A&A 388, 704-711 (2002)
DOI: 10.1051/0004-6361:20020560
V. Vujnovic 1 - K. Blagoev 2 - C. Fürböck 3 - T. Neger 3 - H. Jäger 3
1 - Institute of Physics of the University, PO Box 304, HR-10001 Zagreb, Croatia
2 -
Institute of Solid State Physics, Bul. Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria
3 -
Institut für Experimentalphysik, Technische Universität Graz, Petersgasse 16, 8010 Graz, Austria
Received 29 September 1997 / Accepted 19 March 2002
Abstract
Relative intensity measurements of Al I and Al II
spectral lines in the visible and ultraviolet spectral ranges are
performed using a capacitively coupled high frequency double hollow
electrode discharge. Branching ratios and intensity ratios within multiplets
are determined. By using selected lifetimes absolute transition
probabilites are calculated.
Key words: atomic data - lines: formation - methods: laboratory
Spectral lines of neutral and singly ionized aluminum can be observed in numerous laboratory experiments and are also found in many stellar spectra. Although a number of studies were devoted to lifetimes of upper levels and oscillator strengths of aluminum lines, the availability of reliable data is far from being satisfactory.
In this work a stable source with non-thermal excitation was used for precise relative intensity measurements. We report on measurements of relative intensities of spectral lines of neutral and singly ionized aluminum. The obtained results were partly used to inspect relative intensities within multiplets, i.e. either to justify the ratios expected for LS coupling, or to reveal deviations from these theoretically expected ratios. In case of available lifetimes and branching ratios, measured as completely as possible, absolute transition probabilities were evaluated.
As a light source a capacitively-coupled rf-plasma was produced in a cylindrical chamber, which contained two cylindrical aluminum electrodes. One of these electrodes was grounded, the other one was put at rf-high-voltage, which was produced by a high-frequency generator (Crespo Lopez-Urrutia et al. 1994). The discharge burned in direct contact with the electrodes, therefore electrode material was sputtered off by ions and electrons. During the high-frequency period the electrodes act as hollow cathodes as well as hollow anodes, depending on their momentary polarity.
In the chosen frequency region the electrons are able to follow the changes of the electric field in phase. High sputtering rates are reached by their relatively high kinetic energy. On the other hand, the electron movement is a reason for a negative dc-voltage of the electrodes compared to the plasma. This negative self bias is a typical feature of rf-discharges. It is the reason for an almost steady bombardment of the electrode surfaces with gas ions produced in the plasma.
Since the ions reach energies of several hundreds of eV during their acceleration in the negative bias region, a sufficient sputtering rate of aluminum atoms out of the electrode surface can be achieved. These atoms are ionized by collisions with electrons or charge exchange processes. Compared with dc-discharges, significantly higher excitation stages are within reach in case of rf-conditions, because the high voltage peak values do not have to be maintained constantly. Moreover, the thermal stress of the electrodes is reduced by a very homogeneous current distribution over the electrode surface, a low operating pressure with an increased current density characterizes this discharge type.
The rf-generator was a home-made construction, which favours the choice of
optimal electrical parameters (Crespo Lopez-Urrutia et al. 1994). It is tunable in the region
from 11 to 28 MHz and operated in the C-mode with a BBC ITK 12-1
water-cooled power triode. Although the rated output cw-power was 20 kW,
only an input power of 1.5 kW was applied in case of the capacitively
coupled plasma, since a resonance transformer had to be used in order to
produce a sufficiently high voltage.
The generator was connected to a
rf-matching box, a 50
coaxial line led to the discharge chamber. This
line consisted of two copper tubes (outer diameter 54 mm, inner diameter 18 mm) with teflon spacers fixing them at a distance.
The discharge vessel is mounted to the matching box, which contains the resonance transformer. To prevent undesirable sparks, this box was filled with SF6 at a pressure of approx. 1.5 bar. With a variable vacuum capacitor the matching box was tuned to provide the desired power rate for the discharge. The length of the hollow electrodes was 20-40 mm, the holes in the electrodes had a diameter of 3.5 mm.
The discharge was operated at pressures between 2 and 20 mbar in a power range between 0.7 and 1.3 kW. It burned stably for several hours, which allowed lengthy detection scans. Helium was used as a buffer gas with a continuous flow rate of a few liters per hour. This flow favoured the reduction of dirt deposition on the quartz windows at the discharge chamber being used for end-on observation. The power deposition into the discharge was monitored calorimetrically.
By use of two concave surface mirrors the plasma was imaged on the entrance slit of a McPherson monochromator (type 209, 1.33 m focal length). This monochromator was equipped with a Czerny-Turner mounting and a holographic grating (2400 lines/mm). So it was possible to resolve a FWHM down to 0.06 Å in first order, a spectral range from 1900 to 6500 Å could be covered. The widths of the entrance and exit slits were adjusted for optimal resolution. The height of the entrance slit was restricted to 2 mm to ensure an axial detection of the plasma.
The radiation was recorded using a fast photomultiplier (EMI 9558 QB) being connected to a photon counting system (Stanford Research SR 400). This arrangement offered a high dynamic range together with a very high signal/noise ratio. The spectral sensitivity was calibrated using an Argon Mini Arc of the National Institute of Standards and Technology for wavelengths between 1900 and 4000 Å, at longer wavelengths a tungsten strip lamp was used. Since the maximum photon-counting rate was about 5 MHz, deviations from linear response caused by dead-time and pile-up effects in the multiplier could be reliably avoided.
By use of a personal computer the photon counter and the step motor for the wavelength scan could be controlled simultaneously via an RS-232 interface. This arrangement provided a full-automatic data aquisition.
The used ultraviolet and visible spectral ranges are well suited for the observation of the spectrum of neutral aluminum, since - owing to the low ionization energy - it shows no spectral lines with wavelengths shorter than 2000 Å. For both neutral and singly ionized aluminum, an extension of the observation region towards the infrared, however, would increase the number of completely measurable branching fractions.
We made a review of the published lifetimes and oscillator strengths. In
the case of neutral aluminum, the lifetimes were measured by the Hanle effect,
hook, beam foil, phase shift and LIF techniques, and calculated by several
authors. Many measurements were undertaken of 4s and 3d lifetimes, and
s, p, d level series were experimentally covered in a wide range: ns for
n = 4-10 (Jönsson & Lundberg 1983; Buurman et al. 1986), np for
n = 4 - 12
(Buurman et al. 1986; Jönsson et al. 1984),
for
n = 3 - 13 (Jönsson & Lundberg 1983; Buurman et al. 1986; Davidson et al. 1990).
For the nf levels only
theoretical values exist (Theodosiou 1992).
In the framework of the Opacity Project (OP) (Mendoza et al. 1995) a large number of transition
probabilities of allowed transitions and radiative lifetimes of Al I excited levels were calculated. These values agree well with experimental data. There is a difference for (
)
excited states with theoretical data of C. E.
Theodosiou (Theodosiou 1992), since in the latter
paper the electron configuration interaction effect (CI) had not been taken into account.
This effect originates from the influence of all nd series by the
level, but mainly of the low
lying
levels. Theoretical works
must also be mentioned in which, employing different approaches,
oscillator strengths of
;
and
were calculated (Lavin et al. 1997; Ozdemir &
Karal 1997). In CI approximation, oscillator strengths of 11 transitions were obtained (Marcinek & Migdalek 1993), including
transitions.
Although the Al I spectrum
is rich in spectral lines originating from the complex part of the term
diagram, only the lifetimes of the levels
,
which are also
liable to autoionization, were measured (Lombardi et al. 1981).
Al I | Al II | ||||
Level des. |
![]() |
Source | Level des. |
![]() |
Source |
![]() |
6.85(6) | Buurman et al. (1986) |
![]() |
0.690(13) | Kernahan et al. (1979) |
Klose (1997) | Smith (1970) | ||||
![]() |
19.8(5) | Buurman et al. (1986) | Berry et al. (1970) | ||
![]() |
60.5(9) | Buurmann & Dönszelmann (1990) | Head et al. (1976) | ||
![]() |
65(2) | Buurmann & Dönszelmann (1990) |
![]() |
![]() |
Trabert et al. (1999) |
![]() |
275(8) | Buurman et al. (1986) | Johnson et al. (1986) | ||
![]() |
14.0(2) | Buurman et al. (1986) |
![]() |
6.4(5) | Andersen et al. (1971) |
![]() |
29.5(7) | Buurman et al. (1986) |
![]() |
14(2) | Andersen et al. (1971) |
![]() |
13.2(3) | Davidson et al. (1990) |
![]() |
15(1) | Andersen et al. (1971) |
![]() |
14.0(2) | Davidson et al. (1990) |
![]() |
5.0(5) | Andersen et al. (1971) |
![]() |
3.5(3) | Andersen et al. (1971) |
Experimental data for radiative lifetimes of singly ionized aluminum are scarce in literature. They were calculated for
the
levels (n = 4-7) in the simple part of the energy diagram, and for the
level in the complex part (Chang & Wang 1987). For the same F levels lifetimes were
measured by the beam-foil technique (Andersen et al. 1971), unfortunately
with low precision.
In CI approximation the radiative lifetimes of a large number
of excited levels of Al II were calculated (Chang & Fang 1995),
namely
(n=4-6);
;
;
,
levels. The main part of the papers, however, are dedicated to radiative lifetimes of singlet
and triplet
levels. Radiative lifetime of the
level was measured in a storage ring (Trabert et al. 1999) and
in a ion trap experiment (Johnson et al. 1986). The obtained data are in excellent agreement (within a 1%
limit). The average experimental data are presented in Table 1. There are four experimental works in which, by use of the beam - foil method, the radiative lifetime of the singlet
level was measured (Kernahan et al. 1979; Smith 1970; Berry et al. 1970; Head et al. 1976). The agreement of experimental data is within a 2% limit and the average experimental value is also presented in Table 1. Radiative lifetimes of singlet and triplet
levels are calculated in a number of theoretical papers (Jonsson & Froese-Fisher 1997; Zon & Froese-Fisher 2001; Stanek et al. 1996; Chang & Fang 1995; Chou et al. 1973; Das & Idrees 1990; Huang & Johanson 1985; Cowan et al. 1982; Chang & Wang 1987; Hibbert & Keenan 1987; Laughlin & Victor 1979) some of them published recently. The agreement of theoretical data for the singlet
level is very good (within a 3% limit) and there is also good agreement between experimental and theoretical data for this level. The theoretical results for the triplet
level agree within an 18% limit (except for data of Das & Idrees (1990), where the value is 3 times smaller and probably due to a misprint) and the agreement with average experimental value is within a 5% limit. The theoretical calculations in the framework of Opacity Project also concern radiative constants of the Al II spectrum (Butler et al. 1993). The agreement of these data with experimental values is within 15% except for the
level. The radiative constants of excited Al I and Al II levels are included in the review of Fuhr & Wiese (1996), where a critical compilation was made.
Singly ionized aluminum has
prominent transitions between displaced and normal levels - as in the case
for the neutral atom - and there is a need for experimental data. Employing the beam-foil method in the work of Baudinet-Robinet et al. (1979) along with resonance
and
excited states, the
states were measured using the
-
transition. The obtained data, however, are larger than results of other authors (Berry et al. 1970; Smith 1970). With respect to our branching ratio measurements an experimentally determined lifetime of the
level would be of special importance.
We have selected the lifetimes (Table 1) of the upper levels of transitions which we measured. As criteria the reliability of the experimental method (laser induced fluorescence preferred), the consideration of the experimental conditions, the agreement between different authors' results, and the estimated uncertainties have been applied. For theoretical results a coincidence of the oscillator strengths and the corresponding lifetimes, obtained in the length and velocity formalism, may eventually serve as a sign of quality.
We obtained the lifetime of the Al I 4s level as an average of the values
obtained by Klose (1997) and Buurman et al. (1986). In
the same laboratory Buurmann & Dönszelmann (1990) found
deviations from the LS expected equality of the lifetimes of the fine
structure levels
,
.
The lifetime of 3d equal to
14.0(2) ns was taken over from Buurman et al. (1986). It should be mention that the average of seven measurements
(Jönsson & Lundberg 1983; Buurman et al. 1986; Andersen et al. 1969; Cunningham 1968; Smith & Liszt 1971; Marek & Richter 1973; Hannaford & Lowe 1981) is equal to
13.9 ns. This confirms the reliability of the 3d lifetime within the
stated accuracy.
Table 2 contains the findings of intensity ratios and of
transition probabilities of the simple Al I spectrum. The
resonance doublet at
and 3962 Å is an example that demonstrates our procedure. The relative error of the transition
probability is given as the sum of the relative errors of the ratio and of
the lifetime. Taking the theoretical ratio, the transition probability would
equal the error of the lifetime measurement (1%).
Upper | Lower | ![]() |
Intensity ratio | Branching |
![]() |
||||||
level | level | measured | LS | ratio | this work other authors | ||||||
![]() |
![]() |
3944.006 | 0.48 | 9% | 0.5 | 0.325 | 9% | 0.47 | 10% | 0.493 | Wiese & Martin (1980) |
![]() |
3961.520 | 1 | 5% | 1.0 | 0.675 | 5% | 0.99 | 6% | 0.98 | Wiese & Martin (1980) | |
|
![]() |
2652.484 | 0.50 | 2% | 0.5 | 0.142 | 12% | 0.133 | Wiese & Martin (1980) | ||
![]() |
2660.393 | 1 | 1% | 1.0 | 0.284 | 11% | 0.264 | Wiese & Martin (1980) | |||
![]() |
21093.04 | 0.5 | 0.030 | >30% | |||||||
![]() |
21163.75 | 1.0 | 0.060 | >30% | |||||||
![]() |
![]() |
13150.76 | 0.5 | 0.160 | 3% | 0.169 | Buurman et al. (1986) | ||||
|
![]() |
13123.41 | 1.0 | 0.150 | 5% | 0.169 | Buurman et al. (1986) | ||||
|
![]() |
3082.153 | 0.53 | 10% | 0.56 | 0.83 | 10% | 0.59 | 12% | 0.63 | Wiese & Martin (1980) |
![]() |
3092.839 | ![]() |
0.11 | 0.17 | (0.12) | ||||||
|
![]() |
3092.710 | 1 | 5% | 1 | ![]() |
0.71 | 2% | 0.74 | Wiese & Martin (1980) | |
|
![]() |
2567.984 | 0.51 | 9% | 0.56 | <0.28 | 0.23 | Wiese & Martin (1980) | |||
0.192 | Davidson et al. (1990) | ||||||||||
![]() |
2575.393 | 0.12 | 25% | 0.11 | <0.05 | 0.044 | Wiese & Martin (1980) | ||||
0.038 | Davidson et al. (1990) | ||||||||||
|
![]() |
2575.094 | 1 | 6% | 1 | <0.34 | 0.28 | Wiese & Martin (1980) | |||
0.230 | Davidson et al. (1990) | ||||||||||
|
![]() |
2367.025 | 0.75 | 8% | 0.56 | <0.63 | 0.72 | Wiese & Martin (1980) | |||
0.526 | Davidson et al. (1990) | ||||||||||
|
![]() |
2373.349 | 0.13 | 37% | 0.11 | <0.13 | 0.14 | Wiese & Martin 1980) | |||
0.105 | Davidson et al. (1990) | ||||||||||
|
![]() |
2373.124 | 1 | 7% | 1.00 | <0.76 | 0.86 | Wiese & Martin (1980) | |||
0.631 | Davidson et al. (1990) | ||||||||||
|
![]() |
2263.462 | 0.50 | 14% | 0.56 | <0.59 | 0.66 | Wiese & Martin (1980) | |||
0.576 | Davidson et al. (1990) | ||||||||||
![]() |
2269.220 | ![]() |
0.11 | <0.12 | 0.13 | Wiese & Martin (1980) | |||||
0.115 | Davidson et al. (1990) | ||||||||||
|
![]() |
2269.096 | 1 | 7% | 1.00 | <0.71 | 0.79 | Wiese & Martin (1980) | |||
0.691 | Davidson et al. (1990) |
The level 5s has two decay channels,
p and
p.
Buurman et al. (1986) measured their branching ratios, without resolving the fine
structure, with an error of 10%. For
transitions we evaluated the
transition probabilities assuming theoretical intensity ratios of the
component lines.
The branching ratios of
,
were measured indirectly by (Buurmann & Dönszelmann 1990). They found 2-3% of de-excitation going into
the 3d term. Accounting for this loss we obtained transition probabilities
for the doublet
from respective lifetimes.
Among the triplet
we could observe only the principal lines 3082.153
and 3092.710 Å; the transition probability of the line 3092.839 Å was
then obtained following the LS intensity ratio. Since the transition
3092.710 Å is a single decay channel from the level
,
its transition probability is by definition equal to the reciprocal value of
the lifetime and has got the same error of 2%.
The intensity ratios within multiplets of the simple part of the Grotrian diagram closely follow LS coupling ratios.
For some other
transitions, theoretical multiplet oscillator strengths were published in Buurman et al. (1986); Weiss (1974); Taylor et al. (1988); Trefftz (1988), and experimental ones in Davidson et al. (1990).
The observed multiplets (septets) in the complex part of the Al I
spectrum (Table 3) have, in principle, components of very
different intensities, therefore the accuracy is strongly dependent on the
intensity.
Upper | Lower | ![]() |
Intensity ratio | ||
level | level | measured LS | |||
|
![]() |
3059.924 | 0.01 | 67% | 0.08 |
![]() |
3064.290 | 0.14 | 43% | 0.40 | |
|
![]() |
3054.679 | 0.02 | 53% | 0.04 |
![]() |
3059.029 | 0.01 | 40% | 0.13 | |
![]() |
3066.145 | 0.14 | 24% | 0.43 | |
|
![]() |
3050.073 | 0.41 | 11% | 0.43 |
![]() |
3057.144 | 1 | 25% | 1.00 | |
|
![]() |
2311.035 | 0.11 | 21% | 0.08 |
![]() |
2313.526 | 0.34 | 19% | 0.40 | |
|
![]() |
2312.491 | 0.22 | 23% | 0.40 |
![]() |
2314.983 | 0.09 | 36% | 0.13 | |
![]() |
2319.057 | 0.25 | 21% | 0.43 | |
|
![]() |
2317.482 | 0.35 | 21% | 0.43 |
![]() |
2321.562 | 1 | 16% | 1.00 | |
|
![]() |
2368.112 | 0.20 | 24% | 0.21 |
![]() |
2370.726 | 0.05 | 39% | 0.04 | |
|
![]() |
2367.611 | 0.23 | 35% | 0.21 |
![]() |
2370.225 | 0.27 | 30% | 0.27 | |
![]() |
2374.496 | 0.03 | 44% | 0.03 | |
|
![]() |
2369.304 | 0.57 | 15% | 0.52 |
![]() |
2373.571 | 0.22 | 30% | 0.23 | |
|
![]() |
2372.070 | 0.50 | 17% | 1.00 |
Two of the multiplets show a severe decrease of the relative
intensity for transitions having upper levels
.
The decrease
is caused by autoionization, since these upper levels have the same orbital
and inner quantum numbers and parity as the continuum
.
A weakening of the transitions
was
observed by Eriksson & Isberg (1962). The decrease in intensity
of autoionizing components is not so obvious in the case of the multiplet
due to generally lower
intensities of the transition from this higher term. In case of this
multiplet only components not affected by autoionization obey the
LS coupling ratio. A property of all three multiplets is the absence of
other transitions from their upper terms. By accounting for autoionization,
branching ratios would immediately follow from the intensity ratios.
A very interesting deviation from the LS coupling ratios is found in the
multiplet
.
The component
,
which should be the
strongest, has half of the regular intensity, while all other components
closely follow the LS coupling model. The exceptional component is blended
by a component of another multiplet with much lower intensities, and the quoted
intensity needed only a slight correction.
Considering all multiplets of the observed ionized aluminum (Table 4), the LS coupling ratios are confirmed within the error brackets.
Only principal multiplet lines (
)
have been observed.
Upper | Lower | ![]() |
Intensity ratio | ||
level | level | measured | LS | ||
![]() |
![]() |
3900.675 | 8.04 | ||
![]() |
2081.481 | 0.68 | 9% | 0.6 | |
![]() |
2086.864 | 1 | 5% | 1.0 | |
|
![]() |
2195.502 | 0.42 | 9% | 0.47 |
|
![]() |
2194.245 | 0.65 | 11% | 0.69 |
|
![]() |
2192.604 | 1 | 8% | 1.00 |
|
![]() |
5100.34 | 0.44 | 19% | 0.47 |
|
![]() |
5093.65 | 0.63 | 13% | 0.69 |
|
![]() |
5085.02 | 1 | 8% | 1.00 |
|
![]() |
3587.450 | 0.46 | 13% | 0.47 |
|
![]() |
3587.068 | 0.71 | 10% | 0.69 |
|
![]() |
3586.557 | 1 | 4% | 1.00 |
|
![]() |
2638.690 | 0.51 | 12% | 0.47 |
|
![]() |
2638.155 | 0.60 | 18% | 0.69 |
|
![]() |
2637.689 | 1 | 8% | 1.00 |
|
![]() |
2326.406 | 0.43 | 24% | 0.47 |
|
![]() |
2325.494 | 0.68 | 16% | 0.69 |
|
![]() |
2324.199 | 1 | 13% | 1.00 |
|
![]() |
5867.81 | 0.48 | 39% | 0.47 |
|
![]() |
5861.53 | 0.62 | 39% | 0.69 |
|
![]() |
5853.62 | 1 | 26% | 1.00 |
|
![]() |
2095.140 | 0.48 | 19% | 0.47 |
|
![]() |
2094.790 | 0.67 | 13% | 0.69 |
|
![]() |
2094.264 | 1 | 10% | 1.00 |
In Table 5 we present Al II-branching ratios and transition probabilities obtained in this work. A special precaution was paid to the decay of the level
.
Tayal & Hibbert (1984) calculated, because of
severe cancelations, a very small oscillator strength for the line
3900.68 Å. The length form gives
,
the velocity form
.
We tentatively took the length form with the corresponding
transition probability equal to
.
Then, from our
branching ratios, the lifetime could be estimated to be
s (we cannot assign an error
bracket due to an inherent uncertainty of the theoretical
value; OP gives
s, Butler et al. 1993). Four other theoretical treatments
(Weiss 1967; Zare 1967; Victor et al. 1976; Froese-Fischer & Godefroid 1982) gave very disparate values of
the same oscillator strength.
Upper | Lower | ![]() |
Branching |
![]() |
|||
level | level | ratio | this work other authors | ||||
![]() |
![]() |
3900.675 | 0.79 | 13% |
![]() |
||
![]() |
2081.481 | 0.08 | 48% |
![]() |
|||
![]() |
2086.864 | 0.13 | 42% |
![]() |
|||
|
![]() |
2195.502 | 0.70 | 13% | <2 | 2.14 | Chang & Wang (1987) |
![]() |
5100.34 | 0.30 | 31% | <0.85 | 0.012 | Chang & Wang (1987) | |
|
![]() |
2194.245 | 0.76 | 13% | <2.2 | 2.25 | Chang & Wang (1987) |
![]() |
5093.65 | 0.24 | 40% | <0.7 | 0.013 | Chang & Wang (1987) | |
|
![]() |
2192.604 | 0.72 | 10% | <2.1 | 2.54 | Chang & Wang (1987) |
![]() |
5085.02 | 0.28 | 26% | <0.8 | 0.015 | Chang & Wang (1987) | |
|
![]() |
3587.450 | 0.98 | 3% | <
![]() |
1.98 | Chang & Wang (1987) |
![]() |
3587.3 | 0.02 | 100% | ||||
![]() |
3587.1 | ![]() |
|||||
|
![]() |
3587.068 | 0.96 | 5% | <
![]() |
2.07 | Chang & Wang (1987) |
![]() |
3587.9 | 0.04 | 100% | ||||
![]() |
2635.020 | ![]() |
|||||
|
![]() |
3586.557 | <1.55 | 2.33 | Chang & Wang (1987) | ||
|
![]() |
2638.690 | ![]() |
<0.7 | 0.25 | Chang & Wang (1987) | |
|
![]() |
2638.255 | ![]() |
<0.7 | |||
|
![]() |
2637.689 | ![]() |
<0.7 | 0.29 | Chang & Wang (1987) | |
|
![]() |
2326.496 | 0.18 | 26% | <0.12 | 0.31 | Chang & Wang (1987) |
![]() |
5867.81 | 0.82 | 6% | <0.54 | 0.10 | Chang & Wang (1987) | |
|
![]() |
2325.494 | 0.32 | 15% | <0.21 | 0.33 | Chang & Wang (1987) |
![]() |
5861.53 | 0.68 | 7% | <0.45 | 0.11 | Chang & Wang (1987) | |
|
![]() |
2324.199 | 0.41 | 20% | <0.27 | 0.37 | Chang & Wang (1987) |
![]() |
5853.62 | 0.59 | 20% | <0.39 | 0.12 | Chang & Wang (1987) | |
|
![]() |
2095.140 | 0.97 | 7% | <2 | 1.47 | Chang & Wang (1987) |
![]() |
4589.742 | 0.03 | 100% | ||||
![]() |
![]() |
2094.790 | 0.98 | 4% | <2 | 1.57 | Chang & Wang (1987) |
![]() |
4588.191 | 0.02 | 100% | ||||
|
![]() |
2094.264 | 0.98 | 3% | <2 | 1.74 | Chang & Wang (1987) |
![]() |
4585.817 | 0.02 | 100% | ||||
The OP data for this level are uncertain (Butler et al. 1993). The strong mixing of
and
levels is the reason that the designation was not possible. This confirms the conclusion of Tayal & Hibbert (1984).
For settling the query and to shed more light on
the theoretical treatment, the measurement of the lifetime of the
level would be very important.
The decay of the
(
)
term has several channels
into four lower terms
.
Therefore only a tentative
upper limit to transition probabilities could be set for the measured spectral
lines. We compared the maximum possible experimental value with the theory
of Chang & Wang (1987). The lifetimes obtained by the beam foil
method suffer from statistical errors larger than 10%.
Wiese & Martin (1980) have quoted transition probabilities of ionized aluminum of only two spectral lines in the range from 2000 - 6000 Å, and no line of our Table 5 is on their list.
sublevels can depopulate to 3d and
sublevels, but
transitions to n = 4 are further in the infrared region and likely very weak.
It is interesting that there was also a very weak transition from
to
,
but on the verge of perception. The line
3586.557 Å is the only transition from the
sublevel
and has no error assigned.
Terms
,
,
can
depopulate to several lower terms and we can give only an estimate of the upper
limit of transition probabilities of some lines. A comparison of these upper
estimates with theoretically derived transition probabilities (Chang & Wang 1987)
shows, in some cases, the inadequacy
of the calculated values. It should be noted that we found the depopulation
branches
less intense than branches to the
higher term
,
in contrast to the
calculations of Chang & Wang (1987).
For convenience we added, in Table 6, transition probabilities for some prominent lines which we derived from selected literature data. The lifetime of
the triplet metastable level
has an accuracy within 1% as is shown by agreeing experimental results (Table 1). As it is mentioned above, the lifetime of the singlet resonant level
presented in Table 1 is the average value of the experimental data with an accuracy within 2%.
Upper level | Lower level | ![]() |
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|
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2669.157 |
![]() |
Trabert et al. (1999) |
Johnson et al. (1986) | ||||
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1670.787 | 14.5 | Kernahan et al. (1979) |
Smith (1970) | ||||
Berry et al. (1970) | ||||
Head et al. (1976) | ||||
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2816.185 | 3.80 5% | Tayal & Hibbert (1984) |
Weiss (1967) | ||||
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1990.531 | 13.8 10% | Tayal & Hibbert (1984) |
Weiss (1967) | ||||
Zare (1967) | ||||
Froese-Fischer & Godefroid (1982) | ||||
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4663.056 | 0.575 5% | Tayal & Hibbert (1984) |
Zare (1967) | ||||
Victor et al. (1976) | ||||
Chang & Wang (1987) |
In a similar way we noted the agreement between different theoretical approaches
resulting in the transition probabilities of the Al II spectral lines 2816.185 Å (
), 1990.531 Å (
)
and 4663.056 Å (
).
In most cases we approved LS intensity ratios. Known lifetimes gave us the
possibility to evaluate absolute transition probabilities, either by using
the branching ratios we measured, or using those found in the literature. We
mostly justified LS ratios with some error and are tempted to
consider intensity ratios within multiplets equal to the theoretical ones.
Accounting for the inevitable experimental error of intensity ratios, and
adding it to the error of lifetimes, we likely overestimate the error of the
transition probabilities. The question is, can we deliberately consider the
intensity ratios equal to the theoretical ones? We found some deviations
from LS ratios, sometimes caused by autoionization. Puzzling deviation is
found in neutral aluminum for the component
,
which instead of being the most intense, acquires only half of its regular relative intensity.
Our study showed the importance of further lifetime measurements.
Transitions arising from the complex part of the neutral aluminum term diagram
are prominent and well resolved, and could serve for diagnostic purposes,
but lifetimes of
,
and
are missing. Only theoretical OP data exist (Mendoza et al.
1995), but not for all of these states (for example
). For ionized aluminum, especially
because of the problems associated with the cancelation of integrals during the
calculations, the experimental lifetime of
is urgently needed.