Optical studies on free-standing polypyrrole films by the photopyroelectric method

Optical spectra of free-standing polypyrrole films were investigated by a novel PhotoPyroElectric (PPE) method. The reflection and absorption spectra in the range 2000–25000 cm^–1 of TsO^–- and ClO ₄ ^su– -doped polypyrrole films were obtained with an experimental setup comprising two pyroelectric sensors. The absorption bands due to electronic transitions to the bonding and antibonding bipolaronic levels appear at 11 000 and 21 500 cm^–1, respectively. Some relevant details in the polypyrrole spectrum in the 2000–5000 cm^–1 range were observed. The absorption band located at 3400 cm^–1 is characteristic for NH stretching vibration. The bands in the 2300–2500 cm^–1 range are ascribed to the vibration of N⁺H groups containing positively charged nitrogen. The PPE spectra also reveal changes of the electronic and vibrational absorption bands after polymer reduction and HCl treatment.     

The absorption of pulsed CO2-laser radiation by tetrafluorosilane at various wavenumbers,fluences, pulse durations,temperatures, optical path lengths and pressures of absorbing and non-absorbing gases

The absorption of three linesP(30) (1037.4 cm^–1),P(36) (1031.5 cm^–1) andP(40) (1027.4cm^–1) of the pulsed CO₂ laser (00⁰1–02⁰0 transition) by SiF₄ was examined at various pulse energies, pulse durations, temperatures, optical path lengths and pressures of this compound and several non-absorbing gases. In addition, the low-intensity infrared absorption spectrum of tetrafluorosilane was compared with high-intensity absorption data for all lines of the laser. The experimental dependences demonstrate nonlinear features of the absorption phenomena originating from the high power of the incident radiation and collisions of absorbing molecules with surroundings. These effects are included in the analytical formula, being an extended form of the Lambert-Beer law. which reasonably approximates all experimental data. The importance of the results obtained for understanding general features of multiphoton absorption and for revealing potential applications of SiF₄ as a sensitizer for the infrared region is presented in brief.     

Xe2Cl fluorescence and absorption in self-sustained discharge XeCl lasers

Fluorescence at 490 nm from the triatomic excimer Xe₂Cl^* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe₂Cl^* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl^–1. Xe₂Cl^* absorption is estimated to reach 0.6% cm^–1 under non-lasing conditions but decreases to a non-saturable 0.2% cm^–1 for intracavity laser intensity>1 MW cm^–2. XeCl^* and Xe₂Cl^* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer.     

Cross section of supercooled238UF6 in multiphoton absorption induced by 16 micrometer Raman-laser radiation

Measurements of multiphoton absorption of 16 µm Raman-laser radiation in supercooled²³⁸UF₆ at 90 K were performed by using a pulsed Laval nozzle with an optical path length of 50 cm. The laser fluence was varied between 50 and 500 mJ/cm² for four frequencies in the range from 625 to 629 cm^–1. The energy absorbed by²³⁸UF₆ molecules was investigated as a function of laser frequency or fluence, and highly accurate results were obtained with the use of the nozzle whose optical path length is much greater than that of nozzles used before. The results indicated that the absorption cross section at the peak absorption frequency (627.8cm^–1) was proportional to the –1/3 power of the fluence.     

Low-Temperature Midinfrared Absorption in GaSe

We report the measurement of the temperature dependence of the absorption spectra of - GaSe over the temperature range 300 K to 5 K. Measurements have been made for both the e-ray (polarized parallel to the crystals c-axis) and the o-ray (polarized perpendicular to the c-axis), over the spectral range 4000 to 10 cm^–1. Nine absorption lines at 417, 440, 499, 546, 891, 945, 1015, 1093, 1270 cm^–1 were recorded at 300 K for the e- ray spectra. Some of these lines were identified using the results of a modified single layer, linear chain model of GaSe. The lines at 417, 440 and 499 cm^–1 were assigned to local impurity absorption originating from N, Mg and O, respectively. The weak lines at 945, 1015 and 1093 cm^–1 were assigned to hole transitions from the acceptor levels to the top of the valence band. Two absorption lines at 891 cm^–1 and 1270 cm^–1 were assigned to hole transitions from the quasi-local acceptor levels to the double degenerate valence sub-bands ₅ or ₆. The origin of lines recorded in the far IR absorption spectra at 20, 37 cm^–1 and 362 cm^–1 were also identified.     

The UV absorption spectrum of the phenyl radical isolated in solid argon

Phenyl radicals have been generated by photodecomposition of nitrosobenzene isolated in solid argon at 12 K. In this medium the origin of the first UV absorption band of nitrosobenzene corresponding to theS ₀ S ₂ transition is found at 30260 cm^–1. Excitation with an excess energy of 2200 cm^–1 results in very efficient photodissociation. The fragments NO and phenyl are stable for many hours. Annealing of the sample above 35 K led to partial recombination and recovery of the absorption spectrum of nitrosobenzene. The UV spectrum of the phenyl radical was obtained in the wavenumber range 25000–45000 cm^–1, apparently without contamination by other species. It shows sharp lines at 25220, 33880, and 34820 cm^–1 and two broader maxima at 38360 and 41060 cm^–1. Of these only the line at 34820 cm^–1 had been assigned to the phenyl radical in earlier work. These data are compared to recent molecular orbital calculations.Dedicated to Prof. F. P. Schäfer on the occasion of his 65th birthday.     

Determination of the gap distribution in YBa2Cu3O7 using a far-infrared reflection-Fabry-Pérot device

This paper reports on far-infrared measurements of YBa₂Cu₃O₇ films oriented with the c-axis perpendicular to the surface, by using a silicon reflection Fabry-Pérot interferometer as a multireflection device. From these we could derive the dielectric function, the refractive index, the field penetration depth and the surface impedance of the material. The one order of magnitude higher sensitivity of the method compared to a direct reflectance measurement allowed to find an almost continuous gap distribution in the 70–215 cm^–1 region together with a separate gap at about 330 cm^–1. A quasizero gap absorption is found down to 20 cm^–1 even at low temperatures (10 K).     

Excited Even-Symmetry States of Metallocomplexes of Porphin and Its Derivatives

Based on calculations by the CNDO/S method, data on the excited molecular states of even parity of the magnesium complexes of porphin (P), tetraazaporphin (TAP), tetrabenzoporphin (TBP), and phthalocyanine (Phc) are obtained. It is only in MgP that the first excited g-state 1¹ B _2g (29,000 cm^–1) is located 300 cm^–1 higher than the B level (28,700 cm^–1). In MgTBP, the two states 1¹ B _1g (24,700 cm^–1) and 1¹ B _2g (25,500 cm^–1) are found to be near the B level (27,500 cm^–1), while the states 1¹ B _2g (25,500 cm^–1) in MgTAP and 1¹ B _2g (21,000 cm^–1) and 1¹ B _1g (23,100 cm^–1) in MgPhc are located much lower than the B level; the energy of the latter is 31,900 and 32,400 cm^–1 in MgTAP and MgPhc respectively. The results obtained are in good agreement with experimental data on two-photon absorption: in the zinc complex of tetraphenylporphin (TPhP), the g-state is detected in the region of the B level, while in ZnPhc, two bands at 20,400 and 21,700 cm^–1 show up.     

Characteristic bands of the valence vibrations of the NO2 group in infrared absorption spectra. Part 1. Experimental data and band assignments

Frequency and intensity (integral and peak) are given for bands in the IR absorption spectra of 71 nitrocompounds; these are assigned to the symmetric valence vibration (1300–1400 cm^–1) of the C-NO₂ group and to the antisymmetric one (1500–1600 cm^–1). The splitting of the symmetric band is discussed in relation to resonance in the coupled vibrations of the nitro and methyl groups in saturated nitro-compounds.     

The absorption edge of amorphous As2S3

The absorption constantK of amorphous As₂S₃ was determined at the absorption edge in the range 1 to 10⁵ cm^–1. It is shown that at low energy levels up to about 10³ cm^–1 K depends exponentially on the photon energy; at higher absorption levels the energy dependence ofK is quadratic. The significance of these results is discussed. When sulphur is added to As₂S₃ the absorption edge has similar properties but is shifted towards lower energies.     

H2S : First Observation of the (70,0) Local Mode Pair and Updated Global Effective Vibrational Hamiltonian

The absorption spectrum of H₂S has been recorded by intracavity laser absorption spectroscopy in the spectral region 16 180–16 440 cm⁻¹ corresponding to an excitation of the (70^±, 0) local mode pair. Seventy-seven sublevels could be rotationally assigned and fitted with a rms of 0.009 cm⁻¹ by considering the (70^±, 0) local mode pair as isolated. The corresponding vibrational terms combined with all the levels reported in the literature were used to refine the effective vibrational Hamiltonian parameters of H₂³²S. The importance of the Fermi-type interaction is discussed.     

Elementary reactions of formyl (HCO) radical studied by laser photolysis—transient absorption spectroscopy

Several elementary reactions of formyl radical of combustion importance were studied using pulsed laser photolysis coupled to transient UV–Vis absorption spectroscopy: HCO → H + CO (1), HCO + HCO → products (2), and HCO + CH₃ → products (3). One-pass UV absorption, multi-pass UV absorption as well as cavity ring-down spectroscopy in the red spectral region were used to monitor temporal profiles of HCO radical. Reaction (1) was studied over the buffer gas (He) pressure range 0.8–100 bar and the temperature range 498–769 K. Reactions (2a), (2b), (2c), (3a) and (3b) as well as the UV absorption spectrum of HCO, were studied at 298 and 588 K, and the buffer gas (He) pressure of 1 bar. Pulsed laser photolysis (308, 320, and 193 nm) of acetaldehyde, propionaldehyde, and acetone was used to prepare mixtures of free radicals. The second-order rate constant of reaction (1) obtained from the data at 1 bar is: k₁(He) = (0.8 ± 0.4) × 10⁻¹⁰exp(−(66.0 ± 3.4) kJ mol⁻¹/RT) cm³ molecule⁻¹ s⁻¹. The HCO dissociation rate constants measured in this work are lower than those reported in the previous direct work. The difference is a factor of 2.2 at the highest temperature of the experiments and a factor of 3.5 at the low end. The experimental data indicate pressure dependence of the rate constant of dissociation of formyl radical 1, which was attributed to the early pressure fall-off expected based on the theory of isolated resonances. The UV absorption spectrum of HCO was revised. The maximum absorption cross-section of HCO is (7.3 ± 1.2) × 10⁻¹⁸ cm² molecule⁻¹ at 230 nm (temperature independent within the experimental error). The measured rate constants for reactions (2a), (2b), (2c), (3a) and (3b) are: k₂ = (3.6 ± 0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (298 K); k₃ = (9.3 ± 2.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹(298 and 588 K).     

Fourier transform measurements of SO2 absorption cross sections: II.: Temperature dependence in the 29 000–44 000 cm (227–345 nm) region

This paper is the second of a series that reports results on the measurements of the absorption cross section of SO₂ in the UV/visible region at high resolution and that investigates high temperatures in support to planetary applications. Absorption cross sections of SO₂ have been obtained in the 29 000–44 000 cm⁻¹ spectral range (227–345 nm) with a Fourier transform spectrometer at a resolution of 2 cm⁻¹ (0.4500 cm MOPD and boxcar apodisation). Pure SO₂ samples were used and measurements were performed at room temperature (298 K) as well as at 318, 338 and 358 K. Temperature effects in this spectral region are investigated and are favorably compared to existing studies in the literature. Comparison of the absorption cross section at room temperature shows good agreement in intensity with most of the literature data, but shows that most of the latter suffer from inaccurate wavelength scale definition. Moreover, literature data are often given only on restricted spectral intervals. Combined with the data described in the first part of this series of papers on SO₂, this new data set offers the considerable advantage of covering the large spectral interval extending from 24 000 to 44 000 cm⁻¹ (227–420 nm), at the four temperatures investigated.     

Intersubband absorption in highly photoexcited semiconductor quantum wells

Transient mid infrared (MIR) absorption spectroscopy is used to investigate transitions between higher electronic subbands in semiconductor quantum well (QW) structures after interband photoexcitation with intense picosecond pulses in the visible spectral range. Our investigation focuses on the e₂–e₃ intersubband transition in an asymmetric undoped GaAs/AlGaAs QW structure. At an injected nonequilibrium carrier density of 1×10¹³ cm⁻²/QW, an e₂–e₃ absorption band at 99 meV with a spectral width of 5 meV is found. For a higher density studied, 3×10¹³ cm⁻²/QW, the band is broadened and blueshifted by 30 meV. Intersubband absorption signals are distinguished from free-carrier absorption signals in the MIR by their characteristic time behavior.     

Efficient production of13C2F4 in the infrared laser photolysis of CHClF2

We report the isotopically selective decomposition of chlorodifluoromethane. Chlorodifluoromethane is used industrially in high volume for the production of tetrafluoroethylene and its polymers; thereby it is an attractive working substrate for a medium scale isotope separation process, both in terms of its price and availability.We have studied the infrared multiphoton decomposition of carbon-13 substituted chlorodifluoromethane molecules present at their natural abundance (1.11%). A well defined CO₂ laser pulse (80 ns FWHM) was used and both the yield of carbon-13 enriched product and the net absorption of laser radiation were measured. These measurements were made as a function of substrate pressure (10-800 Torr), CO₂ laser line (9P 12–9P 32) and fluence (2–8 J cm^–2) and were used to determine the energy expenditure per carbon atom produced () at specified product carbon-13 content in the range 30%–96%. The results of these parametric studies were interpreted in terms of the kinetics of multiphoton absorption and dissociation, and allowed an initial optimization of the experimental conditions to minimize .Optimum results were obtained at 1046.9 cm^–1, 69 cm^–1 to the red of the¹²CHClF₂ v ₉ band center. Irradiation of 100 Torr of chlorodifluoromethane at 3.5 J cm^–2 gave tetrafluoroethylene containing 50% carbon-13 for an absorption of 140 photons (0.017 keV) per carbon atom produced. This efficiency compares favourably with existing carbon-13 enrichment technologies and would require an absorption pathlength of only 2 m to absorb half the incident photons.Issued as NRCC 20112     

Infrared absorption and far-infrared diffuse reflection spectra in Pb-doped Bi?Sr?Ca?Cu?O high-temperature superconductor

In this paper we study the room-temperature infrared transmission spectra (400–1600cm^–1) and far-infrared diffuse reflection spectra (50–450cm^–1) in Pb-doped Bi–Sr–Ca–Cu–O (2223) single phase (Tc=107k, sp1), multiphase (Tc=110k, sp2) and nonsuperconducting samples (sp3). The spectral features in superconductor are totally different from those in nonsuperconductor, which show the different crystal structure. The correlation existing between a factor group analysis of the phonons in (2223) and (2212) compounds affords a tentative assignment of ir-active modes in Pb-doped (2223) single phase by comparison with reported data in (2212) materials. The Cu–O stretching E_u vibration (605cm^–1) of CuO₂ layers is the characteristic vibrational mode related perovskitelike crystal structures. Two phonon coupling effect emerges in the infrared transmission spectra in Pb-doped superconductor. The Ca–O vibration A_2u (254cm^–1) might be related to superconductivity of Bi-based family.     

Laser investigations at 269 nm for XeF(D-X) in Ne crystals

From line narrowing in amplified spontaneous emission at the D-X transition (269nm) of XeF in solid Ne a gain coefficient of 3.4 cm^–1 has been derived and ground-state losses of 2.8 cm^–1 have been determined by variation of the absorption length. A dielectric laser cavity has been optimized with the reflectivities R₁=100% and R₂=70% for 1 cm long crystals; laser action has been achieved.     

Infrared spectra of catalysts and adsorbed molecules

The infrared absorption spectra of water adsorbed on Al₂O₃ and NiO-Al₂O₃ catalysts have been studied in the range 4000–1200 cm^–1. For all the catalysts, broad bands are observed at 3400 and 1640 cm^–1, characteristic of the stretching and bending vibrations of the OH groups in liquid water. The bands observed in the spectrum of alumina prepared from suspension in isopropyl alcohol are produced by water molecules and isopropyl alcohol molecules absorbed on the catalyst surface. The influence of treatment with heavy water and thermal treatment on the position of the infrared absorption bands in the spectrum of alumina is discussed.     

Electrical characteristics of the rf-excited oxygen plasma-cathodization-grown SiO2/Si interface

The electrical properties of the SiO₂/n-type Si(100) interface, where the silicon-oxide layer was grown by an electrodeless rf oxygen-plasma-cathodization technique, were investigated usingC-V and DLTS methods. Interface traps with high density in the range of 10¹² eV^–1 cm^–2 and a capture cross section as large as 10^–18 cm² were found in the upper region of the silicon forbidden gap. After a post-annealing process, typically at 400°C for 30 min in dry N₂ atmosphere, their densities and capture cross sections were reduced to the range of 1–2 × 10¹¹ eV^–1 cm^–2 and 10^–19 cm^–2, respectively. Apparant differences in DLTS curves before and after thermal annealing were also observed. Results are qualitatively explained by considering the specific oxidation and annealing mechanism of this low-temperature silicon-oxidation technique.