A millimeter/submillimeter
of transient molecules
spectrometer
for !high resolution
studies
L. M. Ziurys,a) W. L. Barclay, Jr.,b) M. A. Anderson,b) and D. A. Fletcher
Department of Chemistry,Arizona State University, Tempe,Arizona 85287-1604
J. W. Lamb
National Radio Astronomy Observatory,c)949 North Cherry Avenue, lkson, Arizona 85721-0655
(Received 26 August 1993; accepted for publication 26 January 1994)
A design is presented for a mlllimeter/submillimeter direct absorption spectrometer for studies of the
pure rotational spectra of metal-bearing free radicals. The spectrometer operates in the frequency
range of 65-550 GHz with an instrumental resolution of 200-1000 kHz and an absorption
sensitivity of a few ppm. The instrument utilizes phase-locked Gunn oscillators as the tunable,
coherent source of radiation from 65-140 GHz. Higher source frequencies are obtained with
Schottky diode multipliers. The gas cell and optics path are designed utilizing Gaussian beam optics
to achieve maximum interaction between molecules and the mm-wave radiation in the reaction
region. Scalar feedhorns and a series of PTFE lenses are used to propagate the source signal. The
gas cell is a cylindrical tube 0.5 m in length with a detachable Broida-type oven. The detector for
the spectrometer is a helium-cooled InSb hot electron bolometer. Phase-sensitive detection is
achieved by FM modulation of the Gunn oscillators and use of a lock-in amplifier. Spectra are
recorded by electrical tuning of the Gum-r oscillator, which is done under computer control. The
millimeter and sub-mm rotational spectra of several free radicals have been observed for the first
time using this instrument, including CaOH, MgOH, CaH, MgF, and BaOH.
I. INTRODUCTION
The pure gas-phase rotational spectra of most simple
two, three, and four atom molecules composed of the cosmically abundant elements usually occur at millimeter and
sub-mm wavelengths. Such spectra are of particular interest
for a variety of scientific disciplines, especially when the
species involved is a free radical, i.e., contains at least one
unpaired electron.
From the aspect of astronomy, the measurement of the
pure rotational frequencies of free radicals is an avenue by
which such species can be identified in the interstellar medium. Free radicals are highly unstable and reactive in the
laboratory. Because of the very cold and diffuse nature of
interstellar gas clouds, however, they are common constituents of such objects.’ Using radio and mm-wave telescopes,
the “fingerprint” rotational spectra of these radicals can be
detected in interstellar gas and their abundances can be
evaluated-one of the major goals of astrochemistry.
Studies of free radicals also impact on the fields of
chemistry and chemical physics. Because they have an unpaired electron, radicals are often intermediates in chemical
reactions. The study of their geometric structure via rotational spectroscopy is important in understanding gas phase
kinetics. Also, because they have unpaired electrons, free
radicals must possess spin and often orbital angular momentum, and perhaps nuclear spin momentum as well. Investigating the rotational spectra of such molecules, especially at
high resolution, enables accurate determination of tine and
hyperfine structure in these species, which shows how these
‘)NSF Presidential Faculty Fellow.
b)NASA Space Grant Fellow.
‘)NRAO is operated under cooperative agreement with the NSF.
angular momenta interact. Such information is extremely
useful in development of the theory of open-shell molecules.
Fine and hyperfine structure can also be used to evaluate
certain bonding characteristics in a species.
It is of scientific interest to measure the pure rotational
spectra of free radicals; however, the instrumentation required to obtain these measurements at millimeter and
sub-mm wavelengths generally is not commercially available. A basic mm-wave spectrometer is just a higher fiequency version of a classic microwave direct absorption
system.2 It consists of three basic components: (1)a source
of tunable, frequency-stable radiation, (2) a cell in which to
contain molecules in their gas phase, and (3) a detector. The
operation of the instrument is quite elementary in principle.
The particular species to be studied is created in the cell, and
radiation is propagated through the cell where it is absorbed
by the molecules. A detector placed at the other end of the
cell monitors the absorption of the radiation, which takes
place at specific frequencies characteristic of the particular
species.
The details of the instrument design are more complex,
however. In order to accurately determine properties of free
radicals, many rotational transitions of a given species must
be studied. Hence, any radiation source and detector must
operate over a large frequency range of at least several hundred GHz, and preferably in wavelength regions accessible
by ground based radio astronomy (-65-600 GHz). The radiation source must have sufficient power to insure maximum interaction with the absorbing molecules. The detector
should be as sensitive as possible to small changes in incident signal, as well. To resolve fine/hyperfine structure in
molecules, any source of mm-wave radiation should be very
narrow band so that molecular linewidthsof lessthan 1 MHz
QQ34-6748/94/65(5)/1517/6/$6.00
0 see
1994 http://ojps.aip.org/rsio/rsicpyrts.html
American Institute of Physics
Downloaded
16 Mar
20011994
to 128.196.209.95.
Redistribution subject to AIP copyright,
Rev. Sci.
Instrum. 65
(5), May
1517
FIG. 1. Block diagram of the millimeter/sub-mm spectrometer described in
this work. The instrument consists of a phase-locked Gunn oscillator source,
a gas cell, and an I&b detector system.
may be measured. The linewidth requirement additionally
means that relatively low pressures (PS25 Pa) must be used
in the cell to reduce pressure broadening.
The fact that chemically unstable species are &died also
restricts the instrument. design. It is difficult to create large
quantities of free radicals; hence, interaction of thi: transient
molecules and the radiation must be as efficient as p.ossible.
The radiation sliould therefore be concentrated into the molecule production region and then into the detector. In the
microwave region, this means that gas cells are typically
constructed from a waveguide of the appropriate frequency
band.”At mm wavelengths, the waveguide is far too small to
be used as gas cells and quasioptical techniques must be used
to direct the radiation through the cell to the detector. A
proper optics design also results in fewer reflections in the
system, which can help minimize baseline instabilities.
Moreover, in addition td conforming to the optics specifications, the cell must be able to accommodate various devices
for production of free radicals, including dc and microwave
discharges, hollow cathode discharges, sputtering, and laser
ablation sources, etc. For example, for the production of
metal-bearing free radicals, an oven may need to be part of
the cell design such that metal vapor can be produced and
flowed into the reaction region.
In this paper, we present a working design for a mm/
sub-mm spectrometer constructed for the study of metalbearing free radicals. This instrument operates currently in
the frequency range 65-550 GHz, but could be used up to
800 GHz, and was designed using Gaussian beam optics.
The rotational spectra of several metal-bearing hydroxide,
hydride, and fluoridk free radicals, including MgOH, CaOH,
CaH, MgH, MgF, and BaOH, have already been successfully
measured with this spectrometer.
II. INSTRUMENT DESIGN
A. Overview
A block diagram of the spectrometer is shown in Fig. 1.
,The instrument consists of four basic parts: the radiation
source, gas cell, detector, and a data collection scheme. Data
obtained with this instrument is stored on computer disk,
after processing through a lock-in amplifier.
The mm-wave radiation sources for this instrument are
Gunn oscillators,3. which provide 65-140 GHz frequency
coverage. These oscillators are phase-locked to a harmonic
of a reference frequency near 2 GHz from a signal generator
(Fluke 6082A). The phase-lock intermediate frequency (IF)
is 100 MHz and the IF tone is monitored on a spedtrum
analyzer. To obtain higher frequencies (125-550 GHz), the
Gunn oscillator signal is doubled, tripled, or quadrupled using Schottky diode multipliers (Millitech Corporation). As
Fig. 1 shows, the output from the Gundmultiplier is directed
from a feedhorn through a P?FE lens and polarizing grid
into the gas cell. The grid is aligned with the linear polarization from the feedhorn. PTFE lenses at the ends of the cylindrical cell fdcus the incident radiation and seal the chamber. At the far end of the cell, a rooftop reflector reflects the
radiation back through the cell, rotating its polarization by
90”. After a second pass through the cell, the beam is focused
back on the grid, which now acts as a mirror and reflects the
radiation through a fourth lens and into a detector. Several
quartz windows on the cell allow monitoring of chemiluminescence, which is often observed when radicals form.
The cell is evacuated by a Roots-type blower pump (Edwards EH500/E2M40), which has a displacement of -7000
/min-I.
The cell itself is a stainless steel tube 0.5 m in
length and 0.1 m in diameter. The pump is attached to the
cell on the end near the signal source. To produce metal
vapor for creating free radicals, a Broida-type4 oven is incorporated into the cell near the end which has the rooftop reflector. The oven system is -0.4 m in length and is attached
to the bottom of the cell about 14 cm fi-om the cell end.
The detector is a helium-cooled InSb hot electron bolometer (Cochise Instruments). Since the bolometer is tin ac
device, the input signal must be chopped. This is accomplished by FM modulation of the Gunn oscillators at a 25
kHz rate. The signal at the bolometer is synchronously demodulated by a lock-in amplifier (EG&G PAR 5301) ai twice
the modulation rate.
The spectrometer is operated under computer control
through an IEEE-488 bus which allows communication between a 486 PC, the signal generator and the lock-in amplifier. To take data, the particular species to be studied is created in the cell. This procedure will vary according to
molecuie. For the radicals investigated with this sytem, the
procedure usually involves heating a metal in the oven until
it starts to vaporize, and adding a reactant gas. Reactant species such as free hydrogen atoms have also been produced
using a 2.4 GHz microwave discharge tube attached to a side
of the cell. To carry out a spectral scan, the Gunn oscillator is
manually tuned to the nominal frequency of operation and its
output power maximized. If a multiplier is used, its output is
maximized as well. The center frequency for a given scan .is
input to the PC, which computes the reference frequency
(near 2 GHz) and sends the value to the signal generator. The
Gunn is then manually adjusted to phase-lock it to a harmonic (typically 35th-60th) of the reference signal. A scan
is carried out by stepping the frequency of the reference signal generator to which the Gunn is phase-locked. The com-
1518
Rev. Sci. Instrum., Vol. 85, No. 5, May 1994
Sub-millimeter
spectromeber
Downloaded 16 Mar 2001 to 128.196.209.95. Redistribution subject to AIP copyright, see http://ojps.aip.org/rsio/rsicpyrts.html
puter records the bolometer output at every frequency step as
it is processed by the lock-in amplifier. The frequency is
changed about every second, with a 300 ms time constant on
the lock-in amplifier. Scans up to 200 MHz in range can be
successfully recorded in this manner without loss of phaselock. To achieve wider scans, the frequency of the Gunn
must be manually adjusted. Background scans are not necessary to record spectra.
B. Individual
components
1. Signal source
Indium phosphide Gunn oscillators3 are used as the
source of millimeter-wave radiation. They have several advantages over klystrons, which have been typically used in
the past as spectrometer sources.2 First of all, they operate at
voltages of -10 V, instead of several kilovolts. Second, they
are relatively simple to tune electrically and phase-lock. In
principle, they can be electrically tuned over a 200 MHz
range.a Finally, a micrometer backshort allows easy coarse
manual tuning over a wider range. Gunn oscillators are also
relatively wideband, and have sufficient power to drive frequency multipliers. For example, three oscillators used in the
spectrometer cover the frequency range 65-140 GHz, with
typical power levels ranging from 15-100 mW.
Gunn oscillators utilize a relatively simple phase-lock
scheme, shown in Fig. 1. The reference signal (usually in the
GHz) is multiplied close to the desired
range -1.8-2.02
Gunn frequency, using a Schottky diode harmonic mixer (Pacific Millimeter). Ten percent of the Gunn output power is
coupled into the harmonic mixer and mixed down to an IF
frequency of 100 MHz. The phase-lock box (X-L Microwave) compares this signal with a 100 MHz reference and
varies the Gunn bias voltage around 10.3 V to synchronize
the IF to the reference. Monitoring the phase-lock IF signal
on a spectrum analyzer greatly simplifies the Gunn tuning.
Frequency modulation of the Gunn oscillator is accomplished by variation of the reference signal t3 kHz around
the nominal reference frequency at a rate of 25 kHz using an
external source. The source is a variable phase square wave
generator. Two outputs are added in quadrature to produce a
bidirectional square wave.* An additional 25 kHz output is
used as the reference for the lock-in amplifier, which detects
signals modulated at 50 kHz so that the second derivative of
molecular absorption signals is measured. The bidirectional
square wave modulation scheme is used to suppress baseline
effects in spectra.’
2. Optics
Figure 2 shows the layout of the spectrometer optics,
which are built around the absorption cell. A nearly Gaussian
beam is produced by a corrugated horn H and focused by
lens Ll to a beam waist at a polarizing grid G. The signal is
transmitted, apart from cross-polar components which are
terminated in absorber A. From the grid, the beam is expanded to lens L2, which focuses it at the middle of the
absorption cell and then through to lens L3 on the other end
of the cell. A rooftop reflector R, with its apex at 45” to the
incident polarization, rotates the plane of polarization 90”
FIG. 2. Layout of the spectrometer optics that are designed around the gas
absorption cell shown in the figure. The letters indicate the various components as follows: H: feedhorn; Ll, LZ, L3, LA: teflon lenses; G: polarizing
grid; R: rooftop reflector; B: detector lightcone; A: absorber.
and returns the beam through the cell. When the beam exits
through L2, it is reflected by the grid and focused by L4 into
the bolometer light cone B.
Standard Gaussian beam techniques were used to design
the optics.6 Figure 3 shows the optics “unfolded.” It is assumed that the pathlength between the lens and the rooftop is
small, so that lens L3 is replaced by L3’ which has half the
focal length to account for the two passes through LZ& L2’
represents L2 for the second pass of the beam. The system is
arranged as a Gaussian beam telescope,6 where the beam
waists are located at the focii of the lenses. Beam-waist radii
are alternately fixed and proportional to the wavelength X. To
minimize undesirable diffraction and reflection loss, the apertures are designed to be greater than four beam radii in
diameter.
The design is constrained by the cell dimensions. If the
length of the cell is 6’ and the diameter is d, then it may be
shown that the longest wavelength which may be transmitted
with the required clearance is
h
nd’
=-max Ihl .
(1)
For the cell used here, this corresponds to about 76 GHz
which is taken as the lowest design frequency although it can
in practice be used to 65 GHz. Lens L2 has a focal length
f,=kl2,
and L3 h as a focal length f3=-/. The corresponding
beam-waist size in the center of the cell is found to be
h
(2)
3
wo3==
or 18 mm in this case. This waist size is frequency independent but the waist radius before L2 is
*02-
-- ifi
TWO3
(3)
’
~~y!Yjzfyjq
WC,,we2
WC2
WC4
We WLS
FIG. 3. “Unfolded” uptics of the design shown in Fig. 2. The various beam
waists, focal lengths, and lenses are indicated in the figure.
1519
Sub-millimeter
spectrometer
Rev. Sci.
Instrum., Vol.
65, 2001
No. 5,toMay
1994
Downloaded
16 Mar
128.196.209.95.
Redistribution subject to AIP copyright, see http://ojps.aip.org/rsio/rsicpyrts.html
and Ll is used to match this beam waist to the feedhorn H.
The optimum waist size at the horn aperture is7
wo1=U.64a,
the system which is periodic with the signal frequency. This
appears in the spectra as a sinusoidal baseline with a period
of
(4)
where a is the horn aperture radius. A lens, Ll, of focal
length,
(5)
matches the waists wol and wnz.
On the detector end, lens L4 focuses the beam into the
light cone of the bolometer. Its focal length was chosen to
make the beam diameter about the same size as the lightcone aperture (12.7 mm).
Some specific details of the components are given in the
next sections.
Feedhorns. These are electroformed from copper and
gold plated. Several feedhorns are used to cover the required
frequencies. Corrugated horns are used as they can produce
highly Gaussian beams over about a 40% bandwidth.’ The
aperture is chosen to be -4h at the low end of the band and
the flare angle 4.7”, placing the beam waist close to the aperture.
G-id. The grid is wound from 25 pm diam gold-plated
tungsten wire with a pitch of 110 ,um. Measured losses are
less than a few percent including dissipative and imperfect
polarizing losses.
Lenses. All the lenses are made from PTFE which has
very low loss at millimeter wavelengths9 Its index of refraction is low [1.44, (Ref. 9)] so that interface retlections are
only about 3% of the power per surface. The lenses on the
spectrometer cell have no antireflection treatment since they
are used over a very wide frequency range, but the lenses
used with each of the feedhorns have grooves cut to minimize reflection over the given band.
Rectangular grooves with a slot-to-ridge ratio of one and
a depth of l/4,/n at midband (where IZ is the refractive index
of the dielectric) were used, giving a return loss of better
than -15 dB. To minimize diffraction effects, the groove
pitch is made less than X/3 for lenses up to 200 GHz, and
less than A/2 above that.
Because of the curvature of the lens surface, the grooves
are cut in a circular pattern that gives rise to some cross
polarization from the differential phase delay between components of the electric field parallel and perpendicular to the
grooves. The fraction of the power converted to cross polar
is about the same as would be reflected by an ungrooved
lens,” but it is properly terminated by the grid and absorber
A and therefore preferable to reflections which could introduce poor baselines.
Rooftop rejlector. As well as rotating the plane of polarization of the incident beam, the rooftop is used as a path
length modulator to reduce the effects of standing waves.
Standing waves can exist between discontinuities in the
optics such as reflections from windows and feedhorns. If
there are two discontinuities separated by an electrical distance p, then there will be a variation in the transmission of
where c is the speed of light. Several discontinuities give rise
to a number of sinusoidal waves which can produce irregular
baselines and make detection of weak absorption lines difficult.
Several techniques can be used to reduce these problems. In a well-designed system, reflections may be kept low
by using matching layers on lenses as described above, by
allowing ample clearance for the beam and avoiding interfaces which are on equiphase surfaces of the beam. If the
spectrometer is structurally stable, spectra can be taken with
and without the gas being measured and ratioed to remove
the instrumental effects.
Another strategy is to vary the pathlength in the optics so
that the standing wave is smeared out. If the pathlength is
varied evenly in time over a distance of X/2, then the effect
of the standing wave is completely removed at that frequency. In practice, varying the pathlength over several halfwavelengths gives good cancellation over the range of frequencies scanned by the Gunn.
In the spectrometer, the rooftop reflector is mounted on
linear bearings and driven with a lead-screw and electric motor, a mechanism originally developed by Payne and Ulich’r
for a radiotelescope. It is scanned at a rate of about 10 mm/s.
The effect of the moveable rooftop reflector is at least a
tenfold reduction of the instrumental spectral features, as
shown in Fig. 4.
3. Detector
The detector used for the spectrometer is an InSb “hot
electron” bolometer. The bolometer dewar has an inner liquid helium container and an outer liquid nitrogen jacket, and
is cooled to 4.2 K for spectrometer operation. The liquid
helium usually lasts in the dewar for 36 h. The signal from
the InSb device passes through a preamplifier before going bo
the lock-in amplifier. The predetection bandwidth of the bolometer is greater than 600 GHz, and the video bandwidth is
1 MHz. Its sensitivity (NEP) is lo-l3 W Hz-r”. The very
low l/f corner frequency of the detector allows the use of
relatively low modulation rates.
4. Oven and cell
The cell is composed of several cylindrical pieces made
of 304 stainless steel attached with O-rings and clamps so
that it is easily taken apart and cleaned after extensive operation. The total length of the cell is 0.5 m and is generally
0.1 m in diameter. Where the oven is attached, the cell is
composed of a cross piece. The oven attaches to the bottom
of this cross. On the top of the cross piece, there is a flange
through which reactant gases can be added to the cell.
The oven itself consists of a stainless steel cylindrical
chamber 76 mm high by 95 mm in diameter. The outer 6 mm
of the oven sides is a water-cooling jacket. A tungsten basket
Rev. Sci. Instrum., Vol. 65, No. 5, May 1994
1520
Sub-millimeter
spectrometer
Downloaded 16 Mar 2001 to 128.196.209.95. Redistribution subject to AIP copyright, see http://ojps.aip.org/rsio/rsicpyrts.html
ments varies from metal to metal and with basket size. Vapor
production times range from 0.5 h for 15 g of magnesium in
the small crucible up to 4 h for 25-30 g of barium in a large
crucible.
MgOH (X?Z+): N=12 -13
III. RESULTS
J=?+$
385,155.0
385,21
Frequency (MHz)
FIG. 4. Spectra of the N=12-+13 rotational transition of MgOH measured
near 385 GIiz with the spectrometer described here. The two features
present in each spectrum are fine structure components of the transition. The
spectra illustrate the improvement in instrumental baseline effects when the
rooftop reflector is used as a pathlength modulator (bottom panel), as opposed to holding the reflector fixed (top panel).
is mounted inside the oven chamber for vaporization of
samples. IIvo copper rods passing through vacuum
feed&roughs supply electrical current to the tungsten basket.
A stainless steel tube through the bottom flange can supply a
carrier gas (usually Ar) to the oven. A tantalum sheet (0.25
mm thick) is placed on the inner diameter of the watercooled jacket to act as a heat shield. A removable oven lid
with a 38 mm hole contains the vapor to prevent coating of
the nearby lens and chamber with unreacted metal and also
controls heat loss. Reactant gases are usually introduced
through the top of the cell at the cross piece.
Solid metal is vaporized by placing it in a dielectric crucible which fits in the tungsten basket. ?Lvo sizes of tungsten
baskets are currently used with two standard type crucibles
(25 and 38 mm: R. D. Mathis). Typically, Al,O, (Alumina)
crucibles are employed which do not degrade when heated
with most of the metals thus far used, although barium and
ahrminum do corrode them after a few vaporizations. (Both
of these metals liquify before vapor is produced, while others
sublime.) Normal voltage requirements for the oven are approximately 4-8 V and lo-30 A, but the power require-
The millimeter/sub-mm spectra of a wide range of molecules have been investigated using this system. Several results demonstrate the sensitivity and versatility of the spectrometer.
Pure rotational spectra of various metal hydroxide, Buoride, and metal hydride free radicals in their ground and several vibrationally excited modes have been measured. Hydroxides studied thus far include CaOH (Ref. 12), MgOH
(Ref. 13), SrOH (Ref. 14), AlOH (Ref. 15) and BaOH (Ref.
16). These were all created by heating the appropriate metal
in the oven and introducing a reactant gas, in this case hydrogen peroxide. The hydride radicals MgH (Ref. 17) and
CaH (Ref. 18) have been investigated as well. For these species, a microwave discharge was created in a quartz tube
attached to the side of the cell near the oven. This discharge
dissociated H2 molecules into H atoms, which then reacted
with metal vapor to create the desired radical. Here, MgF
(Ref. 19) was created in a manner similar to the hydrides,
using a discharge in F2 instead of Ha.
The frequency range of the spectrometer enables measurement of multiple rotational transitions of a species. ‘This
allows accurate determination of the molecule’s rotational,
fine structure, and hyperfine constants, from which physical
properties of the radical are derived. For example, the frequencies of 23 rotational transitions of BaOH were measured
over the range 77-376 GHz with this instrument and 19
transitions of SrOH were observed in the region 89-370
GHz.
Good resolving power permits the rotational structure in
the spectra of free radicals to be accurately measured.
Linewidths observed with this instrument for radicals and
closed-shell species range typically from 200-900 kHz, with
the linewidth increasing with frequency. The increase in linewidth with frequency results primarily from modulation and
pressure broadening. Lowering the frequency modulation deviation amplitude at the higher frequencies decreases the
linewidth to some extent; however, it also lowers the signalto-noise ratio. Even with the broader linewidths, fine and
hyperfine structures have readily been resolved in the radicals investigated.
Detection of rare isotopomers requires very high sensitivity. Measurements with the spectrometer indicate an ability to detect absorption to a concentration limit of a few parts
per million. This is considerably less than expected from the
NEP of the bolometer. Sensitivity is limited rather by the
amplitude stability of the Gunn oscillator and by other
sources of electrical noise. The sensitivity may be degraded
by a further order of magnitude by baseline instabilities. Despite this, spectra of the rarer metal isotopomers have been
observed with this instrument. For example, many rotational
transitions were measured for BaOH with the ‘37Ba and
‘36Ba nuclei, where 138 isotope is the most prevalent form
Ci3’Ba:71.66%,‘37Ba:11.32%;136Ba:7.81%).Here,MgOH
Rev. Sci. Instrum., Vol. 65, No. 5, Allay 1994
Sub-millimeter
spectrometer
1521
Downloaded 16 Mar 2001 to 128.196.209.95. Redistribution subject to AIP copyright, see http://ojps.aip.org/rsio/rsicpyrts.html
Wide frequency scans are easily made with the spectrometer so that spectral lines may be found, even when the
optical and theoretical predictions of transition frequencies
are poor. Typically, a 100 MHz scan is done by electrical
tuning in -5 min and a 1 GHz scan with additional mechanical tuning in -1.5 h. Because of inaccurate optical data,
several GHz had to be scanned to detect MgOH. The ease of
scanning was a critical factor in obtaining the rotational
spectrum of this radical.
‘37BaOH(x2 IS+) : N=25 - 26
J - 9-2
-2
J=5_1+S
2
2
2
ACKNOWLEDGMENTS
337,600
337,650
Prequency @II-Iz)
FIG. 5. Spectrum of the N=25+26 rotational transition near 337 GHz of
the barium 137 isotopomer of the BaOH radical observed in its natural
abundance (13’Ba: 71.66%; 13’Ba.. 11.32%), measured with the spectrometer
described here. Tbe eight hyperfine components of this transition, indicated
by quantum number F, are clearly resolved in the spectrum.
and MgF were observed with the less abundant 25Mg and
26Mg isotopes (24Mg: 78.60%; 25Mg: 10.11%; 26Mg:
11.29%) and SrOH with the *‘Sr isotope (s8Sr: 82.56%; s6Sr:
9.86%). The a6Mg isotope was seen in MgH as well.
Typical spectra of free radicals studied with this instrument are shown in Figs. 4 and 5. Figure 4 is a spectrum of
the N=12--+13 rotational transition of MgOH near 385 GHz.
The top panel shows the spectrum without the pathlength
modulator (see Sec. II B 2). The bottom panel shows the reduction in baseline ripple with the modulator. Both spectra
are 100 MHz wide and are averages of two 6 min scans, one
taken with increasing frequency and one taken with decreasing frequency. The lines appear to be in emission because of
the phase chosen for the lock-in amplifier. The two features
present in each spectum are the spin rotation components of
the N=12-+ 13 transition. They are separated by 37 MHz
and are clearly resolved.13 Figure 5 shows the N=12--+13 of
‘37BaOH near 337 GHz, measured with the less abundant
137Baisotope in natural abundance. The eight hyperfine components, which arise from the interaction of the 137Banuclear
spin of 312 and the electron spin, are readily resolved in this
spectrum.16
1522
Rev. Sci. Instrum.,
Vol. 65, No. 5, May 1994
This work was supported by NSF Grant Nos. AST-9110701 and AST-92-53682 (Presidential Faculty Fellow
Award) and NASA Grant No. NAGW 2989. The authors
thank J. M. Payne and the staff at NRAO, Tucson, and Jesse
Davis of Cochise Instruments, for their advice and help in
the design of the spectrometer.
‘L. M. Ziurys, Evolution of the interstellar Medium, edited by L. Blitz
‘(Astronomical Society of the Pacific, San Francisco, 1990), p. 22.9.
‘C. H. Townes and A. L. Scbawlow, Microwave Spectroscopy, 2nd ed.
(Dover, New York, 1975).
“J E Carlstrom R. L. Plambeck, and D. D. Thorton, IEEE Trans. Microwave Theory Tkcbnol. m-33,
610 (1985).
“J. B. West, R S. Bradford, J. D. Eversole, and C. R. Jones, Rev. Sci.
Instrum. 46, 164 (1975).
5J. F. Verdieck and C. D. Comwell, Rev. Sci. Instrum. 32, 1383 (1961).
6P. F. Goldsmith, fnfrared and Millimeterwaves, edited by K. Button (Academic, Orlando, 1982), Vol. 6, p. 227.
‘R. J. Wylde, Proc. IEE, Part H, Wl, 258 (1984).
‘B. MacA. Thomas, IEEE Trans. Antennas and Propagat. AP-26, 367
(1978).
‘J. R. Birch, J. D. Dromey, and J. Lesurf, Infrared Pbys. 21,225 (1981).
r”J. W. Lamb (in preparation).
“J. M. Payne and B. L. Ulicb (private communication).
t*L. M. Ziurys, W. L. Barclay, Jr., and M. A. Anderson, Astropbys. J. Lett.
384, L63 (1992).
13W. L. Barclay, Jr., M. A. Anderson, and L. M. Ziurys, Cbem. Pbys. Lett.
196, 225 (1992).
t4M A Anderson W. L. Barclay, Jr., and L. M. Ziurys, Cbem. Pbys. Lett.
196. ;66 (1992):
“A J Apponi, W. L. Barclay, Jr., and L. M. Ziurys, Astrophys. J. Lett. 414,
Li29 (1993).
“M. A. Anderson, M. D. Allen, W. L. Barclay, Jr., and L. M. Ziurys, Cbem.
Phys. L&t. 205, 415 (1993).
“L. M. Ziurys, W. L. Barclay, Jr., and M. A. Anderson, Astrophys. J. Lett.
402. L21 (1993).
“W L. Barclay Jr., M. A. Anderson, and L. M. Ziurys, Astropbys. J. Lett.
4ci3, L65 (1963).
t9M. A. Anderson, M. D. Allen and L. M. Ziurys, J. Cbem. Pbys. 100, 824
(1994).
Sub-millimeter
spectrometelr
Downloaded 16 Mar 2001 to 128.196.209.95. Redistribution subject to AIP copyright, see http://ojps.aip.org/rsio/rsicpyrts.html