Study on chemical reactions of isolated Mössbauer probes in solid gas matrices using in-beam Mössbauer spectroscopy

In-beam Mössbauer spectra of ⁵⁷Fe after ⁵⁷Mn (T _1/2= 1.45 min) implantation into a CH ₄ matrix and mixture matrices of CH ₄ and Ar at 18 K were measured. The spectrum obtained in the CH ₄ matrix was analyzed well with a doublet and a singlet peaks. These components were assigned to two constitutional isomers of Fe(CH ₄)\(_{\mathrm {2}}^{\mathrm {+}}\) as derived from density functional theory calculations and the Mössbauer parameters. In the case of the low concentration of CH ₄ with an Ar matrix, the Fe ⁺ (3d ⁷4s) in the excited state atomic configuration and Fe ⁺ (3d ⁶4s ¹) in the ground state were observed, as observed in our previous implantation experiment into Ar and Xe matrices. The formation yields of Fe(CH ₄)\(_{\mathrm {2}}^{\mathrm {+}}\) are discussed in term of the number of first-nearest-neighbor CH ₄ molecules around an Fe ⁺ ion.     

Optical diagnostics for temperature measurement in a DC arcjet reactor used for diamond deposition

2 a-state in this plume. LIF measurements of CH are complicated by the presence of C₃ and we discuss strategies to deal with this interference. The gas temperature describing the rotational distributions obtained for NO, C₂ and CH agree within experimental error. Optical emission measurements indicate that the rotational and vibrational distributions of the excited A-state of CH and a-state of C₂ are characterized by vibrational and rotational populations which are at least 1000 K above the ground state distributions. The excited states are collisionally quenched before their population distributions equilibrate with the gas temperature. We also determine relative populations of the ground and excited states along the axis of the plume between the arcjet nozzle and the substrate and relative populations for a cross section of the jet, midway between the nozzle orifice and the substrate. The measured relative ground and excited state populations for both CH and C₂ show different trends along the plume axis, indicating that the ground and excited states of these molecules are products of different chemical mechanisms; such mechanisms are discussed. Received: 16 July 1996/Revised version: 24 September 1996     

Single-electron charge transfer in collisions of CF 2 ++ with Ar and Ne at collision energies 3–5 eV

Non-dissociative, single-electron charge transfer processes between the dication CF ₂ ⁺⁺ and Ar and Ne were investigated in crossed beam scattering experiments at collision energies of 3.0 and 4.55eV (c.m.). The reaction with Ar produces CF ₂ ⁺ in its ground state and, to a smaller extent, in the excited CF ₂ ⁺ (²_u) state. The reaction with Ne occurs only with an excited state of CF ₂ ⁺⁺ lying about 4.4 eV above the ground state and present in a small amount in the reactant dication beam; its molecular product is CF ₂ ⁺ in the ground state.     

Complex acceptors in CdTe:Cl investigated by differential spectroscopy

The acceptor states in CdTe:Cl single crystals are investigated using a technique based on the analysis of differential spectra of selectively excited donor-acceptor pair photoluminescence. The splitting of 2P _3/2(Γ₈) and 2S _3/2(Γ₈) states of the Ag_Cd tetrahedral acceptor taking place upon a decrease in the donor-acceptor separation is found. The energies of seven excited states of the complex acceptor with an activation energy of ～121 meV are determined.     

Experimental 2CH excitation in acetylene-containing van der Waals complexes

Spectroscopic results are presented concerning the 2CH excitation around 1.5?µm in van der Waals complexes of acetylene (C₂H₂) with Ar, Kr, N₂, CO₂, N₂O and C₂H₂. Many are reviewed from the literature, with some updates. Previously unpublished results are also presented, concerning the mechanism of formation of C₂H₂–Ar in the supersonic jet, the assignment of new spectral structures in C₂H₂–N₂O, and the first observation of 2CH excitation in C₂H₂–Ne, C₂H₂–H₂O, C₂H₂–D₂O and (C₂H₂) _n . Lifetimes of these 2CH vibrationally excited dimers are discussed.     

Ab initio study of the Ar–CS2(V1B2) intermolecular potential surface: effect of van der Waals interaction on the emission of CS2 molecule

In this work, the excited state intermolecular potential energy surface of the Ar–CS₂(V¹B₂) van der Waals complex was evaluated for the first time. The calculation of more than 4000 single-point interaction energies for the complex using an equation-of-motion coupled-cluster model with single and double substitutions level of theory with extended basis set involving bond functions has been performed. After fitting the interaction energies to analytical functions, the emission spectra of the Ar–CS₂(V¹B₂) complex related to the different stationary points on the potential energy surface were calculated. It was seen that the intensity and the position of the emission spectra are dependent on the orientation of the Ar atom around the bent excited CS₂ and the distance between two components. The information about the structural parameters of the complex related to the global minimum was obtained under the pseudodiatomic approximation with assistance of ab initio potential. The presented investigation could be useful for further theoretical and experimental studies of Ar–CS₂(V¹B₂) complex.     

Penning ionization of NO and O2(1Δg) by argon in the 3P1 state

Electronically excited argon in the ³ P ₁ state was generated in a flow system by resonance absorption of the λ=106·7 nm emission line. This photochemical source is particularly convenient for kinetic studies since Ar(³ P ₁) alone is produced in the primary exciting step. Energy transfer from excited argon may lead to Penning ionization:

This process has been studied for M=NO and M=O₂(¹Δ _g ). Relative concentrations of Ar* were estimated from the Penning ion current, and it is shown that Ar* is probably in the initially excited ³ P ₁ state and not in the metastable ³ P ₂ state. Kinetic studies showed that Penning ionization of NO occurs for about one in every five gas kinetic collisions with Ar(³ P ₁). The Penning process can only compete effectively with radiative loss of Ar(³ P ₁) as a result of extensive imprisonment of resonance fluorescence. Penning ionization of O₂(¹Δ _g ) is shown to proceed at nearly the same rate as ionization of NO. The decrease in Penning ion current on addition of a quenching gas, such as N₂ or O₂, is used to derive rate constants for the quenching process relative to the rate of quenching by NO. Estimated absolute rate constants are presented in a table.     

Structured emission of relaxing systems: molecular excitons in rare-gas crystals

We have computed the emission spectra of relaxing excitated-state systems in solids as a function of the rates of radiative decay (γ) from excited to ground state and non-radiative decay (γ) within the excited state. When these are comparable a pronounced dip is obtained in the central region of the spectrum, which then may have the appearance of two distinct, unrelated emission bands. The computations well reproduce the observed W and M₂ bands in Ar crystals. Accordingly, these arise from the initially excited and near repulsive regions of the A¹Σ⁺ emitting state, respectively.     

Determination of the vibrational temperature of OCS in a supersonic free jet

The rotational temperature of OCS in the v₂ = 1 excited state and the vibrational temperature in supersonic free jets seeded in Ar were measured by a diode-laser spectrometer. From the hot band associated to the ν₁ band, the rotational temperature in the v₂ = 1 state was obtained to be 5.7 K and the vibrational temperature obtained for this state was 213 K.     

Rotational distribution in simultaneously excited electronic states of NH

The rotational structures of the NH(A³Π→X³Σ^-) and (c¹Π→a¹Δ) bands were excited simultaneously in electrical discharges through NH₃ and NH₃ + Ar. The A³Π state exhibits higher rotational temperature than the c¹Π state. The NH (A³Π) excited in an NH₃ + Ar discharge showed non-Maxwellian distribution. Mechanisms for excitation of NH are discussed.     

Test of the intercombination rules on the two excited states (0 and 1) potential parameters of the second group metal atoms perturbed by inert gases

A set of intercombination rules has been used to calculate the two excited (³0 and ³1) state potential parameters ε₁₂ and R₁₂ of Hg, Cd and Zn interacting with inert gases (Xe, Kr, Ar and Ne). The results obtained with these rules are compared with various experimental and theoretical results for these molecules. The rules can be very well used for determination of the position of the potential minimum for the two states of all molecules. Concerning the well depths of the two states (³0 and ³1) of these molecules, it is observed that for the more bounded excited state ³0 some of these rules give results that are in close agreement with experimental data especially for molecules consisting of heavy atoms but for the shallow excited state ³1 these rules cannot be used.     

Beta-neutrino recoil broadening inβ-delayed proton emission of32Ar and33Ar

In high-resolution measurements of delayed protons following theβ-decays of³²Ar and³³Ar we have observed significant line broadening caused by theβ —gn recoil motion of the daughter nuclei. The measured line shapes agree well with the calculation for both Fermi and Gamow-Teller decays and yield new spectroscopic information on the transitions and excited states involved. For the Gamow-Teller strength in theβ ⁺-decay of³³Ar to the isobaric analogue state in³³Cl an upper limit of B_GT<0.029 was determined.     

Near threshold effects in positron-atom (molecule) scattering

The energy dependence of the total cross-sections for ionization by positron impact has been measured for He, Ne, Ar, CO₂ and O₂. The interplay between this and other scattering channels has been investigated. The data for He, Ne and Ar has been interpreted using a combination ofR-matrix and threshold theories in the neighbourhood of the Ps formation threshold. In CO₂, an enhancement of the Ps formation probability is found when the remnant ion is left in an excited state. In O₂, structure has been observed which indicates channel-coupling with excitation to the Schumann-Runge continuum.     

CALCULATIONS OF STRETCHING VIBRATIONAL ENERGY LEVELS OF THE CH3I MOLECULE BY A NONLINEAR MODEL

A nonlinear model, i.e. the quantized discrete self-trapping equation, is applied to calculate the highly excited CH stretching vibrational energy levels of the CH₃I molecule in the liquid phase at the electronic ground state up to n=8. The obtained results agree well with the experimental data and with those obtained from local mode model calculations. We note that the dominant feature of the methyl CH stretching vibrational energy levels of the CH₃I molecule is a pattern of local mode pairs. When n≥7, all the vibrational energy of the CH₃ group can nearly be localized on a single CH bond.     

Excited state geometry optimisation using Fock-space multi-reference coupled cluster method

The Fock-space multi-reference coupled cluster method is used for the geometry optimisation of the low-lying excited states of the molecules. Molecular geometries of the carbon monohydride cation (CH)⁺, water (H₂O), ozone (O₃) and formaldehyde (HCHO) in their low-lying excited states are optimised. Excited state gradients are calculated using finite field multi-reference coupled cluster method. We compare our results with other theoretical and/or experimental results, wherever available.     

Excited state positronium formation in low density gases

Excited state Ps atoms formed in low density Ne, Ar and H₂ gases have been observed for the first time. The maximum yield was estimated to be ≈ 5.7 × 10^?2 excited Ps atoms per stopped positron of energy ≈ 16 eV in H₂. This is about 14 times greater than previous maximum yields.     

Electrooptical Absorption Measurements (EOAM) Testify Existence of two Conformers of Prodan and Laurdan with Different Dipole Moments in Equilibrium Ground and Franck-Condon Excited State

The results from the electrooptical absorption measurements (EOAM) on the equilibrium ground and excited Franck-Condon state dipole moments of Prodan and Laurdan in 1,4-dioxane are presented. As follows from experiments Prodan and Laurdan in the equilibrium ground and excited Franck-Condon state have two conformers with considerably different dipole moments. The electrical dipole moments and the transition dipole moment, obtained from the short-wavelength region of the absorption spectrum are parallel. The electrical dipole moments measured at the long-wavelength spectral region are parallel to each other but not parallel to the transition dipole moment m _a. The angle θ between the transition dipole moment m _a and the dipole moment in the equilibrium ground state μ _g of the long-wavelength conformer is about 30⁰ for both probes. Obtained results evidence that donor-acceptor pairs of the short-wavelength and long-wavelength conformers are not located on the same axis. Two low-energy conformers of Prodan have been found by density functional theory (DFT) calculations, differing in the orientation of the carbonyl group towards the naphthalene system.     

Red Emitting Coumarin—Azo Dyes : Synthesis,Characterization, Linear and Non-linear Optical Properties-Experimental and Computational Approach

The coumarin molecules with 7-(N,N-diethylamino) substitution and aryl azo (Ar–N=N-) at 3-position were synthesized, by reacting diazonium salt of substituted amines and 7-(N, N-diethylamino)-4-hydroxy coumarin under basic conditions. They were found to be fluorescent despite the presence of azo group. The azo group rotation was blocked by complexing with -BF₂, so as to get a red shift in absorption. The azo molecules show charge transfer, whereas BF₂-complexes do not. The dipole moment ratios between the ground and excited states calculated suggest highly polar excited state and an intra-molecular charge transfer at the excited state in the case of azo dyes. The NLO properties were calculated by solvatochromic method and computationally. Second order hyperpolarizability was found to be 46 to 1083 times more than urea. DFT and TDTDF calculations were performed to understand the electronic properties of the molecules at the ground as well as excited states.     

New model of electron stimulated desorption from physisorbed monolayers application to Ar and N2O on Ru(001)

A new model of electron stimulated desorption from monolayers physisorbed on metal surfaces is proposed in which the excited surface state potential is due to a chemical rather than an image force acting on the adsorbed particle after the initial electronic excitation is completed. The equilibrium positions of the excited and the ground state potentials may nearly coincide and desorption is a purely quantum mechanical effect. It is demonstrated that, unlike for the commonly accepted Antoniewicz model, both, the kinetic energy distributions of desorbing neutral particles and the total desorption yields calculated in the model proposed here are consistent with the existing experimental data for Ar and N₂O physisorbed on Ru(001).     

Numerical study of the effect of gas flow in low pressure inductively coupled Ar/N<Subscript>2</Subscript> plasmas

The effect of gas flow in low pressure inductively coupled Ar/N₂ plasmas operating at the rf frequency of 13.56 MHz and the total gas pressure of 20 mTorr is studied at the gas flows of 5–700 sccm by coupling the plasma simulation with the calculation of flow dynamics. The gas temperature is 300 K and input power is 300 W. The Ar fractions are varied from 0% to 95%. The species taken into account include electrons, Ar atoms and their excited levels, N₂ molecules and their seven different excited levels, N atoms, and Ar⁺, N⁺, N₂ ⁺, N₄ ⁺ ions. 51 chemical reactions are considered. It is found that the electron densities increase and electron temperatures decrease with a rise in gas flow rate for the different Ar fractions. The densities of all the plasma species for the different Ar fractions and gas flow rates are obtained. The collisional power losses in plasma discharges are presented and the effect of gas flow is investigated.