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Stark effect in submillimeter laser lines from optically pumped CH3OH and CD3OD
Volume
40, number
I
OPTICS
STARK EFFECT IN SUBMILLIMETER FROM OPTICALLY
COMMUNICATIONS
Received
1981
LASER LINES
PUMPED CH,OH AND CD,OD
Toshihiko YOSHIDA, Masayoshi KOBAYASHI, Kiyomi SAKAI and Shigeru FUJITA Department
1 December
Tohru YISHIHARA,
of Applied Physics, Osaka University, Suita 56.5, Japan
28 August
198 1
The Stark effect in submillimeter laser lines from optically pumped CHsOH and CDaOD were studied for a low electric field between 0 and 1.3 kV/cm. A Stark splitting and/or an output power enhancement were observed for some lines. Twelve new cw laser lines were observed at zero electric field from optically pumped CDsOD.
1. Introduction When a static electric field is applied to the active media of an optically pumped submillimeter laser, a splitting of laser line and/or an enhancement of output power with the Stark effect are observed for some well-known laser lines of CH30H and CH3F [l-4]. The Stark splitting can be explained by the fact that the upper and lower laser levels are split into Msublevels. For the lines of CH,OH, each M component is not resolved and only the most intense one must lase. An output power enhancement has been explained experimentally as the homogeneous linewidth of ir absorption transition being broadened with the Stark field by Inguscio et al. [5]. These properties of the Stark laser will be useful in some applications including spectroscopy [4,6] and an intense light source in the submillimeter wavelength region, especially as a local oscillator of a heterodyne detection system. Since the Stark laser is composed of a metal-dielectric rectangular waveguide, the output beam is linearly polarized. This property is another advantage as a local oscillator. As the methyl alcohol and its deuterated species are slightly asymmetric rotors with hindered motion, many emission lines have been observed in the submillimeter wavelength region. Because of its rather large electric dipole moment, large Stark effect can be expected. We report the Stark effect in emission lines 0 030-4018/81/0000-OOOO/$
02.75 0 1981 North-Holland
obtained from optically pumped CD,OD in this experiment and a few lines of CH,OH. CD,OD has rather fewer submillimeter laser lines than other deuterated species. Twelve new laser lines have been obtained from optically pumped CD,OD in the absence of an external electric field. Ten of which were obtained by pumping with the emission lines of 9.4 pm band of a CO, laser overlapped with the absorption of CD, deformation vibration band of CD,OD. Most of the lines pumped through this absorption band have not been reported previously.
2. Experiment Experimental set up is shown in fig. 1. A submillimeter laser cavity consists of a metal-dielectric hybrid waveguide in order to investigate the Stark effect. Cross section and dimensions of this waveguide are shown in fig. 2. Two aluminum plates are used as the Stark electrodes, and the breakdown electric field of this guide at about 200 mTorr, which is a typical optimum pressure of a submillimeter laser, is about 800 V/cm. The onset and maximum output of a Stark splitting were observed at about 100 V/cm, hence such breakdown electric field did not prevent our investigation. Our submillimeter laser cavity fs 1.3 m long having two aluminum coated flat mirrors with 4.5
OPTICS COMMUNICATIONS
Volume 40, number 1
ponent of aM= +l and -1, the frequency separation between them were measured by comparison with the free spectral range of the cavity. The Stark frequency tuning versus applied electric field (Stark coefficient) was obtained by the least square method assuming that energy level splitting does not exist at zero electric field [7]. The output power variation of the submillimeter laser versus electric field strength was measured at the maximum by continuous cavity tuning.
PZT
f'ETER
CONCAM MIRROR
1 December 1981
ZNSE IR
SUB-MM SlRRK LASER
Fig. 1. A schematic diagram showing the experimental set-up for our optically pumped submillimeter laser system. 2 mm dia. coupling hole at the center. The submillimeter laser output power was monitored with a Golay cell protected with a sapphire plate for pumping beam. In addition, black polyethylene filters were used to avoid the saturation of Golay detector. The pressure of CH,OH and CD,OD vapor were measured with a Pirani gauge. Measurements of Stark splitting and output variation were performed at optimum pressure for each laser line except when the laser oscillation had not ceased before electrical breakdown occurred. The CD,OD (minimum 99% deuterated) was supplied by Sharp and Dohme and continuously flowed through the laser cavity. A cw CO, laser was used as a pump source. The CO, laser is 1.9 m long with a 150 lines/mm diffraction grating on one end for line selection and a ZnSe meniscus lens having 60% reflectivity on the other end. The ZnSe lens is mounted on a piezoelectric device in order to tune the CO, frequency for the optimum submillimeter signal. This tuning was performed at zero electric field and the output power of CO, laser was monitored by a power meter (Coherent Radiation, model 201). The CO, laser power was about 25 W at most intense transition. When the laser line is splitting into two Stark coiACRYLIC SPACER
hLUMINUM PLATE H (STARK ELECTRODE) 10 mm
Fig. 2. Cross-section of a Stark cell. Dimensions are given in mm.
46
3. Results 3.1. CD,OD Observed submillimeter emission lines of CD,OD are listed in table 1. The pump line and measured submillimeter laser wavelength are listed in the first two columns. Lines with note a) are previously reported lines [8-l 11. The submillimeter laser wavelength was measured with the interferogram by scanning the cavity, and by reading the micrometer calibrated with the well known wavelenght of a few lines of CH,OH. The uncertainty of wavelength determination is about ?l%. The pump frequency offset at which the submillimeter output peaks, measured with respect to the CO, line center, is given in the third column, and the available pump power in the fourth column. Relative output given in the sixth column is the output signal of a Golay detector corrected for the transmittance of the black polyethylene filter. For the lines split into two components, the Stark coefficient, measured by the method described in the above, is given in the eighth column, and only for these lines the investigation with E IIE, mode was made beside E,lE,, which shows t Ke pump field is either parallel or perpendicular to the Stark electric field. The 410 pm line pumped by 1OR( 12) line of a CO, laser with E, IIEs pump mode (denoted by b)) did not develop a splitting before it ceased to lase. The 183 pm line pumped by lOR(26) and the 327 pm line pumped by 9R(28) did not oscillate in Ep IIE, pump mode. For the lines whose output was enhanced with external electric field, the electric field at maximum submillimeter output is given in the last column. The
Volume
40, number
Table 1 The Stark effect
OPTICS
in the submillimeter
co2
pump
1
line
COMMUNICATIONS
laser lines from optically
Measured wavelength
Pump offset
(332
Olm)
(MHz)
iw
pump power
pumped
Optimum pressure (mTorr)
1 December
CDsOD output power (arb. units)
Ep:Es
..~ lOP(10)
149
10
180
0.2
311 a)
-20
19
170
410 a)
-20
16
200
2200
lOR(16)
351 a)
-25
18
270
210
lOR(22)
163 a)
-25
21
190
lOR(24)
183 a)
+30
20
380
291 a)
+30
20
380
+35 +35
20 20 15 15
300 300 150 150
16
420
-10 -10 -10
20 20 20
250 250 250
-20
24
250
+15
21
260
23
180
3.7
1
20
150
0.1
1
12
140
9.9
1
lOR(28) 9P(24)
9P(28)
122 a) 96.6 a) 113 c) 114 62.1
0 ’
1, II
0.4 68
1
0.02 0.3 3.0 1.5
30 27.2 * 1.0 26.4 * 0.1
140
25
1 1
.7
280
1
1
5.0
1
1 1
9P(14)
353
9R(8)
269 a)
9R(16)
316 454
-20 -20
18 18
200 200
6.2 1.9
1
9R(26)
316
-10
11
120
1.2
1
9R(28)
321 a)
1
90
50
1
0
18
120
0.9
1
0
5
110
2.2
1
-
28.1 +
1
3.7 1.5 1.5
16
250 130 115
1
129
63.1 a)
30 60.5 + 1.3 b,
1400
9P(16)
9R(38)
(V/cm)
1
137
0
(MHz/kV/ cm)
1
9P(26)
0
Electric field for maximum output
1
IOR
96.8 a) 119 a) 183 a) 291 a)
29
Measured Stark coefficient
1
lOR(12)
lOR(26)
1981
30.6 * 0.2
65
a) . ~These lines are previously reported [8- 111. O) This means that this line is oscillate with E IIEs pump mode but does not splitted as explained in the text. ‘) Output pokier of these lines are not affectePd by an external electric field through to the breakdown voltage. power of two lines pumped by 9P(34) marked with c) were not affected by an electric field of 0 to 1.3 kV/cm. The wavelengths of the 113 pm and 114 pm lines pumped by 9P(34) coincides with each other within the experimental error. In spite of this fact these two lines can be distinguished because the output variation of them against the electric field are much different. output
3.2. CH30H The Stark effect in a few lines ported previously [7]. The Stark new lines has been observed, and The output power at zero electric by the same procedure as for the
of CH,OH were resplitting of three are given in table 2. field was measured lines of CD,OD. It 47
Volume
40, number
Table 2 The Stark effect
9R(18) 9P(14) 9P(24) ___.._ _
in submillimcter
~~~
laser lines from optically
61.5 118.0 133.1
pumped
470 210 210
decreases monotonically with electric field. The Stark coefficient of the 133.1 pm line pumped by 9P(24) CO, laser was reported by Henningsen [ 121. His result is consistent with our data within experimental error. The Stark coefficient for this line is given by K-l --~__ J
1981
CHjOH Ep : Es
Measured Stark coefficient (MHz/kV/cm)
Outpout power (arb. units)
1 1 1
33.1 ? 0.6 113 * 1.4 97.0 f 1.2
4.0 220 1800 ~_____
9.4 pm emission band of CO, laser. The new submillimeter laser lines of CD,OD pumped with 9.4 pm emission band could be those pumped through this vibration band.
References > [l] M. Inguscio,
assuming that only the most intense M component is observed, where p,, means the component of electric dipole moment parallel to the molecular axis. The 133.1 pm line is assigned as (J, K) = (9, K) + (8, K - 1) by Henningsen [ 131 and Forber et al. [ 141. The value of K is estimated to be 4 ir 1. The calculated Stark coefficient is 60.4 rt 10.1 MHz/kV/cm, which is much smaller than observed value 97.0 MHz/kV/ cm.
[2]
[ 31 [4] 151 [6] [7] [8]
4. Discussion Since th; CD,OD molecule has a smaller rotational constant than CH,OH, the Stark splitting was observed at longer wavelength lines between 183 pm and 410 pm. The Stark splitting will be observed in more lines by using a higher pump power source and a cavity with lower threshold and with higher breakdown electric field. Optically pumping of the CD, deformation vibration band of CD,OD has yielded a large number of submillimeter laser lines [15,16]. The CD, deformation vibration band of CD,OD centered at 1027.8 cm-l [I 7] overlaps with the P and R branches of the
48
1 December
OPTICS COMMUNICATIONS
1
[9]
[lo] [ 1 l] [12] (131 [14] [15] [16] [17]
P. Minguzzi, F. Strumia and M. Tonelli, Appl. Phys. 18 (1979) 261. M. Inguscio, F. Strumia, K.M. Evenson, D.A. Jennings, A. Scarablin and S.R. Stein, Optics Lett. 4 (1979) 9. G. Bionducci, M. Inguscio, A. Moretti and F. Strumia, Infra. Phys. 19 (1979) 297. J.O. Henningsen, J. Mol. Spectra. 82 (1980), in press. M. Inguscio, A. Moretti and F. Strumia, IEEE J. Quantum Electron. QE-16 (1980) 955. N.M. Lawandy, Infra. Phys. 20 (1980) 379. T. Yoshida, A. Saito, M. Kobayashi, K. Sakai and S. Fujita, Jap. J. Appl. Phys. 19 (1980) 2527. S. Kon, E. Hagiward, T. Yano and H. Hirose, Jap. J. Appl. Phys. 14 (1975) 731. H. Herman and B.E. Prcwer, Appl. Phys. 19 (1979) 241. K. Muro, private communication. E.C.C. Vasconcellos, A. Scalabrm, F.R. Petersen and K.M. Evenson, Int. J. IR and MM Waves 2 (1981) 533. J.O. Henningsen, 5th Int. Conf. Submillimeter Waves, Wiirtzburg, Conference Digest pp. 373 (1980). J.O. Henningsen, IEEE J. Quantum Electron. QE-14 (1978) 958. R.A. Forber, J. Tannenbaum and MS. Feld, Int. J. IR and MM Waves 1 (1980) 527. M. Grinda and C.O. Weiss, Optics Comm. 26 (1978) 91. E.J. Danielwicz and C.O. Weiss, IEEE J. Quantum Electron. QE-14 (1978) 458. A. Serrallach, R. Meyer and H.H. Giinthard, J. Mol. Spectra. 52 (1974) 94.
40, number
I
OPTICS
STARK EFFECT IN SUBMILLIMETER FROM OPTICALLY
COMMUNICATIONS
Received
1981
LASER LINES
PUMPED CH,OH AND CD,OD
Toshihiko YOSHIDA, Masayoshi KOBAYASHI, Kiyomi SAKAI and Shigeru FUJITA Department
1 December
Tohru YISHIHARA,
of Applied Physics, Osaka University, Suita 56.5, Japan
28 August
198 1
The Stark effect in submillimeter laser lines from optically pumped CHsOH and CDaOD were studied for a low electric field between 0 and 1.3 kV/cm. A Stark splitting and/or an output power enhancement were observed for some lines. Twelve new cw laser lines were observed at zero electric field from optically pumped CDsOD.
1. Introduction When a static electric field is applied to the active media of an optically pumped submillimeter laser, a splitting of laser line and/or an enhancement of output power with the Stark effect are observed for some well-known laser lines of CH30H and CH3F [l-4]. The Stark splitting can be explained by the fact that the upper and lower laser levels are split into Msublevels. For the lines of CH,OH, each M component is not resolved and only the most intense one must lase. An output power enhancement has been explained experimentally as the homogeneous linewidth of ir absorption transition being broadened with the Stark field by Inguscio et al. [5]. These properties of the Stark laser will be useful in some applications including spectroscopy [4,6] and an intense light source in the submillimeter wavelength region, especially as a local oscillator of a heterodyne detection system. Since the Stark laser is composed of a metal-dielectric rectangular waveguide, the output beam is linearly polarized. This property is another advantage as a local oscillator. As the methyl alcohol and its deuterated species are slightly asymmetric rotors with hindered motion, many emission lines have been observed in the submillimeter wavelength region. Because of its rather large electric dipole moment, large Stark effect can be expected. We report the Stark effect in emission lines 0 030-4018/81/0000-OOOO/$
02.75 0 1981 North-Holland
obtained from optically pumped CD,OD in this experiment and a few lines of CH,OH. CD,OD has rather fewer submillimeter laser lines than other deuterated species. Twelve new laser lines have been obtained from optically pumped CD,OD in the absence of an external electric field. Ten of which were obtained by pumping with the emission lines of 9.4 pm band of a CO, laser overlapped with the absorption of CD, deformation vibration band of CD,OD. Most of the lines pumped through this absorption band have not been reported previously.
2. Experiment Experimental set up is shown in fig. 1. A submillimeter laser cavity consists of a metal-dielectric hybrid waveguide in order to investigate the Stark effect. Cross section and dimensions of this waveguide are shown in fig. 2. Two aluminum plates are used as the Stark electrodes, and the breakdown electric field of this guide at about 200 mTorr, which is a typical optimum pressure of a submillimeter laser, is about 800 V/cm. The onset and maximum output of a Stark splitting were observed at about 100 V/cm, hence such breakdown electric field did not prevent our investigation. Our submillimeter laser cavity fs 1.3 m long having two aluminum coated flat mirrors with 4.5
OPTICS COMMUNICATIONS
Volume 40, number 1
ponent of aM= +l and -1, the frequency separation between them were measured by comparison with the free spectral range of the cavity. The Stark frequency tuning versus applied electric field (Stark coefficient) was obtained by the least square method assuming that energy level splitting does not exist at zero electric field [7]. The output power variation of the submillimeter laser versus electric field strength was measured at the maximum by continuous cavity tuning.
PZT
f'ETER
CONCAM MIRROR
1 December 1981
ZNSE IR
SUB-MM SlRRK LASER
Fig. 1. A schematic diagram showing the experimental set-up for our optically pumped submillimeter laser system. 2 mm dia. coupling hole at the center. The submillimeter laser output power was monitored with a Golay cell protected with a sapphire plate for pumping beam. In addition, black polyethylene filters were used to avoid the saturation of Golay detector. The pressure of CH,OH and CD,OD vapor were measured with a Pirani gauge. Measurements of Stark splitting and output variation were performed at optimum pressure for each laser line except when the laser oscillation had not ceased before electrical breakdown occurred. The CD,OD (minimum 99% deuterated) was supplied by Sharp and Dohme and continuously flowed through the laser cavity. A cw CO, laser was used as a pump source. The CO, laser is 1.9 m long with a 150 lines/mm diffraction grating on one end for line selection and a ZnSe meniscus lens having 60% reflectivity on the other end. The ZnSe lens is mounted on a piezoelectric device in order to tune the CO, frequency for the optimum submillimeter signal. This tuning was performed at zero electric field and the output power of CO, laser was monitored by a power meter (Coherent Radiation, model 201). The CO, laser power was about 25 W at most intense transition. When the laser line is splitting into two Stark coiACRYLIC SPACER
hLUMINUM PLATE H (STARK ELECTRODE) 10 mm
Fig. 2. Cross-section of a Stark cell. Dimensions are given in mm.
46
3. Results 3.1. CD,OD Observed submillimeter emission lines of CD,OD are listed in table 1. The pump line and measured submillimeter laser wavelength are listed in the first two columns. Lines with note a) are previously reported lines [8-l 11. The submillimeter laser wavelength was measured with the interferogram by scanning the cavity, and by reading the micrometer calibrated with the well known wavelenght of a few lines of CH,OH. The uncertainty of wavelength determination is about ?l%. The pump frequency offset at which the submillimeter output peaks, measured with respect to the CO, line center, is given in the third column, and the available pump power in the fourth column. Relative output given in the sixth column is the output signal of a Golay detector corrected for the transmittance of the black polyethylene filter. For the lines split into two components, the Stark coefficient, measured by the method described in the above, is given in the eighth column, and only for these lines the investigation with E IIE, mode was made beside E,lE,, which shows t Ke pump field is either parallel or perpendicular to the Stark electric field. The 410 pm line pumped by 1OR( 12) line of a CO, laser with E, IIEs pump mode (denoted by b)) did not develop a splitting before it ceased to lase. The 183 pm line pumped by lOR(26) and the 327 pm line pumped by 9R(28) did not oscillate in Ep IIE, pump mode. For the lines whose output was enhanced with external electric field, the electric field at maximum submillimeter output is given in the last column. The
Volume
40, number
Table 1 The Stark effect
OPTICS
in the submillimeter
co2
pump
1
line
COMMUNICATIONS
laser lines from optically
Measured wavelength
Pump offset
(332
Olm)
(MHz)
iw
pump power
pumped
Optimum pressure (mTorr)
1 December
CDsOD output power (arb. units)
Ep:Es
..~ lOP(10)
149
10
180
0.2
311 a)
-20
19
170
410 a)
-20
16
200
2200
lOR(16)
351 a)
-25
18
270
210
lOR(22)
163 a)
-25
21
190
lOR(24)
183 a)
+30
20
380
291 a)
+30
20
380
+35 +35
20 20 15 15
300 300 150 150
16
420
-10 -10 -10
20 20 20
250 250 250
-20
24
250
+15
21
260
23
180
3.7
1
20
150
0.1
1
12
140
9.9
1
lOR(28) 9P(24)
9P(28)
122 a) 96.6 a) 113 c) 114 62.1
0 ’
1, II
0.4 68
1
0.02 0.3 3.0 1.5
30 27.2 * 1.0 26.4 * 0.1
140
25
1 1
.7
280
1
1
5.0
1
1 1
9P(14)
353
9R(8)
269 a)
9R(16)
316 454
-20 -20
18 18
200 200
6.2 1.9
1
9R(26)
316
-10
11
120
1.2
1
9R(28)
321 a)
1
90
50
1
0
18
120
0.9
1
0
5
110
2.2
1
-
28.1 +
1
3.7 1.5 1.5
16
250 130 115
1
129
63.1 a)
30 60.5 + 1.3 b,
1400
9P(16)
9R(38)
(V/cm)
1
137
0
(MHz/kV/ cm)
1
9P(26)
0
Electric field for maximum output
1
IOR
96.8 a) 119 a) 183 a) 291 a)
29
Measured Stark coefficient
1
lOR(12)
lOR(26)
1981
30.6 * 0.2
65
a) . ~These lines are previously reported [8- 111. O) This means that this line is oscillate with E IIEs pump mode but does not splitted as explained in the text. ‘) Output pokier of these lines are not affectePd by an external electric field through to the breakdown voltage. power of two lines pumped by 9P(34) marked with c) were not affected by an electric field of 0 to 1.3 kV/cm. The wavelengths of the 113 pm and 114 pm lines pumped by 9P(34) coincides with each other within the experimental error. In spite of this fact these two lines can be distinguished because the output variation of them against the electric field are much different. output
3.2. CH30H The Stark effect in a few lines ported previously [7]. The Stark new lines has been observed, and The output power at zero electric by the same procedure as for the
of CH,OH were resplitting of three are given in table 2. field was measured lines of CD,OD. It 47
Volume
40, number
Table 2 The Stark effect
9R(18) 9P(14) 9P(24) ___.._ _
in submillimcter
~~~
laser lines from optically
61.5 118.0 133.1
pumped
470 210 210
decreases monotonically with electric field. The Stark coefficient of the 133.1 pm line pumped by 9P(24) CO, laser was reported by Henningsen [ 121. His result is consistent with our data within experimental error. The Stark coefficient for this line is given by K-l --~__ J
1981
CHjOH Ep : Es
Measured Stark coefficient (MHz/kV/cm)
Outpout power (arb. units)
1 1 1
33.1 ? 0.6 113 * 1.4 97.0 f 1.2
4.0 220 1800 ~_____
9.4 pm emission band of CO, laser. The new submillimeter laser lines of CD,OD pumped with 9.4 pm emission band could be those pumped through this vibration band.
References > [l] M. Inguscio,
assuming that only the most intense M component is observed, where p,, means the component of electric dipole moment parallel to the molecular axis. The 133.1 pm line is assigned as (J, K) = (9, K) + (8, K - 1) by Henningsen [ 131 and Forber et al. [ 141. The value of K is estimated to be 4 ir 1. The calculated Stark coefficient is 60.4 rt 10.1 MHz/kV/cm, which is much smaller than observed value 97.0 MHz/kV/ cm.
[2]
[ 31 [4] 151 [6] [7] [8]
4. Discussion Since th; CD,OD molecule has a smaller rotational constant than CH,OH, the Stark splitting was observed at longer wavelength lines between 183 pm and 410 pm. The Stark splitting will be observed in more lines by using a higher pump power source and a cavity with lower threshold and with higher breakdown electric field. Optically pumping of the CD, deformation vibration band of CD,OD has yielded a large number of submillimeter laser lines [15,16]. The CD, deformation vibration band of CD,OD centered at 1027.8 cm-l [I 7] overlaps with the P and R branches of the
48
1 December
OPTICS COMMUNICATIONS
1
[9]
[lo] [ 1 l] [12] (131 [14] [15] [16] [17]
P. Minguzzi, F. Strumia and M. Tonelli, Appl. Phys. 18 (1979) 261. M. Inguscio, F. Strumia, K.M. Evenson, D.A. Jennings, A. Scarablin and S.R. Stein, Optics Lett. 4 (1979) 9. G. Bionducci, M. Inguscio, A. Moretti and F. Strumia, Infra. Phys. 19 (1979) 297. J.O. Henningsen, J. Mol. Spectra. 82 (1980), in press. M. Inguscio, A. Moretti and F. Strumia, IEEE J. Quantum Electron. QE-16 (1980) 955. N.M. Lawandy, Infra. Phys. 20 (1980) 379. T. Yoshida, A. Saito, M. Kobayashi, K. Sakai and S. Fujita, Jap. J. Appl. Phys. 19 (1980) 2527. S. Kon, E. Hagiward, T. Yano and H. Hirose, Jap. J. Appl. Phys. 14 (1975) 731. H. Herman and B.E. Prcwer, Appl. Phys. 19 (1979) 241. K. Muro, private communication. E.C.C. Vasconcellos, A. Scalabrm, F.R. Petersen and K.M. Evenson, Int. J. IR and MM Waves 2 (1981) 533. J.O. Henningsen, 5th Int. Conf. Submillimeter Waves, Wiirtzburg, Conference Digest pp. 373 (1980). J.O. Henningsen, IEEE J. Quantum Electron. QE-14 (1978) 958. R.A. Forber, J. Tannenbaum and MS. Feld, Int. J. IR and MM Waves 1 (1980) 527. M. Grinda and C.O. Weiss, Optics Comm. 26 (1978) 91. E.J. Danielwicz and C.O. Weiss, IEEE J. Quantum Electron. QE-14 (1978) 458. A. Serrallach, R. Meyer and H.H. Giinthard, J. Mol. Spectra. 52 (1974) 94.