bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification of a new ion-pair state of Br2 by a sequential two-photon excitation technique
Authors: Ishiwata Takashi , Ohtoshi Hirokazu , Tanaka Ikuzo
Published: February 2003

A new ion-pair state of Br2 has been identified by a sequential two-photon excitation technique through the B 3Π0u+ state. The associated UV emission spectra are analyzed as a function of excitation frequency and the new state can be assigned to f(0+g) which correlates with Br−(1S)+Br+(3P0). The spectroscopic constants for 79Br2 are Te=53 101.7 cm−1, ωe=152.8 cm−1, ωexe=0.42 cm−1, Be=0.042 60 cm−1, and re=3.167 Å.
1
9F NMR chemical shifts due to intermolecular interactions in F2C=CFX. A quantitative measure of the nuclear site effect
Authors: Jameson Cynthia J. , Jameson A. Keith , Oppusunggu D.
Published: February 2003

In CF2=CFX (X=H, Cl, Br, I) there are three probe nuclei for studying the magnitudes of intermolecular effects on nuclear shielding. The values of σ1=limρ→0 (∂σ/∂ρ)T, the change in the nuclear shielding due to interactions between pairs of molecules, have been obtained from the resonance frequencies in medium to low density gas samples. The nuclear site effect gives the most exposed one of the three probe nuclei the largest magnitude of σ1, i.e., the F trans to X=Cl, Br, I or the F cis to X=H. The gas-to-liquid shifts show exactly the same ordering as the σ1 in the dilute gas. A model is described which relates a calculated site factor to the observed σ1 values. With this model, the observed density dependence of the NMR isotope shift in D2/HD is calculated. This model provides a mechanism for nonspecific solvent effects on the NMR isotope shift.
Simultaneous analysis of the ν2, ν3, ν5, and ν6 infrared bands of CD3F
Authors: Halonen L. , Kauppinen J. , Caldow G. L.
Published: February 2003

The infrared spectrum of CD3F from 800 to 1220 cm−1 measured with resolution of about 0.0045 cm−1 has been analyzed using a model which treats the four fundamentals ν2(A1), ν3(A1), ν5(E), and ν6(E) simultaneously. Sixty-two parameters were refined in a least squares fit of microwave data and over 4200 upper state energy levels with J′≤40 obtained from the infrared spectrum. The rms deviation of the infrared data was 0.706×10−3 cm−1. Due to strong vibration-rotation resonances many normally ‘‘forbidden’’ infrared transitions have been observed which has allowed us to determine the ground state constants A0 and DK0 with high accuracy. The energy levels in many of the observed submillimeter-wave laser lines of CD3F can now be identified.
Laser induced fluorescence study of the B̃ 2A″ → X̃ 2A″ transition of the vinoxy radical in a supersonic free jet expansion
Authors: DiMauro L. F. , Heaven M. , Miller Terry A.
Published: February 2003

The vinoxy, CH2CHO, free radical has been produced in a supersonic free jet expansion by ArF excimer laser photolysis of a suitable precursor like ethyl vinyl ether. Electronic spectra involving the B̃ 2A″ ↔ X̃ 2A″ transition have been observed at vinoxy temperatures between 2–30 K. Rotational structure as well as spin splittings have been resolved. Analysis of the spectrum yields the electronic term value, vibrational frequencies, rotational and spin-splitting constants for both electronic states. These molecular parameters provide considerable information about the electronic and geometric structure of the radical.
Vibrational predissociation and infrared spectrum of the Ar–HCl van der Waals molecule
Authors: Hutson Jeremy M.
Published: February 2003

The energies and lifetimes of bound and vibrationally predissociating states of Ar–HCl are calculated, using accurate close coupling calculations on the best available potential energy surface, and the selection rules governing the infrared spectrum are described. The vibrational predissociation lifetimes are of the order of microseconds; thus the resulting spectroscopic lines will be sharp (width <1 MHz), but the excited states will not survive to travel far down a molecular beam. The dominant predissociation product is found to be HCl in the highest accessible rotational level ( j=15). Perturbation theory calculations of the lifetimes and product state distributions are drastically in error.
Infrared laser multiple-photon dissociation of CTCl3: Wavelength dependence, collisional effects, and tritium/deuterium isotope selectivity
Authors: Magnotta Frank , Herman Irving P.
Published: February 2003

Isotopically selective infrared multiple-photon dissociation of CTCl3 in CDCl3 was investigated using a pulsed CO2 laser-pumped NH3 laser in the 12 μm region. The wavelength dependence of CTCl3 MPD in the ν4 absorption band was studied and the observed MPD peak was found to be shifted ∼10 cm−1 to the red of the absorption peak at 835 cm−1, consistent with the anharmonicity of the CTCl3 ν4 mode. The CTCl3 dissociation probability per pulse was investigated under various experimental conditions, including laser energy, pulse duration, and CDCl3 and buffer gas pressure. A rate equation model was used to analyze the CTCl3 experiments assuming various multiple-photon absorption and collisional relaxation mechanisms; this modeling effort was extended to a brief, more general, investigation of multiple-photon absorption by molecules with prespecified dissociation probability profiles. Lower limit single step T/D enrichment factors in chloroform MPD exceeding 15 000 were achieved under optimized conditions of the above parameters. No evidence of any CDCl3 depletion or decomposition product was observed in irradiated 200 ppm CTCl3/CDCl3 mixtures. CDCl3 absorption measurements were made using an optically pumped NH3 laser, and the observed high fluence T/D optical selectivity was found to be sufficiently high for practical tritium recovery.
General-model-space perturbation theory: Excitation and ionization of N2
Authors: Kaldor Uzi
Published: February 2003

Vertical excitation and ionization energies are calculated by diagrammatic many-body perturbation theory (MBPT) for 20 states of N2 and N+2 at R=2.068 bohr. Nondegenerate or degenerate MBPT with complete or incomplete model spaces is employed, depending on the molecular state. A variety of zero-order Hamiltonians (H0) is tried. It is found that [2/1] Padé approximants depend little on the choice of H0, making them superior to straightforward summation to third order. They also provide good approximations to the CI limit for the basis (average error for all states 0.35 eV).
Electronegativities and hardnesses of open shell atoms
Authors: Gázquez José L. , Ortiz Elba
Published: February 2003

A Taylor series expansion of the energy of an atomic system around the neutral atom value, which introduces the first and second derivatives of the energy with respect to the number of electrons (electronegativity χ, and hardness η, respectively) is proposed. The relaxed first derivative and the unrelaxed second derivative of the Xα and hyper-Hartree–Fock methods are used to relate χ and η with the Lagrange multiplier εi, and the self-repulsion integral J(i) of the highest occupied atomic orbital for the case of an open shell. A simple model, based on screening effects, is developed to get a better representation of a relaxed second derivative. This model replaces J(i) by 1/2 〈r−1〉i and leads to η= 1/4 〈r−1〉i. The use of this relation, together with the Xα expression for electronegativity, χ=−εi, and a simple charge transfer model for electronegativity equalization leads to values of molecular electronegativities which are in very good agreement with the values obtained through the use of atomic or molecular experimental information. The relations here derived only need information obtained from a neutral atom calculation.
Generalized Langevin theory for many-body problems in chemical dynamics: The method of partial clamping and formulation of the solute equations of motion in generalized coordinates
Authors: Adelman Steven A.
Published: February 2003

A formulation of the analytical dynamics of an n-atom solute system present at infinite dilution in a monatomic solvent is presented. This treatment, which will aid in the understanding of liquid state spectroscopic, energy-transfer, and chemical reaction processes, is made by combining the methods of classical dynamics with recently developed generalized Langevin equation techniques [S. A. Adelman, Adv. Chem. Phys. 53, 611 (1983)]. The Hamiltonian of the solute system is formulated in generalized coordinates which are related to the underlying Cartesian coordinates by a point canonical transformation. The 3n generalized coordinates are partitioned into a set of p explicit coordinates whose dynamics are of direct interest and a set of q=3n−p implicit coordinates whose motion is of lesser interest. A generalized Langevin equation of motion for the explicit coordinates is formulated by computing the reaction force exerted by the solvent on the explicit coordinates in response to small displacements of these coordinates. This generalized Langevin equation will provide a realistic description of explicit coordinate dynamics if the liquid state motion of these coordinates is oscillatory on subpicosecond time scales. Solvent effects appearing in the generalized Langevin equation may be discussed in terms of the fluctuation spectrum which describes atomic motion in the solvation shells. A rigorous statistical mechanical method for assessing the influence of the implicit coordinate motion on this fluctuation spectrum is presented. The formulas for the liquid state quantities appearing in the generalized Langevin equation may be exactly evaluated for particular solute–solvent systems via molecular dynamics (MD) simulation of the motion of the implicit and solvent coordinates given that the solute is partially clamped, i.e., given that the explicit coordinates are fixed. The method of partial clamping provides an improvement of the method of full solute clamping developed earlier. This improvement should permit one to realistically treat many liquid state processes via generalized Langevin equation techniques for which the solute/solvent mass ratio is less than unity.
Variational fluid theory with inverse 12th power reference potential
Authors: Young David A. , Rogers Forrest J.
Published: February 2003

An inverse 12th power fluid variational theory has been constructed. For this we have used an accurate fit to the reference fluid free energy and have tabulated accurate reference fluid radial distribution functions computed from a new integral equation. The theory has been compared with Monte Carlo thermodynamic data for fluids with Lennard-Jones, exponential-six, and n=9, 6, and 4 inverse power potentials. Agreement is good, especially near the freezing point. The theory is also in close agreement with Ross’ ‘‘soft-sphere’’ theory.
Bond energies of the molecules H2O, SO2, H2O2, and HCl to various atmospheric negative ions
Authors: Böhringer H. , Fahey D. W. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

Thermochemical data for some negative cluster ions of importance in atmospheric ion chemistry have been determined from equilibrium constant measurements in a flowing afterglow apparatus. The ions studied have the structure X−⋅M with X−=Cl−, NO−2, NO−3, and HSO−4 and M=H2O, SO2, H2O2, and HCl. The bond strengths of these cluster ions were found to be in the range ΔH=50 to 102 kJ/mol. ΔH increases as a function of the ligand molecules in the following order (H2O)<(SO2)≲(H2O2)≲(HCl). The equilibrium constants for the association of H2O2 and HCl to negative ions are three to four orders of magnitude higher than those for H2O association at temperatures from 250 to 300 K. This implies that the stratospheric trace gases H2O2 and HCl can cluster to negative ions in the presence of the much higher abundance of water vapor in the stratospheric environment. Negative cluster ions containing H2O2 and HCl molecules may therefore exist in measurable quantities in the stratosphere.
Collisional deactivation of vibrationally excited N+.2
Authors: Kemper P. R. , Bowers Michael T.
Published: February 2003

Collisional deactivation of vibrationally excited N+.2 was investigated using a tandem ion cyclotron resonance spectrometer. Relative rates of deactivation produced by collisions with He, Ne, Kr, Xe, and O2 were determined with respect to N2. Absolute deactivation rate constants were then derived from the known N+.2 /N2 deactivation rate. These derived rate coefficients agree within experimental uncertainty with previous selected ion flow tube measurements. Comparison is made between vibrational deactivation in ion-neutral and neutral–neutral systems. In addition the vibrational energy dependence of the charge transfer rates between N+.2 and Kr, NO., O2, and CO2 was investigated. Significant enhancement of the Kr and NO reaction rates was observed.
Proton production probability in the dissociative photoionization of H2
Authors: Masuoka Toshio
Published: February 2003

We report the proton production probability in the dissociative photoionization of H2 from its threshold to 36 eV. The method depends on differentiating the observed ratios of H+/H+2 with respect to the incident photon energy. The probability presented shows at least three states are involved in this energy region and is attributed to protons formed through the ground state 2Σ+g(1sσg) of H+2, the two-electron excited state 1Σ+u(2pσu2sσg) of H2, and the first excited state 2Σ+u(2pσu) of H+2 by comparing the present results with other observations and theoretical calculations. Further the results indicate that interference between the direct (2Σ+g) and resonant (1Σ+u) processes produces an appreciable amount of protons at photon energies of 20–25 eV.
Competitive reaction and quenching of vibrationally excited O+2 ions with SO2, CH4, and H2O
Authors: Durup-Ferguson M. , Böhringer H. , Fahey D. W. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

Vibrationally excited O+2 ions injected into a He buffered flow tube react rapidly with SO2 and H2O by charge transfer and with CH4 to produce CH3O+2 , CH+3 , and CH+4 . It is found that the rapidly reacting states at thermal energy are O+2 (v≥2) for SO2 and CH4 and O+2 (v≥3) for H2O, while the lower vibrationally excited states are rapidly quenched. When the reactions of SO2 and CH4 are studied in Ar buffer as a function of kinetic energy it is found that the vibrational temperature of O+2 established through collisional excitation by the Ar buffer is perturbed by quenching collisions with the reactant molecules. This leads to observed reaction rate constants that change with reactant gas concentration. For the reaction of O+2 with CH4 the influence of kinetic and vibrational energy on the branching ratio of the reaction channels has been investigated. The present vibrational relaxation data for O+2 (v) by CH4, in conjunction with other recent measurements, allows a rather detailed picture of the mechanism to be drawn for this complicated reaction that involves the making and breaking of four chemical bonds.
Comparison of 12CO+2 and 13CO+2 alignment following photoionization of carbon dioxide
Authors: Guest J. A. , O’Halloran M. A. , Zare R. N.
Published: February 2003

Fluorescence polarizations of the B̃ 2Σ+u–X̃ 2Πg (λ∼289 nm) and Ã 2Πu–X̃ 2Πg (λ>310 nm) transitions in 12CO+2 and 13CO+2 are measured following the photoionization of CO2 at energies ranging from 18.2 to 31 eV. The rotational alignments and channel ratios of the 12CO+2 and 13CO+2 B̃ 2Σ+u state are identical within experimental error and agree well with existing theoretical calculations. Polarizations for the Ã–X̃ emission differ for 12CO+2 and 13CO+2. We present evidence that this behavior can be explained by considering the influence of the extensive perturbations in the Ã and B̃ states of the 12CO+2 photoion. Mixed (Ã, B̃) states in 12CO+2 give rise to red-shifted emission (λ>310 nm) with a quantum yield ΦB, A defined as the fraction of red-shifted emission to all emission originating at the B̃ state energy. Using the present alignment data and published 12CO+2 Ã and B̃ state fluorescence cross sections, ΦB, A is determined to be nearly independent of ionizing energy with a value of 0.36 at 21.0 eV, in good agreement with other quantum yield measurements.
Collisional quenching and depolarization of NO2 Ã 2B2 state fluorescence as studied by Zeeman quantum beat spectroscopy
Authors: Brucat P. J. , Zare R. N.
Published: February 2003

We have studied the effect of collisions on the time-resolved fluorescence arising from a single optically selected fine structure level (F1303) of the Ã 2B2 state of NO2 prepared via absorption near 5933 Å in the presence of a weak magnetic field. The fluorescence modulation induced by the magnetic field (Zeeman quantum beats) is used to monitor the time dependence of the alignment. The alignment decays exponentially and follows simple Stern–Volmer kinetics. The incoherent fluorescence intensity, used to determine population information, fits well a bi-exponential form at nonzero pressure. This is interpreted by a simple kinetic scheme involving cascade to additional fluorescing states. We find that only about 15% of the population removed from the initial level by collision produces subsequent emission that is detected (630–830 nm). We are unable to identify the exact nature of these emitting states. However, it is clear that pure rotationally inelastic collisions do not dominate the energy transfer process. Collisional population and alignment relaxation rate constants for the optically prepared level have been measured for excited state collisions with He, Ne, Ar, Kr, Xe, N2, CO, and NO2 at 295 K and low density (<1×1015 particle/cm3). In particular, we find that the rate for population removal of the initially excited level by collision is faster than the rate for elastic disalignment (depolarization within the same rotational level).
A multichannel distorted wave perturbation treatment applied to molecule–surface scattering
Authors: Whaley K. B. , Light J. C.
Published: February 2003

An approximate quantum mechanical approach to the scattering of systems with several degrees of freedom is developed. It is based on the exact (numerical) solution of the close coupled equations for subsets of the internal states and the coupling of these subsets to each other by perturbation theory. The calculation of the multichannel distorted wave solutions and their perturbative coupling is done simultaneously. Application to diffractive and rotationally inelastic scattering of H2 from a model corrugated surface is given and compared to earlier approximate diffractive sudden close coupled rotation calculations.
Identification of the lowest energy nπ* states in gas-phase polycyclic monoazines: Quinoline and isoquinoline
Authors: Hiraya Atsunari , Achiba Yohji , Kimura Katsumi , Lim Edward C.
Published: February 2003

We report here the first spectroscopic observation of the nπ* singlet states in gas-phase polycyclic monoazines.
On Ostwald’s supersaturation theory of rhythmic precipitation (Liesegang’s rings)
Authors: Smith D.
Published: February 2003

Liesegang’s rings are thin precipitation bands formed when two components in solution diffuse into each other and react to form an insoluble precipitate. Ostwald’s supersaturation theory, in the quantitative form given by Prager and Zeldovitch et al., correctly predicts many features of the process of ring formation in some systems. Anomalous systems not conforming to Ostwald’s theory have been known for some time, and the recent work of Ross and co-workers has demonstrated that an alternative mechanism involving autocatalytic growth of colloidal particles operates at low concentrations. They have also shown that a uniform colloidal precipitate is laid down prior to ring formation in more concentrated systems, which apparently contradicts Ostwald’s assumption that the spaces between rings remain clear of nuclei. The purpose of this paper is to suggest an interpretation of Ostwald’s mechanism which is compatible with the known facts, and to extend the analysis of Prager and Zeldovitch in various ways. Our analysis shows that each ring grows to a finite thickness which, for dilute solutions, is much less than the ring spacing. A quantitative description of ring formation is given for arbitrary values of the ratio of the component concentrations, and is used to derive the form of the phase boundary between metaphases showing Liesegang rings and showing continuous precipitation. The modifications expected to this phase diagram when the Ostwald model is replaced by a more realistic model are also discussed.
Vibrational relaxation of HCl (v=1, 2, 3) in Ar, Kr, and Xe matrices
Authors: Young Linda , Moore C. Bradley
Published: February 2003

The spectroscopy and vibrational relaxation of matrix isolated HC1 (v=1,2,3) have been studied as a function of host (M=Ar, Kr, and Xe) and temperature (9–42 K). FTIR and laser-excited fluorescence excitation spectra were used to determine vibrational frequencies for the v=0−1, 0−2, and 0−3 transitions. The anharmonicity is found to increase relative to the gas phase value with increasing host size and polarizability. Relaxation occurs stepwise, nonradiatively from v=3 to 2, 2 to 1, and 1 to 0. For a given vibrational state, the relaxation rate increases in the series k(Ar)<k(Kr)<k(Xe), i.e., with increasing spectral shift. At 20 K, k10(Ar)=0.9, k10(Kr)=1.2, and k10(Xe)=1.6 ms−1. The deviation from the harmonic approximation, kv,v−1 =vk10, increases in the same sequence with host change. At 20 K, k32/k21/k10=11/4.3/1 for Ar, 53/11/1 for Kr, and 260/33/1 for Xe. These results are in remarkable contrast with those for most other molecules and suggest the importance of an attractive interaction, especially for HC1/Xe, which increases strongly with HC1 bond length. The temperature dependences of the rates are of the same modest magnitude as found in other systems. The smooth trends of rates with vibrational quantum number and host suggest that there are not strong resonances for specific final rotation-local phonon states. Both rotational and translational degrees of freedom probably receive a significant fraction of the vibrational energy transferred.
A
b i
n
i
t
i
o calculations on the He–O2 potential energy surface. Hartree–Fock instability of O2
Authors: van Lenthe J. H. , van Duijneveldt F. B.
Published: February 2003

The interaction energy between He and O2 (3∑−g) is obtained for five angles and eight distances, keeping the O–O distance fixed. The energies are calculated at the Hartree–Fock and MCSCF level, corrected for basis set superposition errors and augmented by an estimate of the dispersion energy from second-order perturbation theory. The Hartree–Fock wave function for O2 is unstable with respect to breaking the D∞h symmetry. This makes a multiconfigurational description of the O2 π system essential to obtain uniform reliability at all angles. The MCSCF+dispersion surface shows minima at 90° (−0.17 mhartree) and 0° (−0.16 mhartree) and a barrier in between at 47° (−0.13 mhartree).
Extended diffusion of rigid asymmetric molecules
Authors: Bull T. E. , Egan William
Published: February 2003

The extended diffusion theory of molecular motions in fluids has been applied to the case of rigid asymmetric molecules. Illustrative calculations of the second rank reorientational correlation time matrix and the correlation factors associated with NMR spin-rotation relaxation are presented as functions of the angular momentum correlation time. Analytic expressions for these correlation times and factors in the diffusion limit are also given.
Electron transport in gaseous ethene and cyclopropane: Temperature and electric field effects
Authors: Gee Norman , Freeman Gordon R.
Published: February 2003

To check whether ethene and cyclopropane display a Ramsauer–Townsend minimum in their electron momentum transfer cross sections, electron drift velocities were measured over much larger temperature ranges than previously. Ethene: 130≤T(K)≤647, 0.25≲n(1025 molecule/m3)≲3.0. Cyclopropane: 185≤T(K)≤644, 0.13≲n(1025 molecule/m3)≲18. At 294 K vd at a given E/n was independent of n up to the highest n used. In both gases vd increases linearly with E/n up to ∼0.3 Td, then tends towards saturation at ∼4 Td. In ethene vsat is ∼17 km/s, and is ∼35 km/s in cyclopropane. The value of vsat in alkanes correlates with the inverse polarizability. Ethene has a Ramsauer–Townsend minimum in its momentum transfer cross section at 0.08 eV, and cyclopropane has one at 0.07 eV.
Structure of D2O ice VIII from i
n s
i
t
u powder neutron diffraction
Authors: Jorgensen James D. , Beyerlein R. A. , Watanabe Noboru , Worlton T. G.
Published: February 2003

Structural parameters for D2O ice VIII have been determined by Rietveld refinement of time-of-flight powder neutron diffraction data collected in situ at 28 kbar and −4 °C. The D2O molecule has a bond length of 0.973 (11) Å and a bond angle of 104(2)°. O–D⋅⋅⋅O bonds are slightly bent with an angle of 177(1)°. Thus, the D2O molecules in ice VIII are essentially identical to those observed in other high pressure ice structures for which accurate structural data are available.
Effective spherical potentials for the thermodynamics of homonuclear diatomic Lennard-Jones liquids
Authors: MacGowan David
Published: February 2003

The predictions of several effective spherical potentials for thermodynamic properties of symmetric diatomic Lennard-Jones molecules are examined. The effective potentials are obtained by splitting the molecular potential in various ways followed by adding the separate angular medians of the two parts. Luckily, the best results are obtained when each site–site interaction is divided into its individual power law terms. It is therefore possible to obtain quite accurate thermodynamic properties over a continuous range of elongations using linear combinations of just two spherical potentials for which accurate analytic fitting functions are provided.
Characteristics of the probability function for three random-walk models of reaction–diffusion processes
Authors: Musho Matthew K. , Kozak John J.
Published: February 2003

A method is presented for calculating exactly the relative width (σ2)1/2/〈n〉, the skewness γ1, and the kurtosis γ2 characterizing the probability distribution function for three random-walk models of diffusion-controlled processes. For processes in which a diffusing coreactant A reacts irreversibly with a target molecule B situated at a reaction center, three models are considered. The first is the traditional one of an unbiased, nearest-neighbor random walk on a d-dimensional periodic/confining lattice with traps; the second involves the consideration of unbiased, non-nearest-neigh bor (i.e., variable-step length) walks on the same d-dimensional lattice; and, the third deals with the case of a biased, nearest-neighbor walk on a d-dimensional lattice (wherein a walker experiences a potential centered at the deep trap site of the lattice). Our method, which has been described in detail elsewhere [P.A. Politowicz and J. J. Kozak, Phys. Rev. B 28, 5549 (1983)] is based on the use of group theoretic arguments within the framework of the theory of finite Markov processes. The approach allows the separate effects of geometry (system size N, dimensionality d, and valency ν), of the governing potential and of the medium temperature to be assessed and their respective influence on (σ2)1/2/〈n〉, γ1, and γ2 to be studied quantitatively. We determine the classes of potential functions and the regimes of temperature for which allowing variable-length jumps or admitting a bias in the site-to-site trajectory of the walker produces results which are significantly different (both quantitatively and qualitatively) from those calculated assuming only unbiased, nearest-neighbor random walks. Finally, we demonstrate that the approach provides a method for determining a continuous probability (density) distribution function consistent with the numerical data on (σ2)1/2/〈n〉, γ1, and γ2 for the processes described above. In particular we show that the first of the above reaction–diffusion models (and probability the second) can be described quantitatively by an exponential distribution function in d=2,3 and by a Pearson type III distribution in d=1.
1
7O NQR study of the pseudo-one-dimensional ferroelectric phase transition in CsH2PO4
Authors: Seliger J. , Žagar V. , Blinc R.
Published: February 2003

The hydrogen bond dynamics of pseudo-one-dimensional CsH2PO4 has been studied in the paraelectric and ferroelectric phases by 17O–proton nuclear double resonance spectroscopy. The results show that the protons in the short one-dimensionally linked O–H(2)--O hydrogen bonds are moving between two equilibrium sites above Tc and freeze into one of the two off-center sites below Tc. The protons in the asymmetric O–H(1)--O bonds are static and ordered in the same way both above and below Tc and seem to play only a minor role in the transition mechanism.
Ultraviolet photoassociative production of XeCl(B,C) molecules in Xe/Cl2 gas mixtures: Radiative lifetime of Xe2Cl(4 2Γ)
Authors: McCown A. W. , Eden J. G.
Published: February 2003

Electronically excited XeCl molecules are produced directly from xenon and chlorine atoms in mixtures of Xe and Cl2 vapor at room temperature by laser photoassociation at λ=308 nm. The peak intensities of both the XeCl(C→A) and Xe2Cl 4 2Γ→2 2Γ spontaneous emission signals at 350 and ∼485 nm, respectively, are linear in laser fluence and Cl2 partial pressure. Consequently, the XeCl excimer production mechanism is one involving Xe, Cl and one 4 eV photon in which Cl2 is photodissociated on the X 1∑→1Π band throughout the XeCl laser pulse. This conclusion is supported by the enhancements of the XeCl(C) state population of more than 15 that are observed when the Xe/Cl2 mixture is irradiated by an additional ultraviolet (UV) laser pulse [of wavelength 351 nm (XeF), 308 or 193 nm (ArF)] that arrives prior to firing the XeCl laser. The effect is much more pronounced at 193 nm than at the other wavelengths, indicating tht the Cl concentration is augmented by photoionizing Xe, followed by XeCl excimer formation and dissociation of the weakly bound ground state. Also, the radiative lifetime of the Xe2Cl 4 2Γ state was measured to be 245±10 ns.
Ultrasonic measurements of the vibrational relaxation of UF6: Mixtures with H2, CH4, and C2H6
Authors: Bass H. E. , Rogers Richard
Published: February 2003

Ultrasonic absorption has been measured in mixtures of 10% and 5% UF6 in H2, CH4, and C2H6 at 273 and 350 K for f/P ratios from 15 kHz/atm to 2 MHz/atm. The measured absorption was corrected for tube, classical losses, absorption due to rotational relaxation and, for mixtures with CH4 and C2H6, vibrational relaxation of the diluent gas. The resulting absorption was analyzed to determine the vibrational relaxation time of UF6 in collisions with the diluent (H2, CH4, or C2H6). The relaxation times were found to be greater than 4×10−8 s atm for H2 at 273 K; 1.2×10−7 and 1.3×10−7 s atm for CH4 at 273 and 350 K, respectively; and 4.9×10−9 and 1.2×10−8 s atm for C2H6 at 273 and 350 K, respectively.
New experimental evidences for dynamical constraints on angular momentum transfer in heavy molecules–light particles collisions: I*2(B)+4He, 3He, H2, D2
Authors: Derouard J. , Sadeghi N.
Published: February 2003

The selective laser excitation and induced fluorescence observation technique has been used to study thermally averaged rotationally inelastic processes: I*2 (B, vi=15, ji)+M→I*2 (B,vi=15, jf )+M with M=4He, 3He, H2, D2. In each case, the transfer rates k( ji→jf) display an exponential-like dropoff vs ‖ jf−ji‖, confirming the importance of dynamical constraints on the transfer of angular momentum by light particles. The onset of this sharp decrease scales like the relative linear momentum of the colliding particles and might be interpreted as resulting from a rotational rainbow effect. Effective anisotropies for I*2 –He and I*2 –H2 potential energy surfaces can thus be estimated.
Radiative and collisional properties of CsH (A 1Σ+) state. III. Rotational energy transfer by collisions with H2
Authors: Ferray M. , Visticot J. P. , Sayer B.
Published: February 2003

Rotational energy transfers occurring in the CsH molecule in its A 1Σ+ (v=6) state, due to collisions with H2 are studied. Two fitting laws (IOS-P and AON) are used to fit the experimental distribution of population among the rotational levels. IOS-P law predicts better than AON the large population of rotational levels lying far from the photoexcited one (ΔE>kT/3). This comparison is discussed. State to state and total cross sections are calculated from the determined parameters of the fitting laws.
Triplet exciton transport in isotopic mixed naphthalene crystals. II. Master equation analysis
Authors: Gentry Stuart T. , Kopelman R.
Published: February 2003

The experimental data on triplet exciton transport in isotopically mixed crystals of naphthalene/perdeuteronaphthalene (paper I) are contrasted with singlet exciton transport in the same samples (20%–100%) and analyzed in terms of incoherent hopping models. The master equation approach is emphasized and extended. We modify the conventional continuum master equations via a physically plausible cutoff of the high-frequency transfer rates. This results in an experimentally acceptable functional form (transport linear with high power of concentration) and nearest-neighbor transfer time (100 ps). We also developed a lattice master equation (numerically soluble), using an experimentally tested exciton superexchange formula. The somewhat surprising result is that the lattice master equations do not fit the experimental functional form. The success of the continuum models and the failure of the lattice models are attributed to the latter’s neglect of the spread in transfer rates for a given intersite distance. We claim that clusterization as well as diagonal homogeneous and/or inhomogeneous disorder cause the above spread. On the other hand, these energy mismatches are small with respect to the thermal energy, in contrast to the singlet exciton transport case, where, due to larger energy mismatches, a percolation-like critical concentration is observed. Thus for the given concentration and temperature regimes, the triplet exciton transport is diffusive while the singlet exciton transport is percolative. Lower temperatures and/or concentrations are required for percolative triplet energy transport in these systems.
The far-infrared laser magnetic resonance spectrum of the SiH radical and determination of ground state parameters
Authors: Brown John M. , Curl Robert F. , Evenson Kenneth M.
Published: February 2003

The far-infrared laser magnetic resonance (LMR) spectrum of the SiH radical in the υ=0 level of its X 2Π state has been recorded. The signals are rather weak. The molecules were generated in the reaction between fluorine atoms and SiH4. Rotational transitions have been detected in both 2Π1/2 and 2Π3/2 spin components but no fine structure transitions between the spin components were observed. Proton hyperfine splittings were resolved on some lines. The measurements have been analyzed, subjected to a least-squares fit using an effective Hamiltonian and the appropriate molecular parameters determined. The weakness of the spectrum and the failure of attempts to power saturate favorable lines are both consistent with a small value for the electric dipole moment for SiH.
New sum rules for electronic absorption spectra
Authors: Coalson Rob D. , Karplus M.
Published: February 2003

The third moment of a zero or finite temperature electronic absorption spectrum is evaluated within the Condon approximation for arbitrary ground and excited potential surfaces. For a transition moment with an arbitrary distance dependence, a corresponding formula is obtained for the first moment of the absorption spectrum. The results reduce to an average of simple functions of position over the appropriate probability density. Numerical examples illustrating the new formulas are presented. The significance of the third moment in ascertaining the skewness of a typical structureless spectrum is demonstrated and the utilization of sum rule information in modeling potential surfaces is discussed.
Photoelectron studies of resonant multiphoton ionization of molecular nitrogen
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: February 2003

Photoelectron studies of three photon resonant, four photon ionization of N2 via the b 1Πu, v′=0–5, c 1Πu, v′=0, 1, and c′ 1Σ+u, v′=0, 1 levels are presented. A qualitative analysis of the observed photoelectron spectra in terms of homogeneous perturbations among the b 1Πu, c 1Πu, o 1Πu states and among the b′ 1Σ+u, c′ 1Σ+u, and e′ 1Σ+u states is partially successful. However, several of the photoelectron spectra exhibit marked deviations from the expected behavior. These deviations are discussed in terms of more complex interactions among the intermediate levels, and in terms of the dynamics of excited state photoionization. In addition, the N2 a 1Πg, v′=0–5 levels have been observed in two photon resonant, four photon ionization, and the results are compared with the results of earlier two-photon laser induced fluorescence studies.
FTIR spectra of (HF)n species in solid neon
Authors: Andrews Lester , Bondybey V. E. , English J. H.
Published: February 2003

Condensation of Ne/HF mixtures at 5 K produces in addition to monomer HF numerous strong, sharp infrared absorptions in the 4000–3600 and 600–300 cm−1 regions and broader bands in the 3500–3100 and 1300–600 cm−1 regions. The relative absorbances of these bands depend upon concentration, spray-on rate, and sample annealing, which are used to identify individual (HF)n species. Sharp bands at 3919, 3848, and 410 cm−1 are assigned to (HF)2. Further infrared evidence is presented for an open chain (HF)3 species. The broader bands assigned to cyclic-(HF)n species increase at the expense of the sharper bands under conditions which favor association of HF in the matrix.
Collision induced electronic energy transfer between the A 2∏u
i (v=4) and X 2∑+g (v=8) rotational manifolds of N+2
Authors: Katayama Daniel H.
Published: February 2003

A two laser, optical–optical double resonance technique has been used as a direct probe of the electronic energy transfer process between the A 2∏ui (v=4) and X 2∑+g (v=8) rotational manifolds of 14N+2. For the first time collisional selection or propensity rules are obtained for a homonuclear molecule which has no perturbations or ‘‘gates’’ between the states involved. The selection rules s→s, a→a, and s⇄a are shown to apply for this electronic energy transfer process. The transfer rate is shown to be comparable to that for rotational energy transfer and in spite of the competition between these two mechanisms it is demonstrated that there is a propensity towards ΔJ≊0 rather than ΔE (energy gap) ≊0. These selection or propensity rules are optical-like and appear to result from collision induced electric dipole transitions. Therefore, these are truly electronic energy transfer processes rather than a form of rotational energy transfer as occurs when the rotational levels are known to be perturbed in the free molecule.
Temperature and pressure dependence of the absolute rate constant for the reactions of NH2 radicals with acetylene and ethylene
Authors: Bosco S. R. , Nava D. F. , Brobst W. D. , Stief L. J.
Published: February 2003

Absolute rate constants for reaction of the NH2 free radical with acetylene and ethylene have been measured as a function of pressure and temperature using the technique of flash photolysis combined with time resolved detection of NH2 via laser induced fluorescence. NH2 radicals were produced by flash photolysis of ammonia highly diluted in argon and the decay of fluorescent NH2 photons was measured by multiscaling techniques. At each temperature employed in the study, the observed pseudo-first-order rate constants were independent of flash intensity (i.e., initial [NH2]) and were a linear function of [C2H2] or [C2H4]. The bimolecular rate constant for the reaction NH2+C2H2 (1) increased with an increase in pressure at 373 and 459 K but not at lower temperatures. Results at or near the high pressure limit are well represented by the Arrhenius expression k∞1 =(1.11±0.36)×10−13 exp[−(1852±100)/T] cm3 molecule−1 s−1 over the temperature range 241–459 K. For the reaction NH2+C2H4 (2), a smaller increase in the bimolecular rate constant with an increase in pressure was observed and only at 465 K. Results representing the high pressure limiting rate constant over the temperature interval 250–465 K are given by k∞2 =(3.41±0.12)×10−14 exp[−(1318±23)/T] cm3 molecule−1 s−1. The observed pressure dependence confirms expectations from previous studies that both reactions involve addition of NH2 to the unsaturated molecule followed by collisional stabilization of the adduct radical. The results are compared with two previous determinations using the techniques of flash photolysis–laser absorption and discharge flow–laser induced fluorescence. There is substantial disagreement between these two previous studies; our results for both NH2+C2H2 and NH2+C2H4 are much more consistent with the flash photolysis–laser absorption results. The implications of these results for atmospheric and combustion chemistry are briefly considered.
Excess energy dependence in radiationless transitions. Triplet to ground state nonradiative rate calculations in benzene
Authors: Hornburger H. , Kono H. , Lin S. H.
Published: February 2003

The T1→S0 intersystem crossing (ISC) rates in benzene are calculated on the assumption that all T1 vibrational states with the same amount of excess energy are equally populated owing to fast intramolecular vibrational redistribution. The results are compared with the experimentally observed tendency that the ISC rate increases rapidly with excess energy and levels off at higher excess energy. The above tendency is seen when vibrational frequencies in T1 are assumed to be the same as in S1. Whether the rapid initial increase appears or not depends on parameters involved in the radiationless process; on the other hand, the saturation of the ISC rate in the high energy range occurs regardless of the parameters. Furthermore, the saturation proves to correlate with the fact that the slope of the density of states in a logarithmic scale becomes smaller as the energy increases.
Optical spectroscopic study of thermal-energy charge transfer between He+ and CO2
Authors: Tsuji M. , Endoh M. , Susuki T. , Mizukami K. , Nishimura Y.
Published: February 2003

The thermal-energy charge transfer between He+ and CO2 has been studied by optical spectroscopy in a flowing afterglow apparatus fitted with pulse modulation equipment. CO+ (A 2Π–X 2Σ+) emission from v′(vi:)=0 to v′(vi:)=11 has been identified in the 300–500 nm region. The excitation rate constant of CO+ (A) was evaluated by comparing the total emission intensity with that for N+2 (B−X) emission in He+/N2. The upper limit to the excitation rate constant for CO+ (A) represents only 1% of all CO+ produced in the He+/CO2 reaction. The CO+(A) vibrational population was approximately exponential corresponding to an effective vibrational temperature of 10 500±900 K. The long range resonant model for the He+/CO2 reaction is discussed in the light of recent information on the product distribution and the excited states of CO+2.
Optical absorption and luminescence induced by x rays in KDP, DKDP, and ADP
Authors: Diéguez E. , Cabrera J. M. , Agulló-López F.
Published: February 2003

The effect of x-ray irradiation on the absorption and luminescence spectra of KDP, DKDP, and ADP has been investigated. σ-Polarized absorption bands at 390 and 550 nm are induced by low temperature irradiation in the three crystals. A π-polarized band has also been observed in KDP and DKDP peaked at 510 and 550 nm, respectively. Thermal bleaching of the σ bands takes place at ∼70 K for KDP and ADP and ∼105 K for DKDP. π bands annihilate at the Curie temperature Tc for both KDP and DKDP. The experimental data allow association of the σ and π bands, respectively, with the radicals B and A, which have been previously identified by EPR. The same x-ray induced luminescence bands have been observed for the three crystals during low temperature (∼10 K) irradiation. At higher temperatures some differences arise. In particular, a luminescence band appears at 350 nm in KDP and DKDP irradiated at T≥70 K which reaches maximum efficiency at Tc. For ADP only the low temperature emissions are detected up to Tc, where they become quenched. Thermoluminescence glow peaks have been observed at ∼70 K (for the three crystals) and at Tc for KDP and DKDP. Tentative mechanisms are discussed to account for the radiation-induced production of centers, their annihilation behavior, and the light emission. For KDP the effect of impurity doping on these processes has been investigated.
The barrier to inversion in NF+3
Authors: Berkowitz J. , Greene J. P.
Published: February 2003

The first band in the He I photoelectron spectrum of NF3 has been obtained at sufficiently high resolution to reveal vibrational structure. The bifurcated nature of the band is interpreted as a transition from pyramidal NF3 to pyramidal NF+3, the latter having a lower barrier to inversion. The span of the Franck–Condon region in the transition is sufficient to surmount the barrier, whose magnitude can be estimated from the spectrum to be ∼0.75 eV. This is one of the very few cases for which an accurate experimental measurement of an inversion barrier is known.
A statistical wave function model for C–H/C–D overtone linewidths: Application to C6H6, C6D6, C6HD5, C6HF5
Authors: Buch V. , Gerber R. B. , Ratner M. A.
Published: February 2003

A simple quantitative model calculation is presented for C–H overtone spectra in polyatomic systems. The model employs a realistic force field Hamiltonian that includes the structure of all modes. The physical basis for the linewidths is a centrifugal coupling to the CCH in-plane wag motions, as first suggested by Sibert, Reinhardt, and Hynes. To avoid exhaustive analysis of the full coupled-mode problem, approximate solutions for the eigenstates are constructed, using a statistical assumption on the wave functions in terms of their expansion coefficients in the decoupled mode basis. The results yield a simple physical interpretation for the observed isotope effects, the fluctuations in linewidth with the quantum number, the anomalously narrow lines of C6HF5, etc. Calculations are presented for the linewidths of C6H6, C6HD5, C6D6, and good agreement with the experiment is found. The statistical assumption for calculation of the density of effectively coupled states appears to be adequate for problems of this type.
Calculation of rovibrational spectra of water by means of particles-on-concentric-spheres models. I. Ground stretching vibrational state
Authors: Natanson Grigory A. , Ezra Gregory S. , Delgado-Barrio Gerardo , Berry R. Stephen
Published: February 2003

A new method is presented to compute bending frequencies and rotational structure in the ground and excited bending states of water-like molecules. As a zeroth-order approximation the water molecule is simulated by the two hydrogens confined to move on a sphere around the oxygen. Even the simplest approximation, choosing the radius of the sphere equal to the equilibrium bond length, gives better results for rotational levels than the rigid bender model does. A further improvement for both bending frequencies and rotational energy levels, especially in excited bending states, has been achieved by introducing an effective radius and an effective mass. These are calculated by averaging over stretching motions to reduce the Schrödinger equation to only the rotational and bending degrees of freedom.