Hyperfine structure of the J = 1 ’ 0 and J = 2 ’ 1 transitions

of D35Cl and D37Cl

G. Cazzoli* and C. Puzzarini

Dipartimento di Chimica ‘‘G. Ciamician’’, Via Selmi 2, I-40126 Bologna, Italy.E-mail: cazzoli@unibo.it

Received 27th August 2004, Accepted 16th September 2004First published as an Advance Article on the web 4th October 2004

For the first time, the deuterium hyperfine structure of the rotational J ¼ 1 ’ 0 and J ¼ 2 ’ 1 transitions ofD35Cl and D37Cl has been resolved by using microwave-spectroscopy. To this purpose, the Lamb-dip techniquehas been employed. The present investigation allowed us to provide for DCl not only accurate values for thedeterminable hyperfine constants but also, joint together with previous data, the most accurate ground staterotational parameters known up to now.

1. Introduction

While the spectroscopic parameters of H35Cl as well as H37Clare known to very high accuracy (see ref. 1 and referencestherein), D35Cl and especially D37Cl are not so well character-ized. In order to fill this gap, the J ¼ 1 ’ 0 and J ¼ 2 ’ 1transitions of both isotopic species have been investigated withsub-Doppler resolution. To this purpose, the Lamb-dip tech-nique has been employed and it allowed us to nearly comple-tely resolve the hyperfine structure due to electric quadrupoleand nuclear spin-rotation interactions of the deuterium (I ¼ 1)and chlorine (I ¼ 3/2) nuclei.

As concerns previous investigations, a highly accurate studyof the hyperfine structure of rotational levels in H35Cl andD35Cl was carried out by Kaiser2 employing molecular-beamelectric resonance (MBER) spectroscopy; in particular, theMBER spectra of D35Cl in the v ¼ 0, J ¼ 1, 2, 3 and v ¼ 1,J ¼ 1 states were analyzed. The first observation of the purerotational spectrum of D35Cl and D37Cl was performed by DeLucia et al. in 1971, who reported the submillimetre-wavespectra of the hydrogen halides.3 In regards to DCl, theyobserved the J ¼ 1 ’ 0 and J ¼ 2 ’ 1 transitions of bothisotopic species of chlorine. Since they investigated all theisotopic species of hydrogen chloride, by joining together therotational constants of H35Cl, H37Cl, D35Cl, D37Cl, T35Cl andT37Cl they were able to determine the equilibrium structure ofhydrogen chloride. In 1981, Guelachvili et al.4 investigated theinfrared bands of HCl and DCl by Fourier transform (FT)spectroscopy evaluating accurate Dunham coefficients for bothisotopomers and predicting those for TCl. The pure rotationalspectra of D35Cl and D37Cl were recorded at Doppler resolu-tion with tunable far infrared spectroscopy (TuFIR) in therange J00 ¼ 0–16 by Fusina and coworkers.5 In that investiga-tion the hyperfine structure due to the quadrupole moment ofCl of some rotational lines was partially resolved. Morerecently, high-resolution sub-Doppler Lamb-dip measure-ments were performed on the J ¼ 1 ’ 0 and J ¼ 2 ’ 1 purerotational transitions of H35Cl as well as H37Cl, and D35Cl aswell as D37Cl in the ground and first excited vibrational states.Furthermore, the Doppler-limited lines of the DCl J ¼ 3 ’ 2transition were also recorded in both the ground and firstexcited vibrational states.6 In that study, the hyperfine struc-ture due to D was not resolved at all. The new frequencies forall the isotopomers investigated were analyzed in a globalfit together with the available FIR data yielding a set of

mass-invariant rotational parameters as well as isotopicallyinvariant hyperfine constants. For the latter, the results fromMBER2 were also included. Finally, in a previous workWinnewisser and coworkers recorded the J ¼ 6 ’ 5 transitionof D35Cl as an application of the terahertz laser sidebandspectrometer set up in their laboratory.7

While HCl is known to have an astrophysical relevance sinceit has been detected in molecular and diffuse clouds,8–10 and inatmospheres of stars,11 Earth,12 and Venus,13 to our knowl-edge the spectra of the deuterated isotopic species have notbeen observed in the interstellar medium yet. Anyway, we areconfident that an improvement of the ground state rotationalparameters as well as of the rest frequencies could be veryuseful for future observations purposes. In fact, for instance,Schilke et al. performing an unbiased line survey of Orion-KLin the frequency range 607–725 GHz with the Caltech Sub-millimeter Telescope observed a weak feature at the frequencyof the J ¼ 2’ 1 transition of DCl but the corresponding DCl/HCl ratio was found too high; consequently, their conclusionwas that the feature observed was not DCl.14 The possibleastrophysical interest on DCl is also confirmed by the deter-mination of the absorption cross sections of HCl and DCl at135–232 nm carried out by Bahou et al.15 As pointed out inref. 15, the large [D]/[H] ratio in the Venusian atmosphere is ofcurrent interest and could be due to the fact that DCl is lessdissociated than HCl, so that the D atom is retained and thismight contribute to the enhancement of the [D]/[H] ratio.

2. Experimental details

In order to resolve the hyperfine structure due to the D and35Cl or 37Cl nuclei of the J ¼ 1 ’ 0 and J ¼ 2 ’ 1 transitionsof D35Cl and D37Cl, the Lamb-dip technique has been em-ployed. The Lamb-dip spectra have been observed with afrequency modulated computer-controlled spectrometerequipped with a liquid He-cooled InSb detector. Since anextensive description of our experimental set up can be foundin various previous papers (see for examples, refs. 16–19) and adetailed description of the spectrometer employed can befound in ref. 20, only the most important details related tothe present investigation will be reported here. The millimetre-wave sources employed are frequency multipliers drivenby Gunn diode oscillators covering the frequency range300–750 GHz. The source is phase-locked to a rubidiumfrequency standard; the frequency modulation is obtained by

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sine-wave modulating (1.666 kHz) the 90 MHz local oscillatorof the synchronization loop. The detector output signal isprocessed by means of a lock-in amplifier tuned at twice themodulation frequency; as a result, the observed profile is givenby the second derivative of the natural line profile. As demon-strated in ref. 21, Lamb-dip spectroscopy in the millimetre-wave region can be performed using a conventional free spacecell. The sensitivity of the spectrometer has been improved bydoubling the radiation pass in the absorption cell (for details,see for examples in refs. 18 and 22).

As concerns the experimental conditions, samples of DClhave been directly prepared inside the absorption cell startingfrom HCl and D2O. More precisely, HCl and D2O have beenseparately introduced in gas phase into the cell approximatelyin the ratio 2 : 1. The consequent isotopic H/D exchange givesan amount of DCl that showed rotational spectra with a verygood signal to noise ratio. In order to improve this ratiofurther, the cell has been cooled down by employing a liquidnitrogen refrigeration system. The spectra of D37Cl have beenrecorded in natural abundance. The measurements have beencarried out in the pressure range 0.1–0.5 mTorr. Such lowvalues of working pressure have been chosen in order to reduceas much as possible the dip half-width as well as to avoid thepressure frequency shift. The modulation depth used has beenadjusted according to the experimental conditions in the range6–12 kHz and 12–42 kHz for the J ¼ 1 ’ 0 and J ¼ 2 ’ 1transitions, respectively. Finally, in order to have narrow andnot distorted dips, it is particularly important to reduce asmuch as possible the source power.

Fig. 1 presents the observed hyperfine structure, due to boththe D and 35Cl nuclei, of the J ¼ 1 ’ 0 transition of D35Cl.The total spectrum has been recorded at a pressure of0.2 mTorr and employing a modulation depth of 30 kHzwhereas in the insets the resolved hyperfine components areshown in more details. In Fig. 2 the J ¼ 2 ’ 1 transition ofD37Cl is depicted. The total spectrum has been recorded at apressure of 3 mTorr and employing a modulation depth of40 kHz; in the insets the strongest resolved hyperfine compo-nents are reported.

3. Spectra analysis and results

The observed frequencies of the J ¼ 1 ’ 0 and J ¼ 2 ’ 1transitions of D35Cl and D37Cl are reported in Tables 1and 2, respectively. The various hyperfine componentsresult from the DF1, DF ¼ þ 1, 0, �1 selection rules, whereF1 and F are the hyperfine quantum numbers comingfrom the coupling schemes F1 ¼ J þ I1 and F ¼ F1 þ I2, whereI1 and I2 are the spin of the 35Cl (or 37Cl) and D nuclei,respectively.The corresponding Hamiltonian can be written as:

H ¼ HROT þ HQ þ HSR þ HSS, (1)

where HROT is the rotational part of the Hamiltonianoperator.

HQ ¼ ClHQ þ DHQ (2)

is the nuclear quadrupole coupling Hamiltonian due to thechlorine and deuterium nuclei, defined as follows:

ClHQ ¼eQq0ðClÞ þ eQqJðClÞJðJ þ 1Þ

2Jð2J � 1ÞIð2I � 1Þ

� 3ðI � JÞ2 þ 3

2I � J � I2J2

� �;

ð3Þ

DHQ ¼eQq0ðDÞ

2Jð2J � 1ÞIð2I � 1Þ 3ðI � JÞ2 þ 3

2I � J � I2J2

� �;

ð4Þ

and

HSR ¼ ClHSR þ DHSR, (5)

is the spin–rotation Hamiltonian:

KHSR ¼ þIKCI(K)J, (6)

where K denotes the chlorine or deuterium nucleus. Finally,

HSS ¼ þIClDID (7)

Fig. 1 The J ¼ 1 ’ 0 transition of D35Cl is shown. The spectrum has been recorded at a pressure of 0.2 mTorr and with a modulation depth of30 kHz. In the three insets, the resolved hyperfine components are reported with a better resolution, obtained using a modulation depth of 8 kHz.

5134 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 3 3 – 5 1 3 9 T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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is the spin–spin Hamiltonian; the constant D is connected tothe S12, which is usually reported in the literature, by thefollowing expression:

D ¼ �3S12. (8)

The frequency of each resolved or partially resolved line hasbeen determined by a line profile analysis (see, ref. 20) insteadof fitting the experimental data points to a parabolic functionas usually done in laboratory spectroscopy. In this analysisfour parameters have been evaluated with their standarderrors: the transition frequency, the sub-Doppler line width,the line intensity and the background offset; while, the modu-lation depth and the Doppler line width have been kept fixed inthe fitting procedure. In the case of multiplets, all the lines areconsidered at the same time in the fitting procedure. Theobserved frequencies have been obtained as averages of a setof measurements, and three times the standard deviation (3s)has been employed as uncertainty affecting our measurements.

It should be remarked that in both transitions and for boththe isotopic species investigated, in addition to the resolvedhyperfine components, crossing resonances, the so-called ghosttransitions, appeared. An example is given in Fig. 3, where theJ,F1 ¼ 1,5/2’ 0,3/2 transition of D35Cl is reported. This effectis due to the saturation of overlapping gaussian profiles of twoor more transitions with a common level,23,24 and it wasobserved in our laboratory various times (see for examples,refs. 16–18, 25). As highlighted in ref. 25, the presence of acrossing resonances can produce a repulsion or an attractionbetween the two Lamb-dips. The first was suggested by Leto-khov and Chebotayev24 and a dispersion term which producesa repulsion between the two Lamb-dips should be taken intoaccount. The second case was proposed by Mandal andGhosh.26 As concerns the ghost transition appearing in theJ ¼ 1 ’ 0 of 13CO, the second case was found to be the rightone in order to correctly interpret the hyperfine spectrum.25 In

the case of DCl, the situation cannot be so easily classified. Infact, since the complicated hyperfine structure due to D and35Cl or 37Cl also determine a high number of ghost transitions,it is not clear if they do produce attraction or repulsion. Inorder to overcome this problem, the presence of ghost transi-tions has been taken into account in the line profile analysiswithout considering any attraction or repulsion effects, whichmeans to consider them very weak or negligible.For D35Cl, we have performed three different fits. The first

one involved only the hyperfine components of the J ¼ 1 ’ 0and J ¼ 2 ’ 1 transitions we observed and it allowed us toevaluate very accurate values of the ground state rotationalconstant B0, the quartic centrifugal distortion constant and thedeterminable hyperfine parameters. On this topic, it should beaddressed that, even though the spin-rotation of deuteriumCI(D) and the spin–spin interaction S12 constants change thehyperfine pattern in an unambiguous manner with and withouttaking them into account, they are too small to be accuratelydetermined by our analysis. There is also a strong correlationbetween them. Consequently, they have been fixed at the valuesfrom ref. 2: while for CI(D) a very accurate value is given, forS12 only a calculated value is available, but anyway it is wellknown that this sort of estimate is very reliable and usuallyagrees with experiment within its uncertainty. It is worthnoting that, as expected from the data available in the litera-ture, the dependence on J of the quadrupole coupling constantof 35Cl, eQqJ(

35Cl), is not only determinable but also funda-mental for correctly reproducing the experimental frequencies.The value obtained has been found in very good agreementwith that evaluable from the literature.In the second fit, the transition frequencies from ref. 5 have

been added to our observed frequencies allowing the determi-nation of the sextic and octic centrifugal distortion constants;in this fit the hyperfine parameters have been kept fixed at thevalues obtained from the first one. In regard to this fit, it should

Fig. 2 The J ¼ 2 ’ 1 transition of D37Cl is shown. The spectrum has been recorded at a pressure of 3 mTorr and with a modulation depth of40 kHz. In the insets, the strongest resolved hyperfine components are reported with a better resolution, obtained using a modulation depthof 12 kHz. The asterisks mark the two weakest hyperfine components for which the Lamb-dip spectrum has also been observed.

P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 3 3 – 5 1 3 9 5135T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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be noted that the octic centrifugal distortion constant has beenfound not only determinable but also necessary to well repro-duce the higher J frequencies by Fusina et al.;5 in fact, the large

(observed � calculated) value for the J ¼ 17 ’ 16 transitionobtained by Fusina et al. can be definitely reduced by includingthe octic centrifugal distortion constant.

Table 1 Observed rotational frequencies of D35Cl

J0 F01 F0 J00 F001 F00 Observed freq./MHz o – c/kHz Ref. 6/MHz

1 3/2 3/2 0 3/2 5/29=;323282:1815ð8Þ

9>>>>>>>>=>>>>>>>>;323282:2085ð30Þ

1 3/2 3/2 0 3/2 1/2 0.7

1 3/2 3/2 0 3/2 5/2

1 3/2 5/2 0 3/2 5/2�323282:2132ð3Þ �0.2

1 3/2 5/2 0 3/2 3/2

1 3/2 1/2 0 3/2 1/2�323282:2501ð15Þ 0.8

1 3/2 1/2 0 3/2 3/2

1 5/2 3/2 0 3/2 3/2�323299:1017ð32Þ �2.0

9>>>>=>>>>;323299:1290ð30Þ

1 5/2 3/2 0 3/2 1/2

1 5/2 7/2 0 3/2 5/2 g323299:1131ð19Þ �0.51 5/2 5/2 0 3/2 5/2

�323299:1561ð7Þ 0.2

1 5/2 5/2 0 3/2 3/2

1 1/2 1/2 0 3/2 1/29>>>>=>>>>;323312:4949ð1Þ

1 1/2 1/2 0 3/2 3/2

1 1/2 3/2 0 3/2 3/2 0.0 323312.4994(30)

1 1/2 3/2 0 3/2 1/2

1 1/2 3/2 0 3/2 5/2

2 3/2 5/2 1 1/2 3/29>>>>=>>>>;646473:8815ð5Þ

2 3/2 3/2 1 1/2 3/2

2 3/2 3/2 1 1/2 1/2 �0.1 646473.881(10)

2 3/2 1/2 1 1/2 3/2

2 3/2 1/2 1 1/2 1/2

2 5/2 5/2 1 5/2 5/29=;646475:2824ð14Þ2 5/2 7/2 1 5/2 7/2 �0.4 646475.282(10)

2 5/2 3/2 1 5/2 3/2

2 3/2 5/2 1 5/2 5/29>>>>=>>>>;646487:2517ð31Þ

2 3/2 3/2 1 5/2 5/2

2 3/2 5/2 1 5/2 7/2 0.6 646487.255(10)

2 3/2 3/2 1 5/2 3/2

2 3/2 1/2 1 5/2 3/2

2 1/2 3/2 1 1/2 3/29>>=>>;646490:6910ð01Þ

2 1/2 3/2 1 1/2 1/20.0 646490.695(10)

2 1/2 1/2 1 1/2 3/2

2 1/2 1/2 1 1/2 1/2

2 5/2 3/2 1 3/2 1/2 g646492:1521ð30Þ �2.0 9>>>>>>>>>>>>>>=>>>>>>>>>>>>>>;

646492:2290ð50Þ

2 7/2 5/2 1 5/2 5/2�646492:1844ð21Þ 0.5

2 5/2 7/2 1 3/2 5/2

2 5/2 5/2 1 3/2 5/29>>>>>>>>=>>>>>>>>;646492:2314ð27Þ

2 7/2 9/2 1 5/2 7/2

2 5/2 3/2 1 3/2 3/2

2 7/2 5/2 1 5/2 3/2 �0.72 7/2 7/2 1 5/2 5/2

2 5/2 5/2 1 3/2 3/2

2 7/2 7/2 1 5/2 7/2

2 3/2 3/2 1 3/2 1/2�646504:1330ð19Þ �0.2

9>>>>>>>>=>>>>>>>>;646504:167ð10Þ

2 3/2 1/2 1 3/2 1/2

2 3/2 5/2 1 3/2 5/2�646504:1612ð20Þ 1.8

2 3/2 3/2 1 3/2 5/2

2 3/2 5/2 1 3/2 3/29=;646504:1999ð24Þ2 3/2 3/2 1 3/2 3/2 1.8

2 3/2 1/2 1 3/2 3/2

2 1/2 1/2 1 3/2 1/29>>>>=>>>>;646520:9747ð18Þ

2 1/2 3/2 1 3/2 1/2

2 1/2 3/2 1 3/2 5/2 �2.6 646520.978(10)

2 1/2 1/2 1 3/2 3/2

2 1/2 1/2 1 3/2 3/2

5136 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 3 3 – 5 1 3 9 T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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Finally, in the third fit we have also determined the hyperfineparameters, i.e., all the determinable parameters have beenfitted. As expected, negligible variations with respect to theresults of the first or second fits have been found, and also theeQqJ(

35Cl) value has been confirmed. The second and third fitshave also been repeated by introducing the frequencies of theJ ¼ 3’ 2 transition from ref. 6 instead of those from ref. 5. Inthis case 2 more frequencies are available, but, as expected

since the frequencies from refs. 5 and 6 agree within theiruncertainties, no variations at all in the determinable para-meters have been found.All these are least-squares fits in which each line is weighted

proportionally to the inverse square of its experimental un-certainty evaluated as explained above. For unresolved hyper-fine components, the calculated frequencies have beendetermined using intensity-weighted averages. All these fits

Table 2 Observed rotational frequencies of D37Cl

J0 F01 F0 J00 F001 F00 Observed freq./MHz o � c/kHz Ref. 6/MHz

1 3/2 3/2 0 3/2 3/29=;322338:9058ð16Þ

9>>>>>>>>=>>>>>>>>;322338:9345ð30Þ

1 3/2 3/2 0 3/2 5/2 3.3

1 3/2 3/2 0 3/2 1/2

1 3/2 5/2 0 3/2 3/2�322338:9344ð4Þ �1.3

1 3/2 5/2 0 3/2 5/2

1 3/2 1/2 0 3/2 1/2�322338:9705ð18Þ �0.7

1 3/2 1/2 0 3/2 3/2

1 5/2 3/2 0 3/2 5/29>>=>>;322352:2509ð4Þ

9>>>>>>=>>>>>>;322352:2639ð30Þ

1 5/2 3/2 0 3/2 1/20.9

1 5/2 3/2 0 3/2 3/2

1 5/2 7/2 0 3/2 5/2

1 5/2 5/2 0 3/2 3/2�322352:2955ð9Þ �0.4

1 5/2 5/2 0 3/2 5/2

1 1/2 1/2 0 3/2 1/29>>>>=>>>>;322362:7966ð2Þ

1 1/2 1/2 0 3/2 3/2

1 1/2 3/2 0 3/2 3/2 �0.2 322362.7986(30)

1 1/2 3/2 0 3/2 1/2

1 1/2 3/2 0 3/2 5/2

2 3/2 5/2 1 1/2 3/29>>=>>;644585:7682ð5Þ

2 3/2 3/2 1 1/2 3/2�1.8 644585.768(10)

2 3/2 3/2 1 1/2 1/2

2 3/2 1/2 1 1/2 1/2

2 5/2 7/2 1 5/2 7/2 g644586:8760ð3Þ 1.6 644586.870(10)

2 3/2 5/2 1 5/2 5/29>>>>=>>>>;644596:3015ð50Þ

2 3/2 3/2 1 5/2 5/2

2 3/2 5/2 1 5/2 7/2 �0.3 644596.301(10)

2 3/2 3/2 1 5/2 3/2

2 3/2 1/2 1 5/2 3/2

2 1/2 3/2 1 1/2 3/29=;644599:0140ð3Þ2 1/2 3/2 1 1/2 1/2 �1.3 644599.017(10)

2 1/2 1/2 1 1/2 3/2

2 5/2 3/2 1 3/2 1/2 g644600:1598ð20Þ �1.79>>>>>>>>>>=>>>>>>>>>>;

644600:2292ð50Þ

2 7/2 5/2 1 5/2 5/2�644600:1916ð13Þ

2 5/2 7/2 1 3/2 5/20.8

2 5/2 5/2 1 3/2 5/29>>>>=>>>>;644600:2311ð5Þ

2 7/2 9/2 1 5/2 7/2

2 5/2 3/2 1 3/2 3/2 �0.72 7/2 5/2 1 5/2 3/2

2 7/2 7/2 1 5/2 5/2

2 3/2 3/2 1 3/2 1/2�644609:6014ð24Þ �0.1

9>>>>>>>>=>>>>>>>>;644609:635ð10Þ

2 3/2 1/2 1 3/2 1/2

2 3/2 5/2 1 3/2 5/2�644609:6282ð3Þ

2 3/2 3/2 1 3/2 5/20.5

2 3/2 5/2 1 3/2 3/29=;644609:6672ð3Þ2 3/2 3/2 1 3/2 3/2 0.4

2 3/2 1/2 1 3/2 3/2

2 1/2 1/2 1 3/2 1/29>>=>>;644622:8798ð22Þ

2 1/2 3/2 1 3/2 5/2�2.8 644622.880(10)

2 1/2 3/2 1 3/2 3/2

2 1/2 1/2 1 3/2 3/2

P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 3 3 – 5 1 3 9 5137T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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have been carried out with Pickett’s SPCAT/SPFIT program.27

Since the third fit is the most exhaustive and the determinableparameters vary less than the standard error from one fit toanother, only the results of the third one are reported inTable 3, where they are compared with those available in theliterature.

As far as D37Cl is concerned, since its spectra have beenrecorded in natural abundance, the S/N ratio is lower than in

the case of D35Cl, and consequently the retrieved frequenciesare less accurate. This results in a lower number of deter-minable parameters; in fact, the eQqJ (37Cl) and the octiccentrifugal distortion constant are not determinable. Conse-quently, while the octic centrifugal distortion constant can benot included in the fitting procedure, eQqJ(

37Cl) should betaken into account. Thus, since for D37Cl hyperfine parametershave not been previously determined with the exception of the

Fig. 3 The J,F1 ¼ 1,5/2 ’ 0,3/2 transition of D35Cl is shown: the observed and calculated (retrieved by line profile analysis) spectra and theresidual (o� c) are given. In addition to the resolved F0 � F00 hyperfine components, a ghost transition is evident. The spectrum has been recorded ata pressure of 0.3 mTorr and with a modulation depth of 8 kHz. The residuals and spectrum use the same intensity scale.

Table 3 Spectroscopic constants of D35Cl and D37Cl

Constant This work Klaus et ala Fusina et alb Molec. beamc

D35Cl

B0 (MHz) 161656.24855(27) 161656.249(14) 161656.2467(41) —

D0 (MHz) 4.195777(30) 4.19574(17) 4.195662(38) —

H0 (kHz) 0.06858(24) 0.06787(66) 0.067639(92) —

L0 (Hz) �0.00201(53) — — —

eQq0(35Cl) (MHz) �67.3894(17) �67.38842(25) �67.461(96) �67.38846(9)e

eQqJ(35Cl) (kHz) �2.27(51) — 0.0d �2.46(1)e

CI(35Cl) (kHz) 27.41(28) 27.423(25) 28.3(58) 27.426(7)

eQq0(D) (MHz) 0.1831(43) 0.18746(64) — 0.18736(30)

CI(D) (kHz) �3.295f �3.285(14) — �3.295(46)S12 (kHz) 0.86f — — 0.86

Standard deviation

s (MHz) 0.123

D37Cl

B0 (MHz) 161183.12953(23) 161183.133(14) 161183.1231(32)

D0 (MHz) 4.171022(27) 4.17111(17) 4.170993(30)

H0 (kHz) 0.066967(80) 0.06727(65) 0.066925(73)

eQq0(37Cl) (MHz) �53.1074(40) �53.10408(20) �53.002(54)

eQqJ(37Cl) (kHz) �1.94f — 0.0d

CI(37Cl) (kHz) 22.76(19) 22.759(21) 19.1(51)

eQq0(D) (MHz) 0.1841(35) 0.18746(64) —

CI(D) (kHz) �3.275f �3.275(29) —

S12 (kHz) 0.72f — —

Standard deviation

s (MHz) 0.202

a Ref. 6: uncertainties given in parenthesis are 3s. b Ref. 5. c Ref. 2: v ¼ 0, J ¼ 1 values with exception of derived values, see below. d Ref. 5:

constrained value. e Ref. 2: derived values. f Values from the literature or, for D37Cl, derived by scaling the D35Cl data: see text.

5138 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 3 3 – 5 1 3 9 T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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isotopically invariant hyperfine constants available by Klauset al.,6 we kept eQqJ(

37Cl) fixed at the scaled value, i.e., thatobtained by multiplying eQqJ(

35Cl) by the Q(37Cl)/Q(35Cl)ratio of the quadrupole moments. With regard to CI(D) andS12, they have been kept fixed at the value reported by Klauset al.6 and at the scaled value, respectively. The scaled value forS12 has been derived from the D35Cl data by using the g factorsratio of 35Cl/37Cl. Finally, for this isotopic species two fitscorresponding to the first and third ones carried out for D35Clhave been performed. As for the main isotopic species, negli-gible variations of the hyperfine parameters have been foundfrom one fit to the other.

From Table 3, we can notice that our results are in very goodagreement with those available in the literature. As concernsthe rotational parameters, for both D35Cl and D37Cl ourdetermined parameters are clearly improved with respect tothe previous available ones: in particular, the uncertaintyaffecting B0 has been reduced by about an order of magnitudeand the octic centrifugal distortion constant for D35Cl has beendetermined for the first time. With regard to the determinablehyperfine parameters of D35Cl, it is clear from Table 3 thatthere is a very good agreement with the MBER spectroscopydata, i.e., they agree within the standard errors. As far asD37Cl, it should be noted that the rotational and centrifugaldistortion constants are affected by errors slightly lower than inthe case of the main isotopic species. This can be ascribedto the frequencies from ref. 5; in fact, the same behaviorcan be noticed for the parameters by Fusina et al. Finally, itshould be remarked that this is the first experimental determi-nation of the deuterium hyperfine structure for this isotopicspecies.

4. Conclusions

The Lamb-dip technique has been applied to the observationof the J ¼ 1 ’ 0 and J ¼ 2 ’ 1 transitions of D35Cl andD37Cl. The high accuracy of this technique allows us to nearlycompletely resolve the hyperfine structure due to 35Cl (or 37Cl)and D. Our measurements together with the available ones upto J00 ¼ 16 allow us to provide the most accurate ground staterotational constants known at the moment. It should beremarked that the knowledge of very accurate hyperfine para-meters for the D35Cl isotopic species from molecular beamdata helped a lot in clarifying the effects of the crossingresonances on the contiguous Lamb-dips and, consequently,allowed us to provide for the first time very accurate andreliable hyperfine parameters for D37Cl.

Since HCl is known to have an astrophysical relevance andthe [D]/[H] ratio in the interstellar medium is of currentinterest, we think that the improvement in the knowledge ofthe ground state spectroscopic parameters as well as of the restfrequencies of the rotational transitions of D35Cl and D37Clcarried out in the present investigation could be very useful forthe purposes of future observations.

Acknowledgements

This work has been supported by MIUR (ex-40%), CNR andUniversity of Bologna (funds for selected research topics andex-60% funds).

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P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 3 3 – 5 1 3 9 5139T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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