Silicon molecules in space and in the laboratory
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MOLECULAR PHYSICS, 2003, VOL. 101, NOS. 4–5, 697–704
Silicon molecules in space and in the laboratory
M. C. McCARTHY, C. A. GOTTLIEB and P. THADDEUS*
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 and Division
of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
(Received 12 August 2002; accepted 22 August 2002)
Despite the highly non-volatile nature of Si, silicon bearing molecules are a significant trace
constituent of the gas associated with late-type stars and star forming regions, representing
nearly 10% of the molecular species that have been identified in space. The early astronomical
detection of SiO and its remarkable maser emission stimulated laboratory studies of other
small silicon species that were crucial to the astronomical identifications. Although the pace of
this laboratory work until recently has been slow, the application of Fourier transform micro-
wave spectroscopy to supersonic molecular beams has overcome many experimental difficul-
ties, yielding the detection of more than 20 new silicon chains and rings of astronomical
interest during the past three years. Three of these, the rhomboidal ring SiC3 and the isovalent
linear molecules SiCN and SiNC, have now been found in space, and with dedicated searches
other recently discovered silicon molecules may soon be found. A number of exotic silicon
molecules of astronomical and chemical interest should be amenable to laboratory detection
with present techniques.
1. Introduction SiO and SiS in space, it led to the detection of only a
Silicon on Earth is considered a highly non-volatile few other silicon molecules in the laboratory, none of
element, and there are essentially no gas-phase silicon which has yet been found in space. While attempting to
containing molecules in the terrestrial atmosphere, improve the spectroscopic constants of SiCC, derived at
which (excepting argon) is composed almost entirely of that time mainly on the basis of astronomical data, we
elements from the first row of the Periodic Table. In discovered that small reactive silicon bearing molecules
striking contrast, however, Si in the interstellar gas is such as SiC, SiC2 , and SiO could be produced efficiently
only slightly more depleted than C, N and O [1] and, in low pressure discharges through mixtures of silane
in some circumstellar shells (the gas streaming outward (SiH4 ) and stable gases such as CO and C2 H2 . In the
from evolved stars), a surprisingly rich silicon chemistry past few years this production method has been adapted
has been discovered, even in regions where the tempera- to a pulsed supersonic discharge nozzle, resulting in the
ture is fairly low. Of the 130 or so gas-phase astronom- detection of many new silicon molecules of astronomical
ical molecules, 10 contain silicon (see figure 1), and it is interest in this laboratory by means of Fourier trans-
likely that with better spectroscopic data from the form microwave (FTM) spectroscopy.
laboratory, many more may be found. Following a brief discussion of the early astronomical
The radio astronomical observations of SiO in space discoveries of silicon molecules and of the supporting
in the early 1970s [2] and the discovery of its remarkable laboratory spectroscopy, we review the recent labora-
maser emission in vibrationally excited states [3–5] pro- tory and astronomical detections of three new silicon
vided an important impetus to the laboratory rotational molecules, and discuss the prospect that other newly
spectroscopy of silicon molecules. Nevertheless the found silicon molecules may be found in space with
laboratory work proceeded slowly in the succeeding sensitive radio telescopes. We end with a discussion of
twenty years, owing to the difficulty of producing these other silicon molecules of astronomical and chemical
highly refractory molecules in the gas phase in adequate interest that may be amenable to laboratory detection.
concentrations. In the early laboratory experiments, SiO
[6–8] and SiS [9] were produced by resistively heating 1.1. SiO
refractory silicon compounds to high temperatures. Silicon monoxide, SiO, was discovered in space from
Although this approach yielded the identification of observation of rotational lines in its ground vibrational
state in 1971 [2], but much stronger lines from excited
* Author for correspondence. e-mail: pthaddeus@cfa. vibrational states were observed accidently three years
harvard. edu later [3], and soon shown to be the result of population
Molecular Physics ISSN 0026–8976 print/ISSN 1362–3028 online # 2003 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/0026897021000035197698 M. C. McCarthy et al.
SiO SiC 2 several oxygen-rich stars [12]. Its lines are so intense
towards IRCþ10216 that SiS has now been observed
Si in the three lowest vibrationally excited states of the
SiC main isotopic species, and in the ground vibrational
state [14] of six rare isotopic species, including the
doubly substituted species 29 Si34 S and 30 Si34 S. Milli-
SiC 3 metre-wave interferometric measurements in IRCþ
SiN 10216 show that SiS is localized near the central star
Si C rather than in the outer circumstellar shell [15]. Owing
to its high abundance and chemical reactivity, SiS is
SiS thought to be the progenitor of silicon carbides such
as SiC, SiC2 , and SiC4 , which are thought to be synthe-
SiC4
sized in the outer circumstellar envelope of carbon-rich
stars by the reactions of the abundant positive ions
C2 Hþ þ þ
2 , C2 H3 , and C with SiS [16].
SiCN
SiH4 1.3. SiC
Silicon carbide, SiC, has been found only in the cir-
cumstellar shell of the well known evolved carbon star
SiNC IRCþ10216. Because SiC is so refractory, and con-
denses out of the vapour only at high temperatures,
generally it was assumed that if it could be found in
Figure 1. Astronomical silicon-bearing diatomic and polya- space it would be close to the surface of a star, and
tomic molecules, all but SiH4 discovered with radio tele- would be of small angular extent [17]. Instead, SiC
scopes. SiH2 has been reported also by Turner [59], but was found at a considerable distance from the central
because of its small dipole moment (Silicon molecules in space and laboratory 699
near 500 nm. Thirty years later SiC2 was observed in the the handling and disposal of unreacted silane gas and its
laboratory and was shown to be the carrier of these well discharge products. Silane is spontaneously flammable
known optical bands [25]; however, it was mistakenly in air at concentrations of 5% or more and, unfortu-
thought to be a linear molecule (i.e., Si—C—C). It nately, high flows with little dilution were required to
was not until 1984 that an elegant two-photon ioniza- produce the strongest signals of SiC and SiC2 with our
tion experiment established the ground state geometry: a free-space millimetre-wave absorption spectrometer.
three-membered ring with C2v symmetry [26]. Approx- The application of FTM spectroscopy to supersonic
imate rotational constants derived by Michalopoulos molecular beams has largely overcome these difficulties,
et al. [26] allowed Thaddeus et al. [27] to assign nine yielding here during the past three years the detection of
previously unidentified lines in space to SiC2 , to greatly more than two dozen silicon molecules of astronomical
improve the accuracy of the rotational constants, and to interest (figure 2). This technique has proved particu-
determine the leading centrifugal distortion constants. larly effective because many types of reactive molecules
The pure rotational spectrum was measured a few can be produced efficiently in an electrical discharge
years later in the laboratory by Gottlieb et al. [28] who source and their rotational spectra are simplified greatly
found indirect evidence for a non-rigid structure. Since at the low rotational temperature of a few degrees
then SiC2 has been investigated in a number of labora- Kelvin that is achieved readily in a supersonic expan-
tories, and has been the subject of several quantum sion. In addition, stringent safety precautions are not
theoretical calculations which have shown that there is needed, because the flow of silane is about 500 times
large amplitude motion in the low lying 3 vibrational less than in our millimetre-wave spectrometer, and the
mode (for a summary see [29]). Many rotational lines of silane is heavily diluted in a buffer gas. Rotational lines
SiC2 , including those of the singly substituted isotopic of many of the new silicon species shown in figure 2 may
species, have now been observed in IRCþ10216. In now be observed with a signal to noise approaching 100
addition, rotational lines in the first vibrational state
in one minute of integration, sufficient to detect rare
of the 3 mode have also been observed in this circum-
isotopic species in natural abundance. Molecular struc-
stellar shell [14].
tures have been determined for about one-quarter of the
new molecules by isotopic substitution, simply by meas-
1.5. SiC4
uring the rare isotopic species in natural abundance.
The linear chain SiC4 was found accidently in
In our spectrometer, reactive silicon molecules are
IRCþ10216, and its presence there was confirmed by
created by a small low-current cw discharge in the
subsequent laboratory detection in the same discharge
throat of the expansion nozzle (figure 3) as a gas pulse
source in which SiC and SiCC are observed [30]. Obser-
consisting of a small amount of silane and an organic
vations of SiC4 at millimetre wavelengths in space and in
the laboratory are sparse, owing to the large rotational vapour heavily diluted (99% or more) in an inert gas (Ar
partition function and the fairly low abundance with or Ne) expands from a commercial pulsed valve. Free
respect to other astronomical silicon carbides. In expansion from the nozzle forms in a short distance an
IRCþ10216, the abundance of SiC4 is ten times less approximately mach 2 supersonic beam with a rota-
than that of SiC and thirty times less than that of SiC2 tional temperature approaching 1 K. As the beam
[30]. passes through a large high-Q Fabry–Perot microwave
cavity, a rotational transition resonant with one of the
2. Laboratory and astronomical discovery of new modes of the cavity is excited coherently by a short pulse
silicon bearing molecules of resonant microwave radiation; the radiation almost
Despite advances that had been made in the labora- immediately decays away, and line radiation by the
tory detection of reactive silicon molecules of astronom- coherently rotating molecules is then detected by a sen-
ical interest in this laboratory and elsewhere, less than sitive microwave receiver. The Fourier transform of this
ten such species had been studied by rotational spectro- free induction decay yields the power spectrum. For an
scopy prior to the mid 1990s. Detection of large silicon axial beam (the configuration adopted for nearly all our
molecules is an experimental challenge because in addi- experiments because it gives the best signal-to-noise and
tion to their high refractivity, the rotational partition resolution) the line shape is double peaked, owing to the
function of these molecules becomes increasing unfa- two Doppler components that result from decomposi-
vourable in a large discharge cell (even when the walls tion of the confocal mode of the Fabry–Perot cavity into
are cooled with liquid nitrogen to about 100 K), because two travelling waves, one parallel to the supersonic
of dilution over many thermally populated levels. beam and the other antiparallel. In practice, this pecu-
Another consideration in our production scheme is liar lineshape is usually only a minor inconvenience, and
safety, since a great deal of care must be exercised in the effective spectral resolution of the spectrometer is700 M. C. McCarthy et al.
SiC SiC 2 H H 2 CCSi
*
1.0
99
117.3˚
1.321 1.703
SiC 2 SiC 4 H
H 2CCCSi
*
Si
SiC 5 H
SiC 3
1.834 1.435
*
1.490
Si C
SiC 6 H H 2 CCCCSi
SiC3 2.022
1.344
1.893
SiCN
SiC 3
* SiC 2 S
1.298
SiNC 1.669 1.554
SiC5 * SiC 3 S
HSiCN
SiC 4 S
SiC6
1.282 1.256 1.277
1.689 1.291 1.300
HSiNC SiC 6 S
SiC7
SiC 8
Si 2 H 3
Figure 2. Silicon bearing molecules detected in this laboratory. With the exception of SiC and SiC2 , all were detected in the past
three years. Asterisks indicate those molecules detected in space as a result of this work. The convention described in figure 1 is
used to denote the composition of each molecule. Bond lengths and angles were derived from isotopic measurements.
determined by the small width of the individual Doppler by the confocal cavity to about 0.5 MHz are stitched
peaks. together, or in a single-scan mode, where individual
Progress in detecting new silicon molecules in this spectra are recorded at many different preselected fre-
laboratory has also benefited from instrumental quencies. Continuous operation over many tens of hours
improvements that have been made during the past or even days is now routine. A second crucial factor is
several years. Of particular importance to the present sensitivity, which has been increased significantly by
work is the capability to survey large ranges in fre- liquid nitrogen cooling a very low noise cryogenic
quency under computer control. The spectrometer is microwave receiver and large confocal Fabry–Perot
now automated to the point that rotational spectra cavity [31].
may be acquired rapidly either in a search mode, One of the first silicon molecules detected with the
where individual spectra each limited in spectral range Fourier transform microwave spectrometer is the rhom-Silicon molecules in space and laboratory 701
wave band [33] in a pulsed discharge through a dilute
Fabry-Perot cavity
mixture of diacetylene and silane. At most four rota-
tional transitions of each isotopic species lie within the
frequency range of our FTM spectrometer, but this
microwave number alone is sufficient to establish unambiguously
receiver the existence of this exotic ring, and to allow a precise
determination of its geometry. If SiC3 were strictly
Supersonic beam planar and rigid, two parameters would suffice to deter-
mine the entire spectrum (the three moments of inertia
Vacuum chamber
constrained by the Pythagorean theorem), but to
describe the actual deformable molecule to the accuracy
needed to conduct a radio astronomical search requires
the three rotational constants and several centrifugal
distortion constants. Using our free-space absorption
diffusion pump
spectrometer, the millimetre-wave spectrum of SiC3
was measured [34] to within a few parts in 107 up to
Pulsed discharge 300 GHz, and the nine leading centrifugal distortion
Mirror nozzle constants were determined to fairly high precision as
well.
copper
electrodes
A total of 11 millimetre-wave rotational transitions of
SiC3 have now been detected in IRCþ10216 with the
12 m NRAO telescope on Kitt Peak. The lines are
Gas fairly weak (5 mK), but all have a mean radial velocity
of 26ð3Þ km s1 , precisely that found for most other
Pulsed molecules in this source. The rotational excitation of
nozzle SiC3 is similar to that of SiCC, with a low rotational
temperature within the K-stacks of 10–20 K, and a high
Teflon
spacers
Teflon rotational temperature across them of roughly 50 K,
housing
which probably is fairly close to the kinetic temperature
of the ambient H2 gas, the main constituent of the
shell. The column density of SiC3 is calculated to be
4:3 1012 cm2 assuming a shell diametre of 60 00 , per-
Figure 3. Schematic diagram of the FTM spectrometer haps 50% larger than that of SiC4 .
showing the Fabry–Perot cavity, the axially oriented
molecular beam, and the pulsed discharge source. The Two other silicon molecules that were detected
stagnation pressure of the gas behind the valve is recently in the laboratory and then in space with radio
1–3 atm, the discharge potential is 1000–1800 V, and the telescopes are the isovalent radicals SiCN and SiNC.
length of the gas pulse is typically 200–350 ms. The dia- Intense rotational FTM lines of both species were
metre of the opening orifice of the nozzle immediately observed first at centimetre-wavelengths in a discharge
following the valve is 1 mm. A pulsed valve working at
6–10 Hz, open less than 1% of the time, is required to through silane and cyanoacetylene, and then at milli-
keep the pressure in the chamber low enough to form a metre-wavelengths with our free-space spectrometer
good molecular beam. [35]. Like SiC3 , determination of the leading centrifugal
distortion constants is required for an astronomical
search because the most intense radio lines of these radi-
boidal ring SiC3 (figure 1), a long postulated but pre- cals are expected in a warm circumstellar source to lie in
viously unknown molecule with a remarkable internal or the 80–120 GHz frequency range, where centrifugal dis-
‘transannular’ carbon–carbon bond. Once we had good tortion is important.
rest frequencies in hand, we quickly detected seven lines SiCN was discovered first in IRCþ10216 [36],
at precisely the right frequencies in the circumstellar followed very recently by SiNC [37], using the powerful
shell of IRCþ10216 [32], one of the best astronomical IRAM 30 m radio telescope. Three rotational transitions
sources of complex molecules. Rhomboidal SiC3 is the of SiCN, each split by lambda-doubling, were detected
sixth and largest reactive ring molecule that has been at the 5 mK level; from the characteristic cusp shapes of
conclusively identified in space. the line profiles, it appears that SiCN, like many other
Rotational transitions of rhomboidal SiC3 and its rare radicals, is largely confined to the outer molecular
isotopic species were first observed in the centimetre- envelope that surrounds the central star. The column702 M. C. McCarthy et al.
density is estimated to be 2 1012 cm2 , which is about SiP H 3 SiSiH
20 times smaller than that of SiC and MgNC, but com-
parable to that of MgCN. The strength of the SiNC lines
and the derived column density of SiNC are similar to
that of SiCN, as one might expect from the comparable SiNN
polarity and stability of these two nearly isoenergetic Si 2 H 5
radicals (SiNC is calculated to lie only 5 kcal mol1
higher in energy). The formation mechanism of SiCN
and SiNC is not well understood, but both may be SiCO
formed via ion–molecule reactions, neutral–neutral
reactions, or reactions on the interstellar dust grains. SiC 2 H
Other silicon molecules of astronomical interest are (i)
two low-lying rearrangements of SiC3 : a triplet chain HSiCCH
and a second rhomboidal isomer [38], (ii) the recently
discovered chain H2 CCSi [39], and (iii) isoelectronic SiC 5
HSiCN and HSiNC [40]. Although H2 CSi has not
been found in space [41], H2 CCSi is a good candidate H 2 SiCN
for astronomical discovery because it is much more
polar ( ¼ 1:21 D versus 0:3 D) and because isova-
lent H2 CCC has been detected in both interstellar and Si 3 H
circumstellar sources. HSiCN and HSiNC were detected
in the same molecular beam discharge source that pro- H 2 SiSi
duces strong lines of SiCN and SiNC, and precise spec-
troscopic constants were derived for each from the
centimetre-wave data. Although there is no evidence
for either species in published astronomical surveys, Si 4 H
H 3 SiSi
HSiCN ( ¼ 3:5 D [42]) and HSiNC ( ¼ 2:4 D) are
highly polar, so dedicated astronomical searches are
warranted. HSiCN is isovalent to HCCN, a well studied
laboratory molecule that possesses a large amplitude
CCH bending motion and a non-rigid structure [43]. A Figure 4. Postulated silicon molecules that are good candi-
structural analysis is underway to determine whether dates for laboratory and astronomical detection. The con-
HSiCN, like HCCN, also possesses large amplitude vention described in figure 1 is used to denote the
composition of each molecule.
bending.
3. Postulated silicon molecules of laboratory and these simple molecules have eluded gas-phase detection,
astronomical interest even though the isovalent triplet chains CCO and CCS
Although many new reactive silicon molecules have have been the subjects of numerous experimental and
been discovered recently, the radio spectra of many astronomical investigations. The SiNN radical is poten-
(some with only a few atoms) remain unknown. Small tially the most interesting of the three, because quantum
molecules that are strong candidates for laboratory and chemical calculations suggested until recently that two
astronomical detection include the diatomic molecule distinct isomers, a tightly bound and a loosely bound
SiP and the triatomic molecules SiNN and SiCO structure, compete for the ground state; at the coupled
shown in figure 4. SiN, PN, and CP have all been cluster level of theory, however, the loose structure is
detected in at least one astronomical source [36, 44, not a minimum on the potential surface [51]. Experi-
45], but the lack of accurate spectroscopic data has pre- mental confirmation of this prediction is highly desir-
cluded an astronomical search for SiP. Strong optical able, as is a precise determination of the N—N bond
spectra of SiP were recently measured in the laboratory length, because the theoretical N—N distance is appar-
[46] using a similar production scheme to the one we are ently sensitive to the level of theory and size of the basis
likely to employ, i.e., a discharge through silane and set.
phosphine (PH3 ). There is very little experimental or theoretical infor-
Despite the availability of high level ab initio calcula- mation on partially hydrogenated silicon molecules such
tions [47–49] and the identification of SiNN, SiCO, and as H2 SiCN, HSiCCH, H2 SiCCH, longer such chains, or
SiCS in an inert matrix by electron spin resonance [50], the related isocyanides, even though these species areSilicon molecules in space and laboratory 703 closely related in structure and composition to known are good candidates for laboratory detection for the astronomical molecules. Most of these chains possess same reasons. linear or nearly linear heavy atom backbones, and as a Another class of silicon molecules that is of interest result rotational spectra of nearly symmetric prolate are silicon hydrides with three or more silicon atoms. tops with nearly harmonically related transitions that Detection of longer chains and larger ring radicals are recognized readily against the background of unre- (e.g., Si3 H2 , Si3 H, Si4 H) should be possible because lated lines present in discharge sources such as ours. In the time-of-flight mass spectrum of a discharge of addition to their astronomical interest, systematic silane measured in our laboratory indicates that Si3 studies of analogous systems such as SiHn CCH and larger silicon clusters are only about 10% less abun- (n ¼ 0–3) and SiHn CN (n ¼ 0–3) might significantly dant than the ubiquitous Si2 dimer, and because the improve our understanding of the production mechan- photodetachment spectrum of some of the proposed isms at work in our discharge source. molecules has been measured by Neumark and co- The astronomical detection of SiC3 implies that larger workers [58]. Many chain and ring isomers of compar- silicon rings exist at detectable abundances in space, and able stability probably exist for silicon hydrides with will be discovered there once precise laboratory frequen- three or more silicon atoms because of the ability of cies are in hand. There are few high level theoretical silicon to be either singly or multiply bound to adjacent calculations for systems larger than SiC3 to guide silicon atoms in the cluster. Direct spectroscopic obser- laboratory searches, but many low lying polar isomers vations of larger silicon hydrides should provide us with of these larger silicon molecules are expected because of a better understanding of silicon cluster structure, and the tendency of silicon to prefer single rather than how such clusters form in the laboratory and in space. double or triple bonds, and to form both cyclic and Such studies may also provide an important challenge linear isomers fairly readily. For example, Gomei et al. for computational techniques, because of the subtle and [52] and Zdetsis et al. [53] concluded on the basis of ab ambiguous character of silicon bonding. initio calculations that silicon carbides up to SiC6 pos- Once rotational lines of the normal isotopic species sess several low lying ring or ring-chain isomers. For have been found, a number of interesting follow-up SiC6 , high level calculations [53] predicted a ring investigations are worth pursuing, including (i) detection isomer with C2v symmetry only 5 kcal mol1 higher in of rare isotopic spectra and determinations of molecular energy than the linear chain that we have already structure, (ii) detection of rotational satellites from low observed with high signal-to-noise. lying vibrational states and, where appropriate, (iii) Many silicon carbide molecules may possess strong millimetre-wave measurements to determine the leading electronic transitions of astronomical interest; SiC, for centrifugal distortion constants of molecules whose example, has strong such transitions [21, 22] and, as strongest radio lines fall in the millimetre-wave band. mentioned, optical bands of SiC2 have long been Such studies provide useful information for subsequent observed in the atmospheres of cool stars [24]. At the experiments at infrared or optical wavelengths, and densities at which chains up to SiC7 are produced in our possible insights into a wide range of important molecular beam, such spectra may be detectable by cur- phenomena in chemical physics, including molecular rent laser techniques such as laser-induced fluorescence structure and rigidity, chemical bonding, and intramo- or cavity ring down spectroscopy. Compared with the lecular vibrational relaxation. isoelectronic pure carbon chains, these chains (or rings) have the advantage that their production may now be We are indebted to E. S. Palmer for technical assis- monitored and optimized in the radio band as described tance in the construction and maintenance of the FTM here. If electronic transitions are detected, it may be spectrometer, and E. W. Gottlieb for assistance with possible to confirm the identifications by means of software for spectral acquisition and analysis. We also microwave-optical double resonance [54]. thank those colleagues who contributed to some of the Silicon hydrides of the form Si2 Hm with m > 2 are original papers referred to here: A. J. Apponi, M. E. also excellent candidates for laboratory detection Sanz, R. Mollaaghababa, J. M. Vrtilek and M. Guélin. because we already have evidence that Si2 H2 [55], H2 Si2 H, and other silicon hydrides are readily produced in a discharge through silane. Laboratory detection of Note added in proof H3 SiSi (figure 4) is a good place to start because this Very recently three new silicon molecules have been molecule is calculated to be polar and nearly isoener- detected in our laboratory: Si3 and a low-lying mono- getic to H2 SiSiH [56, 57], and to have the easily recog- bridged isomer of Si2 H4 have been observed by FTM nizable rotational spectrum of a nearly symmetric spectroscopy, and Si3 C has been observed by resonant prolate top. Polar isomers of Si2 H4 , including H3 SiSiH, two-colour, two-photon ionization spectroscopy.
704 Silicon molecules in space and laboratory
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