Molecular Spectroscopic Techniques Research Articles

Determination of ascorbic acid by molecular spectroscopic techniques

The widespread use of ascorbic acid in medical practice and the food industry is responsible for a variety of determination methods for this substance. The importance of ascorbic acid (vitamin C) for the human immune system and for the prevention of various diseases is a matter of common knowledge. Thus, ascorbic acid is a constituent of multivitamin preparations, food additives, and pharmaceuticals; it is added as an antioxidant to foods, juices, and beverages. Ascorbic acid exhibits reducing properties. Ascorbic acid is rapidly oxidized in aqueous solutions at pH > 6.0 in the presence of atmospheric oxygen. This process is catalyzed by Cu(II) and Fe(III) compounds [1]. Thus, the necessity of the quality control of preparations, in particular, drugs, containing ascorbic acid is beyond question. Along with traditional titrimetric analysis [2], highly sensitive physicochemical techniques, such as spectroscopy, chromatography, and electroanalysis, were proposed for the quantitative determination of ascorbic acid. The aim of this work was to summarize the domestic and foreign experience in the determination of ascorbic acid by molecular spectroscopic techniques over the last 15 years. These techniques are preferable to analytical laboratories because of their high sensitivity and good reproducibility and the availability of instrumentation.

Dissociation rates of benzene at VUV excitation

Photodissociation dynamics of benzene under 193 nm and 157 nm photon excitation have been studied using the molecular beam photofragmentation spectroscopic technique. By detecting the H atom products produced away from the photolysis region, the lifetime of the excited benzene molecule as well as the kinetic energy distribution of the H atom elimination process can be determined simultaneously. Experimental results show that the H atom elimination from benzene occurs on the ground electronic state in which the excess energy is likely fully randomized.

Characteristics of the Phosphorescence Spectra of Benzophenone Adsorbed on Ti−Al Binary Oxides

The excited states of benzophenone adsorbed on well-characterized Ti−Al binary oxides were investigated by means of conventional molecular spectroscopic techniques such as UV and IR absorption as well as photoluminescence spectra measurements. It was found that the photochemical and photophysical properties of benzophenone adsorbed on the oxide surfaces strongly depend on their surface properties, which are systematically changed by changing the composition of the oxides. Specifically, the phosphorescence spectra of benzophenone adsorbed on the oxides have been found to be composed of the hydrogen-bonded benzophenone species and the protonated benzophenone species.

Ultrafast spectroscopy of shock waves in molecular materials.

Recent progress in combining the techniques of time-resolved molecular spectroscopy with shock compression science is reviewed. Shock wave spectroscopy probes the response of molecules to high-speed, large-amplitude mechanical transients and is an important way of studying physical chemical phenomena that involve large-amplitude displacements. A brief discussion of the continuum model for shock compression and a molecular model for the shock front is presented. Methods for generating and detecting shock effects are reviewed. Several applications of shock spectroscopy are reviewed, including high explosives, the nanoshock technique that uses ultrafast lasers, and shock compression of biological molecules.

Optical diagnostics and numerical modelling of arc re-strikes in low-voltage circuit breakers

This paper is devoted to the study of the phenomenon of arc re-striking in low-voltage circuit breakers. The arc re-strike can be described as a sudden re-appearance in the arcing contact region when the arc had been situated in the quenching chamber a few tens of microseconds before. Our experimental investigations have established that the critical arcing contact region is still crossed by a so-called residual current of the order of several amperes. A gas temperature of around 4000 K was derived both from fine electrical measurements and from a molecular spectroscopy technique just before the occurrence of the arc re-strike. We also demonstrate that the re-strike takes place through the growth of the remaining current of several amperes in the arcing contact region. A numerical approach was carried out with a two-dimensional hydrodynamic code. This was found able to describe the arc movement in the model circuit breaker throughout a high-current-interruption operation and, notably, to simulate the arc re-strikes. The simulation exhibits the role of the flow of gas evaporated from the wall in the process of maintaining a slightly conductive medium in the arc ignition region.

Dynamik der Wechselwirkung von Molekülen mit Oberflächen

AbstractA multitude of methods from surface physics in combination with molecular beam and laser spectroscopic techniques provides detailed insight into the various aspects of the dynamics of interactions between molecules and surfaces, reaching from macroscopic kinetics over phenomena of spatiotemporal self‐organisation to the microscopic elementary process. The present state of our knowledge is illustrated by means of selected examples, concerning mainly single crystal surfaces of metals.

Real-time observation of single molecules by confocal fluorescence microscopy

The ability to detect, identify, and manipulate individual molecules offer exciting possibilities in many fields, including chemical analysis, materials research, and the biological sciences. A particularly powerful approach is to combine the exquisite sensitivity of laser-induced fluorescence and the spatial localization and imaging capabilities of diffraction-limited or near-field optical microscopes. Unlike scanning tunneling microscopy (STM) and atomic force microscopy (AFM), which lack molecular specificity, optical spectroscopy and microscopy techniques can be used for real-time monitoring and molecular identification at nanometer dimensions or in ultrasmall volumes.We report the use of confocal fluorescence microscopy coupled with a diffraction-limit laser beam and a high-efficiency photodiode for real-time detection of single fluorescent molecules in solution at room temperature. Rigler and Eigen have also demonstrated single-molecule detection with a confocal microscope and fluorescence correlation spectroscopy. The probe (or sampling) volume is effectively an elongated cylinder, with its radius being determined by optical diffraction and length by spherical aberration.

Evidence for stepwise dissociation dynamics in acetone at 248 and 193 nm

The technique of molecular beam photofragment translational spectroscopy has been used to study the dissociation of acetone following S1←S0 (248 nm) and S2←S0 (193 nm) excitation. Excitation at 248 nm resulted in the production of CH3 and CH3CO with 14.2±1.0 kcal/mole on average of the available energy appearing as translation of the photofragments. Comparison of the measured 〈ET〉 with values reported at 266 nm suggest that the energy partitioning is dominated by the exit barrier caused by an avoided crossing on the potential energy surface. A substantial fraction (30±4%) of the nascent acetyl radicals from the primary dissociation contain sufficient energy to undergo spontaneous secondary decomposition. From the onset of the truncation of the CH3CO P(ET) a threshold of 17.8±3.0 kcal/mole for the dissociation of the acetyl radical has been determined in agreement with recent results on the photodissociation of acetyl chloride. The translational energy release in the dissociation of CH3CO closely matches the experimentally determined exit barrier. At 193 nm the only observed dissociation pathway was the formation of two methyl radicals and carbon monoxide. On average ∼38% of the available energy is found in product translation suggesting that significant internal energy resides in the nascent CH3 fragments consistent with the results of Hall et al. [J. Chem. Phys. 94, 4182 (1991)]. We conclude that the dynamics and energy partitioning for dissociation at 193 nm is similar to that at 248 nm.

Experimental and theoretical study of the dissociation energies D0(H 2NH) and D0(H 2N +H) and other related quantities

Combining the techniques of molecular beam and vacuum ultraviolet synchrotron radiation photoionization mass spectroscopy, we have measured the ionization energy of NH 3 (10.16±0.02 eV) and the appearance potentials of H + (18.57±0.05 eV) and NH 2 + (15.75±0.02 eV). From these data, we have obtained dissociation energy values for D 0(H 2NH) (4.97±0.05 eV), D 0(H 2N +H) (5.59±0.02 eV) and D 0(H 2NH +) (8.41±0.05 eV) and the ionization energy IE(NH 2) (10.78±0.05 eV). High-level ab initio calculations lead to results that are in good agreement with the experimental findings.

Setting up of an Isotope Analyser for Quantitative Determination of Percentage Abundance of15N in Nitrogen Gas

Abstract An 15N isotope analyser has been assembled for the quantitative determination of the percentage abundance of 15N isotope in nitrogen gas employing molecular optical emission spectroscopic technique. The (2,0) band of the second positive system of N2 was excited in a high frequency discharge. The band heads of the isotopic species 14N14N and 14N15N molecules were resolved using a 0.5 m monochromator and their intensity peaks were measured. The ratio of the peak heights enabled the quantitative determination of 15N in the N2 sample. The performance of the isotope analyser assembled was evaluated and it was found to be quite good as inferred from the analysis of primary standards. The analytical error in the 15N concentration-range 0.36 to 24% is <6% and rsd is <4%. A salient feature of the method adopted is that it is direct and does not call for the use of comparision standards.

Industrial problem solving with molecular spectroscopy

We will describe the evolution of molecular spectroscopy techniques, discuss the challenges of integrating and automating analytical procedures within the laboratory, and touch on the challenges facing analytical chemists

Electronic spectroscopy of yttrium monosulfide: molecular beam studies and density functional calculations

The yttrium monosulfide molecule (YS) has been investigated using the techniques of molecular beam fluorescence spectroscopy and density functional theory. Fluorescence spectra in the region of the electronic orgin of the B2∑+ ← X2∑+ system (ν00 = 14 826.07 cm−1) were recorded using a ring dye laser, the experimental resolution being 120 MHz. The B2Σ+ ← X2Σ+ (0,0) band, and a cold band of a hitherto unreported 4Π±1/2 ← X2Σ+ system (νυ′0 = 14 926.02 cm−1) have been rotationally analysed. (The ± 1/2 notation implies that the state has either 4Π1/2 or 4Π−1/2 symmetry.) Improved molecular rotational constants were obtained for the ν = 0 levels of the X2Σ+ and B2Σ+ states (r0(X) = 2.27191(17) Å, r0(B) = 2.32202(19) Å, γ0(B) = −0.15150(14) cm−1 (2σ error bounds)). The magnetic hyperfine and spin rotation parameters determined for the X2Σ+ state were found to be in good agreement with previous work. An accurate bond length has been derived for the upper vibrational level of the 14 926.0 cm−1 band (rυ′ = 2.49510(16) Å). The ν = 1 level of the B2Σ+ state is found to be strongly perturbed by another vibrational level of the 4Π±1/2 state. The spin-forbidden 4Π±1/2 ← X2Σ+ transition gains intensity via spin-orbit mixing between the 4Π state and the B2Σ+ state. Radiative lifetimes of the observed bands were measured by digitizing the fluorescence decay traces obtained upon excitation with a pulsed dye laser. The results of the density functional treatment provide broad confirmation of the experimental measurements. A molecular orbital description of the bonding in the YS molecule is presented.

Surface mobility of C60 on SiO2

The interaction of a collisionless beam of thermal C60 nanoclusters with a silicon dioxide surface has been investigated with modulated molecular beam-mass spectroscopic techniques. Analysis of the amplitude and phase lag of the desorbed C60 shows the interaction mechanism to involve the elementary steps of sticking, desorption, and long-range surface diffusion. Surface diffusion coefficients determined in this measurement indicate that surface C60 nanoclusters approach two-dimensional gas-like behavior. The sticking probability of C60 clusters on SiO2 is determined to be unity. The best fit desorption rate constant kd for C60 from SiO2 is 5×1010 exp(−23 kcal/mol/RT) in agreement with previous temperature programmed desorption experiments. The efficiencies of electron impact ionization of C60 to C60+ and C60++ is measured for electron energies from 10 to 105 eV for neutrals at 875 K.

A high resolution infrared double resonance technique for molecular eigenstate spectroscopy in a free jet

An infrared double resonance technique for the study of jet-cooled polyatomics is reported which offers state selection, access to one-photon-forbidden vibrations, sub-Doppler resolution, and high sensitivity. Molecular eigenstate spectra of the propyne 2ν1 band reveal a predicted doorway state which mediates the two-stage IVR coupling mechanism.

A supersonic molecular beam optical Stark study of CaOH and SrOH

The permanent electric dipole moments of CaOH and SrOH in their X 2Σ+, A 2Π3/2, A 2Π1/2, and B 2Σ+ states have been measured using the technique of supersonic molecular beam optical Stark spectroscopy. For CaOH the values obtained were μ(X 2Σ+)=1.465(61)D, μ(A 2Π1/2)=0.836(32)D, μ(A 2Π3/2)=0.766(24)D, and μ(B 2Σ+)=0.744(84)D, while for SrOH the values were μ(X 2Σ+)=1.900(14)D, μ(A 2Π1/2)=0.590(45)D, μ(A 2Π3/2)=0.424(5)D, and μ(B 2Σ+)=0.396(61)D. The results are compared with values from a recent ab initio calculation for CaOH and with the predictions of a semiempirical electrostatic polarization model.

Phenylation of cationic allyl palladium(II) complexes by tetraphenylborate. Synthesis of α-diimine olefin palladium(0) complexes and mechanistic aspects

The cationic allyl complexes [Pd(η3-2-R1C3H4)(N–N′)]+(N–N′=α-diimine ligand; R1= H or Me) react with BPh4– in the presence of activated olefins to give [Pd(η2-olefin)(N–N′)](olefin = fumaronitrile, dimethyl fumarate or maleic anhydride) and PhCh2C(R1)CH2. The palladium(0) derivatives can be isolated in good yield and have been characterized by elemental analysis, molecular weight measurements and standard spectroscopic techniques. The reaction rates increase with increasing π-accepting ability of the α-diimine, with decreasing steric requirements of the imino carbon substituents and with decreasing stability towards palladium–nitrogen bond breaking in the parent cationic compounds. The rates also increase with decreasing relative permittivity and co-ordinating properties of the solvent. Kinetic measurements in aqueous (2% v/v) methanol provide pseudo-first-order rate constants that are independent of both BPh4– and olefin concentrations. This has been interpreted on the basis of extensive ion pairing between the cationic substrate and the BPh4– anion, followed by rate-determining phenyl transfer to the palladium centre and fast reductive elimination of allylbenzenes.

In SituCharacterization of the Pulsed Laser Deposition of Magnetic Thin Films

ABSTRACTPulsed Laser Deposition (PLD) has been proven as an effective means of depositing films from refractory targets. In our earlier work, either Nd/YAG or excimer lasers, interacting directly with target surfaces, were used to deposit thin films of high Tcsuperconductors, high dielectric constant BaTiO3and ferroelectric PbZr0.53Ti0.47Os3(PZT). Time-resolved molecular beam mass spectrometry and optical emission spectroscopic techniques have been developed to characterize the vapor plumes responsible for film formation. More recently, this work has been extended to the PLD of magnetic thin films of Ag-Fe3O4nanocomposites using excimer (ArF*, 193 nm) laser excitation. Optical emission spectra of the excited vapor phase species, formed during the plume generation and material deposition process, indicate that physically compressed powdered metal targets have inadequate homogeneity for film production, compared to targets that are chemically produced. Anin situLaser-induced Vaporization Mass Spectrometry (LVMS) technique utilizing a Nd/YAG (1064 nm) laser has been used to determine Time-of-Arrival (TOA) profiles of the atomic, molecular, and ionic species produced in the plumes of Ag-Fe3O4The neutral species TOA profiles indicate velocity distributions that are multimodal and not Maxwellian. These observations are in contrast to the TOA profiles observed from one-component targets (Ag or Fe3O4), where a single Maxwellian velocity distribution is found. Mossbauer effect measurements of the thin films have been made for correlation with the gas phase studies.

In SituCharacterization of the Pulsed Laser DepositIon of Magnetic Thin Films

AbstractPulsed Laser Deposition (PLD) has been proven as an effective means of depositing films from refractory targets. In our earlier work, either Nd/YAG or excimer lasers, interacting directly with target surfaces, were used to deposit thin films of high Tcsuperconductors, high dielectric constant BaTiO3and ferroelectric PbZr0.53Ti0.47O3(PZT). Time-resolved molecular beam mass spectrometry and optical emission spectroscopic techniques have been developed to characterize the vapor plumes responsible for film formation. More recently, this work has been extended to the PLD of magnetic thin films of Ag- Fe3O4nanocomposites using excimer (ArF*, 193 nm) laser excitation. Optical emission spectra of the excited vapor phase species, formed during the plume generation and material deposition process, indicate that physically compressed powdered metal targets have inadequate homogeneity for film production, compared to targets that are chemically produced. Anin situLaser-induced Vaporization Mass Spectrometry (LVMS) technique utilizing a Nd/YAG (1064 nm) laser has been used to determine Time of-Arrival (TOA) profiles of the atomic, molecular, and ionic species produced in the plumes of Ag-Fe3O4. The neutral species TOA profiles indicate velocity distributions that are multimodal and not Maxwellian. These observations are in contrast to the TOA profiles observed from one-component targets (Ag or Fe3O4), where a single Maxwellian velocity distribution is found. Mössbauer effect measurements of the thin films have been made for correlation with the gas phase studies.

ChemInform Abstract: Room Temperature Rate Coefficient for the Reaction Between CH3O2 and NO3

The molecular modulation spectroscopic technique was employed to study the kinetics of NO3 radicals produced in the 253.7 nm photolysis of flowing gas mixtures of HNO3/CH4/O2 at room temperature. By computer fitting of the NO3 temporal behaviour, a rate coefficient of (2.30 ± 0.64)xl0-12 cm3 molecule-1 s-1 was obtained for the reaction between CH3O2 and NO3.

Room temperature rate coefficient for the reaction between CH3O2 and NO3

AbstractThe molecular modulation spectroscopic technique was employed to study the kinetics of NO3 radicals produced in the 253.7 nm photolysis of flowing gas mixtures of HNO3/CH4/O2 at room temperature. By computer fitting of the NO3 temporal behavior, a rate coefficient of (2.3 ± 0.7) × 10−12 cm3 molecule−1 s−1 was obtained for the reaction between NO3 and CH3O2 at 298 K.