Millimeter-wave spectroscopy of vibrationally excited ground state
alkaline-earth hydroxide radicals ( X 2 S 1 )
D. A. Fletcher,a) M. A. Anderson,b) W. L. Barclay, Jr., and L. M. Ziurysc)
Department of Chemistry, Arizona State University, Tempe, Arizona 85287-1604
~Received 23 September 1994; accepted 5 December 1994!
Pure rotational spectra of the alkaline-earth monohydroxides have been recorded for vibrationally
excited states ~0 1 0!, ~0 2 0!, ~0 3 0!, and ~1 0 0! of the ground electronic state (X 2 S 1 ) using
millimeter-wave absorption spectroscopy. The radicals MgOH, CaOH, SrOH, and BaOH were
studied. The data for CaOH, SrOH, and BaOH were analyzed with a linear 2 S 1 model, but with the
addition of two terms to account for contamination of the v 2 51 2 P and v 2 52 2 D vibronic levels
with 2 P and 2 D electronic states. The data for MgOH, however, did not fit well to this linear model
and is additional evidence that this species is quasilinear. © 1995 American Institute of Physics.
I. INTRODUCTION
Bonding in the alkaline-earth monohydroxide radicals is
expected to be ionic ~M1OH2!, with the OH2 moiety behaving as a pseudo halide ion. Indeed, assignment of the strong
visible systems of CaOH, SrOH, and BaOH were based on
similarities with the isoelectronic ionic monohalides.1 More
recently, with the advent of the flowing metal vapour production technique of Harris et al.,2 there has been much Doppler
limited visible spectroscopy of these species, particularly of
the A 2 P – X 2 S 1 and B 2 S 1 – X 2 S 1 systems. MgOH,3
CaOH,4 –7 SrOH,8 –10 and BaOH ~Refs. 11–13! have been
studied in this way. The ionic form should have a linear
geometry and the optical systems are consistent with this
structure, with the exception of MgOH. This molecule is
more covalent due to the higher ionization potential of magnesium, thus tending towards a bent geometry. Optical studies by Ni ~Ref. 3! suggest that MgOH has a quartic bending
potential, and hence is quasilinear.
Recently, pure rotational spectra in the millimeter/
sub-mm frequency range ~65– 400 GHz! have been recorded
in this laboratory for CaOH,14 MgOH,15 SrOH,16 and
BaOH.17 These data, including that for MgOH, were analyzed consistently in terms of a linear X 2 S 1 ground state,
again suggestive of ionic bonding. New electronic studies at
rotational resolution confirm these findings.18 –23 More recently, development of a supersonic beam source for the
hydroxides24 has allowed for a determination of the proton
hyperfine structure of the ground electronic states of CaOH
~Ref. 25! and SrOH,26 as well as their dipole moments.27 The
very small values of the proton hf parameters suggest that
the electron density of the unpaired electron chiefly resides
on the metal atom, confirming the M1OH2 ionic structure
for these radicals. In fact, the millimeter-wave studies of
25
MgOH ~Ref. 15! have resulted in the determination of the
hyperfine constants for the 25 Mg nucleus, including a Fermi
contact term of b F 52304.0 MHz, which is much larger
than the same parameter for the proton in CaOH and SrOH.
a!
Present Address: Chemical Database Service, Daresbury Laboratory,
Daresbury, Warrington, Cheshire, WA4 4AD, United Kingdom.
b!
NASA Space Grant Fellow.
c!
NSF Presidential Faculty Fellow.
4334
J. Chem. Phys. 102 (11), 15 March 1995
This value indicates 51% s-orbital occupancy for the unpaired electron at the 25 Mg nucleus.
Ab initio calculations28 –30 predict that all but BeOH will
be linear in their ground electronic states, except that MgOH
is calculated to have a quartic bending potential, as confirmed by experiment.3 Using the quasilinear parameter defined by Yamada and Winnewisser,31 CaOH, SrOH, and
BaOH will be linear but MgOH will be quasilinear. Thus,
both theoretically and experimentally MgOH is certainly different from the hydroxides of the later members of the alkaline earth group.
Only very recent experiments have examined the vibrationally excited levels of the ground electronic states of the
monohydroxides in any detail. The ~0 1 0! and ~0 2 0! vibrational modes have been studied in CaOH ~Refs. 20, 22, 23!,
as well as for SrOH.32
Here we report on millimeter and submillimeter wave
rotational spectra of vibrationally excited MgOH, CaOH,
SrOH, and BaOH. Rotational transitions arising from several
quanta of the bending and heavy-atom stretching modes
were recorded in the frequency range 278 –380 GHz. The
CaOH, SrOH, and BaOH analysis was done assuming linear
geometry. However, for MgOH, the data did not fit well with
this model, presumably because of the quasilinear nature of
the species. A complete analysis of the MgOH spectra will be
presented in a future publication.
II. EXPERIMENT
The rotational transitions of vibrationally excited
MgOH, CaOH, SrOH, and BaOH were measured using a
millimeter wave direct absorption spectrometer, operating in
the range 65– 400 GHz. This instrument has been used in
previous studies of the alkaline-earth hydroxides14 –17 and is
described in detail elsewhere.33
The instrument consists of three sections, radiation generation, sample cell, and detector. The sources of radiation
are tunable Gunn oscillators ~65–115 GHz!, phase-locked to
a signal generator operating near 2 GHz. Frequency multipliers provide additional spectral coverage from 130– 400
GHz. The source is frequency modulated at 25 kHz, for
phase-sensitive detection, and is transmitted quasioptically
using scalar feed horns and Teflon lenses.
0021-9606/95/102(11)/4334/6/$6.00
© 1995 American Institute of Physics
Fletcher et al.: Spectroscopy of earth hydroxide radicals
The sample cell is a 0.5 m section of pipe with the ends
sealed with Teflon lenses. The radiation makes a double-pass
through the cell via a rooftop reflector affixed to one end.
Since the reflection imparts a 90° phase change on the millimeter waves, a wire polarizing grid can be used to separate
out the returning radiation which is then focused onto a liquid helium-cooled InSb detector.
The alkaline-earth hydroxide radicals were produced inside the sample cell via the reaction of metal atoms with
hydrogen peroxide. The metal atoms themselves were generated by heating solid metals in a Broida-type oven. The
metal vapor was entrained in ;20–30 mTorr of argon carrier
gas and reacted with ;50 mTorr of H2O2 .
The transition frequencies were measured to an estimated accuracy of 6100 kHz for the ~0 1 0! modes and
6150 kHz for the other vibrational states. The main sources
of error are in the determination of the center frequency of a
given line, which were fit with Gaussian profiles, and the
intrinsic frequency stability of the 2 GHz signal generator.
4335
FIG. 1. Spectrum of the N527→28 rotational transition originating in the
l-type doubled ~0 22 0! excited vibrational mode of BaOH (X 2 S 1 ) near
362 GHz. Each l-type doublet, which is labeled by the notation ~0 22 0!
lower or ~0 22 0! upper, consists of two fine structure components labeled by
the quantum number J. The spectrum represents one single 5 min scan.
III. RESULTS
The data for the ~0 0 0! measurements have been published previously.14 –17 For the excited vibrational modes,
both spin-rotation components were observed for every transition recorded, as well as both l-type doublets for each
bending vibration measured. For CaOH and BaOH, six transitions were recorded for each vibrational mode, while only
five were observed for SrOH. For MgOH, only four transitions of the ~0 11 0! mode were analyzed, along with the
~0 0 0! data. ~Additional lines in this mode and data from the
other vibrational levels of MgOH could not be successfully
fit and hence were not used in the analysis.! The values for
nobs2ncalc for the vibrationally excited transitions were in the
following ranges ~in MHz!: CaOH, 60.000–0.115; SrOH,
60.000–0.085; BaOH, 60.000–0.052; MgOH, 60.002–
0.029. The actual line frequencies for all hydroxide transitions studied will be published elsewhere.34 The linewidths
of the spectral features varied from about 200 to 800 kHz,
being generally larger at higher frequencies primarily due to
modulation broadening. A typical spectrum is given in Fig. 1,
which shows the N527→28 rotational transition originating in the l-doubled ~0 22 0! mode of BaOH near 362 GHz.
The transition is split into four components because of l-type
doubling and spin–rotation interactions.
The pattern of lines seen for each rotational transition
was fairly similar for each molecule observed except for
MgOH. The strongest pair of lines occurred at higher frequency and were assigned as the F 1 – F 1 and F 2 – F 2 pure
rotational transitions in the ~0 0 0! vibrational level of the
X 2 S 1 electronic ground state. They have been previously
recorded and analyzed for each molecule.14 –16 To lower frequency, a number of weaker features were observed, while
no lines were observed at higher frequency. A typical pattern
is illustrated with a stick diagram in Fig. 2. All of the lines
recorded occurred in spin–rotation pairs, with a splitting approximately equal to the spin–rotation constant of the ~0 0 0!
mode, and were assigned as F 1 – F 1 and F 2 – F 2 rotational
transitions of various vibrationally excited levels of the
ground electronic state. Two pairs occur for the ~0 11 0! vi-
brational level, which is the bending mode and thus is split
by the l-type doubling interaction. Similarly, two pairs are
observed for the ~0 22 0! level, but here the l-type doubling is
much smaller. A set of ~0 20 0! lines and of ~1 0 0! lines
@following the vibrational notation of Ref. 20, ~1 0 0! is the
heavy atom stretch# complete the set of transitions observed
for all of the molecules. Additional features and effects were
noticed for individual molecules as detailed below.
For SrOH additional, weaker lines were present, one pair
near the ~0 20 0! lines and one pair near the ~1 0 0! lines, for
most of the rotational spectra recorded. These are due to
transitions in the ~0 31 0! levels. The approximate rotational
constant is consistent with what is expected from the analysis
of the v 2 50, 1, and 2 data and the l-type splitting is approxi-
FIG. 2. A stick diagram showing the progression of rotational transitions
originating in the ground state ~0 0 0! and vibrationally excited modes
~0 1 0!, ~0 2 0!, ~0 3 0!, and ~1 0 0! observed in this work for the
N524→25 transition of SrOH near 371–373 GHz. The heights of the
sticks qualitatively indicate the observed relative intensities. Each rotational
transition consists of doublets arising from spin–rotation interactions in the
molecule.
J. Chem. Phys., Vol. 102, No. 11, 15 March 1995
Fletcher et al.: Spectroscopy of earth hydroxide radicals
4336
F 1 ~ N ! 5B v @ N ~ N11 ! 2l 2 # 2D v @ N ~ N11 ! 2l 2 # 2
11/2 g v N11/2 g D N 2 ~ N11 ! ,
~1!
F 2 ~ N ! 5B v @ N ~ N11 ! 2l 2 # 2D v @ N ~ N11 ! 2l 2 # 2
21/2 g v ~ N11 ! 21/2 g D N ~ N11 ! 2 .
~2!
The centrifugal correction to the spin–rotation parameter, g D , was only required in the case of BaOH and MgOH.
Additional terms to model the l-type doubling in the
v 2 .0 levels were taken from Presunka and Coxon.32 For the
~0 11 0! level, the l-type splitting is given by
DE ~ l-type! 561/2q v N ~ N11 ! 61/2q v D @ N ~ N11 !# 2 ,
~3!
where the 1 sign refers to the F 1 (e) and F 2 ( f ) levels and
the negative sign to the F 1 ( f ) and F 2 (e) levels.
For v 2 52 level, the interaction between the l52 and
l50 component is modeled by two 333 matrices, one for
the e levels and one for the f levels, with the off diagonal
elements being given by37,38
FIG. 3. A qualitative energy level diagram of lower vibrational manifold of
CaOH (X 2 S 1 ). The quantum numbers ( v 1 v 2 v 3 ) and vibronic symmetry
are given for each level.
^ v 1 , v l2 , v 3 ;N,l u H u v 1 , v l62
2 , v 3 ;N,l62 &
51/4q v $ ~ v 2 7l !~ v 2 6l12 !@ N ~ N11 ! 2l ~ l11 !#
3 @ N ~ N11 ! 2 ~ l61 !~ l62 !# % 1/2 .
mately twice that of the ~0 1 0! lines, consistent with the
v 2 11 dependence of this phenomenon.35
An additional effect for SrOH and CaOH was a slight
difference in the spin–rotation splitting between the two
l-type doublets in the ~0 11 0! vibrational level. The splitting
is larger in the lower frequency set of lines.
For BaOH, an additional pair of lines were present between the ~1 0 0! and ~0 22 0! lines, in the correct position for
the ~0 33 0! lines, although no l-type doubling was observed.
A difference in the spin–rotation splitting of the two l-type
components of the ~0 11 0! level was also present in this
radical, but is substantially larger. In addition, a similar effect
was just apparent in the l-type components of the ~0 22 0!
level for BaOH.
For MgOH, the pattern deviates noticeably from the
other three species. While transitions originating in the
~0 1 0! and ~0 2 0! modes were observed, there were many
other doublets with approximately the correct spin–rotation
splitting which could not be identified. Also, the difference in
the spin–rotation splitting of the l-type doublets of the bending mode was twice as large as that of BaOH.
A representative energy level diagram qualitatively
showing the various vibrational modes observed in this work
for CaOH is shown in Fig. 3.
1
IV. DATA ANALYSIS
The data for each molecule was fit using an effective
Hamiltonian. The data were analyzed separately for each vibrational level and the derived constants combined to produce vibrational dependencies. The basic Hamiltonian for
each vibrational level was a conventional Hund’s case ~b!
2
S Hamiltonian, in which the two energy levels for each
rotational level (N) are given by36
~4!
In this model, the ~0 20 0!–~0 22 0! vibrational energy splitting is required. This is equal to four times g 22 . Values of
g 22 for CaOH ~Ref. 20! and SrOH ~Ref. 31! are available
from the literature. For BaOH, however, the separation between the ~0 20 0! and ~0 22 0! levels is not known. Consequently, the l-type doubling in v 2 52 can only be described
by an effective parameter q v ,eff , which gives rise to a
splitting32
61/2q v ,eff$ N ~ N11 !@ N ~ N11 ! 22 # % .
~5!
To account for the observed difference in effective spin–
rotation constants in the v 2 51 l-type doublets, a p P constant
was introduced with the same functional form as the lambadoubling constant p in a 2 P electronic state, as proposed for
C2H.39,40 For BaOH, it was necessary to introduce a similar
p D parameter for the ~0 22 0! level corresponding to the
~much smaller! lambda-type doubling parameter in an electronic 2 D state.
The parameters determined for the various vibrational
levels of each molecule, analyzed using these Hamiltonian
terms, are given in Table I. The overall rms of the fit for each
species is also given in the table.
The vibrational dependence of the rotational constant,
B v , in a linear triatomic has been considered previously by
Lide and Matsumura.41 A reliable representation of the v 2
dependence has been found from microwave spectroscopy to
be
B v 5B̄ e 2 a 2 ~ v 2 11 ! 1 g 22 ~ v 2 11 ! 2 1 g ll l 2 ,
~6!
where B̄ e 5B e 21/2 a 1 21/2 a 3 . Values of these parameters
derived from the experimentally determined rotational constants are presented in Table II for each molecule. ~The a1
term is also given.! For SrOH, only data up to v 2 52 was
J. Chem. Phys., Vol. 102, No. 11, 15 March 1995
Fletcher et al.: Spectroscopy of earth hydroxide radicals
4337
TABLE I. Molecular constants for alkaline-earth monohydroxide species (X 2 S 1 ).a
( v 1v 2v 3)
~0 0 0!b
~0 11 0!
~0 20 0!
~0 22 0!
~0 31 0!
~0 33 0!~?!
~1 0 0!b
a
MgOH
B
D
g
gD
B
D
g
gD
qv
q vD
pP
B
D
g
gD
B
D
g
gD
qv
q v ,eff
pD
B
D
g
qv
q vD
pP
B
D
g
gD
B
D
g
gD
RMS of fit
CaOH
14 822.516 7~17!
0.026 225~7!
37.602~24!
•••
14 757.177 7~30!
0.026 535~11!
41.79~13!
23.07~32!31023
285.123~6!
3.74~21!31024
2.20~5!
SrOH
10 023.084 1~10!
0.011 570 7~20!
34.765~19!
•••
9 996.751 8~17!
0.011 769 6~29!
35.051~21!
•••
221.649 2~34!
6.4~6!31025
20.050~42!
9 982.838 7~23!
0.011 923 9~40!
35.045~29!
•••
9 969.396 7~16!
0.011 933 9~28!
35.569~20!
•••
221.131~17!
7 470.822 5~6!
0.006 518 6~7!
72.774~16!
•••
7 452.243 0~20!
0.006 623 8~19!
72.240~22!
•••
211.854 6~41!
2.47~37!3 1025
21.03~4!
7 440.989 7~29!
0.006 725 5~27!
71.736~32!
•••
7 433.286 6~20!
0.006 718 3~19!
71.589~22!
•••
211.934~16!
BaOH
6 493.775 1~5!
0.004 924 7~4!
71.325~27!
2.31~20!31024
6 485.264 0~15!
0.005 010 3~11!
68.65~16!
4.8~8!31024
29.493 2~31!
2.33~21!31025
22.66~4!
6 482.486 9~22!
0.005 204 0~15!
68.06~23!
3.0~1.1!31024
6 476.377 0~16!
0.005 038 3~11!
64.04~16!
0.001 08~8!
•••
1.026 9~28!31024
25.5~2.0!31022
7 429.630 7~20!
0.006 886 8~19!
71.135~22!
212.484 0~20!
3.42~19!3 1025
21.67~4!
9 956.359 3~23!
0.011 648 4~39!
34.549~29!
0.018
7 426.907 1~29!
0.006 538 1~26!
72.210~32!
0.035
0.020
6 469.245 0~22!
0.005 187 9~15!
67.52~23!
0.000 54~11!
6 460.661 9~22!
0.004 939 8~15!
70.89~23!
2.5~1.1!31024
0.015
In MHz; number in parentheses represent one standard deviation of the least squares fit in units of the last quoted decimal place.
Data are taken from Refs. 14 –17 and refit to produce ~0 0 0! constants.
b
used, although the predicted rotational constant for the
~0 31 0! level given in Table II is almost identical with that
derived experimentally ~see Table I!.
Although there are only as many pieces of data as parameters, for each molecule the various terms scale approximately with B, suggesting that there is some justification for
the above expression. Moreover, for SrOH, the calculated
TABLE II. Vibrational dependence of B v .a
b
B̃ e
a1
a2
g22
g ll
Predicted:
B(0 3 1 0)
B(0 3 3 0)
a
CaOH
SrOH
BaOH
10 085.12~10!
66.725~4!
31.518~3!
2.849~3!
23.361~1!
7 512.907~10!
43.915~4!
21.865~3!
1.737~3!
21.926~1!
6 519.993~10!
33.113~4!
11.001~3!
1.339~3!
21.527~1!
7 429.359~30!
7 413.953~30!
6 479.333~30!
6 467.113~30!
9 967.902~30!
9 941.018~30!
All values in MHz.
B̃ e 5B e 21/2 a 3 .
b
~0 31 0! rotational constant is almost identical to the experimentally measured one, as mentioned, although this is not
the case for BaOH ~0 33 0! data. However, these lines are not
very intense and may not actually be the ~0 33 0! lines. No
l-type doubling is observed for the possible ~0 33 0! transitions, but based on the value of q v and g 22 determined below, we would not expect to resolve the doublets in this
experiment.
Table III shows a comparison of molecular constants determined in this work with those derived previously from
optical spectra. In general, our parameters agree with the
older data, but are much better determined. In certain cases,
the parameters differ significantly; this is probably due to
correlations with electronically excited state parameters in
the analysis of the optical data.
V. DISCUSSION
One of the more significant points to notice in this analysis is the positive sign for a2 , the vibration-rotation constant
for the bending mode. This parameter is usually negative.42 A
J. Chem. Phys., Vol. 102, No. 11, 15 March 1995
Fletcher et al.: Spectroscopy of earth hydroxide radicals
4338
TABLE III. Comparison of constants with previous work.a
Molecule
CaOH
SrOH
Parameter
This work
B(1 0 0)
B(0 2 0 0)
B(0 2 2 0)
B(0 1 1 0)
D(0 1 1 0)
q v (0 1 1 0)
a1
a2
g ll
B(0 1 1 0)
D(0 1 1 0)
g~0 11 0!
q v (0 1 1 0)
q v D (0 1 1 0)
B(0 2 0 0)
D(0 2 0 0)
g~0 22 0!
B(0 2 2 0)
D(0 2 2 0)
g~0 22 0!
q v (0 2 0)
Optical
9 956.359 3~23!
9 982.838 7~23!
9 969.396 7~16!
9 996.751 8~17!
0.011 769 6~29!
221.649 2~34!
66.725~4!
31.518~3!
23.361~1!
7 452.243 0~20!
0.006 623 8~19!
72.240~22!
211.854 6~41!
2.47~37!3 1025
7 440.989 7~29!
0.006 725 5~27!
71.736~32!
7 433.286 6~20!
0.006 718 3~19!
71.589~22!
211.934~16!
All values in MHz; errors quoted are 1s.
Reference 20.
c
Reference 23.
9 958.8~9!b
9 984.0~1.2!b
9 968.7~9!b
9 999.4~7!c
0.013 7~6!c
221.24~26!c
65.95~30!b
19.5~2!c
23.8~4!c
7 452.22~4!d
7 452.5~1.2!e
d
0.006 624~10!
0.006 56~24!e
72.2~4!d
6~18!e
d
211.843~14!
12.1~4!e
1.8~4!31025d
7 439.71~27!d
0.006 28~11!d
70~5!d
7 433.95~33!d
0.007 00~14!d
93~5!d
214.7~2.1!d
a
d
b
e
positive a2 is consistent with the observations of Presunka
and Coxon for SrOH ~Ref. 32! and CaOH ~Ref. 20! and
those of Lide and Matsumura,41 who observed a similar effect for the alkali hydroxides. They suggest that this effect is
due to a dominant positive anharmonic contribution to a2 ,
arising from the small hydrogen mass and the small force
constant for the M–O bond. This is likely to be the case for
the alkaline-earth hydroxides as well.
The vibrational dependencies of D v and g v are less
meaningful. If they are modeled with equations similar to
Eq. ~6!, the parameters determined are vastly different in
each molecule. This suggests that these two molecular parameters are contaminated to different degrees in each molecule by other effective Hamiltonian terms, and are not
‘‘pure’’ centrifugal distortion and spin–rotation constants.
It is apparent that the main features of the rotational
patterns of these hydroxide radicals are not influenced by
Fermi resonance effects. The ~1 0 0!–~0 2 0! splitting is comparatively large ~100–200 cm21! and the pattern of rotational
constants is not consistent with such interactions.42 As shown
in Fig. 2, the ~0 20 0! rotational lines appear at a higher frequency than the ~0 22 0! mode. If the ~0 20 0! mode was interacting with the ~1 0 0! level, as would be expected in a
Fermi resonance type effect, rotational transitions arising
from this mode would be to the left of the ~0 22 0! lines.
Moreover, according to Jarman and Bernath,22 the ~0 20 0!
and ~0 22 0! orientation is opposite to that which would be
expected from a ~1 0 0!–~0 2 0! Fermi resonance. We do not,
however, have a sufficiently large data set to detect any small
interactions of this type. Such effects would be absorbed into
the effective Hamiltonian constants.
The l-type doubling parameter, q v , is given approximately by the formula42
Reference 32.
Reference 8.
q v 52
2B 2e
v2
S
114
(
i
j 22i
v 22
v 2i 2 v 22
D
.
~7!
Using the B̃ e values from Table II in place of the more correct B e constants and substituting the observed values of q v
into Eq. ~7!, we get estimates for the Coriolis term
4 ( i j 22i v 22 /( v 2i 2 v 22 ) of 0.11 to 0.13 for CaOH, SrOH, and
BaOH. These numbers compare well with the value of 0.1
determined from optical spectra for SrOH ~Ref. 32! but are
significantly smaller than the typical value of 0.3 quoted in
Ref. 42.
The vibrational dependence of q v is somewhat erratic
~see Table I!, but is small compared with q v itself. Therefore,
for BaOH we can assume q v ( v 2 52)5q v ( v 2 51). Using
the known l-type doubling in v 2 52, we can deduce that the
~0 20 0!–~0 22 0! splitting in BaOH to be about 29.3 cm21,
i.e., g 22 57.3 cm21. This compares well with 6.092 cm21 for
CaOH ~Ref. 20! and 7.5646 cm21 for SrOH.31
The need for a p P lambda-doubling type term has been
seen before in the v 2 51 2 P vibronic state of C2H.39,40 In
this case, a small spin–orbit A term was also observed. In
our case we would need to observe the F 1 – F 2 crossover
transitions in order to see the effects of the A term, but these
lines were too weak. The effective p P constant is probably
due to mixing of low-lying electronic 2 P states with the
v 2 51 vibronic 2 P level. The electronic state mixes orbital
angular momentum with l, vibrational angular momentum
arising from the bending motions. The mixing increases in
magnitude as we descend the alkaline-earth group from Ca to
Ba. This trend is expected since the low-lying electronic
states get nearer to the ground state in energy from CaOH
through to BaOH. For example, the A 2 P state of BaOH lies
J. Chem. Phys., Vol. 102, No. 11, 15 March 1995
Fletcher et al.: Spectroscopy of earth hydroxide radicals
11 500 cm21 above ground state,22 while the corresponding
energy for CaOH is 16 000 cm21.20 Also, the spin–orbit parameters in these states become larger in going from calcium
to barium.
The data for MgOH, however, does not fit very well to
this model and the parameters determined do not follow the
trends observed down the alkaline-earth group. Specifically,
there are large residuals for the low N transitions and the
determined p P term for the ~0 11 0! level is around 2 MHz,
rather than virtually zero as expected from the pattern observed in the other molecules. In fact, the 2 P states of
MgOH lies about 25 000 cm21 in energy above ground state3
such that the p P constant should be negligible. As previously
discussed, MgOH is thought to be quasilinear, whilst the
other three hydroxides are expected to be linear in their
ground states. It is not surprising, therefore, that the MgOH
spectra does not fit to our linear model. A more complete
analysis of this data will be presented in a future publication.
VI. CONCLUSIONS
A vibrationally excited, pure rotational data set for
alkaline-earth monohydroxides in their ground electronic
state has been recorded. The spectra for CaOH, SrOH, and
BaOH have been successfully analyzed with a linear 2 S 1
model. A number of ‘‘anomalous’’ effects, however, were
observed. The vibration-rotation constant, a2 , was found to
be positive, as has been determined previously for some of
these molecules and the alkali metal hydroxides. Contamination of the v 2 51 2 P vibronic levels by 2 P electronic states
was also observed for all four species studied, similar to that
seen previously for C2H. In addition, in BaOH there is some
evidence of contamination of the v 2 52 2 D vibronic level by
an excited 2 D electronic state. Finally, it is apparent that a
different model is required for the quasilinear MgOH.
ACKNOWLEDGMENTS
We would like to thank Dr. John Brown for his helpful
suggestions regarding the analysis of the data and comments
on the text. This work was supported by NSF Grant No.
AST-92-53682 and NASA Grant No. NAGW 2989.
C. G. James and T. M. Sugden, Nature ~London! 175, 333 ~1955!.
R. F. Wormsbecher, M. Trkula, C. Martner, R. E. Penn, and D. O. Harris,
J. Mol. Spectrosc. 97, 29 ~1983!.
3
Y. Ni, Ph.D. thesis, University of California, Santa Barbara, 1986.
4
R. C. Hilborn, Z. Qingshi, and D. O. Harris, J. Mol. Spectrosc. 97, 73
~1983!.
5
P. F. Bernath and S. Kinsey-Nielson, Chem. Phys. Lett. 105, 663 ~1984!.
6
P. F. Bernath and C. R. Brazier, Astrophys. J. 288, 373 ~1985!.
1
2
7
4339
R. A. Hailey, C. N. Jarman, W. T. M. L. Fernando, and P. F. Bernath, J.
Mol. Spectrosc. 147, 40 ~1991!.
8
J. Nakagawa, R. F. Wormsbecher, and D. O. Harris, J. Mol. Spectrosc. 97,
37 ~1983!.
9
C. R. Brazier, P. F. Bernath, S. Kinsey-Nielson, and L. C. Ellingboe, J.
Chem. Phys. 82, 1043 ~1985!.
10
C. R. Brazier and P. F. Bernath, J. Mol. Spectrosc. 114, 163 ~1985!.
11
S. Kinsey-Nielson, C. R. Brazier, and P. F. Bernath, J. Chem. Phys. 84,
698 ~1986!.
12
W. T. M. L. Fernando, M. Douay, and P. F. Bernath, J. Mol. Spectrosc.
144, 344 ~1990!.
13
T. Gustavsson, C. Alcaraz, J. Berlande, J. Cuvellier, J.-M. Mestdagh, P.
Meynadier, P. de Pujo, O. Sublemontier, and J.-P. Visticot, J. Mol. Spectrosc. 145, 210 ~1991!.
14
L. M. Ziurys, W. L. Barclay, Jr., and M. A. Anderson, Astrophys. J. Lett.
384, L63 ~1992!.
15
W. L. Barclay, Jr., M. A. Anderson, and L. M. Ziurys, Chem. Phys. Lett.
196, 225 ~1992!.
16
M. A. Anderson, W. L. Barclay, Jr., and L. M. Ziurys, Chem. Phys. Lett.
196, 166 ~1992!.
17
M. A. Anderson, M. D. Allen, W. L. Barclay, Jr., and L. M. Ziurys, Chem.
Phys. Lett. 205, 415 ~1993!.
18
J. A. Coxon, M. Li, and P. I. Presunka, J. Mol. Spectrosc. 150, 33 ~1991!.
19
M. Li and J. A. Coxon, J. Chem. Phys. 97, 8961 ~1992!.
20
J. A. Coxon, M. Li, and P. I. Presunka, Mol. Phys. 76, 1463 ~1992!.
21
C. W. Bauschlicher Jr., S. R. Langhoff, T. C. Steimle, and J. E. Shirley, J.
Chem. Phys. 93, 4179 ~1990!.
22
C. N. Jarman and P. F. Bernath, J. Chem. Phys. 97, 1711 ~1992!.
23
J. A. Coxon, M. Li, and P. I. Presunka, J. Mol. Spectrosc. 164, 118 ~1994!.
24
C. J. Whitham, B. Soep, J.-P. Visticot, and A. Keller, J. Chem. Phys. 93,
991 ~1990!.
25
D. A. Fletcher, K. Y. Jung, C. T. Scurlock, and T. C. Steimle, J. Chem.
Phys. 98, 1837 ~1993!.
26
C. T. Scurlock, D. A. Fletcher, K. Y. Jung, and T. C. Steimle, J. Mol.
Spectrosc. 159, 350 ~1993!.
27
T. C. Steimle, D. A. Fletcher, K. Y. Jung, and C. T. Scurlock, J. Chem.
Phys. 96, 2556 ~1992!.
28
C. W. Bauschlicher, Jr., S. R. Langhoff, and H. Partridge J. Chem. Phys.
84, 901 ~1986!.
29
W. E. Palke and B. Kirtman, Chem. Phys. Lett. 117, 424 ~1985!.
30
C. W. Bauschlicher, Jr., and H. Partridge, Chem. Phys. Lett. 106, 65
~1984!.
31
K. Yamada and M. Winnewisser, Z. Naturforsch, Teil A 31A, 139 ~1976!.
32
P. I. Presunka and J. A. Coxon, Can. J. Chem. 71, 1689 ~1993!.
33
L. M. Ziurys, W. L. Barclay, Jr., M. A. Anderson, D. A. Fletcher, and J. W.
Lamb, Rev. Sci. Instrum. 65, 1517 ~1994!.
34
L. M. Ziurys, D. A. Fletcher, M. A. Anderson, and W. L. Barclay, Jr.,
submitted to Astrophysics J. Suppl. Ser.
35
G. Herzberg, Electronic Spectra and Electronic Structure of Polyatomic
Molecules ~Van Nostrand Rheinhold, New York, 1966!.
36
G. Herzberg, Spectra of Diatomic Molecules ~Van Nostrand Rheinhold,
New York, 1966!.
37
M. Larzilliere and Ch. Jungen, Mol. Phys. 67, 807 ~1987!.
38
Ch. Jungen, K. E. Hallin, and A. J. Merer, Mol. Phys. 40, 65 ~1980!.
39
J. M. Brown and K. M. Evenson, J. Mol. Spectrosc. 131, 161 ~1988!.
40
R. F. Curl, P. G. Carrick, and A. J. Merer, J. Chem. Phys. 82, 3479 ~1985!.
41
D. R. Lide, Jr. and C. Matsumura, J. Chem. Phys. 50, 3080 ~1968!.
42
C. H. Townes and A. L. Schawlow, Microwave Spectroscopy ~Dover, New
York, 1975!.
J. Chem. Phys., Vol. 102, No. 11, 15 March 1995