V
THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 134 : 319È321, 2001 June
( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE MILLIMETER- AND SUBMILLIMETER-WAVE SPECTRUM OF GLYCOLALDEHYDE (CH OHCHO)
2
REBECCA A. H. BUTLER AND FRANK C. DE LUCIA
Department of Physics, The Ohio State University, Columbus, OH 43210-1106
DOUG T. PETKIE
Department of Physics, Ohio Northern University, Ada, OH 45810
HARALD MÔLLENDAL AND ANNE HORN
Department of Chemistry, University of Oslo, Blindern, Oslo, Norway
AND
ERIC HERBST
Departments of Physics and Astronomy, The Ohio State University Columbus, OH 43210-1106 ; herbst=mps.ohio-state.edu
Received 2000 November 28 ; accepted 2001 January 18
ABSTRACT
The simplest monosaccharide, glycolaldehyde (CH OHCHO), has recently been detected toward the
2
Galactic center in the source Sgr B2(N) at Ðve frequencies from 71È104 GHz. None of the individual
lines used in the detection had been measured previously in the laboratory ; rather, their frequencies were
predicted based on lower frequency measurements. We have now recorded and analyzed many new rotational transitions of glycolaldehyde through 354 GHz using two spectrometers. Lines through 48 GHz in
frequency were measured with a spectrometer that uses Stark modulation, while the higher frequency
transitions were measured with a FASSST (Fast Scan Submillimeter Spectroscopic Technique) apparatus. Analysis of the data has allowed us to conÐrm the interstellar identiÐcations and to predict the
frequencies of many additional lines not measured in the laboratory.
Subject headings : ISM : molecules È methods : laboratory È molecular data È radio lines : ISM
On-line material : machine-readable tables
1.
INTRODUCTION
At a low temperature (B10 K), most of the molecules
produced on dust grains remain in the condensed phase and
form large mantles, perhaps 100 monolayers in thickness
(Ehrenfreund & Schutte 2000). During the process of star
formation, however, the dust grains in the vicinity of the
young stellar object can be heated sufficiently to lose their
mantles, changing the molecular composition of the gas
phase dramatically. The picture is not a static one since the
molecules released into the gas phase can then undergo a
gas-phase chemistry at the prevailing temperatures of
100È300 K for the B105 yr lifetime of the hot core.
Two types of models have been used to study the chemistry of hot cores. In one, the grain surface chemistry is
ignored and initial gas-phase abundances are assumed following the period of mantle evaporation such that the Ðnal
gas-phase abundances best Ðt the observed data (Charnley,
Tielens, & Millar 1992). In the second, the gas-phase and
grain chemistries of the cold ambient interstellar cloud are
followed, and the material that evaporates into the gas
when temperatures increase is determined by the chemistry
of the earlier cold era (Caselli, Hasegawa, & Herbst 1993).
In both types of models, the current picture is one in which
more complex species are produced mainly from the precursor molecule methanol via gas-phase ion-molecule, radiative association, and dissociative recombination reactions.
Although gas-phase syntheses of molecules such as methyl
formate have been suggested (Millar, Herbst, & Charnley
1991), the synthesis of glycolaldehyde is currently unknown.
Moreover, it is not obvious that methanol is the most
complex species that is released into the gas with the general
evaporation of grain mantles. Indeed, it is quite possible
that more complex species such as glycolaldehyde are themselves formed on grains. More work is clearly needed to
understand the syntheses of the saturated species found in
hot cores.
Glycolaldehyde (CH OHCHO), an isomer of methyl
formate and acetic acid2and the simplest monosaccharide,
has recently been detected toward the Galactic center cloud
Sgr B2(N) (Hollis, Lovas, & Jewell 2000). Based on previous
studies of molecular abundances toward this source with
interferometric techniques, glycolaldehyde probably exists
in a small object known as the ““ Large Molecule Heimat ÏÏ
(LMH ; Snyder 1997). The LMH is a hot molecular core
about 0.1 pc in diameter, in which a variety of hydrogenrich (saturated) organic molecules, such as methanol,
methyl formate, vinyl cyanide, and ethyl cyanide have
already been detected (Snyder 1997 ; Pei, Liu, & Snyder
2000). It is generally thought that the saturated molecules in
hot cores are synthesized at least partially on interstellar
dust grains in a previous low-temperature era (Brown,
Charnley, & Millar 1988 ; Millar, Herbst, & Charnley 1991).
Unlike low-temperature ion-molecule chemistry, which
leads invariably to unsaturated (hydrogen-poor) species
(Lee, Bettens, & Herbst 1996), surface chemistry can
produce saturated species such as water, ammonia,
methane, and methanol via successive hydrogenation reactions with atomic hydrogen (Hasegawa, Herbst, & Leung
1992). For example, methanol is probably formed from CO
by the following sequence of surface reactions (Charnley,
Tielens, & Rodgers 1997) :
H ] CO ] HCO ,
(1)
H ] HCO ] H CO ,
(2)
2
H ] H CO ] H CO ,
(3)
2
3
H ] H CO ] CH OH ,
(4)
3
3
despite the fact that reactions (1) and (3) possess at least
some activation energy.
319
320
J@
8 ....
34 . . .
34 . . .
19 . . .
21 . . .
21 . . .
19 . . .
32 . . .
8 ....
6 ....
8 ....
7 ....
43 . . .
43 . . .
23 . . .
23 . . .
17 . . .
36 . . .
36 . . .
17 . . .
BUTLER ET AL.
Vol. 134
TABLE 1
TABLE 2
MEASURED TRANSITION FREQUENCIES OF GLYCOLALDEHYDEa
SPECTROSCOPIC PARAMETERS FOR
GLYCOLALDEHYDE
K@
a
3
17
17
8
11
11
8
14
5
1
5
1
19
19
12
12
7
18
18
7
K@
c
5
17
18
12
10
11
11
19
4
6
3
7
24
25
11
12
11
18
19
10
J@@
7
35
35
18
22
22
18
31
9
5
9
6
42
42
24
24
16
37
37
16
K@@
a
4
16
16
9
10
10
9
15
4
2
4
2
20
20
11
11
8
17
17
8
K@@
c
4
20
19
9
13
12
10
16
5
3
6
4
23
22
14
13
8
21
20
9
Frequency
(MHz)
Obs. [ Calc.
(MHz)
7805.830
8154.390
8154.390
8176.950
8181.510
8181.510
8188.740
8228.890
8254.730
8516.690
8738.800
9742.620
9876.970
9876.970
10184.780
10184.780
10204.580
10230.210
10230.210
10252.520
0.060
[0.023
[0.023
0.109
[0.116
0.155
0.003
0.008
0.004
0.027
0.004
[0.014
[0.015
[0.015
0.082
0.144
0.027
0.009
0.009
0.046
Parameter
Valuea
A (MHz) . . . . . . . . . . . . . . .
18446.26097 (81)
B (MHz) . . . . . . . . . . . . . . . .
6525.996543 (316)
C (MHz) . . . . . . . . . . . . . . .
4969.234992 (297)
* (kHz) . . . . . . . . . . . . . . . .
6.2221931 (252)
J
* (kHz) . . . . . . . . . . . . . .
[20.39579 (122)
JK
* (kHz) . . . . . . . . . . . . . . .
47.72286 (187)
K
d (kHz) . . . . . . . . . . . . . . . .
1.834147 (98)
J
d (kHz) . . . . . . . . . . . . . . . .
8.87443 (266)
K
' (mHz) . . . . . . . . . . . . . . .
[6.60491085 (10076415)
J
' (Hz) . . . . . . . . . . . . . . . .
0.156937114 (2502875)
JK
' (Hz) . . . . . . . . . . . . . . . .
[0.770296110 (7150179)
KJ
' (Hz) . . . . . . . . . . . . . . . . .
1.05485373 (555354)
K
/ (mHz) . . . . . . . . . . . . . . .
2.40191260 (5230127)
J
/ (mHz) . . . . . . . . . . . . . .
14.7219844 (22964112)
JK
/ (Hz) . . . . . . . . . . . . . . . . .
0.185831236 (21878345)
K
a Standard error in units of last digit in parenthesis.
NOTE.ÈTable 1 is available in its entirety in the electronic edition of the
Astrophysical Journal. A portion is shown here for guidance regarding its
form and content.
a Lines with frequencies below 50 GHz measured in Oslo.
Previously published laboratory work on the rotational
spectroscopy of glycolaldehyde consists of lines under 25
GHz in frequency and rotational quantum number J ¹ 31
(Marstokk & MÔllendal 1970). The Ðve rotational transitions used to identify glycolaldehyde in Sgr B2(N) are in
the range 71È104 GHz, which means that it was necessary
to predict these frequencies based on the laboratory data.
There is generally some uncertainty involved in predicting
the positions of higher frequency lines, and a Ðrm interstellar identiÐcation of glycolaldehyde should be based on
measured transitions or transitions predicted from a Ðt that
includes higher frequency data. In this paper we report the
measurement of a large number of lines of glycolaldehyde
through 354 GHz. Our results conÐrm the interstellar
assignments and allow radioastronomers to search for this
molecule over a much wider range of frequencies.
2.
EXPERIMENT AND SPECTRAL ANALYSIS
Glycolaldehyde was studied with two di†erent spectrometers. At Oslo, a Stark spectrometer was used over the
ranges 7.5È12.0 and 18È48 GHz. The spectrometer is
TABLE 3
PREDICTED TRANSITION FREQUENCIES OF GLYCOLALDEHYDE
J@
30 . . . . . .
30 . . . . . .
32 . . . . . .
32 . . . . . .
34 . . . . . .
34 . . . . . .
45 . . . . . .
45 . . . . . .
34 . . . . . .
34 . . . . . .
32 . . . . . .
32 . . . . . .
47 . . . . . .
47 . . . . . .
43 . . . . . .
43 . . . . . .
36 . . . . . .
36 . . . . . .
30 . . . . . .
30 . . . . . .
K@
a
15
15
16
16
15
15
20
20
17
17
14
14
23
23
19
19
18
18
13
13
K@
c
16
15
17
16
20
19
26
25
18
17
19
18
25
24
24
25
19
18
18
17
J@@
31
31
33
33
33
33
44
44
35
35
31
31
48
48
42
42
37
37
29
29
K@@
a
14
14
15
15
16
16
21
21
16
16
15
15
22
22
20
20
17
17
14
14
K@@
c
17
18
18
19
17
18
23
24
19
20
16
17
26
27
23
22
20
21
15
16
Frequency
(MHz)
Uncertaintya
(MHz)
4033.961
4033.962
6089.341
6089.342
6144.876
6144.877
7706.551
7706.551
8154.413
8154.413
8228.882
8228.886
8755.532
8755.532
9876.985
9876.985
10230.201
10230.201
10305.759
10305.779
0.006
0.006
0.006
0.006
0.006
0.006
0.009
0.009
0.007
0.007
0.006
0.006
0.010
0.010
0.009
0.009
0.008
0.008
0.006
0.006
E (cm~1)
u
274.48
274.48
311.87
311.87
324.84
324.84
568.38
568.38
351.63
351.63
286.76
286.76
658.34
658.34
517.64
517.64
393.78
393.78
251.05
251.05
E (cm~1)
l
274.35
274.35
311.66
311.66
324.64
324.64
568.13
568.13
351.36
351.36
286.48
286.48
658.05
658.05
517.31
517.31
393.44
393.44
250.71
250.71
k2S
12.44
12.44
13.15
13.15
14.23
14.23
18.83
18.83
13.86
13.86
13.53
13.53
19.15
19.15
18.13
18.13
14.57
14.57
12.82
12.82
NOTE.ÈTable 3 is available in its entirety in the electronic edition of the Astrophysical Journal. A portion is shown
here for guidance regarding its form and content.
a Uncertainties represent one standard deviation.
No. 2, 2001
SPECTRUM OF GLYCOLALDEHYDE
described by Gurigis, Marstokk, & MÔllendal (1991). At
Ohio State, the spectrum was taken in the range of 128È354
GHz with the FASSST system (Fast Scan Submillimeter
Spectroscopic Technique ; Petkie et al. 1997). The source of
radiation for the FASSST system is a voltage-tunable
backward-wave oscillator, which is scanned quickly (B104
MHz s~1) over a large frequency range in order to freeze
frequency instabilities in the source. A Fabry-Perot cavity
with a reference gas is used to calibrate the spectrum. The
detectors are liquid heliumÈcooled InSb bolometers. The
spectrum of glycolaldehyde was calibrated with SO as a
2
reference gas. Glycolaldehyde can be purchased as a solid,
in which phase it exists as the dimer. The gas phase, which
contains the monomer, was studied at room temperature in
Oslo and at temperatures slightly above room temperature
at Ohio State. The vapor pressure at room temperature is
roughly 4 Pa.
The b-component of the dipole moment of glycolaldehyde, k \ 2.33 D, is much larger than the a-component,
k \ 0.26 bD (Marstokk & Mollendal 1973). Therefore, most
ofa the newly assigned lines are the stronger b-type transitions. The assignment of the data was begun by making
predictions based on the lower frequency data with the use
of a standard asymmetric-top Hamiltonian in the Watson
A-reduction (Gordy & Cook 1984 ; Pickett et al. 1998 ;
Watson 1977). Once the new data were assigned, they were
added to the lower frequency transitions to form a global
data set consisting of 1082 measured transitions. These
transitions were analyzed with the same asymmetric-top
Hamiltonian. The root mean square deviation of the Ðt is
107 kHz, which is comparable to the experimental uncertainty of the FASSST spectrometer and double the ¹50
kHz uncertainty of the Oslo spectrometer.
Table 1 contains the rotational quantum numbers for the
measured lines, the observed frequencies, and the residuals
(observed [ calculated frequencies). (Only the Ðrst 20 lines
of Table 1 appear in the paper edition ; for the complete
version, see the electronic edition). The determined spectroscopic parameters, which include the standard rotational
constants as well as fourth- and sixth-order centrifugal distortion constants, are listed in Table 2 along with their
uncertainties.
3.
DISCUSSION
The spectroscopic constants in Table 2 can be used to
predict the frequencies of unmeasured lines both somewhat
321
TABLE 4
INTERSTELLAR TRANSITIONS OF GLYCOLALDEHYDEa
Transition
Previous Frequencyb
(MHz)
New Frequency
(MHz)
7 È6 . . . . . . . . .
71542.7(9)
71542.200(8)
07 16
8 È7 . . . . . . . . .
75347.3(5)
75347.389(8)
17 26
8 È7 . . . . . . . . .
82471.2(14)
82470.670(8)
08 17
9 È8 . . . . . . . . .
93053.3(21)
93052.672(9)
09 18
10
È9 . . . . . .
103392.0(30)c
103391.28(1)
0,10 19
10 È9 . . . . . . .
103667.4(16)
103667.910(9)
19 28
a 2 p uncertainty in units of last digit in parenthesis.
b Hollis et al. 2000.
c Blend with no discernible peak.
outside of the measured frequency range and, to a limited
extent, involving higher rotational quantum numbers than
used in the Ðt. In Table 3, we list predicted transition frequencies, uncertainties (1 p), quantum numbers, intensities
in the form of k2 S (Townes & Schawlow 1975) where k is in
debye, and lower and upper state energies (cm~1). It is our
experience that the predicted uncertainties for extrapolated
rather than interpolated lines are considerably too small.
The measured lines in Table 1 are repeated in Table 3 so
that astronomers can obtain information concerning their
intensities and state energies. (Once again, only the Ðrst 20
lines of Table 3 are presented in the paper version ; the
entire table can be found in the electronic version.) Only
transitions with frequency ¹400 GHz, E
¹ 1000 cm~1,
lower of less than
intensity º10 D2, and a predicted uncertainty
0.5 MHz are included in Table 3.
We have not measured the Ðve transition frequencies
used for the interstellar identiÐcation since they lie in
between the regions studied with the Stark spectrometer
and the FASSST system. Nevertheless, since our data set
comprises much higher quantum numbers and frequencies
than those used for the interstellar detection, the predictions
should be very accurate. In Table 4, we list the quantum
numbers, interstellar frequencies from Hollis et al. (2000),
and our predicted frequencies. It can be seen that the earlier
frequencies are sufficiently accurate for interstellar
identiÐcation.
We would like to thank NASA for their support of laboratory astrophysics at The Ohio State University.
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