High Photon Yield Research Articles

Excited-state formation model for metal-oxide sputtered molecules

At present it has been well established that the continuum and the bands observed in radiation spectra under ion bombardment of refractory and rare-earth metal surfaces are emitted by metal-oxide sputtered molecules. An important feature of this radiation is the anomalously high photon yield, Y λ : Y λ = 10 −2−10 −3photon/ion. This value exceeds Y for sputtered excited atoms: In this article, we propose the excited-state formation model for sputtered molecules based on the following assumptions: (1) a molecule is nondissociatively sputtered if the energy of the relative motion of its atom in the center-of-mass system is less than its bond energy; and (2) the nuclei energy of relative motion of the nuclei is transferred to the electron subsystem excitation by a Landau-Zener-type transition. The model permits us to understand the large value of y λ , because a great number of the molecules undergoes excitation beyond the electron exchange region, S 0: S 0 = 5–10 A ̊ where molecule-surface exchange processes take place.

Readout techniques and radiation damage of undoped cesium iodide

Several readout techniques for undoped CsI have been studied which utilize the fast scintillation component for speed and the high photon yield for good energy resolution. Quantum yields have been measured for samples up to 30 cm in length using photomultiplier tubes, wavelength shifters, and silicon photodiodes. A study has also been made of the scintillation properties of undoped CsI. It is found that the light output and decay time of the 310-nm fast component increases and the emission spectrum shifts to longer wavelengths at lower temperatures. The effects on the optical transmission and scintillation light output due to radiation damage from /sup 60/Co gamma rays have been measured for doses up to approximately 10/sup 6/ rad. It is found that the radiation resistance of undoped CsI is substantially higher than has been reported for thallium-doped CsI. >

Photosynthetic acclimation in a terrestrial CAM bromeliad, Bromelia humilis Jacq.

Photosynthetic acclimation of a terrestrial CAM bromeliad -Bromelia humilis Jacq. to irradiance and nitrogen supply during growth was investigated under controlled conditions. Considerable acclimation to shade was realized through an increase in total chlorophyll, accessory pigments, an increase in chloroplast size and appressed to non-appressed thylakoid membrane ratio, and a reduction in chlorophyll a/b ratio. A relatively high apparent photon yield in shade was associated with a low light-compensation point and low dark-respiration rate. Light saturation of integrated dark CO2 uptake occurred at a relatively low total daily photosynthetic photon flux density (PPFD) and was not influenced by growth PPFD level. Maximum photosynthetic capacity expressed per unit chlorophyll and nitrogen use efficiency were significantly higher in high-PPFD-grown plants. In addition, apparent photon yield was only slightly reduced in high-PPFD-grown plants with nitrogen, while maximum photosynthetic rate remained the same indicating radiationless dissipation of excess light. Very high levels of recycling of respiratory carbon were observed under nitrogen deficiency in plants grown in both high and low PPFD. The extensive photosynthetic acclimation of B. humilis is discussed in terms of contrasting exposure and limitations of its natural environment.

Detailed balance description of energetic photons in heavy-ion collisions

Photon production cross sections in intermediate energy heavy-ion reactions are calculated using detailed balance. The model gives substantial photon yields and reproduces qualitatively the experimental observations. A possible influence of the \ensuremath{\Delta}-isobar degree of freedom on the high energy photon yield is pointed out.

Light Response of CO2 Assimilation, Reduction Stage of Q and Radiationless Energy Dissipation in Intact Leaves

Response curves of the rate of net CO2 uptake and of fluorescence quenching (photochemical, qp, and non-photochemical, qE) to photon irradiance were obtained for leaves of eight species. Their photosynthetic capacity varied greatly due to genetic (sun/shade species) and environmental factors (growth under different light and nutrient regimes). The two fluorescence parameters were studied in response to the proportion of excess light at each irradiance, which is given by 1 -Aa/Ap, where Aa is the actual and Ap the potential rate of photosynthesis (assuming that the high photon yield of CO2 uptake seen at low light occurred at all irradiances). All leaves exhibited a very similar response of either 1-qp or qE to 1 - Aa/Ap indicating that the differences in 1 - qp or qE among leaves at a given irradiance can be explained exclusively by differences in the balance between absorbed light and electron transport. In leaves of all species qE, which is primarily a reflection of radiationless dissipation of excitation energy, responded linearly to 1 -Aa/Ap, except for a component already present under conditions in which light is limiting (i.e. when Aa=Ap). By contrast, the reduction state of the primary electron acceptor of photosystem II, 1 - qp, when expressed as a function of 1 - Aa/Ap, increased in all leaves much less than expected if it were solely a reflection of the balance between light absorption and photochemistry. Radiationless energy dissipation is a process which has the potential to maintain Q in a low reduction state.

Effect of nitrate‐nitrogen limitation on photosynthesis of the diatom Phaeodactylum tricornutum Bohlin (Bacillariophyceae)

Abstract Nitrate limited growth of the diatom Phaeodactylum tricornutum in chemostat cultures produced marked changes in biochemical composition and a six‐fold reduction in the specific growth rate. This was associated with a reduction in the carbon and chlorophyll a specific light saturated rates, with little effect on light limited photosynthesis. Variations in specific growth rate were quantitatively related to carbon specific net photosynthesis and maximum chlorophyll a specific light saturated rates were positively correlated with cell nitrogen contents. The correlation between nitrogen content and photosynthesis for P. tricornutum and the differential effect of nitrogen supply on the light response curve of photosynthesis is qualitatively and quantitatively similar to published results for terrestrial vascular plants. There was little change in the photon (quantum) yield of photosynthesis which was not significantly different from 0.125mol O2 mol photon‐1 the theoretical upper limit based on the Z scheme, even under severe nitrate deficiency. The capacity to maintain a high photon yield under nitrate limitation is discussed in relation to the nitrogen requirements of the stromal and membrane components of the photosynthetic apparatus.

Crossed-beám chemiluminescence. II. Kinetics and mechanisms for reactions of group IIA Metals in ground and metastable states with fluorine

Visible chemiluminescence technique under crossed-beam conditions has been applied to the study of the reactions of the group IIA metal atoms Mg, Ca, Sr and Ba in their ground state (1S0) or in an excited metastable state (3Pi, 3Di or 1D2) with F2. The monofluoride emission bands are the most prominent features in the chemiluminescence spectrum. Using higher fluorine densities radiation is observed from excited alkaline earth difluorides, which are shown to principally originate from secondary reactions. Formations of MF* are determined to be first order with respect to both the metal atom and the fluorine molecule. Total cross sections for removal of ground state metal atoms from the beam by F2 are 115 ± 15Å2 for Ca, 125 ± 15Å2 for Sr, and 160 ± 15Å2 for Ba, which is consistent with an electron jump model. Chemiluminescence cross sections are reported for the reactions involving electronically excited reactants M*. Photon yields of 12 ± 3% for Mg*, 18 ± 5% for Ca*, 20 ± 5% for Sr*, and 15 ± 8% for Ba* reacting with F2 are measured. These high photon yields are remarkable when compared with absolute photon yields for the ground state reactions which indicate that less than 2% of the products are MF* molecules. It was possible to obtain vibrational state distributions for some of the excited monofluorides which are found to be populated in a non-thermal manner. This strongly suggests that the dynamics of the reactions are governed by a direct mechanism. From the crossed-beam chemiluminescence spectra the dissociation energies of the ground state monofluorides are estimated. In addition, improved spectroscopic constants, dissociation energies and dissociation products of some of the excited electronic states of MF are given.

Gas-phase reaction of Ba with N2O. II. Mechanism of reaction

A reaction mechanism and a correlation diagram argument are presented to account for the high photon yield of the chemiluminescent reaction of Ba atoms with N2O. Most BaO molecules appear to be formed initially in the long-lived a3π state and to be transferred subsequently by collisions into the shorter-lived A1Σ+ state. This mechanism accounts for numerous kinetic and spectroscopic observations of BaO. The arguments suggest that reactions of alkaline earth atoms with N2O and with O3 may be suitable for electronic-transition chemical lasers.

Measurement of Glomerular filtration rate using chelated ytterbium-169

Ytterbium-169 chelated with diethylenetriaminepentaacetic acid ( 169Yb-DTPA) was developed and evaluated for possible use as a glomerular substance. The radionuclide has a high photon yield with complex gamma ray spectrum. It has a physical half-life of 32 days, but less than 1 percent of intravenously injected chelated complex is retained in the body, which limits the radiation exposure. Gel fitration studies showed that 169Yb-DTPA complex did not bind to plasma proteins and was excreted unchanged in the urine. The clearance rate of the complex was found to be independent of the plasma concentrations of Yb-DTPA. The renal clearance of 169Yb-DTPA was compared simultaneously with that of 14C-inulin in dogs, normal human subjects and patients with renal disease. Nearly identical results were obtained with both the agents over a wide range of glomerular filteration rate. The radiation doses due to different glomerular agents containing 131I, 125I, 57Co, 99 m Tc, 51Cr, 113 m In, 140La and 169Yb were calculated for normal and impaired renal conditions, 169Yb-DTPA was found to deliver comparatively low radiation doses. It was believed to be a safe, inexpensive and effective new agent for the measurement of glomerular filtration rate.