The Astrophysical Journal, 713:520–523, 2010 April 10
C 2010.
doi:10.1088/0004-637X/713/1/520
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE SUBMILLIMETER SPECTRUM OF AlD (X 1 Σ+ )
D. T. Halfen and L. M. Ziurys
Department of Chemistry, Department of Astronomy, Arizona Radio Observatory, and Steward Observatory, University of Arizona,
933 N. Cherry Avenue, Tucson, AZ 85721, USA
Received 2010 January 19; accepted 2010 March 1; published 2010 March 23
ABSTRACT
The J = 2 ← 1 and 3 ← 2 rotational transitions of AlD (X 1 Σ+ ) near 393 and 590 GHz have been measured
using submillimeter direct absorption spectroscopy. AlD was created in an AC discharge of Al(CH3 )3 and D2 in
the presence of argon. This work is the first direct measurement of the J = 3 ← 2 transition of AlD. Each transition
was found to be split into multiple hyperfine components due to the 27 Al nuclear spin of I = 5/2, some of which
are blended together. The data for AlD were fit with an effective Hamiltonian and rotational, electric quadrupole,
and nuclear spin-rotation constants were determined for the molecule. The accuracy of these constants has been
improved by a factor of 2–3, compared to previous studies. From these data, predictions for the J = 1 → 0 and
4 → 3 transitions of AlD have also been made. Photospheric AlH has been observed via its A 1 Π-X 1 Σ+ electronic
transition, suggesting that this species may be present in circumstellar gas surrounding late-type stars, where
four aluminum-bearing molecules have already been detected. High deuterium enrichment has been observed
in hydride species in molecular clouds, making AlD a feasible candidate for searches in these objects as well.
Key words: astrochemistry – ISM: molecules – line: identification – methods: laboratory – molecular data – stars:
AGB and post-AGB
(1992) used laser–diode spectroscopy to measure the v = 1–0,
2–1, 3–2, 4–3, 5–4, 6–5, and 7–6 vibrational sequence of
AlD at infrared wavelengths. White et al. (1993) subsequently
conducted emission studies of AlD using Fourier transform
infrared spectroscopy at high resolution (±0.0002 cm−1 or
±6 MHz), obtaining rotational and vibrational spectroscopic
parameters for this molecule. The most recent investigation has
been the measurement of the J = 2 ← 1 rotational transition
near 393 GHz by Halfen & Ziurys (2004) using direct absorption
methods. This line is split into multiple quadrupole hyperfine
transitions, and six out of a possible nine components were
recorded in this work. Other rotational transitions have not been
directly measured.
Although the nominal D/H ratio in the ISM is ∼10−5
(Boesgaard & Steigman 1985), past observations of molecules
have shown that high deuterium enrichment occurs often in
colder clouds, in particular for hydrides. This enrichment occurs
because the low temperatures in the ISM heavily favor the
formation of H2 D+ . This ion then exchanges H for D in hydride
species, i.e.,
H2 D+ + AlH → H+3 + AlD.
(1)
1. INTRODUCTION
A variety of molecules containing metallic elements (in the
chemist’s sense) have been observed in the circumstellar envelopes of late-type stars, including asymptotic giant branch
(AGB) stars, protoplanetary nebulae (PPNe), and red supergiants (RSGs). Metal halides, cyanides, and oxides have been
identified, such as MgNC, NaCN, KCl, and AlO (Cernicharo
& Guélin 1987; Turner et al. 1994; Kawaguchi et al 1993;
Tenenbaum & Ziurys 2009). These observations have shown
that aluminum appears to be the dominant metal in astrophysical molecules, with four such species (AlCl, AlF, AlNC, and
AlO) detected in either carbon- or oxygen-rich circumstellar
material (Cernicharo & Guélin 1987; Ziurys et al. 1994, 2002;
Highberger et al. 2001; Tenenbaum & Ziurys 2009). Aluminum
has a cosmic elemental abundance of 3 × 10−6 , as opposed to
4 × 10−5 for both silicon and magnesium (Grevesse & Sauval
1998).
While simple diatomic hydrides are the logical choice of
molecular metal carriers, such species have remained elusive in
the interstellar medium (ISM). The main difficulty in studying
hydrides in interstellar and circumstellar environments is that
their spectra occur at submillimeter and infrared wavelengths,
where atmospheric transmission is poor or non-existent. However, with Herschel and other upcoming space observatories, observations in the submillimeter/far-infrared will likely be more
fruitful, making hydrides viable targets.
The interesting metal hydride AlH has been well characterized in its X 1 Σ+ ground electronic state. The A 1 Π-X 1 Σ+ transition was comprehensively studied by Zeeman & Ritter (1954)
and later by Ram & Bernath (1996), while ro-vibration spectra
of the X 1 Σ+ state, from v = 1–0 to v = 5–4, were recorded and
analyzed by White et al. (1993). More recently, the J = 1 ← 0
rotational transition was measured by Goto & Saito (1995) and
then by Halfen & Ziurys (2004).
AlD, the deuterated form of this molecule, was studied in
the optical region by Holst & Hulthén (1934), who observed
the A 1 Π–X 1 Σ+ transition. More recently, Urban & Jones
Recently, in fact, triply deuterated ammonia, ND3 , has been
identified in Barnard 1 and NGC 1333 (Lis et al. 2002; van der
Tak et al. 2002). Measurement of the spectrum of AlD, therefore,
is of astrophysical interest.
In order to complete the spectroscopic characterization of
AlD, we have recorded the J = 3 ← 2 transition of this species
near 590 GHz, the first direct measurement of this line, as well
as several additional hyperfine components of the J = 2 ← 1
transition near 393 GHz. We have used the data to determine an
accurate set of spectroscopic constants for AlD, and have made
additional frequency predictions. Here we describe our results
and spectral analysis.
2. EXPERIMENTAL DETAILS
The pure rotational spectrum of AlD was measured using
millimeter/submillimeter direct absorption methods. The spec520
THE SUBMILLIMETER SPECTRUM OF AlD (X 1 Σ+ )
No. 1, 2010
521
Table 1
Observed Rotational Transitions of AlD (X 1 Σ+ )
J
←
J
F
←
F
ν obs
(MHz)
ν obs– ν calc
(MHz)
Relative
Intensitya
2
←
1
2.5
3.5
2.5
1.5
4.5
3.5
0.5
2.5
1.5
←
←
←
←
←
←
←
←
←
1.5
3.5
3.5
1.5
3.5
2.5
1.5
2.5
2.5
393651.388
393653.213
393655.198
393658.084
393663.416b
393663.416b
−0.067
0.082
0.011
−0.050
0.420
−0.306
393665.791
393672.413
0.013
−0.045
6.22
9.52
1.59
9.33
33.33
17.14
6.67
12.19
4.00
4.5
1.5
2.5
3.5
0.5
3.5
5.5
4.5
1.5
2.5
←
←
←
←
←
←
←
←
←
←
4.5
0.5
1.5
2.5
0.5
3.5
4.5
3.5
1.5
2.5
590306.198
590310.729b
590310.729b
590313.390
−0.016
0.393
−0.293
−0.149
3
←
2
c
c
590316.070b
590316.070b
590316.070b
590316.070b
c
0.476
0.347
−0.008
−0.358
4.41
2.96
6.86
12.20
3.71
6.45
28.60
19.40
5.42
6.61
Notes.
a Townes & Schawlow (1975).
b Blended lines.
c Contaminated.
trometer in the Ziurys group that was used in these experiments consists of a radiation source, a gas cell, and a detector
(Savage & Ziurys 2005). The sources are various combinations
of Gunn oscillators and Schottky diode multipliers that are used
to generate frequencies from 65 to 850 GHz. The single-pass
spectrometer cell consists of a 0.8 m long glass tube 10 cm in
diameter with two ring discharge electrodes at either end. The
cell is cooled down to −65 ◦ C with chilled methanol. A heliumcooled InSb bolometer is used to detect the radiation after it
passes through the cell.
Aluminum deuteride was produced in the gas phase by the
reaction of Al(CH3 )3 and D2 in the presence of argon carrier
gas and an AC discharge. A mixture of 40 mtorr Ar, 25 mtorr
of D2 , and ∼4 mtorr of Al(CH3 )3 with a discharge power of
200 W was found to optimize signals. Unfortunately, the
J = 1 ← 0 transition of AlD near 196.8 GHz could not
be recorded because of strong contamination by vibrationally
excited AlO, probably created from residual H2 O in the cell.
The rest frequencies of AlD were recorded using pairs of
scans 5 MHz wide; one scan increasing in frequency and one
scan decreasing in frequency. These scans were then averaged
together; usually 5–20 scan pairs were needed to acquire an
adequate signal-to-noise ratio. The lines were modeled with
Gaussian profiles to determine the center frequencies, as well
as the line widths, which ranged from 0.9 to 2.1 MHz over the
region 393–590 GHz. The experimental error is estimated to be
±100 kHz.
3. RESULTS AND DISCUSSION
The J = 2 ← 1 and 3 ← 2 rotational transitions of AlD were
measured near 393.6 GHz and 590.3 GHz, respectively. The 27 Al
nucleus has a spin of I = 5/2, and thus has both quadrupole and
magnetic moments. Therefore, each rotational transition of AlD,
labeled by quantum number J, is additionally split into multiple
aluminum hyperfine components, designated by the F quantum
number, where F = J + I. The rest frequencies recorded for
the hyperfine lines of AlD (X1 Σ+ ) are listed in Table 1. Also
listed in the table are the predicted relative intensities for each
hyperfine component (Townes & Schawlow 1975). Note that
the deuterium nucleus also has a spin of I = 1, but both its
quadrupole and magnetic moments are smaller than those of
27
Al by about 1–2 orders of magnitude (Townes & Schawlow
1975). Therefore, the deuterium contribution to the hyperfine
splitting is negligible.
In Figure 1, the J = 2 ← 1 transition (upper panel) and
the J = 3 ← 2 line (lower panel) of AlD measured in
this work are shown. Each of these transitions consists of
multiple hyperfine components, whose positions and relative
intensities are indicated underneath the spectra. For the J =
2 ← 1 transition, eight out of nine possible hyperfine lines
were measured, two of which were blended. The remaining
component has a low relative intensity (6.67), but in principle
is still strong enough to be detected; unfortunately, it is masked
by the second-derivative profile of the stronger, blended F =
4.5 ← 3.5 and 3.5 ← 2.5 lines. For the J = 3 ← 2 transition,
8 of the 14 possible hyperfine components were observed, six
of which create two blended features. Thus, only four distinct
lines are apparent in the data for the J = 3 ← 2 transition. Of
the remaining six hyperfine components, two are contaminated
by the profile of the four blended lines (see Figure 1). The
remaining four transitions have relative intensities of 1.1 or less
(F – 1 ← F), and are too weak to be recorded.
The spectroscopic parameters for AlD were determined from
a fit to the data using an effective Hamiltonian consisting of
rotational, electric quadrupole, and nuclear spin-rotation terms:
Ĥeff = Ĥrot + ĤeQq + Ĥnsr .
(2)
The spectroscopic constants established from the analysis are
listed in Table 2, along with those derived from the past
submillimeter and infrared measurements. These constants are
522
HALFEN & ZIURYS
J=2←1
1 +
AlD (X Σ )
F = 4.5 ← 3.5
F = 3.5 ← 2.5
F = 2.5 ← 1.5
F = 3.5 ← 3.5
F = 2.5 ← 3.5
F = 1.5 ← 1.5
393653
Vol. 713
1 +
J=1→0
F = 2.5 → 2.5
F = 2.5 ← 2.5
F = 1.5 ← 2.5
393663
J=3←2
F = 3.5 ← 2.5
F = 1.5 ← 0.5
F = 2.5 ← 1.5
F = 4.5 ← 4.5
393673
F = 1.5 → 2.5
196833
196863
J=4→3
F = 3.5 ← 3.5
F = 5.5 ← 4.5
F = 4.5 ← 3.5
F = 1.5 ← 1.5
F = 5.5 → 5.5
590304
590314
590324
Frequency (MHz)
Figure 1. Laboratory spectra of the J = 2 ← 1 (upper panel) and
3 ← 2 (lower panel) rotational transitions of AlD (X 1 Σ+ ) near 393.6 and
590.3 GHz, respectively. Each transition consists of multiple aluminum hyperfine components, whose positions and relative intensities are indicated by
vertical lines underneath the data. The most intense features are blends of the
several strong hyperfine components. These spectra are averages of 48 and 80
separate scans, respectively, each with a duration of about 30 s and covering
55 MHz, then cropped to display a 30 MHz region.
Table 2
Spectroscopic Constants for AlD (X 1 Σ+ )
Parameter
B
D
eQq
CI
rms
AlD (X Σ )
F = 3.5 → 2.5
This Work
Previous Worka
FTIRb
98439.108(32)
2.9609(20)
−48.64(37)
0.108(22)
0.072
98439.140(21)
2.96178 c
−48.48(88)
0.156(41)
0.059
98439.147(99)
2.96178(23)
Notes. Values are in MHz; errors quoted are 3σ in the last quoted digits.
a Halfen & Ziurys (2004).
b White et al. (1993).
c Held fixed.
the most accurate for AlD to date. With the additional transitions
of AlD measured in this work, the values of B, eQq, and CI
could be refined by factors of 2–3, and the centrifugal distortion
constant D was able to be independently established from pure
rotational data for the first time. The rms of the fit is 72
kHz. Note that the FTIR measurements of White et al. (1993)
did not have the spectral resolution to resolve the hyperfine
splittings. However, because of the uncertainties in the hyperfine
constants and the fact that only two rotational transitions were
786724
196893
F = 2.5 → 1.5
F = 3.5 → 1.5
F = 1.5 → 0.5
F = 4.5 → 4.5
F = 4.5 → 3.5
F = 6.5 → 5.5
F = 5.5 → 4.5
F = 3.5 → 3.5
F = 1.5 → 1.5
F = 2.5 → 2.5
786754
786784
Frequency (MHz)
Figure 2. Simulated spectra of the J = 1 → 0 (upper panel) and 4 → 3
(lower panel) rotational transitions of AlD (X 1 Σ+ ) near 196.8 and
786.7 GHz, respectively. The frequencies and relative intensities of the hyperfine
components are indicated by vertical lines underneath the data. The individual
hyperfine components in each transition are modeled with a Gaussian line shape
assuming a line width of 1 km s−1 . The three hyperfine components of the
J = 1 → 0 transition are well resolved, while most of the components of the
J = 4 → 3 transition are blended together into a single feature.
measured, the distortion constant determined here may have a
small systematic deviation. Fixing the value of D to that from
White et al. (1993), changes the hyperfine constants by a few
kHz; the rms of the fit is then 75 kHz.
With these improved spectroscopic constants, the rotational
rest frequencies of the J = 1 → 0 and 4 → 3 transitions near
196.8 and 786.7 GHz have been predicted. These frequencies
are listed in Table 3, along with the relative intensities of the
hyperfine components. Simulated spectra of these transitions
are shown in Figure 2, assuming line widths of 1 km s−1
Three hyperfine components could possibly be resolved for the
J = 1 → 0 transition (top panel), while most of the hyperfine
lines for the J = 4 → 3 transition are blended into one feature
(bottom panel). Using the constants determined from the fit with
the D value of White et al., the predictions for the J = 1 → 0
and 4 → 3 transitions shift in frequency by a maximum of 30
and 140 kHz, respectively, not significant for most astronomical
observations.
A reliable set of rest frequencies is now available for AlD to
be used for submillimeter observations. The v = 0–0, 1–0, and
1–1 bands of the A 1 Π–X 1 Σ+ transition of AlH near 4241,
4066, and 4353 Å, respectively, have been observed in the
THE SUBMILLIMETER SPECTRUM OF AlD (X 1 Σ+ )
No. 1, 2010
Table 3
Predicted Rotational Rest Frequencies of AlD (X 1 Σ+ )
J
→
J
F
→
F
ν obs (MHz)
Relative Intensitya
1
→
0
2.5
3.5
1.5
→
→
→
2.5
2.5
2.5
196858.48
196869.07
196872.81
33.33
44.44
22.22
4
→
3
5.5
2.5
3.5
1.5
4.5
4.5
6.5
5.5
3.5
1.5
2.5
→
→
→
→
→
→
→
→
→
→
→
5.5
1.5
2.5
0.5
4.5
3.5
5.5
4.5
3.5
1.5
2.5
786746.20
786752.84
786753.32
786753.57
786754.07
786754.56
786755.52
786755.71
786757.48
786757.79
786758.25
2.53
7.14
10.40
4.76
3.85
14.60
25.90
19.70
4.16
2.38
3.63
523