Journal of Molecular Spectroscopy209,66–70 (2001)doi:10.1006/jmsp.2001.8394, available online at http://www.idealibrary.com on

Millimeter- and Submillimeter-Wave Spectra of CuF and AgFin the X 1Σ+ Electronic Ground States

Toshiaki Okabayashi, Emi Yamazaki, Takeshi Honda, and Mitsutoshi Tanimoto

Department of Chemistry, Faculty of Science, Shizuoka University, 836 Oya, Shizuoka 422-8529, Japan

Received March 16, 2001; in revised form June 12, 2001; published online August 30, 2001

The millimeter- and submillimeter-wave spectra of CuF and AgF in theX 16+ states were observed by employing a source-modulated microwave spectrometer. The CuF and AgF molecules were generated in a free space cell by a dc glow dischargein CF4 and Ar. The copper and silver atoms were respectively supplied by sputtering from a copper cathode and silver sheetsplaced on the inner surface of a stainless-steel cathode. Rotational transitions of63CuF, 65CuF, 107AgF, and109AgF in highlyexcited and ground vibrational states were measured in the region between 200 and 400 GHz. The transition frequencies wereanalyzed by a least-squares method using a combined-isotopomer Dunham-type term energy expression.C© 2001 Academic Press

Key Words:copper fluoride; silver fluoride; rotational spectrum; Born–Oppenheimer breakdown constant.

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INTRODUCTION

Transition-metal compounds often possess a large numblow-lying electronic excited states owing to theird-electrons.These electronic states yield complex electronic spectra, ais difficult to analyze them and to understand the physical snificance of the spectroscopic constants obtained. Since it isdifficult to produce most of the transition-metal compounds whigh melting points efficiently in the gas phase, many previostudies have measured the electronic spectra of these compousing a high sensitivity spectrometer.

The diatomic transition metal halides MX (X=F, Cl, Br,and I) are highly ionic species represented as M+X−. All un-paired electrons mainly reside on the metal atom becausthe closed shell structure of the X− ion; thus, their electronicstates strongly reflect the character of the metallic ion M+. Sincecopper and silver halides have simpleX 16+ electronic groundstates and relatively low melting points, many experimentaltheoretical studies have been carried out for these specieparticular, copper and silver chlorides, bromides, and iodihave extensively been studied by microwave spectroscopy: C(1–4), CuBr (3, 5, 6), CuI (7, 8), AgCl (4, 9–13), AgBr (11,14–16), and AgI (11, 17–19). In contrast, there are not manmicrowave studies on CuF and AgF. Microwave spectra of Cand AgF in thev = 0, 1, and 2 states were observed by Hoet al. (20) using a Stark modulated molecular beam spectromter. They produced gaseous beams of CuF and AgF from sCuF2 and AgF, respectively, in an oven heated to 650–850◦C.Honerjager and Tischer (21) measured the Zeeman effect in thJ = 1–0 microwave rotational transition in the vibronic grounstate of CuF. Very recently, Evans and Gerry (22) observed theJ = 1–0 transition of CuF using a Fourier transform microwa(FTMW) spectrometer, where the molecule was producedthe reaction of laser-ablated Cu with SF6.

660022-2852/01 $35.00Copyright C© 2001 by Academic PressAll rights of reproduction in any form reserved.

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The spectroscopic properties of CuF and AgF have bethe subject of recent theoretical investigations for molecucontaining heavy atoms. The coinage metal fluorides (CAgF, and AuF) were discussed using the coupled clusCCSD(T) method combined with the no-pair one-componeDouglas–Kroll–Hess (DK) approximation (23) and using thezeroth-order regular relativistic approximation (ZORA) fomolecular density functional calculations (24). The ground andseveral excited electronic states of AgF were studied throucomplete active space self-consistent field (CASSCF) acomplete active space second-order perturbational (CASPcalculations (25). The Cu and F electric field gradients in Cuwere accurately calculated by a point charge model for tnuclear quadrupole moment tensor (PCNQM) (26). Severalsingle-reference (SR) many-electron theories were usedestimate the molecular properties of CuF (27).

Sputtering reactions from metal electrodes or from metalcompounds placed on the electrodes have been used to obsthe microwave spectra of compounds involving transition-meatoms. Examples are TiO (28), TiN (28), CrF (29), CrCl (30, 31),MnO (32), FeCl (33), NiO (34), NiF (35), NiCl (36), CuH (37),CuO (38), CuOH (39, 40), AgH (41), AgO (42), and AgOH(39, 40). In the present paper, we report the millimeter- ansubmillimeter-wave spectra of CuF and AgF generated bysimilar method. The measured data include transition frequcies in highly excited vibrational states of isotopic CuF (v = 0to 7) and AgF (v = 0 to 18) that have enabled us to determinseveral higher-order Dunham constants and Born–Oppenheibreakdown constants.

EXPERIMENTAL

The present experiment was carried out using a sourmodulated microwave spectrometer at Shizuoka University (43).

MICROWAVE SPECTRA OF CuF AND AgF 67

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FIG. 1. The rotational transitions of107AgF and 109AgF observed near285 GHz.

Millimeter- and submillimeter-wave radiation was generatedmultiplying the output of a klystron. The radiation transmittthrough a free space cell was detected by an InSb detector coto 4.2 K by liquid helium. The cell contained a pair of cylindricelectrodes for a dc glow discharge. The cell was surroundea cooling jacket made of copper through which liquid nitrogwas circulated.

The CuF and AgF species were generated in the free scell by a dc glow discharge in CF4 and Ar. The Cu or Ag atomswere supplied by sputtering from a copper cathode or a stainsteel cathode with silver sheets lining its inner surface. Thewas cooled to the liquid nitrogen temperature and the disch

TABLE 1Molecular Constants of CuF and AgF

a Molecular constants of63CuF and107AgF were obtained from the multi-isotopomer fit. Constantsof 65CuF and109AgF were derived from those of63CuF and107AgF, respectively. Values in parenthesesare one standard deviations.

b Reference (20). Constants were obtained from data on each isotopomer.c Fixed in the analysis.

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current was set to about 450 mA. Optimum sample pressure w1 mTorr of CF4 and 4 mTorr of Ar.

Under these experimental conditions, the line intensitiesCuF and AgF in the vibrationally excited states were found toextremely strong. The highest vibrational energy levels observfor CuF and AgF werev = 7 and 18, respectively. In total, weobserved 66 lines of63CuF and65CuF in the rangeJ ′′ = 8 to17, and 132 lines of107AgF and109AgF in the rangeJ ′′ = 14 to26. Figure 1 displays a sample of the observed spectrum.

ANALYSIS

The observed transition frequencies were fitted by a leasquares analysis using a combined-isotopomer Dunham-tterm energy expression (44). The energy formula for theαthisotopomer of species A–B formed from atoms of massMα

A andMα

B was represented as

Eα(v, J) =∑

(l ,m)6=(0,0)

Y1l ,m

(µ1

µα

)m+l/2

(v + 1/2)l [ J(J + 1)]m

+∑

(l ,m)≥(0,0)

{1Mα

A

MαA

δAl ,m +

1MαB

MαB

δBl ,m

}(µ1

µα

)m+l/2

× (v + 1/2)l [ J(J + 1)]m, (1)

where1MαA = Mα

A − M1A, andα = 1 identifies the reference

isotopomer. In the present work, the most abundant63CuF and107AgF species were selected as the reference isotopomers

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68 OKABAYASHI ET AL.

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this expression, the Dunham constants of other isotopomersYαl ,m

were expressed as follows:

Yαl ,m =

{Y1

l ,m +1Mα

A

MαA

δAl ,m +

1MαB

MαB

δBl ,m

}(µ1

µα

)m+l/2

. (2)

The parameters for the breakdown of the Born–Oppenheimapproximation,δA

l ,m andδBl ,m, are related to the familiar Watson

type breakdown terms (45),1Al ,m and1B

l ,m, through the equations

1Al ,m = −δA

l ,m

(M1

A

/me)/(

Y1l ,m + δA

l ,m + δBl ,m

), (3)

and

1Bl ,m = −δB

l ,m

(M1

B

/me)/(

Y1l ,m + δA

l ,m + δBl ,m

), (4)

whereme is the mass of an electron.The analysis of our millimeter- and submillimeter-wave da

combined with microwave data in Ref. (20) led to the constantsin Table 1 using a least-squares analysis. Observed microwfrequencies and residuals of the fit are summarized in Table

TABLE 2Observed Transition Frequencies of CuF and AgF in MHza

a Values in parentheses represent the residuals (Obs.-Calc.) in kHz.b Reference (20).

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RESULTS AND DISCUSSION

In the present study, lines from states with energies of ab4000 and 8000 cm−1 above the ground vibrational states weobserved for CuF and AgF, respectively. These observationsbe explained in terms of vibrational excitation in the discharplasma. The effective vibrational temperature for CuF and Awas estimated to be about 1500 K under the assumption oBoltzmann distribution.

The present measurement has led to an improvement omolecular constants of CuF and AgF. Our Dunham constaare about a hundred times more accurate than the previousues (20). Experimental data including those of highly excitestates give much more precise information on the higher-orDunham terms such asY1,2, which were not determined in Ref(20). The Born–Oppenheimer breakdown constantsδCu

0,1 andδAg0,1

are determined for the first time. In the fluorides, theδCu0,1 con-

stant has a sign opposite toδAg0,1 as observed in the correspondin

hydrides, CuH and AgH (46).

TABLE 2—Continued

69

70 OKABAYASHI ET AL.

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The spectral lines of AgF were found to be much stronthan those of CuF under the present experimental conditiThe electric dipole moment (20) of CuF (5.77 D) is as largeas that of AgF (6.22 D), which means that AgF was much mabundantly generated by the sputtering reaction than CuF.intensity of the CuF spectrum had decreased significantly wthe copper cathode had been used for two days. Howeverintensities of AgF lines were not weakened so much, even wthe silver-lined cathode had been used for four days. Afterexperiment, the surface of the copper electrode was corroand discolored. On the other hand, the surface of the silver lindid not lose its burnish so much and was covered by manyflakes of silver. The difference of generation of CuF and AgFprobably due to corrosion of the surface of Cu and Ag sputtertargets.

ACKNOWLEDGMENT

The authors thank the Japan Society for the Promotion of Science for suthrough Grant-in-Aid for Scientific Research 12740316.

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