High resolution rotational spectroscopy has long been central to remote sensing techniques in atmospheric sciences and astronomy. As such, laboratory measurements must supply the required data to make direct interpretation of data for instruments which sense atmospheres using rotational spectra. Spectral measurements in the microwave and far infrared regions are also very powerful tools when combined with infrared measurements for characterizing the rotational structure of vibrational spectra. In the past decade new techniques were developed which have pushed high resolution spectroscopy into the wavelength region between 25 micrometers and 2 mm. Techniques to be described include: (1) harmonic generation of microwave sources, (2) infrared laser difference frequency generation, (3) laser sideband generation, and (4) ultrahigh resolution interferometers

Pickett, Herbert M.

English

NASA Technical Reports Server

N90-267 6
HIGH RESOLUTION SPECTROSCOPY IN THE MICROWAVE
AND FAR INFRARED
HERBERT M. PICKETT
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
ABSTRACT
High resolution rotational spectroscopy has long been central to remote sensing tech-
niques in atmospheric sciences and astronomy. As such, laboratory measurements must
supply the required data to make direct interpretation of data for instruments which sense
atmospheres using rotational spectra. Spectral measurements in the microwave and far
infrared regions are also very powerful tools when combined with infrared measurements
for characterizing the rotational structure of vibrational spectra. In the past decade new
techniques have been developed which have pushed high resolution spectroscopy into the
wavelength region between 25 #m m_d 2 ram. Techniques to be described include: (1) har-
monic generation of microwave sources, (2) infrared laser difference frequency generation,
(3) laser sideband generation, and (4) ultra high resolution interferometers.
INTRODUCTION
The rotational spectra of molecules provide a very sensitive probe for remote sensing
in the atmospheres of planets and the interstellar medium. The rotational lines are very
narrow and uniquely characteristic of the species observed. Since the lines can be observed
in thermal emission, temperature can can often be determined. In addition, the profiles
of the lines can be used to infer pressure and species abundance. When observations are
made close to the planet, limb observations can be made with great sensitivity. Weight
and power limitations have made limb observations impractical for observations of planets
other than the Earth. However, nadir observations are still of great utility provided that
there is sufficient contrast between sources of continuum emission (ground and clouds)
and the molecular emission. A consequence of this limitation is that observations are most
sensitive to areas of planetary atmospheres which are at a different temperature than the
ground or the cloud tops.
To interpret the data, or indeed to determine the sensitivity of a potential instrument, it
is essential for laboratory spectroscopy to determine the transition frequencies of the lines,
their pressure broadening coefficients, and their strengths. It is also important to determine
the spectra of all plausible molecules so that interfering lines are avoided. These needs are
common to all regions of the spectra where high resolution observations are made. In fact,
determinations of rotational spectra and infrared vibrational spectra have been combined
in a powerful way to provide more accurate information for use in both spectral regions. 1
The trend for planetary instruments which observe rotational lines is to evolve to higher
frequency. The antenna, optics become smaller with smaller wavelength and the absorption
27
https://ntrs.nasa.gov/search.jsp?R=19900017430 2020-03-24T03:05:29+00:00Z
coefficientsof molecules generally become larger. Thus, there is both a size premium and
a sensitivity premium in moving to higher frequency. In addition, many of the hydride
molecules such as NH3, PH3, H20, HDO, CH3D, and HCN have their predominant spectra
in the far infrared. Other molecules such as CO and S02 have strong spectra throughout
the far infrared. The difficulty with this region is that water in the Earth's atmosphere
absorbs strongly, and so observations are best made from space. Technology for high res-
olution submillimeter and far infrared instruments is evolving rapidly and will form an
important part of planetary observations in the future. In the next decade, application
will be found in high altitude observatories, aircraft and balloons. Astrophysics missions
such as the Large Deployable Reflector (LDR) and its precursors will provide significant
measurement capability in the submillimeter wavelength region. Such measurement ca-
pability will be of little use to the planetary community unless an adequate laboratory
spectral data base exists.
The emphasis of this paper will be on the new techniques for submillimeter and far
infrared laboratory spectroscopy which have appeared in the past decade. With encour-
agement, these techniques can become important tools for laboratory measurements of
planetary molecules.
ACllVE FnEOUE_Cy
_ t _g
[ heJL_nI_uER I 6/ _ AN_PT_ON CELLsxnr_rEn -/:ran 4_ G_4:-
i "OLVg T_YLENE
• LE_'S
I _ _ _ I LO_.... 1
_ ........ r'
C12_
_ROWXVE.
_SC_GE
J
A_L_OU_nUT
Fig. 1" Submillimeter Spectrometer Using Harmonic Generation
HARMONIC GENERATION
Harmonic generation is one of the oldest of the far infrared spectral techniques, and
was developed by Walter Gordy and his students at Duke University. 2 Recently, several
developments have made this technique more sensitive for routine measurements. The
28
method involves illuminating a diode with microwave power and detecting absorption with
the harmonics of the microwave source. The general scheme for this is illustrated in Fig. 1.
The first development has been to replace the W-Si point contact diodes with monolithic
Schottky diodes. The diodes we have used most successfully for this application have been
produced by R. Mattauch at the University of Virginia. a Interestingly, the best types
have been high frequency mixer diodes. Varactor diodes, while theoretically better for
multiplication, suffer from excess diode capacitance. The best mount for high harmonic
generation is still the simple crossed waveguide mount originally developed by Gordy.
While more efficient mounts can be made for doubling or tripling, the task of terminating
all the harmonics and idler frequencies becomes a very difficult design problem at higher
multiples. Our experience is that the harmonic power in the simple crossed waveguide
mount drops by 6-10 dB per harmonic.
The second development has been the use of sensitive liquid He cooled detectors. The
best in the 0.3 - 2 mm region is the InSb hot electron bolometer. 4 Unlike most bolometers,
the InSb bolometer has its limiting thermal conductance between the conduction electrons
(which are heated by the far infrared radiation) and the lattice. Because of this the InSb
bolometer is best mounted in good thermal contact with the liquid He cold surface. The
response time is < 1#see, making the detector more immune to 1/f noise in the source,
but the sensitivity falls off above 20 cm -1.
The third development, has been the use of a dichroic plate to filter out the lower
harmonics of the harmonic generator. These plates are designed after similar but much
larger plates used to separate S and X band at the JPL Deep Space Network. In our
application the plates are made of 1-2 mm thick aluminum and have a close-packed array
of circular holes. Each of the holes act like a small circular waveguide. The plate reflects
frequencies which are below the cut-off frequency of the waveguide. The plates have better
than 80% transmission up to about twice the cut-off frequency, where diffraction starts
becoming important. The highest frequency cutoff filter we have made has a 500 GHz
(16.7 cm -1) cutoff employing about 2000 holes of 0.25 mm diameter.
These developments have allowed us to make many studies of spectra in the 10-30 cm -1
region. An unretouched example is shown in Fig. 2. It is the J,K = 3,3 inversion transition
of ammonia in the Y2 = 1 excited state at 1073 049.708 MHz (35.79308 cm -1).
LASER DIFFERENCE FREQUENCY GENERATION
Laser difference frequency generation is a technique developed by Ken Evenson and his
co-workers at NBS (now NIST) in Boulder, CO. s The apparatus for two photon generation
is shown in Fig. 3. Central to this technique is the W-NiO-Ni tunnel junction called a MIM
diode which provides the non-linearity fi)r mixing the two CO2 lasers. One of these lasers
is fixed in frequency and Lamb dip stabilized to an accuracy of 10 kHz. The second
waveguide laser can be tuned over -_ 100 MHz. Its frequency is determined by beating
a portion of the output with a Lamb dip stabilized laser, which is lasing on the same
CO2 line, in a HgCdTe high speed detector. By choosing different laser transitions wide
29
...... : ,,li llll " i_i _ili i [
.... ' _ _'_ _ijEJiii_;_i i_,ii!!jI i_,Ji_ r ,_r:_,_-_,_,_:.LL_ -t. tf,li- ;ii! I i!:
I _i Ii i_; ::!I 'i_: :: i;;ii.i! !i;;ii_iill]-Hl[t-,ltl,lilil___,',i [I_ tlii l_,i,i_,,.. i,; I_,.'".
¢[i r[ii !rl: F.:" ':: : i ':;11!_1,/,i1_!1:1IiIIil i I ;:_ !_' IV:
............ =---_-.-:-:-r--:_ r. T_I ii7_ :_i !-_ii-: i!:._ii" :ii
I. - i! /:; i .:rll ii!ili-ii _i_11 i,i,..i 'lti''ii-,ii' 'i;'"iii""':'"
.... .... _. b-;.- i- :" "": li_4'_H4 tI_W/I:iH
...... ;. . ":- i ;.- " _,.: i_ fir . [ !!:llii:i:!:.tl;.:l:.!!!i:.
: : :- F i- " , t : i_ :-rTi t,-T]-,,I i:
':l:'+, ,_l_ ;;:i '!r_ ;I , l,
:i)i[!:li:ihH_:MmriiiH_rfl:H:i-it:l}[ _liF_i:_HI
Fig. 2:NH3 _2 inversion line for J,K = 3,3 at 1073 049.708 MHz
I rf Sweep Osc. I
"-il-'
,2 v°±vs
Servo
CO;_ laser rl I
f
I
Waveguide LJ
C02 Laser
v I
" ' CO2 Laser I
MIM _I_ I/FIR I
diode _ : i Abs. Cell
]
i
i
I
I
I,
Plollet 1
-5-,
I 1 I I I I' I
ly
I
#
I
I
I Lock.in Amp.
I
I
I
i
t- J
Fig. 3: Two Photon Laser Difference Generation
frequency coverage can be obtained in selected 100 MHz patches. Frequency accuracy is
limited by ability to measure the peak position of the far infrared absorptions or _ 200
kHz.
In order to fill in the frequency coverage, Evenson has developed a three photon method
in which two mid-infrared photons are combined with a microwave photon. * In this method,
the two CO2 lasers are both Lamb dip stabilized, and the tuning comes from the microwave
30
source. The optical layout is shown in Fig. 4. Frequency accuracy is comparable to the two
photon method, but getting three wave mixing from the MIM diode is much more difficult.
Computer Control
and
Data AcquisitionI
I
I
I
t
o,ooe I A°s'Ce" i
Fig. 4: Three Photon Laser and Microwave Difference Generation
Actually three far infrared frequencies are generated: one from the difference of the two
infrared lasers, and two which are above and below the two photon frequency by an amount
equal to the microwave frequency. These are readily distinguished by frequency modulating
one of the lasers and observing the sense of the first derivative lineshape obtained under
phase-sensitive detection while sweeping the microwave source.
LASER SIDEBAND GENERATION
Laser sideband generation involves mixing the output of a fixed-frequency far infrared
laser with a microwave source to produce sidebands above and below the laser frequency.
Early demonstrations of this technique were made by Dymanus in the Netherlands _ and by
a group at Lincoln Laboratory. s Our improvement of the technique at JPL proceeded con-
currently with Evenson's work using infrared lasers. The goal of our work was to produce a
source which had an accuracy of 100 kHz and had adequate power to make sensitive spec-
troscopic measurements. The far infrared laser is obtained by pumping a molecule such as
methanol with a high power CO2 laser and observing lasing from rotational transitions in
the pumped vibrational state. While the output frequency is nominally at the rotational
frequency, cavity and pump pulling effects make the laser frequency uncertain by as much
as several MHz. Our strategy was to measure "today's" laser frequency by measuring a
transition of known frequency before or after the unknown transition is measured. This
required development of a very stable far infrared laser using Invar and active temperature
stabilization. 9 A diagram of it is shown in Fig. 5. Interestingly, this laser also turned out
31
CO2 LASER(APOLLO1.50)
lm FOCALLENGTH/,
I
I
RDERBEAM I_ HgCdTe
"_ FABRY-PEROT DETECTOR-
E'TALON
(BURLEIGH)
POLY
WINDOW COOLINGWATER FIR LASER
(SCIENTECH_I) r^c_.,.,_ " ...../. ...... _ FLATOR
v_b INLEI II_V_I_ _UU ru_vw BREWSTER
Fig. 5: Far Infrared Laser
to be a very high power laser, and our measured power of 1.25 W at 118 #m wavelength
is probably a far infrared world record. More importantly, the frequency stability is < 100
kHz / hour.
In order to have usable sideband output power, it is essential to have a good way of
separating sidebands from the laser. The way we chose to do this is shown in Fig. 6.1° The
PARABOLIC
MIRROR
FERRIIE
MODULATOR
CORNER
CUBE MIXER
MILL ih'_[ I ER-WA',"_.
PHASL-LOC KED
KLYSTROI'_
POLARIZING
MICHELSON
INTERFEROMETER
ABSORPTION CELL
I
I
I
I
j LOCK-IN iAMPLIFIER I
I
I
!
(InSb OR lI I°' I
Fig. 6: Laser Sideband Generator
output from the laser passes through the analyzer polarizer and the polarizing Martin-
32
ORIGINAL PAGE IS
OF POOR .QUALITY'
Puplett interferometer such that it is not attenuated and arrives at the corner cube diode
mount with the correct polarization. At the diode, the laser is mixed with the output of
a mm wavelength klystron. The sidebands and the unused laser power are reflected from
1
the corner cube back through the interferometer. If the optical path difference is _ _x
the wavelength of the microwave source, then the sideband polarization is rotated by 90 °
with respect to the laser. As a consequence, the sidebands are reflected off the analyzer
polarizer toward the sample cell and detector. Frequently we use a Fabry-Perot resonator
to discriminate between the sidebands and to provide further rejection of the laser. A
typical length scan of this Fabry-Perot, shown in Fig. 7, clearly shows the two sidebands
and the original laser frequency.
163 F.m 163 _m
iii I "I I " I I I
c_
D
173 F.m 173 F.m
.._1
'< 155 ,y.m
I--
<
_J
u.l
if" + -
900 950 1000 1050 1100 1150 1200
SEPARATION BETWEEN FABRY-PEROT REFLECTORS
Fig. 7: Fabry-Perot Scan of Laser Sideband Output
Fig. 8 shows spectra obtained of ammonia and HDO using two cells in series. Accurate
frequency measurements require a reference gas whose frequencies are known to the 100
kHz level, and whose doppler width facilitates accurate measurements. HDO is not a good
reference gas in this sense, but CO is better since extensive laser difference measurements
have been made on this gas. 11 An even better choice is a gas like SO2, which has a narrower
doppler width and a denser spectrum. We have calibrated SO2 against the CO standard,
and in addition have made several measurements of two SO2 lines in the same laser sideband
scan. In many cases, for example if the lines are P and R branch lines of different K, the
difference in frequency is almost as valuable to the fit as the absolute frequency. The fit
combining these far infrared determinations with microwave data is providing a very useful
secondary standard for the laser sideband technique which is accurate to the level of our
ability to measure the frequencies.
33
oo
o
d
Q:
O
176.3.5
| .... i .... , ! , l ,
HDO /_
Jx='r'° = 303 "'" 404 / /
.... /-x, / /
.... ! .... ! .... ! .... ! .... L.-,-, , , i ....
176.3.51 176,,3.52 1763.53 t753.54 176,3.55 1763.56
FREOUENCY (GHz)
1763.57
Fig. 8: Laser Sideband Spectrum of NH3 and HDO
INTERFEROMETERS
There has been a steady improvement of both user-constructed and commercial far
infrared interferometers. In the far infrared it is important to have large optics and high
throughput so that resolution is not limited by diffraction. In the long wavelength region,
the advantages of the polarizing Martin-Puplett design (in which a wire-grid polarizer forms
the beam splitter) have been employed by B. Carli at IROE in Italy. 12 More common
are the mylar beam splitter Michelson interferometers represented by the Bomem and
Bruker instruments. The Bomem instrument in its highest resolution form has an optical
path difference of 2.5 m, although J. Johns at the Hertzberg Institute in Canada has a
modification to effectively double this difference. 13 The Brucker instrument in its highest
resolution form has an optical path difference of 6 m, which represents a resolution of
0.0016 cm -1 Last November JPL received delivery of the first Bruker of this resolution in
this country. We are still in the process of characterizing this instrument, but an example
of the spectra we have obtained is shown in Fig. 9. Unfortunately, the pressure is too high
to display the spectra at the doppler limit, but an indication of the resolution is given by
the splitting at 101.6 cm -1 which is 0.005 cln -1. While the ultimate resolution of 50 MHz
is not comparable to laser-based methods, it is very good for survey work and linewidth
measurements.
CONCLUSIONS
The four techniques described here are both complementary and powerful. Harmonic
generation is most useful for the millimeter wavelength end of the spectral region. High
resolution interferometers are diffraction limited in this region, but are very accurate and
34
1.1022
0.9885
0.8247
0.761
0.6423
0.5336
0.4199
0.3062
0.1924
0.0282
-0.035
100.0
I I
100.2 100.4
I I I I I I I
00.6 100.8 101.9 101.2 101.4 101.6 101.8 102.0
WFIVENUMBERS CM-I
Fig. 9: Bruker Spectrum of HDO at 2 Torr Pressure
sensitive survey instruments at shorter wavelengths. The two methods for laser generation
of tunable far infrared radiation are capable of higher resolution measurements on selected
lines. Difference frequency generation has high absolute accuracy in frequency, but laser
sideband generation is probably easier once suitable standards are available.
ACKNOWLEDGEMENT
Part of the research described in this paper was performed by Jet Propulsion Labora-
tory, California Institute of Technology, under contract with the National Aeronautics and
Space Administration.
REFERENCES
1. H. M. Pickett, E. A. Cohen, L. R. Brown, C. P. Rinsland, M. A. H. Smith, V.
Malathy Devi, A. Goldman, A. Bathe, B. Carli, and M. Carlotti, J. Mol. Spec. 128,
151 (1988).
2. W. Gordy, and R. L. Cook, Microwave Molecular Spectra, Techniques of Chemistry,
Wiley, N. Y., Vol. XVIII, (1984).
3. T. W. Crowe, and R. J. Mattauch, IEEE Trans. Microwave Theory Tech. 27_ 159
( 9s7)
4. M. A. Kinch, and B. V. Rollin, Brit. Y. Appl. Phys. 14, 672 (1963).
5. K. M. Evenson, D. A. Jennings, and F. R. Peterson, Appl. Phys. Left. 44,576 (1984).
35
o K. M. Evenson, D. A. Jennings, L. R. Zink, and K. R. Leopold, 11th Conf. Infrared
and Mm. Waves, Pisa, Italy, 267 (1986).
7. D. D. Bicanic, B. F. J. Zuiberg, and A. Dymanus, Appl. Phys. Lett. 32,367 (1978).
. H. R. Fetterman, P. E. Tannenwald, B. J. Clifton, W. D. Fitzgerald, and N. R.
Erickson, Appl. Phys. Lett. 33_ 151 (1978).
9. J. Farhoomand, and H. M. Pickett, Int. Y. Infrared and Mm. Waves 8, 441 (1987).
10.
11.
12.
J. Farhoomand, G. A. Blake, M. A. Frerking, and H. M. Pickett, J. Appl. Phys. 57,
1763 (1985).
I. G. Nolt, J. V. Radositz, G. DiLonardo, K. M. Evenson, D. A. Jennings, K. R.
Leopold, M. D. Vanek, L. R. Zink, A. Hinz, and K. V. Chance, Y. Mol. Spec. 125,
274 (1987).
M. G. Baldecchi, B. Carli, M. Carlotti, G. DiLonardo, F. Forni, F. Mencaraglia, and
A. Trombetti, Int. Y. Infrared and Mm. Waves 5,381 (1984).
13. J. W. C. Johns, and J. Vander Auwera, submittted to Y. Mol. Spec. , (1989).
36


High resolution spectroscopy in the microwave and far infrared

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