Intramolecular motion and isotope effects in muonium‐substituted chloroalkyl radicals

Muon irradiation of pure liquid 3‐chloropropene, CH₂=CH-CH₂Cl, yields a primary radical, \dot\mboxCH₂-CHMu-CH₂Cl, and a secondary radical, MuCH₂-\rm\dot\mboxCH-CH₂Cl. 2‐methyl‐3‐chloropropene yields only the tertiary radical, MuCH₂-\rm\dot\mboxC(CH₃)-CH₂Cl. These three chloroalkyl radicals have been characterized by μSR and μLCR, and the hyperfine coupling constants (hfcs) have been determined over a range of temperatures, either in the pure liquid precursor or in concentrated solution. The temperature variation of the hfcs has been analyzed to obtain estimates of the barrier to internal rotation about the C_\alpha-C_\beta axis for various alkyl groups, and also their minimum energy conformations, i.e. their orientations with respect to the axis of the 2p_z orbital of the unpaired electron. The tertiary radical is particularly interesting because all three methyl‐like groups, -CH₃,-CH₂Cl and -CH₂Mu, are represented. The results can be compared to electron spin resonance data for analogous radicals, to provide information on the effects of Mu substitution for H. This revised version was published online in August 2006 with corrections to the Cover Date.     

EPR of γ-irradiation-induced free radicals in chicken, duck, and quail egg shells

Chicken, duck, and quail egg shells were γ-irradiated and investigated by electron paramagnetic resonance (EPR) at ambient temperature and at 113 K. The EPR-active species after γ-irradiation were attributed to the ĊO₃⁻, ĊO₃³⁻, and ĊO₂⁻ radicals at ambient temperature and at 113 K. Furthermore, when the samples were stored in an oven at 423 K for about 1 h new EPR signals appeared at ambient temperature in addition to the above ones and these signals were attributed to the (CH₃)₂Ċ-R radical. These signals appear also at 113 K, but they are broadened likely due to the freezing of some of their motions. Any significant difference between the egg shells of poultry and those of village-grown chickens could not be observed in EPR spectra. The duck and quail egg shells exhibit the same signals and therefore indicate the inducement of the same radicals. The spectroscopic splitting and the hyperfine interaction values were found to be consistent with the literature data.     

Finite-temperature properties of the muonium substituted ethyl radical CH2MuCH2: nuclear degrees of freedom and hyperfine splitting constants

The finite-temperature (T) properties of the muonium substituted ethyl radical CH₂MuCH₂ have been theoretically studied by Feynman path integral quantum Monte Carlo (PIMC) simulations. To derive the ensemble averaged expectation values we have combined the PIMC formalism with an efficient tight-binding (TB) Hamiltonian and a density functional operator of the B3LYP type in the EPRIII basis. The TB operator has been used to calculate the potential energy surface (PES) of the ethyl radical in the doublet ground state, the harmonic and anharmonic vibrational wave numbers as well as several probability density functions of the nuclei. The harmonic linear response approximation, which makes use of the Feynman centroid density, has been adopted to evaluate the anharmonic wave numbers. The large anharmonicities in the nuclear potential lead to bond lengths in thermal equilibrium which exceed the vibrationless parameters at the PES minimum. This enhancement is particularly strong for the C–Mu bond. It is responsible for the suppression of the intramolecular rotation for temperatures below room temperature. In C₂ H₅ the rotation is allowed down to 10?K. The dissimilar rotational dynamics for H₂MuCH₂ and C₂ H₅ has been studied with the help of TB-based probability density functions. The nuclear configurations of CH₂MuCH₂ and C₂ H₅, which are populated in thermal equilibrium, have been used to evaluate the isotropic and anisotropic hyperfine splitting (hfs) constants under explicit consideration of the nuclear vibrations and the internal rotation. The hfs constants have been determined with the help of the B3LYP-EPRIII Hamiltonian. The hindered low-temperature rotation in the Mu isomer is responsible for roto-vibrational corrections to the isotropic hfs constants which are smaller than the corrections in C₂ H₅. The shortcomings of single-configuration approaches for the evaluation of isotropic hfs constants have been demonstrated for both radicals. The ensemble corrections to the isotropic hfs parameters are correlated with fluctuations in the atomic spin densities. Differences in the absolute values of the isotropic hfs parameters in CH₂MuCH₂ and C₂ H₅ can be traced back to differences in the nuclear degrees of freedom. The ensemble shift for each isotropic hfs parameter can be explained by characteristic nuclear motions. For this discussion we make use of the distribution functions of the isotropic hfs constants. Roto-vibrational corrections to the anisotropic hfs constants are rather small. PIMC simulations have been performed between 25 and 1000?K, i.e. in a T interval that is large enough to consider nuclear effects beyond zero-point motions. The TB and B3LYP-EPRIII based physical quantities of CH₂MuCH₂ and C₂ H₅ have been compared with experimental findings whenever possible.     

Generation of CdS clusters using laser ablation: the role of wavelength and fluence

The formation of cationic clusters in the laser ablation of CdS targets has been investigated as a function of wavelength and fluence by mass spectrometric analysis of the plume. Ablation was carried out at the laser wavelengths of 1064, 532, 355, and 266 nm in order to scan the interaction regimes below and above the energy band gap of the material. In all cases, the mass spectra showed stoichiometric Cd _n S _n ⁺ and nonstoichiometric Cd _n S _n−1⁺, Cd _n S _n+1⁺, and Cd _n S _n+2⁺ clusters up to 4900 amu. Cluster size distributions were well represented by a log-normal function, although larger relative abundance for clusters with n=13, 16, 19, 34 was observed (magic numbers). The laser threshold fluence for cluster observation was strongly dependent on wavelength, ranging from around 16 mJ/cm² at 266 nm to more than 300 mJ/cm² at 532 and 1064 nm. According to the behavior of the detected species as a function of fluence, two distinct families were identified: the “light” family containing S₂⁺ and Cd⁺ and the “heavy” clusterized family grouping Cd₂⁺ and Cd _n S _m ⁺. In terms of fluence, it has been determined that the best ratio for clusterization is achieved close to the threshold of appearance of clusters at all wavelengths. At 1064, 532, and 355 nm, the production of “heavy” cations as a function of fluence showed a maximum, indicating the participation of competitive effects, whereas saturation is observed at 266 nm. In terms of relative production, the contribution of the “heavy” family to the total cation signal was significantly lower for 266 nm than for the longer wavelengths. Irradiation at 355 nm in the fluence region of 200 mJ/cm² has been identified as the optimum for the generation of large clusters in CdS.     

Energy efficiency of femtosecond laser ablation of polymer materials

We present the results of an experimental study of the ablation spectral energy thresholds for a number of polymer materials ((C₂F₄) _n , (CH₂O) _n ) exposed to femtosecond (τ_0.5 ~ 45–70 fs) laser pulses (λ ~ 266, 400, 800 nm) under atmospheric conditions and under vacuum (p ~ 10–2 Pa). We have analyzed the energy thresholds and the efficiency of optical, thermophysical, and gasdynamic processes in laser ablation vs. the laser pulse duration and photon energy.     

Photodesorption from ultra-thin metal films – a comparison of SO<Subscript>2</Subscript> and NO<Subscript>2</Subscript> on Ag/Si(100)

The photochemistry of SO₂ on thin epitaxial Ag films (5–60 nm) deposited on Si(100) has been studied using laser light with the wavelengths of 266, 355, and 532 nm. SO₂ desorbs with cross sections of 1.7×10^-19,1.7×10^-20 and 2.9×10^-21 cm², respectively. The average translation energy, 〈E_trans/2k〉, is 440 K for 266 and 355 nm light, and 270 K for 532 nm light. Cross sections for a 60 nm thick Ag film are practically identical to the ones for Ag(111) as the substrate. An increase by a factor of ∼3.5 is observed when the film thickness is reduced to 5 nm for 266 and 355 nm light. No significant change is observed for 532 nm excitation. The film thickness has no significant influence on the translational energy of the photodesorbed molecules. The data are discussed in connection with the change of absorptivity of the metal film–semiconductor system. A model is put forward which takes into account the light absorption in the Si substrate and the reduced relaxation of excited electrons in Si. Modelling indicates that electrons excited in the Si substrate with energies and parallel momenta not allowed in Ag contribute to the surface chemistry after crossing the gap in the projected band structure of Ag(111). PACS 82.45.MP; 73.63.-b; 82.50.Bc     

Study of opto-mechanical characteristics of interaction of ultrashort laser pulses with polymer materials

The opto-mechanical characteristics, such as the specific mechanical recoil momentum, the specific impulse, and the energy efficiency, of the laser ablation of flat polymer targets ((C₂F₄) _n , (CH₂O) _n ) have been determined experimentally for the first time for the case of excitation with femtosecond pulses (τ ∼ 45–70 fs) of UV-IR (λ ∼ 266, 400, 800 nm) laser radiation (I ₀ up to 10¹⁵ W/cm²) under normal atmospheric and vacuum (p ∼ 10⁻⁴ mbar) conditions. The efficiency of mechanical recoil momentum generation is analyzed for various regimes of the laser irradiation.     

Examination of the laser-induced variations in the chemical etch rate of a photosensitive glass ceramic

Previous studies in our laboratory have reported that the chemical etch rate of a commercial photosensitive glass ceramic (Foturan^TM, Schott Corp., Germany) in dilute hydrofluoric acid is strongly dependent on the incident laser irradiance during patterning at λ=266 nm and λ=355 nm. To help elucidate the underlying chemical and physical processes associated with the laser-induced variations in the chemical etch rate, several complimentary techniques were employed at various stages of the UV laser exposure and thermal treatment. X-ray diffraction (XRD) was used to identify the crystalline phases that are formed in Foturan following laser irradiation and annealing, and monitor the crystalline content as a function of laser irradiance at λ=266 nm and λ=355 nm. The XRD results indicate the nucleation of lithium metasilicate (Li₂SiO₃) crystals as the exclusive phase following laser irradiation and thermal treatment at temperatures not exceeding 605 °C. The XRD studies also show that the Li₂SiO₃ density increases with increasing laser irradiance and saturates at high laser irradiance. For our thermal treatment protocol, the average Li₂SiO₃ crystal diameters are 117.0±10.0 nm and 91.2±5.8 nm for λ=266 nm and λ=355 nm, respectively. Transmission electron microscopy (TEM) was utilized to examine the microscopic structural features of the lithium metasilicate crystals. The TEM results reveal that the growth of lithium metasilicate crystals proceeds dendritically, and produces Li₂SiO₃ crystals that are ∼700–1000 nm in length for saturation exposures. Optical transmission spectroscopy (OTS) was used to study the growth of metallic silver clusters that act as nucleation sites for the Li₂SiO₃ crystalline phase. The OTS results show that the (Ag⁰)_x cluster concentration has a dependence on incident laser irradiance that is similar to the etch rate ratios and Li₂SiO₃ concentration. A comparison between the XRD and optical transmission results and our prior etch rate results show that the etch rate contrast and absolute etch rates are dictated by the Li₂SiO₃ concentration, which is in turn governed by the (Ag⁰)_x cluster concentration. These results characterize the relationship between the laser exposure and chemical etch rate for Foturan, and permit a more detailed understanding of the photophysical processes that occur in the general class of photostructurable glass ceramic materials. Consequently, these results may also influence the laser processing of other photoactive materials. PACS  42.62.-b; 61.43.Fs; 81.05.Kf; 81.10.-h; 83.80.Ab     

Energy efficiency of femtosecond laser ablation of refractory metals

We present the results of an experimental study of the ablation energy thresholds and ablated mass for a number of refractory metals (Ti, Zr, Nb, Mo) by femtosecond (τ _0.5 = 45–70 fs) exposed to laser pulses in the ultraviolet — near infrared range (λ = 266, 400, 800 nm) under atmospheric conditions and under vacuum (p ~ 10^–2 Pa). We have analyzed the ablation efficiency (mass yield per unit energy of the acting coherent radiation) and ablation energy thresholds vs. the laser pulse duration and photon energy.     

Amplitude gratings generation by selective defect bleaching in KCl:CO<Subscript>3</Subscript>K<Subscript>2</Subscript> crystals

Volume grating formation by spatially-selective defect bleaching is demonstrated in coloured KCl:CO₃K₂ crystals. The sample is coloured by a train of pulses with a wavelength of λ= 266 nm and bleached with a cw Ar-laser with a wavelength of λ= 514.5 nm. Diffraction efficiency for a 1.6 μm grating period is studied both experimentally and theoretically, using the evolution of colour centre concentration. Hologram recording parameters are obtained and the evolution of the spatial grating profile is discussed. Diffraction efficiencies three times higher than previously published efficiencies are obtained. PACS 42.40.Ht; 42.40.Lx; 42.70.Ln     

Experimental study of optical gas-dynamic processes of polymeric material ablation by ultrashort laser pulses

Optical gas-dynamic processes occurring in polymeric targets ((CH₂O) _n , (C₂F₄) _n ) exposed to ultrashort laser pulses (τ _0.5 ∼ 45 − 70 fs; λ _I,II,III = 266, 400, 800 nm; and E/S ∼ 0.1 − 40 J/cm² at r ₀ ∼ 20 μm) were studied under normal conditions and in vacuum (p ∼ 10⁻² Pa). The dynamics of the mass flow from the target surface (m′ ∼ 10⁻⁵ − 10⁻⁴ g/J) was studied and the spectral-energy thresholds of laser ablation, the electron density distribution (n _e ∼ 10¹⁴ − 10¹⁸ cm⁻³), the mass-averaged velocity of the material flow from the target surface (∼ 10³ m/s), and the chemical composition and average temperature in the near-surface plasma formation (T ∼ 5000 K) were determined using interference microscopy, emission spectroscopy, and shadowgraphy.     

Spin relaxation studies of the muonium substituted ethyl radical in the gas phase

This short communication draws attention to the power of μSR and related measurements in providing an unusually complete characterisation of muonium substituted organic radicals in the gas phase. Spectroscopic information is available from muon spin rotation and muon level crossing resonance, giving all the nuclear hyperfine coupling constants, just as in the liquid phase. In addition, measurements of the relaxation time of the muon Zeeman energy become possible; these are potentially informative on the molecular collision dynamics. Demonstration results are presented in summary for the muonium substituted ethyl radical, ĊH₂CH₂Mu, in ethene gas.     

Hydroxyl radical consumption following photolysis of vapor-phase hydrogen peroxide at 266?nm: Implications for?photofragmentation laser-induced fluorescence measurements of?hydrogen peroxide

The decay of OH concentration following photolysis of room-temperature vapor-phase hydrogen peroxide is studied as a function of photolysis fluence at 266 nm in an open air environment. The rate of decay is found to increase with increasing photolysis fluence, i.e., with increasing number of photodissociated H₂O₂(g) molecules. Single-exponential functions approximate the OH concentration decay rather well, even for higher photolysis levels, and the decay time is shown to be inversely proportional to the H₂O₂(g) concentration. For fluences of about 450 mJ/cm² the difference between a single-exponential decay and measured data is becoming evident after approximately 150 μs. Calculations based on a chemical kinetics model agree well with experimental data also for times >150 μs. By combining the model with measurements, the actual photolysis levels used in experiments are estimated. The best fit between measured data and the model suggests that about 1.1% of the H₂O₂(g) molecules are dissociated with a photolysis fluence of ∼450 mJ/cm², in reasonable agreement with a Beer–Lambert based estimation. Excitation scans did not unfold any differences between OH spectra recorded at different photolysis fluences.     

Collision-energy-resolved Penning ionization electron spectroscopy of styrene, 2-vinylpyridine, and 4-vinylpyridine with He*(23S) metastable atoms

Collisional ionization of styrene (phenylethylene), 2-vinylpyridine, and 4-vinylpyridine with metastable He*(2³S) atoms were studied by means of collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy. Collision energy dependence of partial ionization cross-sections, which reflects the anisotropic interactions between a He*(2³S) atom and the target molecules, indicates that attractive interaction for the out-of-plane access of a He*(2³S) atom to phenyl group is stronger than that for the out-of-plane access to vinyl group. Moreover, it was found for vinylpyridines that the attractive interaction around π electrons became weaker than that for styrene, and that the attractive interaction for the in-plane access to the nitrogen atom is stronger than that for out-of-plane π-directions. However, in 2-vinylpyridine, the hydrogen atom of vinyl group prevents a He*(2³S) atom from approaching to the nitrogen atom along in-plane directions, and thus the attractive interactions around the nitrogen atom were shielded by the vinyl group. The experimentally observed anisotropic interactions were qualitatively supported with ab initio model interaction potential calculations between a Li (He*(2³S)) atom and the target molecule. Concerning with electronic structures of investigated molecules, the assignment of Penning ionization electron spectrum for 4-vinylpyridine was discussed on the basis of different behavior of collision-energy dependence of partial ionization cross-sections, and the satellite ionization band in Penning ionization electron spectra was also reported for styrene.     

Structuring of aluminum films by laser-induced local oxidation in water

A simple method for patterning of thin (15–650 nm) aluminum films on glass substrates by direct, low-power, laser-thermal oxidation in water under common laboratory conditions is demonstrated. Local heating of the metal film enhances the formation of aluminum oxide (hydrargillite, Al₂O₃–3H₂O) and provokes breakdown of the passivation layer followed by local corrosion at temperatures close to the boiling point of water. Moving the focus of an Ar-ion laser (λ=488 nm) over the aluminum film with a speed of several μm/s yields grooves flanked by hydrargillite. Upon through oxidation of the metal these structures act as electrically insulating domains. Depending on the film thickness, the minimum width of the line structures measures between 266 nm and 600 nm. The required laser irradiation power ranges from 1.7 mW to 30 mW. It is found that the photo-thermal oxidation process allows for writing of two-dimensional electrode patterns. Received: 16 July 2001 / Accepted: 23 July 2001 / Published online: 2 October 2001     

Mass and kinetic energy distribution of the species generated by laser ablation of La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>

The mass distributions of the species generated by laser ablation from a La_0.6Ca_0.4MnO₃ target using laser irradiation wavelengths of 193 nm, 266 nm and 308 nm have been investigated with and without a synchronized gas pulse of N₂O. The kinetic energies of the species are measured using an electrostatic deflection energy analyzer, while the mass distributions of the species were analyzed with a quadrupole mass filter. In vacuum (pressure 10⁻⁷ mbar), the ablation plume consists of metal atoms and ions such as La, Ca, Mn, O, LaO, as well as multiatomic species, e.g. LaMnO⁺. The LaO⁺ diatomic species are by far the most intense diatomic species in the plume, while CaO and MnO are only detected in small amounts. The interaction of a reactive N₂O gas pulse with the ablation plume leads to an increase in plume reactivity, which is desired when thin manganite films are grown, in order to incorporate the necessary amount of oxygen into the film. The N₂O gas pulse appears to have a significant influence on the oxidation of the Mn species in the plume, and on the creation of negative ions, such as LaO⁻,O⁻ and O₂⁻.     

Fluorescence spectroscopy of naphthalene at high temperatures and pressures: implications for fuel-concentration measurements

Fluorescence of the S ₁→S ₀ transition of naphthalene vapour after laser excitation at 266 nm was studied in a heated cell. Experiments were carried out for temperature in the range 350–900 K, at pressure between 0.1 and 3.0 MPa and for oxygen molar fraction from 0 to 21%. The absorption cross section of naphthalene showed a non-monotonic dependence upon temperature, which may be attributed to the spectral structures present in the absorption spectrum of naphthalene. Under nitrogen atmosphere, naphthalene fluorescence bi-exponentially decreased by an order of magnitude as temperature increased, whereas it increased by about 10% with pressure. Strong influence of quenching by O₂ on naphthalene fluorescence was observed and Stern–Volmer plots were found to be linear for temperatures between 450 and 750 K. The dependence of naphthalene fluorescence on oxygen concentration suggests one to use this molecule for fuel-concentration measurements in turbulent flows.     

Variable UV laser exposure processing of photosensitive glass-ceramics: maskless micro- to meso-scale structure fabrication

A novel variable UV laser processing technique was developed that enables the concurrent fabrication of structures in photosensitive glass-ceramic (PSGC) materials that range from the micro-scale to the meso-scale domains. This technique combines the advantages of direct-write volumetric laser patterning and batch chemical processing. The merged non-thermal laser fabrication approach relies on the ability to precisely and selectively alter the chemical etch rate of the PSGC by varying the laser exposure during pattern formation. The present study determined that the chemical etch rate of a commercial photosensitive glass-ceramic (Foturan^TM, Schott Corp., Germany) in dilute hydrofluoric (HF) acid is strongly dependent on the incident laser irradiance during patterning at λ=266 nm and λ=355 nm. For low laser irradiances, the etch rate ratio (R_exposed/R_unexposed) increased nearly linearly with laser irradiance. The slopes of the linear ranges of the etch rate ratios were measured to be 435.9±46.7 μm²/mW and 46.2±2.3 μm²/mW for λ=266 nm and λ=355 nm, respectively. For high laser irradiances, the measured etch rate ratio saturated at ∼30:1 with a maximum absolute etch rate of 18.62±0.30 μm/min. The maximum absolute chemical etch rate was independent of the exposure wavelength. Consequently, variation of the laser exposure during direct-write patterning permits the formation of variegated and proximal high and low aspect ratio structures on a common substrate. The results show that adjacent microstructures with aspect ratios ranging from <1:1 to ∼30:1 can be fabricated in a single, simultaneous batch chemical etch step without the need for a complex masking sequence or post-process ablation step. This new technique facilitates rapid prototype processing with pattern and component uniformity, and achieves material processing over large areas without incurring high cost. PACS 42.62.-b; 42.79.-e; 81.05.Kf; 81.20.-n     

Spectra of high-frequency waves in hot magnetized plasmas

On the basis of numerical solution of the dispersion equation, we obtain the spectra of weakly damped high-frequency waves in a hot magnetized plasma for the case where the electron cyclotron frequency ω_He is below the plasma frequency ω_pe. It is shown that the longitudinal wave propagating at an angle to the magnetic field evolves into the slow extraordinary wave for the refractive index n ≤ 1. For n ≫ 1, the longitudinal-wave frequency increases with the refractive index, and the wave evolves into the wave with anomalous dispersion if the angle θ between the wave vector and the magnetic field is close to 90°. In the same range of θ angles, Bernstein modes appear in the spectrum of plasma eigenmode oscillations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 3, pp. 258–266, March 2006.     

Tailoring the optical properties of MgxZn1?xO thin films by nitrogen doping

Thin films of Mg _x Zn_1−x O and Mg _x Zn_1−x O doped with nitrogen were deposited by Radio Frequency plasma beam assisted Pulsed Laser Deposition (RF-PLD) in oxygen or oxygen-nitrogen discharge with different nitrogen/oxygen ratios. A Nd:YAG laser working at a wavelength of 266 nm, having a 10 Hz repetition rate was used for the depositions. The energy density of the incident beam was 3 J/cm² and the RF power was set to 100 W for all the samples. X-ray Diffraction (XRD) and Spectroscopic Ellipsometry (SE) were employed to investigate the samples. The degree of crystallinity is fount to decrease with increasing the Mg concentration, while the solubility of Mg in ZnO increases by 30% in the N-doped Mg _x Zn_1−x O thin films grown by RF-PLD. Segregation of MgO phase at a Mg concentration of 30% for Mg _x Zn_1−x O thin film is detected both by XRD and SE. The band gap of the samples increases from 3.37 up to 3.57 eV with increasing the Mg concentration and the nitrogen/oxygen ratio for each Mg concentration. A dependence of the dielectric function (refractive index) on both stoichiometry and degree of crystalinity is also found, the refractive index having values between 1.7 and 2 in visible spectral range.