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Waveguide and diode heterodyne measurements with CO2 laser and assignment of the CW FIR laser emission of OCS
JOURNAL• FMOLECULARSPECTROSCOPY
140,252-258(1990)
Waveguide and Diode Heterodyne Measurements with CO2 Laser and Assignment of the CW FIR Laser Emission of OCS A. FAYT AND J. G. LAHAYE Molecular Speclroscopy Laboratory, Catholic Universityof Louvain, Chemin du cyclotron,2 81348 Louvain-la-Neuve,Belgium AND
J. LEMAIRE, F. HERLEMONT, AND J. G. BANTEGNIE Laboratoire de SpectroscopicHertzienne associe’au CNRS No. 249, Universitede LiNe I, FS9655 Villeneuve d’ascq Cedex, France Heterodyne spectra ofcarbonyl sulfidehave been obtainedwith saturation in a COz waveguide laser and without saturation with a diode laser. The absolute uncertainties ofthe measured positions lie between lOA and 7 X 1O-6 cm-‘. The CW submillimeter laser line of OCS at 378 pm has been assigned to the J = 65 -+ 64 transition in the 0220e state of ‘60’2C3’S, with a pumping through the R(64) line of 02*0e + 0000, a A-Z transition weakly allowed by the I-type resonance. 0 1990 Academic Press, Inc. INTRODUCTION
Good secondary frequency standards are necessary to calibrate the infrared spectra obtained by Fourier transform (FT) spectroscopy or with diode lasers. If an absolute accuracy of 1O-4 cm-’ is enough for diode laser spectra, FT spectra require a 10W5 cm-’ accuracy to take full advantage of their internal coherence. Carbonyl sulfide is one of the molecules usable as secondary standards, on account of its spectrum which is of good intensity, moderately dense, of relatively simple structure, and broadly spread through the infrared domain. The goal of heterodyne spectroscopy is to transfer the accuracy of some primary standards to a laser which is locked to the center of the absorption line to be measured. Generally some accuracy is lost in the transfer. A conventional low-pressure CO2 laser, which emits in the range of the 2~2 band of OCS, is an excellent primary standard (I), highly monochromatic, and easily stabilized within about 30 kHz when locked to the 4.3~pm CO2 fluorescence Lamb-dip. To observe the absorption line, we need another laser to be tuned to the corresponding frequency before counting the beat frequency between the two lasers. The CO2 waveguide laser is quite suitable for this purpose (2): it is quite monochromatic, and its power allows the saturation of the absorption line giving rise to the observation of a Lamb-dip of width about 1 MHz on which the laser can be stabilized within 100 kHz or better. The essential disadvantage of the waveguide CO, laser is the limitation of its tuning range within a few hundred megahertz around the center of each COz 252
0022-2852190 $3.00 Copyright
0
1990 by Academic
All rights of reproduction
Press, Inc.
in any form reserved.
HETERODYNE
MEASUREMENTS
253
ON OCS
laser line. That is the reason why no more than two such heterodyne measurements are reported in this paper, but they are the most accurate frequency measurements on OCS in the infrared range, except in Stark spectroscopy ( 3). Broadly tunable diode lasers allow the observation of many more lines but their accuracy is limited to about 3 MHz for two reasons: first, the frequency noise of these lasers is in the range of a few megahertz; second, their low power does not saturate the absorption lines which are thus measured on the basis of their Doppler profiles ( - 50 MHz width). We report here the diode laser heterodyne measurements of 20 OCS lines. CO2 WAVEGUIDE
LASER
HETERODYNE
MEASUREMENTS
The experimental setup has been previously described elsewhere ( 4). The absorbing gas is set inside the COZ waveguide laser cavity where it is submitted to the laser standing wave. The saturation resonances are observed by monitoring the power emitted by the laser as its frequency is tuned. The measurements are made relative to a conventional CO* laser locked to the 4.3-pm CO* fluorescence-detected Lamb-dip. The waveguide laser is locked to the saturated absorption dip and the beat note between the two lasers is counted. Absolute frequency accuracy is estimated to be between 0.1 and 1.O MHz. We have applied this technique to measure two OCS absorption lines previously observed without saturation (5). They appear in Table I. The 02°0-OOo0 P( 5 ) line is the most accurate frequency measurement on OCS in the infrared. Its accuracy is confirmed by saturation Stark measurements (3) whose accuracy reaches 20 kHz. This transition has been used by Schlossberg and Fetterman (6) to generate 20 pm pulsed emission from OCS. The other line is also observed by Stark spectroscopy and it is involved in the discussion about the assignment of the CW submillimeter emission line of OCS ( 7). DIODE
LASER
HETERODYNE
MEASUREMENTS
The Pb salt diode laser used is mounted in its commercially available (Laser Analytics) close cycle refrigerator. As shown in Fig. 1, a conventional setup is used to select the different diode laser modes. The beam is then split to allow the absorption line monitoring and heterodyne frequency measurements. The calibration of the spectrum is obtained using varactor mixing of the diode laser wave with a conventional CO* laser beam locked to the 4.3~pm fluorescence-detected CO2 Lamb-dip. Fast HgCdTe photodiodes from the S.A.T. (Soci& Anonyme des T&communications) are set in a microwave package that allows them to be biased TABLE
I
Saturation Heterodyne Measurements on OCS inside the CO2 Waveguide Laser Cavity Cq law line (cd)
Shill (MHz)
ocs line(em’)
9P(22) 1045.021670
+ 129.16(20)
1045.025978(7)
-4
02%
oooo
P(5)
9R(8)
+
1070.463719(50)
80
O&Of
02%
R(28)
1070.462308
42.3(15)
Obs-Cak (106 cm-‘)
AsipmmI
reference
CO2
I
FIG.
laser
\
I
Cell 1
I
sweep
To
I
m
photodiode
Hg Cd
I
I. Schematic of the experimental setup for heterodyne calibration of the diode laser spectra.
Monochromatol
\
ocs
frequency
I
P
ul
N
HETERODYNE
MEASUREMENTS
255
ON OCS
by a microwave signal (8,9).Efficient varactor mixing can be obtained. A frequency marker is obtained when the microwave signal is equal to the frequency difference between the two lasers plus or minus the TV tuner received frequency ( 180 MHz). When the laser frequency difference is higher than 12.4 GHz, the second or third harmonics of the microwave signal is used to generate the frequency markers. In this way, although the microwave frequency is kept lower than 12.4 GHz, markers can be obtained almost everywhere in between successive CO2 laser lines. Measurements of OCS line frequencies are made by adjusting the frequency markers on the line to be measured. The microwave frequency is determined quite accurately and the accuracy of the measurement is mainly limited by the appreciation of the top of the OCS line. The accuracy is typically a few megahertz. The 20 measured OCS lines are listed in Table II with their assignments. These lines, as well as the two from Table I, have already been introduced in the global rovibrational analysis of carbonyl sulfide (3. 10). ASSIGNMENT
OF THE CW SUBMILLIMETER
EMISSION LINE FROM
OCS
By optically pumping OCS with the 9-ym R( 8) COz laser radiation, Landsberg ( 7) has observed a CW submillimeter emission line at a wavelength of 378.4 + 0.7 pm which corresponds to 26.43 -+_0.05 cm-’ or 792.3 f 1.6 GHz. The 04*0~‘“-02~0f R( 28 ) line (Table I) is shifted by no more than 44.8 MHz from the R( 8) CO* laser line but this transition is not appropriate for optical pumping on account of the weak TABLE II Diode Laser Heterodyne Measurements on OCS C% laserline(cm-‘)
Shift (MHz)
OCS line(cd)
ObCak
(lW5 cm-‘)
A.+llmenI
9R(36) 1087.94831
8 147(3)
1087.676.55(10)
27
03’M
9RU6) 1075.98782 *, ,, *,
3 lZO(3)
1075.X8376(10)
-2
03’M
4 10x3)
1075.85091(10)
13
4 39x3)
1075.84121(10)
6
8 128(3)
1075.71670(10)
,, ,, *, II I, II ,, ,, ,, .I ,, II ,, ,, *, ,, ,, ,, II ,, II 9R(6) 1069.01409 *, ,, ,, ,, ,. ,, 9P(8) 1057.3C016 ,, ,. ,, ,, *, ,. ,, ,, ,. II ,, ,* ,I ,, ,* II ,, II
-2
OI’M
R(75)
Ol’Of
R(51)
02%
oooo
R(75)
05’Of -
03’Of
R(31)
04%
02%
R(40)
-
-
9 290(3)
1075.67793(10)
02%
OC?% R(63)
10 686(3)
1075.63140(10)
6
0530
0330
R(29)a
11 314(S)
1075.61@43(17)
15
05%
03%
R(31)
17 341(3)
1075.40940(10)
17
03’Of
01%
R(50)
19 llo(3)
1075.35038(10)
0
Moo
0200
R(40)
8 5X1(3)
1068.72786(10)
I7
04%
02%
R(24)
8 796(3)
1068.72069(10)
25
04%
02%
R(24)
8 981(3)
lc68.71454(10)
~2
04%
-
0200
R(Z)
17 463(3)
1056.71765(10)
-8
029)
.
O@%
R(22)
19 ill(3)
1056.66269(10)
-6
03’Of -
OllOf R(8)
19 768(3)
1056.64078(10)
-9
03’oe
20 726(3)
lO56.60881(10)
-
-
Ol’oe
R(8)
0420
02%
P(4)b
03’Oe P(l4)
21405(3)
1056.58613(10)
13
05’oe
21 679(3)
1056.57704(10)
26
0530
03%
P(l6)C
21 957(3)
lOS6.56774(10)
II
04%
02%
P(O3)
a 0.33 X 10e3 cm-’ doublet. ’ tl X 10d6 cm-’ doublet. ’ 5 X 10e6 cm-’ doublet.
256
FAYT ET AL.
population in its lower state lying at 1200 cm-‘. Furthermore, as already pointed out by Landsberg, the observed wavelength of emission is incompatible with this assignment. Finally Landsberg tentatively concluded that the pumping and lasing transitions were, respectively, due to the 02°0-OOo0 R(66) and to P(67) in the 02OO state of “Ocs. We have identified the pumping transition to be a forbidden A + Z transition, weakly allowed due to I-type resonance: the 02 *O-00’0 R( 64) line of natural OCS. The 22~ energy levels involved in that experiment are presented in Fig. 2. At J = 65, the 2: state is 5.567 cm-r above the A state, and the Z-type interaction energy between them is 0.906 cm-’ (IO). The effects of this resonance are first a 0.143 cm-’ shift down of the 02 *Oe level (responsible for the major part of the A splitting), and second a mixing of the interacting levels’ wavefunctions rc/(02*0e) = 0.1 56tio( 02’0) + 0.988$o( 02*0) giving rise to a 2.4% intensity transfer from the allowed 02°0-OOo0 R( 64) transition. The FIR laser emission occurs from J = 65 to J = 64 in the 02*0e vibrational state at a frequency which is calculated to be 791 600.2 f 0.2 MHz, in perfect agreement with Landsberg’s experimental value. In our study of the Stark spectrum of OCS (3)) we have observed with the 9 R( 8 ) COz laser line many Stark components of both 04 *Of-02 2Of R (28 ) and 02 20e-OOo0
EIcm
J-65
SUBMILLIMETER
FIG. 2. Upper energy levels of OCS involved in the CW submillimeter emission obtained by Landsberg (7)
HETERODYNE
MEASUREMENTS ON OCS
257
R( 64) transitions, and we have determined their shifts from the laser line, respectively, +44.8 t 0.5 and +32.0 -t 0.5 MHz. These two transitions are thus candidates for optical pumping, but, as has been observed, the second one is more efficient. First, the population is 14 times higher in 00’0 J = 64 than in 02”Of.J = 28, due to the Boltzmann factor and the degeneracy of the levels. Second, the coincidence is better for the A-Z transition and, as the absorption line is quite inhomogeneous (the homogeneous broadening is less than 1 MHz at the 60-mTorr experimental pressure), the axial velocity class interacting with the pump laser beam includes 2.35 times more molecules in the case of the best coincidence. So, combining those two effects, the number of molecules involved in the pumping process is 33 times higher for 02’OeOO’OR(64) than for 0420f-0220fR(28). What concerns the transition probability is that it is 55 times greater for R( 28) than for R(64): a factor 42 from the 2.4% intensity transfer, a factor 3 from the vibrational term, and a factor l/2.28 from the rotational term. The low transition probability of the 02°0e-OOo0 R( 64) line is not a problem in Landsberg’s experiment on account of the high power (more than 5 W) of the CO2 pump laser. On the other hand, this point is critical in the CO2 waveguide laser experiment described in the present work: no Lamb-dip is observed for the R( 64) line by lack of saturation. In our diode laser heterodyne measurements, we also observed the R( 75 ) line of the same 02 ‘Oe-00’0 forbidden transition. The intensity transfer which is proportional to J2( J f I )’ reaches 4.5% in this case. The R( 36) line of the same transition, with an intensity transfer of 0.26%, was too weak to be observed. ABOUT THE BEST STANDARD
We report in the present work the best zero field measurement on OCS, the 02’000’0 P( 5 ) line observed at 1045.025978 (7) cm -‘. Nevertheless, the saturation Stark spectra (3) of this 2~2 band are sensitively more accurate. In those experiments, the CO? laser was stabilized with an accuracy of 15 kHz, and the centers of the OCS Lamb-dips (FWHM 1: 500 kHz) were measured within 220 kHz, giving an overall accuracy estimated to be 30 kHz or 10 P6 cm-‘. The critical point is naturally the problem of the transfer of this accuracy to the zero held frequencies. However, the accuracy seems to be maintained due to the following points: -there are 22 Stark components observed for the 02°0-OOo0 band for J G 5; -the analysis uses simultaneously many kinds of available data, particularly microwave data and LMDR and MBER data in addition to the Stark data; -the Stark model avoids any approximations by dealing simultaneously with all kinds of off-diagonal terms (anharmonic, l-type, Stark, and anisotropy of polarizability ), So, at the end of the analysis, the band center of the 02°0-OOo0 band is calculated with a standard deviation of 0.7 X 10 -6 cm-’ (Table VIII of Ref. (10)). Taking 2.5 times this value as the uncertainty, we consider that the saturation Stark measurements determine the low J levels of 02’0 with an uncertainty of 2 X low6 cm-r. The corresponding calculated value for the 02°0-OOo0 P( 5) line is 1045.025982(2) cm-‘, more accurate than the experimental zero field value 1045.025978( 7) cm-’ and in perfect agreement with it. RECEIVED:
December
6. 1989
FAYT
ET AL.
REFERENCES I. G. GUELACHVILI AND K. NAFUHARIRAO, “Handbook of InfraredStandards,”Academic Press,San Diego, 1986. 2. CH. BORDE,M. OUHATOUN,A. VAN LERBERGHE, C. SALOMON,S. AVRILLIER, C. D. CANTRELL, AND J. BORDE,“Laser SpectroscopyIV” (H. Walther and K. W. Rothe, Eds.), Springer-Verlag,Berlin, 1979. 3. J. G. LAHAYE,R. VANDENHAUTE, AND A. FAYT, J. Mol. Spectrosc. 119,267-279 ( 1986). 4. F. HERLEMONT, J. FLEURY,J. LEMAIRE, ANDJ. DEMAISON, J. Chem. Phys. 76( lo), 4705-4714 ( 1982). 5. F. HERLEMONT, M. LYSZYK,AND J. LEMAIRE,J. Mol. Spectrosc. 77, 69-75 ( 1979). 6. H. R. SCHLOSSBERG AND H. R. FETTERMAN, Appl. Phys. Lett. 26,3 16-3 18 ( 1975 ). 7. B. M. LANDSBERG, IEEE J. Quantum. Electron. QE-16(7), 704-706 ( 1980). 8. M. LYSZYK,J. C. DEPANNEMAECKER, J. G. BANTEGNIE, F. HERLEMONT, J. LEMAIRE, AND Y. RIANT, Opt. Commun. 37,53-55 (1981). 9. J. LEMAIRE,J. C. DEPANNEMAECKER, F. HERLEMONT, Y. RIANT,AND J. FLEURY,SPZE Recent Dev. Mater. Detect. Infrared 588, 26-28 ( 1985). 10. A. FAYT, R. VANDENHAUTE, AND J. G. LAHAYE,J. Mol. Spectrosc. 119,233-266 (1986).
140,252-258(1990)
Waveguide and Diode Heterodyne Measurements with CO2 Laser and Assignment of the CW FIR Laser Emission of OCS A. FAYT AND J. G. LAHAYE Molecular Speclroscopy Laboratory, Catholic Universityof Louvain, Chemin du cyclotron,2 81348 Louvain-la-Neuve,Belgium AND
J. LEMAIRE, F. HERLEMONT, AND J. G. BANTEGNIE Laboratoire de SpectroscopicHertzienne associe’au CNRS No. 249, Universitede LiNe I, FS9655 Villeneuve d’ascq Cedex, France Heterodyne spectra ofcarbonyl sulfidehave been obtainedwith saturation in a COz waveguide laser and without saturation with a diode laser. The absolute uncertainties ofthe measured positions lie between lOA and 7 X 1O-6 cm-‘. The CW submillimeter laser line of OCS at 378 pm has been assigned to the J = 65 -+ 64 transition in the 0220e state of ‘60’2C3’S, with a pumping through the R(64) line of 02*0e + 0000, a A-Z transition weakly allowed by the I-type resonance. 0 1990 Academic Press, Inc. INTRODUCTION
Good secondary frequency standards are necessary to calibrate the infrared spectra obtained by Fourier transform (FT) spectroscopy or with diode lasers. If an absolute accuracy of 1O-4 cm-’ is enough for diode laser spectra, FT spectra require a 10W5 cm-’ accuracy to take full advantage of their internal coherence. Carbonyl sulfide is one of the molecules usable as secondary standards, on account of its spectrum which is of good intensity, moderately dense, of relatively simple structure, and broadly spread through the infrared domain. The goal of heterodyne spectroscopy is to transfer the accuracy of some primary standards to a laser which is locked to the center of the absorption line to be measured. Generally some accuracy is lost in the transfer. A conventional low-pressure CO2 laser, which emits in the range of the 2~2 band of OCS, is an excellent primary standard (I), highly monochromatic, and easily stabilized within about 30 kHz when locked to the 4.3~pm CO2 fluorescence Lamb-dip. To observe the absorption line, we need another laser to be tuned to the corresponding frequency before counting the beat frequency between the two lasers. The CO2 waveguide laser is quite suitable for this purpose (2): it is quite monochromatic, and its power allows the saturation of the absorption line giving rise to the observation of a Lamb-dip of width about 1 MHz on which the laser can be stabilized within 100 kHz or better. The essential disadvantage of the waveguide CO, laser is the limitation of its tuning range within a few hundred megahertz around the center of each COz 252
0022-2852190 $3.00 Copyright
0
1990 by Academic
All rights of reproduction
Press, Inc.
in any form reserved.
HETERODYNE
MEASUREMENTS
253
ON OCS
laser line. That is the reason why no more than two such heterodyne measurements are reported in this paper, but they are the most accurate frequency measurements on OCS in the infrared range, except in Stark spectroscopy ( 3). Broadly tunable diode lasers allow the observation of many more lines but their accuracy is limited to about 3 MHz for two reasons: first, the frequency noise of these lasers is in the range of a few megahertz; second, their low power does not saturate the absorption lines which are thus measured on the basis of their Doppler profiles ( - 50 MHz width). We report here the diode laser heterodyne measurements of 20 OCS lines. CO2 WAVEGUIDE
LASER
HETERODYNE
MEASUREMENTS
The experimental setup has been previously described elsewhere ( 4). The absorbing gas is set inside the COZ waveguide laser cavity where it is submitted to the laser standing wave. The saturation resonances are observed by monitoring the power emitted by the laser as its frequency is tuned. The measurements are made relative to a conventional CO* laser locked to the 4.3-pm CO* fluorescence-detected Lamb-dip. The waveguide laser is locked to the saturated absorption dip and the beat note between the two lasers is counted. Absolute frequency accuracy is estimated to be between 0.1 and 1.O MHz. We have applied this technique to measure two OCS absorption lines previously observed without saturation (5). They appear in Table I. The 02°0-OOo0 P( 5 ) line is the most accurate frequency measurement on OCS in the infrared. Its accuracy is confirmed by saturation Stark measurements (3) whose accuracy reaches 20 kHz. This transition has been used by Schlossberg and Fetterman (6) to generate 20 pm pulsed emission from OCS. The other line is also observed by Stark spectroscopy and it is involved in the discussion about the assignment of the CW submillimeter emission line of OCS ( 7). DIODE
LASER
HETERODYNE
MEASUREMENTS
The Pb salt diode laser used is mounted in its commercially available (Laser Analytics) close cycle refrigerator. As shown in Fig. 1, a conventional setup is used to select the different diode laser modes. The beam is then split to allow the absorption line monitoring and heterodyne frequency measurements. The calibration of the spectrum is obtained using varactor mixing of the diode laser wave with a conventional CO* laser beam locked to the 4.3~pm fluorescence-detected CO2 Lamb-dip. Fast HgCdTe photodiodes from the S.A.T. (Soci& Anonyme des T&communications) are set in a microwave package that allows them to be biased TABLE
I
Saturation Heterodyne Measurements on OCS inside the CO2 Waveguide Laser Cavity Cq law line (cd)
Shill (MHz)
ocs line(em’)
9P(22) 1045.021670
+ 129.16(20)
1045.025978(7)
-4
02%
oooo
P(5)
9R(8)
+
1070.463719(50)
80
O&Of
02%
R(28)
1070.462308
42.3(15)
Obs-Cak (106 cm-‘)
AsipmmI
reference
CO2
I
FIG.
laser
\
I
Cell 1
I
sweep
To
I
m
photodiode
Hg Cd
I
I. Schematic of the experimental setup for heterodyne calibration of the diode laser spectra.
Monochromatol
\
ocs
frequency
I
P
ul
N
HETERODYNE
MEASUREMENTS
255
ON OCS
by a microwave signal (8,9).Efficient varactor mixing can be obtained. A frequency marker is obtained when the microwave signal is equal to the frequency difference between the two lasers plus or minus the TV tuner received frequency ( 180 MHz). When the laser frequency difference is higher than 12.4 GHz, the second or third harmonics of the microwave signal is used to generate the frequency markers. In this way, although the microwave frequency is kept lower than 12.4 GHz, markers can be obtained almost everywhere in between successive CO2 laser lines. Measurements of OCS line frequencies are made by adjusting the frequency markers on the line to be measured. The microwave frequency is determined quite accurately and the accuracy of the measurement is mainly limited by the appreciation of the top of the OCS line. The accuracy is typically a few megahertz. The 20 measured OCS lines are listed in Table II with their assignments. These lines, as well as the two from Table I, have already been introduced in the global rovibrational analysis of carbonyl sulfide (3. 10). ASSIGNMENT
OF THE CW SUBMILLIMETER
EMISSION LINE FROM
OCS
By optically pumping OCS with the 9-ym R( 8) COz laser radiation, Landsberg ( 7) has observed a CW submillimeter emission line at a wavelength of 378.4 + 0.7 pm which corresponds to 26.43 -+_0.05 cm-’ or 792.3 f 1.6 GHz. The 04*0~‘“-02~0f R( 28 ) line (Table I) is shifted by no more than 44.8 MHz from the R( 8) CO* laser line but this transition is not appropriate for optical pumping on account of the weak TABLE II Diode Laser Heterodyne Measurements on OCS C% laserline(cm-‘)
Shift (MHz)
OCS line(cd)
ObCak
(lW5 cm-‘)
A.+llmenI
9R(36) 1087.94831
8 147(3)
1087.676.55(10)
27
03’M
9RU6) 1075.98782 *, ,, *,
3 lZO(3)
1075.X8376(10)
-2
03’M
4 10x3)
1075.85091(10)
13
4 39x3)
1075.84121(10)
6
8 128(3)
1075.71670(10)
,, ,, *, II I, II ,, ,, ,, .I ,, II ,, ,, *, ,, ,, ,, II ,, II 9R(6) 1069.01409 *, ,, ,, ,, ,. ,, 9P(8) 1057.3C016 ,, ,. ,, ,, *, ,. ,, ,, ,. II ,, ,* ,I ,, ,* II ,, II
-2
OI’M
R(75)
Ol’Of
R(51)
02%
oooo
R(75)
05’Of -
03’Of
R(31)
04%
02%
R(40)
-
-
9 290(3)
1075.67793(10)
02%
OC?% R(63)
10 686(3)
1075.63140(10)
6
0530
0330
R(29)a
11 314(S)
1075.61@43(17)
15
05%
03%
R(31)
17 341(3)
1075.40940(10)
17
03’Of
01%
R(50)
19 llo(3)
1075.35038(10)
0
Moo
0200
R(40)
8 5X1(3)
1068.72786(10)
I7
04%
02%
R(24)
8 796(3)
1068.72069(10)
25
04%
02%
R(24)
8 981(3)
lc68.71454(10)
~2
04%
-
0200
R(Z)
17 463(3)
1056.71765(10)
-8
029)
.
O@%
R(22)
19 ill(3)
1056.66269(10)
-6
03’Of -
OllOf R(8)
19 768(3)
1056.64078(10)
-9
03’oe
20 726(3)
lO56.60881(10)
-
-
Ol’oe
R(8)
0420
02%
P(4)b
03’Oe P(l4)
21405(3)
1056.58613(10)
13
05’oe
21 679(3)
1056.57704(10)
26
0530
03%
P(l6)C
21 957(3)
lOS6.56774(10)
II
04%
02%
P(O3)
a 0.33 X 10e3 cm-’ doublet. ’ tl X 10d6 cm-’ doublet. ’ 5 X 10e6 cm-’ doublet.
256
FAYT ET AL.
population in its lower state lying at 1200 cm-‘. Furthermore, as already pointed out by Landsberg, the observed wavelength of emission is incompatible with this assignment. Finally Landsberg tentatively concluded that the pumping and lasing transitions were, respectively, due to the 02°0-OOo0 R(66) and to P(67) in the 02OO state of “Ocs. We have identified the pumping transition to be a forbidden A + Z transition, weakly allowed due to I-type resonance: the 02 *O-00’0 R( 64) line of natural OCS. The 22~ energy levels involved in that experiment are presented in Fig. 2. At J = 65, the 2: state is 5.567 cm-r above the A state, and the Z-type interaction energy between them is 0.906 cm-’ (IO). The effects of this resonance are first a 0.143 cm-’ shift down of the 02 *Oe level (responsible for the major part of the A splitting), and second a mixing of the interacting levels’ wavefunctions rc/(02*0e) = 0.1 56tio( 02’0) + 0.988$o( 02*0) giving rise to a 2.4% intensity transfer from the allowed 02°0-OOo0 R( 64) transition. The FIR laser emission occurs from J = 65 to J = 64 in the 02*0e vibrational state at a frequency which is calculated to be 791 600.2 f 0.2 MHz, in perfect agreement with Landsberg’s experimental value. In our study of the Stark spectrum of OCS (3)) we have observed with the 9 R( 8 ) COz laser line many Stark components of both 04 *Of-02 2Of R (28 ) and 02 20e-OOo0
EIcm
J-65
SUBMILLIMETER
FIG. 2. Upper energy levels of OCS involved in the CW submillimeter emission obtained by Landsberg (7)
HETERODYNE
MEASUREMENTS ON OCS
257
R( 64) transitions, and we have determined their shifts from the laser line, respectively, +44.8 t 0.5 and +32.0 -t 0.5 MHz. These two transitions are thus candidates for optical pumping, but, as has been observed, the second one is more efficient. First, the population is 14 times higher in 00’0 J = 64 than in 02”Of.J = 28, due to the Boltzmann factor and the degeneracy of the levels. Second, the coincidence is better for the A-Z transition and, as the absorption line is quite inhomogeneous (the homogeneous broadening is less than 1 MHz at the 60-mTorr experimental pressure), the axial velocity class interacting with the pump laser beam includes 2.35 times more molecules in the case of the best coincidence. So, combining those two effects, the number of molecules involved in the pumping process is 33 times higher for 02’OeOO’OR(64) than for 0420f-0220fR(28). What concerns the transition probability is that it is 55 times greater for R( 28) than for R(64): a factor 42 from the 2.4% intensity transfer, a factor 3 from the vibrational term, and a factor l/2.28 from the rotational term. The low transition probability of the 02°0e-OOo0 R( 64) line is not a problem in Landsberg’s experiment on account of the high power (more than 5 W) of the CO2 pump laser. On the other hand, this point is critical in the CO2 waveguide laser experiment described in the present work: no Lamb-dip is observed for the R( 64) line by lack of saturation. In our diode laser heterodyne measurements, we also observed the R( 75 ) line of the same 02 ‘Oe-00’0 forbidden transition. The intensity transfer which is proportional to J2( J f I )’ reaches 4.5% in this case. The R( 36) line of the same transition, with an intensity transfer of 0.26%, was too weak to be observed. ABOUT THE BEST STANDARD
We report in the present work the best zero field measurement on OCS, the 02’000’0 P( 5 ) line observed at 1045.025978 (7) cm -‘. Nevertheless, the saturation Stark spectra (3) of this 2~2 band are sensitively more accurate. In those experiments, the CO? laser was stabilized with an accuracy of 15 kHz, and the centers of the OCS Lamb-dips (FWHM 1: 500 kHz) were measured within 220 kHz, giving an overall accuracy estimated to be 30 kHz or 10 P6 cm-‘. The critical point is naturally the problem of the transfer of this accuracy to the zero held frequencies. However, the accuracy seems to be maintained due to the following points: -there are 22 Stark components observed for the 02°0-OOo0 band for J G 5; -the analysis uses simultaneously many kinds of available data, particularly microwave data and LMDR and MBER data in addition to the Stark data; -the Stark model avoids any approximations by dealing simultaneously with all kinds of off-diagonal terms (anharmonic, l-type, Stark, and anisotropy of polarizability ), So, at the end of the analysis, the band center of the 02°0-OOo0 band is calculated with a standard deviation of 0.7 X 10 -6 cm-’ (Table VIII of Ref. (10)). Taking 2.5 times this value as the uncertainty, we consider that the saturation Stark measurements determine the low J levels of 02’0 with an uncertainty of 2 X low6 cm-r. The corresponding calculated value for the 02°0-OOo0 P( 5) line is 1045.025982(2) cm-‘, more accurate than the experimental zero field value 1045.025978( 7) cm-’ and in perfect agreement with it. RECEIVED:
December
6. 1989
FAYT
ET AL.
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