Observation and spectroscopy of high-lying states of the CaOH radical: Evidence for a bent, covalent state

Article abstract of DOI:10.1063/1.472844

The CaOH radical has been produced in a supersonic molecular beam by the reaction of water vapor with laser desorbed calcium. Three new electronic states, the D ²∑⁺, E ²∑⁺, and F states have been observed by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. The D and E states are linear, but the F state is bent, the first bent and strongly covalent state of CaOH to be observed. Vibrational constants for the D state have been determined and a partially rotationally resolved spectrum has provided the rotational constant and Ca-OH bond length in this state. The D and E states are more strongly bound than the ground state, perhaps indicating some covalent contribution to the bonding. Additional vibrational constants of the ground electronic state including the CaO-H vibrational frequency have been determined from the emission spectra.

Full text of DOI:10.1063/1.472844

Observation and spectroscopy of highlying states of the CaOH radical: Evidence for a
bent, covalent state
R. Pereira and Donald H. Levy
Citation: The Journal of Chemical Physics 105, 9733 (1996); doi: 10.1063/1.472844
View online: http://dx.doi.org/10.1063/1.472844
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Observation and spectroscopy of high-lying states of the CaOH radical:
Evidence for a bent, covalent state
R. Pereiraa) and Donald H. Levy
The James Franck Institute and The Department of Chemistry, University of Chicago, Chicago,
Illinois 60637
͑
Received 31 July 1996; accepted 3 September 1996͒
The CaOH radical has been produced in a supersonic molecular beam by the reaction of water vapor
2
ϩ
2 ϩ
with laser desorbed calcium. Three new electronic states, the D ⌺ , E ⌺ , and F states have been
observed by laser induced fluorescence and resonance enhanced multiphoton ionization
spectroscopy. The D and E states are linear, but the F state is bent, the first bent and strongly
covalent state of CaOH to be observed. Vibrational constants for the D state have been determined
and a partially rotationally resolved spectrum has provided the rotational constant and Ca–OH bond
length in this state. The D and E states are more strongly bound than the ground state, perhaps
indicating some covalent contribution to the bonding. Additional vibrational constants of the ground
electronic state including the CaO–H vibrational frequency have been determined from the emission
spectra. © 1996 American Institute of Physics. ͓S0021-9606͑96͒00746-5͔
I. INTRODUCTION
states ‘‘may have a non-negligible contribution from cova-
lent configurations.’’ The X, A, B, and C states are all linear
as would be expected of ionic states.
The spectroscopy of the free radical alkaline earth mono-
hydroxides, CaOH, SrOH, etc. is a subject that has been of
interest since 1983 when Hilborn et al. reported the obser-
It should be noted that although CaOH is ionic, the di-
pole moments of the lower states are not particularly large
1
2
2
vation and spectroscopy of CaOH produced in a Broida-type
͑
the dipole moments of the X, A ⌸ , A ⌸ , and B states
1/2 3/2
2
8
oven. These species are interesting because they appear in
are 1.465, 0.836, 0.766, and 0.744 D, respectively͒ due to
3
4
flames, are supposed to appear in circumstellar shells, and
the large polarization of the unpaired electron away from the
1
Ϫ
can act as reaction intermediates. Hilborn and co-workers
carried out the first spectroscopic study of the A ⌸–X ⌺
OH .
2
2
ϩ
Supersonic jet spectroscopy has proven to be an excel-
lent technique to obtain simplified spectra of polyatomic
molecules.22 The use of laser desorption coupled with super-
sonic expansions has allowed the study of molecules which
were not accessible by other means due to their very low
vapor pressures and/or their fragility at temperatures neces-
sary to give a detectable signal.23 The production of free
radicals in a supersonic jet environment has been carried out
electronic transition of CaOH. They found the molecule to
be linear in both ground and electronic excited states and
gave a set of spectroscopic parameters for both the proto-
5
nated and deuterated radicals. Wormsbecher et al. observed
2
ϩ
2
ϩ
the B ⌺ –X ⌺ transition in the chemiluminescence re6-
action of Ca with water, and Bernath and Kinsey-Nielsen
determined the rotational structure of the 000–000 and 100–
7
in several ways.2
4–27
Recently, Whitham et al. produced
27
1
00 transitions of this system. Jarman and Bernath observed
2
2
ϩ
the electronically forbidden C ⌬–X ⌺ transition which
is made allowed through vibronic coupling.
CaOH and other radicals through the reaction of the laser
desorbed metal vapor with water and other gases. The same
8
Many experimental studies have been carried out to
characterize these states.8 Transitions between the ground
state and the A, B, and C states all occur in the visible, the
technique was used by Steimle et al. to carry out an optical
–17
Stark study of the radical and has been used in this work to
study the high lying electronic states.
electronic transition energy being 15 998, 18 022,6 and
1
In this paper we report the observation of three near-
ultraviolet transitions between the ground and higher lying
electronic states of the CaOH radical. The highest of these
three states is bent and is the first electronic state of CaOH
that is largely nonionic.
Ϫ1
2
1 907 ͑Ref. 7͒ cm for the A–X, B–X, and C–X transi-
tions, respectively. Prior to this work the X, A, B, and C
states were the only electronic states of CaOH to have been
observed.
There have been several theoretical calculations of the
excited electronic states of CaOH.1
8–21
The ground elec-
II. EXPERIMENT
tronic state of CaOH is largely ionic consisting of a closed
Ϫ
ϩ
shell OH anion and a Ca cation containing a 4s unpaired
electron and a closed shell argon core. The low-lying excited
electronic states of CaOH are also ionic involving excitation
The free radical CaOH was produced by laser desorbing
calcium metal into a helium carrier gas which contained wa-
ter vapor. The high pressure mixture was expanded through a
supersonic nozzle and both resonance enhanced multiphoton
ionization ͑REMPI͒ and laser induced fluorescence ͑LIF͒
were used to investigate the system.
ϩ
of the unpaired electron to excited orbitals of Ca , although
Allouche and Aubert-Fr e´ con20 propose that the A and B
a͒Present address: Instituto Pluridisciplinar. Universidad Complutense. Paseo
Juan XXIII, 1.28040 Madrid, Spain.
The experimental apparatus used in this work has been
described in detail elsewhere23 and only a brief description
J. Chem. Phys. 105 (22), 8 December 1996
0021-9606/96/105(22)/9733/7/$10.00
© 1996 American Institute of Physics
9733
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9734
R. Pereira and D. H. Levy: High-lying states of CaOH
will be given here. In the REMPI experiments the supersonic
jet produced by a pulsed solenoid valve was skimmed with a
1.0 mm skimmer placed about 4 cm from the nozzle. The
molecular beam formed by the skimmer passed into a second
chamber containing a reflectron time of flight mass spec-
trometer. The output of a Nd-YAG pumped dye laser was
doubled or mixed with the 1.06 ␮ fundamental of the Nd-
YAG laser and crossed with the molecular beam in the center
of the acceleration region of the mass spectrometer. Several
dyes ͑Fluorescein 548, DCM, LDS698͒ and a dye mixture
͑DCM/LDS698͒ were used to probe the spectral region of
interest. Spectra were taken monitoring the parent mass of
the radical.
Fluorescence experiments were done using an un-
skimmed free jet with the time-of-flight mass spectrometer
replaced by fluorescence collection optics. Fluorescence ex-
citation spectra were measured using f/1 optics which im-
aged the fluorescence onto a slit with an RCA 8575 photo-
multiplier tube immediately behind it. Dispersed
fluorescence was also detected using f/1 optics which imaged
the fluorescence onto the entrance slit of a Spex 1 m mono-
chromator having an RCA 4501 photomultiplier tube at the
exit slit. Both total and dispersed fluorescence could be col-
lected at the same time. Excitation of the free jet occurred
about 2 cm from the nozzle. For the rotationally resolved
scans an excimer pumped dye laser ͑DMQ dye͒ with 0.2
FIG. 1. Fluorescence excitation ͑top͒ and one color REMPI ͑bottom͒ spectra
Ϫ1
of the CaOH radical in the range 28 150–29 850 cm showing the band
Ϫ1
cm bandwidth was used for excitation.
system D 2
ϩ
–X 2
ϩ
of CaOH and the C 1
ϩ
–X 1
ϩ
⌺
⌺
⌺
⌺
system of CaO
A calcium metal rod of 1 in. diam and 1 in. length was
prepared by melting Ca pieces inside an alumina holder us-
ing an argon atmosphere to avoid oxidation of the metal. A
disk of about 2 mm thickness was cut and attached to a
sample holder using cyanoacrylate ester. The calcium disk
was then machined to make sure that the holder and the Ca
surfaces were parallel. The disk could be rotated and trans-
lated at the same time to provide fresh areas of sample for
vaporization, but translation was required only when a deep
groove in the sample occurred. Failure to have the holder and
metal surfaces parallel produced periodic signal fluctuation
as the calcium disk was rotated. When these surfaces were
parallel, the signal fluctuation was less than 15%.
͑indicated by asterisks͒. In the case of CaOH the quantum numbers of the
Ca–OH stretch and the bend are indicated. The third quantum number cor-
responding to the CaO–H stretch is omitted. When the vibrational quantum
numbers are listed for only one electronic state, the initial state of the tran-
sition is the zero-point level of the ground electronic state.
is 0.96 eV endothermic.28 It is possible ͑though unlikely͒ that
calcium desorbs with this much kinetic energy, but it is al-
most certainly relaxed by collisions with helium before re-
acting to form CaOH. Moreover, calcium produced in a
Broida oven is known to react with water vapor to produce
1
CaOH, and in these circumstances the calcium could not
have sufficient energy to react directly. The puzzle regarding
the reaction of alkaline-earth metals with water to produce
metal hydroxides and even electronically excited metal hy-
droxides has been previously noted.5
Calcium from this disk was vaporized using the second
harmonic of a Nd-YAG laser that was attenuated to 1 mJ per
pulse using a 0.1 M CoSO solution. The desorption laser
4
In addition to CaOH, we also observe spectral features
from CaO produced in the reaction of calcium with water.
The CaOH and CaO spectra are not optimized under the
same conditions. CaO is favored by relatively high ͑e.g., 28
atm͒ helium pressure and CaOH is favored by relatively low
was focused onto the calcium disk with a 50 cm focal length
lens. The vapor of the metal was injected into a mixture of
helium gas and water vapor. For most of the experiments the
helium pressure was around 8 atm and the water vapor was
at its room temperature vapor pressure.
͑e.g., 8 atm͒ helium pressure. There is also a question regard-
ing the mechanism of production of CaO since the reaction
III. RESULTS AND DISCUSSION
A. CaOH production
2
CaϩH OϭCaOϩH2
is 0.92 eV endothermic.29
It should be noted that there is some uncertainty regard-
ing the mechanism responsible for CaOH production. The
most obvious reaction
B. The D state of CaOH
Figure 1 shows the REMPI and fluorescence excitation
spectra of the CaOH radical in the range 28 150–29 850
Ϫ1
CaϩH OϭCaOHϩH
cm , and the wave numbers of the transitions of these spec-
2
J. Chem. Phys., Vol. 105, No. 22, 8 December 1996
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R. Pereira and D. H. Levy: High-lying states of CaOH
TABLE I. Wave numbers ͑cmϪ1͒ for the assigned bands of CaOH ͑except
9735
as noted͒ that appear in the excitation spectra of Figs. 1 and 2. Vibrational
modes are listed in the order, Ca–OH stretch, bend, CaO–H stretch.
Wave
number D 2
⌺
ϩ
–X 2
⌺
ϩ
E 2
⌺
ϩ
–X 2
⌺
ϩ
F 2⌸–X 2
⌺
ϩ
C 1 –X 1
⌺ ⌺
ϩ
ϩ
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
8 153
8 207
8 261
8 565
8 776
8 828
8 949
9 225
9 269
9 486
9 874
9 879
0 134
0 215
0 617
000–000
010–010
020–020
010–000
0–0, CaOa
100–000
020–000
110–000
1–0, CaOa
2–0, CaOa
200–000
300–000
000–000
100–000
000–000
aReference 30.
tra are given in Table I. The total fluorescence spectrum is
shown at the top of the figure while the REMPI spectrum is
shown at the bottom to facilitate comparison. The mass se-
lected spectrum reproduces most of the features appearing in
total fluorescence. The peaks marked with asterisks appear-
FIG. 2. Emission spectra of the three band origins of the three electronic
2
ϩ
states observed in this work. ͑Top͒ Emission from D ⌺ ; ͑middle͒ emission
Ϫ1
2 ϩ
ing at 28 776, 29 269, and 29 874 cm in total fluorescence
from E ⌺ ; and ͑bottom͒ emission from F. The quantum numbers for the
ground electronic state of the Ca–OH stretch, the bend, and the CaO–H
stretch are indicated. Where only two quantum numbers appear, the third
quantum number of the CaO–H stretch is zero and has been omitted.
are assigned to the 0–0, 1–0, and 2–0 bands, respectively, of
1
ϩ
1
ϩ
the C ⌺ –X ⌺
electronic transition of the CaO
3
0
molecule. Changing the helium backing pressure allowed
us to optimize the production of CaOH against CaO.
Ϫ1
The band appearing at 28 153 cm is assigned as the
origin of a transition to a previously unobserved electronic
state. The emission spectrum produced by excitation of this
band is shown in the top spectrum of Fig. 2, and the wave
numbers of the features in this spectrum are given in Table
II. We observed no features to the red of this band which
could not be assigned to transitions involving previously
identified states. This band is shown at higher resolution in
Fig. 3. The rotational structure shows a P branch head and a
violet degraded R branch with no Q branch. This is consis-
sitions have been previously observed in other electronic
transitions of CaOH.
2
ϩ
Figure 4 shows the vibrational levels of the D ⌺ state
that have been directly observed in this work. Table III gives
the molecular parameters obtained for this electronic state
which were determined from the energy level structure given
in Fig. 4 and a fit to the formula31
2
1
2
2
EϭT ϩw v ϩw v ϩx v ϩx v ϩx v v .
0
1
1
2
2
11
22
12 1 2
2
ϩ
2 ϩ
tent with a transition from the X ⌺ state to a new ⌺
2
2
state. The spin-rotation splitting characteristic of a ⌺– ⌺
transition is just observable in some of the R branch features.
We neglect the terms corresponding to the CaO–H stretching
mode, as well as terms involving the angular momentum of
the degenerate bending vibration. The w and x molecular
1
8
2 ϩ
Ortiz calculated a D ⌺ state with transition energy of
2
1
11
6 495 cmϪ1. The 28 153 cm band being the closest new
Ϫ1
constants listed in Table III have been determined with one
feature to the calculation of Ortiz, we assign this band as the
degree of freedom in the estimated error, while w , x , and
x12 have been estimated with no degrees of freedom in the
estimated error.
2
22
2
ϩ
2
ϩ
origin of the D ⌺ –X ⌺ transition. In addition to the
origin transition, we observe progressions involving both the
Ca–OH stretching vibration and the bending vibration and
sequences in the bending vibration. These are marked on Fig.
The rotational structure shown in Fig. 3 has been ana-
1
2
lyzed. Assuming
a value for the ground state of
2
Ϫ1
1. For nontotally symmetric vibrations such as the bend, the
BЉ(X ⌺)ϭ0.334 33 cm , the rotational constant of the ex-
2
Ϫ1
selection rule for an allowed transition is ⌬vϭeven. In ad-
dition to such allowed transitions, we also see two weak
cited state is BЈ(D ⌺)ϭ0.364 45 cm . Using the relation6-
ship between the bond lengths and the rotational constant1
and assuming that the value of the bond length R͑CaO–H͒ is
the same in the D state as it is in the A state16 ͑0.9572 Å͒, the
⌬
v2ϭ1 transitions presumably made allowed by vibronic
coupling in the excited state. Such vibronically allowed tran-
J. Chem. Phys., Vol. 105, No. 22, 8 December 1996
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9736
R. Pereira and D. H. Levy: High-lying states of CaOH
TABLE II. Wave numbers ͑cmϪ1͒ of the bands appearing in the emission spectra for different excitatiion
features of CaOH. Wave numbers are given as red-shifts relative to the excitation wave number.
Vibrational
assignment
a
28 153b
29 879
30 215
30 311
30 366
350
30 617
347
Average
0
1
0
1
2
1
0
2
1
3
2
1
3
2
4
3
4
5
0
1
2
1
3
2
4
3
10
00
20
10
00
20
40
10
30
00
20
40
10
30
00
20
10
00
01
01
01
21
01
21
01
21
356
608
700
363
611
690
354
606
686
604
679
603
671
688
951
954
952
1204
1207
1280
1213
1289
1211
1295
1207
1296
1389
1543
1626
1794
1871
1968
2128
1201
1270
1207
1286
1389
1539
1626
1800
1872
1966
2126
2203
2381
2457
2702
2961
3778
4384
4969
5029
5556
5604
6133
6196
1533
1541
1797
1794
1865
1806
1875
1967
1803
1876
1964
2124
2203
2383
2457
1806
1872
2376
2452
2384
2459
2459
2702
2961
3779
4384
4974
2961
3777
4963
5029
5545
5604
6133
6196
5567
a
Vibrational quantum numbers of the X electron state in the order Ca–OH stretch, bend, CaOH–H stretch.
Wave number of the excitation transition.
b
Ca–OH bond length in the D state is 1.894 Å, approximately
% less than the Ca–OH bond length in the ground state
1.982 Å͒.
Finally, we should comment on the rotational and vibra-
tional temperatures inferred from the D–X fluorescence ex-
citation spectrum. Since the ground electronic state has a
bending frequency of 352 cmϪ1, we were surprised to see
relatively intense sequence transitions in the bending mode
in the cold environment of a supersonic free jet. The inten-
sities of the sequence transitions are fairly well described by
a vibrational temperature of 277 K. As is usually the case,
the intensities of the rotational structure in the 0–0 band are
not well described by a single rotational temperature, the
5
͑
FIG. 3. Fluorescence excitation rotationally resolved spectrum of the band
2
ϩ
2
ϩ
2 ϩ
origin of the D
⌺
–X
⌺
system of CaOH. The value of J for several of
FIG. 4. Schematic diagram of the vibrational levels of the D ⌺ state that
have been directly observed in this work and that give the molecular con-
stants listed in Table III.
the P(J) and R(J) branch features is indicated. The intensity pattern of the
rotational levels yields a rotational temperature of 40–50 K.
J. Chem. Phys., Vol. 105, No. 22, 8 December 1996
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R. Pereira and D. H. Levy: High-lying states of CaOH
9737
2
ϩ
TABLE III. Molecular parameters for the excited state D ⌺ of CaOH
determined from the observed energy levels shown in Fig. 3.
Parameters
Value/cmϪ1
Std. Err/cmϪ1
w1
w2
x11
x22
x12
681.0
425.6
Ϫ6.9
Ϫ13.6
Ϫ13.9
1.5
2.1
high-J, R-branch transitions having too much intensity for a
Boltzmann distribution at a temperature that fits the low-J
transitions. If the rotational distribution is to be characterized
by a single rotational temperature fit to the low-J transitions,
that temperature is between 40 and 50 K. Again, this is a
higher rotational temperature than is usually seen in a super-
sonic expansion.
However, such high temperatures are not unknown. Cai
et al.32 reported a rotational temperature of 14 K and a vi-
brational temperature of 4000 K for jet cooled AlO prepared
by the reaction of laser vaporized aluminum with various
oxidants. Vibrational relaxation is an inefficient process re-
quiring many collisions with the carrier gas. If the hot seed
gas is present in the original gas mixture, it undergoes a
relatively large number of collisions between the throat of
the nozzle and the point of observation. However if the hot
seed gas is formed by means of a chemical reaction some-
where downstream of the throat, the number of collisions
with the carrier gas is reduced producing less complete vi-
brational cooling. Rotational relaxation requires fewer colli-
sions, but again, if the rotationally hot seed is formed down-
stream in the expansion, there will be less rotational cooling.
It is possible that this is also the reason for our observa-
tion of vibrationally and rotationally hot CaOH. However,
we note that we also observed vibrationally hot CaO ͑up to
FIG. 5. Fluorescence excitation ͑top͒ and one color REMPI ͑bottom͒ spectra
of CaOH in the range 29 760–30 720 cm . A feature due to CaO is indi-
cated by an asterisk. The quantum numbers of the Ca–OH stretch and the
bend are indicated. The third quantum number corresponding to the CaO–H
stretch is omitted. The initial state of the marked transitions is the zero-point
level of the ground electronic state.
Ϫ1
stretch, the first time this mode has been observed in a CaOH
emission spectrum. Both the rotational profile and the emis-
sion spectrum resemble those of the D–X origin transition,
but the intensity and frequency of this feature fit neither the
intensity profile nor the expected frequency of a progression
Ϫ1
vЉϭ6 in the 732 cm vibration͒ when the CaO was laser
desorbed from solid CaO prior to any expansion. In this case
there was no chemical reaction, and the hot vibrational dis-
tribution must have been produced in the laser desorption
process and not completely relaxed even by the large number
of collisions between the desorption in the nozzle and the
point of observation.
18
built on the D–X origin. Ortiz calculated the E state as a
2
ϩ
Ϫ1
⌺
state at 29 860 cm which further supports our assign-
Ϫ1
ment of the 29 879 cm feature as the origin of the E–X
transition.
We have not been able to unambiguously assign vibra-
tional progressions built on the E–X origin because the
spectrum becomes crowded at higher frequencies, and the
possibility of vibronic mixing of the E and F ͑see below͒
states makes interpretation of the spectrum more difficult.
C. The E state of CaOH
Figure 5 continues the REMPI and fluorescence excita-
tion spectra of CaOH to higher wave numbers than shown in
Fig. 1. There is an intense feature at 29 879 cm that we
Ϫ1
2
ϩ
Ϫ1
assign as the origin of a transition to a new ⌺ electronic
state. Although we have not observed this feature at high
enough resolution to resolve rotational structure, the band
profile shows a P-branch head and a violet degraded R
branch with no Q branch. The emission spectrum observed
upon excitation of this feature is shown in the middle spec-
trum of Fig. 2 and consists of a long progression in the
Ca–OH stretch with only very weak features involving even
harmonics of the bend. This emission spectrum also shows
the first member of a progression involving the CaO–H
There is a feature at 30 617 cm whose rotational profile
2
ϩ 2 ϩ
resembles that of a ⌺ – ⌺ transition and which might be
the first member in the Ca–OH stretching progression. The
emission spectrum obtained by exciting this feature is shown
in Fig. 6. The emission spectrum shows a lot of activity in
the Ca–OH stretch but also shows activity in the bend in-
cluding transitions to odd vЉ states perhaps caused by mixing
with the F state. Moreover, this emission spectrum also
shows the first member of a progression in the CaO–H
stretch and combination bands built on this third fundamen-
J. Chem. Phys., Vol. 105, No. 22, 8 December 1996
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9738
R. Pereira and D. H. Levy: High-lying states of CaOH
FIG. 6. Emission spectrum of the feature at 30 617 cmϪ1 tentatively as-
signed to the first member of the Ca–OH stretching progression of the
2
ϩ
2
ϩ
E
⌺
–X
⌺
system. The quantum numbers for the ground electronic
state of the Ca–OH stretch, the bend, and the CaO–H stretch are indicated.
Where only two quantum numbers appear, the third quantum number of the
CaO–H stretch is zero and has been omitted.
2
ϩ
FIG. 7. Schematic diagram of the vibrational levels of the X ⌺ state that
have been observed in this work and that give the molecular constants listed
in Table IV.
tal. The fluorescence excitation spectrum of CaOD has cor-
responding features at 29 905 and 30 628 cm which might
Ϫ1
be assigned to the E–X origin and first Ca–OD stretching
progression. Emission from both of these states also shows
activity in all three vibrations. If these assignments are cor-
Since the E and F states are only separated by 336 cmϪ1
,
Ϫ1
rect, the vЈϭ0ϪvЈϭ1 spacing is 738 cm in CaOH and
extensive mixing between these two states would be ex-
pected as well as possible mixing with other ionic and cova-
lent states that might be in this region. Therefore, further
assignment of the spectrum may be difficult without consid-
erably more work.
Ϫ1
7
23 cm in CaOD indicating that the Ca–OH stretching
frequency is higher in the E state than in any of the previ-
ously observed states. This might be an indication of in-
creased covalent bonding in the E state.
D. The F state of CaOH
The feature at 30 215 cmϪ1 is quite different from any of
E. Ground state vibrational constants
the bands at lower frequencies. First, the rotational profile of
this band shows a strong central Q branch in addition to the
P and R branches. Second, the emission spectrum produced
by excitation of this feature ͑bottom spectrum of Fig. 2͒
shows not only extensive activity in the Ca–OH stretching
vibration but also extensive activity in the bending vibration.
Transitions to even vЉ states are more intense, but transitions
Coxon9,14 and co-workers have determined the frequen-
cies for a number of emission features and have used this
information to extract vibrational constants for the ground
electronic state of CaOH. The precision of the previous work
was greater than ours. However, using emission from the
origins of the D–X, E–X, and F–X transitions, and from
Ϫ1
the features at 30 311 and 30 366 cm , we were able to
2
to odd vЉ states are clearly visible. Ortiz calculated a ⌸ state
observe transitions to more and higher vibrational levels of
the ground state as shown in Fig. 7. The extensive activity in
the bending mode observed in emission from the features at
Ϫ1
at 30 010 cm . The rotational P, Q, R rotational profile
2
2 ϩ
would be consistent with a ⌸– ⌺ transition, although one
would expect a second strong transition to a second fine-
structure state. However, the extensive activity in the bend-
ing vibration is not consistent with a transition between two
Ϫ1
30 215 ͑F origin͒, 30 311, and 30 366 cm ͑see Table II͒
were particularly important in allowing us to measure previ-
ously unobserved vibrational levels in the ground electronic
state. We were therefore able to obtain estimates of addi-
tional anharmonic constants. The vibrational spectroscopic
constants extracted from our data and the values from Refs. 9
and 14 are given in Table IV.
Ϫ1
linear states. It seems likely that the feature at 30 215 cm
is the F–X origin, but that the F state is bent. CaOH has
always been regarded as an ionic molecule and therefore
only linear states have been considered. At some point of
excitation one would expect bent, covalent states involving p
Of particular importance is the first measurement of the
CaO–H fundamental frequency obtained from the emission
Ϫ1
orbitals on the oxygen, and the feature at 30 215 cm ap-
Ϫ1
pears to be a transition to the first of these states.
The region above the F–X origin is congested and ͑with
the possible exception discussed above͒ remains unassigned.
spectra of the E state. This frequency is 3778 cm in CaOH
Ϫ1
and 2790 cm in CaOD. This is very close to the vibrational
frequency33 of OH which is 3738 cm . We note that the
Ϫ
Ϫ1
J. Chem. Phys., Vol. 105, No. 22, 8 December 1996
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R. Pereira and D. H. Levy: High-lying states of CaOH
9739
TABLE IV. Molecular parameters for the ground state of CaOH determined
from the observed energy levels shown in Fig. 7.
1
2
3
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Parameters
Value/cmϪ1
Std. Err./cmϪ1
␯
␯
w1
w2
x11
x22
x12
͑CaO–H͒
͑CaO–D͒
3778
2790
610.4͑612.8͒a
350.0͑352.9͒
Ϫ3.7͑Ϫ3.74͒
Ϫ0.8
2.0
2.5
0.8
0.6
0.7
4
5
b
a
6
7
8
Ϫ7.1
a
Reference 9.
Reference 14.
b
9
0
1
1
1
assumption of Li and Coxon16 that the CaO–H stretching
12
Ϫ
frequency is the same as the OH frequency is correct.
13
D. A. Fletcher, M. A. Anderson, W. L. Barclay, and L. M. Ziurys, J.
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IV. CONCLUSIONS
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15
The electronic spectrum of the CaOH radical in the UV
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16
1
1
1
2
7
8
9
0
2
ϩ
The vibrational structure of the D ⌺ excited state has been
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21
2
2
2
3
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ϩ
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2
ϩ
E ⌺ states are linear and more strongly bound than the
ground state. These states are probably largely ionic but may
have a non-negligible covalent component. The F state is
bent and the first strongly covalent state observed for CaOH.
The dispersed emission spectra have allowed the determina-
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24
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2
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9
0
ACKNOWLEDGMENTS
R.P. thanks the Direccion General de Investigacion Ci-
entifica y Tecnica and the Fulbright for support. We thank
Dr. Joe Batka for helping in the identification of the CaOH
and Xin Wang for help with the experiments. The work was
supported by the National Science Foundation under Grant
No. CHE-9319958.
31
3
2
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33
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J. Chem. Phys., Vol. 105, No. 22, 8 December 1996
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