The microwave spectrum of the cesium monoxide CsO radical

Article abstract of DOI:10.1063/1.480291

The microwave spectrum of CsO has been observed and analyzed, not only in the ground vibrational state, but also in the ν=1-3 excited vibrational states. The CsO radical was generated by the reaction of N₂O with Cs vapor, which was produced by the reaction of Li metal with CsCl at 500-530°C. The observed spectra were found to conform to those expected for a ²Σ diatomic molecule, thereby establishing the ground electronic state of CsO to be of ²Σ. The observed rotational and centrifugal distortion constants yielded the equilibrium bond length and the harmonic vibrational frequency to be 2.300 745 (16) Aand 356.78 (11) cm^-1, respectively, based on the Born-Oppenheimer approximation. A careful examination of the observed spectral pattern definitely concluded that the spin-rotation interaction constant was positive, at variance with the expectation from a simple ²Σ/²π two-states interaction. This observation was interpreted by assuming positive contributions from higher excited electronic states which superseded a negative contribution from the ²π lowest excited state; the latter state was responsible for the large dependence of the spin-rotation interaction constant on the vibrational quantum number and was estimated from this vibrational dependence to be located at 1225 cm^-1 above the ground electronic state. In reverse to the spin-rotation splitting, the hyperfine splitting was found to increase with the vibrational excitation; in the υ=3 state the hyperfine structure was found completely resolved. However, the hyperfine coupling constants did not vary much with the vibrational quantum number, namely the vibrational dependence of the hyperfine splitting was caused primarily by that of the spin-rotation splitting. The observed hyperfine interaction constants indicated that CsO was an ionic molecule.

Full text of DOI:10.1063/1.480291

The microwave spectrum of the cesium monoxide CsO radical
Chikashi Yamada and Eizi Hirota
Citation: The Journal of Chemical Physics 111, 9587 (1999); doi: 10.1063/1.480291
View online: http://dx.doi.org/10.1063/1.480291
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 21
1 DECEMBER 1999
The microwave spectrum of the cesium monoxide CsO radical
Chikashi Yamadaa)
The Institute for Molecular Science, Okazaki 444-8585, Japan
Eizi Hirota
The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, Japan
͑
Received 19 July 1999; accepted 9 September 1999͒
The microwave spectrum of CsO has been observed and analyzed, not only in the ground vibrational
state, but also in the vϭ1–3 excited vibrational states. The CsO radical was generated by the
reaction of N O with Cs vapor, which was produced by the reaction of Li metal with CsCl at
2
2
5
00–530 °C. The observed spectra were found to conform to those expected for a ⌺ diatomic
molecule, thereby establishing the ground electronic state of CsO to be of ⌺. The observed
2
rotational and centrifugal distortion constants yielded the equilibrium bond length and the harmonic
Ϫ1
vibrational frequency to be 2.300 745 ͑16͒ Å and 356.78 ͑11͒ cm , respectively, based on the
Born–Oppenheimer approximation. A careful examination of the observed spectral pattern
definitely concluded that the spin-rotation interaction constant was positive, at variance with the
expectation from a simple 2⌺/ ⌸ two-states interaction. This observation was interpreted by
assuming positive contributions from higher excited electronic states which superseded a negative
contribution from the 2⌸ lowest excited state; the latter state was responsible for the large
dependence of the spin-rotation interaction constant on the vibrational quantum number and was
2
Ϫ1
estimated from this vibrational dependence to be located at 1225 cm above the ground electronic
state. In reverse to the spin-rotation splitting, the hyperfine splitting was found to increase with the
vibrational excitation; in the vϭ3 state the hyperfine structure was found completely resolved.
However, the hyperfine coupling constants did not vary much with the vibrational quantum number,
namely the vibrational dependence of the hyperfine splitting was caused primarily by that of the
spin-rotation splitting. The observed hyperfine interaction constants indicated that CsO was an ionic
molecule. © 1999 American Institute of Physics. ͓S0021-9606͑99͒00845-4͔
I. INTRODUCTION
tio calculations on CsO and other members of alkali monox-
ides. They reported the bond length and the vibrational fre-
Cesium monoxide is the last member of the series of
alkali monoxides, which we have been investigating system-
atically by using microwave and infrared diode laser spec-
2
2
quency in addition to the ⌸– ⌺ energy separation.
The present paper describes the microwave spectrum of
CsO and reports the molecular constants derived from there.
1
,2
troscopy. We have already reported the results on LiO,
3
4
5
NaO, and RbO, and Hirota has briefly described the spec-
troscopic information so far obtained on KO, RbO, and CsO
in a review paper.
II. EXPERIMENT
We generated cesium monoxide radicals by the reaction
of N O with Cs vapor, which we produced by heating a
Very little has been known on the CsO radical in the gas
2
6
mixture of lithium metal and CsCl in a stainless steel absorp-
tion cell. We continuously pumped dinitrogen monoxide gas
through the cell at the pressure of 10–12 mTorr. Absorption
lines which were presumably due to CsO radicals started to
appear when the cell temperature reached 350 °C and they
became strong at 500–530 °C. Lithium metal of 20 g lasted
for 2 days.
The microwave spectrometer employed in the present
study was identical to that used for RbO ͑Ref. 4͒ and was
described in detail in Ref. 13.
phase. Sarkas et al. determined the A–X separation in CsO
Ϫ1
to be 0.135Ϯ0.025 eV (1089Ϯ202 cm ) by applying pho-
Ϫ
todetachment spectroscopy to CsO and confirmed the
2
ϩ
ground electronic state of CsO to be of ⌺ . Spiker and
7
,8
Andrews observed the fundamental band of CsO isolated
in a nitrogen and an argon low-temperature matrix at 314
Ϫ1
and 322 cm , respectively. They produced radicals by the
reactions of cesium metal with N O and ozone in the matri-
2
9
ces, respectively. Lindsay, Herschbach, and Kwiram re-
ported the observation of electron spin resonance ͑ESR͒
spectra of CsO isolated in nitrogen, argon, or krypton matri-
ces at 4 K. The production of the radical was similar to that
used in the infrared studies.
III. OBSERVED SPECTRA
Search for the spectra of the CsO radical was initiated in
the frequency region from 339 to 383.4 GHz, and several
lines were observed and were tentatively assigned to the N
ϭ26� 25, 27� 26, and 28� 27 transitions. Later the mea-
surement was extended to lower frequency region in order to
1
0,11
and Langhoff et al.12 performed ab ini-
Allison et al.
a͒Present address: The University of Electrocommunications, 1-5-1 Chof-
ugaoka, Chofu, Tokyo 182-8585, Japan.
0021-9606/99/111(21)/9587/6/$15.00
9587
© 1999 American Institute of Physics
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1

9588
J. Chem. Phys., Vol. 111, No. 21, 1 December 1999
C. Yamada and E. Hirota
TABLE I. Part of the observed frequencies of rotational transitions of CsO in the vϭ0, 1, 2, and 3 ͑in MHz͒.
vϭ0
vϭ1
NЈ� NЉ
JЈ� JЉ
8.5
FЈ� FЉ
12.0
obs
oϪc
wt
obs
oϪc
wt
9
8
12
16
7.5
11.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
119 779.752
119 781.954
119 783.195
119 783.808
119 783.808
119 783.195
119 781.954
0.128
0.015
0.030
0.094
0.094
0.030
0.015
1.00
0.41
0.36
0.32
0.28
0.24
0.28
119 108.007
119 111.073
119 113.316
119 115.263
119 115.263
119 115.263
119 113.316
0.051
0.156
0.044
0.219
0.219
0.219
0.044
1.00
1.00
0.32
0.28
0.24
0.21
0.19
1
1
1.0
0.0
9
8
7
6
5
.0
.0
.0
.0
.0
9.5
8.5
13.0
12.0
11.0
10.0
9.0
120 208.412
120 206.357
120 205.356
120 205.356
120 205.356
120 206.357
120 207.380
120 208.929
0.019
0.001
0.036
0.036
0.036
0.001
0.086
0.037
1.00
0.40
0.36
0.32
0.29
0.26
1.00
1.00
1
1
1
2.0
1.0
0.0
119 471.768
119 469.889
119 468.191
119 468.191
119 468.191
0.040
0.112
0.018
0.018
0.018
1.00
1.00
0.32
0.29
0.26
9
8
7
6
.0
.0
.0
.0
8.0
7.0
6.0
5.0
1
3
12.5
11.5
12.5
15.5
16.5
16.0
15.0
14.0
13.0
12.0
11.0
10.0
9.0
173 062.421
173 063.523
173 064.362
173 064.362
173 064.362
173 064.362
173 063.523
173 062.421
Ϫ0.089
Ϫ0.056
Ϫ0.034
Ϫ0.034
Ϫ0.034
Ϫ0.034
Ϫ0.056
Ϫ0.089
0.42
0.39
0.36
0.33
0.30
0.28
0.26
0.24
172 077.807
172 079.333
172 080.398
172 081.112
172 081.112
172 081.112
172 079.739
172 077.178
0.010
0.067
0.050
0.038
0.038
0.038
0.004
0.089
1.00
1.00
1.00
0.33
0.30
0.28
1.00
1.00
15.0
14.0
13.0
12.0
11.0
10.0
9.0
8.0
13.5
17.0
16.0
15.0
14.0
13.0
12.0
11.0
10.0
9.0
173 491.085
173 490.003
173 489.338
173 489.338
173 489.338
0.029
Ϫ0.046
Ϫ0.022
Ϫ0.022
Ϫ0.022
0.42
1.00
0.36
0.33
0.31
172 443.906
172 442.565
172 441.440
172 440.686
172 440.686
172 440.686
172 440.686
172 441.440
Ϫ0.124
0.090
Ϫ0.262
0.118
0.118
0.118
1.00
1.00
0.36
0.33
0.31
0.28
0.26
0.24
16.0
15.0
14.0
13.0
12.0
11.0
10.0
0.118
Ϫ0.262
173 491.085
0.029
0.22
1
7
16.5
20.0
19.0
18.0
17.0
16.0
15.0
14.0
13.0
12.0
226 293.483
226 294.667
226 294.667
226 294.667
226 294.667
226 294.667
226 294.667
226 293.483
Ϫ0.152
0.073
0.073
0.073
0.073
0.073
0.073
Ϫ0.152
0.40
0.38
0.36
0.33
0.31
0.29
0.28
0.26
224 995.845
224 996.704
224 997.613
224 997.613
224 997.613
224 997.613
224 996.704
224 995.264
Ϫ0.089
Ϫ0.083
Ϫ0.024
Ϫ0.024
Ϫ0.024
Ϫ0.024
Ϫ0.083
Ϫ0.075
1.00
0.38
0.35
0.33
0.31
0.29
0.28
1.00
19.0
18.0
17.0
16.0
15.0
14.0
13.0
1
7.5
21.0
20.0
19.0
18.0
17.0
16.0
15.0
14.0
13.0
226 720.701
226 719.870
226 719.870
226 719.870
226 719.870
226 719.870
Ϫ0.030
Ϫ0.060
Ϫ0.060
Ϫ0.060
Ϫ0.060
Ϫ0.060
1.00
0.38
0.35
0.33
0.32
0.30
225 359.949
225 359.130
225 358.190
225 358.190
225 358.190
Ϫ0.214
Ϫ0.014
0.021
0.021
0.021
1.00
0.38
0.35
0.34
0.31
20.0
19.0
18.0
17.0
16.0
15.0
14.0
225 358.190
225 359.130
0.021
Ϫ0.014
0.28
0.27
226 721.145
0.150
1.00
vϭ2
vϭ3
1
3
12
12.5
11.5
16.0
15.0
14.0
13.0
12.0
11.0
10.0
9.0
171 097.584
171 099.997
171 101.932
171 103.419
171 104.348
171 104.348
171 102.611
171 096.137
0.020
Ϫ0.008
Ϫ0.044
Ϫ0.041
0.005
0.005
0.044
Ϫ0.160
1.00
1.00
1.00
1.00
0.52
0.48
1.00
1.00
170 128.710
170 132.806
170 137.000
170 141.146
170 145.602
170 150.858
0.050
Ϫ0.076
0.029
0.006
Ϫ0.049
0.014
1.00
1.00
1.00
1.00
1.00
1.00
15.0
14.0
13.0
12.0
11.0
10.0
9.0
8.0
170 125.583
Ϫ0.151
1.00
13.5
12.5
17.0
16.0
15.0
14.0
171 375.146
171 372.817
171 370.922
Ϫ0.123
Ϫ0.003
0.040
1.00
1.00
1.00
170 258.005
170 253.981
170 250.284
Ϫ0.092
0.026
0.066
1.00
1.00
1.00
16.0
5.0
1
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J. Chem. Phys., Vol. 111, No. 21, 1 December 1999
The cesium monoxide radical
9589
TABLE I. ͑Continued.͒
vϭ0
vϭ1
NЈ� NЉ
JЈ� JЉ
FЈ� FЉ
4.0
3.0
2.0
1.0
0.0
obs
o-c
wt
obs
o-c
wt
1
1
1
1
1
13.0
12.0
11.0
10.0
9.0
171 369.437
171 368.285
171 367.857
171 368.285
171 370.098
0.037
0.012
0.000
0.012
1.00
0.54
1.00
0.46
1.00
170 246.706
170 242.963
170 238.816
170 233.908
170 227.942
0.067
0.008
Ϫ0.045
Ϫ0.073
Ϫ0.022
1.00
1.00
1.00
1.00
0.00
Ϫ0.033
1
7
16
16.5
15.5
20.0
19.0
18.0
17.0
16.0
15.0
14.0
13.0
12.0
223 700.053
0.093
1.00
222 410.206
224 413.013
222 415.686
222 418.266
222 420.767
222 422.977
222 423.850
222 407.877
0.051
0.056
0.046
0.022
0.007
Ϫ0.021
0.007
Ϫ0.212
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
19.0
18.0
17.0
16.0
15.0
14.0
13.0
223 702.650
223 703.604
223 703.604
223 703.604
223 702.650
223 698.940
0.185
0.009
0.009
0.009
0.185
0.56
0.35
0.33
0.31
0.44
1.00
Ϫ0.138
17.5
16.5
21.0
20.0
19.0
18.0
17.0
16.0
15.0
14.0
13.0
223 974.384
223 972.961
223 971.794
223 970.836
223 970.281
223 970.281
223 970.281
223 972.165
Ϫ0.127
Ϫ0.007
0.050
1.00
1.00
1.00
1.00
0.35
0.33
0.31
1.00
222 532.825
222 530.128
222 527.650
222 525.233
222 522.910
222 520.647
222 518.657
222 517.949
Ϫ0.119
Ϫ0.048
0.015
Ϫ0.001
0.003
0.010
0.062
0.075
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
20.0
19.0
18.0
17.0
16.0
15.0
14.0
0.001
Ϫ0.025
Ϫ0.025
Ϫ0.025
0.201
confirm the rotational assignment. The observed spectral pat-
tern completely conforms with that expected for a ⌺ di-
is primarily caused by the decrease of the spin-rotation inter-
action constant with vibrational excitation.
2
atomic molecule.
Table I includes some representative transitions ob-
served for the four vibrational states. Figure 1 shows the N
ϭ13� 12 transitions; the left and right insets depict, respec-
tively, the lower- and higher-frequency spin components for
vϭ0–2 and those for vϭ3 are reproduced in the upper and
lower insets, respectively.
Because the cesium atom consists of a single stable iso-
tope with the nuclear spin quantum number Iϭ7/2, the ob-
served set of absorption lines was readily ascribed to the
ground vibrational state. This set was accompanied by satel-
lites, which were assigned to the excited vibrational states of
vϭ1–3. Some additional lines were observed, which were
probably due to the vϭ4 state, but they seemed to be much
perturbed and we could not make any definite assignment for
them. The observed rotational transitions are as follows:
vϭ0: Nϭ7� 6, 8� 7, 9� 8, 13� 12, 14� 13, 17
IV. ANALYSIS
The observed transition frequencies were subjected,
separately for each vibrational state, to the least-squares
2
�
16, 18� 17, 19� 18, 26� 25, 27� 26, and 28� 27, v
ϭ1: Nϭ9� 8, 13� 12, 17� 16, 18� 17, 19� 18, 26� 25,
7� 26, 28� 27, and the lower-frequency spin component
analysis using the Hamiltonian for a ⌺ diatomic free radi-
4
cal, which was identical to that used for RbO. The sign of
2
the spin-rotation interaction constants required special atten-
tion; both the positive and negative signs yielded almost
equally good fitting in spectral line frequency. However, as
Fig. 2 clearly indicates, the hyperfine structure of the N
ϭ17� 16 transition in the vϭ3 state led us to favor the
positive sign to the negative one; the positive and negative
signs of the ␥ constant gave quite different intensity patterns
and only a positive ␥ constant yielded the pattern in good
agreement with the observed data. An analysis of this tran-
sition assuming the negative sign for ␥ resulted in the fol-
of 29� 28, vϭ2: Nϭ13� 12, 14� 13,17� 16, 19� 18, 26
�
�
25, 27� 26, 28� 27, and 29� 28, vϭ3: Nϭ13� 12, 14
13, and 17� 16.
Each rotational transition was found to consist of two
groups of lines or, in a case of high-N transition, of a dou-
blet, splitting being caused by the spin-rotation interaction.
The spin splitting, which is about 430 MHz in the ground
vibrational state, decreases rapidly with the vibrational quan-
tum number v. Most of the spin components except for some
high-N transitions of the ground vibrational state were ob-
served further split into hyperfine components. The hyperfine
splitting increases drastically with the vibrational quantum
number; for the vϭ3 state even the Nϭ17� 16 transition
was completely resolved into hyperfine components. The
analysis described below shows that the hyperfine coupling
constants do not vary much with the vibrational quantum
number and that the drastic increase of the hyperfine splitting
lowing constants: ␥ϭϪ144.3(2), b ϭ432.4(60), and c
F
ϭϪ267(19) in MHz, which may be compared with the posi-
tive case: ␥ϭ143.88(84), bFϭ448.84(165), and
c
ϭ119.9(22) in MHz. A negative c constant implies that the
unpaired electron occupies a p␲ orbital, which is certainly
not the case for the ground state of CsO. For the rest of the
vibrational states vϭ0, 1, and 2 also, only the positive sign
for ␥ led to reasonable sets of hyperfine constants.
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9590
J. Chem. Phys., Vol. 111, No. 21, 1 December 1999
C. Yamada and E. Hirota
FIG. 2. Hyperfine patterns for the Nϭ17� 16 transition of CsO in the v
ϭ3 state; the F2 and F1 spin components were displayed in the upper and
lower insets, respectively. The observed intensity pattern agrees better with
that calculated with a positive ␥ constant than with that obtained from a
negative one.
Oppenheimer approximation holds for CsO. The difference
1
4
between Y01 and B is 0.0095 MHz for CsF, and the cor-
e
FIG. 1. Nϭ13� 12 transitions of CsO in vϭ0, 1, 2, and 3. For the first
three vibrational states the F and F components are shown in the left and
right insets, respectively, whereas they appear in the upper and lower insets,
rection for the breakdown of the Born–Oppenheimer ap-
proximation will not affect the results on CsO much. The
2
1
equilibrium bond length r is thus readily calculated from the
e
respectively, for the vϭ3 state.
B constant to be 2.300 745 ͑16͒ Å. This result may be com-
e
pared with ab initio calculated lengths, 2.67 ͑Refs. 10, 11͒
and 2.425 Å.12 A similar fit for the centrifugal distortion
constants led to the following result:
Table II summarizes molecular constants derived from
the observed spectra.
1
D ϭ0.010 456 1͑58͒ϩ0.000 022͑12͒͑vϩ ͒
v
2
V. DISCUSSION
1
2
2
ϩ0.000 020 9͑29͒͑vϩ ͒ ,
͑2͒
The observed rotational spectra have established that the
2
ground electronic state of CsO is of ⌺, in agreement with
again with standard deviations in parentheses. The equilib-
rium rotational constant Be and the equilibrium centrifugal
distortion constant De , the first term of Eq. ͑2͒, when in-
serted in the following conventional expression for the har-
monic frequency ␻e :
the conclusion of previous studies.6 The observed data pro-
vide us with further information on the molecular structure
of the radical, as listed below.
,9
A. Molecular structure and vibrational frequency
␻eϭ͑4B /D ͒1/2,
3
͑3͒
e
e
The observed rotational constants were fitted to a power
1
2
result in ␻ ϭ356.78(11) cmϪ1, which is somewhat larger
series in (vϩ ),
e
than ab initio values, 286 ͑Refs. 10, 11͒ and 334 ͑Ref. 12͒ in
1
2
B ϭ6687.310͑92͒Ϫ38.47͑19͒͑vϩ ͒
v
Ϫ1
cm . For a reference, we cite Y10 and ␻e of CsF:
1
2
2
Ϫ1 14
Ϫ0.258͑46͒͑vϩ ͒ ,
͑1͒
352.616 20͑91͒ and 352.615 58͑82͒ cm .
where the values in parentheses denote standard deviations,
which essentially represent the consistency among the four
constants. The first term of the series corresponds to the
B. Lowest excited electronic state
We may anticipate to estimate the excitation energy of
the lowest electronic state from the observed spin-rotation
equilibrium rotational constant B , provided that Born–
e
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J. Chem. Phys., Vol. 111, No. 21, 1 December 1999
TABLE II. Molecular constants of CsO ͑in MHz͒.a
The cesium monoxide radical
9591
Constant
vϭ0
vϭ1
vϭ2
vϭ3
B
D
H
␥
␥D
␥H
b
6 668.031 65͑77͒
0.010 473 8͑16͒
0.000 000 003 8͑10͒
432.263͑48͒
0.005 026͑66͒
Ϫ0.000 000 043͑39͒
426.30͑36͒
6 628.964 9͑14͒
6 589.588 3͑17͒
6 549.490 9͑20͒
0.010 789 6͑40͒
0.010 532 9͑28͒
Ϫ0.000 000 005 0͑17͒
371.61͑15͒
0.007 45͑17͒
0.000 000 168͑95͒
427.3͑22͒
0.010 646 8͑40͒
0.000 000 008 5͑25͒
286.02͑16͒
0.011 23͑23͒
Ϫ0.000 000 27͑13͒
411.5͑15͒
143.88͑84͒
0.018 7͑26͒
Ϫ0.000 004 8͑35͒
408.9͑15͒
c
118.16͑63͒
111.9͑37͒
126.9͑31͒
119.9͑22͒
eQq
T00
Ϫ23.4͑33͒
25.͑11͒
Ϫ0.079͑12͒
17.͑15͒
Ϫ0.015͑14͒
30.8͑85͒
0.069 9͑96͒
aValues in parentheses denote standard deviations and apply to the last digits of the constants.
interaction constant ␥. However, as mentioned above, a care-
ful examination of the observed spectra resulted in a positive
sign for the constant. This observation precluded the possi-
bility of simply applying the formula ␥ϭϪ4AB/⌬E which
was derived by a second-order perturbation treatment on a
basis of the pure precession approximation.15 We have rather
interpreted the observed ␥ constants to consist of at least two
sources; the positive sign of the ␥ constants was ascribed to
contributions of higher excited electronic states which super-
seded a negative contribution of the lowest excited ⌸ state.
The marked dependence of the ␥ constant on vibrational ex-
citation was presumed to be mainly caused by the interaction
with the lowest ⌸ state.
vϭ4 state can be buried in the manifold of the ⌸ state. The
Ϫ1
excitation energy of 1225 cm6 we estimated agrees also
Ϫ1
with the result of Sarkas et al., 1089Ϯ202 cm
.
The positive term ␥ of 881.42 MHz may be accounted
0
for by the contribution of an excited electronic state at 4539
Ϫ1
cm above the ground state, provided that the effective
Ϫ1
spin–orbit coupling constant is 150 cm
.
C. Electronic structure
9
Lindsay et al. reported the hyperfine constants obtained
from the ESR spectra; aϭ139 G and T ϭ28 G, which are
ʈ
converted to b ϭbϩc/3ϭ390 and cϭ118 MHz to be com-
F
In order to substantiate this interpretation, we have car-
pared with the present results, b ϭ465.7(4) and c
F
1
2
ried out a calculation using ab initio potential functions
ϭ118.2(6) MHz.
which were fitted to Morse potentials for ease of evaluating
the Franck–Condon factors between the ground and the low-
est excited ⌸ states. We have then found that the observed
decrease of the ␥ constant with the vibrational quantum num-
4
As in the case of RbO, it is difficult to extract informa-
tion on the unpaired electron from these hyperfine constants.
When we compare the observed b constant with the atomic
F
17
value 2467 MHz, as compiled by Morton and Preston for a
ber was reproduced when the excitation energy Ϫ⌬E of the
6
s electron, the s character becomes 18.9%. However, we
eff
⌸
state and the effective spin–orbit interaction constant A
should use the value for a 5s electron, rather than for a 6s
electron, and then the s character would reduce to 1% or so.
Ϫ1
were 1225 and Ϫ29.83 cm , respectively, leaving a
vibration-independent positive term ␥0 of 881.42 MHz as
contributions from higher excited electronic states. Figure 3
compares the observed minus calculated spin-rotation inter-
eff
action constants obtained by using this set ͓1͔ of Ϫ⌬E, A ,
and ␥0 with those derived from two other sets; ͓2͔ Ϫ⌬E
Ϫ1
eff
Ϫ1
ϭ1250 cm , A ϭϪ32.80 cm , and ␥ ϭϩ918.80 MHz,
0
Ϫ1
eff
Ϫ1
and ͓3͔ Ϫ⌬Eϭ1200 cm , A ϭϪ27.06 cm , and ␥0
ϭϩ846.31 MHz.
The excitation energy 1225 cmϪ1 of the ⌸ state may be
rationalized as follows. As mentioned above, we observed a
few additional lines and suspected that they were assigned to
the vϭ4 excited vibrational state; we could not have made
any proper assignment for them presumably because they
were subjected to severe perturbations. A plausible explana-
tion for this observation is the interaction of the vϭ4 state
with the ⌸ electronic state. An equation derived by
1
6
Pekeris,
2
2
FIG. 3. Discrepancies between the observed and calculated spin-rotation
interaction constants. The three sets ͓1͔, ͓2͔, and ͓3͔ of the calculated con-
stants designated respectively by circles, squares, and triangles were ob-
␻ex ϭB ͓͑␣ ␻ /6B ͒ϩ1͔
͑4͒
led to ␻ x to be 1.432 cmϪ1. The energy of the excited
e
e
e
e
e
e
e
Ϫ1
eff
eff
eff
tained from the following parameters: ͓1͔ Ϫ⌬Eϭ1225 cm
,
A
A
vibrational state measured from the ground state was thus
calculated to be 353.92, 704.97, 1053.16, and 1398.49 cm
for the vϭ1, 2, 3, and 4 states, respectively. Therefore, the
Ϫ1
Ϫ1
ϭϪ29.83 cm
, and ␥ ϭϩ881.42 MHz, ͓2͔ Ϫ⌬Eϭ1250 cm ,
0
Ϫ1
Ϫ1
Ϫ1
ϭϪ32.80 cm , and ␥ ϭϩ918.80 MHz, and ͓3͔ Ϫ⌬Eϭ1200 cm , A
0
Ϫ1
ϭϪ27.06 cm , and ␥ ϭϩ846.31 MHz.
0
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1

9592
J. Chem. Phys., Vol. 111, No. 21, 1 December 1999
C. Yamada and E. Hirota
6
The positive sign of the c constant is consistent with the
electronic state being of ⌺. It is, however, not easy to esti-
mate the spin density from this constant.
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7
8
9
The Cs nuclear quadrupole coupling constant is Ϫ23.4
͑
33͒ MHz in the ground vibrational state and changes its sign
in the excited vibrational states; ϩ25 ͑11͒ for vϭ1, ϩ17
15͒ for vϭ2, and ϩ30.8 ͑85͒ MHz for vϭ3. These data
315 ͑1974͒.
1
1
0
1
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͑
͑
1984͒.
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12
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1
8
1
1
4
5
CsF, which is known as a typical ionic molecule.
1
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18
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1