ALBERT

Notes: The photoabsorption spectrum of ArN+ has been studied in the visible wavelength region by means of photofragment kinetic energy spectroscopy. The mass selected ions were accelerated to 25 keV and the ion beam was merged or crossed with a laser beam. The photoproducts Ar+/N and, weakly, Ar/N+ were observed as ArN+ was photodissociated with photons in the range 1.8–2.5 eV. This is the first observation of electronic transitions in ArN+. In order to interpret the results, potential energy curves of ArN+ were calculated with complete active space self-consistent field and multireference configuration interaction methods. The observed spectra are assigned to the A 3Π←X 3Σ−, C 3Σ−←X 3Σ− and C 3Σ−←A 3Π transitions. From these assignments a value of 2.16 eV for the dissociation energy of ArN+ is inferred.

Notes: A photopredissociation spectrum of the A 1Πu–X 1Σ+g (8,2) transition in N2+2 is reported. The spectrum was recorded by detecting the photofragment signal as a function of excitation wavelength in a merged laser-ion beam apparatus. Line positions for the (8,2) band were measured and reduced to molecular constants. The natural linewidths, and hence predissociation rates, of all observed v'=8 levels were measured. The predissociation rates were found to be e/f symmetry dependent and, in particular, for the f levels, also J dependent. Somewhat surprisingly it appears as if tunneling does not contribute to the observed predissociation; rather, the v'=8 level of the A 1Πu state is depleted by coupling to the continua of the 1Σ−u, 3Σ+u, and 3Δu states.

Notes: A study of the NO2+ ion by means of Auger spectroscopy, fast ion beam laser spectroscopy and ab initio calculations is reported. The photon induced Auger spectrum of NO was recorded. Potential curves for a number of electronic states of NO2+ were calculated by the complete active space SCF method in order to facilitate an analysis of the Auger spectrum. A photoabsorption spectrum of NO2+ was observed by means of photofragment kinetic energy spectroscopy and assigned to the A 2Π←X 2Σ+ transition. The two different experimental methods both give a value of 38.6 eV for the appearance energy of NO2+, which is entirely consistent with recent photoionization and double charge transfer results.

Notes: High quality, strained Si/Si1−xGex layered structures have been grown at temperatures in the range 400–625 °C, using a solid-source molecular-beam epitaxy (MBE) system with a mass-spectrometry-based loop-control to improve the accuracy and stability of the evaporation rates. Good control of the growth parameters has been achieved as verified by, e.g., high-resolution x-ray diffraction. Very high intensities and extremely small peak widths, down to 2.7 meV for the XNP transition at low excitation levels of photoluminescence spectra, indicate high crystalline quality of the layers. It is shown that some previously reported defect-related luminescence from MBE-grown SiGe layers is not intrinsic to the MBE process. © 1994 American Institue of Physics.

Notes: The fluorescence lifetime of the v'=0 level of the N2+2 D 1 ∑+u electronic state has been measured, with the high frequency deflection technique, for the first time. The result, τ=6.0±0.5 ns, is in good agreement with a recent photoion–photon of fluorescence coincidence measurement which yields a lifetime, averaged over an unspecified number of vibrational levels, of 8±3 ns. The electronic transition moment function for the N2+2 D 1 ∑+u–X 1 ∑+g transition was calculated with nonorthogonal complete active space SCF wave functions. Potential energy curves for the X 1 ∑+g and D 1 ∑+u states were calculated with a multireference contracted CI treatment and these potential curves were used, together with the electronic transition moment function, to obtain vibrationally averaged transition moments and hence a theoretical radiative lifetime. The calculated lifetime, 8.0 ns, is in good agreement with the experimentally determined value indicating that the decay of D 1 ∑+u, v'=0 occurs primarily via fluorescence to the X 1 ∑+g state.

Notes: The complete active space SCF (CASSCF) method has been used to calculate the SiH A 2Δ–X 2Π electronic transition moment curve. The radiative lifetime deduced for v'=0 on basis of this transition moment curve and averaged over a number of rotational levels was 508 ns, which is in very good agreement with experimental results. The contracted CI (CCI) method was used to obtain the electronic potential functions of the A 2Δ and X 2Π states and the ground state electric dipole moment function. The dipole moment calculated at the equilibrium geometry (re=1.520 A(ring)), μe=0.123 D, is in excellent agreement with previous theoretical results. The ground state dissociation energies of SiH and CH were calculated with comparable basis sets in order to resolve a discrepancy regarding the SiH dissociation energy which prevails in the literature. The calculated dissociation energy D0(SiH)=3.00 supports the value obtained from observation of a predissociation in the B 2∑+ state of SiH, D0≤3.06 eV. Very recently this upper bound to the SiH dissociation energy also found support from a photoelectron spectroscopy experiment. The previously observed predissociation in the A 2Δ state, which suggested a 10% higher dissociation energy, is discussed in some detail and it is shown that the prior interpretation of this predissociation as an interaction between the A state and the ground state continuum is reasonable, but only yields an upper limit to the dissociation energy.

Notes: Lifetimes have been measured for a large number of rotational levels of the A 2Δ and B 2Σ− states in CD using the high frequency deflection technique. Four B-state levels are found to be influenced by predissociation by rotation which demands a lower value of the CD dissociation energy. The B state predissociations have also been studied using ab initio calculations. The A state levels above the dissociation limit are found to be weakly predissociated through interaction with the ground state continuum. The results are compared with earlier lifetime investigations of CH.

Notes: Dissociative recombination of the polyatomic ions D3O+ and H3O+ with electrons have been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Absolute cross sections have been determined from 0.001 eV to 0.25 eV center-of-mass energy for D3O+ and from 0.001 eV to 28 eV for H3O+. The cross sections are large (7.3×10−13 cm2 for D3O+ and 3.3×10−12 cm2 for H3O+ at 0.001 eV). At low energies, the cross sections for D3O+ are E−1 energy dependent whereas it is slightly steeper for H3O+. A similar E−1 energy dependence was also observed by Mul et al. [J. Phys. B 16, 3099 (1983)] with a merged electron-ion beam technique for both H3O+ and D3O+ and by Vejby-Christensen et al. [Astrophys. J. 483, 531 (1997)] with the ASTRID storage ring in Denmark, who presented relative cross sections for H3O+. A resonance has been observed around 11 eV for H3O+. It reflects an electron capture to Rydberg states converging to an excited ionic core. A similar structure was reported by Vejby-Christensen et al. Our absolute measurements are in fairly good agreement with those from Mul et al., which were first divided by 2 (Mitchell, 1999, private communication) and from Heppner et al. [Phys. Rev. A 13, 1000 (1976)] for H3O+. Thermal rates were deduced from the measured cross sections for electron temperatures ranging from 50 K to 30 000 K. At 300 K, the thermal rate is equal to 7.6×10−7 cm3 s−1 for H3O+ and to 3.5×10−7 cm3 s−1 for D3O+. Complete branching ratios for all the possible product channels have been determined from 0 eV to 0.005 eV center-of-mass energy for D3O+ and at 0 eV for H3O+, using a well-characterized transmission grid in front of an energy-sensitive surface-barrier detector. No isotope effect was observed within the experimental uncertainties. The three-body break-up channel OX+X+X (where X stands for H or D) is found to occur for 67%–70% of the dissociations. Water or heavy water is produced with an 18%–17% probability and the production of oxygen atoms is negligible. These results support the three-body break-up dominance already found by Vejby-Christensen et al. for the DR of H3O+ in a similar heavy-ion storage ring experiment. However, even if the general trend is the same for both storage rings, significant differences have been observed and will be discussed. © 2000 American Institute of Physics.

Notes: Wheat (Triticum aestivum L., cv. Drabant) seedlings were grown in a ‘split root’ system where either the whole root system or one root half was subjected to osmotic stress for 24 h, using 200 g polyethylene glycol (PEG, molecular weight 4000) dm−3 nutrient solution. 15N-Labelled nitrate was fed to one of the root compartments and total N and 15N-labelling were measured in plant material and xylem sap. Untreated plants translocated 87% of the N taken up to the shoot, and 10% of this was then retranslocated back to the root. Recalculated on a root nitrogen basis, 36% of the label recovered in the root after 24 h had passed through the shoot. Significant labelling of xylem sap collected from non-labelled roots indicated cycling of organic N through the roots. PEG-treatment of the whole root system caused significant water loss in both roots and shoots. Uptake of nitrate and retranslocation of N to roots were inhibited, whereas cycling of organic nitrogen through the root was still measurable. Treatment of half the root system with PEG had minor effects on shoot water content, but reduced the water content of the treated root part. The total uptake of nitrate by the root system was unaffected, and the effect on the treated root half was comparatively small. Nitrate reductase activity (NRA) declined in PEG-treated roots even if high nitrate uptake rates were maintained. Shoot NRA was unaffected by osmotic stress. The data indicate that the reduction in water content of the root per se has only small effects on nitrate uptake. Major inhibition of nitrate uptake was observed only after treatment of a sufficiently large portion of the root system to given an effect on shoot water content.

Notes: Several pathogenic Pythium spp. were isolated from spinach roots in a 4-year disease survey in commercial spinach fields in southern Sweden. Heterothallic and zoospore-producing isolates belonging to the filamentous, non-inflated zoosporangia (F) group of van der Plaats-Niterink were most abundant, followed by P. sylvaticum, P. ultimum var. ultimum and P. heterothallicum. The heterothallic hyphal swelling (HS) group and P. tracheiphilum were found in a few plants. Two types of pathogenicity tests were performed, to measure the effects of seedling infection and of root infection of older plants. These tests showed P. ultimum var. ultimum to be the most severe spinach pathogen inducing pre- and post-emergence damping off as well as root rot of older plants. Also P. heterothallicum and P. tracheiphilum damaged both seedlings and older plants, Pythium sylvaticum, Pythium‘group HS’ and Pythium‘group F’were pathogenic only to seedlings, P. ultimum var. ultimum and Pythium group F were isolated in significantly higher frequencies from spinach grown in the autumn season than in the spring. No clear relationships were found between Pythium prevalence and disease severity index of surveyed field plants, between Pythium prevalence and plant developmental stage, or between prevalence of Pythium and other pathogens isolated. This is the first report of P. heterothallicum and P. sylvaticum being pathogenic on spinach.