共查询到20条相似文献,搜索用时 15 毫秒
1.
M.A. Bouchiat J. Guéna Ph. Jacquier M. Lintz A.V. Papoyan 《Applied physics. B, Lasers and optics》1999,68(6):1109-1116
We demonstrate that the walls of glass cells become electrically conductive when exposed to a dry cesium vapor, glasses of
different compositions leading to similar effects. We find dramatically different results for monocrystalline sapphire cells,
which exhibit typical resistances of a few GΩ, nearly five orders of magnitude larger than glass cells in similar conditions.
In spectroscopy experiments requiring the application of an electric field, low cell resistances imply leakage currents generating
stray magnetic fields. The latter, at the origin of harmful effects in precise tests of the fundamental laws of physics performed
in cesium cells, will thus be suppressed in sapphire cells. Moreover, with such cells it becomes possible to place the electrodes
outside.
A tentative interpretation of the surface electrical conductivity of glass cells is propounded by establishing a connection
with the surface coverage by cesium atoms physically adsorbed on the glass surface. This results from the observed dependences
of the cell conductance versus Cs vapor density in the mtorr range and versus the wall temperature up to 200 °C, which indicates
an activation energy of 0.66±0.05 eV. In the sapphire cell, where there is no hint of surface effects, the conductivity looks
instead attributable to collisional processes occuring inside the vapor for Cs number densities ≳1014 at/ cm3.
Received: 4 November 1998 / Published online: 7 April 1999 相似文献
2.
jet =18600). Here, PLIF images reveal a CH layer of thickness typically <1 mm from flame base to tip. Furthermore, in these permanently
blue flames, we observe instantaneous flamefront strain rates – derived from the PIV data – in excess of ±104 s-1 without flame extinction.
Received: 16 October 1997/Revised version: 30 October 1997 相似文献
3.
M.M. Ivanenko H. Handreck J. Göthel W. Fuss K.-L. Kompa P. Hering 《Applied physics. B, Lasers and optics》1997,65(4-5):577-582
2 to the CHClF2/He mixture irradiated by a Q-switched CO2 laser leads to oxidation of the dissociation product according to the reaction: CF2+NO2→COF2+NO. The resulting COF2 with a 13C content near 50% is easy to convert to CO2 or CO for further enrichment by a nonlaser process. We measured the dependence of the fraction of dimerised CF2 on NO2 pressure pNO2 and the amount of NO2 required to suppress dimerisation on the dissociation yield. Both agree with a kinetic model using known rate constants.
For the range of the dissociation parameters (13CF2 yield of 10% per pulse, isotope selectivity of 130) of practical interest, 95% of the CF2 produced is oxidized at pNO2≈1/2pCHClF2. In the absence of NO2, major (20%–35%) losses of CF2 at the metal walls of the irradiation system were observed. Addition of NO2 suppresses them. For comparison, we also used O2 as a scavenger in CHClF2 dissociation. NO2 is by orders of magnitude more efficient.
Received: 21 January 1997/Revised version: 23 March 1997 相似文献
4.
F.E. Tichy T. Bjorge B.F. Magnussen P.E. Bengtsson F. Mauss 《Applied physics. B, Lasers and optics》1998,66(1):115-119
2 H2O2). Laser-induced fluorescence spectra from glyoxal vapor using the same excitation wavelength of 428 nm showed the same strongest
lines as the signal from the flame. Glyoxal was visualized in two different modes; two-dimensional imaging and a spatial-spectral
mode where spectra were obtained at different spatial positions in the flame simultaneously. For the premixed laminar rich
flame it is shown that glyoxal is produced early in the flame, before the signals for C2 and CH appear. For the turbulent non-premixed flames it is shown that glyoxal is produced in a layer on the fuel rich side
of the flames. Here the fuel is premixed with ambient air. This layer is thin and has a high spatial resolution.
The general trend was that the glyoxal signal appeared in regions with a lower temperature compared with the emission from
C2 and CH. The imaging of glyoxal in turbulent acetylene flames is a promising tool for achieving new insight into flame phenomena,
as it gives very good structural information on the flame front. Tests so far do not indicate that the detected glyoxal is
a result of photo-production. To our knowledge, this is the first detection of glyoxal in flames using laser-induced fluorescence.
Received: 19 December 1996/Revised version: 26 May 1997 相似文献
5.
A method for the holographic determination of the chemical kinetic order of irradiance in linear-chain polymerization rates
using phase gratings recorded over dry photopolymer films is derived. This method, based in the initial slope of the recorded
diffraction efficiency, does not depend on any model of the grating growth, and it allows to collect information on the kinetic
mechanisms involved in the polymerization. An acrylamide-based linear-chain photopolymer is analysed.
Received: 2 August 1999 / Published online: 8 March 2000 相似文献
6.
7.
Modeling of chemical-mechanical polishing with soft pads 总被引:10,自引:0,他引:10
Received: 8 Dezember 1997/Accepted: 5 January 1998 相似文献
8.
G. Bialolenker E. Polacco C. Rizzo G. Ruoso 《Applied physics. B, Lasers and optics》1999,68(4):703-706
-9 .
Received: 20 July 1998 / Revised version: 27 November 1998 / Published online: 24 February 1999 相似文献
9.
F. Brandi F. Della Valle A.M. De Riva P. Micossi F. Perrone C. Rizzo G. Ruoso G. Zavattini 《Applied physics. B, Lasers and optics》1997,65(3):351-355
-7 rad to 2×10-6 rad.
Received: 28 January 1997 相似文献
10.
H. Delbarre C. Przygodzki W. Chen D. Boucher 《Applied physics. B, Lasers and optics》1998,66(2):169-173
2 crystal. This method, based on group velocity measurement, allows the determination of the principal axes, the indices, the
group velocities and the group velocity dispersion (GVD).
Received: 11 February 1997/Revised version: 23 May 1997 相似文献
11.
F. Demming J. Jersch K. Dickmann P. I. Geshev 《Applied physics. B, Lasers and optics》1998,66(5):593-598
Received: 9 April 1997/Accepted: 7 October 1997 相似文献
12.
M. Versluis N. Georgiev L. Martinsson M. Aldén S. Kröll 《Applied physics. B, Lasers and optics》1997,65(3):411-417
1 (6) rotational line in the A2Σ+(v′=0)←X2Π(v′′=0) band of OH at 309 nm. The requirements for obtaining a good signal-to-noise ratio for the technique are discussed and
the possibilities of single-shot measurements are investigated.
Received: 31 October 1996/Revised version: 3 December 1996 相似文献
13.
N. Kalivas L. Costaridou I. Kandarakis D. Cavouras C.D. Nomicos G. Panayiotakis 《Applied Physics A: Materials Science & Processing》1999,69(3):337-341
The quality of a medical image depends, among other parameters, on quantum noise. Quantum noise is affected by the fluctuations
in the number of optical quanta produced within the phosphor, per absorbed X-ray (i.e. phosphor intrinsic-gain fluctuations).
This effect is considered by means of a factor, called in this study intrinsic-gain noise factor, IGNF(E). In existing theoretical
models of quantum noise, the corresponding factor is taken to be equal to one. In this paper, an expression that accounts
for the coefficient of variation of the phosphor intrinsic gain is introduced. This expression takes into account the process
of electron–hole pair conversion to optical photons and the frequency distribution function of the emitted optical photon
energy. Subsequently IGNF(E) is expressed in terms of this coefficient of variation. IGNF(E) has been calculated for several
phosphors and for various energies. For all medical X-ray energies studied, phosphors that exhibit a high relative fluctuation
of emitted optical photon energy, IGNF(E) exceeds by 2% to over 17% the corresponding factor of the existing theoretical models
of quantum noise.
Received: 25 March 1999 / Accepted: 29 March 1999 / Published online: 14 July 1999 相似文献
14.
Characteristics of hydrogenated fiber Bragg gratings 总被引:1,自引:0,他引:1
C. L. Liou L. A. Wang M. C. Shih 《Applied Physics A: Materials Science & Processing》1997,64(2):191-197
By utilizing both hydrogen loading for photosensitivity enhancement and a phase
mask for holographic exposure, we have fabricated fiber Bragg grating with controllable reflectance and bandwidth. The evolution
of hydrogen diffusion into a single-mode optical fiber before exposure, and out of the fiber after exposure was characterized,
and the results were consistent with the theoretical modeling. The shifting of Bragg wavelength and thermal reliability were
found related to the hydrogen behavior inside the fiber. One solution to prevent Bragg wavelength from drifting after the
grating was formed and hence to increase the grating's reliability was to anneal it so that the residual hydrogen was forced
out of the optical fiber in a short time.
Received: 12 June 1996/Accepted: 17 June 1996 相似文献
15.
A study of some optical properties of hafnium dioxide (HfO2) thin films and their applications 总被引:1,自引:0,他引:1
M. Fadel O.A. Azim M. O.A. Omer R.R. Basily 《Applied Physics A: Materials Science & Processing》1998,66(3):335-343
2 ), a series of films ranging in thickness from 50 to 10000 nm was prepared by using an electron beam gun inside an evacuated
coating chamber of pressure 1×10-5 mbar. The films were obtained on optical glass substrate by using oxygen with a backfill pressure of 2.4×10-4 mbar during the deposition processes. The optical constants of the films were computed in the spectral wavelength region
(350–2000 nm) from the transmission, reflection and thickness measurements. A computer program was created to determine two
optical parameters n and k of the films, and this was achieved by entering the practical results into the computer program,
which solved a series of equations for each wavelength. The effects created by changing various evaporation conditions (thickness,
substrate temperature and evaporation rate) were studied in the spectral wavelength range, and the optimum values of the various
conditions were obtained while achieving the best optical performance. According to the investigations of the HfO2 material, two applications of the anti-reflection (AR) multi-layer coatings were achieved in two different spectral wavelength
ranges. The first application was measured in the visible and near infra-red (VIS/NIR) range from 500 nm to 850 nm deposited
on the glass substrate. The second application was measured in the infrared (IR) range from 7500 nm to 11500 nm deposited
on germanium substrate. Computer modelling for designing the optical multi-layer system has been presented. The theoretical
formulation and experimental results with the same specification were achieved. The correlation between the theoretical and
the experimental results reveals a close agreement that offers a convenient method for predicting and controlling the multi-layer
coating. By continuous measurement of the optical and mechanical (durability) performances of the coating process, high-quality
films were produced in the manufacture of various optical devices.
Received: 16 April 1997/Accepted: 5 August 1997 相似文献
16.
I. Kandarakis D. Cavouras P. Prassopoulos E. Kanellopoulos C.D. Nomicos G.S. Panayiotakis 《Applied Physics A: Materials Science & Processing》1998,66(5):521-525
2 SiO4:Mn phosphor was evaluated for use in radiation detectors of medical imaging systems. Zn2SiO4:Mn was used in the form of laboratory-prepared fluorescent layers (screens) with coating weights from 18 to 150 mg/cm2. The phosphor was excited to luminescence by low-energy X-raysusing X-raytube voltages ranging from 15 to 50 kVp. The number
of emitted optical photons per incident X-rayquantum was thus determined for various X-rayenergies and phosphor coating weights.
The optical emission spectrum was also measured and it was used to evaluate the spectral compatibility of Zn2SiO4:Mn with radiographic films, photocathodes and the Si photodiode. Finally, phosphor optical properties were estimated by fitting
a theoretical model to experimental data. Results showed that Zn2SiO4:Mn is more efficient for low-energy X-rays. Its intrinsic conversion efficiency was found equal to 0.08, which is comparable
to that of actually used phosphors. Zn2SiO4:Mn was also adequately compatible with orthochromatic films and the ES-20 photocathode, thus being appropriate for low-voltage
radiography and fluoroscopy.
Received: 31 July 1998/Accepted: 3 August 1998 相似文献
17.
D. Cavouras I. Kandarakis T. Maris G.S. Panayiotakis C.D. Nomicos 《Applied Physics A: Materials Science & Processing》2001,72(1):67-72
In information theory, entropy expresses the information gain obtained after detection of a signal concerning the state of
a parameter of interest. In this study, entropy has been expressed in terms of physical quantities (emitted optical fluence
and MTF) related to the imaging performance of phosphor materials, which are employed in medical imaging radiation detectors.
Four phosphor materials, used in the form of laboratory-prepared fluorescent layers (screens), were compared on the basis
of their entropy performance. Measurements were performed using 30- and 80-kVp X-ray beams often employed in X-ray imaging.
Results showed that phosphor materials with high density and effective atomic number exhibit high entropy performance, especially
at the higher X-ray tube voltage of 80 kVp. Entropy values are also affected by the type of activator, which determines the
intrinsic X-ray-to-light conversion efficiency, and the spectrum of emitted light. The proximity of the incident X-ray quanta
energy to the energy of the K-shell threshold for photoelectric absorption is an additional important factor which increases
entropy. This effect was more apparent in the performance of yttrium-based phosphors at the lower voltage of 30 kVp.
Received: 7 January 2000 / Accepted: 28 March 2000 / Published online: 23 August 2000 相似文献
18.
Received: 27 October 1997/Revised version: 12 January 1998 相似文献
19.
Received: 10 November 1997 相似文献
20.
R.O. Johnson S.J. Karis G.P. Perram W.B. Roh 《Applied physics. B, Lasers and optics》1998,66(4):411-415
2 laser operating on the 1001-1000 transition at λ=4.3 μm and pumped by E –V energy transfer from Br(2P1/2) has been demonstrated. The dynamics and performance of this device were characterized by observing the time-resolved stimulated
emission and the steady-state spontaneous side fluorescence after photolysis of IBr or Br2 by a frequency-doubled Nd:YAG laser or Ar+ laser, respectively. Although the E –V excitation kinetics are favorable, rapid vibrational relaxation limits laser action
to CO2 pressures of less than 1 Torr. Numerical modeling of laser pulse shapes and the dependence on IBr and CO2 pressure and photolysis energy establish a relatively high gain of 0.33%/cm, a CO2-pressure-dependent optical loss of 0.04–0.06%/cm, and an efficiency of 2×10-5 4.3-μm-laser photons per incident photolysis photon. The CO2 fluorescence after photolysis of a fixed Br2/CO2 gas mixture decreases as a function of photolysis time by about 30%/h, indicating the photolytic production of an important
quencher.
Received: 23 June 1997/Revised version: 23 September 1997 相似文献