A&A 406, 759-764 (2003)
S. Djenize1,2,3 - S. Bukvic1,2 - A. Sreckovic1,2 - S. Kalezic1
1 - Faculty of Physics, University of Belgrade 11001 Belgrade, POB 368, Serbia
2 - Isaac Newton Institute of Chile, Yugoslavia Branch, Belgrade, Serbia
3 - Hungarian Academy of Sciences, Budapest, Hungary
Received 31 January 2003 / Accepted 23 April 2003
On the basis of the relative line intensity ratio (RLIR) method transition probability values of the spontaneous emission (Einstein's A values) of 41 astrophysically important transitions (in 15 multiplets) in the doubly (O III) and 7 transitions (in 5 multiplets) in triply (O IV) ionized oxygen spectra have been obtained relative to the reference A values related to the 326.085 nm O III and 340.355 nm O IV, most intensive transitions in the O III and O IV spectra. Fourteen of the investigated O III lines belong to the cascades in the astrophysically important Bowen fluorescence mechanism. Most of the O III transition probability values are the first data obtained experimentally using the RLIR method. Stark shift values (d) of the mentioned lines are also measured. Twenty three of them were not known and represent the first data in this field. Our A and d values are compared to available experimental and theoretical data. A linear, low-pressure, pulsed arc was used as an optically thin plasma source operated in oxygen discharge at a 42 000 K electron temperature and m-3 electron density.
Key words: plasmas - line: profiles - atomic data
A significant number of papers are dedicated to the investigations of the O III and O IV A values (NIST 2003, and references therein). The Stark shifts (d) of the O III and O IV spectral lines are also of an interest in the astrophysics. However, they are poorly known (Lesage & Fuhr 1999; Konjevic et al. 2002).
The aim of this work is to present 41 O III (in 15 multiplets) and 7 O IV (in 5 multiplets) A and d values obtained on the basis of accurately measured spectral line intensities and line center positions using the step-by-step technique for the line profile recording. The well-known relative line intensity ratio (RLIR) method was used for transition probability determination. We have already been applied this method in case of the Ar III, Ar IV, O II, Ne II, N III, N IV, N V and Si III spectra (Djenize & Bukvic 2001; Djenize et al. 2002a,b,c; Sreckovic et al. 2001a, 2002).
The experimental values are obtained relatively to the reference A values. Most of the O III transition probability values are the first data obtained experimentally using the RLIR method. Stark shift values of 22 O III and one O IV lines are the first experimental data and many of them are the first data in this field. Experimental A values have been compared to the transition probabilities from the references which contain Adata corresponding to our chosen reference O III and O IV transitions only.
|Figure 1: Part of the recorded spectrum with several investigated O III and O IV spectral lines in the 5 s after the beginning of the discharge.|
One can notice that the investigated spectral lines are well isolated while the continuum is very close to zero within the wavelengths range of interest. These facts are important for an accurate determinations of the total line intensities and correspondingly, for a reliable determination of A values. All O III and O IV lines are recorded by same experimental arrangement needed for the use of the RLIR method. Total line intensity (I) corresponds to the area under the line profile (within 3%-8% accuracy). Great care was taken to minimize the influence of self-absorption on line intensity determinations. Using a technique described in Djenize & Bukvic (2001) the absence of self-absorption was obtained in the case of the investigated O III and O IV spectral lines.
The electron temperature was determined from the ratios of the relative intensities (Saha equation) of O III (326.08 nm, 372.09 nm and 375.99 nm) and O II (327.05 nm, 372.73 nm and 374.95 nm) spectral lines with an estimated error of 8%, assuming the existence of LTE, according to the criterion from Griem (1964). In the s after the beginning of the discharge electron temperature has also been obtained on the basis of the Boltzmann plot method using the investigated 8 O IV relative line intensities. All the necessary atomic data were taken from NIST (2003). The electron temperature decay is presented in Fig. 2. The electron density decay was measured using a well-known single laser interferometry technique for the 632.8 nm He-Ne laser wavelength with an estimated error of 6%. The electron density decay is presented also in Fig. 2.
|Figure 2: Temporal evolutions of the electron temperature (T) and electron density (N). , O III/O II line intensity ratio method; Boltzmann-plot method (see text).|
In the case when plasma remains at LTE the well-known
The Stark shifts were measured relative to the unshifted
spectral lines emitted by the same plasma (Djenize et al. 2002a,c
and references therein). Stark shift data
are corrected to the electron temperature decay (Popovic et al. 1992).
Our measured ()
Stark shifts are presented in Tables 3 and 4.
Our experimentally obtained and values are given in Tables 1-4 together with all existing d values of the other authors. In the case of the transition probabilities we have compared our results to the experimental A values obtained by RLIR technique and to the recently calculated A values only. The data tabulated by NIST (2003) are also given in Tables 1 and 2. In Table 5 the transition probability ratios have been given for the O III transitions belonging to the primary and secondary cascades in the Bowen fluorescence mechanism.
Generally, most of our O III values lie above the cited calculated data and ).The best agreement, within 9%, has been found for the theoretical values calculated by Froese Fischer (1994) using the MCHF + Breit-Pauli approximation (averaged over 23 transitions). It should be mentioned that the calculated values related to the 326.720 nm, 328.183 nm and 328.445 nm transitions, in the multiplet, are small and because of that they are excluded from the average procedure. Very tolerable agreement (within 14% averaged over 27 transitions) for the values calculated by Aggarwal et al. (1997) has been found. These results are obtained using the CIV3 program where extensive configuration interaction and relativistic effects have been included. The agreement between our values and those presented by Bhatia & Kastner (1993) is within 28% (averaged over 27 transitions). The values have been calculated applying the Super-Structure program of Eissner et al. (1974).
Values tabulated by NIST (2003), , are in agreement with our data within 11% (averaged over 41 transitions). The agreement between our values and previously published experimental data by Berg (1967) (B) and Truhan & Kiseljevskij (1967) (TK) are within 18% (averaged over 16 transitions) and 25% (averaged over 6 transitions) which can be considered as a reasonable agreement taking into account uncertainties cited in these works and our estimated accuracies. In the case of the 369.871 nm O III transition, our value is practically equal to the (Pinnington et al. 1970) value. The lowest scattering for presented experimental and theoretical data is found for 375.469 nm, 375.987 nm, 304.710 nm, 331.232 nm, 334.076 nm, 303.541 nm, 305.927 nm, 302.342 nm 326.720 nm, 326.532 nm, 301.761 nm and 313.279 nm O III transitions. Especially, small scatter is seen in the case of the 326.532 nm (within 6%), 334.076 nm (within 15%) and 331.232 nm (within 19%) transitions (see Table 1).
In the case of the O IV spectrum we have found excellent agreement, within 5.7% on average, between our data set and appropriate values tabulated by NIST (2003) . Experimental (TK) values presented by Truhan & Kiseljevskij (1967) are lower than our data for 49% on average (see Table 2).
Transition probability ratios related to the O III lines that belong to the cascades of the astrophysically important Bowen fluorescence mechanism (Bowen 1934) are presented in Table 5. One can see that our transition probability ratios show the best agreement with Froese Fischer's (1994) values (FF). We recommend the transition probability ratio related to the 331.232 nm and 334.076 nm transitions as the ratio with the highest accuracy ( %) within the cascades in the Bowen fluorescence mechanism.
One can notice that all O III values are small and have a negative sign. Theoretical data (Sreckovic et al. 2001b) also display negative sign, but are several times smaller than our data. Taking into account the experimental accuracy of our values and the uncertainties of the calculations (see Sreckovic et al. 2001b) one can conclude that reasonable agreement exists among the and mentioned values corresponding to the 331.232 nm and 334.076 nm O III lines. Experimental values of Sreckovic et al. (2001b) are measured at 54 000 K electron temperature within pm uncertainties in nitrogen (83%) - oxygen (17%) plasma. Our values are in good agreement with values measured earlier by Puric et al. (1988) in oxygen plasma. For 22 O III lines our values are the first published data. Our O IV values have a positive sign and lie above (also positive) data (Blagojevic et al. 1996 and also Dimitrijevic & Sahal-Bréchot 1995; Sahal-Bréchot 1969a,b) at about 26% (on average). Experimental d data published recently by Blagojevic et al. (1996) have a positive sign also, but absolute values are higher by approximately 40% than mentioned theoretical data. The shift of the 338.552 nm O IV line, presented in this paper, is the first published value.
On the basis of the precisely measured spectral line intensities we have obtained 41 O III and 7 O IV transition probability values relative to the reference 326.085 nm O III and 340.355 nm O IV transitions. We have found that the relative transition probability values of the 326.532 nm, 334.076 nm and 331.232 nm transitions in O III and 341.363 nm, 341.169 nm transitions in O IV are convenient and we recommend these for astrophysical applications, especially the A values related to the 334.076 nm and 331.232 nm O III transitions which are important in the Bowen fluorescence mechanism. In the case of the Stark shifts we recommend the 331.232 nm and 334.076 nm O III and 306.343 nm and 307.160 nm O IV lines, due to convenient Stark shift values, for plasma diagnostics in astrophysical applications.
This work is a part of the project "Determination of the atomic parameters on the basis of the spectral line profiles'' supported in part by the Ministry of Science, Technologies and Development of the Republic of Serbia. S. Djenize is grateful to the Foundation "Arany János K zalapitvány'' Budapest, Hungary.