The Astrophysical Journal, 488:L137–L140, 1997 October 20
q 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.
LABORATORY DETECTION AND SUBMILLIMETER SPECTRUM OF THE NaS RADICAL (X2Pi)
B.-Z. Li1 and L. M. Ziurys1,2
Received 1997 May 1; accepted 1997 July 25; 1997 September 25
ABSTRACT
The sodium sulfide radical NaS has been observed in the laboratory for the first time using millimeter/
submillimeter direct absorption spectroscopy. Twelve rotational transitions originating in both the Q 5 1/2 and
3/2 sublevels of this species in its X2Pi ground state were recorded in the frequency range 200–400 GHz. Splittings
arising from lambda-doubling interactions were resolved in both spin-orbit components and were particularly
large (∼2.9 GHz) in the Q 5 1/2 substate. The data were analyzed using a 2P Hamiltonian, and rotational, spinorbit, and lambda-doubling parameters were determined. NaS may be detectable in the late-type star IRC 110216,
given the observation of CS, SiS, NaCl, and NaCN in this object.
Subject headings: ISM: molecules — line: identification — molecular data
be a large fraction of sulfur available to form other molecules
in the outer shell.
Another possible carrier of sulfur in circumstellar envelopes
is the refractory species NaS. Two other sodium-bearing molecules have thus far been detected toward IRC 110216, NaCl,
and NaCN (Cernicharo & Guélin 1987; Turner, Steimle, &
Meerts 1994). Both species appear to arise from the inner envelope of this star and have confined, central distributions (Guélin et al. 1996). Therefore, other sodium-containing molecules
may be present in IRC 110216, especially in the outer shell,
where there are no known carriers as yet. Metal sulfide molecules have to date not been detected in IRC 110216, but
accurate rest frequencies have only existed for a few species,
namely, MgS, CaS, and AlS (Takano, Yamamoto, & Saito 1989,
1991).
Up to the present time, NaS had never been studied in the
laboratory by any spectroscopic method, although high-resolution measurements had been conducted for other small sodium-bearing species. For example, millimeter-wave spectra of
NaO (Yamada, Fujitake, & Hirota 1989) and NaH (Sastry,
Herbst, & DeLucia 1981) have been recorded. Although astronomical searches have been carried out for such molecules
(e.g., Plambeck & Erickson 1982), the lack of spectroscopic
information for NaS made any study of this radical in IRC
110216 not possible.
Here we present the first laboratory detection of NaS and
the measurement of its pure rotational spectrum in the submillimeter wavelength region. Twelve rotational transitions
were recorded for this radical in the frequency range 200–390
GHz in both Q 5 1/2 and Q 5 3/2 spin-orbit components,
verifying that indeed the electronic ground state for this species
is 2Pi. Lambda-doubling interactions were observed in both
spin-orbit ladders as well. We have analyzed these data and
have determined rotational, spin-orbit, and lambda-doubling
parameters for NaS. In this Letter we describe our measurements and present rest frequencies and spectroscopic constants
for sodium sulfide.
1. INTRODUCTION
One of the very important phases in stellar evolution is that
involving high mass loss, i.e., the asymptotic giant branch
(AGB) phase. It is significant because mass loss from AGB
stars is a major source of the heavy elements to the interstellar
medium (e.g., Glassgold 1996). The expelled matter from such
stars is primarily in the form of H2, which cannot easily be
studied. Hence, determining mass-loss rates in circumstellar
shells has been accomplished instead through measurements of
millimeter-wave lines of CO and infrared studies of dust. In
addition, detailed information about the structure of such envelopes has been obtained through high-resolution observations
of numerous chemical species, such as those produced from
the Plateau de Bure interferometer (e.g., Guélin, Lucas, & Neri
1996). Toward the envelope of the late-type carbon star IRC
110216, for example, emission from molecules such as C4H,
C2H, CN, CS, and SiC2 indicates the presence of multiple shells,
clumps, and arclike structures not apparent in CO. Such data
can yield useful information about mass loss from the envelope
of IRC 110216, if the chemistry could be understood.
One chemical puzzle in the shell of IRC 110216 is the
distribution of CS. Plateau de Bure maps (Guelin et al. 1996)
suggest a central source for this molecule and an additional
ring with a radius extending no more than about 100 from the
star. If present, such a ring would be significantly smaller than
the shell-like structures observed in the emission of other species such as C2H, MgNC, and HNC, which all have radii near
200. CS is a common constituent of molecular clouds and is
not very refractory. Therefore, its confined distribution in IRC
110216 is unusual. The sulfur in CS must be channeled into
other molecules in the outer shell. SiS cannot be a possibility,
because this species is even more confined in this object than
CS. The other candidates for sulfur carriers in the outer envelope are C2S, C3S, and possibly H2S, although the detailed
distributions of these molecules in IRC 110216 are at present
not known. However, the combined column densities of these
three species are at least an order of magnitude less than that
of CS (Cernicharo et al. 1987). Consequently, there appears to
2. EXPERIMENTAL
The measurements for NaS were carried out using one of
the millimeter/submillimeter direct absorption spectrometers of
the Ziurys group. This instrument consists of a phase-locked
millimeter/submillimeter source (Gunn oscillator/Schottky diode multiplier), a gas cell containing a Broida-type oven, and
1
Department of Chemistry, Arizona State University, Tempe, AZ 852871604.
2
Departments of Astronomy and Chemistry, and Steward Observatory,
University of Arizona, Tucson, AZ 85721.
L137
L138
LI & ZIURYS
Vol. 488
TABLE 1
Observed Transition Frequencies of NaS (X2Pi)a
2
2
P3/2
P1/2
J r J9
Parity
nobs
nobs 2 ncalc
nobs
nobs 2 ncalc
20.5 r 21.5 . . . . . .
e
f
e
f
e
f
e
f
e
f
e
f
e
f
e
f
e
f
e
f
e
f
261771.159
261767.975
273910.631
273907.085
286045.152
286041.253
298174.589
298170.254
310298.680
310293.977
322417.158
322412.090
334529.834
334524.382
346636.530
346630.705
358736.983
358730.716
370831.029
370824.330
382918.385
382911.251
0.110
0.201
0.075
0.123
0.002
0.030
20.020
20.079
20.027
20.088
20.068
20.113
20.102
20.134
20.088
20.080
20.063
20.067
0.040
0.049
0.151
0.188
261204.163
263133.566
273362.444
275289.324
285515.771
287439.862
297663.888
299585.071
309806.640
311724.634
321943.666
323858.340
334074.866
335985.936
346199.970
348107.191
358318.753
360221.847
370430.977
372329.608
382536.417
384430.213
20.140
20.153
20.092
20.061
20.031
20.044
0.008
0.026
0.092
0.060
0.089
0.089
0.108
0.096
0.093
0.107
0.065
0.077
20.031
20.020
20.182
20.195
21.5 r 22.5 . . . . . .
22.5 r 23.5 . . . . . .
23.5 r 24.5 . . . . . .
24.5 r 25.5 . . . . . .
25.5 r 26.5 . . . . . .
26.5 r 27.5 . . . . . .
27.5 r 28.5 . . . . . .
28.5 r 29.5 . . . . . .
29.5 r 30.5 . . . . . .
30.5 r 31.5 . . . . . .
a
In MHz.
a helium-cooled InSb detector. Phase-sensitive detection is accomplished through modulation of the source. For more details,
see Ziurys et al. (1994).
Sodium sulfide was created in a DC discharge by the reaction
of carbon disulfide and sodium vapor. The metal vapor was
produced with a Broida-type oven operated near 1007C and
entrained in approximately 20 mtorr of argon carrier gas. About
2–4 mtorr of CS2 was then added through the oven to this
mixture, which was then discharged. The discharge current used
was 50 mA, and the voltage ranged between 100 and 200 V,
depending on the amount of waste product coating the cell
walls. The sodium/argon mixture glowed a bright yellow when
discharged, probably from sodium D line emission; addition
of CS2 caused a weaker blue glow. NaS was also produced by
using H2S instead of C2S as a precursor, but the observed lines
were not as strong.
The transition frequencies were measured by fitting Gaussian
curves to the line profiles, recorded in scans 5 MHz in total
coverage. Line widths were typically 500–800 kHz over the
range 200–400 GHz.
3. RESULTS AND ANALYSIS
The transition frequencies obtained for NaS are listed in
Table 1. As the table illustrates, 11 rotational transitions were
measured for NaS in each of the two spin-orbit ladders, Q 5
1/2 and Q 5 3/2. Each transition consists of two lines because
of lambda doubling. Hence, 44 separate frequencies were recorded. Each set of lambda doublets is labeled by the parity
notation “e” and “f.” As is also evident from the table, the
lambda doubling in the Q 5 3/2 sublevel is small, ranging from
3 to 7 MHz for the transitions studied, and increasing in magnitude with increasing J. In contrast, the splitting in the Q 5
1/2 levels is much larger (∼1.9 GHz) and decreases with increasing J value. No evidence of hyperfine (hf) interactions
was observed in the data, which would arise from the sodium
nuclear spin of I 5 3/2. The absence of hf splitting is not
unexpected; it was seen only at very low J values (J ≤ 9/2) in
NaO (Yamada et al. 1989).
Spectra illustrating these interactions are shown in Figure 1,
which displays the J 5 61/2 r 63/2 transition of NaS near
382–384 GHz. The closely spaced set of lines in the lower
spectrum are the lambda doublets of the Q 5 3/2 ladder, labeled
by e and f. The top spectrum, which has a large frequency gap,
shows the same transition in the Q 5 1/2 sublevel. Here the
lambda doublets, again indicated by e and f, are separated by
1.893 GHz. The bottom spectrum covers 110 MHz in frequency
and was taken in one 50 s scan; the top panel is two catenated
scans, each taken in approximately 30 s.
The y-axis scale on the two spectra in Figure 1 is arbitrary.
In reality, the Q 5 3/2 lines are stronger than the Q 5 1/2
transitions. This effect clearly identifies the 2P ground state of
NaS to be inverted i.e., the Q 5 3/2 component lies lower in
energy than the Q 5 1/2 sublevel. (To first order, lambda doubling enters in only as an off-diagonal term in the Hamiltonian
for Q 5 3/2 substate, as opposed to a diagonal one for the Q
5 1/2 levels; hence, the lines with the smaller L-doubling are
readily assigned to the Q 5 3/2 substate). A 2Pi ground state
for NaS is consistent with its K (sj)2 (pj)2 (pp)3 electron configuration; also, its isovalent counterpart, NaO, has an inverted
2
P ground state (Yamada et al. 1989).
The spectra were analyzed using an effective 2P Hamiltonian
of the following form:
Hef f 5 Hrot 1 Hso 1 HLD
(1)
(e.g., Brown et al. 1978; Brown & Merer 1979). The terms in
this expression define the rotation (Hrot ), the spin-orbit coupling
(Hso ), and the lambda-doubling (HLD ) interactions in the molecule. The individual parts of this Hamiltonian can be expressed
No. 2, 1997
DETECTION AND SPECTRUM OF THE NaS RADICAL (X2Pi)
L139
TABLE 2
Spectroscopic Constants for NaS (X2Pi)a
Parameter
NaS
NaOb
A ..............
AD . . . . . . . . . . . . .
AH . . . . . . . . . . . . .
B ..............
D ..............
H # 108 . . . . . .
p ..............
pD . . . . . . . . . . . . .
pH # 106 . . . . . .
q ..............
qD . . . . . . . . . . . . .
28,005,000 (11,000)
20.275 (13)
0.0001880 (84)
6100.797 (30)
0.009169 (42)
22.7 (1.9)
1,949.8 (3.0)
20.0198 (22)
20.0026 (13)
2.30 (17)
0.000038 (90)
23,212,300 (18,000)
0.06 (54)
)
12662.6762 (36)
0.037783 (27)
4.7 (6.3)
2650.11 (22)
20.27323 (87)
15.2 (2.7)
18.687 (81)
20.00105 (21)
a
In MHz; errors are 3 j and apply to the last quoted decimal
places.
b
From Yamada, Fujitake, & Hirota 1989.
Fig. 1.—Spectra of the J 5 61/2 r 63/2 transition of NaS observed in this
work near 382–384 GHz. The bottom spectrum is the Q 5 3/2 spin-orbit
component, which is split into two features separated by about 7 MHz because
of lambda doubling. The top panel shows the Q 5 1/2 component, which also
consists of two lines arising from lambda doubling but which are split by
nearly 2.9 GHz. (The spectrum has a frequency gap in it.) The doublets in
each Q component are labeled by “e” and “f” parity designations.
as
Hrot 5 B(J 2 L 2 S) 2 2 D(J 2 L 2 S) 4 1 H(J 2 L 2 S) 6,
(2)
Hso 5 AL · S 1 1/2AD
# [(J 2 L 2 S) 2L Z SZ 1 L Z SZ (J 2 L 2 S) 2 ]
1 1/2AH [(J 2 L 2 S) 4L Z SZ 1 L Z SZ (J 2 L 2 S) 4 ],
4. DISCUSSION
(3)
HLD 5 1/2( p 1 2q)(J1 S1 1 J2 S2 ) 2 1/2q(J12 1 J22 ).
and q are the lambda-doubling parameters described by Brown
et al. (1978), which are basically the Mulliken & Christy (1931)
definitions but contain higher order contributions. Several centrifugal distortion terms (pD, pH, and qD) were also found necessary to fit to the lambda-doubling splittings. The pD and qD
parameters are first-order corrections to p and q, and their corresponding matrix elements can be found in Brown et al.
(1978). The pH constant is the second-order correction to p; its
matrix elements were derived from those for p and pD. Similar
pD, pH, and qD terms were used to analyze the X2P spectrum
of NaO of Yamada et al. (1989), where very large L-doubling
was observed in the Q 5 1/2 ladder, similar to NaS.
The spectroscopic parameters derived using this Hamiltonian
in a nonlinear least-squares analysis are listed in Table 2. It
was found in fitting the data that the q and pH parameters were
highly correlated, as were A and AD. Hence, A and pH were
held fixed while q and AD were allowed to float, and then vice
versa. This procedure was done over several iterations. In the
final fit, q and AD were fixed to values determined from the
past iterations, and the values of all other constants varied. As
the table shows, the final analysis resulted in rotational and
spin-orbit constants, as well as the lambda-doubling parameters, being well determined. Unfortunately, there are no other
constants, optical, theoretical, or otherwise, for comparison.
However, the constants for NaS scale as expected from those
of NaO, also shown in the table. Furthermore, they reproduce
the observed frequencies to nobs 2 ncalc ≤ 200 kHz, with the residuals for the majority of lines being ≤100 kHz, the estimated
experimental accuracy. The rms of the data fit is 99 kHz.
(4)
In the first equation, B is the rotational constant, and D and H
are the first- and second-order centrifugal distortion parameters.
In the second expression, A is the spin-orbit constant, and
AD and AH define first- and second-order centrifugal corrections
to the spin-orbit energy. The AH term was included because its
use resulted in a significantly better fit; it is also justified because of the high J levels involved in the analysis. For HLD, p
NaS has usually large lambda doubling in the Q 5 1/2 sublevel, very similar to NaO (Yamada et al. 1989). The p parameter is in fact 1.9498 GHz. The large splitting is very likely
due to perturbations by a low-lying 2S electronic state, whose
energy above the X2P state can be estimated. If the case of
pure precession is assumed, then the energy of the sigma state
above the ground state can be calculated from the lambdadoubling parameter p via the following expression:
p 5 4AB/(E P 2 E S ).
(5)
This equation yields E S 2 E P ∼ 3342 cm21. In contrast,
L140
LI & ZIURYS
E S 2 E P for NaO is 2050 cm21, using identical assumptions
(Yamada et al. 1989).
Although the sodium 23 nucleus has a spin (I 5 3/2), no hf
splittings were observed in the data. Such a result is expected,
as mentioned, because only higher J transitions were observed
in this study, and the hf structure should collapse with increasing J. Moreover, the hf constants found for NaO were very
small. In fact, the hf parameters found for NaO indicate that
only 0.7% of the electron density in this radical lies on the Na
atom, indicating a very ionic bond. Similar bonding is expected
for NaS. The r0 bond length derived for NaS is 2.488 Å.
Another point to note in this study is that higher order spinorbit and lambda-doubling terms were necessary to obtain an
Vol. 488
acceptable fit. Although addition of nonphysical higher order
parameters is certainly not desirable, the fact that transitions
with high J and a rather wide range of J values (J 5 41/2 to
J 5 63/2) were analyzed probably justifies use of the AH, pD,
pH, and qD terms.
Finally, these frequency measurements will allow astronomical searches for NaS to be conducted in the interstellar medium, especially toward the envelope of IRC 110216. Observations of SiS, CS, NaCl, and NaCN in this object suggest that
NaS may be present as well.
This research was supported by NSF grant AST-95-03274
and NASA grant NAGW 2989.
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