Journal of Molecular Spectroscopy 195, 147–153 (1999)
Article ID jmsp.1999.7823, available online at http://www.idealibrary.com on
The b1S1(b01) 3 X 3S2(X101, X21) and a1D(a2) 3 X21 Transitions
of SbF, SbCl, SbBr, and SbI
M. Beutel, K. D. Setzer, and E. H. Fink
Physikalische Chemie-Fachbereich 9, Bergische Universität-Gesamthochschule Wuppertal, D-42097 Wuppertal, Germany
E-mail: [email protected]
Received September 21, 1998; in revised form January 26, 1999
Emission spectra of the a 1 D(a2) 3 X 2 1 and b 1 S 1 (b0 1 ) 3 X 3 S 2 (X 1 0 1 , X 2 1) transitions of SbF, SbCl, SbBr, and SbI
have been observed in the near-infrared spectral region. The antimony halide radicals were generated and excited in a fast-flow
system by reaction of antimony vapor (Sbx ) with the halides and microwave-discharged oxygen. The NIR chemiluminescence
was measured with a Fourier-transform spectrometer equipped with Ge and InSb detectors. The spectra contain the known
b 1 S 1 (b0 1 ) 3 X 3 S 2 (X 1 0 1 , X 2 1) transitions in the range 730 –910 nm and the hitherto unknown a 1 D(a2) 3 X 2 1 transitions
in the range 1600 –1900 nm. Vibrational analyses have yielded improved molecular constants for the X 1 0 1 , X 2 1, and b0 1
states and the following constants of the a2 states (in cm21): 121SbF: T e 5 6815.6(5), v e 5 615.75(3), v e x e 5 2.62(1);
121
Sb35Cl: T e 5 6546.3(2), v e 5 379.8(1), v e x e 5 1.20(2); 121Sb79Br: T e 5 6496.4(4), v e 5 265.9(2), v e x e 5
0.55(3); 121SbI: T e 5 6366.7(3), v e 5 214.20(5), v e x e 5 0.430(9), where the numbers in parentheses are the standard
deviations of the parameters. © 1999 Academic Press
Key Words: antimony halides; metastable states; chemiluminescence; Fourier-transform spectroscopy.
1. INTRODUCTION
The antimony monohalide radicals (SbX; X 5 F, Cl, Br, I)
belong to the molecules with X 3 S 2 ground states and lowlying, metastable a 1 D and b 1 S 1 excited states. Whereas the
b 1 S 1 (b0 1 ) 3 X 3 S 2 (X 1 0 1 , X 2 1) spectra in the near-infrared
range were observed for all 20 group V halides (1–3), until
recently, the a 1 D(a2) 3 X 3 S 2 (X 1 0 1 , X 2 1) transitions were
known only for the three nitrogen halides NF (4), NCl (5), and
NBr (5). Stimulated by theoretical predictions of rather large
transition probabilities of the a2 3 X 2 1 transitions of several
heavy atom-containing hydrides and halides (BiH (6), BiI (7),
SbI (8)), we have searched for such transitions of the bismuth,
antimony, and arsenic halides in chemiluminescence systems
and dc discharges. In a previous paper, we reported on the
observation of the a2 3 X 2 1 spectra of BiCl, BiBr, and BiI
with a Fourier-transform spectrometer (9). In continuation of
this work, we have also found the a2 3 X 2 1 transitions of all
four antimony halides. Out of these molecules, only SbF has
been studied in greater detail and at high spectral resolution
(1). The electronic energy and the vibrational and rotational
constants of the a 1 D state of SbF have been deduced from uv
spectra (1). The b 1 S 1 (b0 1 ) 3 X 3 S 2 (X 1 0 1 , X 2 1) transition
in the near infrared and a number of uv/vis spectra from higher
lying states have been measured at high resolution (10 –12).
But in most studies, only a few bands could be analyzed.
Therefore, the vibrational constants of the X 1 0 1 , X 2 1 ground
states of SbF are not well known. The b 1 S 1 (b0 1 ) 3
X 3 S 2 (X 1 0 1 , X 2 1) spectra of SbCl (13), SbBr (14), and SbI
(15) have only been measured at low resolution. Except for one
transition of SbCl (16), no high-resolution studies of uv/vis
spectra have been reported for these molecules. Therefore, like
for SbF, the vibrational constants of the ground states X 1 0 1
and X 2 1 are not well known.
In the present paper, we report on medium-resolution measurements of emission spectra of the b 1 S 1 (b0 1 ) 3
X 3 S 2 (X 1 0 1 , X 2 1) and a 1 D(a2) 3 X 2 1 transitions of all four
antimony halides with a Fourier-transform spectrometer. Analyses of the b 3 X 1 , X 2 spectra were used to deduce improved
constants of the X 2 1 states which were needed to derive the
electronic energy and vibrational constants of the a2 states
from the a 3 X 2 bands.
2. EXPERIMENTAL DETAILS
As in previous low-resolution studies (11, 12), the emission
spectra of antimony halide molecules were observed from
chemiluminescence and energy transfer reactions in fast-flow
systems. The flow system was made of pyrex glass and consisted of a tube, 100-cm long and 4-cm in diameter, with quartz
windows at both ends and equipped with a pumping port and
three side arms for gas inlet. In one side arm, argon carrier gas
was passed over antimony metal heated in a quartz tube to
above its melting point ('1000 K). The halide molecules
mixed with Ar carrier gas were added to the Sbx /Ar flow
through the second inlet of the observation tube. Metastable
oxygen molecules, O2(a 1 D g ), were generated in the third inlet
system by passing O2 through a microwave discharge. They
147
0022-2852/99 $30.00
Copyright © 1999 by Academic Press
All rights of reproduction in any form reserved.
148
BEUTEL, SETZER, AND FINK
FIG. 1.
Survey spectra of the b0 1 3 X 1 0 1 , X 2 1 spectra of SbF (a), SbCl (b), SbBr (c), and SbI (d) at a spectral resolution of 0.5 cm21.
served as energy carriers and were used to excite the SbX
molecules to their b0 1 and a2 states by electronic-to-electronic energy exchange and energy pooling processes.
Spectra were recorded in the region 3000 –14 000 cm21 with
a Bruker IFS 120 HR Fourier-transform spectrometer using
liquid nitrogen-cooled germanium (Applied Detector Corp.,
Model 403 S) and InSb (Cincinnati Electronics Corp., Model
IDH 100) detectors. All gases and chemicals were research
grade and were used without further purification. The wavenumber scale of the spectrometer was calibrated by use of Ne
I or Ar I reference lines (17). In the medium-resolution measurements reported here, the precision of the measured wavenumbers is on the order of 0.1 cm21.
3. RESULTS AND ANALYSES
Figures 1a– d show survey spectra of the b1S1(b01) 3
X S2(X101, X21) transitions of the antimony monohalides
measured at a resolution of 0.5 cm21 with the Ge detector.
The spectra are not corrected for the relative sensitivity of
the detection system in the corresponding wavenumber
ranges. As is typical for chemiluminescence measurements,
the spectra do not contain any atomic lines which are
3
inevitable in discharge sources. Except for that of SbF, the
b 3 X spectra contain band sequences of both subtransitions
b 3 X1 and b 3 X2. The very weak 0 – 0 band of the b 3 X1
system of SbF was observed in spectra measured at higher
resolution only. For SbF, SbCl, and SbBr, the b 3 X1
subtransitions are much weaker than the b 3 X2 transitions,
for SbI the opposite is true. When accounting for the decrease of the sensitivity of the Ge detector with increasing
wavenumber, the ratios of the integrated intensities of the
Dv 5 0 sequences of the b 3 X1 and b 3 X2 transitions are
found to be 0.012 (SbF), 0.30 (SbCl), 0.025 (SbBr), and 17
(SbI), approximately.
Figure 2a shows the Dv 5 0 sequence of the b 3 X 2
transition of SbF. The bands consist of broad P and R branches
and narrow Q branches with strong maxima. The sequence
extends to the 13–13 band showing a very hot vibrational
population in the b0 1 state. The sequence of bands first
extends to higher wavenumbers, then forms a head of bands
and turns around. As is seen from the equation for the band
wavenumbers in a Dv 5 0 sequence [1], this can happen if ( v 9e
2 v e x9e ) . ( v 0e 2 v e x 0e ) and v e x9e . v e x 0e , i.e., if the terms
linear and quadratic in v have different signs,
Copyright © 1999 by Academic Press
ANTIMONY HALIDES
149
FIG. 2. Spectra of the Dv 5 0 sequences of the b 3 X 2 systems of SbF (a), SbCl (b), and SbBr (c), and the b 3 X 1 system of SbI (d) with assignments
of the Q-branch maxima. The resolution is 0.2 cm21.
n v 3v 5 T9v 2 T 0v 5 n 00 1 @~ v 9e 2 v e x9e!
2 ~ v 0e 2 v e x 0e!#v 2 @ v e x9e 2 v e x 0e#v 2.
@1#
The equation and the conditions become somewhat more difficult if terms cubic in v 1 21 with parameters v e y e are considered in the energy formulas. With more than a factor of 100
lower intensity, some weak bands of the Dv 5 21 sequence are
observed near 12 250 cm21. The vibrational analysis of the
system is made more difficult by the fact that antimony consists
of two stable isotopes, 121Sb and 123Sb, with relative abundances of 57.3 and 42.7%. The vibrational isotope effect leads
to a broadening of the Q-branch maxima in the Dv 5 0 bands
(Fig. 2a) and to a splitting of the maxima in all bands with Dv
Þ 0.
For all antimony halides, the new a 1 D(a2) 3 X 2 1 transitions were observed under experimental conditions optimized
for maximum intensity of the b 3 X bands. Except for SbI,
only the Dv 5 0 sequences of the a 3 X 2 systems were
observed, indicating that the a2 and X 2 1 states have very
similar potential curves. Figure 3a shows the Dv 5 0 sequence
of the a 1 D(a2) 3 X 2 1 transition of SbF. The bands consist of
broad P and R branches and very narrow Q branches, which
can easily be assigned up to the 7–7 band. The narrow Q
branches reveal that the difference of the rotational constants
B v in the a2 and X 2 1 states is small and the potential curves
have very similar r e values.
Rotational analyses have been performed for a few uv/vis
bands of SbF only (10 –12); therefore, no accurate vibrational
constants are known for any of the low-lying states. In our
analyses, we have chosen to fix the vibrational parameters of
the X 2 1 state to the values v 0e 5 612.6 cm21, v e x 0e 5 2.6
cm21 as recommended by Wang et al. (10) and Prévot et al.
(11). The wavenumbers of the maxima of the Q branches read
from the spectra were corrected by 20.3 cm21 to get approximate band origins (Table 1). For the 0 – 0 and 1–1 bands of the
b 3 X 2 system, the more accurate band origins from the
rotational analyses of Wang et al. (10) were used. The results
of the three-parameter (a 3 X 2 ) and four-parameter (b 3 X 2 )
fits are given in Table 2. The T e values of the X 2 1, a2, and
b0 1 states were calculated from the fitted parameters T e (a) 2
T e (X 2 ) and T e (b) 2 T e (X 2 ) by use of the vibrational constants
of the X 1 0 1 state ( v e 5 609.0 cm21, v e x e 5 2.6 cm21), and
Copyright © 1999 by Academic Press
150
BEUTEL, SETZER, AND FINK
TABLE 1
Vacuum Wavenumbers of the Band Origins of the a 3 X2 and b 3 X2 Bands of
121
SbF (in cm21)
a
Numbers in parentheses are the obs-calc differences in units of the last digit.
b
From Ref. 10.
the origin of the 0 – 0 band of the b 3 X 1 transition (n00 5
13 653.7 cm21) given by Wang et al. (10), the latter value
having been confirmed by our spectra.
Figure 2b shows the Dv 5 0 sequence of the b 3 X 2
transition of SbCl measured at a resolution of 0.2 cm21. In both
subsystems, the bands show the expected structure and have
heads in the P branches. The small splitting of the P-branch
head of the 0 – 0 band of the b 3 X 2 system suggests that the
X 2 1 state exhibits V doubling. The P-branch heads of the weak
0 –1 and 1–2 bands show isotope splitting corresponding to the
four isotopic species 121Sb35Cl, 121Sb37Cl, 123Sb35Cl, and
123
Sb37Cl. The vacuum wavenumbers of the band origins of
121
Sb35Cl deduced from the spectra are collected in Table 3.
For SbCl, SbBr, and SbI, the a 1 D(a2) 3 X 2 1 spectra lie
outside the sensitivity range of the Ge detector (5900 –14 000
cm21) and had to be measured with the InSb detector, which is
about a factor of 10 less sensitive. Figure 3b shows the Dv 5
0 sequence of the a 3 X 2 system of SbCl. The bands consist
of broad R and P and rather narrow Q branches. The origins
of five bands deduced from spectra measured at different
spectral resolution are given in Table 3.
In the analysis of the SbCl spectra, we fixed the vibrational
constants of the X 1 0 1 ground state to the values v e 5 372.55
cm21, v e x e 5 1.15 cm21 deduced from analysis of highresolution spectra of a DV 5 0 system near 500 nm by Balfour
and Ram (16). From a three-parameter fit of the b 3 X 1 bands,
the vibrational constants of the b state were obtained. These
were fixed in the fit of the b 3 X 2 bands to derive v e and v e x e
values of the X 2 state, which finally were fixed in the fit of the
a 3 X 2 bands. The results are given in Table 2.
Figure 2c shows the Dv 5 0 sequence of the b 3 X 2
transition of SbBr. The b 3 X 1 transition is very weak, the
only bands observed are 0 – 0, 0 –1, 1–2, and 2–3. In the latter
three bands, the P-branch heads again are split into four
belonging to the four isotopic species 121Sb79Br, 121Sb81Br,
123
Sb79Br, and 123Sb81Br, which have similar abundances. The
b 3 X 2 subsystem is stronger, and six bands, 0 – 0, 1–1, 2–2,
0 –1, 1–2, and 2–3, are observed. The P-branch heads of the Dv
5 21 bands again show isotope splitting. Vacuum wavenumbers of the band origins of 121Sb79Br are collected in Table 4.
In the Dv 5 0 sequence of the a 1 D(a2) 3 X 2 1 transition (Fig.
3c), Q-branch maxima of four bands, 0 – 0, 1–1, 2–2, and 3–3,
are observed. The P branch of the 0 – 0 band is narrower than
for SbF and SbCl and forms a head. Due to overlapping of the
branches, the accuracy of the wavenumbers of band origins
(Table 4) derived from the Q-branch maxima is low (about
60.5 cm21).
There are no reliable literature data for the vibrational constants of the X 1 and X 2 states of SbBr. Although our band
wavenumbers are not very accurate, they allow calculation of
a complete set of electronic energies and vibrational constants
for the low-lying states of 121Sb79Br. From a fit of the b 3 X 2
bands, v e and v e x e values of the b and X 2 states were
obtained. These parameters were fixed in the fits of the b 3 X 1
and a 3 X 2 bands to get the electronic energies and vibrational constants of the X 1 and a states. The results are collected
in Table 2.
Due to the smaller vibrational constants, a larger number of
bands show up in the spectra of SbI. Figure 2d shows the Dv
5 0 sequence of the b 3 X 1 system. In both subsystems, the
bands display sharp heads in the P branches which in the Dv 5
11, 21, and 22 sequences split into two, belonging to the
121
SbI and 123SbI isotopic species. A total of 18 bands of the
b 3 X 1 and 10 bands of the b 3 X 2 system of 121SbI could
be measured (Table 5). Sharp P-branch heads likewise are the
characteristic features in the a 3 X 2 bands of SbI (Fig. 3d).
Different from the lighter molecules, weak Dv 5 11 and Dv 5
21 sequences are observed, such that a total of 11 bands could
be measured (Table 5). Least-squares fits allowed determination of a complete set of electronic energies and vibrational
constants of the four low-lying states involved (Table 2).
4. DISCUSSION
The identification of the transitions and the assignment of
the bands observed in the present chemiluminescence work is
unambiguous. Therefore, the electronic energies and vibra-
Copyright © 1999 by Academic Press
151
ANTIMONY HALIDES
TABLE 2
Spectroscopic Constants of the X101, X21, a2, and b01 States of
121
Sb35Cl, 121Sb79Br, and 121SbI (in cm21)
tional constants derived represent a first complete and reliable
set of molecular constants for the four low-lying states of the
antimony halides. The absolute accuracy of the wavenumbers
TABLE 3
Vacuum Wavenumbers of the Band Origins of the b 3
X1, b 3 X2, and a 3 X2 Bands of 121Sb35Cl (in cm21)
a
Numbers in parentheses are the obs-calc differences in units of
the last digit.
121
SbF,
of band origins derived from the medium-resolution spectra is
estimated to be 60.5 cm21. The overall error limits of the
molecular parameters obtained from the fits likely are larger by
a factor of two or three than the statistical standard deviations
given in Table 2. So the T e and v e values are accurate to about
61 cm21. Within the error limits, the new data agree with the
less accurate results of our previous measurements performed
with low-resolution monochromators (14, 15). The T e and v e
values show similar systematic trends like in the previously
studied bismuth halides (9). For 121SbF, our results are in good
agreement with the literature data summarized by Prévot et al.
(11). Boustani et al. (18) and Das et al. (8) have performed
detailed relativistic spin– orbit configuration interaction studies
of the potential curves and transition probabilities for the
electronic states and transitions of SbF and SbI. For both
molecules, the calculated energies of the a2 and b0 1 states are
by 1500 –2000 cm21 too high and the vibrational constants
mostly are by about 10% too small.
As in the case of the previously studied bismuth halides
BiCl, BiBr, and BiI (9), the new a 1 D(a2) 3 X 2 1 spectra of the
antimony halides show surprisingly high intensities. When
accounting for the much lower sensitivity of the InSb detector,
Copyright © 1999 by Academic Press
152
BEUTEL, SETZER, AND FINK
FIG. 3. Spectra of the Dv 5 0 sequence of the a 3 X 2 systems of SbF (a), SbCl (b), SbBr (c), and SbI (d) with assignments of the Q-branch maxima. The
resolution is 0.2 cm21 (a, b) and 0.5 cm21 (c, d).
the a 3 X 2 bands are found to have intensities similar to those
of the strongest b 3 X bands measured with the Ge detector.
From the theoretical work (8, 18), the radiative lifetimes of the
TABLE 4
Vacuum Wavenumbers of the Band Origins of the b 3
X1, b 3 X2, and a 3 X2 Bands of 121Sb79Br (in cm21)
a
Numbers in parentheses are the obs-calc differences in units of
the last digit.
a2 states of SbF and SbI are predicted to be 490 and 9.2 ms and
those of the b0 1 states 0.057 and 0.016 ms, respectively. So
the transition probabilities of the a 3 X 2 bands are by factors
of '8600 (SbF) and '580 (SbI) smaller than those of the b 3
X bands. Therefore, the observed intensities of the a 3 X 2 and
b 3 X bands show that the concentrations of the excited
species in the a2 states are by about a factor of 1000 higher
than those of molecules in the b0 1 states. This suggests that
the long-living a2 states likewise are stable toward collisional
quenching in the gas phase and at the wall of the reaction tube.
Such behavior would be quite similar to what is well known for
the metastable a 1 D ( g) states of O2 and NF. Like these species,
antimony halide molecules in the metastable a2 states, therefore, could be interesting energy carriers in energy transfer
processes and chemical reactions. Due to the low frequency
response of the sensitive Ge and InSb detectors, the weak
a2 3 X 2 emissions are not suited for time-resolved studies of the a2 states. For SbF, several higher lying states
are known, which combine with the a2 state (1) and thus
can be used to monitor the concentration of a2 state molecules
by LIF in kinetic experiments. Knowledge of the electronic
energies and vibrational constants of the a2 states of SbCl,
Copyright © 1999 by Academic Press
153
ANTIMONY HALIDES
TABLE 5
Vacuum Wavenumbers of the Band Origins of the b 3
X1, b 3 X2, and a 3 X2 Bands of 121SbI (in cm21)
ACKNOWLEDGMENTS
Financial support of this work by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie is gratefully acknowledged. Special
thanks are due to Professor R. J. Buenker and Dr. A. B. Alekseyev for making
their results available to us prior to publication and for many discussions.
REFERENCES
a
Numbers in parentheses are the obs-calc differences in units of
the last digit.
SbBr, and SbI will help to find such transitions also for these
molecules and to develop spectroscopic methods for kinetic
studies.
1. K. P. Huber and G. Herzberg, “Molecular Spectra and Molecular Structure, Vol. 4, Constants of Diatomic Molecules,” Van Nostrand–Reinhold,
New York, 1979.
2. M. Bielefeld, G. Elfers, E. H. Fink, H. Kruse, J. Wildt, R. Winter, and F.
Zabel, J. Photochem. 25, 419 – 438 (1984).
3. O. Shestakov, R. Gielen, K. D. Setzer, and E. H. Fink, J. Mol. Spectrosc.
192, 139 –147 (1998).
4. W. E. Jones, Can. J. Phys. 45, 21–26 (1967).
5. A. T. Pritt, Jr., D. Patel, and R. D. Coombe, J. Mol. Spectrosc. 87,
401– 415 (1981).
6. A. B. Alekseyev, R. J. Buenker, H.-P. Liebermann, and G. Hirsch,
J. Chem. Phys. 100, 2989 –3001 (1994).
7. A. B. Alekseyev, K. K. Das, H.-P. Liebermann, R. J. Buenker, and G.
Hirsch, Chem. Phys. 198, 333–344 (1995).
8. K. K. Das, A. B. Alekseyev, H.-P. Liebermann, G. Hirsch, and R. J.
Buenker, Chem. Phys. 196, 395– 406 (1995).
9. M. Beutel, K. D. Setzer, O. Shestakov, and E. H. Fink, J. Mol. Spectrosc.
175, 48 –53 (1996).
10. D. K. W. Wang, W. E. Jones, F. Prévot, and R. Colin, J. Mol. Spectrosc.
49, 377–387 (1974).
11. F. Prévot, R. Colin, and W. E. Jones, J. Mol. Spectrosc. 56, 432– 440 (1975).
12. R. Vasudev and W. E. Jones, J. Mol. Spectrosc. 59, 442– 458 (1976).
13. F. Prévot, Thesis, Université Libre de Bruxelles, 1974.
14. H. Kruse, R. Winter, E. H. Fink, J. Wildt, and F. Zabel, Chem. Phys. Lett.
93, 475– 479 (1982).
15. R. Winter, H. Kruse, E. H. Fink, J. Wildt, and F. Zabel, Chem. Phys. Lett.
104, 383–388 (1984).
16. W. J. Balfour and R. S. Ram, J. Mol. Spectrosc. 130, 389 –394 (1988).
17. V. Kaufman and B. Edlén, J. Phys. Chem. Ref. Data 3, 825– 895 (1974).
18. I. Boustani, S. N. Rai, H.-P. Liebermann, A. B. Alekseyev, G. Hirsch, and
R. J. Buenker, Chem. Phys. 177, 45–52 (1993).
Copyright © 1999 by Academic Press