1756
J. Phys. Chem. 1980, 84, 1756-1757
Quadrupole Coupling Constant of Deuterium in DC15N by Lamb Dip Microwave Spectroscopy G. Cazzoli, C. Degli Esposti, and P. G. Favero" Laboratorio di Spettroscopia Molecolare del C.N.R., 40 126 Bologna, M y , and Isfituto Chimico "G. Ciamician" Universita ' di Bologna, 40 126 Bologna, Italy (Received August 7, 1979)
A high-Q semiconfocal Fabry-Perot cavity has been used to resolve the hyperfine structure of deuterium in DCI5N. The observed values of the deuterium quadrupole coupling constant and of the rotational constants of DC1'N are xD = -0.207 (4) MHz, Bo = 35169.809 (1) MHz, and Do = 0.0560 (2) MHz.
Costain in the centimeter region' and Winton and Gordy2 in the millimeter region first used the Lamb dip technique to improve the precision of frequency measurements and resolution power. f. = fk, - 2 fkl
the radiation must pass linearly in opposite directions through the cell, the power in each beam must be sufficient to produce saturation, and all other broadening factors must be small in comparison with Doppler broadening. M u l t .
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TABLE I: Transition Frequencies and Spectroscopic Constants of DC"N (in MHz) S +- J" F' C F" this work a
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70 339.292 ( 2 ) 70 339.382 ( 2 ) 70 339,445 ( 2 ) 1 4 0 677.339 ( 4 ) 1 4 0 677.437 ( 4 ) 140 677.503 (4)
b
70 339.394 (2)'
70 339.48 (10)
1 4 0 677.442 (4)'
1 4 0 677.446 (45)
3 5 169.791 ( 2 0 ) B o = 35169.809 (1)' 0.0538 ( 2 0 ) D o = 0.0560 (2)" X D = -0.207 (4)' E. F. Pearson, R. A. Creswell, M. Winnewisser, and G. Winnewisser, 2. Naturforsch. A , Unsplitted line frequencies. 31, 1894 (1976). ' Standard deviations obtained from the least-squares fit.
*
As pointed out by Lamb3 some conditions must be satisfied in order that a highly monocromatic radiation produces a sharp dip in the center of the line. Especially, *Author to whom correspondence should be addressed at the Wniversita' di Bologna. 0022-3654/80/2084- 1756$0 1.OO/O
These conditions are readily realized in the millimeter wave region by use of the high-Q Fabry-Perot interferometer recently developed in our laboratory and described e l ~ e w e r e .The ~ characteristics of this cavity are the following: 10-cm diameter of the mirrors; 30-cm distance between the. mirrors; 60-cm radius of curvature of the 0 1980 American Chemical Society
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Quadrupole Coupling Constant of Deuterium in DC”N __-.- .
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The Journal of Physical Chemistry, Vol. 84, No. 14, 1980
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Figure 2. (a) J - 10
8 transition of OCS at 121624 MHz. The broad upward curve is the second derivative of the Doppler-broadenedline while the downward sharp curve is the second derivative of the Lamb dip. (b) Second derivative of the dip of the OCS J = 10 9 transition at high dispersion.
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concave reflector; Q = 280 000 measured at 140 GHz. The block diagram of the spectrometer used is shown in Figure , 1. Since the control and the stability of the spectrometer are important for observation and measurement of Lamb dip effects, the external reference, which is provided by a GR 1115 high-stability crystal oscillator, has been carefully checked against a cesium beam standard. The IF of the millimetric klystron syncronizer has been frequency modulated with a sine wave of 1.67 kHz while the lock-in amplifier connected to the detector has been tuned to 3.33 kHz in order to obtain the second derivative of the actual signal. The frequency scanning has been performed through a frequency synthesizer as shown in Figure 1. In order to test the performance of the spectrometer the J = 10 9 transition of OCS has been recorded and the result is shown in parts a and b of Figure 2. The broad upward curve in Figure 2a is the second derivative of the Doppler-broadened line while the downward sharp curve is the second derivative of the Lamb dip. Figure 2b shows the central part of Figure 2a at higher
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Figure 3. The J = 2 1 transition of DC15Nat 140 677 MHz showing the hyperfine structure due to the deuterium nucleus.
dispersion. The accuracy of the frequency measurement in this case has been proved to be better than 1 kHz a t 121624 MHz. We then decided to try to observe with this spectrometer the deuterium hyperfine structure in DC15N since a conventional spectrometer does not have the required resolution power and because the quadrupole coupling constant of deuterium for this molecule is not known. Moreover, the value at 1 / 2 of the nuclear spin for 15N simplifies the pattern of the hyperfine structure which in this case is only due to the presence of the deuterium. The J = 1 0 and J = 2 1transition of DC15N have been measured and the resulting transition frequencies and spectroscopic constants are shown in Table I. The resolution power of our spectrometer can be judged 1 transition of from Figure 3 which shows the J = 2 DC15N. In conclusion our results show that the use of the Lamb-dip technique in millimeter wave spectroscopy allows an improvement of the resolution power of about a factor of 10 with respect of conventional spectroscopy in this region without any major alteration of the experimental apparatus.
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Acknowledgment. We thank Mr. R. Pezzoli and L. Cludi for their continuous technical support. References and Notes (1) C. C. Costain, Can. J . Phys., 47, 2431 (1969). (2) R. S. Winton and W. Gordy, Phys. Left., 32, 219 (1970) (3) W. E. Lamb, Phys. Rev. A , 1429, 134 (1964). (4) Manuscript to be submitted for publication.
