Ultraviolet absorption spectrum and cross-sections of ethynyl (C2H) radicals

This work reports the ultraviolet absorption spectrum and cross-sections of ethynyl (C₂H) radicals in the wavelength range 235-260 nm, determined at T=298 K and at a total pressure of about 20 kPa (150 Torr). Ethynyl radicals were produced from the single photon 193 nm excimer laser photolysis of dilute mixtures of C₂HCF₃ or C₂H₂ in He. Gas chromatographic/mass spectroscopic analysis of the photolyzed samples showed diacetylene (C₄H₂) as the major stable product. Addition of methane in the photolysis mixtures resulted in reduction of diacetylene and production of methylacetylene and ethane, products of the reaction of ethynyl radicals with methane. Survey of the ultraviolet spectral region, employing time-resolved UV-absorption spectroscopy, resulted in detection of a transient absorption centered at about 243 nm. The spectra obtained from the 193-nm photolysis of both C₂HCF₃/He and C₂H₂/He radical precursors were nearly identical, suggesting that the absorption feature can be attributed to the ethynyl radicals. The observed ultraviolet spectrum exhibits a relatively broad absorption feature with some structures and an absorption peak at about 243.5 nm. The absorption cross-sections for ethynyl radicals have been determined in this work for the first time. The C₂H cross-section, at the maximum absorption, at 243.5 nm is . The stated uncertainty includes the random and systematic measurement errors. The UV absorption feature, detected in this work, can be assigned to the transitions from the ground electronic state and also possibly from coupled ground and lowest electronic states to the electronic state B (or 32A′) of C₂H. This assignment is based on previously reported high level ab initio molecular orbital calculations and results of recent laser-induced fluorescence studies of the ethynyl radical.     

Sub-Doppler high-resolution excitation spectroscopy of the S1 ← S0 transition of dibenzofuran

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S₁ ← S₀ transition of dibenzofuran have been observed using the technique of crossing a collimated molecular beam and the single-mode UV laser beam. 3291 rotational lines of the band and 3047 rotational lines of the band have been assigned. The band has been found to be a b-type transition, in which the transition moment is along the twofold symmetry axis of this molecule, and only the ΔK_a = ± 1 transitions were observed. The excited state is identified to be the S₁¹A₁(ππ^∗) state. In contrast with this, the band has been found to be an a-type transition in which the transition moment is along the long axis in plane. It indicates that the intensity of this vibronic band arises from vibronic coupling with the S₂¹B₂(ππ^∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.     

Probing Rb MOT in 5P3/2(F = 4) → 5D5/2(F′) transitions. In search for effective Rabi frequency

The aim of this work was to provide a simple justification and applicability limits for the concept of effective Rabi frequency being related to an average atom-field interaction in MOT. We sampled ⁸⁵Rb MOT with a weak probe beam tuned across the 5P_3/2 (F′ = 4) → 5D_5/2(F″ = 3, 4, 5) hyperfine transitions, while the 5S_1/2(F = 3) → 5P_3/2(F′ = 4) transition was driven by the red-detuned trapping beam. The probe absorption spectra were registered for a number of detunings Δ and intensities P of the trapping beam. The Autler-Townes splitting δ of the clearly dominating F′ = 4 → F″ = 5 line was the subject of analysis. The character of the space-dependent interactions of atoms with MOT fields is of a complex nature, which brings the notion of the effective Rabi frequency for MOT into challenge. However, we argue that for the range of the typical values of P and Δ, it is justified to characterize MOT with an effective Rabi frequency Ω_0eff, by using the intuitive formula , where is a mean scaling factor experimentally determined, basing on predictions of a straightforward 3-level model. We postulate that our simple procedure, providing both the value and the applicability limits of the approach, should be repeated with each new implementation of MOT (e.g., with trap beams realignment), which may change conditions experienced by cold atoms.     

Laser diode beam shaping with GRIN lenses using the twisted beam approach and its application in pumping of a solid-state laser

We report a novel beam shaping method for laser diodes based on the concept of twisted Gaussian Schell-model beams using GRIN lenses. This method enables a more symmetric pump as well as a compact setup for pumping of solid-state lasers. In the experiment with a standard 2 W diode, the beam quality factors in the two orthogonal directions were equalized to and , respectively, which pumped an intra-cavity frequency-doubled Nd:YAG laser at 946 nm with an output of 28 mW at 473 nm.     

Sapphire (0 0 0 1) surface modifications induced by long-pulse 1054 nm laser irradiation

We have investigated modifications of sapphire (0 0 0 1) surface with and without coating, induced by a single laser pulse with a 1054 nm wavelength, 2.2 s duration, 7.75 mm spot and energy of 20-110 J. A holographic optical element was used for smoothing the drive beam spatially, but it induced small hotspots which initiated damage on the uncoated and coated surfaces. The individual damage effects of hotspots became less pronounced at high fluences. Due to high temperature and elevated non-hydrostatic stresses upon laser irradiation, damage occurred as fracture, spallation, basal and rhombohedral twinning, melting, vitrification, the formation of nanocrystalline phases, and solid-solid phase transition. The extent of damage increased with laser fluences. The formation of regular linear patterns with three-fold symmetry ( directions) upon fracture was due to rhombohedral twinning. Nanocrystalline -Al₂O₃ formed possibly from vapor deposition on the coated surface and manifested linear, triangular and spiral growth patterns. Glass and minor amounts of -Al₂O₃ also formed from rapid quenching of the melt on this side. The - to -Al₂O₃ transition was observed on the uncoated surface in some partially spalled alumina, presumably caused by shearing. The nominal threshold for laser-induced damage is about 47 J cm⁻² for these laser pulses, and it is about 94 J cm⁻² at the hotspots.     

LIF excitation spectra for S0 → S1 transition of deuterated anthranilic acid COOD, ND2 in supersonic-jet expansion

Laser induced fluorescence (LIF) excitation spectrum for the S₀ → S₁ transition of anthranilic acid molecules deuterated in the substituent groups (COOD, ND₂) was investigated. Analysis of the LIF spectrum allowed for the assignment of the six most prominent fundamental in-plane modes of frequencies up to ca. . The experimental results show good correlation with the frequency changes upon deuteration computed with CIS (CI-Singles) and TD-DFT for the S₁ state. Deuteration induced red-shifts of the identified fundamental bands are used for examination of the alternative assignments proposed in earlier studies. Potential energy distributions (PED) and overlaps of the in-plane normal modes with frequencies below indicate that the correspondence of the respective vibrations of the deuterated and non-deuterated molecule is very good. A blue-shift of the 00 transition due to the isotopic substitution, is equal to . This relatively large value is caused primarily by a significant decrease of the N-H stretching frequency associated with the increase of strength of the intramolecular hydrogen bond upon the electronic excitation. The deuteration shift of the 00 band was interpreted in terms of the differences of the zero point energy (ZPE) between the S₀ and S₁ electronic states, computed with DFT and TD-DFT methods, respectively.     

Absorption spectra of AsH2 radical in 435-510 nm by cavity ringdown spectroscopy

The absorption spectra of jet-cooled AsH₂ radicals were recorded in the wavelength range of 435-510 nm by cavity ringdown spectroscopy. The AsH₂ radicals were produced by pulsed DC discharge in a molecular beam of a mixture of AsH₃, SF₆, and argon. Seven vibronic bands with fine rotational structures have been identified and assigned as the , , and (n = 1-3) bands of the electronic transition. Based on the previous studies of AsH₂ radical, rotational assignments and rotational term values for each band were obtained, and the molecular parameters including vibrational constants, rotational constants, centrifugal distortion constants, and spin-rotation interaction constants were also determined.     

CO oxidation on Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces studied by infrared chemiluminescence spectroscopy

The infrared (IR) chemiluminescence studies of CO₂ formed during steady-state CO oxidation over Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces were carried out. Analysis of their emission spectra indicates that the order of the average vibrational temperature () values of CO₂ formed during CO oxidation was as follows: Pd(1 1 1) > Pt(1 1 1) > Rh(1 1 1), and the order is coincident with the potential energy in the transition state expected by the theoretical calculations. Furthermore, it is suggested that the bending vibrational temperature () can also be influenced by the angle of O-C-O (∠_OCO) of the activated complex in the transition state, which has also been proposed by the theoretical calculations.     

Electric field induced hyperfine level-crossings in (nD)Cs at two-step laser excitation: Experiment and theory

The pure electric field level-crossing of m_F Zeeman sublevels of hyperfine F levels at two-step laser excitation is described theoretically and studied experimentally for the nD_3/2 states in Cs with n = 7, 9 and 10, by applying a diode laser in the first 6S_1/2 → 6P_3/2 step and a diode or dye laser for the second 6P_3/2 → nD_3/2 step. Level-crossing resonance signals are observed in the nD_3/2 → 6P_1/2 fluorescence. A theoretical model is presented to describe quantitatively the resonance signals by correlation analysis of the optical Bloch equations in the case when an atom simultaneously interacts with two laser fields in the presence of an external dc electric field. The simulations describe well the experimental signals. The tensor polarizabilities α₂ (in ) are determined to be 7.45(20) × 10⁴ for the 7D_3/2 state and 1.183(35) × 10⁶ for the 9D_3/2 state; the electric field calibration is based on measurements of the 10D_3/2 state, for which α₂ is well established. The α₂ value for the 7D_3/2 state differs by ca. 15% from the existing experimentally measured value.     

Epitaxial growth of Nd2Hf2O7(1 1 1) thin films on Ge(1 1 1) substrates by pulsed laser deposition

Nd₂Hf₂O₇ (NHO) thin films have been epitaxially grown by pulsed laser deposition (PLD) on Ge(1 1 1) substrates. In situ reflection high-energy electron diffraction (RHEED) evolution of the (1 1 1)-oriented NHO during the deposition has been investigated and shows that the epilayer has a twin-free character with type-B stacking. Interfacial structure of NHO/Ge has been examined by high-resolution transmission electron microscopy (HRTEM). The results indicate a highly crystalline film with a very thin interface, and the orientation relationship between NHO and Ge can be denoted as (1 1 1)_NHO//(1 1 1)_Ge and . Finally, twin-free epitaxial growth of NHO with type-B orientation displays temperature dependence and the type-B epitaxy is favored at high temperature.     

Modelling of phase transitions and reaction at CO adsorption on oxygen precovered Pd(1 1 1)

Using the interaction parameters up to the third neighbors and activated form of O and CO diffusion and their reaction, the model has been proposed for Monte-Carlo simulations describing the catalytic O + CO → CO₂ reaction and occurring phase transitions on Pd(1 1 1) surface. Upon adsorption of CO the pre-adsorbed oxygen transforms from p(2 × 2)_O phase into and phases in the limit of room and moderate temperatures, respectively. We demonstrate that the kinetic effects determine both the occurrence of the p(2 × 1)_O and disappearance of the phases at moderate and low temperatures, respectively. Using reaction rate as a fit parameter, we show that at room temperature the start of the reaction can be synchronized with the occurrence of phase.     

Physicochemical, structural and fluorescence properties of Er-doped Ge-S-Ga glasses

Erbium-doped (GeS₂)_x(Ga₂S₃)_100−x (x=75, 80, 85, 90 mol%) glasses have been characterized by some basic parameters, which are important from a practical point of view. The influence of Er by introduction of 0.3, 0.6, and 0.9 mol% Er₂S₃ on the properties has been studied. The glasses have relatively high glass transition temperatures and high thermal stability, the maximal being at x=80 (the difference between the crystallization and glass transition temperatures has been found to be 150 °C. The values of Vickers microhardness and density increase with increasing GeS₂ content, slightly depending on the presence of the Er³⁺ ions. The distribution and changes of the structural units, caused by addition of Ga₂S₃ and Er₂S₃ to GeS₂, have been specified by the Raman scattering in the range 50-550 cm⁻¹. The intensity dependence of the luminescence on glass composition has been evaluated. The glasses have shown a good chemical durability and their resistance to the moisture is relatively high. The obtained results have supported possible applications of these glasses in rare-earth doped devices.     

Ab initio calculations of generalized-stacking-fault energy surfaces and surface energies for FCC metals

The ab initio calculations have been used to study the generalized-stacking-fault energy (GSFE) surfaces and surface energies for the closed-packed (1 1 1) plane in FCC metals Cu, Ag, Au, Ni, Al, Rh, Ir, Pd, Pt, and Pb. The GSFE curves along (1 1 1) direction and (1 1 1) direction, and surface energies have been calculated from first principles. Based on the translational symmetry of the GSFE surfaces, the fitted expressions have been obtained from the Fourier series. Our results of the GSFEs and surface energies agree better with experimental results. The metals Al, Pd, and Pt have low γ_us/γ_I value, so full dislocation will be observed easily; while Cu, Ag, Au, and Ni have large γ_us/γ_I value, so it is preferred to create partial dislocation. From the calculations of surface energies, it is confirmed that the VIII column elements Ni, Rh, Ir, Pd, and Pt have higher surface energies than other metals.     

Surface defect states of CdTexS1 − x quantum dots

Properties of surface defect states of CdTe_xS_1 − x quantum dots with an average diameter of 7 nm are investigated experimentally. The stoichiometric ratio is found to be for by use of the energy dispersive analysis of x-ray. The photoluminescence spectrum, the photoluminescence excitation spectrum, and the surface passivation are adopted to characterize the properties of surface defect states. The energy levels of surface defect states of CdTe_xS_1 − x quantum dots are also determined.     

Spectroscopic characterization of the potential energy functions of Ne2 Rydberg states in the vicinity of the Ne(S0) + Ne(4p′) dissociation limits

The gerade autoionizing Rydberg states of Ne₂ have been studied in the range 162 000-172 000 cm⁻¹ by 1 + 1′ resonant two-photon excitation from the Ne₂ X ground state via different vibrational levels of the Ne₂ C state. A rotationally resolved part of the spectrum allowed the determination of the potential energy functions of two states of 1_g and characters in the vicinity of the Ne(2p⁶¹S₀) + Ne (2p⁵4p′) dissociation limit. The presence of maxima in these potential energy functions is interpreted as originating from a repulsive interaction between the Rydberg electron and the neutral atom.     

On the optical properties of amorphous Ge-Ga-Se-KBr films prepared by pulsed laser deposition

Amorphous thin films (1 − x)(4GeSe₂-Ga₂Se₃)-xKBr (x = 0, 0.1, 0.2, 0.3) were prepared by the pulsed laser deposition (PLD) technique. The optical parameters were calculated using the Swanepoel method from the optical transmission spectra. The optical band gap () of the studied films increased while the index of refraction decreased when increased the content of KBr. The Tauc slopes were discussed as an indicator of the degree of structural randomness of amorphous semiconductors. The index of refraction decreased and increased after annealing of as-deposited films below the glass transition temperature. The thermal-bleaching and thermal- contraction effects were observed, which are discussed in relation to the reduction in the density of homopolar bonds confirmed by the Raman spectra analysis and the decreased amount of fragments of the as-deposited films, respectively.     

Microstructure of epitaxial scandium nitride films grown on silicon

Epitaxial scandium nitride films (225 nm thick) were grown on (1 1 1)-oriented silicon substrates by molecular beam epitaxy (MBE), using ammonia as a reactive nitrogen source. Film microstructure was investigated using X-ray diffraction (XRD). The (1 1 1) ω-scan FWHM of 0.551° obtained for films grown at 850 °C is the lowest reported so far for ScN thin films. The principal orientation of ScN with respect to Si is (1 1 1)_ScN//(1 1 1)_Si and []_ScN//[]_Si, representing a 60° in-plane rotation of the ScN layer with respect to the Si substrate. However, some twinning is also present in the films; the orientation of the twinned component is (1 1 1)_ScN//(1 1 1)_Si and []_ScN//[]_Si, representing a ‘cube-on-cube’ orientation. The volume percentage of these twins in the films decreases with increasing film growth temperature.     

High-temperature measurements of the rates of the reactions CH2O + Ar → Products and CH2O + O2 → Products

The two-channel thermal decomposition of formaldehyde [CH₂O], (1a) CH₂O + Ar → HCO + H + Ar, and (1b) CH₂O + Ar → H₂ + CO + Ar, was studied in shock tube experiments in the 2258-2687 K temperature range, at an average total pressure of 1.6 atm. OH radicals, generated on shock heating trioxane-O₂-Ar mixtures, were monitored behind the reflected shock front using narrow-linewidth laser absorption. 1,3,5 trioxane [C₃H₆O₃] was used as the CH₂O precursor in the current experiments. H-atoms formed upon CH₂O and HCO decomposition rapidly react with O₂ to produce OH via H + O₂ → O + OH. The recorded OH time-histories show dominant sensitivity to the formaldehyde decomposition pathways. The second-order reaction rate coefficients were inferred by matching measured and modeled OH profiles behind the reflected shock. Two-parameter fits for k_1a and k_1b, applicable in this temperature range, are:

     

High-temperature UV absorption of methyl radicals behind shock waves

The absorption of ultraviolet narrow-line laser radiation by methyl radicals (CH₃) in the electronic system has been studied at high temperatures behind shock waves. Methyl radicals at high temperatures were generated by the shock heating of methyl precursors: azomethane, methyl iodide, and ethane. The spectral shape and intensity of the broadband absorption feature from 211.5 to 220 nm at high temperature (1565 K) has been measured. The absorption coefficient of CH₃ at 216.62 nm, the wavelength of peak absorption at high temperatures in the P+Q band, has been determined from 1200 to 2500 K. Additionally, the absorption coefficients of several interfering UV-absorbing combustion species (, and C₃H₆) have been determined at 216.62 nm.