On a semiclassical approach to energy transfer in polyatomic molecules

A semiclassical method for energy transfer in polyatomic molecules is suggested. The method is based on a partial quantization of the vibrational degrees of freedom. The remaining degrees of freedom are treated classically. As an example a system with three quantum coordinates is considered. The Coriolis coupling terms are taken into account in the quantum mechanical part of the system and discussed for the N₂ + CO₂ system.     

Looking Back and Looking Ahead in Electrochemical Reduction of CO2

Electrochemical reduction of carbon dioxide (CO₂) to valuable organic compounds is promising as to recycling of carbon source of CO₂ and technical compatibility with systems using renewable energy resources. In recent years, considerable efforts have been devoted to the research field of CO₂ conversion using electrocatalysis. This personal account particularly focuses on the recent progress that has been achieved by the Ertl Center and a number of groups in South Korea that becomes one of the larger CO₂ emitters. The research trends of catalyst development divided into different categories according to the primary products are presented first. Afterwards, several studies on theoretical calculations and electrolytic reactors are reviewed taking into account the fundamental understanding and feasibility of the CO₂ electroreduction. Finally, a perspective on the challenges and needs in achieving the advanced level of research and development is presented.     

Ab initio computation of vibrational relaxation rate coefficients for the collisions of CO2 with helium and neon atoms

Ab initio calculations of rate coefficients are reported for the vibrational relaxation of CO₂ molecules in collision with helium and neon atoms. Self consistent-field computations have been performed to parameterise simple three-dimensional potential energy functions which have been used in vibrational close-coupling, rotational infinite-order-sudden calculations of rate coefficients. Excellent agreement is obtained between the calculated and experimental rate coefficients for the deactivation of the (01₁0) vibrational level in the He + CO₂ system at temperatures of 300 K and above. The ab initio predictions of rate coefficients for relaxation of CO₂ vibrational levels such as (10⁰0) and (02⁰0) should be useful in computer simulations of CO₂ lasers.     

Enhancement of the bimolecular reaction of hydrogen iodide by vibrational excitation

The bimolecular reaction of HI in CO₂, which was excited vibrationally by irradiation of a continuous-wave CO₂ laser light, was investigated in the temperature range of 721–980 K. An enhancement of the reaction rate by a factor of about 2.5 was observed in the 1:1 HI? CO₂ mixture in comparison with the rate in pure HI when both sample gases were irradiated by a CO₂ laser (50 W) at 1 torr. However, in the HI-SF₆ mixtures the decomposition rate of HI was not accelerated by irradiation of the CO₂ laser. Thus the enhancement is attributed to vibrational excitation of HI through collisional energy transfers from laser-excited CO₂ (00°1). At lower total pressures or at lower partial pressures of HI in HI-CO₂ mixtures the enhancement was more significant because of inefficient vibrational deactivation of excited HI. A model calculation gave the result in agreement with the experimental one if the effective activation energy is assumed as E_a? = E_a - αE_vib, where E_a is the activation energy for the thermal reaction, E_vib is the vibrational energy of two colliding HI molecules, and α is estimated to be about 0.7. This means that a part of the vibrational energy of reacting HI is employed to reduce the activation energy for the translational or rotational degree of freedom.     

The dynamics of the SiF4 - sensitized processes initiated by a pulsed CO2 laser

A model enabling the examination of the dynamics of the SiF₄ - sensitized processes initiated by a pulsed CO₂ laser is outlined, taking into account three effects : energy absorption, energeticity of chemical reactions and expansion of hot gas created upon interaction of molecules with the infrared photon field. This approach was used to elucidate some aspects regarding the decomposition of PH₃ and GeH₄.     

Vibrational energy transfer from C2H4 to CO2 studied at low temperatures in a jet by infrared double resonance

In a mixed CO₂-C₂H₄ jet the populations of rotational levels in the ground andv ₂ (667 cm^?1) vibrational state of CO₂ are probed by color center laser absorption. The rotational distributions are well described by a rotational temperature. On the timescale of the expansion thev ₂ state does only relax through resonant deexcitation by CO₂ clusters. A 10% fraction of C₂H₄ molecules is excited into thev ₇ (949 cm^?1) level by CW CO₂ laser absorption, subsequently releasing energy into the expansion due to relaxation to lower C₂H₄ vibrational states and to thev ₂ mode of CO₂. Observed are the induced rise of rotational temperature and the increase ofv ₂ level population. By probing at several distances from the nozzle the temperature range from 30–155 K is covered. The vibrational transfer of the excited C₂H₄ to thev ₂ level of CO₂ has a near quadratic inverse temperature dependence, increasing by a factor of 40 when the temperature is lowered from 155 K to 30 K.     

Spectroscopic properties of the methyl radical calculated from UHF SCEP wavefunctions

The potential energy surface of symmetric stretching and out-of-plane bending motions for the methyl radical has been calculated from UHF SCEP wavefunctions. Anharmonic vibrational frequencies are computed by a variational method and transition dipole moments and Einstein coefficients of spontaneous emission are reported. Isotropic hyperfine coupling constants are obtained in agreement with experiment to within 4% when calculated by differentiation of the perturbed CEPA-1 energy and taking vibrational averaging into account. Also, the temperature dependence of the proton hyperfine coupling constant compares well with experimental results. The vibrational fine structure of the first band of the photoelectron spectra of CH₃ and CD₃ is calculated in good agreement with experiment.     

Triatom-triatom collisions: Fixed angle close-coupling study of vibrational excitation in the 12CO2+13CO2 system

A close-coupling approach to the calculation of quantal vibrational transition probabilities for the fixed angle scattering of a linear triatomic molecule with another linear triatomic molecule is described. The method is applied to the ¹²CO₂+¹³C0₂ collisional system. For a calculated inelastic transition probability to have an appreciable magnitude, it is found that the amount of energy transferred in a transition must be very small and just one quantum of energy must be exchanged between either the symmetric stretch or the asymmetric stretch vibrational modes of ¹²C0₂ and ¹³CO₂. For collisional energies away from threshold, the probabilities for transitions involving the symmetric stretch ¹²CO₂ and ¹³CO₂ modes are insensitive to long range multipole terms in the potential energy surface, while the probabilities for energy exchange between the asymmetric stretch modes are considerably diminished when the long range terms are removed from the potential energy surface. A brief discussion is presented on the possibilities of extending the technique to the calculation of vibrational excitation cross sections for three-dimensional triato—triatom collisions.     

Vibrational population distributions in infrared multiple-photon excitation probed by coherent anti-stokes Raman spectroscopy

Coherent anti-stokes Raman spectroscopy has been used as a probe for molecules pumped by high-intensity CO₂ laser radiation. Ozone, sulfur hexafluoride, and chloroethane have been investigated. The intensity of the ground-state CARS signal is decreased by IR multiphoton excitation, but signals at new vibrational origins do not appear. The observations on chloroethane are interpreted in terms of a rate-equation model for IRMPE, modified to take vibrational redistribution into account.     

Rate constants for the reactions of Ar+ with CO2 and SO2 as a function of temperature (300–1500 K)

We have measured the rate constants for the reactions of Ar⁺ with CO₂ and SO₂ from 300 to 1500 K in a high temperature flowing afterglow. For the reaction with CO₂, we have found that all modes of energy, i.e., translation, rotation, and vibration, affect the rate constant to the same degree up to a total energy of 0.4 eV. Above 0.4 eV total energy, internal energy decreases the rate constants more effectively than does translational energy. For the reaction of Ar⁺ with SO₂, the rate constants go through a minimum at about 900 K. By comparing our results to drift tube data, we derive rate constants for reaction from the υ=0 and υ>0 vibrational levels. At low energy, the vibrationally excited SO₂ molecules react with Ar⁺ approximately twice as fast as the ground state molecules. Both vibrational modes have similar temperature dependences.     

Double step-ladder model of activation in the processes of high-temperature dissociation of polyatomic molecules

A unified mechanism of the interaction of vibrational relaxation and dissociation of polyatomic molecules working in a wide temperature range (from 2000 to 10000 K) is proposed, which is described by a double step-ladder model. Relaxation due to collisions with the transfer of small and large portions of energy is taken into account. The transfer efficiency of the portions of thermal energy in the high-temperature decomposition upon the collisions of CO₂ molecules with atomic and molecular partners is determined. The reaction rate constant of high-temperature dissociation of carbon dioxide is calculated. The data presented in the article suggest a new method for elucidating the mechanism of energy exchange in the absence of vibrational and translational equilibrium and at ultrahigh temperatures when the dissociation takes place during the time of several collisions.     

Tests of Sharma's theory of near resonant energy exchange using the molecules CO2 and COS

Sharma's theory of near resonant energy exchange has been tested by measuring the rates of vibrational energy excitation in the molecules CO₂ and COS using hydrogen and its isotopes and isomers as collision partners. The calculated rates agree well with experiment in the CO₂ systems, but fail for those using COS.     

Raman spectroscopic study of CO2 sorption process in poly methyl methacrylate

Raman spectra of poly methyl methacrylate (PMMA) in contact with compressed CO₂ were measured, to investigate effects of CO₂ sorption on structure of the PMMA chain. The solubility and diffusivity of CO₂ in PMMA were estimated from temporal variations of intensities of CO₂ peaks. The PMMA structure was analyzed from temporal variations of vibrational energies of PMMA peaks. The results show that the vibration energies of C H stretching modes of PMMA increase with CO₂ sorption, whereas those for skeletal vibration modes decrease. These energy shifts are attributed to elongational deformation of PMMA. The PMMA structure is deformed with stretching of the chains as a bundle. From energy shifts of the CO₂ peaks, the size of the CO₂ accommodated space between the bundles is estimated to be 1.6–1.9 nm. Furthermore, it was observed that the vibrational energies of the PMMA modes in foamed glassy PMMA differ from the values in glassy PMMA without foams. This result suggests that the local structure of the PMMA chain changes with the process of the CO₂ sorption and/or foaming. The local structure of the PMMA chain might be one of the dominant factors governing the properties of cellular materials. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 831–842, 2008     

Infrared laser enhancement of the reaction of sulfur hexafluoride with sodium vapor

The vibrational energy dependence of the rate of the gas phase reaction of Na with SF₆ has been determined in a diffusion cloud experiment using CO₂ laser excitation. The “conversion efficiency” of ca. 40% for vibrational energy suggests a preference for vibrational over the translational energy when compared with “prior” statistical expectation.     

Diamondoids approach to electronic,structural, and vibrational properties of GeSi superlattice nanocrystals: a first-principles study

Germanium silicide diamondoids are used to determine electronic, structural, and vibrational properties of GeSi superlattice nanocrystals and bulk as their building block limit. Density functional theory at the generalized gradient approximation level of Perdew, Burke, and Ernzerhof (PBE) with 6-31G(d) basis including polarization functions is used to investigate the electronic structure of these diamondoids. The investigated molecules and diamondoids range from GeSiH₆ to Ge₆₃Si₆₃H₉₂. The variation of the energy gap is shown from nearly 7 eV toward bulk value which is slightly higher than the average of Si and Ge energy gaps. Variations of bond lengths, tetrahedral, and dihedral angles as the number of atoms increases are shown taking into account the effect of shape fluctuations. Localized and delocalized electronic charge distribution and bonds for these molecules are discussed. Vibrational radial breathing mode (RBM) converges from its initial molecular value at 332 cm^?1 to its bulk limit at 0 cm^?1 (blue shift). Longitudinal optical-highest reduced mass mode (HRMM) converges from its initial molecular value 332 cm^?1 to experimental bulk limit at 420.7 cm^?1 (red shift). Hydrogen vibrational modes are nearly constant in their frequencies as the size of diamondoids increases in contrast with lower frequency Ge–Si vibrational modes. GeSi diamondoids can be identified from surface hydrogen vibrational modes fingerprint, while the size of these diamondoids can be identified from Ge–Si vibrational modes.     

Capture of CO<Subscript>2</Subscript> from flue gas by vacuum pressure swing adsorption using activated carbon beads

Vacuum pressure swing adsorption (VPSA) for CO₂ capture has attracted much research effort with the development of the novel CO₂ adsorbent materials. In this work, a new adsorbent, that is, pitch-based activated carbon bead (AC bead), was used to capture CO₂ by VPSA process from flue gas. Adsorption equilibrium and kinetics data had been reported in a previous work. Fixed-bed breakthrough experiments were carried out in order to evaluate the effect of feed flowrate, composition as well as the operating pressure and temperature in the adsorption process. A four-step Skarstrom-type cycle, including co-current pressurization with feed stream, feed, counter-current blowdown, and counter-current purge with N₂ was employed for CO₂ capture to evaluate the performance of AC beads for CO₂ capture with the feed compositions from 15–50% CO₂ balanced with N₂. Various operating conditions such as total feed flowrate, feed composition, feed pressure, temperature and vacuum pressure were studied experimentally. The simulation of the VPSA unit taking into account mass balance, Ergun relation for pressure drop and energy balance was performed in the gPROMS using a bi-LDF approximation for mass transfer and Virial equation for equilibrium. The simulation and experimental results were in good agreement. Furthermore, two-stage VPSA process was adopted and high CO₂ purity and recovery were obtained for post-combustion CO₂ capture using AC beads.     

Non-equilibrium vibrational kinetics of CO pumped by vibrationally excited nitrogen molecules: A comparison between theory and experiment

Temporal evolution of CO vibrational distribution pumped by vibrationally excited nitrogen molecules has been experimentally determined by side-on infrared emission spectroscopy. Cooled and uncooled radiofrequency N₂ discharges have been utilized to pump vibrational energy in N₂, which then does excite CO in the post-discharge regime. CO₂ formed during the contact time has been detected by a mass-spectrometric technique. The experimental results show the different stages of the relaxation of CO, merely the pumping, the redistribution and the loss of vibrational quanta introduced by N₂. The results have been rationalized by means of a theoretical model which qualitatively reproduces the experimental relaxation of CO distributions as well as the production of CO₂.     

Nonadiabatic Corrections To The Vibrational Energies Of The Hydrogen Molecule

The nonadiabatic corrections to the vibrational levels of H₂ and D₂ are evaluated for the X¹∑⁺_g state by taking into account the coupling with the E, F¹∑⁺_g electronic state. The computed corrections diminish the existing discrepancy between the experimental and theoretical vibrational quanta by about 20%.     

Sensitized multiphoton dissociation of UF6 IN SF6-UF6 mixtures by a tea CO2 laser

UF₆ undergoes decomposition in the presence of SF₆ when mixtures of both are irradiated with a TEA CO₂ laser. The mechanism for UF₆ decomposition may involve vibrational energy transfer from excited SF₆ and laser absorption from the same laser pulse by excited UF₆ in its vibrational quasi-continuum     

Collisionless intramolecular energy transfer in vibrationally excited SF6

Infared-infrared double resonance measurements of the ν₃ absorption band of SF₆ using a CO₂ TEA pump laser (0.3 J/cm² maximum excitation) and a cw CO₂ probe laser have been carried out. These measurements probe both the coherent and quasi-continuum phases of the multiphoton dissociation process in SF₆. The spectral and temporal dependences of the induced absorption are obtained. The results give the first direct measurement of collisionless intramolecular vibrational energy redistribution times in a vibrationally excited complex molecule; for an excess energy of three quanta deposited in the ν₃ mode of SF₆, the ν₃ mode comes into equilibrium with all the remaining vibrational degrees of freedoom with a 3 ± 1 μs time constant.