Helium recombination

The results are summarized in Figure 1. The different symbols denote the spectral classification of the individual stars, whereas four groups are distinguished WNE-A, WNE-B, WNL and WC stars. The size of the symbols indicates the mass-loss rates. The uncertainties of the results are estimated to be 0.1 dex in T, 0.4 dex in M and 0.5 dex in L. For the 11 stars in common with the sample of Abbott et al. (1986) we find our mass-loss rates to be compatible with their radio flux if the correct ionization equilibrium in the radio emitting region is applied (Schmutz and Hamann, 1986). The model calculations show that for all but the WN2 and WN3 stars helium recombines to He before the ions enter the radio-emitting region. 

Figure 7. Schematic diagram of a flowing-afterglow electron-ion experiment. The diameter of flow tubes is typically 5 to 10 cm and the length is 1 to 2 meters. The carrier gas (helium) enters through the discharge and flows with a velocity of 50 to 100 m/s towards the downstream end of the tube where it exits into a fast pump. Recombination occurs mainly in the region 10 to 20 cm downstream from the movable reagent inlet, at which the ions under study are produced by ion-molecule reactions. The Langmuir probe measures the variation of the electron density in that region. A differentially pumped mass spectrometer is used to determine which ion species are present in the plasma.

The recombination of He is a special case. We include it here because of the similarities with H3 and because it is the only known example where three-body recombination of a diatomic molecular ion dominates over the binary process. The literature on the helium afterglow is quite large and we will not be able to do justice to all aspects of this problem. Mulliken71 had predicted that fast dissociative recombination of Hej should not occur due to a lack of a suitable curve crossing between the ionic potential curve and repulsive curves of He. Afterglow experiments in pure helium, at sufficient pressure to enable formation of Hej ions, have confirmed this expectation. It does not appear that the true binary recombination... 

The second step in the reaction, dissociation of the Hej Rydberg molecule, is similar to dissociative recombination of He with a free electron. For this reason, Bates73 called this recombination mechanism Rydberg dissociative recombination. It enhances the overall loss rate of free electrons because the stabilization of He2 prevents the return of weakly bound electrons to the population of free electrons. The reaction plays the same role as the reaction of H with H2 that we discussed in Section IV.C. As has been discussed by Bates, the mechanism also provides an explanation for spectroscopic observations of atomic and molecular emissions in helium afterglows. There is direct evidence for the existence of a substantial population of weakly bound electrons in helium afterglows.74 Most likely, the weakly bound electrons are Rydberg electrons in He2 molecules. 

Figure 6. Rate constant for the recombination of Cr(CO)4 with CO as a function of bath gas (helium) pressure. The various symbols correspond to data obtained...

Abstract. New results of the Primordial Helium abundance (Yp) measurement by radio recombination line (RRL) observations from five galactic HII regions are presented. The RRL observations were carried out with two telescopes RT32 (22.4 and 8.3 GHz, Medicina, Italy) and RT22 (36.5 and 22.4 GHz, Pushchino, Russia). The results of the first run of the low frequency RRL observations (408 MHz) with the Croce del Nord radiotelescope (Medicina Observatory, Italy) are also presented. 

The theory of recombination for hydrogen and helium seems to be well understood, at least to 1 or 2 per cent, and the line intensities are not very sensitive to temperature or density in particular, there is no exponential factor. 

Temperature gradients and local temperature fluctuations usually parameterized by t2 (Peimbert 1967) lead to a systematic bias when the electron temperature determined conventionally from [O m] X 4363/a 5007 is substituted into the expressions for effective recombination coefficients of hydrogen and helium. 

Following hydrogen recombination the luminosity rises at a rate that is very sensitive to the explosion energy, the envelope mass, and to the opacity in the helium core. The ultimate source of the energy here is the decay of Co to 56Fe, a reaction that has powered the light curve since late March and especially through the peak and tail. Because the amount of 5 Ni synthesized is artificially constrained to be the same (0.07 Me V) in all our models, the peak... 

Abstract. We use a semi-analytical model of supernova light curves that extends Arnett s scheme [1,2] to include the effect of recombination of hydrogen, helium and heavy elements in the expanding ejecta. Introducing in the model the physical parameters of Sanduleak -69 202, the salient characteristics of the light curve of SN 1987 A are reasonably reproduced over 100 days. 

Chlorine atoms were produced by flowing a mixture of 5 % Cl2 in helium through a quartz tube, coated with a thin film of baked phosphoric acid to inhibit Cl atom recombination, and enclosed in a 2.45 GHz microwave cavity operating at 35 W. The purity of reactants was 99.5 to 97 %, and they were frequently subjected to several freeze-pump-thaw cycles. The reactants were flowed inside the reactor neat or diluted in helium (3% mixtures). 

D.R. DeWitt, E. lindroth, R. Schuch, H. Gao, T. Quinteros, W. Zong, Spectroscopy of highly doubly excited states of helium through dielectronic recombination, J. Phys. B 28... 

Helium-4 can be observed in galactic and extragalactic HII regions - regions of hot and ionized gas - using either optical or radio recombination lines. However the best determinations come from observations of Hell —> Hel recombination lines in extragalactic HII regions. 

Photo- and cathodoluminescence (PL, CL) measurements at liquid helium temperature are sensitive tools for investigation of the optical recombination properties [62] of the PLD ZnO films. In ZnO, the information depth of PL using a 325 nm He-Cd laser is only about 60 nm [63], corresponding to... 

A solid state Si(Li) detector is used to maximise the flux of helium-like vanadium transitions, to minimise contamination from undesirable charge states and to monitor dielectronic recombination transitions. A small amount of pure nitrogen gas is leaked into the trap (injection pressure 5x 10 7Torr) to increase the proportion of lower charge states via evaporative cooling. 

Essentially different from the reactions studied is the MB + S02 system.17 Whereas in the BC + Cl2 system the degree of chemical conversion did not exceed 10-15%, in the MB + S02 copolymerization reaction the conversion was practically complete. The small degree of conversion in the BC + Cl2 system is likely to be connected with the heating up of the wave front to the devitrification temperature, which leads to the effective decay of active centers as a result of their recombination. Perhaps this reason, along with the others discussed earlier, may be called to account for the increase in the degree of conversion in the wave front on passing from the nitrogen to helium temperature. In other words, in the reactions of chlorination and hydro-... 

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