Journal of Molecular Spectroscopy 256 (2009) 186–191
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Journal of Molecular Spectroscopy
journal homepage: www.elsevier.com/locate/jms
The pure rotational spectrum of ZnO in the X1R+ and a3Pi states
L.N. Zack, R.L. Pulliam, L.M. Ziurys *
Department of Chemistry, Steward Observatory, University of Arizona, Tucson, Arizona 85721, USA
Department of Astronomy, Steward Observatory, University of Arizona, Tucson, Arizona 85721, USA
a r t i c l e
i n f o
Article history:
Received 21 February 2009
In revised form 1 April 2009
Available online 10 April 2009
Keywords:
Zinc oxide (ZnO)
Rotational spectroscopy
Excited electronic state
a b s t r a c t
The pure rotational spectrum of ZnO has been measured in its ground X1R+ and excited a3Pi states using
direct-absorption methods in the frequency range 239–514 GHz. This molecule was synthesized by reacting zinc vapor, generated in a Broida-type oven, with N2O under DC discharge conditions. In the X1R+
state, five to eight rotational transitions were recorded for each of the five isotopologues of this species
(64ZnO, 66ZnO, 67ZnO, 68ZnO, and 70ZnO) in the ground and several vibrational states (v = 1–4). Transitions
for three isotopologues (64ZnO, 66ZnO, and 68ZnO) were measured in the a3Pi state for the v = 0 level, as
well as from the v = 1 state of the main isotopologue. All three spin–orbit components were observed in
the a3Pi state, each exhibiting splittings due to lambda-doubling. Rotational constants were determined
for the X1R+ state of zinc oxide. The a3Pi state data were fit with a Hund’s case (a) Hamiltonian, and rotational, spin–orbit, spin–spin, and lambda-doubling constants were established. Equilibrium parameters
were also determined for both states. The equilibrium bond length determined for ZnO in the X1R+ state
is 1.7047 Å, and it increases to 1.8436 Å for the a excited state, consistent with a change from a p4 to a
p3r1 configuration. The estimated vibrational constants of xe 738 and 562 cm1 for the ground and
a state agreed well with prior theoretical and experimental investigations; however, the estimated dissociation energy of 2.02 eV for the a3Pi state is significantly higher than previous predictions. The
lambda-doubling constants suggest a low-lying 3R state.
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
Diatomic oxides of 3d transition metals have received attention
from both theoreticians and experimentalists due to the complexity of their spectra, as well as their importance in numerous fields
such as astrophysics and high-temperature chemistry [1–3]. To
date, high-resolution spectroscopy has been used to study all of
the 3d transition metal monoxides [1,4–12], with the exception
of ZnO. This absence is surprising, as the predicted ground state
for ZnO is closed-shell, 1R+, and should yield a relatively simple
spectrum. Also, ZnO is used in a broad range of applications,
including nanotechnology, semiconductors, thin films, and solar
cells [13–17]. Studying the fundamental properties of ZnO by spectroscopy would seem imperative.
Because of its extensive use in materials science, most experimental work concerning ZnO has focused on the molecule’s bulk
properties (e.g. dielectric constant, band gap, and exitonic energies
[13–17]). On the single molecule level, early studies of the oxide
utilized mass spectrometry to determine thermochemical properties, such as bond and ionization energies [18–20]. Spectroscopic
* Corresponding author. Fax: +1 520 621 5554.
E-mail addresses: [email protected] (L.N. Zack), [email protected]
(L.M. Ziurys).
0022-2852/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jms.2009.04.001
studies of ZnO, however, are incredibly sparse. In 1996, Chertihin
and Andrews [21] used FTIR to determine an equilibrium bond
length, re, and fundamental vibrational frequency, xe, for ZnO in a
solid argon matrix. Photoelectron spectroscopy (PES) has also been
used to measure re and xe in the X1R+ and a3Pi states, as well as the
electron affinity, EA, and dissociation energy, D0, in the X1R+ ground
state [22–24]. The PES studies also produced an estimate of the energy differences between the ground and low-lying excited states.
Despite the lack of gas-phase experimental data for ZnO, theoretical studies of this molecule are rather ubiquitous. The earliest
of these investigations was performed by Bauschlicher and Langhoff in 1986 [25], where CPF methods predicted a 3P excited state
lying roughly 0.026 eV above the 1R ground state; re and xe were
also calculated in this study. Over a decade later, in accompaniment to the PES work done by Fancher et al. [22], Bauschlicher
and Partridge [26] performed more accurate calculations, which
produced a (slightly) higher D0, Te, and xe and a shorter re than
the original prediction. Recent calculations by Boughdiri et al.,
which included spin–orbit interactions [27], resulted in similar values to Bauschlicher and Partridge. Other theoretical investigations
have examined ZnO in relation to bonding trends of the 3d metal
monoxides in their ground [28] and/or excited states [29–31], or
in comparison to similar molecules like CaO or ZnF [32]. All of
these studies predict a 1R+ ground state, a low-lying 3P excited
state, and re 1.7 Å.
Here we report the first gas-phase high-resolution study of ZnO
using millimeter/sub-millimeter direct-absorption methods. Rotational transitions, including vibrational satellite lines, were measured for ZnO in its X1R+ ground state, as well as the a3Pi
excited state. The spectra of several zinc isotopologues were also
recorded. Spectroscopic constants and equilibrium parameters
were determined for both electronic terms. Bond lengths and
stretching frequencies were estimated from the derived spectroscopic parameters. Here we report our measurements and results.
2. Experimental
R
The pure rotational spectrum of ZnO was measured using one of
the Ziurys group mm/sub-mm spectrometers, described in detail
elsewhere [33]. Briefly, the instrument is a double-pass, directabsorption system, which uses Gunn oscillators and Schottky diode
multipliers to produce radiation over a frequency range of approximately 65–850 GHz. Gaussian-beam optics, including a rooftop
reflector, were used to focus the radiation through a water-cooled,
steel reaction chamber containing a Broida-type oven, and into a
helium-cooled InSb hot-electron bolometer detector.
ZnO was produced by reacting zinc vapor with N2O gas in the
presence of a DC discharge. Zinc vapor was generated by melting
zinc pieces (99.9% Aldrich) in an alumina crucible in a Broida-type
oven. On average, signal strength was optimized when approximately 4 mTorr of N2O gas was introduced into the reaction chamber above the oven, with 12–20 mTorr of Argon carrier gas added
through the bottom of the oven. ZnO was also synthesized using
roughly 30 mTorr of 30% H2O2 or 1–4 mTorr of H2O as a precursor
gas; however, signal strengths were several times weaker than
with N2O.
Because no prior gas-phase spectroscopic studies of ZnO have
been published, the range of 385–485 GHz (approximately 7B)
was initially scanned continuously in order to locate and assign
transitions. Harmonic patterns of the v = 0 and vibrational satellite
lines of the main isotopologue, 64ZnO, were initially identified in
the X1R+ state. Similar patterns were then located for the four
weaker zinc species: 66ZnO, 67ZnO, 68ZnO, and 70ZnO. Quadrupole
hyperfine splittings were not observed for the 67Zn nucleus. After
the ground state species was identified, a weaker series of harmonically related doublets with varying separation remained. The pattern suggested a species with a 3P electronic state, and the lines
appeared under identical chemical conditions as the ground state
of ZnO. These doublets were thus attributed to the a3Pi state of
ZnO, which is predicted to lie 2000 cm1 above the 1R+ ground
state [26]. Given the recent detection of the analogous state in
MgO by millimeter spectroscopy [34], appearance of rotational
transitions from this metastable level is not unexpected.
Transition frequencies were determined by fitting Gaussian
curves to lines obtained by averaging an equal number of scans
in increasing and decreasing frequency over a 5-MHz scan width.
Typical line widths found were 0.5–1.3 MHz over the range 241–
511 GHz and 0.7–1.8 MHz over 320–511 GHz for the X1R+ and
a3Pi states, respectively. The instrumental accuracy is estimated
to be ±50 kHz.
3. Results and analysis
Eight rotational transitions were
of the X
188
L.N. Zack et al. / Journal of Molecular Spectroscopy 256 (2009) 186–191
corded in the v = 3 and v = 4 states for 64ZnO, 66ZnO, and 68ZnO. The
complete set of transition frequencies is presented in Table 1. A total of 116 lines of this molecule were measured, as shown in the
table.
Fig. 1 presents representative spectra of ZnO (X1R+) in the
J = 18
17 transition near 478 GHz. In this figure, the v = 2 satellite line of the main isotopologue, 64ZnO, is shown, as well as the
v = 1 line of 67ZnO and the ground vibrational state of 70ZnO. The
inset, showing the v = 0 transition of 64ZnO, illustrates the high signal-to-noise ratio. Single lines are observed in every case, which is
consistent with the predicted 1R+ ground state of ZnO. There is no
evidence for hyperfine structure due to the 67Zn nuclear spin
(I = 5/2) in this or any other spectrum studied in this work.
For the excited a3Pi state of ZnO, eight rotational transitions
were recorded for three isotopologues (64ZnO, 66ZnO, and 68ZnO)
in the ground vibrational state, as well as for the v = 1 state of
the most abundant isotopologue. Transition frequencies are listed
in Table 2. Each transition consists of three spin–orbit components
(X = 0, 1, and 2), each of which is further split by K-doubling, labeled by e and f. The K-doubling was smallest in the X = 2 level
(1–6 MHz), and largest in the X = 1 spin component (70–
102 MHz), consistent with the relative contributions of diagonal
vs. off-diagonal matrix elements for this interaction [35]. In total,
204 lines were measured for the a3P state.
Fig. 2 displays spectra of each spin component of the J = 20
19
transition of ZnO (main isotopologue) near 461 GHz. The K-doublets in each component are indicated by e and f. All three spectra
are displayed on the same scale, and the differences in K-doubling
splittings are clearly illustrated in these data.
The spectra for all five isotopologues of ZnO in the X1R+ state
were analyzed with the program SPFIT [36], and the resulting rotational constants, B0 and D0, are listed in Table 3. Higher order centrifugal distortion terms were not needed in order to successfully
fit the data.
The a3Pi state was analyzed using the following Hund’s case (a)
effective Hamiltonian:
Heff ¼ Hrot þ HSO þ HSS þ Hld
ð1Þ
where Hrot, HSO, HSS, and Hld represent the rotational kinetic energy,
spin–orbit interaction, electron spin–spin interaction, and the lambda-doubling, respectively. Both the spin–orbit and spin–spin constants, A and k, could be determined from the data, as well as
lambda-doubling constants q and p+2q. Note that the lambda-doubling term o+p+q has no J dependence and cannot be fit from rotational spectra. Also, A < 0, as expected for a state with a p3r1 openshell electron configuration. Centrifugal distortion terms, including
(o+p+q)D, were used to fit all interactions. The results from this analysis are listed in Table 4.
Equilibrium parameters (Be, De, ae, and be) were established for
each ZnO isotopologue (except 70ZnO) in the ground state, and for
64
ZnO in the excited state. These values were calculated using a
least-squares analysis of the following equations [37]:
1
Bv ¼ Be ae V þ
2
1
Dv ¼ De be V þ
2
ð2Þ
ð3Þ
The harmonic vibrational frequency, xe, anharmonic correction,
xexe, equilibrium dissociation energy, DE,e, and ground state dissociation energy, DE,v=0, were approximated from Be and De using the
expressions [37]:
sffiffiffiffiffiffiffiffi
4B3e
xe De
xe xe DE;e ae xe
6B2e
x2e
4 x e xe
DE;v ¼0 ¼ DE;e ð4Þ
!2
þ1
Be
ð5Þ
ð6Þ
xe
2
þ
xe xe
4
ð7Þ
Fig. 1. Representative spectra of several isotopologues of ZnO (X1R+) in different vibrational states of the J = 18
17 rotational transition near 478 GHz observed in natural
abundance. Signals originating from 67ZnO (v = 1), 70ZnO (v = 0), and 64ZnO (v = 2) are clearly visible and appear as single lines, as expected for a 1R+ state. The inset shows the
64
v = 0 state of ZnO in this transition. The main spectrum is a composite of two 110 MHz wide scans, each of which was acquired in 60 s.
189
L.N. Zack et al. / Journal of Molecular Spectroscopy 256 (2009) 186–191
Table 2
Observed rotational transition frequencies of ZnO (a3Pi).a
J0
J00
X
Parity
64
66
ZnO
v=0
mobs
v=1
mobs–calc
mobs
68
ZnO
ZnO
v=0
mobs–calc
mobs
v=0
mobs–calc
mobs
mobs–calc
14
13
0
0
1
1
2
2
e
f
e
f
e
f
326
326
323
323
320
320
076.528
097.361
661.259
591.774
743.048
744.249
0.023
0.151
0.114
0.029
0.011
0.044
322
322
320
320
317
317
658.962
672.537
279.105
215.163
422.763
423.945
0.034
0.100
0.099
0.004
. . .b
. . .b
324
324
321
321
318
318
092.077
110.733
705.292
636.743
822.516
823.696
. . .b
. . .b
0.110
0.018
0.012
0.083
322
322
319
319
222.923
239.252
862.570
794.908
0.028
0.139
0.103
0.047
15
14
0
0
1
1
2
2
e
f
e
f
e
f
349
349
346
346
343
343
324.947
347.253
740.295
666.335
619.133
620.895
0.050
0.094
0.050
0.016
0.128
0.151
345
345
343
343
340
340
663.171
677.421
116.215
048.113
061.138
062.812
0.083
0.083
0.055
0.031
0.037
0.025
347
347
344
344
341
341
199.318
219.303
645.110
572.177
561.778
563.480
0.030
0.099
0.036
0.012
0.136
0.155
345
345
342
342
339
339
197.132
214.676
671.199
599.194
623.369
624.906
0.005
0.063
0.056
0.004
0.051
0.102
16
15
0
0
1
1
2
2
e
f
e
f
e
f
372
372
369
369
366
366
564.692
588.358
811.144
732.855
488.304
490.353
0.018
0.008
0.025
0.008
0.111
0.079
368
368
365
365
362
362
658.672
673.466
945.062
872.933
692.505
694.554
<0.000
0.001
0.011
0.007
0.010
0.069
370
370
367
367
364
364
297.987
319.153
576.822
499.599
294.162
296.170
0.091
0.022
0.023
0.011
0.168
0.068
368
368
365
365
362
362
162.747
181.516
471.843
395.608
226.897
228.783
0.002
0.037
0.003
0.007
0.096
0.049
17
16
0
0
1
1
2
2
e
f
e
f
e
f
395
395
392
392
389
389
794.987
820.030
873.234
790.718
349.928
352.413
0.035
0.090
0.068
0.021
0.087
0.094
391
391
388
388
385
385
644.793
660.009
765.220
689.027
316.159
318.547
0.128
0.074
0.152
0.018
0.015
0.027
393
393
390
390
387
387
387.155
409.651
499.911
418.468
019.080
021.586
0.002
0.007
0.084
0.057
0.193
0.110
391
391
388
388
384
384
119.273
139.025
264.011
183.592
823.127
825.467
0.083
0.021
0.109
0.016
0.043
0.130
18
17
0
0
1
1
2
2
e
f
e
f
e
f
419
419
415
415
412
412
015.271
041.575
926.028
839.430
203.487
206.459
0.021
0.033
0.086
0.007
0.088
0.057
414 620.654
414 636.419
411 575.760
411 495.810
407 931.600
407 934.504
0.004
0.087
0.051
0.006
0.031
0.065
416 466.547
416 490.299
0.011
0.018
409 736.163
409 739.104
0.102
0.130
414
414
411
410
407
407
065.874
086.970
047.054
962.553
411.386
414.225
0.010
0.093
0.137
0.085
0.090
0.080
0
0
1
1
2
2
e
f
e
f
e
f
442
442
438
438
224.982
252.686
969.002
878.371
0.006
0.107
0.090
0.043
437 586.070
437 602.169
0.024
0.095
439
439
436
436
0.017
0.089
0.103
0.041
437 002.164
437 024.508
0.097
0.096
430 538.373
430 541.565
0.089
0.417
429 991.319
429 994.746
0.115
0.053
19
18
535.505
560.565
317.989
228.576
20
19
0
0
1
1
2
2
e
f
e
f
e
f
465
465
462
461
457
457
423.588
452.574
001.608
907.121
884.569
888.753
0.001
0.095
0.067
0.007
0.053
0.018
460
460
457
457
453
453
540.268
556.544
166.809
079.386
136.105
141.109
0.005
0.038
0.054
0.004
0.037
0.784
462
462
459
459
455
455
593.492
619.729
211.949
118.706
144.511
148.495
0.013
0.052
0.092
0.017
0.052
0.043
459
459
456
456
452
452
927.749
951.154
583.742
491.676
562.485
566.503
0.008
0.062
0.035
0.016
0.080
0.007
21
20
0
0
1
1
2
2
e
f
e
f
e
f
488
488
485
484
480
480
610.461
640.677
023.243
925.030
710.394
716.041
0.006
0.007
0.049
0.005
. . .b
. . .b
483
483
479
479
475
475
482.775
499.293
946.060
855.106
724.153
729.124
0.026
0.016
0.015
0.018
. . .b
. . .b
485
485
482
481
477
477
639.834
667.300
095.129
998.189
834.764
839.358
0.023
0.020
0.044
0.036
0.072
0.124
482
482
479
479
475
475
841.792
866.373
336.429
240.614
124.438
129.101
0.012
0.013
0.010
0.040
0.051
0.026
22
21
0
0
1
1
2
2
e
f
e
f
e
f
511
511
508
507
503
503
785.024
816.456
033.508
931.662
527.823
533.397
0.028
0.127
0.126
0.069
0.196
0.139
506
506
502
502
498
498
412.833
429.511
713.787
619.324
302.155
307.541
0.066
0.072
0.086
0.004
0.098
0.302
508
508
504
504
500
500
674.074
702.812
966.843
866.418
515.163
520.606
0.010
0.032
0.201
0.048
0.134
0.131
505
505
502
501
497
497
743.868
769.553
077.917
978.638
676.616
681.923
0.028
0.107
0.117
0.107
0.200
0.149
a
b
In MHz.
Blended line: not included in fit.
These quantities are listed in Table 5, along with the equilibrium bond distance, re, for four ZnO isotopologues. It should be
noted that, although the Kratzer relationship (Eq. (4)) applies to
most potentials, Eq. (5) is limited to only Morse potentials.
4. Discussion
The pure rotational spectrum of ZnO in its ground state, as well
as in the a metastable excited state, has been measured in the first
high-resolution experimental study of this molecule. These measurements have confirmed the 1R+ ground state and the presence
of a low-lying 3Pi excited state. These results are consistent with
theoretical predictions and previous low-resolution experiments
[22–27,30–32].
In this study, the equilibrium bond distance for 64ZnO (X1R+)
was established from five vibrational states and calculated to be
re = 1.7047 Å. This value agrees well with predictions of bond
lengths ranging from 1.689 to 1.733 Å made at several different
190
L.N. Zack et al. / Journal of Molecular Spectroscopy 256 (2009) 186–191
Table 4
Rotational constants for ZnO (a3Pi).a
66
64
ZnO
Table 3
Spectroscopic constants for ZnO (X1R+).a
64
B0
D0
ZnO
ZnO
67
ZnO
68
ZnO
70
ZnO
13 536.4361(62) 13 454.5569(62) 13 415.3475(62) 13 377.4195(62) 13 304.6299(62)
0.020563(11)
0.020314(11)
0.020196(11)
0.020082(11)
0.019861(11)
rms 0.005
a
66
0.011
0.011
0.010
0.017
In MHz; values in parentheses are 3r errors.
levels of theory [26,27,31,32]. Although the equilibrium bond
length, re = 1.8436 Å, for the triplet state was based on only the
v = 0 and v = 1 vibrational states, it is also in close agreement with
theoretical predictions, which ranged from 1.827 to 1.873 Å
[26,27,31,32]. The PES data by Moravec et al. determined that the
bond length for the triplet state is 0.126(5) Å longer than that of
the singlet state [23], while Kim et al. determined a value of
1.850 Å [24]. Both are in good agreement with this study, as well.
Because ZnO is the final 3d transition metal monoxide to be
measured via high-resolution spectroscopy, bonding trends across
this group can now be more fully understood. The bond length in
68
ZnO
v=0
v=1
v=0
v=0
B
D
A
AD
(o+p+q)D
(p+2q)
(p+2q)D
q
qD
k
kD
11561.8646(40)
0.0222293(55)
1582359(2300)
70.05(24)
1.664(18)
148.2(1.3)
0.01194(64)
1.526(36)
0.000393(29)
23048(103)
1.144(33)
11441.4666(82)
0.022453(11)
1562844(4500)
70.46(48)
1.657(32)
139.9(2.4)
0.0165(12)
1.547(65)
0.000201(55)
22002(191)
1.035(65)
11491.9795(47)
0.0219653(62)
1584334(2600)
69.04(28)
1.672(18)
146.7(1.3)
0.01105(78)
1.486(36)
0.000410(33)
23095(125)
1.144(37)
11426.1394(42)
0.0217150(57)
1584969(2500)
68.21(26)
1.694(15)
145.7(1.2)
0.01026(64)
1.466(32)
0.000430(29)
23096(111)
1.140(35)
rms
0.077
0.147
0.085
0.078
a
Fig. 2. Representative spectra of the J = 20
19 rotational transition of 64ZnO in the
a3Pi excited state near 461 GHz. Each panel displays one of three spin components,
labeled by X, which comprise this transition. Each spin component is further split
into K-doublets, labeled e and f, with varying separation, as is characteristic of a 3P
state. Each panel is one 110 MHz wide scan, obtained in 60 s.
ZnO
In MHz; values in parentheses are 3r errors.
ZnO, 1.7047 Å, is 0.02 Å shorter than that of CuO, 1.73 Å, as has
been predicted by theory [28]. This decrease is thought to result
from an increased oxygen 2pp contribution to the 4p bonding orbital in the zinc compound [28]. Comparison of the 3d monoxide
group to their monofluoride counterparts shows that the halides
have longer bond lengths by 0.1–0.2 Å [38], but there are no
apparent similarities in bonding trends. For example, the bond
length is longest for TiF and MnF [39,40] and shortest for CoF
[41], whereas, in the oxide series, the longest bond distance is for
CuO [1] and shortest for VO [6]. Another difference is the
0.02 Å increase in bond length from CuF to ZnF [38].
Theoretical values for the vibrational frequency, xe, in the X1R+
state range from 727 to 766 cm1 [26,27,31,32], while PES studies
have yielded xe in the range 720(20) to 805(40) cm1 [22–24].
Theoretical predictions of the anharmonic constant, xexe, are
5.83 [26] and 6.04 cm1 [27]. This study estimates xe = 738 cm1,
which agrees reasonably well with theory and the PES values;
however, in this work, xexe = 4.88 cm1 – slightly lower than theory. Somewhat better agreement is found for the a3Pi state, where
theoretical values fall in the range xe 525–590 cm1
[26,27,31,32] and xexe 4.36–4.76 cm1 [26,27]. In this work,
the constants are estimated to be xe 562 cm1 and
xexe 4.76 cm1, respectively. Our vibrational frequency also
agrees with PES data by Moravec et al. and Kim et al.
(xe = 550(20) and 540(80) cm1) [23,24].
The dissociation energy, DE, for 64ZnO (X1R+) is calculated to be
3.41 eV, which is close to the CCSD(T) value of 3.59 eV predicted by
Boughdiri et al. [27] and 3.54 eV determined by Harrison et al.
using RCCSD(T) methods [32]. Bauschlicher and Partridge [26] also
used CCSD(T) calculations, which resulted in a much lower value
(DE = 1.63 eV), but only because a different a different choice of dissociation limit was used. Boughdiri et al. and Bauschlicher and Partridge are actually in good agreement. Other work, both theoretical
and experimental, tend to underestimate DE, placing it in the range
of 1.3–2.9 eV [18,19,28,29]. In the a3Pi excited state, DE for 64ZnO is
estimated to be 2.02 eV – higher than 1.36 eV determined from PES
data [22], as well as theoretical predictions of 1.14–1.38 eV
[26,27,32].
In addition to the 3Pi excited state, two nearby 3R states, arising
from p2r2 (3R) and r1p4r1 (3R+) electron configurations, are also
predicted for ZnO [31]. The 3R state is probably dissociative, such
that the likely contributor to the lambda-doubling in the a state is
the 3R+ term. Because the 3P term arises from a r2p3r1 configuration, pure precession cannot be realistically assumed, and it is difficult, therefore, to evaluate the 3P–3R energy difference from the
lambda-doubling constants. However, PES measurements estimate the energy difference to be 12 582 cm1 [23], and several
191
L.N. Zack et al. / Journal of Molecular Spectroscopy 256 (2009) 186–191
Table 5
Equilibrium parameters for ZnO (X1R+ and a3Pi).a
X1 R+
ZnO
Be (MHz)
ae (MHz)
De (MHz)
be (MHz)
re (Å)
xe (cm1)
xexe (cm1)
DE,v=0 (eV)
a
64
66
67
13 593.5679(66)
114.3062(29)
0.020539(12)
0.0000494(50)
1.7047(2)
738
4.88
3.41
13 511.1791(66)
113.2769(30)
0.020296(12)
0.0000435(50)
1.7047(2)
735
4.85
3.41
13 471.7303(84)
112.8029(66)
0.020172(15)
0.000047(11)
1.7047(2)
734
4.84
3.41
ZnO
ZnO
68
ZnO
13 433.5958(66)
112.3677(30)
0.020070(12)
0.0000375(50)
1.7047(2)
733
4.82
3.41
a3Pi
64
ZnO
11 622.0636(72)
120.3980(91)
0.022118(10)
0.000224(12)
1.8436(2)
562
4.76
2.02
Values in parentheses are 3r errors.
theoretical calculations predict that the 3P and 3R+ states lie
11 900 cm1 apart in energy [31,32]. The 3R+ state thus appears
to be sufficiently close to the a3P state to account for the observed
lambda-doubling.
5. Conclusions
High-resolution rotational spectroscopy of the 3d diatomic oxides has been completed with the study of ZnO in its X1R+ and a3Pi
states. From these data, spectroscopic constants and equilibrium
parameters in both electronic states have been determined. The
ZnO bond length has been found to be smaller relative to CuO, consistent with the addition of an electron to a p bonding orbital. The
lambda-doubling constants of the 3P state suggest a close-lying 3R
state, as predicted by theory. Studies of zinc oxide by electronic
and FTMW spectroscopy will aid in the further characterization
of this molecule.
Acknowledgments
This work was supported by NSF Grant CHE-07-18699. We
thank Professor Leah O’Brien for suggesting the presence of the
a3P state.
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