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71.
A time-dependent quantum model involving two wave packets on two excited-state surfaces is presented to describe absorption (or emission) from the transition state of a chemical reaction. The connection between the quantum result and existing classical theories is shown. The model ia applied to the direct dissociation of ICN* and gives results in good agreement with experiment. The dissociation time - the time to half-maximal absorption - is almost invariant with the pulse width but is dependent on the probe wavelength. A lower absorption plateau and a longer dissociation time is predicted for probe energies above the asymptotic resonance energy. 相似文献
72.
Light-driven rotary molecular motors based on overcrowded alkenes can be substituted with electron-donating and electron-withdrawing substituents (R = OMe, Cl and CN) in direct conjugation with the central double bond (the axis of rotation) without having a significant influence on the rate-limiting, thermal isomerisation step of their rotary cycle. This indicates that in this system, it is predominantly steric factors that determine the barrier to the thermal helix inversion. In contrast, the quantum yield and photoequilibria in the photochemical step were found to be quite sensitive to the combination of substituent and solvent employed. 相似文献
73.
Laser-induced Breakdown Spectroscopy (LIBS) in combination with Principal Components Regression (PCR) has been applied to determine the elemental composition of a series of run-of-mine (ROM) iron ore samples. The samples were presented for measurement both as compressed pellets and as loose chipped material. The present paper details the results of the measurements of the compressed pellets. Results from ore chips will be reported separately. LIBS spectral data was recorded in three separate spectral regions to measure major, minor and trace components of the iron ore sample pellets. Background stripping, normalization and spectral cleaning were applied to minimize the relative standard deviations of the LIBS data. PCR analysis was then applied to produce calibration models for iron, aluminum, silicon, manganese, potassium and phosphorous. These calibration models were then validated using independent LIBS measurements. Robust calibration models were determined for iron, aluminum, silicon and potassium, whilst the results for manganese were encouraging. Phosphorous, present at low levels in the ores measured, remained the most difficult element to determine accurately. The combination of LIBS and PCR shows potential for in-situ on-line determination of ore composition. 相似文献
74.
Pollard JA Zhang D Downing JA Knorr FJ McHale JL 《The journal of physical chemistry. A》2005,109(50):11443-11452
Resonance Raman spectra are reported for Ru(4,4'-dicarboxylic acid-2,2'-bipyridine)2(NCS)2 (commonly called "N3") in ethanol solution and adsorbed on nanoparticulate colloidal TiO2 in ethanol (EtOH) and in acetonitrile (ACN), at wavelengths within the visible absorption band of the dye. Raman cross sections of free N3 in EtOH are found to be similar to those of N3 adsorbed on colloidal TiO2 in EtOH, and are generally lower than those of N3 on TiO2 in ACN. Strong electronic coupling mediated by surface states results in red-shifted absorption spectra and enhanced Raman signals for N3 adsorbed on nanocolloidal TiO2 in ACN compared to EtOH. In contrast, the absorption spectrum of N3 on nanocrystalline TiO2 in contact with solvent is similar for ACN and EtOH. Wavelength-dependent depolarization ratios for N3 Raman bands of both free and adsorbed N3 reveal resonance enhancement via two or more excited electronic states. Luminescence spectra of N3 adsorbed on nanocrystalline films of TiO2 and ZrO2 in contact with solvent reveal that the quantum yield of electron injection phi(ET) into TiO2 decreases in the order ACN > EtOH > DMSO. Dye-sensitized solar cells were fabricated with N3 adsorbed on nanocrystalline films of TiO2 in contact with ACN, EtOH, and DMSO solutions containing LiI/LiI3 electrolyte. Photoconversion efficiencies eta were found to be 2.6% in ACN, 1.3% in DMSO, and 0.84% in EtOH. Higher short circuit currents are found in cells using ACN, while the maximum voltage is found to be largest in DMSO. It is concluded that the increased photocurrent and quantum yield of interfacial electron transfer in acetonitrile as compared to ethanol and DMSO is primarily the result of faster electron injection of N3 when adsorbed on TiO2 in the presence of ACN as opposed to EtOH or DMSO. 相似文献
75.
Andrew J. Pollard Edward W. Perkins Dr. Nicholas A. Smith Alex Saywell Gudrun Goretzki Dr. Anna G. Phillips Stephen P. Argent Dr. Hermann Sachdev Priv.‐Doz. Frank Müller Dr. Stefan Hüfner Prof. Stefan Gsell Dr. Martin Fischer Matthias Schreck Dr. Jürg Osterwalder Prof. Thomas Greber Prof. Simon Berner Dr. Neil R. Champness Prof. Peter H. Beton Prof. 《Angewandte Chemie (International ed. in English)》2010,49(10):1794-1799
76.
G. H. Pollard 《International Journal of Mathematical Education in Science & Technology》2013,44(3):429-432
Methods of scoring in multiple choice examinations are outlined in which the examinee's expected change in score because of random guessing must be negative for all situations of partial knowledge. Thus, it can be argued that the guessing component in multiple choice examinations can be completely removed. The examinee is instructed to tick the correct box or, if the correct box is unknown, to cross boxes known to be incorrect, or to leave the question blank. Under the proposed scoring system, it is argued that an examinee will not attempt to demonstrate more knowledge on any particular question than the examinee in fact has. Also, the examiner has quite a degree of freedom as to the size of the rewards for correctly crossed boxes; rewards which need not be equal. There is also quite a range in the possible values of the penalties for incorrect responses and appropriate recommendations are made. 相似文献
77.
D.L. Death A.P. Cunningham L.J. Pollard 《Spectrochimica Acta Part B: Atomic Spectroscopy》2009,64(10):1048
In the mining industry the quality and extent of an ore body is determined on the basis of routine assays conducted on drill core and chip samples. Both the elemental composition and the mineralogical classification are important in the characterisation of an ore body for commercial exploitation. Mining industry laboratories typically analyse large numbers of samples from both exploration and mine production environments.At CSIRO we have explored the application of chemometric methods of analysis in combination with laser induced breakdown spectroscopy (LIBS) in order to produce routine quantitative analysis of several ore types including iron, nickel and lead/zinc ores. In particular, principal components regression (PCR) has been applied to perform multi-element analysis of iron ore samples from Australia and West Africa. Calibration models for iron (4.8% Av. Relative Error), aluminium (2.2%), silicon (3.7%) and potassium (1.4%) were determined for the Australian ores. In addition phosphorous measurements were made at trace level for samples from West Africa (5.5% Av. Relative Error). LIBS measurements of segments of a nickel drill core were also analysed using PCR.Mineralogical classification using a combination of LIBS and principal components analysis (PCA) has also been explored. Broad discrimination of ore mineralogy was demonstrated on the basis of the PCA of LIBS spectra in selected emission wavelength bands. The combination of PCA and PCR offers potential for both broad mineralogical and elemental analysis for the minerals industry in exploration and in mine production for the on-line monitoring of ore quality. 相似文献
78.
Sayo O. Fakayode Pamlea N. Brady David A. Pollard Abdul K. Mohammed Isiah M. Warner 《Analytical and bioanalytical chemistry》2009,394(6):1645-1653
We report the first combined use of analytical spectroscopy, guest–host chemistry, and multivariate regression analysis for
determination of enantiometric composition of multicomponent samples of chiral analytes. Sample solutions containing multicomponent
analytes of ephedrine, tryptophan, propranolol, and proline of varying enantiomeric composition with beta-cyclodextrin (BCD)
or methyl-beta-cyclodextrin (Me-BCD) as chiral host molecules were investigated using ultraviolet (UV)–visible spectroscopy.
The interactions of enantiomers of chiral analytes with chiral hosts resulted in the formation of transient diastereomeric
inclusion complexes with varying spectral properties. Multivariate analysis using partial-least-square (PLS) regression was
used to correlate subtle changes in the UV–visible spectra of the guest–host complexes with the enantiomeric composition of
the calibration samples. These PLS regressions were carefully optimized and then used to predict the enantiomeric composition
of multicomponent chiral analytes of validation samples. The results of these validation studies demonstrate the predictive
ability of the regression models for determination of future enantiomeric composition of samples. The accuracy of the models
to correctly predict the enantiomeric composition of samples, evaluated by use of the root mean square percent relative error
(RMS%RE) was analyte and chiral host dependent. In general, better prediction of enantiomeric composition of samples and low
RMS%RE values were obtained when Me-BCD was used as the chiral host. The analyses procedure reported here is simple, rapid,
and inexpensive. In addition, this approach does not require prior separation of chiral analytes, thus reducing analysis time
and eliminating the need for expensive chiral columns. 相似文献
79.
De Geest BG Willart MA Lambrecht BN Pollard C Vervaet C Remon JP Grooten J De Koker S 《Angewandte Chemie (International ed. in English)》2012,51(16):3862-3866
Immunizing: to evoke highly potent immune responses against recombinant antigens, hollow capsules consisting of layers of dextran sulphate and poly-L-arginine that encapsulate the antigen ovalbumin (orange circles) were coated with immune-activating CpG-containing oligonucleotides (green). These capsules were readily internalized by dendritic cells and showed activity in further immunization experiments. 相似文献
80.
Browne WR Pollard MM de Lange B Meetsma A Feringa BL 《Journal of the American Chemical Society》2006,128(38):12412-12413
The fully reversible three-state blue/red/off emission from photo-/electrochromic substituted bis-thiaxanthylidenes is reported. The blue luminescence of the most stable (anti-folded) conformer of dimethyl- and dimethoxy-bis-thiaxanthylidene can be switched off by photochemical conversion to the meta-stable (syn-folded) conformer and switched on again by thermal reversion to the anti-folded state. The red luminescence of the bis-thiaxanthylium dication can be switched on by oxidation at approximately 1.0 and 1.2 V vs SCE of the syn- and anti-folded conformers respectively and switched off or to blue by reduction at approximately 0.35 V vs SCE. 相似文献