bluuebunny/crossref_metadata_2025 huggingface dataset:
Confidence intervals for a spatially generalized, continuous simulation flood frequency model for Great Britain
Authors: Lamb Robert , Kay Alison L.
Published: July 2004

There is growing interest in the application of “continuous simulation” conceptual rainfall‐runoff models for flood frequency estimation as an adjunct to event‐based or statistical design methodology. The approach has advantages that stem from the use of models with continuous water balance accounting. Conceptual rainfall‐runoff models usually require calibration, which in turn requires gauged rainfall and flow data. One of the key challenges is therefore to develop ways of generalizing models for use at ungauged sites. Recent work has produced a prototype scheme for achieving this aim in Great Britain for two catchment models by relating model parameters to spatial catchment properties, such as soils, topography, and geology. In this paper we present an analysis of the uncertainty associated with one of the generalized models (the “probability distributed model”) in terms of confidence intervals for simulations at test sites that are treated as if they were ungauged. This is done by fitting regression relationships between hydrological model parameters and catchment properties so as to estimate the parameters as distribution functions for the ungauged site case. Flood flow outputs are then simulated from the parameter distributions and used to construct approximate confidence intervals. Comparison with gauged data suggests that the generalized model may be tentatively accepted. Uncertainty in the modeled flood flows is often of a similar order to the uncertainty surrounding a more conventional statistical model, in this case a single‐site generalized Pareto distribution fitted to the gauged data.
Inferring time‐varying recharge from inverse analysis of long‐term water levels
Authors: Dickinson Jesse E. , Hanson R. T. , Ferré T. P. A. , Leake S. A.
Published: July 2004

Water levels in aquifers typically vary in response to time‐varying rates of recharge, suggesting the possibility of inferring time‐varying recharge rates on the basis of long‐term water level records. Presumably, in the southwestern United States (Arizona, Nevada, New Mexico, southern California, and southern Utah), rates of mountain front recharge to alluvial aquifers depend on variations in precipitation rates due to known climate cycles such as the El Niño‐Southern Oscillation index and the Pacific Decadal Oscillation. This investigation examined the inverse application of a one‐dimensional analytical model for periodic flow described by Lloyd R. Townley in 1995 to estimate periodic recharge variations on the basis of variations in long‐term water level records using southwest aquifers as the case study. Time‐varying water level records at various locations along the flow line were obtained by simulation of forward models of synthetic basins with applied sinusoidal recharge of either a single period or composite of multiple periods of length similar to known climate cycles. Periodic water level components, reconstructed using singular spectrum analysis (SSA), were used to calibrate the analytical model to estimate each recharge component. The results demonstrated that periodic recharge estimates were most accurate in basins with nearly uniform transmissivity and the accuracy of the recharge estimates depends on monitoring well location. A case study of the San Pedro Basin, Arizona, is presented as an example of calibrating the analytical model to real data.
Magnitude‐frequency analysis of bed load data in an Alpine boulder bed stream
Authors: Lenzi M. A. , Mao L. , Comiti F.
Published: July 2004

The purpose of this study is to investigate the connection between bed load, channel processes, and sediment sources in mountain basins using data from the Rio Cordon basin (northeastern Italian Alps). The main channel is a steep, boulder bed, step pool stream, and bed load volumes are measured at a special facility where particles >20 mm are trapped. Results from a combined frequency analysis of peak water discharges and total bed load volumes based on 17 years of field data are presented, focusing on discrepancies between recurrence intervals of peak discharge and bed load volume for each event. A cause of major disturbance is a high‐magnitude, low‐recurrence event that occurred in 1994. Dimensional and nondimensional bed load intensity‐duration curves are also reported, which emphasize differences between short‐ and long‐duration events. The relationship flow–bed load rates (investigated using a total of 180 data) shows no breakpoints, and two different curves are evident, representing pre‐1994 and post‐1994 floods, respectively. The analysis demonstrates both the control exerted by sediment availability on bed load transport rates and the persisting long‐term impact of major floods on mountain streams.
Coseismic deformation of the May 21st, 2003, Mw= 6.8 Boumerdes earthquake, Algeria, from GPS measurements
Authors: Yelles K. , Lammali K. , Mahsas A. , Calais E. , Briole P.
Published: July 2004

On May 21st, 2003, a Mw= 6.8 earthquake struck the central part of northern Algeria causing extensive damage in the Boumerdes area, 40 km east of Algiers. It is among the largest events to occur in the western Mediterranean over the past 25 years. We present GPS measurements of horizontal coseismic displacements that provide new constraints on the rupture geometry. Modeling the data with a uniform dislocation on a rectangular fault in an elastic half‐space, we find that the rupture occurred on a reverse fault dipping 42°S, with its upper edge 6 km offshore and lower edge 4 km inland. The amplitude distribution of the coseismic displacements indicates that the rupture did not reach the surface, at least in its western part, and ended to the west around 3.4°E. Offshore faults like that of the Boumerdes earthquake could account for part of the Africa‐Eurasia relative plate motion in the western Mediterranean and represent a significant seismic threat for Algeria.
Correlation of LBH intensities with precipitating particle energies
Authors: Carbary J. F. , Sotirelis T. , Newell P. T. , Meng C.‐I.
Published: July 2004

Precipitating particle data from the DMSP F12 and F13 satellites has been merged with image data from the Ultraviolet Imager (UVI) on the Polar satellite. UVI images were combined within single transpolar passes of the DMSP F12 and F13 satellites. Pixel irradiances were then averaged into 1° × 1° bins along the satellite tracks, thus generating along‐track irradiance profiles. Similarly, energy fluxes of precipitating electrons were averaged into 1° bins along the trajectory. The peaks in each set of profiles were isolated in order to compare photon irradiance with electron energy flux. The irradiance and energy flux data of hundreds of peaks were linearly correlated for January–February 1997 when the northern polar regions were in darkness and dayglow effects were minimal. In the pre‐midnight and dawn sectors, good correlations (r > 0.7) arise between the photon irradiance and electron energy fluxes, while correlations for other sectors are not as good. The good correlation suggests that UVI fluxes can be directly calibrated to measure energy input without the additional complication of modeling, at least for some MLT sectors. Using this calibration, global maps of energy input into the aurora may be derived empirically.
Postseismic deformation and stress changes following the 1819 Rann of Kachchh, India earthquake: Was the 2001 Bhuj earthquake a triggered event?
Authors: To A. , Bürgmann R. , Pollitz F.
Published: July 2004

The 2001 Mw 7.6 Bhuj earthquake occurred in an intra‐plate region with rather unusual active seismicity, including an earlier major earthquake, the 1819 Rann of Kachchh earthquake (M7.7). We examine if static coseismic and transient postseismic deformation following the 1819 earthquake contributed to the enhanced seismicity in the region and the occurrence of the 2001 Bhuj earthquake, ∼100 km away and almost two centuries later. Based on the Indian shield setting, great rupture depth of the 2001 event and lack of significant early postseismic deformation measured following the 2001 event, we infer that little viscous relaxation occurs in the lower crust and choose an upper mantle effective viscosity of 1019 Pas. The predicted Coulomb failure stress (DCFS) on the rupture plane of the 2001 event increased by more than 0.1 bar at 20 km depth, which is a small but possibly significant amount. Stress change from the 1819 event may have also affected the occurrence of other historic earthquakes in this region. We also evaluate the postseismic deformation and ΔCFS in this region due to the 2001 event. Positive ΔCFS from the 2001 event occur to the NW and SE of the Bhuj earthquake rupture.
Dental hygienists in Korea
Authors: Kim S‐H
Published: July 2004

Abstract The purpose of this paper is to give basic information about the status of dental hygienists in Korea. This paper examines the changing process of the dental hygiene education system in Korea, from its start in 1965 until the present, 2003, the composition of dental personnel in Korea, the specialised areas and roles of dental hygienists after they receive their licenses, employment of dental hygienists, dental hygiene‐related organisations, and the status and direction of dental hygienists in Korea. This paper shows the organisational, educational, governmental and individual efforts to increase the level of professionalism, education and quality of care delivered to Korean citizens nationwide.
Comparison of modified Bass technique with normal toothbrushing practices for efficacy in supragingival plaque removal
Authors: Poyato‐Ferrera M , Segura‐Egea JJ , Bullón‐Fernández P
Published: July 2004

Abstract  

Objectives: This study was designed to compare the efficacy in supragingival plaque removal of normal toothbrushing practices and a particular toothbrushing technique, the modified Bass method.

Subjects and methods: The research consisted of two identical experiments with two toothbrushing methods: the normal toothbrushing practices and the modified Bass technique. Forty‐six secondary, non‐dental students (10 males and 36 females) with ages ranging from 18 to 30 years were selected. Dental plaque was assessed according to the Turesky modification of Quigley–Hein Index. Subjects were requested not to brush their teeth 48 h prior to the baseline record of plaque index. Participants were instructed to brush twice daily during 3 min for the duration of the 3‐week trial using their usual toothpaste. Plaque index was recorded at 2, 7 and 21 days.

Results: The modified Bass (Mod‐Bass) technique was significantly (P < 0.05) more effective in removing supragingival plaque than normal toothbrushing practices both in all, buccal and lingual sites. After 21 days, the normal practices did not significantly decrease mean plaque indices compared to the scores calculated after 7 days (P > 0.05), but did so with the modified Bass technique (P < 0.05). The modified Bass technique was specially efficient at the lingual sites where it was 2.9‐fold more effective than normal practices in reducing plaque scores (P < 0.01).

Conclusion: The results show that a particular toothbrushing technique as modified Bass method is significantly superior to normal toothbrushing practices in supragingival plaque removal. The introduction of a specific toothbrushing technique, such as modified Bass technique represents an important improvement in the oral hygiene of the patients.
Oral health trends in the US
Authors: Danner V
Published: July 2004

Abstract In the US, dental hygienists are an integral part of the health care workforce. Dental hygienists are prevention specialists who understand that recognising the association between oral and total health can prevent disease. In the US, they also consider it as a part of their job to detect the presence of systemic illnesses. Although they cannot make a medical diagnosis, they can explain their findings to the patients and urge them to see a physician. This report explains the educational requirements for a dental hygienist, barriers and access‐to‐care issues, some oral health preventive methods, among other things.
Dental hygiene in Hong Kong: a global perspective
Authors: Luciak‐Donsberger C , Chan CKP
Published: July 2004

Abstract  

Purpose: This article examines how dental hygiene status in Hong Kong compared to global developments of the profession. The aim is to address access to cost‐effective, qualified preventive care.

Methods: Information for this study was obtained using questionnaires and followed up by e‐mail correspondence with International Federation of Dental Hygienists (IFDH) delegates and additional experts, supporting studies and reference literature. All experts consulted are involved in the professional and educational organisation of dental hygiene on a national level.

Results: Results show that dental hygiene is practised in about 30 countries, generally as a licensed profession, studied at institutions of higher education. Average study duration is 3 years. Globally, low access to qualified dental hygiene care tends to be a result of a lack of social and cultural awareness of the benefits of preventive care and insufficient funding for preventive services as well as supervision requirements, which tie dental hygiene care to private practice dentistry settings. In several countries, a reduction of supervision requirements has opened the door to provide care at alternative settings and public health institutions.

Conclusion: In Hong Kong, owing to a small number of practising dental hygienists, access to qualified preventive care is scarce. Public awareness of the benefits and cost‐effectiveness of preventive measures need to be raised to achieve higher acceptance of the profession. Global and scientific developments mandate an extension of study duration. Access to qualified care could be addressed by public health initiatives, which utilise the expertise of dental hygienists in setting and attaining preventive health goals.
Dental hygiene's interdisciplinary relationship with dental public health: a position paper
Authors: Nathe C
Published: July 2004

Abstract:  Dental hygienists' collaboration with social institutions, health‐care workers and organisations, educational institutions, and dentistry can prevent oral diseases. Dental hygienists working in these organisations can reach an unprecedented number of people, thereby increasing preventive oral care to many undeserved. Working within a dental public health career increases choices and opportunities for the dental hygienist and benefits the public.
Partnerships for better oral health
Authors: Beaulieu E
Published: July 2004

Abstract Improving oral health in populations who do not easily access the private dental office or the available community care site is a challenge to dental hygienists and others concerned with the health and well‐being of all. Partnerships for improved oral health have been part of the community health efforts for many years and in many countries. With the knowledge, skills, and resources that are held by specific groups and organisations combined into a larger entity of a partnership or coalition, greater impact on oral health issue may be possible. Agencies and individuals interested in making improvements in oral health status in any particular target group may begin a process of working with others who have an interest in improving the health and well being of that target group. In a world that is increasingly synergistic and mutually dependent, improvements in oral health can be advanced by considering the elements of successful coalition building and forming partnerships with multiple organisations and individuals.
A dental hygienist's role in tobacco cessation
Authors: Parker D R
Published: July 2004

Abstract Dental professionals are uniquely poised to contribute to the battle against tobacco use. When tobacco users receive a consistent anti‐tobacco message from a variety of health care providers, the likelihood of success in the battle to quit is increased. Most people receive many warnings in the media about the negative health effects of tobacco, especially those regarding cancer and heart disease. Smokers and smokeless tobacco users may not be as knowledgeable about the negative oral manifestations of tobacco use, and dental hygienists can be instrumental in educating their patients on these effects. Including questions about tobacco use in the health history is a vital first step to identifying patients who may benefit from tobacco intervention counselling in the dental office. Once identified, the hygienist can proceed to discover the patient's motivation to quit, and assist in providing information that may be of help to the patient in ceasing tobacco dependency. Helping and encouraging patients in their battle against tobacco use is an important way that dental hygienists can contribute to their patients' improved health.
Accurate theoretical prediction of vibrational frequencies in an inhomogeneous dynamic environment: A case study of a glutamate molecule in water solution and in a protein-bound form
Authors: Speranskiy Kirill , Kurnikova Maria
Published: July 2004

We propose a hierarchical approach to model vibrational frequencies of a ligand in a strongly fluctuating inhomogeneous environment such as a liquid solution or when bound to a macromolecule, e.g., a protein. Vibrational frequencies typically measured experimentally are ensemble averaged quantities which result (in part) from the influence of the strongly fluctuating solvent. Solvent fluctuations can be sampled effectively by a classical molecular simulation, which in our model serves as the first, low level of the hierarchy. At the second high level of the hierarchy a small subset of system coordinates is used to construct a patch of the potential surface (ab initio) relevant to the vibration in question. This subset of coordinates is under the influence of an instantaneous external force exerted by the environment. The force is calculated at the lower level of the hierarchy. The proposed methodology is applied to model vibrational frequencies of a glutamate in water and when bound to the Glutamate receptor protein and its mutant. Our results are in close agreement with the experimental values and frequency shifts measured by the Jayaraman group by the Fourier transform infrared spectroscopy [Q. Cheng et al., Biochem. 41, 1602 (2002)]. Our methodology proved useful in successfully reproducing vibrational frequencies of a ligand in such a soft, flexible, and strongly inhomogeneous protein as the Glutamate receptor.
Continuum equations for magnetic and dielectric fluids with internal rotations
Authors: Rosensweig Ronald E.
Published: July 2004

Several authors have attempted with varying success to derive a complete set of basic equations for the motion of polar fluids having internal rotations and hence in a state of polarization disequilibrium. This work develops a complete set of governing equations derived on the basis of dynamic balance relationships with the dissipation function determined from thermodynamic consideration. The magnetization relaxation equation is thereby determined from requirement of positive entropy production along with a complete set of constitutive laws including antisymmetric terms of the total stress tensor. The analysis employs the Minkowski expression of electromagnetic momentum and assumes that the product of electromagnetic stress and velocity contributes to the energy balance on the same footing as contact stresses of pressure and viscous origin. The work refines the treatment of our earlier effort carrying out the analysis to first order in the ratio of fluid velocity to light speed throughout.
Observation and analysis of the 2g(1D) ion-pair state of I2: The g/u mixing between the 1u(1D) and 2g(1D) states
Authors: Nakano Yukio , Ukeguchi Hiromi , Ishiwata Takashi
Published: July 2004

We report the analysis of the 2g(1D) ion-pair state of I2 by perturbation-facilitated optical-optical double resonance. The present study began with the observation of the 2g(1D)–A′ 3Π(2u) emission at around 230 nm during the analysis of the ultraviolet emissions originating form the 1u(1D) ion-pair state. The identification of this new transition helped us to specify the wavelengths for detecting the 2g(1D) state by emission, and also to estimate its absolute position. The intermediate states used to observe the 2g(1D) state were the B 3Π(0u+)–b′ 2u mixed states by the hyperfine interaction, which allowed us to combine the X 1Σg+ ground state with the 2g(1D) state in the (1+1) photon excitation following the optical selection rules for one-photon transitions: 2g(1D)←b′ 2u–B 3Π(0u+)←X 1Σg+. Our analysis covered the 2g(1D) state in the 0⩽v⩽12 and 9⩽J⩽40 ranges. The molecular constants and Rydberg–Klein–Rees (RKR) potential of the 2g(1D) state were reported. We discussed the occurrence of the 2g(1D)–A′ 3Π(2u) emission, when exciting to the 1u(1D) v=0 state, and attributed it to the g/u mixing between the 2g(1D) and 1u(1D) states by the hyperfine interaction. The effect of the perturbation on measured line intensities and lifetimes was evident.
New ab initio potential energy surface for the (HOCO+-He) van der Waals complex
Authors: Hammami K. , Jaidane N. , Ben Lakhdar Z. , Spielfiedel A. , Feautrier N.
Published: July 2004

A three-dimensional potential energy surface has been calculated for the ground electronic state of the HOCO+–He system. The calculations were performed at the coupled electron pair approximation level with an extended basis set which ensures a balance between accuracy and feasability. The validity of the method and of the basis set was tested through calculations of the polarizability of the He atom and of the spectroscopic constants of the HOCO+ ion. The calculated potential energy surface has been fitted to a spherical harmonic expansion to facilitate calculations of rotational excitation of HOCO+ by collisions with He.
Rotational energy transfer in NO (A 2Σ+,v′=0) by N2 and O2 at room temperature
Authors: Lee Seonkyung , Luque Jorge , Reppel Julie , Brown Austin , Crosley David R.
Published: July 2004

State-to-state rotational energy transfer (RET) rate coefficients for NO (A 2Σ+, v′=0, J=5.5, 11.5, 17.5) were measured for N2 and O2 at room temperature using a pump-probe method. The NO A 2Σ+ state is prepared by 226 nm light and the RET is monitored by fluorescence from the D 2Σ+ v′=0 state, following excitation by a time-delayed laser at ∼1.1 μm. Additionally, total collisional removal and final state distributions were measured exciting in the Q1+P21 band head, to simulate an NO laser-induced fluorescence atmospheric monitoring scheme. Time-resolved modeling is used to understand relaxation mechanisms and predict relaxation times in ambient air. H2O at atmospherically relevant concentrations does not affect the degree of RET in ambient air.
Phosphorus hyperfine structure in the electronic spectrum of the HPCl free radical
Authors: Tackett Brandon S. , Clouthier Dennis J. , Adam Allan G. , Shepard Scott A.
Published: July 2004

The 444 nm 201 bands of the Ã 2A′–X̃ 2A″ transition of the jet-cooled HP 35Cl and HP 37Cl radicals have been studied at high resolution using the pulsed electric discharge technique with a precursor mixture of PCl3 and H2. Spectra recorded with linewidths of ∼360 MHz revealed resolved hyperfine structure in both isotopomers arising from the excited state Fermi contact interaction of the unpaired electron with the magnetic moment of the P31 nucleus, with aF′=0.0641(10) cm−1 and 0.0636(31) cm−1 for HP 35Cl and HP 37Cl, respectively. No contribution from the ground state, or excited state contributions from the hydrogen or chlorine nuclei were resolved, confirming ab initio predictions that HPCl is a pπ radical in the X̃ state, and an sσ radical with a substantial contribution from the phosphorus 3s atomic orbital in the Ã state. The free atom comparison method has been used to estimate that the singly occupied molecular orbital in the excited state has 14% phosphorus 3s character.
Molecular alignment of rigid rods in nonrigid spherical pores
Authors: Chen Yong , Shew Chwen-Yang , LeFave Clare V. , Ajavon Antoinette , Fynewever Herbert
Published: July 2004

We have investigated the orientation ordering of two shish-kebab chains confined by spherically harmonic potentials through Monte Carlo simulations and asymptotic analysis. The rigid rod is modeled as shish-kebab chains consisting of tangent hard spheres aligned in the same axis, and the harmonic potential is chosen to model nonrigid cavities. We first show that the interactions between a rod and the spherically harmonic potential are independent of chain orientation, indicating that the alignment of two confined rods arises from the excluded volume interactions alone. In the strong fields, the order parameter of two confined rods converges to different values, depending on the parity of chain length. From asymptotic order parameters, we find that the rods of odd-number beads rotate more freely even under the limiting strong confinement. However, the two rods of even-number beads are essentially trapped in a configuration of perpendicular alignment through intercalation of their central grooves. We attribute the dependence of the parity of chain length to the different locations of the center-of-mass in a rod for these two cases. Furthermore, we compare the shish–kebab chains with different rod models in the simulations, and utilize these models to explore the effect of the local rod smoothness on molecular alignment. Our findings suggest that increasing local rod smoothness enhances the rotational degree of freedom for confined rods, and the effect of local rod roughness emerges under strong enough applied potentials.
Second-order Møller–Plesset theory with linear R12 terms (MP2-R12) revisited: Auxiliary basis set method and massively parallel implementation
Authors: Valeev Edward F. , Janssen Curtis L.
Published: July 2004

Ab initio electronic structure approaches in which electron correlation explicitly appears have been the subject of much recent interest. Because these methods accelerate the rate of convergence of the energy and properties with respect to the size of the one-particle basis set, they promise to make accuracies of better than 1 kcal/mol computationally feasible for larger chemical systems than can be treated at present with such accuracy. The linear R12 methods of Kutzelnigg and co-workers are currently the most practical means to include explicit electron correlation. However, the application of such methods to systems of chemical interest faces severe challenges, most importantly, the still steep computational cost of such methods. Here we describe an implementation of the second-order Møller–Plesset method with terms linear in the interelectronic distances (MP2-R12) which has a reduced computational cost due to the use of two basis sets. The use of two basis sets in MP2-R12 theory was first investigated recently by Klopper and Samson and is known as the auxiliary basis set (ABS) approach. One of the basis sets is used to describe the orbitals and another, the auxiliary basis set, is used for approximating matrix elements occurring in the exact MP2-R12 theory. We further extend the applicability of the approach by parallelizing all steps of the integral-direct MP2-R12 energy algorithm. We discuss several variants of the MP2-R12 method in the context of parallel execution and demonstrate that our implementation runs efficiently on a variety of distributed memory machines. Results of preliminary applications indicate that the two-basis (ABS) MP2-R12 approach cannot be used safely when small basis sets (such as augmented double- and triple-ζ correlation consistent basis sets) are utilized in the orbital expansion. Our results suggest that basis set reoptimization or further modifications of the explicitly correlated ansatz and/or standard approximations for matrix elements are necessary in order to make the MP2-R12 method sufficiently accurate when small orbital basis sets are used. The computer code is a part of the latest public release of Sandia’s Massively Parallel Quantum Chemistry program available under GNU General Public License.
Perturbational relativistic theory of electron spin resonance g-tensor
Authors: Manninen Pekka , Vaara Juha , Ruud Kenneth
Published: July 2004

We carry out a complete treatment of the leading-order relativistic one-electron contributions, arising from the Breit–Pauli Hamiltonian, to the g-tensor of electron spin resonance spectroscopy. We classify the different terms and discuss their interpretation as well as give numerical ab initio estimates for the F2−, Cl2−, Br2−, and I2− series, using analytical response theory calculations with a multiconfigurational self-consistent field reference state. The results are compared to available experimental data.
The low-lying electronic excited states of NiCO
Authors: Horný Ľuboš , Paul Ankan , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2004

Highly correlated coupled cluster methods with single and double excitations (CSSD) and CCSD with perturbative triple excitations were used to predict molecular structures and harmonic vibrational frequencies for the electronic ground state X̃ 1Σ+, and for the Δ,3 Σ+3, Φ,3 1 3Π, 2 3Π, Σ+1, Δ,1 and Π1 excited states of NiCO. The X̃ 1Σ+ ground state’s geometry is for the first time compared with the recently determined experimental structure. The adiabatic excitation energies, vertical excitation energies, and dissociation energies of these excited states are predicted. The importance of π and σ bonding for the Ni–C bond is discussed based on the structures of excited states.
The vibrational spectrum of NF3 and the manifestation of resonant dipole–dipole interaction in NF3 solutions in liquid argon
Authors: Kolomiitsova T. D. , Shchepkin D. N. , Tokhadze K. G. , Herrebout W. A. , van der Veken B. J.
Published: July 2004

Vibrational spectra of trifluoramine, NF3, dissolved in liquid Ar were studied at 90 K in the concentration range between 2×10−5 and 0.1 mole fraction, using Fourier transform spectroscopy. The concentration dependence of the band shapes in the region of the combination transitions ν1+ν3, ν2+ν3, and 2ν3 involving the strong ν3 mode was studied and the absorption associated with NF3 dimers was isolated. This absorption is compared with spectra of NF3 dimers calculated on the basis of resonant dipole–dipole interaction between two doubly degenerate oscillators. Spectra of pure liquid NF3 were recorded for comparison. Using the ν1+ν3 absorption band of the NF3 dimer the distance R between two NF3 molecules was determined to be R=4.5(1) Å in solution in liquid Ar. This distance is compared with the separation between two NF3 molecules in liquid NF3 and with the value calculated from the pair distribution function obtained from Monte Carlo simulations.
Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis→trans tunneling rate
Authors: Maçôas E. M. S. , Khriachtchev L. , Pettersson M. , Fausto R. , Räsänen M.
Published: July 2004

Rotational isomerization of acetic acid (CH3COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans→cis reaction is promoted using resonant excitation of a number of modes in the 3500–7000 cm−1 region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier (⩾4400 cm−1), the measured quantum yields are in average 2%–3%, and this is one order of magnitude smaller than the corresponding values known for formic acid (HCOOH). This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans→cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis→trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans→cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans→cis reaction quantum yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis→trans dark reaction, the observed temperature and medium effects indicate the participation of the lattice phonons in the tunneling-induced process.
Perturbation theory corrections to the two-particle reduced density matrix variational method
Authors: Juhász Tamás , Mazziotti David A.
Published: July 2004

In the variational 2-particle-reduced-density-matrix (2-RDM) method, the ground-state energy is minimized with respect to the 2-particle reduced density matrix, constrained by N-representability conditions. Consider the N-electron Hamiltonian H(λ) as a function of the parameter λ where we recover the Fock Hamiltonian at λ=0 and we recover the fully correlated Hamiltonian at λ=1. We explore using the accuracy of perturbation theory at small λ to correct the 2-RDM variational energies at λ=1 where the Hamiltonian represents correlated atoms and molecules. A key assumption in the correction is that the 2-RDM method will capture a fairly constant percentage of the correlation energy for λ∈(0,1] because the nonperturbative 2-RDM approach depends more significantly upon the nature rather than the strength of the two-body Hamiltonian interaction. For a variety of molecules we observe that this correction improves the 2-RDM energies in the equilibrium bonding region, while the 2-RDM energies at stretched or nearly dissociated geometries, already highly accurate, are not significantly changed. At equilibrium geometries the corrected 2-RDM energies are similar in accuracy to those from coupled-cluster singles and doubles (CCSD), but at nonequilibrium geometries the 2-RDM energies are often dramatically more accurate as shown in the bond stretching and dissociation data for water and nitrogen.
Electron-impact ionization of CCl4 and CCl2F2
Authors: Lindsay B. G. , McDonald K. F. , Yu W. S. , Stebbings R. F. , Yousif F. B.
Published: July 2004

Absolute partial and total cross sections for electron-impact ionization of CCl4 and CCl2F2 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for production of CCl3+, CCl2+, CCl+, C+, Cl2+, and CCl32+ from CCl4; and for production of CCl2F+, CClF2+, CClF+, (CCl++CF2+), Cl+, CF+, F+, and C+ from CCl2F2. Data are also reported for formation of (CCl2+,Cl+) and (CCl+,Cl+) ion pairs from CCl4. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in the absolute cross sections for most of the singly charged ions is ±5–7 %. The present partial cross sections for lighter fragment ions are found to be considerably greater than had been previously reported but the most recent total cross section measurements agree well with those reported here. Neither the binary-encounter-Bethe theory nor the Deutsch–Märk theory reproduces the experimental cross sections correctly for both targets.
Structure of liquid and glassy methanol confined in cylindrical pores
Authors: Morineau Denis , Guégan Régis , Xia Yongde , Alba-Simionesco Christiane
Published: July 2004

We present a neutron scattering analysis of the density and the static structure factor of confined methanol at various temperatures. Confinement is performed in the cylindrical pores of MCM-41 silicates with pore diameters D=24 and 35 Å. A change of the thermal expansivity of confined methanol at low temperature is the signature of a glass transition, which occurs at higher temperature for the smallest pore. This is evidence of a surface induced slowing down of the dynamics of the fluid. The structure factor presents a systematic evolution with the pore diameter, which has been analyzed in terms of excluded volume effects and fluid–matrix cross correlation. Conversely to the case of Van der Waals fluids, it shows that stronger fluid–matrix correlations must be invoked most probably in relation with the H-bonding character of both methanol and silicate surface.
Abnormal IR effects of Pt nanostructured surfaces upon CO chemisorption due to interaction and electron-hole damping
Authors: Wu Chen-Xu , Lin Hai , Chen You-Jiang , Li Wen-Xuan , Sun Shi-Gang
Published: July 2004

The abnormal IR effects (AIREs) characterized by a positive-going peak of platinum (Pt) nanostructured surface generated in a square-wave potential treatment upon CO molecule chemisorption was observed and analyzed with a consideration of the interparticle interaction and electron-hole damping between nanoislands and CO molecules. A theoretical simulation shows that the islanded nanostructured Pt surfaces, which gives rise to interparticle interaction, coupling with electron-hole mechanism, may contribute to the origins of positive-going peak (AIREs) observed by in situ Fourier transformation IR (FTIR) experiments.
Rate constant for diffusion-influenced ligand binding to receptors of arbitrary shape on a cell surface
Authors: Dudko Olga K. , Berezhkovskii Alexander M. , Weiss George H.
Published: July 2004

The theory of ligand binding to receptors on a cell surface suggested by Berg and Purcell and generalized by Zwanzig and Szabo uses the assumption that receptors are circular absorbing disks on an otherwise reflecting sphere. One of the key ingredients of this theory is a solution for the rate constant for ligand binding to a single circular receptor on a reflecting plane. We give an exact solution for the rate constant for binding to a single elliptic receptor and an approximate solution for binding to a single receptor of more general shape. The latter was tested by Brownian dynamics simulations. We found that the approximate formula predicted the rate constant with better than 10% accuracy for all studied receptor shapes. Using our solutions one can find the rate constant for ligand binding to a cell covered by N noncircular receptors by means of the Zwanzig–Szabo formula.
Long-range ordered structures in diblock copolymer melts induced by combined external fields
Authors: Feng Jie , Ruckenstein Eli
Published: July 2004

The structure of diblock copolymer melts under a single external electric or shear field, as well as under combined orthogonal external fields was investigated using a cell dynamic system. The phase structure was determined by coupling the effects of the external fields with the original structure of the bulk free of external fields. The single electric or shear field generated long-range cylinders in asymmetric A4mB6m diblock copolymers and distorted lamellae in symmetric A5mB5m diblock copolymers. Successive orthogonal shear followed by an electric external field generated long-range lamellae in symmetrical A5mB5m systems. However, the simultaneous orthogonal electric and shear fields could more easily form long-range lamellae than the sequential orthogonal fields. The dynamical processes in diblock copolymer melts under orthogonal fields have been also examined.
Specific heat and Lindemann-like parameter of metallic clusters: Mono- and polyvalent metals
Authors: Lai S. K. , Lin W. D. , Wu K. L. , Li W. H. , Lee K. C.
Published: July 2004

The Brownian-type molecular dynamics simulation is revisited and applied to study the thermal and geometric properties of four mono- and two polyvalent metallic clusters. For the thermal property, we report the specific heat at constant volume CV and study the solid-liquid-like transition by scrutinizing its characteristic. For the geometric property, we calculate the root mean square relative bond-length fluctuation δ as a function of increasing temperature. The thermal change in δ reflects the movement of atoms and hence is a relevant parameter in understanding the phase transition in clusters. The simulated results for the CV of alkali and aluminum clusters whose ground state structures exhibit icosahedral symmetry generally show one phase transition. In contrast, the tetravalent lead is quite often seen to exhibit two phase transitions, a premelting process followed by a progressive melting. In connection with the premelting scenario, it is found here that those (magic number) clusters identified to be of lesser stability (among other stable ones) according to the second energy difference are clusters showing a greater possibility of undergoing premelting process. This energy criterion applies to aluminum clusters nAl=28 and 38. To delve further into the thermal behavior of clusters, we have analyzed also the thermal variation of δ(T) and attempted to correlate it with CV(T). It turns out that the premelting (if exist) and melting temperatures of the smaller size clusters (n≲50) extracted from CV do not always agree quantitatively with that deduced from δ.
Charge and energy dynamics in photo-excited poly(para-phenylenevinylene) systems
Authors: Gisslén L. , Johansson Å. , Stafström S.
Published: July 2004

We report results from simulations of charge and energy dynamics in poly(para-phenylenevinylene) (PPV) and PPV interacting with C60. The simulations were performed by solving the time-dependent Schrödinger equation and the lattice equation of motion simultaneously and nonadiabatically. The electronic system and the coupling of the electrons to the lattice were described by an extended three-dimensional version of the Su-Schrieffer-Heeger model, which also included an external electric field. Electron and lattice dynamics following electronic excitations at different energies have been simulated. The effect of additional lattice energy was also included in the simulations. Our results show that both exciton diffusion and transitions from high to lower lying excitations are stimulated by increasing the lattice energy. Also field induced charge separation occurs faster if the lattice energy is increased. This separation process is highly nonadiabatic and involves a significant rearrangement of the electron distribution. In the case of PPV coupled to C60, we observe a spontaneous charge separation. The separation time is in this case limited by the local concentration of C60 molecules close to the PPV chain.
Deflection and deceleration of hydrogen Rydberg molecules in inhomogeneous electric fields
Authors: Yamakita Y. , Procter S. R. , Goodgame A. L. , Softley T. P. , Merkt F.
Published: July 2004

Hydrogen molecules are excited in a molecular beam to Rydberg states around n=17–18 and are exposed to the inhomogeneous electric field of an electric dipole. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H2 molecules in the inhomogeneous electric field. The trajectories of the molecules are monitored using ion-imaging and time of flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole, respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k and of the duration of the applied field. It is also shown that a large deflection is observed when populating the (17d2)1 state at zero field and switching the dipole field on after a delay. With the dipole mounted perpendicular to the beam direction, the molecules are either accelerated or decelerated as they move towards the dipole. The Rydberg states are found to survive for over 100 μs after the dipole field is switched off before being ionized at the detector and the time of flight is measured. A greater percentage change in kinetic energy is achieved by initial seeding of the beam in helium or neon followed by inhomogeneous field deceleration/acceleration. Molecular dynamics trajectory simulations are presented highlighting the extent to which the trajectories can be predicted based on the known Stark map. The spectroscopy of the populated states is discussed in detail and it is established that the N+=2, J=1, MJ=0 states populated here have a special stability with respect to decay by predissociation.
Infrared–infrared double resonance spectroscopy of cyanoacetylene in helium nanodroplets
Authors: Merritt J. M. , Douberly G. E. , Miller R. E.
Published: July 2004

Infrared–infrared double resonance spectroscopy is used as a probe of the vibrational dynamics of cyanoacetylene in helium droplets. The ν1 C–H stretching vibration of cyanoacetylene is excited by an infrared laser and subsequent vibrational relaxation results in the evaporation of approximately 660 helium atoms from the droplet. A second probe laser is then used to excite the same C–H stretching vibration downstream of the pump, corresponding to a time delay of approximately 175 μs. The hole burned by the pump laser is narrower than the single resonance spectrum, owing to the fact that the latter is inhomogeneously broadened by the droplet size distribution. The line width of the hole is characteristic of another broadening source that depends strongly on droplet size.
Molecular beam scattering of NO+Ne: A joint theoretical and experimental study
Authors: Kim Y. , Meyer H. , Alexander M. H.
Published: July 2004

The collision dynamics of the NO+Ne system is investigated in a molecular beam scattering experiment at a collision energy of 1055 cm−1. Employing resonance enhanced multiphoton ionization of NO, we measured state-resolved integral and differential cross sections for the excitation to various levels of both spin-orbit manifolds. The dependence of the scattered intensity on the laser polarization is used to extract differential quadrupole moments for the collision induced angular momentum alignment. The set of cross section data is compared with results of a full quantum mechanical close coupling calculation using the set of ab initio potential energy surfaces of Alexander et al. [J. Chem. Phys. 114, 5588 (2001)]. In previous work, it was found that the positions and rotational substructures for the lowest bend-stretch vibrational states derived from these surfaces agree very well with the observed spectrum of the NO–Ne complex. For the same potential, we find that the calculated cross sections show a less satisfactory agreement with the experimental data. While the overall Jf dependence and magnitude of the integral and differential cross sections are in good agreement, noticeable discrepancies exist for the angle dependence of the differential cross sections. In general, the calculated rotational rainbow structures are shifted towards larger scattering angles indicating that the anisotropy of the potential is overestimated in the fit to the ab initio points or in the ab initio calculation itself. For most states, we find the measured alignment moments to be in excellent agreement with the results of the calculation as well as with predictions of sudden models. Significant deviations from the sudden models are observed only for those fine-structure changing collisions which are dominated by forward scattering. Results of the full quantum calculation confirm the deviations for these states.