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1.
The wavelength dependence of laser-produced breakdown in air, CO and CO2 has been studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm and 1064 nm) and the ArF-excimer laser (193 nm). Breakdown thresholds at these wavelengths are reported for air, CO and CO2. A significant reduction in the breakdown thresholds for both CO and CO2 is apparent when comparing 193 nm with the four Nd:YAG harmonics. This reduction is attributed to the resonance-enhanced two-photon ionization of metastable carbon atoms generated in the laser focus at the ArF-laser wavelength. In addition to reporting breakdown thresholds, the laser-produced plasmas in CO and CO2 are characterized in terms of plasma temperatures and electron densities which are measured by time-resolved emission spectroscopy. Electron densities range from 9 × 1017 cm–3 to 1 × 1017 cm–3. Excitation temperatures range from 22 000 K at 0.2 µs to 11 000 K at 2 µs. Ionization temperatures range from 22 000 K at 0.1 µs to 16 000 K at 2 µs. Evidence is presented to indicate that, like ArF-laser-produced plasmas, Nd:YAG-laser-produced plasmas formed in CO and CO2 are in or near a state of Local Thermodynamic Equilibrium (LTE) soon after their formation.  相似文献   

2.
The absorption of 250 fs KrF laser pulses incident on solid targets of aluminum, copper and gold has been measured for normal incidence as a function of laser intensity in the range of 1012–1014 W cm–2 and as a function of polarization and angle of incidence for the intensity range of 1014–2.5×1015 W cm–2. As the intensity increases from 1012 W cm–2 the reflectivity at normal incidence changes from the low-intensity mirror reflectivity value to values in the range of 0.5–0.61 at 1014 W cm–2. For this intensity maximum absorption of 63–80% has been observed for p-polarized radiation at angles of incidence in the range of 54°–57°, increasing with atomic number. The results are compared with the expected Fresnel reflectivity from a sharp vacuum-plasma interface with the refractive index given by the Drude model and also to numerical calculations of reflectivity for various scale length density profiles. Qualitative agreement is found with the Fresnel/Drude model and quantitative agreement is noticed with the numerical calculations of absorption on a steep density profile with normalized collision frequencies, v/, in the range of 0.13–0.15 at critical density and normalized density gradient scale lengths, L/0, in the range of 0.018–0.053 for a laser intensity of 1014 W cm–2.At 2.5×1015 W cm–2 a small amount of preplasma is present and maximum absorption of 64–76% has been observed for p-polarized radiation at angles of incidence in the range of 45°–50°.Dedicated to Prof. Dr. Rudolf Wienecke on the occasion of his 65th birthdayOn leave from: Department of Electrical Engineering, University of Alberta, Edmonton, T6G 2G7, Canada  相似文献   

3.
Two-color, two-step resonant photoionization has been used to produce low-density (∼106 – 107cm−3) barium photoplasma in an atomic beam. The two-photon induced specific absorption coefficient at 355nm for the photoionization process has been measured. The motion of the finite size photoplasma bounded by vacuum is experimentally studied in a static electric field in parallel-plate electrode configuration. It is observed that at sufficiently high electric fields all electrons leave the plasma and the evolution of the left over ion bunch is understood on the basis of independent particle motion.  相似文献   

4.
An optical frequency standard based on frequency stabilization to Ramsey fringes observed in a Ca atomic beam is described. The important operation parameters influencing the uncertainty of such a standard are studied experimentally. Frequency shifts due to phase errors of separated traveling wave excitation can be strongly reduced by means of laser beam reversal. It is expected that the present fractional uncertainty of approximately 2.3×10–12 to realize the true line center can be reduced below the 10–14 level by applying atomic beam cooling and improved phase alignment.  相似文献   

5.
The up-conversion of infrared radiation at 808 nm, emitted by a diode laser, into blue emission centered at 480 nm in 1 at.% Tm, 5 at.% Yb: YVO4; 1 at.% Tm, 8 at.% Yb: YVO4 and 2 at.% Tm, 5 at.% Yb: YVO4 has been studied. The highest intensity of blue emission is found for the 1 at.% Tm, 8 at.%Yb: YVO4 system. The power dependence of up-converted emission upon continuous-wave excitation as well as the time evolution of its intensity upon short-pulse excitation were found to be consistent with a two-step excitation mechanism in which the forward Tm3+–Yb3+ energy transfer is followed by the back Yb3+–Tm3+ energy transfer. The effect of dopant concentrations on the up-conversion process is interpreted taking into account dynamics of the excited states involved.  相似文献   

6.
The time resolved emission spectrum of the blue band of Ti:sapphire laser crystal has been investigated as a function of temperature (range 10–290 K) and UV (266 nm) laser excitation intensity. Two blue emission bands, centred at 420 nm and 460 nm, have been detected. The 420 nm band is attributed to Ti4+ centres whereas the 460 nm one is proposed to be due to Ti3+ ions. The evolution of the emission spectrum vs the UV excitation intensity has shown that the concentration of Ti4+ centres is increased under UV irradiation at the cost of the centres responsible for the 460 nm band.  相似文献   

7.
Plasma supported combustion   总被引:9,自引:0,他引:9  
Oxidation of molecular hydrogen and different hydrocarbons in stoichiometric mixtures with air and oxygen in the pulsed nanosecond discharges was studied at room temperature, and the detailed kinetics of the process has been numerically investigated. In the discharge afterglow, the reactions including electron-excited particles play a dominant role for the time up to 100 ns, ion–molecular reactions—for the time of microsecond range, and reactions including radicals mostly contribute for the time interval of several milliseconds. The principal role of processes with formation of excited components that support the development of the chain mechanism of oxidation has been shown. The spatial uniformity of the gas-mixture combustion initiated by a high-voltage nanosecond volume discharge is investigated at gas pressures of 0.3–2.4 atm and temperatures of 1000–2250 K. The self-ignition time and the time of discharge-induced ignition are determined. It is found that the discharge significantly (by 600 K) decreases the ignition temperature with very low energy in the discharge (10−2 J/cm3). The influence of gas excitation by a pulsed nanosecond discharge with a high-voltage pulse amplitude up to 25 kV on the properties of a premixed propane–air flame has been investigated over a wide range of the equivalence ratios (0.4–5). It was experimentally found that the flame’s blow-off velocity increased more than twice at a discharge energy input less than 1% of the burner power. Efficient production of active radicals under the action of a barrier discharge has been observed. The increase in the flame’s propagation velocity is explained by the production of atomic oxygen in a discharge by the quenching of electronically excited molecular nitrogen N2 and the dissociation of molecular oxygen on electron-impact. A numerical model has been developed, which describes the influence of pulsed electric discharges on the ignition, combustion, and flame propagation.  相似文献   

8.
Gain coefficients have been calculated for transitions of singlet levels ns–np of orbital n=4 and n=5 in magnesium-like ions with atomic numbers Z=18, 19, 20, 21, 22 and 23. Population inversions for 4p and 5p levels in these ions were also calculated, via electron collisional excitation, for electron temperature range of 93–231 eV and electron density range of 1016–1017 cm−3. Under these plasma conditions, the maximum gain that occurred for 4s4p transition was at electron temperature of 231 eV and electron density of 4×1017 cm−3. Scaling of the maximum gain coefficients with atomic number Z and the plasma parameters is also presented.  相似文献   

9.
The concentration and drift mobility of charge carriers in Cu1–x Ti x Fe2O4 ferrite are calculated, over a wide range of temperatures (300–773 K), employing d.c. conductivity and thermoelectric power data. With increasing temperature the concentration of charge carriers decreases whilst the drift mobility exhibits an exponential increase. Over the above-mentioned temperature range, the obtained density of charge carriers varies between 1021 and 1022 cm–3 whereas the drift mobility has values between 10–8 and 10–4 cm2/V s. The results are discussed on the basis of a small-polaron hopping conduction. The activation of the d.c. conductivity has been attributed to the thermal activation of the mobility.  相似文献   

10.
The laser-induced fluorescence (LIF) excitation spectrum of jet-cooled 2, 6-dicyano-3,5-dimethylaniline (DCDMA) has been measured in the spectral range of 29,750–32,250cm–1. The band origin at 29,860.8 cm–1 and as many as 250 vibrational bands have been identified in the excitation spectrum. The analysis of the excitation spectrum of DCDMA gives more than 28 vibrational modes involving aromatic ring oscillations and oscillations related to the substituent groups. DCDMA is nonplanar in the ground state, with the NH2 plane at about 9° with respect to the molecular plane (RHF/6-31G*). The singlet excited molecule is planar (CIS/6-31G*). Both CIS/6-31G* and CASPT2 calculations predict that the lowest excited state of DCDMA involves a dominant HOMO-LUMO excited configuration. The characteristic feature of the excitation spectrum of DCDMA is the presence of progressions in the low-frequency mode, 112 cm–1. The calculations suggest that this mode and some other active modes involve motions of the amino group and strongly interacting adjacent cyano substituents.  相似文献   

11.
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).  相似文献   

12.
A mechanism of the laser action of 5d 8 6s 2 –5d 9 6p HgIII transitions is proposed. The explanation is based on atomic constants of the transitions and the predominant role of direct electron excitation of the upper laser level. The kinetic models of electron beam and hollow cathode discharge sources are calculated. The theoretical estimations are compared with experimental data and possible laser transitions are also proposed. The role of electron impact excitation in the formation of inverse population for two-electron transitions in CuII and AgII obtained in hollow cathode discharges is discussed.  相似文献   

13.
Conversion electrons, and X rays from102Rhm (6+,T 1/2=1057 d) have been studied. An electromagnetic branch (2.3±0.3) ·10–3 feeding the102Rh first excited state has been observed in the 6+ isomeric decay. The excitation energy of the 6+ isomer turns out to be 140.7 keV above the 2 ground state.We wish to thank P. Del Carmine, M. Ottanelli, and A. Pecchioli for their skillful collaboration during the measurements and the Technical Staffs of LNS and LNL for operation of the Tandem Accelerators.  相似文献   

14.
The effect of selective excitation of gold atoms in a nonequilibrium oxygen plasma of a combined glow discharge (CGD) has been revealed. When working with alumina oxide as a matrix where the content of aluminum is about 30 wt.%, the atomic lines of Al are not seen at all in the CGD spectrum. But the atomic lines of gold have a very high intensity. Even when the aluminum and gold atomic lines overlap, gold can be detected at a content of 10–7%. This effect has been registered both in synthetic and natural ore samples.  相似文献   

15.
We report on high-resolution spectroscopy with two different samples of calcium atoms, in a laser-cooled and deflected beam and in a magneto-optical trap. The atomic beam was excited by spatially separated laser fields. For spectroscopy with stored atoms in a magneto-optical trap we used a multiple-pulse excitation scheme. The resolution as low as 2.5 kHz was limited by residual frequency fluctuations of our dye-laser spectrometer. The results should allow to establish a frequency standard with a relative uncertainty below 10–14.  相似文献   

16.
Solutions (10–5 to 5×10–2 M) of UO2(NO3)2 have been used in the presence of 2 wt.% Na4P4O12, with excitation by a mercury arc over the range 3200–3800 Å. The emission and absorption spectra of the complex are reported; the first is narrow and has peaks at 4910, 5150, and 5400 Å. This spectrum is the same whether the tetrametaphosphate anion has the chair configuration or the boat one.  相似文献   

17.
Low temperature (5 K) high resolution (0.15 and 0.03 cm−1) absorption spectra of 13CO2 have been recorded in neon, argon, krypton, and xenon matrices, in the ν3 and ν2 regions. Diffusion experiments have been performed in krypton and xenon in order to identify vibrational traps which could be responsible for the decrease and shortening of the emission observed after laser excitation: high-frequency structures in the ν3 region are assigned to dimers and a doubling of the monomer line is due to a site effect. In neon, only a double substitutional site, with a splitting of the degenerate ν2 vibration, is observed. In argon, as previously reported, a single and a double site are characterized. In krypton and xenon, where ν2 is not split, only single sites would be predicted. As one of them exhibits a ν3 line highly sensitive to temperature, we expect a large coupling with the lattice and a fast vibrational relaxation. This site is very likely the vibrational trap we are looking for.  相似文献   

18.
The successive phase transitions of BaZnGeO4 have been studied on meltsolidified samples. A new solid phase (named phase VI) has been found below 186.1 K in samples of large particle size (diameter:D0.1 mm). The higher temperature crystalline phase V can be supercooled easily down to liquid helium temperature. On heating, however, it transforms into phase VI above 95 K in a slow exothermic process. Heat capacities have been measured by adiabatic calorimetry between 14 and 300 K. The enthalpy and entropy of the V–VI phase transition are 187.1 Jmol–1 and 0.971 J K–1 mol–1, respectively. The corresponding data for the IV–V phase transition at 199.8 K are 229.3 J mol–1 and 1.168 JK–1 mol–1. The phase VI does not appear in samples of smaller particle size (D0.1 mm).  相似文献   

19.
Phosphors of nanoparticles LaSrAl3O7:RE3+(REEu, Tb) have been prepared by a sol–gel method. The structure and luminescent properties of LaSrAl3O7:Eu3+ and LaSrAl3O7:Tb3+ phosphors were characterized by X-ray diffraction and atomic force microscopy (AFM), photoluminescence excitation and emission spectra were utilized. From X-ray diffraction (XRD) patterns, it is indicated that the phosphor LaSrAl3O7 forms without impurity phase at 900 °C. From atomic force microscopy (AFM) images, it is shown that the crystal size of the phosphores are about 60–80 nm. Upon excitation with UV irradiation, it is shown that there is a strong emission at around 617 nm corresponding to the forced electric dipole 5D07F2 transition of Eu3+, and at around 545 nm corresponding to the 5D47F5 transition of Tb3+. The dependence of photoluminescence intensity on Eu3+(or Tb3+) concentration and annealing temperature were also studied in detail.  相似文献   

20.
The absorption, reflection, emission, and luminescence spectra of polycrystalline cadmium sulfide films have been studied at 77 deg K at 4500–5500 A. The films were prepared by sublimination of the powder in argon, hydrogen sulfide, or vacuum, followed by crystallization on a heated or unheated substrate.Specimens deposited on substrates below 350 deg C had simple absorption and emission spectra no matter which medium was used, but ones coated on substrates above 350 deg C had absorption, reflection, luminescence, and emission spectra with fine structure, which was due to transitions between the 5s21S0 ground state and 5s5p3P1, 5s5p3P2, and 5s5p3P0 excited states of atomic excess cadmium.Four maxima were found in the excitation spectrum of the blue luminescence (4545, 4605, 4670, and4740 A). The spectrum of the luminescence is independent of exciting wavelength in the range 2200–4900 A.This structure was observed only for films with hexagonal lattice symmetry.  相似文献   

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