Millimeter- and submillimeter-wave spectroscopy of AuFin the ground X1Rþ state

Toshiaki Okabayashi *, Yuki Nakaoka, Emi Yamazaki, Mitsutoshi Tanimoto

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

Received 24 August 2002; in final form 12 September 2002

Abstract

The millimeter- and submillimeter-wave spectrum of AuF in the X1Rþ state was observed by employing a source-

modulated microwave spectrometer. The AuF molecule was generated in a free space cell by the sputtering reaction

from a gold sheet lining the inner surface of a stainless steel cathode using a dc glow plasma of CF4 and Ar. Rotational

transitions in the highly excited vibrational states enabled to determine Dunham-type molecular constants by a least-

squares fit. Other constants like dissociation energy De, vibrational wavenumber xe and its anharmonic term xexe were

also derived from the obtained constants.

� 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

The AuF molecule was one of the most elusive

molecules in the last century. After several un-

successful attempts to prepare AuF and the ther-

modynamic discussion on its stability, it was once

deemed impossible to synthesize this unstablemolecule [1]. Except for an uncertain spectroscopic

observation [2], there was no positive experimental

or theoretical evidence for the existence of AuF. In

1989, Schwerdtfeger et al. [3] carried out a detailed

ab initio calculation including relativistic effects to

elucidate anomalies of several Au-containing

molecules. They predicted that AuF would be less

stable than its congeners CuF and AgF because of

the relativistic destabilization but stable enough

for spectroscopic identification in the gas phase. A

few years later, Saenger and Sun [4] investigated

emission bands from the RF discharge plasma of

O2=SF6 or O2=CF4 mixture on gold film using the

low-resolution visible electronic spectrum near 500nm. They tentatively assigned the observed spec-

trum to 1P–1R transitions of AuF but could not

rule out the possibility that the carrier was AuFþ,

AuO, or AuOþ. Their identification was immedi-

ately supported by an ab initio calculation of

Schwerdtfeger et al. [5]. Subsequently, Schr€ooder

et al. [6] observed neutral AuF using neutraliza-

tion–reionization mass spectrometry (NRMS) andindirectly estimated the dissociation energy to be

in the range of 2.9–3.2 eV through reactions of

Auþ with fluorine-containing organic compounds.

Finally, the Fourier-transform-microwave (FTMW)

Chemical Physics Letters 366 (2002) 406–411

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* Corresponding author.

E-mail address: sctokab@ipc.shizuoka.ac.jp (T. Okabay-

ashi).

0009-2614/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0009 -2614 (02 )01586 -5

spectroscopic observation by Evans and Gerry [7]

confirmed the existence of this labile molecule.

They generated AuF by reaction of laser-ablated

gold with SF6 in the supersonic jet and observed its

J ¼ 1–0 transitions in the first vibrational excited

state as well as the ground state. They calculatedthe preliminary equilibrium internuclear distance

under the assumption that both centrifugal dis-

tortion and higher-order vibration–rotation effects

were negligible. Andreev and BelBruno [8] re-

measured the electronic transition of AuF near 500

nm using the sputtering reaction of a gold anode

with SF6 without O2. They re-assigned a part of

the transitions observed by Saenger and Sun [4] tothe transitions from two different electronic states,1R–X1R as well as 1P–X1R. However, a recent ab

initio calculation [9] suggests that these electronic

transitions are assignable to two spin components

of 3P–X1R, contrary to the experimental results.

In addition to this theoretical work, many ab initio

calculations have been published in the last decade

[3,5,8–17]. This is because the AuF molecule is oneof the suitable molecules to study the importance

of relativistic effects on quantum calculations.

Recently, our group [18–20] and Saito�s group

[21] demonstrated that sputtering reactions from

metal cathodes or from metallic compounds

placed on the cathodes were good sources to ob-

serve the microwave spectra of compounds in-

volving transition-metal atoms. Since molecules ina discharge plasma were efficiently put into vib-

rationally excited states, this was one of the most

suitable methods to observe high-resolution spec-

tra of transition-metal species in their excited

states [22]. In our previous papers, we reported the

efficient generation in this sputtering system and

determination of spectroscopic properties of other

coinage metal fluorides, CuF and AgF [23], andhydrides, CuH [24] and AgH [25]. As an extension

of our interest in coinage metal compounds, we

report in the present Letter the millimeter- and

submillimeter-wave spectrum of AuF generated by

a similar method of production. The measured

data include transition frequencies in highly ex-

cited vibrational states of AuF (v ¼ 0–13) that give

several higher-order Dunham constants. Fromthese constants we have determined several mo-

lecular parameters of AuF including the equilib-

rium internuclear distance, re, with improved

accuracy, and compared with those predicted by

ab initio calculations.

2. Experimental

The present experiment was carried out using a

source-modulated microwave spectrometer at

Shizuoka University [26]. Millimeter- and submil-

limeter-wave radiations were generated by multi-

plying the output of a klystron. The radiation

transmitted through a free space cell was detected

by an InSb detector cooled at 4.2 K by liquid he-lium. The cell contained a pair of cylindrical

electrodes for a dc glow discharge. The cell was

surrounded by a cooling jacket made of copper

through which liquid nitrogen was circulated.

The AuF species were generated in the free

space cell by a dc glow discharge in CF4 with Ar.

The Au atoms were supplied by sputtering from a

small piece of gold sheet lining the inner surface ofa stainless steel cathode. The cell was cooled to the

liquid nitrogen temperature for the AuF genera-

tion. Optimum sample pressure was 1 mTorr of

CF4 with 4 mTorr of Ar. Under this experimental

_

_

Fig. 1. Rotational transitions of AuF in the 8th and 13th ex-

cited states observed near 387 GHz.

T. Okabayashi et al. / Chemical Physics Letters 366 (2002) 406–411 407

condition, the line intensities of AuF in the excited

states were strong enough to be observed easily.

The discharge current was set below 100 mA for

the observation of the ground and low excited

states, and up to 500 mA for states above v ¼ 10.

The highest vibrational energy level observed forAuF was v ¼ 13. In total, we observed 94 lines of

AuF between 189 and 402 GHz. Fig. 1 displays a

sample of the observed spectrum.

3. Results and discussion

The observed transition frequencies were fittedby a least-squares analysis using a conventional

Dunham Yl;m term expression [27],

Eðv; JÞ ¼Xl;m

Yl;mðvþ 1=2Þl½JðJ þ 1Þ�m: ð1Þ

The analysis of our millimeter- and submillimeter-

wave data combined with microwave data in [7]

led to the constants listed in Table 1, where the

previous FTMW [7] and ab initio results [11] are

also included. The ab initio calculation reproducedthe observed values quite well. The observed ro-

tational transition frequencies and residuals of the

fit are summarized in Table 2.

The present measurement has led to an im-

provement of the molecular constants of AuF. Our

Y01 Dunham constant, 7924.89453(45) MHz, dif-

fers from the previous value, 7924.83328(57) MHz

[7], by about 0.06 MHz, which exceeds the S.D. bytwo orders of magnitude; this is because our ex-

perimental data have enabled determination of the

higher-order Dunham terms ignored in [7]. The

effective rotational constant for the J ¼ 1–0 rota-

tional transition in the ground vibrational state is

derived from the Dunham energy expression as

Beff0ðJ¼1–0Þ ¼

XlP 0; mP 1

2ð�lþm�1ÞYl;m: ð2Þ

The effective rotational constant, Beff0ðJ¼1–0Þ ¼

7896:8189ð5Þ MHz, derived from the present

Dunham constants is consistent with the previous

FTMW value, 7896.81976(47) MHz. The equilib-

rium internuclear distance, re, re-determined from

Y01 to be 1.91844206(12) �AA agrees with the previ-

ous FTMW value 1.918449(5) �AA within the latterprecision.

If Y01=Y10 is small enough, the sextic centrifugal

distortion constant Y03 can be approximated by

[27]

Y03 ¼�2Y02

3Y 210

12Y 201

�þ Y10Y11

�: ð3Þ

Using this equation, Y03 is estimated to be

)3.17889(88) mHz, which agrees with the present

experimental value )4.10(57) mHz within the 3

S.D.

The vibrational wavenumber xe and its anhar-monic term xexe of a diatomic molecule with the

small Y01=Y10 value are represented as [27]

xe ¼ Y10 ¼

ffiffiffiffiffiffiffiffiffiffi4Y 3

01

�Y02

sð4Þ

Table 1

Molecular constants of AuFa

This work FTMWb Ab initioc

Y01 7924.89453(45) 7924.83328(57) 7760 mHz

Y11 )56.15575(29) )56.02704(66) )55.2 mHz

Y21 0.064740(78) 0.060 mHz

Y31 )0.2937(84) kHz

Y41 )21.80(30) Hz

Y02 )6.97121(78) )7.11 kHz

Y12 )9.66(14) Hz

Y22 )0.3199(92) Hz

Y03 )4.10(57) mHz

a Values in parentheses represent 1 S.D.b [7].c [11].

408 T. Okabayashi et al. / Chemical Physics Letters 366 (2002) 406–411

and

xexe ¼ �Y20 ¼ Y01

�Y11Y10

6Y 201

�þ 1

�2

: ð5Þ

The xe and xexe values are calculated to be

563.697(32) and 3:27235ð26Þ cm�1, respectively.These values are compared with the previous ex-

perimental and theoretical values in Table 3. The

present xe value is fully consistent with the valueof Saenger and Sun, 560 cm�1, through the low-

resolution electronic spectrum [4]. Our xe value is

also consistent with that estimated from the in-

tensity ratio of the FTMW spectrum (’500 cm�1)

[7]. Ab initio calculations also suggest the xe val-

ues to be 500–600 cm�1, which all agree well with

the present value.

Table 2

Observed transition frequencies of AuF in MHza

J 0–J 00 v ¼ 0 v ¼ 1 v ¼ 2 v ¼ 3

1–0 15793.640(2)b 15681.586(3)b

12–11 189475.764(2)

13–12 205256.326()12) 203799.541(5) 202345.992()19) 200895.687()1)

15–14 236810.794(6) 235129.805()24) 233452.645()2) 231779.166(7)

16–15 252584.307()20) 250791.260()24) 249002.254()14) 247217.191(0)

19–18 299888.191(2) 297758.865()3) 295634.330(5) 293514.450()3)

24–23 315649.991()7) 313408.569()6) 311172.180(1) 308940.685()15)

25–24 394405.413(20) 391603.402(20) 388807.635()7) 386018.024()8)

26–25 401421.544(9)

v ¼ 4 v ¼ 5 v ¼ 6 v ¼ 7

13–12 199448.476()5) 198004.294(4) 196562.977()24) 195124.514(28)

15–14 230109.287(23) 228442.861(15) 226779.795(20) 225119.909(6)

16–15 245435.976(31) 243658.427(19) 241884.457(19) 240113.885(5)

19–18 291399.130(4) 289288.213(15) 285078.817()34)

20–19 306714.004(2) 304491.926()6) 302274.351(34) 300060.949()5)

21–20 319692.278()8) 317363.725()16) 315039.681(17)

25–24 383234.366()19) 380456.528(20)

26–25 398526.457()21) 395637.401()17) 392754.139(11) 389876.380(27)

v ¼ 8 v ¼ 9 v ¼ 10 v ¼ 11

13–12 193688.602()3) 192255.185()18) 190824.109()2) 189395.144()4)

15–14 205492.109()30) 203953.220()10)

16–15 238346.562(2) 236582.286()1) 234820.815()39) 233062.039(2)

17–16 253227.311()22) 251352.762(4) 249481.217(18) 247612.391()28)

20–19 297851.614()19) 295646.110()4) 293444.137()1) 291245.422()3)

21–20 312719.821()5) 308091.822()25) 305783.165(24)

26–25 387003.790()22) 384136.202(7) 381273.183(18) 378414.355()4)

27–26 401847.995()24) 398870.023(2) 395896.799(17)

v ¼ 12 v ¼ 13

14–13 202416.410(8) 200881.436(5)

16–15 231305.578()18)

17–16 245746.174(12)

20–19 289049.701(27)

21–20 303477.573(30) 301174.719(4)

22–21 317901.799()9) 315489.270(16)

27–26 389963.088(41) 387001.672()38)

28–27 401290.924()20)

a Values in parentheses represent the residuals (observed) calculated) in kHz.b Frequency without hyperfine splitting. Calculated from [7].

T. Okabayashi et al. / Chemical Physics Letters 366 (2002) 406–411 409

The dissociation energy De of a diatomic mol-ecule with the small Y01=Y10 value is estimated from

the present result using [27]

De ¼Y 2

10

�4Y20

: ð6Þ

The obtained dissociation energy is about 3.01 eV,

which well agrees with indirect estimation by the

NRMS study as well as ab initio predictions, as

shown in Table 3.

In the present study, lines from vibrationalstates about 7000 cm�1 above the ground state

have been observed for AuF. This observation is

probably explained in terms of vibrational excita-

tion in the discharge plasma [22]. The effective vi-

brational temperature in the discharge plasma can

be roughly estimated from the relative intensity

ratio of two different vibrational states under the

assumption of the Boltzmann distribution. Whenv ¼ 8 and 13 lines displayed in the Fig. 1 are em-

ployed, the vibrational temperature is estimated to

be about 3000 K. Although the vibrational tem-

perature depends on the vibrational quantum

number v, the temperatures are roughly estimated

to be about 1000 and 4000 K under the discharge

current of 100 and 500 mA, respectively.

The intensities of AuF lines were not weakenedduring sputtering, even after a gold sputtering

target had been used for five days. In our previous

work [23], the AgF lines behaved similarly to the

AuF lines, whereas the CuF lines significantly de-creased their intensities when a copper sputtering

target was used for two days. After the experiment,

the surfaces of the gold and silver targets lost their

brilliance only slightly, but that of the copper

target was corroded and discolored. This finding

means that corrosion of the metal surface proba-

bly prevents the sputtering reaction as observed

for CuF, but not for AgF/AuF.

Acknowledgements

The research was supported by Japan Society

for the Promotion of Science through Grant-in-

Aid for Scientific Research (No. 12740316). T.O.

thanks the Kawasaki Steel 21st Century Founda-tion for financial support. E.Y. thanks the Japan

Science Society through the Sasagawa Scientific

Research Grant. T.O. and E.Y. also acknowledge

financial support from the Hamamatsu Founda-

tion for Science and Technology Promotion.

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