Laser separation of oxygen isotopes by IR multiphoton dissociation of (CH3)2O

Isotopically selective (with respect to ¹⁸O) one- and two-frequency multiphoton dissociation of dimethyl ether (CH₃)₂O by pulsed TEA CO₂ laser radiation has been studied. The maximum primary selectivity, 16, is attained with the dissociation yields of the desired component (CH₃) ₂ ¹⁸ O ₁₈=5×10^–4 and 1.7×10^–2 for one- and two-frequency excitation, respectively. The dependences of MPD yields and selectivity on laser radiation frequency, (CH₃)₂O pressure, buffer gas (N₂) pressure and temperature have been measured. Multiphoton absorption coefficients have been measured and the average number of absorbed quanta calculated. The laser photon energy consumed for separating one ¹⁸O atom has been estimated: 11 and 4 keV/¹⁸O atom for one- and two-frequency excitation, respectively.     

Evolution of the homogeneous IR spectrum of a highly vibrationally excited molecule as a result of a change in its vibrational energy

Using a photodissociation technique, we have measured the IR spectrum of thev ₂₁ mode of the (CF₃)₃ CI molecule with a vibrational energy ofE ₂=42500±3500 cm^–1 which is more than two times the dissociation energy. The experimental spectrum of a Lorenzian shape with a halfwidth of ₂=10.8±1.5 cm^-1 has been analyzed simultaneously with the results of the preceding work (₁=8.6±0.6 cm^-1) that were obtained at a lower vibrational energy (E ₁=36500±2500 cm^–1).     

Intensity anomaly in the A2∑+←X2∏ resonant-(2+2) multiphoton ionization spectra of NO

The two-photon resonant four-photon ionization spectra of NO via A²⁺X² (v=0,1) states have been measured. Some intensity-anomalous lines have been found. Based on the signal-power dependence measurement for the normal and anomalous lines, a double resonance and collision ionization model is suggested to elucidate the intensity anomaly.     

Chemical reactions between CF2HCl and NH3 induced by IR double-beam excitation

Infrared multiphoton dissociation experiments with two wavelengths in different mixtures of chlorodifluoromethane and ammonia have been carried out. It is shown that the presence of ammonia in the sample induces a decrease in the chlorodifluoromethane dissociation yield. It has been observed that the distinct chemical reaction channels are differently activated as the time delay between the two laser pulses is varied. The ratio of the obtained products in the infrared multiphoton dissociation changes with the composition of the initial mixture and are not compatible with the mechanism suggested by Sugita and Arai for this reaction in single IR wavelength excitation if it is assumed to be the only contributing mechanism other than that for direct CF₂HCl dissociation and subsequent C₂F₄ formation from the resulting CF₂ radicals. It appears that, although for simultaneous two-wavelength irradiation the presence of an accumulated solid NH₄Cl deposit does not significantly influence the reaction, this is no longer the case when time delays are introduced between the two beams.     

Laser-induced fluorescence thermometry and concentration measurements on NOA–X (0-0) transitions in the exhaust gas of high pressure CH4/air flames

Laser-Induced Fluorescence (LIF) excitation spectra in the NOA–X (0-0) band were used for temperature measurements in the postflame region of high-pressure CH₄/air flames. To improve the quality of the measured spectra and to perform reliable line-shape measurements, the initial mixture was doped with approximately 400 ppm NO. At pressures up to 18 bar, excellent agreement was obtained between NO LIF temperatures and NARS/rotational Raman temperatures. Effective broadening coefficients were also determined in these flames. Problems with quantitative concentration measurements of NO and single-pulse temperature measurements at high pressures are discussed.     

Raman studies of nonlinear phenomena in fullerene crystallites

The resonant Raman scattering of C₆₀ crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, a strong line at 1468 cm^–1 is observed. No feature at 1459 cm^–1 is observable. With increasing intensities, the 1468 cm^–1 line shifts continuously and reversibly to lower frequencies. The mode softening is nonlinearly dependent on the input intensity and is accompanied by a nonlinear increase in the Raman intensity. The spectral changes are discussed in terms of a nonlinear reduction in force constant and increase in bond polarisability as a result of an increased intermolecular delocalisation of the -electron cloud. The nonlinear changes are associated with the nonlinear luminescence and photoconductive response observed in fullerenes and are contrasted to the irreversible phototransformation recently discussed.     

OH laser-induced fluorescence at high pressures: spectroscopic and two-dimensional measurements exciting the A–X (1,0) transition

Received: 11 January 1996/Revised version: 25 July 1996     

TEA CO2 laser-induced photodissociation of UF6 via interspecies V-V energy transfer from multiple photon excited halomethanes

The dissociation of UF₆ sensitized by multiple photon excitation of a series of halomethanes: CF₄, CF₃Cl, and CF₂Cl₂ has been investigated. The roles of various experimental parameters like exciting frequency, fluence and pressures of sensitizer/UF₆ on the dissociation yield were studied to examine (1) the characteristics of the sensitizer/UF₆ system and (2) the coupling of vibrational energy between two molecular systems. The efficiency of the energy transfer process was estimated on the basis of long range dipole-dipole interaction to gain an understanding of the dissociation process.     

Vibrational predissociation of UF6 clusters in supersonic Laval nozzle flow

The time-dependent change in the concentration of UF₆ monomers populated in the ground state was monitored in a supersonic Laval nozzle flow with an infrared diode-laser spectrometer in which the frequency of the laser beam was fixed at the = 1 0 transition (627.7 cm^–1) of the ₃ vibrational mode of the²³⁸UF₆ monomer. The concentration of UF₆ monomers in the ground state increased immediately after a single shot from a Raman laser tuned to a vibrational mode of UF₆ clusters (614.8 cm^–1) was applied to the gas in the nozzle. Subsequently, this concentration leveled off and slowly returned to the previous level. These results indicate that the population of UF₆ monomers in the ground state increased as a result of the predissociation of UF₆ clusters vibrationally excited with Raman laser radiation. It is demonstrated that one can utilize this procedure for vibrational predissociation of UF₆ clusters as a technique to increase the concentration of UF₆ monomers in an irradiation zone for molecular laser isotope separation of uranium.     

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.     

Contrast ratio and separation factor in multiphoton dissociation of supercooled UF6 irradiated with multifrequency 16 μm Raman-laser radiation

In infrared multiphoton dissociation of supercooled UF₆ irradiated with multifrequency para-H₂ Raman laser beams, separation factors and contrast ratios were measured by selective multiphoton ionization of the photoproduct UF₅ at 532 nm followed by time-of-flight mass spectrometric analysis. The relationship between separation factor and contrast ratio is discussed theoretically and quantitatively investigated in the experiments. From this relationship, we obtain the irradiation conditions of the laser-beam fluences for attaining a high separation factor and estimate the value of the intrinsic separation factor in multifrequency dissociation of UF₆.Abbreviations C _r (235) contrast ratio with respect to²³⁵UF₆ - _{q s5} yield of isotopically selective dissociation with respect to²³⁵UF₆ - _{q s8} yield of isotopically selective dissociation with respect to²³⁸UF₆ - _{q n5} yield of isotopically non-selective dissociation with respect to²³⁵UF₆ - _{q n8} yield of isotopically non-selective dissociation with respect to²³⁸UF₆ - S separation factor - S ₀ intrinsic separation factor - i fluence of individual beam i [J/cm²] - ₁ Raman-laser beam for exciting²³⁵UF₆ molecules selectively - ₂ Raman-laser beam for pumping UF₆ molecules excited by ₁ - ₃ Raman-laser beam for pumping excited UF₆ molecules further to a dissociative state     

The collisional energy transfer between the D1Π and d3Π states of the NaK dimer induced by argon buffer gas

1 Π→X¹Σ⁺ transitions and the continuum spectrum of d³Π→a³Σ⁺ transitions of the NaK dimer in a heat pipe could be observed in the range of 500–700 nm. The collision-induced enhancement effects of the intensities I_d→a and the quenching effect of the intensities I_D→X by collisions with argon buffer gas at different pressures were measured experimentally. Based on the stationary collisional model and lifetime measurements, the quenching cross sections and cross sections for collision-induced energy transfer between the D¹Π and d³Π states could be estimated. Received: 13 January 1997/Revised version: 20 October 1997     

Lifetimes,population and deactivation mechanisms of theC 1 ∏ u , 23 ∏ g and 21∑ u + states of Na2

The UV and visible fluorescence of Na₂ molecules in a heat-pipe, excited by the frequency doubled radiation of a tunable mode-locked cavity-dumped cw dye laser was observed through a monochromator, using time-resolved single-photon counting. The effective lifetimes of the directly excitedC ¹ _u state and the collisionally populated 2³ _g and 2¹ _u ⁺ states have been measured as a function of sodium-vapor pressure and argon pressure which yields the radiative lifetimes, the population mechanisms and the collisional quenching cross sections of the three states.     

The collisional energy transfer between the D 1Π and d 3Π states of the NaK dimer induced by argon buffer gas H. Zhiwei et al.: Appl. Phys. B 66, 471 (1998)

It is shown in this Comment that the method proposed by Zhiwei et al. to extract cross sections for collisional energy transfer between molecular states mediated by argon atoms, is neither correct nor suitable, as it fails to reproduce the molecular behaviour they observe. An alternative model based on a consecutive scheme is put forward, and it is shown that a minimal assumption of one intermediate collisionally activated step is enough to satisfactorily reproduce the observations by Zhiwei et al. However, the extracted parameters are functions of the involved rate constants that are too complex to leave any hope of attaining information on the desired cross sections from them by just varying the argon buffer gas pressure, as done by Zhiwei et al. Received: 30 November 1998 / Published online: 14 July 1999     

Generation of excimer emission in dielectric barrier discharges

Dielectric barrier discharges (silent discharges) are used to excite a large number of excimers radiating in the VUV, UV or visible spectral range. The excited species include rare-gas dimers, halogen dimers as well as rare-gas halogen excimers and mercury halogen excimers. In many cases narrow-band UV radiation of typically 1–17 nm halfwidth and remarkable efficiency (1–10%) could be generated. Thus, dielectric barrier discharges provide a simple, versatile arrangement to study the basic reaction kinetics of excimer formation and also bear a substantial potential for large-scale industrial UV processes.     

Self-induced polarization rotation of a laser beam in resonance media

The single-photon effect of the self-induced optical activity (SIOA) has been investigated experimentally for the first time in the vicinity of potassiumD _1,2 lines. Experimental data on self-induced gyrotrophy and dichroism are presented and compared with the theoretical model proposed earlier. The limiting action of SIOA on the efficiency of resonance parametric generators and polarization spectroscopy is outlined.     

Time-resolved spectroscopy of the fluorescence quenching of a donor — acceptor pair by halothane

Donor (anthracene) sensitized acceptor (perylene) fluorescence is quenched more efficiently by halothane than is intrinsic perylene fluorescence. The underlying process of dynamic fluorescence quenching is investigated by time-resolved fluorescence spectroscopy.     

Studies of vibrational relaxation in CDF3 following intense laser excitation

The V-T/R relaxation time of CDF₃ was measured studying the laser-induced infrared fluorescence emitted by vibrationally excited CDF₃. Following excitation by the 10R(12) line of a TEA CO₂ laser infrared fluorescence has been detected without spectral resolution in the 1100–700 cm^–1 range. A decay rate of 28.8 ms^–1 Torr^–1 was obtained for pure CDF₃ when it is excited with a fluence of 390 mJ/cm². Measurements have also been made in the presence of different bath gases (He, Ne, Ar, Xe, and CHF₃).     

Measurements of235U/238U isotopic ratio in the photoproduct UF5 by multiphoton ionization and time-of-flight mass spectrometry

A MultiPhoton-Ionization Time-Of-Flight Mass Spectrometry (MPI/TOFMS) apparatus was developed for real-time measurement of the uranium isotopic ratio in nascent UF₅ formed by the 266 nm photolysis of effusive UF₆ ( < 300 K, 1.3 × 10^–4 Pa). The UF₅ was selectively and efficiently multiphoton ionized by 532 nm radiation at appreciably low fluences ( < 10 J/cm²). The main ions observed, U⁺ and U²⁺, were subsequently analyzed with a TOFMS with mass resolution of 1190 to separate²³⁵U ⁿ⁺ and²³⁸U ⁿ⁺ completely. The isotopic ratio measurements showed good precision resulting from the excellent agreement which was observed between the isotopic ratios in UF₅ products and those in a parent UF₆ sample. These results suggested that the MPI/TOFMS method can be applied to the real-time analysis of separation factors in the molecular laser isotope separation of uranium by ionization of UF₅ following the infrared photodissociation of UF₆.     

Isotopic selectivity of IR laser photodissociation of CF3I molecules

The selectivity dependence of multiphoton dissociation of CF₃I on the conditions of excitation (pulse duration, concentration of the isotope under excitation and gas pressure) has been studied. It has been shown that the main mechanism of selectivity loss isV-V exchange during a laser pulse. The elaborated model is in good agreement with the experiment.