Pure rotational spectrum of gold monoxide (AuO) in the X 2Π3/2 state
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Transcript of Pure rotational spectrum of gold monoxide (AuO) in the X 2Π3/2 state
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Chemical Physics Letters 403 (2005) 223–227
Pure rotational spectrum of gold monoxide (AuO) in the X 2P3/2 state
Toshiaki Okabayashi a,*, Fumi Koto b, Kazuhiro Tsukamoto b,Emi Yamazaki b, Mitsutoshi Tanimoto b
a Center for Instrumental Analysis, Shizuoka University, Oya 836, Shizuoka 422-8529, Japanb Department of Chemistry, Faculty of Science, Shizuoka University, Oya 836, Shizuoka 422-8529, Japan
Received 25 November 2004; in final form 17 December 2004
Available online 18 January 2005
Abstract
The pure rotational spectrum of AuO in the X 2P3/2 state was observed by employing a source-modulated microwave spec-
trometer. The AuO radical was generated in a free space cell by the sputtering reaction from a gold sheet lining the inner sur-
face of a stainless steel cathode using a dc glow plasma of O2 and Ar. Observation of the rotational transitions in both the
ground and first excited vibrational states enabled to determine molecular constants by a least-squares fit. Other spectroscopic
parameters such as equilibrium bond length, re, and vibrational wavenumber, xe, were also derived from the obtained molec-
ular constants.
� 2005 Elsevier B.V. All rights reserved.
1. Introduction
Study on the relativistic effect in diatomic goldcompounds is of much interest, because the pro-
nounced local maximum contraction of the 6s shell
makes gold a unique element [1]. The relativistic effect
of such compounds has recently been a subject of
many theoretical studies, whereas experimental investi-
gations have thus far been quite limited. As for the
gaseous species investigated by high resolution spec-
troscopic methods, there are only few works exceptfor the closed-shell 1R+ species, e.g., gold monohalides
and monohydride. To the best of our knowledge, the
AuSi radical [2] is a sole open-shell species bearing a
gold atom studied by the high resolution spectroscopy
in the gas phase.
Millimeter- and submillimeter-wave spectroscopy is
a good method to investigate transient molecules bear-
ing transition metals. We have studied the gaseous
0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.01.003
* Corresponding author. Fax: +81 54 237 3384.
E-mail address: [email protected] (T. Okabayashi).
gold monohalides using a source-modulation micro-
wave spectrometer with a sputtering method [3,4].
The determined molecular constants have allowed thephysico-chemical properties of these closed-shell spe-
cies to be probed and have helped to understand the
nature of bonding of gold. To improve the under-
standing of diatomic Au species, we have extended
our studies to gold monoxide AuO in the open-shell2Pi electronic state.
The AuO radical was first observed spectroscopically
in 1977 by Griffiths and Barrow [5], who reported elec-tronic transitions of AuO in inert gas matrices in the vis-
ible region. Recently, Citra and Andrews [6] investigated
its infrared bands in inert gas matrices and determined
the vibrational constants. However, no spectroscopic
study was reported for the free AuO radical in the gas
phase. Three theoretical studies have been carried out
on the electronic ground and excited states of AuO [6–
8], giving estimates of several equilibrium molecularconstants.
In this Letter, we report on the first measurement of
the pure rotational spectrum of AuO in the electronic
224 T. Okabayashi et al. / Chemical Physics Letters 403 (2005) 223–227
ground state. Rotational and centrifugal distortion con-
stants have been determined and used to evaluate the
equilibrium bond distance, re, and the harmonic vibra-
tional frequency, xe.
2. Experimental
The present experiment was carried out using a
source-modulated microwave spectrometer [9]. Milli-
meter- and submillimeter-wave radiation was gener-
ated by multiplying the output of a klystron. The
radiation transmitted through a free space cell was de-
tected by an InSb detector cooled by liquid helium.The cell was equipped with a pair of cylindrical elec-
trodes for a dc glow discharge and was covered by a
cooling jacket made of copper through which liquid
nitrogen was circulated. To cancel the magnetic field
of the earth, electromagnetic coils were wound outside
the cell.
The AuO radical was generated in the free space cell
by a dc glow discharge in O2 with Ar just like in our pre-vious experiment on PtO [10]. Gold atoms were supplied
by sputtering from a gold sheet lining the inner surface
of a stainless steel cathode. The cell was cooled to the li-
quid nitrogen temperature for the efficient AuO genera-
tion. Optimum sample pressure was 1 mTorr of O2 with
4 mTorr of Ar. Under these experimental conditions, the
lines of AuO in the ground state were strong enough to
be observed easily. The spectrum showed hyperfine split-ting due to the nuclear spin of gold. The discharge cur-
rent was set to about 400 mA. Weaker lines in the first
excited vibrational state were also observed, and the
line-intensity in the v = 1 state was about seven times
weaker than that in the ground state. In spite of a care-
ful examination, spectral lines in the higher spin substate
X 2P1/2 could not be detected. In total, we observed 13
rotational transitions of AuO between 149 and388 GHz. Fig. 1 displays a sample of the observed
spectrum.
149210 149217(MHz)
F=6–5F=7–6
F=8–7F=9–8
AuO (X 2
Π3/2) J=7.5–6.5
Fig. 1. Rotational spectrum of AuO observed near 149 GHz.
3. Analysis
The observed transition frequencies were analyzed by
a least-squares analysis using a Hund�s case (c) effective
Hamiltonian. The Hamiltonian employed is
H eff ¼ H rot þ Hhf ; ð1Þwhere Hrot represents rotational energy including cen-
trifugal distortion, and Hhf hyperfine interaction.
The matrix elements are described as follows [11]:
hJXF jH rotjJXF i ¼ BvJðJ þ 1Þ � DvJ 2ðJ þ 1Þ2 ð2Þand
hJ 0XF jHhf jJXF i¼ fhX þ hXD½JðJ þ 1Þ þ J 0ðJ 0 þ 1Þ�=2g
� ð�1ÞJ0þIþF I J 0 F
J I 1
� �ð�1ÞJ
0�X J 0 1 J
�X 0 X
� �
� ½ð2J 0 þ 1Þð2J þ 1ÞIðI þ 1Þð2I þ 1Þ�1=2
þ eQq4
ð�1ÞJ0þIþF I J 0 F
J I 2
� �ð�1ÞJ
0�X J 0 2 J
�X 0 X
� �
� ð2J 0 þ 1Þð2J þ 1ÞðI þ 1Þð2I þ 1Þð2I þ 3ÞIðI � 1Þ
� �1=2; ð3Þ
where I is the nuclear spin quantum number of gold
(I = 3/2). The molecular constants were determined by
a least-squares calculation, in which data of overlapped
lines were less weighted. The standard deviations of the
fit were about 10 kHz for both the ground and firstexcited vibrational states. The molecular constants
determined are listed in Table 1. The observed and cal-
culated transition frequencies with their residuals are
summarized in Table 2.
4. Results and discussion
In the present study, we observed the rotational spec-
trum of AuO in the X 2P3/2 substate but not that in the
X 2P1/2 substate. The present molecular constants,
therefore, are effective constants perturbed by the
X 2P1/2 substate. However, in a usual 2P radical, the
molecular constants in the X 2P3/2 substate have similar
values to those in the X 2P1/2 substate. For example, in
Table 1
Molecular constants of AuO in the X 2P3/2 statea
v = 0 v = 1
Bv (MHz) 9948.9266(12) 9856.8209(24)
Dv (kHz) 11.4247(29) 11.4917(62)
h3/2 (MHz) �46.50(37) �46.38(83)
h3/2D (kHz) �17.9(45) �11.6(41)
eQq (MHz) 201.9(29) 201.9b
a Values in parentheses represent 1 SDb Fixed in the analysis.
Table 2
Observed transition frequencies of AuO in MHz
J 0–J00 F 0–F00 v = 0 v = 1
Obs. Freq. O � Ca Obs. Freq. O � Ca
7.5–6.5 6–5 149212.281 �0.006
7–6 149214.814 �0.021b
8–7 149215.188 0.031b
9–8 149215.188 0.105b
8.5–7.5 7–6 169102.019 0.000
8–7 169103.994 0.125b
9–8 169103.994 �0.039b
10–9 169103.994 �0.095b
10.5–9.5 9–8 208873.612 0.016 206939.041 �0.014
10–9 208874.718 0.036b 206940.271 0.122b
11–10 208874.718 0.001b 206940.271 0.079b
12–11 208874.718 �0.137b 206940.271 �0.067b
11.5–10.5 10–9 228755.044 �0.006 226636.210 0.011
11–10 228755.909 �0.006b 226637.162 0.089b
12–11 228755.909 �0.014b 226637.162 0.073b
13–12 228755.909 �0.159b 226637.162 �0.080b
12.5–11.5 11–10 248633.300 0.004 246330.120 0.002
12–11 248633.995 �0.004b 246330.885 0.056b
13–12 248633.995 0.004b 246330.885 0.055b
14–13 248633.995 �0.139b 246330.885 �0.096b
14.5–13.5 13–12 288378.993 �0.141b 285707.177 �0.062b
14–13 288379.593 �0.027b 285707.785 0.052b
15–14 288379.593 �0.003b 285707.785 0.066b
16–15 288379.593 �0.131b 285707.785 �0.070b
15.5–14.5 14–13 308245.970 �0.225b 305389.777 �0.131b
15–14 308246.529 0.100b 305390.330 0.016b
16–15 308246.529 �0.050b 305390.330 0.020b
17–16 308246.529 �0.169b 305390.330 �0.107b
19.5–18.5 18–17 387669.318 0.223b
19–18 387669.318 �0.001b
20–19 387669.318 0.025b
21–20 387669.318 �0.058b
a Observed minus calculated frequencies.b Blended line. Less weighted.
T. Okabayashi et al. / Chemical Physics Letters 403 (2005) 223–227 225
the IO radical [12], the difference between the equilib-
rium internuclear distances, re, in the X 2P1/2 and
X 2P3/2 substates is about 1%, and that in the vibra-
tional wavenumber, xe, is about 5%. Hence, the present
molecular constants in the X 2P3/2 substate can be used
to judge the reliability of theoretical calculations for the
AuO radical in the X 2Pi ground state.
The equilibrium rotational and centrifugal distortionconstants are obtained from the observed B0, B1, D0 and
D1 values: Be = 9994.9795(22) MHz and De = 11.3912
(53) kHz. The equilibrium internuclear distance, re, is
thus 1.84876171(23) A, which is in rather good agree-
ment with the recent theoretical calculations, 1.83–
1.87 A [6,8]. The earlier ab initio calculation [7] provided
a 0.1 A larger value.
The vibrational wavenumber xe and its anharmonicterm xexe of a diatomic molecule are represented as
[13,14]
xe ¼
ffiffiffiffiffiffiffiffi4B3
e
De
sð4Þ
and
xexe ¼ Be
aexe
6B2e
þ 1
� �2
: ð5Þ
The xe and xexe values are calculated to be 624.59(15)
and 5.0122(14) cm�1, respectively. The Kratzer equa-
tion, Eq. (4), usually applies to an unperturbed mole-
cule. Although the X 2P3/2 substate of AuO may be
slightly affected by the X 2P1/2 substate, the present xe
value is probably acceptable for the real AuO molecule.
In contrast, the Pekeris relationship, Eq. (5), applies
only to a Morse oscillator, and it is likely that the pres-
ent xexe value is subject to a systematic error.
The present xe value is compared with the previous
experimental and theoretical values in Table 3. The
Table 3
Comparison of molecular parameters of AuO
re (A) xe (cm�1) De ðkJ=molÞ References
Experimental (X 2P3/2)
mmW 1.84876171(23)a 624.59(15)a,b (233)c This work
Matrix-Vis. 699d [5]
Matrix-IR 619.2d [6]
MS 225 ± 15e [15]
Theoretical (X 2P)
BPW91 1.869 640.7 [6]
BDF 1.831 695 271 [8]
ZORA(MP) 1.864 632 274 [8]
CISD/SC 1.953 516 108.0 [7]
CEPA-1 1.946 497 146.5 [7]
a Values in parentheses represent 1 SDb Calculated value with the Kratzer relation, equation (4).c Estimated value under assumption of a Morse oscillator, equation (6).d Effective value including anharmonic terms.e D0
0 value excluding the zero-point energy.
226 T. Okabayashi et al. / Chemical Physics Letters 403 (2005) 223–227
present xe value is qualitatively consistent with the value
of matrix-IR (619 cm�1) [6], but somewhat smaller than
that of the matrix-Vis. (699 cm�1) [5]. The recent
BPW91 [6] and ZORA(MP) [8] calculations suggestthe xe value of about 640 cm�1, which qualitatively
agrees with the present value. On the other hand, the
earlier ab initio calculations [7] reported the somewhat
poor results of about 500 cm�1.
The dissociation energy De of a Morse oscillator is
estimated from the present vibrational parameters using
[13,14]
De ¼x2
e
4xexe: ð6Þ
The obtained dissociation energy is about 233 kJ/mol,which agrees well with both the indirect estimation by
the mass spectrometric method, 225 ± 15 kJ/mol [15],
and the recent BDF and ZORA(MP) calculations,
about 270 kJ/mol [8], but not with the earlier ab initio
study [7] as shown in Table 3.
In the present study, lines in the first excited vibra-
tional state, which locates at about 600 cm�1 above
the ground state, have been observed for AuO. Theeffective vibrational temperature in the discharge plasma
is roughly estimated to be about 500 K from the relative
intensity ratio of the lines in the v = 0 and 1 states under
the assumption of the Boltzmann distribution. The spin
temperature often has a quite different value from the
vibrational temperature, but if spin temperature is as-
sumed to be the same with the vibrational one (about
500 K), the unobserved X 2P1/2 substate is estimatedto lie at least 800 cm�1 above the X 2P3/2 substate.
Hyperfine splitting arising from the nuclear spin of
gold was partly resolved in the present experiment.
The hyperfine parameters of Frosch and Foley [16], a,
bF and c, are represented as
a ¼ 2lBgNlN
1
r3
� o
; ð7Þ
bF ¼ 8p3gslBgNlN jWð0Þj2
D Es; ð8Þ
and
c ¼ 3
2gslBgNlN
3cos2h� 1
r3
� s
: ð9Þ
However, we could not determine the Frosch and Foley
hyperfine parameters separately, for the transitions were
observed only in the X = 3/2 substate. Instead of these
three parameters, an effective magnetic hyperfine con-
stant h3/2 = a + (b + c)/2 was determined, where
hX = aK + (b + c)R and b = bF � c/3.
It is difficult to estimate the molecular hyperfineparameters for metal-containing molecules from the
atomic values without a high-quality ab initio work,
because the molecular states are by no means simply
related to the atomic states. However, we dare to
estimate the hyperfine constants a, bF and c from
the hyperfine constants P(Au) = 132.0 MHz and
A(Au) = 2876 MHz of atomic gold [17]. If Æ1/r3æo in
Eq. (7) is approximately equal to Æ1/r3æs in Eq. (9),a and c constants are related to a unique atomic
constant P(Au). The angular factor h3cos2h� 1i5dpis taken to be 2/7 [17]. The h3/2 value is then derived
as follows:
h3=2 ¼ aþ 1
2bF þ
1
3c
¼ cspinP ðAuÞ þ cs2AðAuÞ þ cspin
7P ðAuÞ
¼ 8cspin7
P ðAuÞ þ cs2AðAuÞ; ð10Þ
T. Okabayashi et al. / Chemical Physics Letters 403 (2005) 223–227 227
where cspin represents the contribution of the 5d orbital
to the unpaired orbital and cs the s-character of the un-
paired electron. However, it is impossible to reproduce
the observed value h3/2 = �46 MHz, because the atomic
P(Au) and A(Au) have positive values. This finding
means that the behavior of an unpaired electron inAuO cannot be described using such a simple model.
The similar anomalies were also found in the cases of re-
lated species, CuO [18] and AgO [19]. Steimle et al.
[18,19] explained these anomalies in CuO and AgO in
terms of the perturbations from electronic exited states.
Similar phenomenon may also occur in the AuO radical.
For further discussion in detail, it is necessary to observe
the spectral lines in the other spin substate 2P1/2.The electric quadrupole coupling constant eQq is ob-
tained to be 201.9(29) MHz, which is rather larger than
those of AuF (�53.2 MHz [20]), AuCl (9.6 MHz [21]),
AuBr (37.3 MHz [21]), and AuI (78.3 MHz [22]). Since
the eQq constant of a gold compound such as AuCl
[23] is remarkably affected by the relativistic effect, it is
difficult to discuss the eQq constants quantitatively with-
out a high-level ab initio calculation. Theoretical inter-pretation of these gold hyperfine constants including
eQq is thus highly desired.
Acknowledgments
The research was supported by Japan Society for the
Promotion of Science through Grants-in-Aid for Scien-tific Research (Nos. 12740316 and 15656184). T.O.
thanks the Kawasaki Steel 21st Century Foundation
for financial support. E.Y. thanks the Japan Science
Society through the Sasagawa Scientific Research Grant
and the Hayashi Memorial Foundation for Female Nat-
ural Scientists through the Hayashi Fellowship. T.O.
and E.Y. also acknowledge the financial support from
the Hamamatsu Foundation for Science and Technol-
ogy Promotion.
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