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: [email protected] (T. Okabay-
ashi).
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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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