants, kq, were found to be in the range 1.50–2.24 × 109 M−1s−1. Singlet oxygen quantum yield values, ΦΔ, are found to vary from 0.40 to 0.60 and the efficiency, fΔT, with which excited singlet oxygen, O2 (1Δg), is thereby produced was found to be in the range 0.55–0.85. Treatment of the data with charge transfer and non-charge transfer pathways using Schmidt’s model shows that partial charge transfer parameter was in the range of 0.18 to 0.58. It has also been found that fΔT decreases and oxygen quenching rate constants, kq, increases as pCT increases.

Direct Dissociative Excitation of Heteronuclear and Homonuclear Ions of Inert Gases by Electron Impact

Abstract—A theoretical description of the process of dissociative excitation of a molecular ion by an electron impact under the condition of effective population of a large number of its vibrational–rotational levels has been given. The approach is based on the quantum version of the theory of non-adiabatic transitions between electronic terms of a molecular ion and on the replacement of the summation with respect to $${v}J$$ -levels with the integration with respect to $${v}$$ and $$J$$ . Semi-analytical formulas for the integral contribution of the entire vibrational–rotational quasicontinuum to the cross sections $$\sigma _{T}^{{{\text{de}}}}(\varepsilon )$$ and rate constants $${{\alpha }^{{{\text{de}}}}}(T,{{T}_{e}})$$ of the process under study in a plasma with the $${{T}_{e}}$$ and $$T$$ temperatures of its electronic and ionic components have been obtained. The developed theory has been used to study the processes of dissociative excitation of heteronuclear (HeXe+ and ArXe+) and homonuclear (Ar $$_{2}^{ + }$$ and Xe $$_{2}^{ + }$$ ) ions of inert gases. A strong dependence of the results on the ion dissociation energy and large differences in the behavior and characteristic values of $$\sigma _{T}^{{{\text{de}}}}(\varepsilon )$$ and $${{\alpha }^{{{\text{de}}}}}(T,{{T}_{e}})$$ for these systems in different energy, $$\varepsilon $$ , and temperatures, $${{T}_{e}}$$ and $$T$$ , has been established. The regions of dominance of the contributions of two competing channels: direct dissociative excitation and dissociative recombination to the total cross sections and the rate of destruction of molecular ions of inert gases by electron impact have been determined. The analysis of the behavior of the differential rate constants of dissociative excitation in a unit electron energy interval in the final reaction channel has demonstrated qualitative differences in the dynamics of this process in the cases of weakly and moderately bound molecular ions.

Steady-State Fluorescence Signatures of Intramolecular Singlet Fission from Stochastic Predictions.

The advent of new multichromophoric systems capable of undergoing efficient intramolecular singlet fission (iSF) has greatly expanded the range of possible motifs for multiexciton generation approaches for organic light energy harvesting materials. Transient absorption (TA) spectroscopic probes are typically used to characterize singlet fission processes that may place limitations on sensitivity and time resolution on scales comparable to the full lifespan of spin-forbidden triplets and interactions. Here, we investigate the ability of fluorescence-based spectroscopic probes to detect iSF activity in isolated dyads based on large substituted conjugated acenes (e.g., tetracene and pentacene derivatives). Photophysical models are simulated from several iSF-active dyad systems reported in the literature using a stochastic approach to assess the sensitivity of steady-state fluorescence to the presence of triplet excitons. The results demonstrate large fluctuations in expected fluorescence yields with varying excitation rate constants for systems with ΦiSF > 0.5 (assuming weak interchromophore coupling). Exciton-exciton interactions are also investigated, and we further demonstrate how treating iSF dyads stochastically (i.e., finite number of chromophores) accentuates dependences of photophysical yields with excitation rates. Last, our approach reveals the potential ability of single molecule level fluorescence spectroscopy to detect iSF activity that can aid efforts to design and optimize candidate iSF systems.

A global potential energy surface for the ground electronic state of SSiH

An analytical global potential energy surface for the ground electronic state of SSiH is here reported. The function is based upon ab initio energies calculated at Davidson-corrected multi-reference configuration interaction level with the aug-cc-pV(T + d)Z and aug-cc-pV (Q + d)Z basis sets and subsequently extrapolated to the complete basis set (CBS) limit. The analytical representation follows the Aguado and Paniagua many-body expansion methodology. A total of 2487 ab initio points were used in the fitting. The topological features of the so constructed potential are here discussed. A quasi-classical trajectory study for the SH + Si → SSi + H is carried out using the new function. Excitation function, rate constants and details of the reactive collisions are also discussed.

Two-temperature Collisional-radiative Modeling of Partially Ionized O2-Ar Mixtures over 8000-10,000 K Behind Reflected Shock Waves.

The collisional excitation kinetics of atomic oxygen was studied behind reflected shock waves using tunable diode laser absorption spectroscopy. A test gas mixture of 1% O2/Ar was shock-heated to temperatures between 8000 and 10,000 K and pressures between 0.15 and 1 atm. The time evolution of the atomic oxygen population in the 3 s 5S0 state was monitored by laser absorption at 777.2 nm. The measured O(3 s 5S0) population revealed multistage behavior that was not observed in previous measurements over a temperature range of 5300-7200 K. To interpret the multistage behavior, a three-level collisional-radiative model for atomic oxygen excitation kinetics was developed. The model utilized two independent temperatures, that is, heavy particle translational temperature Ttr and electron translational temperature Te, to describe the fundamental rate constants of atomic oxygen excitation because of collisions with heavy particles and electrons, respectively. The heavy particle excitation rate was inferred from the early stage of the measurement to be k(3P →5S0) = 3.4 × 10-27 (T/K)0.5(1.061 × 105 + 2 (T/K)) exp(-1.061 × 105 K/T) ± 50% m3 s-1. The electron impact excitation rate constant of oxygen, electron impact, and heavy particle impact ionization rate constants of Argon were modified in the model to match the experimental population time histories. The modified rate parameters are also reported for the temperature range explored in the current study.

Прямое диссоциативное возбуждение гетероядерных и гомоядерных ионов инертных газов электронным ударом

A theoretical description of the process of dissociative excitation of a molecular ion by electron impact in the case of effective population of a huge number of its vibrational-rotational levels is presented. Our consideration is based on the quantal version of the theory of non-adiabatic transitions between the electronic terms of a molecular ion combined with replacing the summation over vJ-levels by integration over v and J. Semianalytical formulas are derived for the integral contribution of the entire vibrational-rotational quasicontinuum to the cross sections, $\sigma_T^{\mathrm{de}}(\varepsilon)$, and the rate constants, $\alpha^{\mathrm{de}}(T, T_e)$ of the process under study in a plasma with temperatures $T_e$ and $T$ of its electronic and ionic components. The developed theory is used to study the processes of dissociative excitation of heteronuclear (HeXe$^{+}$ and ArXe$^{+}$) and homonuclear (Ar$_2^{+}$ and Xe$_2^{+}$) ions of inert gases. We demonstrate a strong dependence of the results on the ion dissociation energy and large differences in the behavior and characteristic values of $\sigma_T^{\mathrm{de}}(\varepsilon)$ and $\alpha^{\mathrm{de}}(T, T_e)$ for these systems in different ranges of electron energy, $\varepsilon$, and temperatures $T_e$ and $T$. The regions of dominance of the contributions of two competing channels are determined: direct dissociative excitation and dissociative recombination into total cross sections and the rate constants of destruction of rare gas molecular ions by electron impact. Analyzing behavior of the differential rate constants of dissociative excitation per unit range of electron energy in the final channel of reaction we demonstrate qualitative differences in the dynamics of this process for weakly bound and moderately bound molecular ions.

Intramolecular H-bond design for efficient orange–red thermally activated delayed fluorescence based on a rigid dibenzo[f,h]pyrido[2,3-b]quinoxaline acceptor

Tune the molecular excited state and rate constants of radiative intersystem crossing by constructing intramolecular H-bonds to achieve high-efficiency orange–red TADF-OLEDs.

Fast-pulsing LED-enhanced NMR: A convenient and inexpensive approach to increase NMR sensitivity.

Low-concentration photochemically induced dynamic nuclear polarization (LC-photo-CIDNP) has recently emerged as a powerful technology for the detection of aromatic amino acids and proteins in solution in the low-micromolar to nanomolar concentration range. LC-photo-CIDNP is typically carried out in the presence of high-power lasers, which are costly and maintenance-heavy. Here, we show that LC-photo-CIDNP can be performed with light-emitting diodes (LEDs), which are inexpensive and much less cumbersome than lasers, laser diodes, flash lamps, or other light sources. When nuclear magnetic resonance (NMR) sample concentration is within the low-micromolar to nanomolar range, as in LC-photo-CIDNP, replacement of lasers with LEDs leads to no losses in sensitivity. We also investigate the effect of optical-fiber thickness and compare excitation rate constants of an Ar ion laser (488 nm) and a 466 nm LED, taking LED emission bandwidths into account. In addition, importantly, we develop a novel pulse sequence (13C RASPRINT) to perform ultrarapid LC-photo-CIDNP data collection. Remarkably, 13C RASPRINT leads to 4-fold savings in data collection time. The latter advance relies on the fact that photo-CID nuclear hyperpolarization does not suffer from the longitudinal-relaxation recovery requirements of conventional NMR. Finally, we combine both the above improvements, resulting in facile and rapid (≈16 s-2.5 min) collection of 1 and 2D NMR data on aromatic amino acids and proteins in solution at nanomolar to low micromolar concentration.

Comparative analysis of various indolocarbazole-based emitters on thermally activated delayed fluorescence performances

Abstract A detailed comparative analysis of thermally activated delayed fluorescence (TADF) performances of indolocarbazole-based emitters consisting of ortho, meta and para-isomeric materials are described. Their photophysical and electronic properties are thoroughly compared using their natural transition orbitals, absorption and photoluminescence analyses. Excited state dynamics and rate constants are compared to know the ease of reverse intersystem crossing process. Finally, blue TADF organic light emitting diode fabricated with meta-isomer exhibited maximum external quantum efficiency of 28.0% with low efficiency roll-off. It is found that the meta-isomer is the ideal material over other isomeric indolocarbazole-based materials for designing of future TADF materials.

Influence of metastable atoms on the formation of nonlocal EDF, electron reaction rates, and transport coefficients in argon plasma

The influence of metastable atoms on formation of the nonlocal electron distribution function (EDF) and other characteristics of gas-discharge plasma in argon is investigated in this paper. Superelastic collisions of electrons with metastable atoms strongly affect the shape of the EDF, the behavior of reaction rate constants, and other plasma characteristics. An interesting phenomenon of EDF replication emerges with a period of excitation threshold; this effect can increase the rate constants of excitation and ionization processes by several orders of magnitude, especially at the periphery of the plasma volume. Thus, collisions between electrons and metastable atoms are important in the formation of a nonlocal EDF, and neglecting them can lead to large errors in calculations of plasma characteristics.

Inelastic Processes in a Gas-Discharge Plasma of Inert Gases

The excitation rate constants of the lower excited states of inert gas atoms by electron impact in the gas-discharge plasma are determined with taking into account the self-consistent character of this process and the rates of similar processes in the plasma of alkali metals. The rate constants of the stepwise ionization in a uniform gas-discharge plasma of inert gases are determined. They are also determined in a nonunifom plasma if the nonuniformity is a result of the passage of a gas-discharge current. Based on the rates of these processes, the populations of lower excited levels are calculated in the case where the equilibrium in the gas-discharge plasma of inert gases is determined by inelastic collisions of electrons with atoms.

Actinometry of O, N and F atoms

The applicability of actinometry for measuring the absolute concentration of O, N and F atoms in discharge plasma was studied. For this purpose, concentrations of these atoms were measured downstream of an ICP plasma by means of the actinometry method and of appearance potential mass-spectrometry (APMS). Comparison of the results showed good agreement between the two methods. Since the excitation cross sections of electron states O(3p 3P) and O(3p 5P) applied in actinometry are well tested, this allows using the APMS method for absolute calibration of the theoretical excitation cross sections for N and F atoms. As a result, total excitation cross sections of the atomic levels N(3p 4Po), F(3p 2Po) and F(3p 4Do) have been obtained for the first time. Since different types of electron energy distribution function (EEDF) were observed (Maxwellian, bi-Maxwellian and Druyvesteyn) the influence of these possible EEDF types on actinometric coefficients (X = O, N, F), that link the ratio of the atom and actinometer intensities with that of their concentrations [X]/[Ar], was also analyzed. It was shown that at the same ionization rate (effective electron temperature) the excitation rate constants are highly sensitive to the shape of EEDF, whereas actinometric coefficients depend on it only slightly. Dependence of actinometric coefficients on electron temperature is positive if the emitting level of the X-atom is lower than that of the actinometer, and negative if vice versa. The energy difference between the emitting states of O and Ar atoms is maximal (~3 eV), so that is not constant for a whole range of electron temperatures typical for discharge plasmas (~2–8 eV). For nitrogen atoms varies considerably with Te only when Te < 4 eV. In the case of fluorine atoms the energy difference of emitting F and Ar states is only ~1 eV and coefficient is nearly constant in a wide region of Te > 1.5 eV.

Dynamics of the O + ClO reaction: reactive and vibrational relaxation processes.

Classical trajectories have been integrated to study the O + ClO reaction, both reactive and vibrational energy transfer processes, for the range of temperatures 100 ≤ T/K ≤ 500 using momentum Gaussian binning. The employed potential energy surface is the recently proposed single-sheeted double many-body expansion potential energy surface for the (2)A" ground-state of ClO2 based on multireference ab initio data. A capture-type regime with a room-temperature rate constant of (17.8 ± 0.5) × 10(-12) cm(3) s(-1) and temperature dependence of k(T/K)/cm(3) s(-1) = 22.4 × 10(-12) × T(-0.81) exp(-39.2/T) has been found. Although the value reported here is half of the experimental and recommended one, tentative explanations are given. Other dynamical attributes are also examined for the title reaction, with state-to-all and state-to-state vibrational relaxation and excitation rate constants reported for temperatures of relevance in stratospheric chemistry.

The interstellar gas-phase chemistry of HCN and HNC

We review the reactions involving HCN and HNC in dark molecular clouds to elucidate new chemical sources and sinks of these isomers. We find that the most important reactions for the HCN-HNC system are Dissociative Recombination (DR) reactions of HCNH+ (HCNH+ + e-), the ionic CN + H3+, HCN + C+, HCN and HNC reactions with H+/He+/H3+/H3O+/HCO+, the N + CH2 reaction and two new reactions: H + CCN and C + HNC. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio significantly above one as long as the carbon atom abundance remains high. However, the reaction of HCN with H3+ followed by DR of HCNH+ acts to isomerize HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio close to or slightly greater than 1. This agrees well with observations in TMC-1 and L134N taking into consideration the overestimation of HNC abundances through the use of the same rotational excitation rate constants for HNC as for HCN in many radiative transfer models.

The relationship between excitation-rate constants and limits of detection in femtosecond laser-induced breakdown spectroscopy

The electron-impact excitation-rate constants for atoms and ions in the ground state have been calculated. It is shows that, using data on the excitation-rate constants, it is possible to compare the limits of detection of the corresponding elements in femtosecond laser-induced breakdown spectroscopy.

Computational Studies on the Excited States of Luminescent Platinum(II) Alkynyl Systems of Tridentate Pincer Ligands in Radiative and Nonradiative Processes

Platinum(II) alkynyl complexes of various tridentate pincer ligands, [Pt(trpy)(C≡CR)](+) (trpy = 2,2':6',2″-terpyridine), [Pt(R'-bzimpy)(C≡CR)](+) (R'-bzimpy = 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine and R' = alkyl), [Pt(R'-bzimb)(C≡CR)] (R'-bzimb = 1,3-bis(N-alkylbenzimidazol-2'-yl)benzene and R' = C4H9), have been found to possess rich photophysical properties. The emission in dilute solutions of [Pt(trpy)(C≡CR)](+) originated from a triplet alkynyl-to-tridentate pincer ligand-to-ligand charge transfer (LLCT) excited state, with mixing of a platinum-to-tridentate pincer ligand metal-to-ligand charge transfer (MLCT) excited state, while that of [Pt(R'-bzimb)(C≡CR)] originated from a triplet excited state of intraligand (IL) character of the tridentate ligand mixed with a platinum-to-tridentate ligand MLCT character. Interestingly, both emissions were observed in [Pt(R'-bzimpy)(C≡CR)](+) in some cases. In addition, [Pt(R'-bzimb)(C≡CR)] displayed a photoluminescence quantum yield higher than that of [Pt(R'-bzimpy)(C≡CR)](+). Computational studies have been performed on the representative complexes [Pt(trpy)(C≡CPh)](+) (1), [Pt(R'-bzimpy)(C≡CPh)](+) (2), and [Pt(R'-bzimb)(C≡CPh)] (3), where R' = CH3 and Ph = C6H5, to provide an in-depth understanding of the nature of their emissive origin as well as the radiative and nonradiative processes. In particular, the factors governing the ordering of the triplet excited states and radiative decay rate constants of the emissive state ((3)ES) have been examined. The potential energy profiles for the deactivation process from the (3)ES via triplet metal-centered ((3)MC) states have also been explored. This work reveals for the first time the potential energy profiles for the thermal deactivation pathway of square planar platinum(II) complexes.

Stochastic Lindemann Kinetics for Unimolecular Gas-Phase Reactions

Lindemann, almost a century ago, proposed a schematic mechanism for unimolecular gas-phase reactions. Here, we present a new semiempirical method to calculate the effective rate constant in unimolecular gas-phase kinetics through a stochastic reformulation of Lindemann kinetics. Considering the rate constants for excitation and de-excitation steps in the Lindemann mechanism as temperature dependent empirical parameters, we construct and solve a chemical master equation for unimolecular gas-phase kinetics. The effective rate constant thus obtained shows excellent agreement with experimental data in the entire concentration range in which it is reported. The extrapolated values of the effective rate constant for very low and very high concentrations of inert gas molecules are in close agreement with values obtained using the Troe semiempirical method. Stochastic Lindemann kinetics, thus, provides a simple method to construct the full falloff curves and can be used as an alternative to the Troe semiempirical method of kinetic data analysis for unimolecular gas-phase reactions.

Femtosecond laser-induced breakdown spectroscopy of sea water

The composition of the line and band spectra of the plasma induced by a femtosecond laser pulse on the surface of sea water is determined. The temporal behaviors of the intensity of the continuum and the Ca II, Mg II and Na I lines are investigated. It is shown that the time dependence of the intensity of the Na I line is described by a monoexponential function. The characteristic decay times of the line intensities of Mg II and Na I were used to estimate the three-body recombination times. Using these values, we estimate the electron number density and the feasibility of Local Thermodynamic Equilibrium (LTE) criterion. A method involving excitation rate constants is proposed for the comparison of detection limits. For a plasma generated on a liquid surface, the following relation among detection limits will be obtained: LOD(Na)<LOD(K)<LOD(Ca)<LOD(Al)<LOD(Mg)<LOD(Zn).

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 21 P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.