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Spectroscopic application of a ‘tunable source’ in the 10 μM region: Laser-microwave two-photon spectroscopy
SPECTROSCOPIC APPLICATION OF A ‘TUNABLE SOURCE’ IN THE 10,uM REGION: LASER-MICROWAVE TWO-PHOTON SPECTROSCOPY HAROLD Department
of Physical
Chemistry.
JONES
University
of Ulm, 7900 Ulm, W. Germany
Abstract-A tunable microwave frequency was added to. or subtracted from fixed frequency ‘3C’h02 and ‘2C’802 laser lines using the nonlinearity of molecular absorption. In this way the frequency of 37 transitions of 14NH, and I1 transitions of “NH3 have been measured. Under favourable conditions Lamb-dip signals were observed and in these cases absolute frequency measurements with a precision of + 3 MHz were made. In the case of the exact coincidence between the R(l8) “CO2 laser line and the v2 ..Q(5.4) transition of “NH, signals produced by multiple-photon processes were observed. Transitions in fundamental and hot-bands of the linear molecules HCCF and FCN have also been measured. New systems for the generation of submillimetre waves are proposed.
INTRODUCTION
Despite its otherwise desirable properties the CO, laser has one great disadvantage for its application in spectroscopy, namely its essentially fixed frequency output. Lasermicrowave two-photon spectroscopy presents the possibility of loosening this restriction. An effective tunable infrared source for spectroscopic purposes can be produced by the addition or subtraction of a tunable microwave frequency to or from a fixed infrared frequency. The non-linear mixing element used in this technique is the molecular transition itself, and consequently all that is necessary is that the gas which is to be studied is subjected to sufficiently intense microwave and laser fields with suitable frequencies. The first example of systematic spectroscopy being carried out by this method has been reported by Freund and Oka (l) in their study of the v,-band of 14NHj and 15NH, using N20 and normal COz lasers. We have extended this work in two ways. Firstly, the use of 13C’602 and 12V1802 lasers has allowed further transitions of the v,-band of both 14NH3 and 15NH3 to be measured. Secondly it has been shown that under favourable circumstances very low microwave power-levels (cu. 20mW) are sufficient to allow two-photon transitions up to 11 GHz off-resonant with the laser frequency to be observed. This latter point is of considerable experimental importance since it means that the output of the average microwave sweeper is su%cient for this work, and this type of source used with a wide-band intracavity cell of the type previously described (3) allows wide microwave ranges to be easily and quickly searched. In addition, two-photon measurements have been carried out in the O+ v3 and v5 + (v3 + v5) bands of fluoroacetylene, and 0 - vl, v2 --t (v2 + vl) and (2vz + vl) bands of FCN. The theory of this type of two-photon spectroscopy is well established”. 2, and we consider the case of interest here-a three-level system with a microwave transition $l -+ tiz and an infrared transition $z --+ $3. For the case where the sample is subjected to a microwave and a laser field of frequencies o, and w,, respectively, s.uch that (ho, _+ ho,) is equal to the energy interval between w1 and 03, the transition moment of the two-photon transition o1 -+ -+ o3 is given’” by h4 = <1(4,&,)2> where E, and E; are respectively, p,, is the and w. is the resonant is favoured when &
the electric permanent frequency and p, are
<2/p,
E,/3>/2h2(o,
- 0,)
field strengths of the microwave and laser radiation, dipole moment, P, is the vibrational dipole moment of the infrared transition. Thus two-photon absorption large, which is the case in ammonia, when E,,, and. 449
E, are large, i.e. high microwave and laser powers and when ccjO- cl, is small. Thus conditions for a given molecule the upper limit of o 0 - o, is set by the experimental determining the microwave and laser power densities. EXPERIMENTAL
In a11 measurements discussed an X-band cell of 30 cm length with a 6 mm diameter laser aperture was used within the resonator of a semi-seaIed-off laser with an active length of 1 m. (3) The laser was operated at -2O’C and was stabilized at the centre of its gain profile in the usual manner. The sample was subjected to laser power densities ranging from 60 W/cm* on the strongest lines to only 6 W/cm2 on the weakest lines. Microwave power densities in the range 5-lOmW/cm* were produced by a Marconi model 6600 A sweeper. Signals were produced by frequency modulating the microwave output at 90 kHz and phase-sensitive detection of the output of a Pb-Sn-Te infrared detector. The Doppler-limited measurements were carried out with ammonia pressures ranging from 200mtorr to 1.5 torr, for the Lamb-dip measurements the pressure was reduced to 50mtorr. The accuracy to which the centre of the Doppter-limited two-photon signals couid be positioned was. as pointed out by Freund and Oka,“’ essentially limited by how seriously the signal line-shape was distorted by microwave resonances within the waveguide absorption cell. As previously reported (j) the type of cell used here was largely free from strong microwave transmission disturbances and in most cases a good lineshape was observed, despite some localized disturbances near the cut-off frequency of the K--+X-band tapers attached permanently to the cell. As a result of the generally good line-shape it is considered that the frequencies of most of the two-photon transitions could be located with an accuracy of +_ 15 MHz (= 0.0005 cm _ ‘). In the case of infrared transitions up to 7 1 GHz off-resonant with the laser line the two-photon process was sufficiently strong to allow the transition to be saturated, thus allowing sharp inverse Lamb-dip signals to be observed. In this case the centre of the transition could be located to better than 0.1 MHz, but the absolute accuracy was limited by the reproducability of the laser frequency to E 3 MHz (0.~1 cm-i). MEASUREMENTS
In the case of i4NH3, 32 transitions frequencies were obtained with an accuracy of 0.0005 cm- ’ from Doppler-limited measurements and a further four with an accuracy of 0.0001 cm- ’ from Lamb-dip measurements. ‘5) The frequency of the ,,R(3,3) transition has been measured as 1011.2035 Ir 0.0001 cm-’ by observation of a Lamb-dip signal produced by the P(S) ’ 3C0, laser line at 1011.2009 cm- ‘. The frequency difference between this transition and the P(8) laser line was measured to be 80.3 + 3.0 MHz, and is, with the exception of the coincidence between the R(2.0) transition and the Rf42) normal CO,-laser line,‘5’ the closest near-resonance between a transition of 14NH, and any CO, laser line so far shown to exist. If the measurements of Freund and Oka’” are included this raises the total number of transitions of the v:, band of i4NH3 so far measured by this technique to 75. Using the very low microwave power avaiIable from the sweeper the maximum value of ((a, - 0,) for which a two-photon transition was observed was 11.1 GHz. Thus for 14NH, our ‘tunable source’ has a tuning range of + 11 GHz, and by increasing the microwave power used there is no doubt that this range can be extended. were “NH3 was not as well suited for this work as 14NH, and only 11 transitions observed, two of which were measured using the Lamb-dip technique.‘6’ However, in had not been determined and contrast to lJNH,, the molecular constants of “NH3 by combining these results with those of Freund and Oka”’ and with the corrected constants was frequencies of Shimizu’s Stark laser work,“’ a set of accurate molecular determined~6) During this work an exact coincidence between the R(18) “CO, laser line and the .,Q(5,4) transition was located. In this special case in addition to very strong double resonance and two-photon signals extra signals were observed which
Spectroscopic
application
of a ‘tunable
source’
451
were interpreted as being produced by Doppler-tuned multiple-photon processes. The mechanism for the production of these signals was similar to that described by Freund et ~1.“’ Signals were observed which were explained as being produced in one case by a single laser photon combining with a laser-microwave double photon and in another case by a laser triple photon combining with a laser-microwave double photon. Table
Laser line C’802
P(30)
HCCF C”Oz P(26) C’802 P(I0) COz R(20)
C1802 FCN
P(l?)
< C’“O*
~ C”Oz
u = upper.
R(8)
R(4)
I. Observed
two-photon
transitions
Two-photon transition J” -+ J’ 1’ (GHz) AV (GHz) O-0 I-+1 2-+0 2-4 I&, --+ 19, l-3 3-3 2-+2 2-2 1-I 2-O 2-2 I+1 2-O 2, - 3, I,+ 1,
18.277 18.200 39.966 54.523 7.543 42.217 63.325 63.065 4 I.899 42.053 21.150 42.268 42.512 21.083 4 I.444 42.269
-1.135 -1.138 - 1.142 -3.714 -0.120 -0.091 -0.108 -0.095 -0.144 -0.163 -0.129 0.06 I 0.070 0.053 0.855 0.861
IR-transition Band
1%(cm _ ‘)
O+I’,
P(l)
1060.7977
o1’3 l’s - (\.S + \‘3)
R(3) R( Ix),,
1064.0 I IO 1076.5698
o-
R(2)
1079.5871
O-+V,
P(2)
IO75.0806
21’2 -+ (2\12 + 11,)
P(2 )
1089.7430
V2 -+(l.>
R(l),
1087.1436
18,
+ 1.1)
I = lower
The application of three-level two-photon spectroscopy to molecules other than NH3 is restricted by the fact that no other molecule has such a large number of microwave transitions concentrated in an accessible microwave region. This problem is shown by our measurements on the linear molecules HCCF and FCN. With the available apparatus, frequencies up to 63 GHz could be measured and consequently in these two molecules normal rotational transitions only up to J = 2 --+ 3 can be measured. Nevertheless a few infrared transitions of the fundamental C-F stretching bands of these molecules were measured by this method. Table 1 summarizes a few preliminary results for these molecules. A much more promising situation is encountered in the hot-bands of HCCF Here so called direct l-type and FCN vg -+ 11~+ v3 and v2 -+ r1 + v,, respectively. doubling microwave transitions are available and transitions ranging up to J = 30 are to be found at microwave frequencies below z 20 GHz. The draw-back with such transitions is that they are relatively weak, and consequently two-photon transitions with large values of (wO - w,) can be expected to be difficult to observe. The feasibility of this work is shown by the observation of a signal in the hot-band vg ---f (v5 + r3) of HCCF using the C’*O* P(10) laser line at 1076.5738 cm-’ (Fig. 1). The upper frequency l-type doublet of the R(18) of this band lies _ 120 MHz below the frequency of this laser line and a relatively strong two-photon signal was observed. This work is being extended to the measurement of other transitions of this band and to the vq-+(vq + v3) band of HCCF. This work has several aspects which are relevant to the generation of submillimetre waves by optical pumping. Most directly, the new coincidence between the R(18) 13C02 laser line and the Q(5,4) transition of 15NH3 can certainly be used to generate submillimetre output. This combination has several advantages over the well-known coincidence between the P(13) of the N20 laser and the Q(8,7) transition of 14NH3. The absorption coefficient in this new case is very much larger than that of the other case and the 13C0, laser can produce output powers considerably in excess of that achievable from the N,O laser. Secondly, and probably more importantly, this work shows that even with low microwave power levels an infrared-microwave two-photon process can be used to saturate
HAROLD JONES
“5
75’43 MHz
L_---_
Fig. I. Energy level scheme and observed signal of the two-photon transition in the hot-band I’$- i’i + r3 of HCCF produced using the Y.4pm P(10) CLROl Laser line.
an infrared transition. Consequently it seems very likely that if a wave-guide suitable coupling devices for both microwave and CO, laser radiation were two-photon resonant pumping could be used to generate a considerable new lasing transitions in 14NH,, 15NH3 and other molecular species. Since molecular constants for these molecules would be known the exact lasing could be accurately predicted before the experiment was carried out. REFERENCES
3. 4. 5. 6. 7. 8.
pp. 53 I 569. J~NI s. H.. dppi. Pfys. 14. 169 (19771. Jmi.s. H. .4p$ Pity.\. 15, 261 (1973). MOKITA. N.. S. KAF~O. Y. ULUA & T. SHIMWI . .I. C/WIII. Pl~!,.s. 66. 2726 (1977). JONIS. H.. J. rmlrc~. Spwmc. 70, 279 (1978). SHIMIZL,. F.. .I. Chrm. Phj,s. 53. I 149 (1970). FREUNIX S. M.. M. RCiMHELD& T. OKA. PII!\. Rrr,. Lvrr. 35. 1497 (1975).
laser with constructed, number of the accurate frequencies
of Physical
Chemistry.
JONES
University
of Ulm, 7900 Ulm, W. Germany
Abstract-A tunable microwave frequency was added to. or subtracted from fixed frequency ‘3C’h02 and ‘2C’802 laser lines using the nonlinearity of molecular absorption. In this way the frequency of 37 transitions of 14NH, and I1 transitions of “NH3 have been measured. Under favourable conditions Lamb-dip signals were observed and in these cases absolute frequency measurements with a precision of + 3 MHz were made. In the case of the exact coincidence between the R(l8) “CO2 laser line and the v2 ..Q(5.4) transition of “NH, signals produced by multiple-photon processes were observed. Transitions in fundamental and hot-bands of the linear molecules HCCF and FCN have also been measured. New systems for the generation of submillimetre waves are proposed.
INTRODUCTION
Despite its otherwise desirable properties the CO, laser has one great disadvantage for its application in spectroscopy, namely its essentially fixed frequency output. Lasermicrowave two-photon spectroscopy presents the possibility of loosening this restriction. An effective tunable infrared source for spectroscopic purposes can be produced by the addition or subtraction of a tunable microwave frequency to or from a fixed infrared frequency. The non-linear mixing element used in this technique is the molecular transition itself, and consequently all that is necessary is that the gas which is to be studied is subjected to sufficiently intense microwave and laser fields with suitable frequencies. The first example of systematic spectroscopy being carried out by this method has been reported by Freund and Oka (l) in their study of the v,-band of 14NHj and 15NH, using N20 and normal COz lasers. We have extended this work in two ways. Firstly, the use of 13C’602 and 12V1802 lasers has allowed further transitions of the v,-band of both 14NH3 and 15NH3 to be measured. Secondly it has been shown that under favourable circumstances very low microwave power-levels (cu. 20mW) are sufficient to allow two-photon transitions up to 11 GHz off-resonant with the laser frequency to be observed. This latter point is of considerable experimental importance since it means that the output of the average microwave sweeper is su%cient for this work, and this type of source used with a wide-band intracavity cell of the type previously described (3) allows wide microwave ranges to be easily and quickly searched. In addition, two-photon measurements have been carried out in the O+ v3 and v5 + (v3 + v5) bands of fluoroacetylene, and 0 - vl, v2 --t (v2 + vl) and (2vz + vl) bands of FCN. The theory of this type of two-photon spectroscopy is well established”. 2, and we consider the case of interest here-a three-level system with a microwave transition $l -+ tiz and an infrared transition $z --+ $3. For the case where the sample is subjected to a microwave and a laser field of frequencies o, and w,, respectively, s.uch that (ho, _+ ho,) is equal to the energy interval between w1 and 03, the transition moment of the two-photon transition o1 -+ -+ o3 is given’” by h4 = <1(4,&,)2> where E, and E; are respectively, p,, is the and w. is the resonant is favoured when &
the electric permanent frequency and p, are
<2/p,
E,/3>/2h2(o,
- 0,)
field strengths of the microwave and laser radiation, dipole moment, P, is the vibrational dipole moment of the infrared transition. Thus two-photon absorption large, which is the case in ammonia, when E,,, and. 449
E, are large, i.e. high microwave and laser powers and when ccjO- cl, is small. Thus conditions for a given molecule the upper limit of o 0 - o, is set by the experimental determining the microwave and laser power densities. EXPERIMENTAL
In a11 measurements discussed an X-band cell of 30 cm length with a 6 mm diameter laser aperture was used within the resonator of a semi-seaIed-off laser with an active length of 1 m. (3) The laser was operated at -2O’C and was stabilized at the centre of its gain profile in the usual manner. The sample was subjected to laser power densities ranging from 60 W/cm* on the strongest lines to only 6 W/cm2 on the weakest lines. Microwave power densities in the range 5-lOmW/cm* were produced by a Marconi model 6600 A sweeper. Signals were produced by frequency modulating the microwave output at 90 kHz and phase-sensitive detection of the output of a Pb-Sn-Te infrared detector. The Doppler-limited measurements were carried out with ammonia pressures ranging from 200mtorr to 1.5 torr, for the Lamb-dip measurements the pressure was reduced to 50mtorr. The accuracy to which the centre of the Doppter-limited two-photon signals couid be positioned was. as pointed out by Freund and Oka,“’ essentially limited by how seriously the signal line-shape was distorted by microwave resonances within the waveguide absorption cell. As previously reported (j) the type of cell used here was largely free from strong microwave transmission disturbances and in most cases a good lineshape was observed, despite some localized disturbances near the cut-off frequency of the K--+X-band tapers attached permanently to the cell. As a result of the generally good line-shape it is considered that the frequencies of most of the two-photon transitions could be located with an accuracy of +_ 15 MHz (= 0.0005 cm _ ‘). In the case of infrared transitions up to 7 1 GHz off-resonant with the laser line the two-photon process was sufficiently strong to allow the transition to be saturated, thus allowing sharp inverse Lamb-dip signals to be observed. In this case the centre of the transition could be located to better than 0.1 MHz, but the absolute accuracy was limited by the reproducability of the laser frequency to E 3 MHz (0.~1 cm-i). MEASUREMENTS
In the case of i4NH3, 32 transitions frequencies were obtained with an accuracy of 0.0005 cm- ’ from Doppler-limited measurements and a further four with an accuracy of 0.0001 cm- ’ from Lamb-dip measurements. ‘5) The frequency of the ,,R(3,3) transition has been measured as 1011.2035 Ir 0.0001 cm-’ by observation of a Lamb-dip signal produced by the P(S) ’ 3C0, laser line at 1011.2009 cm- ‘. The frequency difference between this transition and the P(8) laser line was measured to be 80.3 + 3.0 MHz, and is, with the exception of the coincidence between the R(2.0) transition and the Rf42) normal CO,-laser line,‘5’ the closest near-resonance between a transition of 14NH, and any CO, laser line so far shown to exist. If the measurements of Freund and Oka’” are included this raises the total number of transitions of the v:, band of i4NH3 so far measured by this technique to 75. Using the very low microwave power avaiIable from the sweeper the maximum value of ((a, - 0,) for which a two-photon transition was observed was 11.1 GHz. Thus for 14NH, our ‘tunable source’ has a tuning range of + 11 GHz, and by increasing the microwave power used there is no doubt that this range can be extended. were “NH3 was not as well suited for this work as 14NH, and only 11 transitions observed, two of which were measured using the Lamb-dip technique.‘6’ However, in had not been determined and contrast to lJNH,, the molecular constants of “NH3 by combining these results with those of Freund and Oka”’ and with the corrected constants was frequencies of Shimizu’s Stark laser work,“’ a set of accurate molecular determined~6) During this work an exact coincidence between the R(18) “CO, laser line and the .,Q(5,4) transition was located. In this special case in addition to very strong double resonance and two-photon signals extra signals were observed which
Spectroscopic
application
of a ‘tunable
source’
451
were interpreted as being produced by Doppler-tuned multiple-photon processes. The mechanism for the production of these signals was similar to that described by Freund et ~1.“’ Signals were observed which were explained as being produced in one case by a single laser photon combining with a laser-microwave double photon and in another case by a laser triple photon combining with a laser-microwave double photon. Table
Laser line C’802
P(30)
HCCF C”Oz P(26) C’802 P(I0) COz R(20)
C1802 FCN
P(l?)
< C’“O*
~ C”Oz
u = upper.
R(8)
R(4)
I. Observed
two-photon
transitions
Two-photon transition J” -+ J’ 1’ (GHz) AV (GHz) O-0 I-+1 2-+0 2-4 I&, --+ 19, l-3 3-3 2-+2 2-2 1-I 2-O 2-2 I+1 2-O 2, - 3, I,+ 1,
18.277 18.200 39.966 54.523 7.543 42.217 63.325 63.065 4 I.899 42.053 21.150 42.268 42.512 21.083 4 I.444 42.269
-1.135 -1.138 - 1.142 -3.714 -0.120 -0.091 -0.108 -0.095 -0.144 -0.163 -0.129 0.06 I 0.070 0.053 0.855 0.861
IR-transition Band
1%(cm _ ‘)
O+I’,
P(l)
1060.7977
o1’3 l’s - (\.S + \‘3)
R(3) R( Ix),,
1064.0 I IO 1076.5698
o-
R(2)
1079.5871
O-+V,
P(2)
IO75.0806
21’2 -+ (2\12 + 11,)
P(2 )
1089.7430
V2 -+(l.>
R(l),
1087.1436
18,
+ 1.1)
I = lower
The application of three-level two-photon spectroscopy to molecules other than NH3 is restricted by the fact that no other molecule has such a large number of microwave transitions concentrated in an accessible microwave region. This problem is shown by our measurements on the linear molecules HCCF and FCN. With the available apparatus, frequencies up to 63 GHz could be measured and consequently in these two molecules normal rotational transitions only up to J = 2 --+ 3 can be measured. Nevertheless a few infrared transitions of the fundamental C-F stretching bands of these molecules were measured by this method. Table 1 summarizes a few preliminary results for these molecules. A much more promising situation is encountered in the hot-bands of HCCF Here so called direct l-type and FCN vg -+ 11~+ v3 and v2 -+ r1 + v,, respectively. doubling microwave transitions are available and transitions ranging up to J = 30 are to be found at microwave frequencies below z 20 GHz. The draw-back with such transitions is that they are relatively weak, and consequently two-photon transitions with large values of (wO - w,) can be expected to be difficult to observe. The feasibility of this work is shown by the observation of a signal in the hot-band vg ---f (v5 + r3) of HCCF using the C’*O* P(10) laser line at 1076.5738 cm-’ (Fig. 1). The upper frequency l-type doublet of the R(18) of this band lies _ 120 MHz below the frequency of this laser line and a relatively strong two-photon signal was observed. This work is being extended to the measurement of other transitions of this band and to the vq-+(vq + v3) band of HCCF. This work has several aspects which are relevant to the generation of submillimetre waves by optical pumping. Most directly, the new coincidence between the R(18) 13C02 laser line and the Q(5,4) transition of 15NH3 can certainly be used to generate submillimetre output. This combination has several advantages over the well-known coincidence between the P(13) of the N20 laser and the Q(8,7) transition of 14NH3. The absorption coefficient in this new case is very much larger than that of the other case and the 13C0, laser can produce output powers considerably in excess of that achievable from the N,O laser. Secondly, and probably more importantly, this work shows that even with low microwave power levels an infrared-microwave two-photon process can be used to saturate
HAROLD JONES
“5
75’43 MHz
L_---_
Fig. I. Energy level scheme and observed signal of the two-photon transition in the hot-band I’$- i’i + r3 of HCCF produced using the Y.4pm P(10) CLROl Laser line.
an infrared transition. Consequently it seems very likely that if a wave-guide suitable coupling devices for both microwave and CO, laser radiation were two-photon resonant pumping could be used to generate a considerable new lasing transitions in 14NH,, 15NH3 and other molecular species. Since molecular constants for these molecules would be known the exact lasing could be accurately predicted before the experiment was carried out. REFERENCES
3. 4. 5. 6. 7. 8.
pp. 53 I 569. J~NI s. H.. dppi. Pfys. 14. 169 (19771. Jmi.s. H. .4p$ Pity.\. 15, 261 (1973). MOKITA. N.. S. KAF~O. Y. ULUA & T. SHIMWI . .I. C/WIII. Pl~!,.s. 66. 2726 (1977). JONIS. H.. J. rmlrc~. Spwmc. 70, 279 (1978). SHIMIZL,. F.. .I. Chrm. Phj,s. 53. I 149 (1970). FREUNIX S. M.. M. RCiMHELD& T. OKA. PII!\. Rrr,. Lvrr. 35. 1497 (1975).
laser with constructed, number of the accurate frequencies