The Microwave Spectrum of the FeS Radical · 2 INTRODUCTION Several transition metal oxides have...
Transcript of The Microwave Spectrum of the FeS Radical · 2 INTRODUCTION Several transition metal oxides have...
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Journal of Molecular Spectroscopy 224, 137 (2004)
The Microwave Spectrum of the FeS Radical
Shuro Takano,* Satoshi Yamamoto,† and Shuji Saito‡ *Nobeyama Radio Observatory1, and Department of Astronomical Science, The Graduate University for Advanced Studies (Sokendai), Minamimaki, Minamisaku, Nagano, 384-1305 Japan; †Department of Physics, The University of Tokyo, Hongo, Bunkyo, Tokyo, 113-8654 Japan; ‡Research Center for Development of Far-Infrared Region, Fukui University, Bunkyo, Fukui, 910-8507 Japan
ABSTRACT
The rotational spectrum of the iron monosulfide radical, FeS, was measured in the frequency
region of 220 to 390 GHz with a source-modulated millimeter/submillimeter-wave spectrometer.
The radical was efficiently produced in a free space absorption cell by a dc discharge in a mixture of
Ar and H2S with a stainless-steel hollow cathode. Several series of paramagnetic lines were
detected with intervals of about 12 GHz. The four series having relatively strong intensity were
assigned to FeS in the vibrationally ground state of the X5∆i electronic state, two series to that in the
vibrationally excited state, and five series presumably to FeS in the electronically excited state, A5Σ+.
The effective molecular constants were determined for FeS in the X5∆i electronic state. The Ω=2
components of the vibrationally ground state showed an apparent shift from the typical pattern of
the 5∆i state. In addition, the fine structure of the A5Σ+ state was found to be far from a regular
pattern expected for a 5Σ state. A trial analysis including electronic interaction between the 5∆i and 5Σ states was carried out, but it was not possible to explain the spectral lines of both electronic states
simultaneously. Reasons for the heavily perturbed spectral patterns are discussed.
Keywords: microwave spectrum, free radical, iron monosulfide, perturbation
1 Nobeyama Radio Observatory is a branch of the National Astronomical Observatory, an inter-university research institute operated by the Ministry of Education, Culture, Sports, Science and Technology.
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INTRODUCTION
Several transition metal oxides have been studied by microwave spectroscopy: e.g., TiO (1),
MnO (2), FeO (3, 4), CoO (5), NiO (6), CuO (7), AgO (8), and ZrO (9). Detailed molecular constants
in multiplet ground electronic states give much new information on the nature of the metal oxide
bond and various interactions within the state and between the electronic states. Transition metal
sulfides have been less studied by high-resolution spectroscopy than corresponding oxides. When
oxygen is replaced by sulfur, substantial change is expected in bonding between transition metal
and sulfur and in the complicated electronic structure, because sulfur has smaller electronegativity
than oxygen, and the 3p of sulfur has larger distribution than the 2p of oxygen, changing s-d-p
hybridization in the bonding.
Among transition metal oxides listed above, FeO has been well studied for ages by optical
spectroscopy, which established the ground electronic state of FeO to be X5∆i (10, 11). Its molecular
properties were determined in details by various methods such as microwave spectroscopy (3, 4),
molecular beam laser spectroscopy (12), MODR spectroscopy (13), LIF spectroscopy (14, 15), and
photoelectron spectroscopy (16). Optical spectroscopy and photoelectron spectroscopy gave evidences
on the existence of low-lying electronic states, A5Σ+ (16) and a7 Σ + (14). Contrary to FeO, there are
few spectroscopic studies of iron sulfide. DeVore and Franzen (17) studied vibrational spectra of
FeS in Ar and OCS matrices, and Zhang et al. (18) measured photoelectron spectra. Few
quantum chemical calculations are available for its bond length (re), harmonic frequency (ωe),
dissociation energy (D0), and dipole moment (µ) (19, 20, 21). Hübner et al. (20), who report
high-level calculations for the electronic energy levels of FeS as well as an interpretation of the
photoelectron spectrum (18), strongly suggest that its state order near the ground state is 5Σ+ (0.0
eV), 5∆ (0.13 eV), and 7Σ+ (0.26 eV).
Iron bearing molecules are interesting for their possible existence in stellar and interstellar
sources, because iron is the most stable element produced in the nuclear syntheses in stars and it
has the highest cosmic abundance among heavier metals, comparable to that of silicon. Several
silicon-bearing molecules were detected in circumstellar envelopes and star forming regions. The
rotational spectral lines of FeO were searched for toward stars and even toward interstellar clouds
since their line frequencies were measured in the laboratory (3). Only upper limits to the
abundance were reported (22, 23). In cold interstellar space the iron bearing molecules are
significantly depleted in dust grains, but they may exist in the gas phase in hot regions such as
circumstellar envelopes of late-type stars and star forming regions. Very recently Walmsley et al.
(24) reported a tentative detection of FeO. Its J=5-4 line was observed in absorption at 153 GHz
toward the hot and relatively low-density gas in the region of Sgr B2. This absorption line was
further confirmed by interferometric observations toward the same object (25). Tsuji (26) predicted
that FeS is more abundant than FeO in cool stellar atmospheres. However, no searches have been
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carried out for FeS, because no spectral line frequencies are available.
In the present study, we observed the rotational spectrum of FeS in the ground electronic state
of X5∆i for the first time in the laboratory, and also detected spectral lines which were presumably
assigned to a low-lying electronically excited state of A5Σ+.
EXPERIMENTAL
The 100 kHz source-modulated microwave spectrometer of Nagoya University was used to
measure spectral lines of FeS (27). For production of metal-bearing molecules in the gas phase, the
dc sputtering method was employed: a mixture of Ar (or He) and H2S (or OCS, CS2) was discharged
in a 2 m long free space absorption cell equipped with a 1.4 m long stainless steel hollow cathode.
Iron atoms were supplied from the surface of the stainless steel hollow cathode to the gas phase.
Since no theoretical prediction was available for FeS at the time of the experiments, the rotational
constant of FeS was estimated to be between 5.1 and 6.3 GHz from bond lengths of related
molecules. The ground electronic state was assumed to be 5∆i, as suggested from FeO, and
paramagnetic lines were searched for in a wide frequency range of more than 20 GHz.
Consequently, several series of paramagnetic lines were found as schematically shown in Figure 1,
where the frequency axis is given with an interval of 12 GHz, close to the interval of two adjacent
lines of each series. The interval is close to two times of the expected rotational constants for FeS.
The lines indicated by closed circles show relatively large Zeeman effect: The line width (FWHM)
increases from ~0.7 MHz to ~2.0 MHz as the magnetic field applied to the cell is increased from zero
to several Gauss. This is in sharp contrast to other series of lines, where the line width (FWHM)
increases to ~2.0 MHz only at the magnetic field of more than 50 Gauss. The different behavior
against the magnetic field is, therefore, rather striking. Two series showing a doublet pattern were
indicated in Figure 1. These doublings were considered to be Λ–type doubling.
These paramagnetic lines changed in intensity with production conditions. (1) The intensity
decreased as a trace amount of oxygen was introduced into the cell. (2) The lines were observable
even by discharging Ar (He) with only a trace amount of H2S (or OCS, CS2). (3) The intensity
decreased severely when the discharge current changed from 500 mA to 50 – 100 mA. The
optimum condition for these lines was obtained at a discharge current of 500 mA in a mixture of Ar
(1.3 Pa or He, 2.7 Pa) and H2S (or OCS, CS2, 0.1 Pa). The intensity increased as the current
increased, but, for maintaining stable discharge, a discharge current of 500 mA was used
throughout measurements. Change in the cell temperature did not affect much the production,
and the temperature was kept between 190 and 270 K.
Several lines of series with relatively strong intensity were investigated to see whether they
were due to an iron bearing free radical. On replacing the stainless-steel hollow cathode by an
aluminum electrode, they disappeared. Subsequently, ferrocene, (C5H5)2Fe, was introduced into
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the cell with the aluminum hollow cathode, and a discharge in an addition of OCS reproduced
weakly the paramagnetic lines. Therefore, we concluded that these lines are due to an iron and
sulfur bearing free radical, most likely FeS. An example of the observed spectral lines is shown in
Figure 2, which was observed at 387.7 GHz and later assigned to the J=32-31 transition of FeS in
the X5∆4 substate.
ANALYSIS
First, each line frequency of the series was divided by a tentatively assumed rotational quantum
number for the upper level, and the derived value (two times the effective rotational constant) was
plotted against the rotational quantum number as shown in Figure 3 (a) and (b). The derived
effective rotational constant shows a harmonic relation with the quantum number (Figure 3 (a))
except for the five series indicated by closed circles in Figure 1 (Figure 3 (b)). As a result, the series
given in Figure 1 are classified into two groups: six series with harmonic relations and five series
without harmonic relations against assumed quantum numbers.
The 5∆i state
Two weak series among six ones with harmonic relations show doublet structures suggesting
Λ–type doubling, and two series at the lower frequency side are considered to belong to the v=1 state
from their weaker intensity. For the assignment of the Ω substate, doubling separations were
examined for the two series. When the doubling separation was plotted against the rotational
quantum number, the separation for the lower frequency series was found to be roughly
proportional to J4 and that for the upper frequency series approximately to J1.5. According to
Brown, Cheung, and Merer (28), the Λ–type doubling separation is nearly proportional to
[J(J+1)]Ω /J ∼ J2Ω- 1 for the ∆ state. Therefore, the Ω values were calculated to be 2.5 and 1.3 for
the lower and upper frequency series, respectively. These features suggest that the observed
spectral lines are due to a molecule in a degenerate electronic state with two or more electronic spin
quantum number and with a negative spin-orbit coupling constant, that is, inverted. Thus, these
series with harmonic relations were assigned to the spectral lines of FeS in the v=0 and 1 states of
the 5∆i electronic state, as the ground electronic state of FeO suggested. The Ω value assigned to
the fine structure component is indicated in Figure 1. The spectral lines of the Ω=0 substate were
not clearly identified in this study, because they should be very weak and might have irregular
pattern due to perturbation.
The spectral lines assigned to the X5∆i electronic state were analyzed by the least-squares
method using the matrix elements for a Hamiltonian appropriate to the 5∆ electronic state (14, 28).
At first the spectral lines of the Ω =4, 3, 2, and 1 substates were analyzed simultaneously, but those
of the Ω =2 substate showed systematic residuals, which were significantly larger than the
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frequency measurement errors. Therefore, the spectral lines of the Ω =2 substate were not
included in the final fit. The correlations among the parameters were rather high only between
ASO and AD, but both of them could be determined. The spectral lines of the v=1 state were
analyzed similarly where B, D, and ASO were employed as floating parameters, because only the
spectral lines of the Ω =4 and 3 substates were observed for the v=1 state. The correlations among
the parameters were not so high to prevent to determine the parameters. The observed and
calculated line frequencies for the v=0 state are listed in Table 1, and those for v=1 in Table 2. The
determined molecular constants are listed in Table 3.
The 5Σ+ state
As noted above, the effective rotational constants for the five series indicated by closed circles
in Figure 1 show no harmonic relation against the assumed quantum number. Since every set of
the five lines have similar intensity (but significantly weaker than those of the 5∆4 and 5∆3 states in
the vibrationally ground state, see Figure 1) and no other series lines with comparable intensities
are observed around the five lines, we conclude that these five series lines are due to FeS in the A5Σ+
state, as suggested from the excited electronic state of FeO (16). The observed spectral lines for
these series were analyzed using a Hamiltonian for the 5Σ state (29). However, it was not
straightforward to assign each line to the component of the 5Σ state, and after several trial
calculations the observed pattern was roughly reproduced with a set of molecular constants:
Beff=6227.6 MHz, λ= –103000 MHz, γ= –733 MHz, and θ=17500 MHz. The standard deviation of
the least-squares fit was about 100 MHz, which was far from the measurement errors of observed
lines. Since the molecular constants derived are so poor to explain the observed spectral lines that
only a tentative assignment is given for each spectral line as listed in Table 4.
DISCUSSION
Production of FeS with sputtering method
In the present study the relatively poorly studied transition metal sulfide, FeS, was
characterized for the first time by using a high-sensitivity millimeter/submillimeter-wave
spectrometer combined with the dc sputtering production method (30). Nonvolatile heavy metal
bearing molecules have been usually produced in the gas phase with high-temperature ovens such
as heat-pipe oven or King furnace. The high-temperature oven evaporates heavy metal directly in
the gas phase and produced molecules remain at high temperature in general. On the other hand,
the sputtering method produces gaseous metal containing molecules at relatively low temperature.
This feature of the sputtering method is adequate for microwave spectroscopy without a very high
temperature cell, and has been applied to spectroscopy of several transition metal oxides (2, 5-8, 31,
32).
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Electronic states of FeS
According to the photoelectron spectroscopy of FeS (18), the order and energ ies of the
electronic states are as follows: 0 eV (5∆), 0.49 eV (5Σ+), 0.66 eV (3Σ+), and 1.03 eV ( 5∆), where they
thought that the ground electronic state of FeS– was 4∆. On the other hand, the detailed ab initio
calculations (CASSCF/ACPF) (20) predicted that the ground electronic state of FeS– was 6∆. They
reassigned the bands obtained with the photoelectron spectroscopy based on their calculations.
The order and energies of the electronic states of FeS obtained were 0 – 0.1 eV (5Σ+, 5∆, 7Σ+), 0.49 eV
(5Π, 5Φ), 0.66 eV (7Π, 7Φ), and 1.03 eV (5Σ–, 7∆). Therefore, there are many electronic states in FeS
in low energy. Our experiment of microwave spectroscopy indicates that the 5∆ and 5Σ+ states
actually exist in low energy. As mentioned before, their properties are rather different from each
other in the aspects of Zeeman effect and harmonic relation with respect to the rotational quantum
number. These different properties indicate that the mixing is not severe between the 5∆ and 5Σ+
states. Based on the observed line intensities, the 5∆ state is most probably the electronic ground
state. We could not find other states with comparable intensity. If several electronic states of FeS
actually exist in quite low energy as predicted by the above-mentioned ab initio calculations, the 5∆
and 5Σ+ states, which we observed, near (or at) the ground state are probably severely perturbed
each other and by other nearby states. In this case the properties of the 5∆ and 5Σ+ states should be
similar due to severe mixing of the states by perturbations, and the Ω value assignments may have
little sense. However, we conclude that the perturbations are not so severe as to prevent the Ω
value assignments based on the results of our measurements.
Molecular constants
Since the molecular constants of FeS (5∆) were determined spectroscopically for the first time, it
may be worthy to compare them with those predicted by an ab initio calculation. The observed
rotational constant B0 of 6106.16(10) MHz is well compared with the predicted value of 6058 MHz
for Be, which is calculated from re of 2.025 Å (CASSCF) (19). Almost the same re of 2.03 – 2.04 Å
(CASSCF, DFT-B3LYP) is obtained by other ab initio calculation (20).
The same calculation above (19) predicts the harmonic frequency of FeS ωe to be 521 cm-1, which
is nearly equal to the fundamental frequency ω0 of 506 cm-1, derived from the present centrifugal
distortion constant D0 (28). Other ab initio calculation (20) predicts ωe to be 519 cm-1
(CASSCF/ACPF) and ~530 cm-1 (DFT-B3LYP). In addition, DeVore and Franzen (17) reported ω0
of FeS in OCS and Ar matrices to be 539 and 542 cm-1, respectively.
The spin-orbit coupling constant ASO was determined to be about –45 cm-1. Although this is an
effective value, it is nearly half of that of FeO (14). Generally sulfides have larger spin-orbit
coupling constants than corresponding oxides. For example, CuO (X2Πi) has the ASO of –276.16
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cm-1 (34) and CuS (X2Πi) that of –433.20 cm-1 (35). The small effective spin-orbit coupling constant
in the present study implies that not only the 5∆2 sublevel but the whole 5∆ state is perturbed. The
spin-orbit coupling constant of the v=1 state is significantly different from that of the v=0 state.
This is also the effect of perturbation, and an effective spin-orbit coupling constant is derived.
The Ω =2 substate lines in the X5∆i state
The Ω =2 component of the 5∆i state show large residuals in the least-squares fit. The
Λ–type doubling separation is abnormally large, and does not obey the typical pattern of the 5∆i state,
as noted above. The low-lying Σ electronic states are the most likely candidates as the perturbing
state. The ∆ and Σ electronic states interact through the spin-spin interaction. According to the
selection rule of ∆S=0, ±1, ±2, ∆Ω=0, ∆Λ=±2, and ∆Σ= m2 for the spin-spin interaction (36), the most
possible perturbing state was considered to be A5Σ+ or a7Σ+. As a trial analysis, a least-squares
program including both the 5∆ and 5 Σ states and the homogeneous spin-spin coupling term between
them was developed and used. However, the inclusion of the electronic interaction between the two
states did not improve the residuals of the 5 Σ state. We wonder that the A5 Σ+ state also suffers
from strong perturbations from a nearby electronic state, for an example, a7 Σ state. So as to
analyze perturbations in the A5 Σ + state as well as in the Ω =2 substate of the 5∆ state, further
detailed information for relative energy differences of the nearby electronic states and their
molecular constants is highly desirable.
Still assignments of part of series lines and analysis of perturbation have ambiguity, because
we cannot have a definite image of the perturbers. For further analysis detections and
measurements in microwave spectroscopy of the additional series lines such as in the Ω=0 state, in
the v=2 state, and of the 54FeS isotopic species (54FeS/56FeS ∼ 1/16) will also be very helpful. In
addition, direct measurements of electronic transitions mainly by optical spectroscopy will be
important for unraveling the electronic states.
Astronomical search
In the circumstellar envelope of the late-type carbon star, IRC+10216, the abundance of SiS is
higher than that of SiO by a factor of 10 times or more (37, 38). This is because the silicon atom
first combines with the oxygen atom, forming SiO, which is more stable than SiS, and the residual
silicon produces a larger amount of SiS in the less oxygen-abundant carbon star (30). The thermal
equilibrium calculation for carbon star predicts a possibility that the abundance of metal sulfides is
larger than that of the corresponding oxides in the relatively cool stellar envelope of the carbon star
(39). Hence, FeS may be detected in the carbon star envelope.
The spectral lines of the J=7-6 and 8-7 transitions in the 5∆4 substate, which is the lowest
substate, were searched for toward one carbon star envelope, IRC+10216 and three giant molecular
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clouds including hot star-forming region, Orion-KL, Sgr B2, and W51. The 45 m radiotelescope of
the Nobeyama Radio Observatory was used for the observations. However, lines corresponding to
FeS were not detected at the noise level of 11 – 29 mK in antenna temperature.
Acknowledgment
We thank the staff members of the 45 m radiotelescope of the Nobeyama Radio Observatory for
their support during the observations. We also thank an anonymous referee for helpful comments.
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TABLE 1
Observed and calculated transition frequencies of
the FeS radical in the v=0 state (X5∆i) (MHz)
J’ – J Ω νobs.a νobs. –νcalc.
19 – 18 4 230383.569(17) -0.004
20 – 19 4 242497.470(26) 0.000
21 – 20 4 254609.589(7) -0.001
22 – 21 4 266719.857(22) 0.015
23 – 22 4 278828.119(7) -0.020
24 – 23 4 290934.432(15) 0.043
25 – 24 4 303038.498(14) -0.008
26 – 25 4 315140.389(7) -0.009
27 – 26 4 327239.984(13) 0.006
28 – 27 4 339337.110(15) -0.046
29 – 28 4 351431.877(28) 0.035
30 – 29 4 363523.901(29) -0.047
31 – 30 4 375613.419(14) 0.035
32 – 31 4 387700.062(19) 0.002
19 – 18 3 230980.325(21) -0.046
20 – 19 3 243125.506(23) 0.013
21 – 20 3 255268.847(20) 0.040
22 – 21 3 267410.235(10) 0.010
23 – 22 3 279549.608(16) -0.046
24 – 23 3 291687.058(26) 0.052
25 – 24 3 303822.226(31) 0.037
26 – 25 3 315955.100(11) -0.013
27 – 26 3 328085.671(6) -0.016
28 – 27 3 340213.814(7) -0.006
29 – 28 3 352339.416(41) -0.007
30 – 29 3 364462.341(41) -0.063
31 – 30 3 376582.644(7) -0.028
32 – 31 3 388700.210(12) 0.072
J’ – J Ω νobs.a νobs. –νcalc.
18 – 17 2 219664.503(61) 130.011b
2 219666.863(47) 135.199
19 – 18 2 231859.268(55) 139.100
2 231862.157(29) 145.306
20 – 19 2 244052.544(21) 148.435
2 244056.082(17) 155.829
21 – 20 2 256244.535(21) 158.311
2 256248.678(42) 166.903
22 – 21 2 268434.889(19) 168.467
2 268439.870(37) 178.547
23 – 22 2 280623.677(35) 179.068
2 280629.571(29) 190.767
24 – 23 2 292810.836(19) 190.142
2 292817.794(41) 203.671
25 – 24 2 304996.254(17) 201.668
2 305004.388(23) 217.200
26 – 25 2 317179.928(22) 213.736
27 – 26 2 329361.779(22) 226.359
28 – 27 2 341541.789(50) 239.612
2 341554.688(35) 262.774
29 – 28 2 353719.841(26) 253.470
2 353734.839(23) 279.818
30 – 29 2 365895.983(19) 268.072
2 365913.371(25) 297.967
31 – 30 2 378070.080(22) 283.377
2 378090.227(25) 317.257
32 – 31 2 390242.182(10) 299.528
2 390265.469(41) 337.844
11
(table 1 continued)
J’ – J Ω νobs.a νobs. –νcalc.
28 – 27 1 342625.6 c 0.082
1 342719.2 c -0.148
29 – 28 1 354834.7 c -0.047
1 354933.6 c 0.239
30 – 29 1 367041.0 c -0.097
1 367144.6 c -0.035
31 – 30 1 379244.5 c 0.030
1 379353.0 c -0.074
32 – 31 1 391444.8 c 0.033
1 391558.6 c 0.017
a Values in parentheses indicate one standard
deviation of the frequency measurements
in kHz. b Weight is 0.0 for all Ω=2 transitions.
c Roughly measured frequency.
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TABLE 2
Observed and calculated transition frequencies of
the FeS radical in the v=1 state (X5∆i) (MHz)
J’ – J Ω νobs. a νobs. –νcalc.
27 – 26 4 325712.7 b -0.101 c
28 – 27 4 337753.3 b -0.089 c
29 – 28 4 349791.530(30) 0.046
30 – 29 4 361826.971(5) -0.025
31 – 30 4 373859.856(14) 0.020
32 – 31 4 385889.7 b -0.216 c
27 – 26 3 326562.5 b -0.215 c
28 – 27 3 338634.390(26) 0.013
29 – 28 3 350703.523(14) 0.021
30 – 29 3 362769.976(9) -0.024
31 – 30 3 374833.785(34) 0.006
32 – 31 3 386894.8 b 0.050 c
a Values in parentheses indicate one standard deviation
of the frequency measurements in kHz.
b Roughly measured frequency. c Weight is 0.1.
13
TABLE 3
The molecular constants of the FeS radical in the X5∆i electronic state (MHz) a
Constants v=0 (Ω=4,3,1) v=1 (Ω=4,3)
ASO – 1340000(31000) – 2245100(230)
B 6106.16(10) 6069.416(10)
D 0.0039529(35) 0.0038465(58)
λ – 47000(5100) ---
λD – 0.232(79) ---
AD 2.38(33) ---
ñ∆ 10.73(29) ---
õ∆ 1.053(55) ---
a Values in parentheses are three standard deviation and apply
to the last digits of the constants.
14
TABLE 4
Observed frequencies and tentative assignments of the FeS
spectral lines in the A5Σ+ state (MHz) a
N’ – N F1 F2 F3 F4 F5
17 – 16 223176.888(3)
18 – 17 234912.782(12)
19 – 18 246649.625(8)
20 – 19 258396.986(23)
21 – 20 270162.747(22)
22 – 21 281953.151(24)
23 – 22 293772.992(6) 277527 b
24 – 23 305625.384(4) 301097.8 b 295881.5 b 298281.1 b
25 – 24 317512.039(23)
26 – 25 329433.684(44) 325628.1 b 321089.7 b 323004.3 b
27 – 26 341389.943(79)c 337905.6 b 333671.2 b 335369.7 b
28 – 27 353379.604(24) 350189.880(22) 342916.4 b 346238.598(16) 347736.898(34)
29 – 28 365401.285(16) 362480.421(9) 355859.4 b 358792.332(8) 360105.104(35)
30 – 29 377452.879(16) 374776.380(18) 368757.9 b 371333.321(13) 372473.982(11)
31 – 30 389532.251(14) 387076.9 b 381613.707(45) 383861.6 b 384843.09 b
32 – 31 394428.2 b
a Values in parentheses indicate one standard deviation of the frequency measurements in kHz. b Roughly measured frequency.
c Disturbed.
15
Figure 1. The spectral lines of FeS are shown schematically. The abscissa indicates observed
frequency regions with the interval of 12 GHz. The ordinate corresponds to relative intensity,
which has an error of about 10-20%. The lines indicated by closed circle show relatively large
Zeeman effect and were assigned to the lines due to a low-lying Σ electronic state. The lines
indicated by asterisk or open square are not well established to make series, and are not yet
assigned.
16
Figure 2. The J=32-31 rotational spectral line of FeS in the 5∆4 substate at 387.7 GHz. The
radical was produced by 500 mA dc discharge with a stainless-steel hollow cathode
electrode in a mixture of H2S (0.1 Pa) and Ar (1.3 Pa). The integration time is about
12 seconds with time constant of 1 ms for the lock-in amplifier.
17
Figure 3 (a)
Figure 3. Each line frequency of the series was divided by rotational quantum number for the
upper level, and the derived value (two times the effective rotational constant) was
plotted against the rotational quantum number. (a) Plot related to the 5∆ state.
The derived value shows a harmonic relation with the quantum number. The series
indicated by asterisk or open square are the same as those indicated by the same
marks in Figure 1. (b) Plot of the five series in the 5Σ state indicated by closed
circles in Figure 1. The derived value does not show a harmonic relation with the
quantum number.
18
Figure 3 (b)