Experimental selection of molecules for isotope separation by multiple-photon dissociation in an intense IR field

Consideration is given to selectivity estimation methods during polyatomic molecules dissociation by multiple photon absorption. Taking the CH₃NO₂ molecule as an example, the possibilities of selectivity estimations made on the electron-excited dissociation products (luminescence) are shown, as well as on the measurements of the energy absorbed in a strong IR field being transformed into heat. Some advantages of the last method of the selectivity estimation are discussed. The experiments on nitrogen-isotopes separation in the mixture of CH₃ ¹⁴NO₂ and CH₃ ¹⁵NO₂ molecules are carried out when exciting thev ₇ vibration with the isotope shift of about 7 cm⁻¹ and thev ₁₃ vibration with no isotope shift in the linear absorption spectrum. The contribution of secondary chemical reactions to the separation process is discussed.     

Electronic states of C2H6 +

The complete photoelectron spectrum of ethane has been measured in the valence region using Ne, He I, and He II resonance radiation. The resolution of these spectra is sufficient to partially resolve vibronic structure accompanying the transition to the ground ionic state. The similarity of this structure with that obtained from model calculations using the Jahn-Teller theorem strongly suggests that the active vibration in this transition is a doubly degenerate CH₃ deformation mode and that the ground ionic state is a Jahn-Teller split ² E _g state. These experiments suggest a ² A _1g term for the first excited ionic state. The transition to the ² A _2u state of the ion contains evidence for two active vibrations v ₁ (C–H stretch) and v ₃ (C-C stretch).     

Kinetic aspect of CO2 reforming of CH4 on Ni(1 1 1): A density functional theory calculation

Reaction pathways of CO₂ reforming of CH₄ on Ni(1 1 1) were investigated by using density functional theory calculation. The computed kinetic parameters agree with the available experimental data, and a new and simplified mechanism was proposed on the basis of computed energy barriers. The first step is CO₂ dissociation into surface CO and O (CO₂ → CO + O) and CH₄ sequentially dissociation into surface CH and H (CH₄ → CH₃ → CH₂ → CH). The second step is CH oxygenation into CHO (CH + O → CHO), which is more favored than its dissociation into C and hydrogen (CH → C + H). The third step is the dissociation of CHO into surface CO and H (CHO → CO + H). Finally, H₂ and CO desorb from Ni(1 1 1) and form free H₂ and CO. The rate-determining step is the CH₄ dissociative adsorption, and the key intermediate is surface adsorbed CHO. Parameters, which might modify the proposed mechanism, have been analyzed. In addition, the formation, deposition and elimination of surface carbon have been discussed accordingly.     

Calculation of the carrier concentration dependence of the Auger lifetime in degenerate n-type (Hg,Cd)Te

The Auger 1 lifetime expression for degenerate n-type (Hg_1?xCd_x)Te has been derived by replacing the Fermi-Dirac distribution functions for the pair of Auger collision electrons with step functions. The lifetime is calculated as a function of carrier concentration, _n0 for several values of energy gap and temperature using a non-parabolic band structure. We find that for strong degeneracy, the lifetime, τ_AlsuD varies as n₀^?γ where 0.7 ?γ? 1 and that γ is dependent upon the energy gap. The relatively slow dependence of τ_Al^D upon n₀, compared to the non-degenerate case () is due to the dependence of τ_Al on the threshold energy which for the degenerate case is a linear function of the Fermi energy, hence, a function of n₀.     

Investigations of multiphoton selective dissociation of (CH3)2O in the field of a pulsed CO2 laser

Experimental investigations of multiphoton selective dissociation of (CH₃)₂O induced by a pulsed CO₂ laser have been conducted. Separation of H, C, and O isotopes was performed in enriched mixtures and in samples with the natural abundance. The following coefficients of selectivity have been obtained:K _D/K_H=4.0,K ₁₃/K₁₂=1.7, andK ₁₈/K₁₆≧1.6. We studied the dependences of the selectivity coefficient on ether pressure, on the laser energy and frequency as well as the influence of secondary chemical reactions on the dissociation selectivity. Estimations made by using the RRKM theory have indicated that ether molecules that decompose have an average excitation energy above the dissociation threshold of ∼1.5 kcal/mole.     

In‐situ Raman spectroscopy study of Ru/TiO2 catalyst in the selective methanation of CO

Raman spectroscopic technique has been used to characterize a Ru/TiO₂ catalyst and to follow in situ their structural changes during the CO selective methanation reaction (S‐MET). For a better comprehension of the catalytic mechanism, the in‐situ Raman study of the catalysts activation (reduction) process, the isolated CO and CO₂ methanation reactions and the effect of the composition of the reactive stream (H₂O and CO₂ presence) have been carried out. Raman spectroscopy evidences that the catalyst is composed by islands of TiO₂–RuO₂ solid solutions, constituting Ru–TiO₂ interphases in the form of Ru_xTi_{1 − x}O₂ rutile type solid solutions. The activation procedure with H₂ at 300 °C promotes the reduction of the RuO₂–TiO₂ islands generating Ru^o–Ti³⁺ centers. The spectroscopic changes are in agreement with the strong increase in chemical reactivity as increasing the carbonaceous intermediates observed. The selective methanation of CO proceeds after their adsorption on these Ru^o–Ti³⁺ active centers and subsequent C―O dissociation throughout the formation of CH_x/C_nH_x/C_nH_xO/CH_x―CO species. These intermediates are transformed into CH₄ by a combination of hydrogenation reactions. The formation of carbonaceous species during the methanation of CO and CO₂ suggests that the CO presence is required to promote the CO₂ methanation. Similar carbonaceous species are detected when the selective CO methanation is carried out with water in the stream. However, the activation of the catalysts occurs at much lower temperatures, and the carbon oxidation is favored by the oxidative effect of water. Copyright © 2015 John Wiley & Sons, Ltd.     

Mobility–diffusivity relationship for heavily doped organic semiconductors

The relationship between the diffusivity D _n and the mobility μ _n of chemically doped organic n-type semiconductors exhibiting a disordered band structure is presented. These semiconductors have a Gaussian-type density of states. So, calculations have been performed to elucidate the dependence of D _n /μ _n on the various parameters of this Gaussian density of states. Y. Roichman and N. Tessler (Appl. Phys. Lett. 80:1948, 2002), and subsequently Peng et al. (Appl. Phys. A 86:225, 2007), conducted numerical simulations to study this diffusivity–mobility relationship in organic semiconductors. However, almost all other previous studies of the diffusivity–mobility relationship for inorganic semiconductors are based on Fermi–Dirac integrals. An analytical formulation has therefore been developed for the diffusivity/mobility relationship for organic semiconductors based on Fermi–Dirac integrals. The D _n /μ _n relationship is general enough to be applicable to both non-degenerate and degenerate organic semiconductors. It may be an important tool to study electrical transport in these semiconductors.     

Influence of cooperativity on the frequency shift of the Ar–H stretch vibration in HArF complexes

CH₃Li–FArH–X (X?=?H₂, OC, N₂, P₂, CO₂, CO, BeH₂) trimers have been investigated using quantum chemical calculations at the QCISD/6-311++G(2d,2p) level. The results show that the lithium bonding has a prominent effect on the strength and properties of the hydrogen bonding. The hydrogen-bonding interaction energy is increased by 160–340% due to the presence of lithium bonding. The Ar–H stretch vibration shows a blue shift in the FArH–X (X?=?H₂, OC, N₂, CO₂, CO) dimer, but a red shift in the FArH–X (X?=?P₂, BeH₂) dimer. The red shift is increased in the corresponding trimer, while the blue shift shows a different change. The blue shift is also increased in CH₃Li–FArH–X (X?=?H₂, OC, N₂, CO₂) trimers, but it changes to a red shift in the CH₃Li–FArH–CO trimer. The shift change is consistent with the explanation given by Joseph and Jemmis.     

Voltage-current characteristics of superatmospheric pressure CO2 laser discharges

The IR NH₃ laser radiation was applied for selective dissociation of CCl₄ molecules. The dissociation yield, its pressure dependence and isotopic selectivity were measured. Two-frequency dissociation of CCl₄ by the NH₃ laser and the CO₂ pump laser radiations was carried out. It was shown that the NH₃ laser is very effective for CCl₄ dissociation.     

Isotopically selective infrared multiphoton dissociation of 2-chloro-1,1,1-trifluoroethane: 13C selectivity and mechanism

13 C-selective infrared multiphoton dissociation of CF₃CH₂Cl has been studied by analyzing the distribution of ¹³C concentrations of the main products CF₂=CHCl, CF₂=CH₂, CF₂=CHF, C₂F₆, and the trace products CF₃CH₂CF₃ and CF₃CH=CHF₃. The mechanism mainly concerns the dissociation of energized CF₃CH₂Cl, the collisional stabilization of excited CF₃CH and CF₃CH₂ and the recombination of the nascent radicals. No significant radical–molecule reactions degrade the intrinsic ¹³C dissociation selectivity. High ¹³C production yield and ¹³C concentration can be attained at a laser fluence of 1.6 J/cm². Such low fluence can be used to improve focus condition and enhance photon utilization efficiency for practicable ¹³C separation. Received: 10 March 1998/Revised version: 17 September 1998     

Synchrotron-radiation-induced deposition of boron and boron carbide films from boranes and carboranes II: Nido-2,3-diethyl-2,3-dicarbahexaborane

Boron-carbon thin films have been successfully deposited on Si (111) from the synchrotron radiation induced decomposition of the nido-2,3-diethyl-dicarbahexaborane, (CH₃CH₂)₂C₂B₄H₆. There are indications that molecular precursor states to complete dissociation exist, and that dissociation is the rate limiting step. As with deposition of boron from decaborane, there is an activation barrier to dissociation of diethylcarborane on Si (111). The composition of the growing film, as determined by the boron to carbon ratio, is strongly dependent upon the boron concentration at the surface of the substrate. The boron concentration of the film increases with increasing film thickness.Part one appeared in [1]     

Matter collineation classification of Bianchi type II spacetime

In this paper we classified the matter collineations (MCs) of Bianchi type II spacetime according to the degenerate and non-degenerate energy-momentum tensor. It is shown that when the energy-momentum tensor is degenerate, most of the cases yield infinite dimensional MCs whereas some cases give finite dimensional Lie algebras in which there are three, four or five MCs. For the non-degenerate matter tensor cases we obtained that the Lie algebra of MCs is finite dimensional, in which the number of MCs are also three, four or five. Furthermore, we discussed the physical implications of the obtained MCs in the case of perfect fluid as source.     

Torsion-vibration interaction in CH3OH

The strong torsion-vibration interaction in CH₃OH has traditionally been dealt with by letting the torsional barrier height depend on vibrational excitation, and letting the vibrational energy depend on torsional excitation. By including an explicit interaction term in the Hamiltonian this is avoided, and apart from an anomaly which is presumably caused by the OH bending mode, the relative location of the vibrational ground state and the CO stretch state is well reproduced for torsional states n = 0, 1, and 2 by adjusting a single interaction constant.     

TDS and EELS observations for CO,O2, and CH3OH bound to Ru(0001)/Cu

The electronic properties of monolayers of copper atoms adsorbed onto a Ru(0001) single crystal surface have been studied with thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS) utilizing carbon monoxide (CO), dioxygen (O₂), methanol (CH₃OH), and to some extent water (H₂O) as chemical probes. Whereas a three-monolayer-thick film exhibits most properties of a Cu(111) crystal distinct deviations are found at lower Cu coverages. TDS as well as EELS show a weakened RuCO bond and a strengthened CuCO bond as a result of metal-metal interaction. The stronger CuCO bond is accompanied by a higher probability for O₂ dissociation. The mobilities of copper and oxygen atoms are such that annealing to 650 K produces an overlayer structure which is independent of adsorption sequence: Cu/O₂ or O₂/Cu, but where RuO as well as CuO vibrations can be identified. Methanol adsorbs reversibly on a monolayer of copper atoms. Metal bound methoxy species are formed in the presence of oxygen atoms. The decomposition paths of such methoxy intermediates alter towards more formaldehyde (CH₂O) relative to CO with increasing copper and methoxy coverages.     

Isotope-selective IR multiphoton dissociation of CHClF2 in the presence of NO2

2 to the CHClF₂/He mixture irradiated by a Q-switched CO₂ laser leads to oxidation of the dissociation product according to the reaction: CF₂+NO₂→COF₂+NO. The resulting COF₂ with a ¹³C content near 50% is easy to convert to CO₂ or CO for further enrichment by a nonlaser process. We measured the dependence of the fraction of dimerised CF₂ on NO₂ pressure p_NO2 and the amount of NO₂ required to suppress dimerisation on the dissociation yield. Both agree with a kinetic model using known rate constants. For the range of the dissociation parameters (¹³CF₂ yield of 10% per pulse, isotope selectivity of 130) of practical interest, 95% of the CF₂ produced is oxidized at p_NO2≈1/2p_CHClF2. In the absence of NO₂, major (20%–35%) losses of CF₂ at the metal walls of the irradiation system were observed. Addition of NO₂ suppresses them. For comparison, we also used O₂ as a scavenger in CHClF₂ dissociation. NO₂ is by orders of magnitude more efficient. Received: 21 January 1997/Revised version: 23 March 1997     

Determination of dipole moments in vibrationally excited CH3OH by combined far infrared laser Stark effect and transferred Lamb dip spectroscopy

Optically pumped fir laser lines in CH₃OH are studied by combined fir emission Stark effect and transferred Lamb dip spectroscopy. Effective dipole moments are determined, which are interpreted as being associated with hybridization of CO stretch states and states of a different vibrational mode, with same J but different K. Far-infrared laser lines originating from large offset absorption lines, which are Stark-tuned into coincidence, are assigned. The results suggest that the mode interacting with the CO stretch is the CH₃ symmetric rocking mode.     

The effect of CO2 on the speciation of bromine in low‐temperature geological solutions: an XANES study

CO₂‐rich solutions are common in geological environments. An XANES (X‐ray absorption near‐edge structure) study of Br in CO₂‐bearing synthetic fluid inclusions has revealed that Br exhibits a strong pre‐edge feature at temperatures from 298 to 423 K. Br in CO₂‐free solutions does not show such a feature. The feature becomes smaller and disappears as temperature increases, but reappears when temperature is reduced. The size of the feature increases with increasing X(CO₂) in the fluid inclusion, where X(CO₂) is the mole fraction of CO₂ in the solution [n_CO2/(n_CO2 + n_H2O + n_RbBr); n indicates the number of moles]. The pre‐edge feature is similar to that shown by covalently bonded Br, but observed and calculated concentrations of plausible Br‐bearing covalent compounds (Br₂, CH₃Br and HBr) are vanishingly small. An alternative possibility is that CO₂ affects the hydration of Br sufficiently that the charge density changes to favour the 1s–p level transitions that are thought to cause the pre‐edge peak. The distance between the first two post‐edge maxima in the XANES also decreases with increasing X(CO₂). This is attributed to a CO₂‐related decrease in the polarity of the solvent. The proposed causes of the observed features are not integrated into existing geochemical models; thus CO₂‐bearing solutions could be predicted poorly by such models, with significant consequences for models of geological processes such as ore‐formation and metamorphism.     

Dissociation of CO on supported Rh

The adsorption and dissociation of CO have been investigated on alumina supported Rh at atmospheric pressure. The dissociation of CO was detected above 473 K. The carbon formed reacted with H₂ even at 300–473 K yielding CH₄. A significant aging of surface carbon occurred above 573 K.     

Solvent‐dependent structural dynamics of 2(1H)‐pyridinone in light absorbing S4(ππ*) state

The B‐band resonance Raman spectra of 2(1H)‐pyridinone (NHP) in water and acetonitrile were obtained, and their intensity patterns were found to be significantly different. To explore the underlying excited state tautomeric reaction mechanisms of NHP in water and acetonitrile, the vibrational analysis was carried out for NHP, 2(1D)‐pyridinone (NDP), NHP–(H₂O)_n (n = 1, 2) clusters, and NDP–(D₂O)_n (n = 1, 2) clusters on the basis of the FT‐Raman experiments, the B3LYP/6‐311++G(d,p) computations using PCM solvent model, and the normal mode analysis. Good agreements between experimental and theoretically predicted frequencies and intensities in different surrounding environments enabled reliable assignments of Raman bands in both the FT‐Raman and the resonance Raman spectra. The results indicated that most of the B‐band resonance Raman spectra in H₂O was assignable to the fundamental, overtones, and combination bands of about ten vibration modes of ring‐type NHP–(H₂O)₂ cluster, while most of the B‐band resonance Raman spectra in CH₃CN was assigned to the fundamental, overtones, and combination bands of about eight vibration modes of linear‐type NHP–CH₃CN. The solvent effect of the excited state enol‐keto tautomeric reaction mechanisms was explored on the basis of the significant difference in the short‐time structural dynamics of NHP in H₂O and CH₃CN. The inter‐molecular and intra‐molecular ESPT reaction mechanisms were proposed respectively to explain the Franck–Condon region structural dynamics of NHP in H₂O and CH₃CN.Copyright © 2015 John Wiley & Sons, Ltd.     

Comparison for σ-hole and π-hole tetrel-bonded complexes involving cyanoacetaldehyde

Ab initio calculations have been performed for the complexes of cyanoacetaldehyde (CA) with TH₃F (T = C, Si, Ge and Sn) and F₂TO (T = C and Si). The σ-hole and π-hole tetrel-bonded complexes are formed for TH₃F and F₂TO, respectively. In general, three minima complexes (N, O–A and O–B) are obtained for each tetrel donor. Most complexes are stabilised by a primary tetrel bond and a secondary hydrogen bonding. TH₃F–N/F₂CO–N has greater stability than TH₃F–O/F₂CO–O, but a reverse result is found in the complexes of F₂SiO although they have comparative interaction energies. Charge transfer from the lone pair on the N/O atom of CA into the T–F σ^* antibonding orbital leads to the stabilisation of the TH₃F complexes. Accordingly, the T–F bond is extended and its stretch vibration displays a redshift. A breakdown of the individual forces involved attributes the stability of the complex mainly to electrostatic energy, with relatively large dispersion term in the CH₃F complexes and relatively large polarisation energy in the F₂SiO complexes.