Electronic and vibrational structure of copper dibromide

Article abstract of DOI:10.1021/jp014441w

Absorption and laser-induced fluorescence spectra of copper dibromide in a solid neon matrix are reported. Similar to those for copper dichloride, electronic transitions between the low-lying so-called ligand field states states are forbidden by the u?g rule, appear as a result of vibronic Herzberg-Teller coupling. The observed transition energies are in good agreement with the adiabatic state energies derived from a recent gas-phase photodetachment study by Wang and co-workers. The matrix study with its much higher resolution yields detailed information about the copper halide electronic states and their vibrational structures.

Full text of DOI:10.1021/jp014441w

J. Phys. Chem. A 2002, 106, 5429-5436
5429
Electronic and Vibrational Structure of Copper Dibromide
Martin Lorenz and Vladimir E. Bondybey*
Institute for Physical and Theoretical Chemistry, Technical UniVersity of Munich,
Lichtenbergstrasse 4, 85747 Garching, Germany
ReceiVed: December 7, 2001; In Final Form: February 28, 2002
Absorption and laser-induced fluorescence spectra of copper dibromide in a solid neon matrix are reported.
Similar to those for copper dichloride, electronic transitions between the low-lying so-called ligand field
states states are forbidden by the uTg rule, appear as a result of vibronic Herzberg-Teller coupling. The
observed transition energies are in good agreement with the adiabatic state energies derived from a recent
gas-phase photodetachment study by Wang and co-workers. The matrix study with its much higher resolution
yields detailed information about the copper halide electronic states and their vibrational structures.
Introduction
between them are forbidden, we have shown that an interesting
vibronic Herzberg-Teller coupling23 makes them weakly al-
Despite the deceptively simple electronic structure of ground-
state Cu atoms, with a closed d electron shell and a single 4s
electron, copper exhibits complex redox chemistry for a
transition metal. Both monovalent and divalent compounds of
copper are common, and the Cu(I)/Cu(II) oxidation plays an
important role in numerous biological processes.1-3 Higher
oxidation states of copper are also known and appear to be
important in high-temperature superconductors.4 A number of
years ago, a visible spectrum observed in low-temperature
matrices5 and initially believed to be due to diatomic CuO was
shown to be actually due to an interesting linear and centrosym-
metric copper dioxide, OCuO, with a formally tetravalent copper
atom.6-8 This result was subsequently confirmed both by
observing the linear isomer in the gas phase by photodetachment
experiments from the corresponding anion9 as well as by
deriving its properties by computational techniques.10
lowed. Excitation of the visible charge transfer is followed by
an efficient nonradiative relaxation that populates the lower-
lying ligand-field states, and extensive fluorescence between
these states can then be observed.
Very recently, Wang and co-workers have reported elegant
-
-
photodetachment experiments on the CuCl2 and CuBr2
anions.24 These yielded information about the electron affinities
and energies of the individual electronic states of the neutral
copper dihalides, but their vibrational structure was not resolved.
The appearance of these gas-phase results has now motivated
us to report our solid neon matrix results for copper dibromide
as well. The matrix data yield not only more accurate energies
of the low-lying electronic states but also extensive information
about their vibrational structure. They also provide interesting
insights into the vibronic coupling, which makes the appearance
of the forbidden transitions in the matrix possible. In this paper,
we therefore report the spectra of copper dibromide in solid
neon and compare the results with the gas-phase data and
theoretical results, and we compile and tabulate the available
information about copper dihalides.
Although copper monohalides were studied fairly extensive-
ly,11-14 much less is known about the corresponding dihal-
ides.15-20 Notwithstanding the 3d104s 2S Cu ground state of the
copper atom, the ground states of molecular dihalides can be
viewed as ionic, so-called “ligand-field” X- Cu2+X- states,
formally arising from a d9 Cu2+ core whose degeneracy is split
by the field of the negatively charged chloride ligands.15 High-
quality theoretical relativistic ab initio calculations of Baus-
Experimental Section
The copper dibromide fluorescence spectra were first ob-
served25 in our laboratory with the help of a self-igniting pulsed
discharge technique26 employing a dilute mixture of CFBr3 with
neon (1:400) and copper electrodes. When the discharge
products were trapped at 5 K, the samples exhibited a structured
absorption spectrum with an origin near 11 500 cm-1. Excitation
of this absorption by a Ti-sapphire laser produced strong near-
infrared fluorescence beginning slightly below 8700 cm-1. The
same spectrum could also be produced using elemental bromine
instead of CFBr3, with the subsequent analysis suggesting that
the emitter is the triatomic copper dibromide, CuBr2.
In most of the experiments reported here, the pulsed laser
vaporization technique was used.27,28 The matrix-isolated bro-
mide could be produced either by vaporizing a copper target in
the presence of a neon carrier gas doped with bromine or by
direct vaporization of copper dibromide. The CuBr2 powder was
pressed into a 13-mm diameter, 2-mm thick disk, which was
then vaporized by frequency-doubled Nd:YAG laser pulses (3
2
chlicher and Roos have predicted Πg ground states for the
halides and gave the energies and ordering of the low-lying
states,20 but for a long time, there was no experimental
2
verification. Besides the X Πg ground state, the ligand-field
2
+
2
splitting should also yield low-lying Σg and ∆g states. An
absorption from the X2Πg state to a higher-lying 2Πu state near
15 500 cm-1 is the most extensively studied transition of the
dichloride. The excited so-called charge transfer state can be
viewed as a superposition of the XCu+X- and X-Cu+X ionic
structures and arises formally from binding between an ionic
d10 Cu+ core with two halogens.15,20
Several years ago, we investigated the halogen halides in rare
gas matrices and showed that detailed information about their
low-lying states can be obtained.21,22 Although all the ligand-
field states are of gerade symmetry and electronic transitions
* Corresponding author. E-mail: bondybey@uci.edu.
10.1021/jp014441w CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/14/2002

5430 J. Phys. Chem. A, Vol. 106, No. 22, 2002
Lorenz and Bondybey
Figure 1. Near-infrared E2Πu-X2Πg absorption spectrum of laser-vaporized copper dibromide isolated in solid neon at about 7 K. Note the hint
of fine structure for the lowest-energy bands.
mJ, 10 ns). The laser pulses were synchronized with the opening
of a pulsed piezoelectric valve controlling the carrier gas flow.
The vaporization products swept by the gas pulse expanded into
the vacuum of the cryostat and were deposited onto a silver-
plated copper substrate cooled to ∼6 K by a Leybold RGD 580
closed cycle refrigerator. Typically, the pellet was ablated at a
10 Hz repetition rate for 1-2 h using a neon backing pressure
of 5 bar.
The absorption and fluorescence spectra were measured on
a Bruker IFS 120-HR Fourier transform spectrometer. The
absorption was measured in the range of 600-30 000 cm-1 with
a resolution of 1 cm-1 or better. For the fluorescence experi-
ments, the samples were excited by an argon laser-pumped
tunable CW dye laser or a Ti-sapphire laser. Depending on
the spectral range of interest, either a photomultiplier or a liquid
nitrogen-cooled germanium or indium antimonide detector was
used.
Results and Discussion
Near-Infrared Absorption Spectrum. As noted above,
matrices containing copper halides can be generated by several
different methods: by laser vaporization of copper in the
presence of a carrier gas doped with small amounts of the
halogen, by electric discharge through a dilute Br2- or Cl2-
Ne mixture using copper metal electrodes, or by direct laser
vaporization of the solid copper halide. The results are relatively
independent of the method used; in each case, one obtains rather
intense near-infrared absorptions due to the copper II halides.
As shown in Figure 1, in the case of CuBr2, the absorption
spectrum starts around 11 212.34 cm-1 and consists of an
extended but rather irregular progression of broad bands with
an approximately 180 cm-1 spacing. The individual absorption
bands have different shapes, and some (in particular, the ones
at the low-energy end) seem to exhibit fine structure. When
one tries to fit the measured absorption maxima to a single
progression, a vibrational frequency of about 180 cm-1 is
obtained, but the fit is poor and the positions of individual bands
show appreciable random deviations from the computed posi-
tions. On the basis of the absorption intensities, the transition
involved must be quite strong and probably fully allowed, but
when it is excited by a laser tuned to the region of absorption,
there seems to be very little or no resonant fluorescence back
into the ground state, suggesting that the excited state prefer-
entially relaxes nonradiatively.
To facilitate the interpretation of the spectra, we have carried
out a series of computations on a Pentium III-based Linux
system using the Gaussian 95 suite of programs. For most of
the computations, we have employed the hybrid B3LYP method
as implemented there, using the pseudorelativistic effective core
basis set of the Stuttgart-Dresden group (SDD).29,30 Specifically
relevant to the present work are the vibrational stretching
fundamentals of ν1 ) 201 and ν3 ) 376. The ground X2Πg
state of the linear, open-shell CuBr2 is subject to Renner-Teller
splitting, and quantum mechanical computations yield two
different values in place of the doubly degenerate bending mode.
As will be shown, the components of the nominal 2Πg state are
widely separated because of large spin-orbit coupling in the
heavy CuBr2 molecule, and the vibrational structure we observe
provides no evidence of the Renner-Teller effect. In discussing
On the other hand, when an infrared detector is used, strong
fluorescence that starts at a much lower energy (below 9000
cm-1) is observed, which leaves an appreciable gap between
the origins of the absorption and emission spectra. By modulat-
ing the laser source and using lock-in detection of this infrared
fluorescence while scanning the laser frequency in the region
of the absorption origin, one obtains the excitation spectrum
shown in Figure 2. This spectrum clearly parallels the structured
the results, we will use a bending frequency of ν2 ) 65 cm-1
which is the average of the two computed values, 82 and 48
cm-1
,
.

Electronic and Vibrational Structure of CuBr2
J. Phys. Chem. A, Vol. 106, No. 22, 2002 5431
Figure 2. Excitation (yield) spectrum of CuBr2 near the E2Πu-X2Πg origin. It was obtained by monitoring the overall intensity of the near-
infrared fluorescence and scanning the Ti-sapphire laser in the region of the lowest-energy bands of Figure 1.
absorption, proving that the emission is due to the same carrier
that is responsible for the near-infrared absorption spectrum.
As will be discussed below, a detailed analysis of the fluores-
cence spectrum unambiguously identifies this carrier as linear,
centrosymmetric copper dibromide, CuBr2. The excitation
spectrum, compared with the direct absorption, exhibits a
considerably improved resolution and signal-to-noise ratio. It
shows that the lowest energy absorption bands exhibit a
reproducible sharp structure, which is at least partially due to
site effects, and reveals that the origin of the major site
spacing. This spacing is appropriate for assignment to a Cu-
Br stretching frequency but clearly too high to be attributed to
a bending frequency of a triatomic molecule like CuBr2. In bent
molecules, usually the bending angle is the parameter that is
most sensitive to electronic excitation, resulting in prominent
progressions in the bending frequency in the electronic spectrum.
This result suggests that if the spectrum is attributed to CuBr2,
the molecule should be linear, similar to the CuCl2 and CuF2
species.
Although the first band of the main progression at 6870 cm-1
(denoted 0,1 in Figure 3) is relatively sharp, the lower-energy,
higher members of the progression broaden, and starting with
the third (2,1 in Figure 3) band, they show signs of a triplet
structure. This structure becomes much better resolved in the
following bands of the progression, with the bands denoted 4,1,
5,1, and 6,1 being clearly resolved triplets with approximate
intensity ratios of about 1:2:1. This is what should be expected
for the V1 vibrational progression of a linear centrosymmetric
CuBr2 molecule, with the 79BrCu79Br, 79BrCu81Br, and 81BrCu81Br
isotopomers in natural isotopic abundances. Although the
presence of some of the weaker bands can be shown to be due
to inhomogeneous “site effects” that are caused by imperfect
site selection and incomplete suppression of the CuBr2 molecules
isolated in the spectrally shifted “sites”, other bands cannot be
explained in this way, and we will return to a more detailed
analysis of the emission spectrum in a later section.
absorption is located at 11212.3 ( 0.1 cm-1
.
Laser-Induced Fluorescence Spectrum. When the laser
excites the strong, broader bands near the intensity maximum
of the absorption spectrum, the emission bands are relatively
broad and seem to be doubled. This “doubled” appearance can
be shown to be due to nonselective, simultaneous excitation of
CuBr2 trapped in several trapping sites. When, on the other hand,
the laser is tuned to one of the weak maxima in the sharp
structure near the absorption origin, the emission lines sharpen
considerably and the band doubling disappears, which then
greatly facilitates the analysis of the spectrum. As is usually
the case with molecules of this type in matrices, CuBr2 is trapped
in solid neon in several sites that can be selectively excited by
the laser. The one responsible for the 11 212.3 cm-1 origin gives
rise to the most intense spectrum, with another slightly less
intense site being shifted 27.4 cm-1 to lower energies. This “red”
site exhibits, except for the uniform 27.4 cm-1 shift of all
vibronic bands, spectroscopy and molecular constants identical
to those of the more intense “blue” site on which the following
discussion will now concentrate.
Selective excitation of the blue site results in a relatively
sharp, well-resolved fluorescence spectrum, which is presented
in Figure 3. Its overall intensity is suggestive of a vibrationally
relaxed emission from the V ) 0 level of some excited electronic
state. The spectrum extends from an origin in the neighborhood
of 8700 cm-1 to well beyond the ∼7000 cm-1 cutoff of the
germanium detector. Note that the intensity scale in the second
panel of Figure 3 is expanded by about a factor of 10. The most
intense “main” bands in the spectrum form a regular, nearly
harmonic progression starting with a strong, sharp line at 8689.0
( 0.1 cm-1 followed by a series of bands with an ∼226 cm-1
When an indium antimonide detector with a response
extending further into the infrared is employed instead, a second,
lower-energy spectrum extending between about 6900-6100
cm-1 is observed, as shown in Figure 4. Its structure appears
relatively simple and similar to the higher-energy Ge detector
region spectrum described above. It consists of a single
progression of at least four rather strong bands with an origin
at 6870 cm-1 and an ∼225 cm-1 spacing. As in the higher-
energy spectrum, the first band at 6870 cm-1 is relatively sharp,
but the higher members of the series progressively broaden and
when examined more carefully, again suggest the triplet structure
with about a 1:2:1 intensity ratio. This result is indicative of a
linear-linear electronic transition of a centrosymmetric CuBr2.

5432 J. Phys. Chem. A, Vol. 106, No. 22, 2002
Lorenz and Bondybey
Figure 3. Infrared C2∆g-X2Πg (3/2) emission spectrum of CuBr2 excited at 11 382 cm-1. The strongest progression starting with the false origin
at 8689.7 cm-1 appears because of vibronic coupling with the πu V2 promoting mode. Note the clearly resolved 1:2:1 triplets for the higher vibrational
levels due to bromine isotopic splitting. The true origin at 8756 cm-1 appears weakly and is denoted by an arrow. Note the weak progressions
involving the 3V2 and 5V2 levels. Bands marked by asterisks are due to CuBr2 in a spectrally shifted site.
Figure 4. Lower-energy infrared fluorescence assigned to the C2∆g-A2Πg (1/2) transition of CuBr2 in solid neon at 7 K measured with an InSb
detector. The sharp band at 6535.8 cm-1 is due to an unidentified impurity.
There may be at least three different interpretations explaining
the fact that two separate band systems, presented in Figures 3
and 4, that are both due to the same CuBr2 molecule are
observed:
Possibility (a), involving the same lower state, can be easily
eliminated because even though the lower states of the two
transitions have very similar vibrational frequencies, detailed
analysis reveals that these frequencies are well outside the
experimental error. The spacings between the first and second
bands of the main progressions are 226.48 and 225.04 cm-1
for the two transitions, which gives a difference of 1.44 cm-1
whereas the mean error of the entire fit of the spectrum of more
than 30 observed isotopic bands30 (to be described below) is
∼0.16 cm-1. It seems, therefore, that possibility (b) must be
preferred, that is, a nonradiative relaxation of the absorbing state
at 11 212.3 cm-1 populates a single state at 8689.6 cm-1, which
(a) The band systems represent emissions from two different
fluorescing states, separated in energy by 1820 cm-1, into the
same lower state, presumably the CuBr2 ground state.
(b) The band systems represent emissions from one fluoresc-
ing state into two lower-lying states, presumably the CuBr2
ground state, and a low-lying excited state at about 1820 cm-1
.
(c) It is much less likely that two different emitting states as
well as two different lower states are involved.

Electronic and Vibrational Structure of CuBr2
J. Phys. Chem. A, Vol. 106, No. 22, 2002 5433
then fluoresces into the ground state as well as into a low-lying
excited state some 1820 cm-1 higher in energy.
Electronic Structure of CuBr2 and the Absorbing State.
There is little experimental information about the triatomic
copper dihalides; also, theoretical calculations for species of
this type are difficult. The open d electron shell of transition
metals still represents a considerable challenge, and for a
molecule containing heavier halogen relativistic effects, it
become increasingly important. Although copper dibromide was
not extensively studied, about a decade ago Bauschlicher and
Roos20 carried out detailed computations for the lighter CuCl2
and CuF2 species, which we found a few years ago to be
extremely useful in interpreting our experimental laser-induced
fluorescence and absorption spectra of matrix-isolated copper
dichloride.21 Both the energies of the low-lying electronic states
and their vibrational structures proved to be very consistent with
the observed spectra. Also, the results of a recent low-resolution
photodetachment study of CuCl2 anions showed satisfactory
agreement with the results of Bauschlicher and Roos.20 Although
their computations did not extend to the isovalent dibromide,
in general its electronic structure can be expected to be quite
similar.
Figure 5. Energy levels of CuBr2. The left-hand side shows a 193-
nm photodetachment spectrum of CuBr2 that was adapted from Figure
2 of ref 24. The upward-aiming arrow denotes the E2Πu-X2Πg laser
excitation. Following nonradiative relaxation of the E state (denoted
by the wiggly arrow), probably via the D2∆g state, emission is observed
from C2∆g into both the A and X components of the 2Πg ground
electronic state.
The CuBr2 ground state and its lowest excited state should
again be the so-called ligand-field states arising formally from
an ionic Cu2+ cation with two Br- anions. Ligand-field theory
shows that depending on the orientation of the hole in the d9
value for the absorbing state, supporting its assignment to the
2
lower spin-orbit component of the Πu electronic state.
core the degeneracy will be split, resulting in Σg+, Πg, and
2∆g electronic states, with ab initio computations predicting the
X2Πg to be the ground state. In the heavy halides, the fine-
structure components of the latter two states should split far
apart because of spin-orbit coupling. Obviously, because all
these states are of gerade symmetry, transitions between them
should be strictly forbidden. It is again expected that levels
arising from a closed-shell 3d10 Cu+ configuration will be
slightly higher in energy. These levels are, perhaps somewhat
inconsistently, called charge-transfer states Because they can
be viewed as a superposition of the BrCu+Br- and Br-Cu+Br
structures, at least in a formal sense, there is less charge transfer
than in the ligand-field states. The lowest of these states will
2
2
Laser-Induced Fluorescence Spectrum and Identities of
the States Involved. Because, as already noted and in contrast
with CuCl2,22 no emission from the E2Πu state back to the X2Πg
ground state is observed despite the fully allowed character of
the transition, apparently E2Πu must efficiently relax nonradia-
tively. The intense fluorescence starting more than 2000 cm-1
further into the infrared must then be assigned to emission from
electronic states populated by this relaxation process. Because
no states other than the ligand-field states discussed above are
expected below the E state, it is clear that transitions between
these states must be involved even though these are, as already
mentioned, forbidden.
According to a standard treatment23 that can be found in most
physical chemistry textbooks, one writes for a transition moment
between two electronic states
2
be a Πu state, with a fully allowed transition into the ground
state. It seems inevitable that the strong absorption we observe
starting at 11 212 cm-1 must be assigned to this transition.
The photodetachment spectrum of the CuBr2- anion, reported
recently by Wang et al.,24 is useful in discussing the CuBr2 state
assignments. The spectrum is reproduced in a vertical orientation
on the right-hand side of Figure 5. The authors24 assigned the
two bands denoted in Figure 5 as E and F to the (1/2) and (3/2)
spin-orbit components, respectively, of this 2Πu electronic state
and report energies of 10 550 and 11 800 cm-1 for these states.
The features observed in the photodetachment spectrum are,
however, quite broad; furthermore, the bands assigned to
different states exhibit different widths and shapes. Therefore,
measuring the separations of the band maxima may not be the
most reliable way of determining the state adiabatic energies.
Rev ) e′,V′′| µ | e′′,V′′ ) e′ | µe | e′′ V′ | V′′ ) Re V′ | V′′
with the transition intensity being proportional to its square,
Rev2. According to the Franck-Condon principle, the Born-
Oppenheimer approximation allows a separation of vibrational
and electronic motion and factoring of the electronic transition
moment Re. Here, the first factor, Re, determines if the electronic
transition is allowed, with the second and the Franck-Condon
2
factors V′ |V′′ determining the intensity distribution between
various vibrational levels. For a given band to appear, the
integrand in V′ |V′′ has to be totally symmetric, and because
most molecules are usually in the vibrationless level, only totally
symmetric vibrations appear in the spectrum.
We have attempted to reanalyze the photodetachment spectra
of both CuCl2 and CuBr2 by an alternative method. We have
first extrapolated the steeply rising portion of each observed
photodetachment band to find an intercept with the baseline,
as suggested by the dotted lines in Figure 5. We have then
converted the separations between intercepts obtained in this
way into the adiabatic state energies and have obtained
considerably different values of about 11 500 ( 200 and 12 700
( 200 cm-1 for the E and F states, respectively. The former
value is in very good agreement with the matrix ∼11 200 cm-1
As realized by Herzberg and Teller,23 the separation of the
electronic and nuclear motion is not necessarily rigorous, and
the dipole and the electronic transition moment Re will change
with the motion of the nuclei. To include this effect of vibronic
coupling and the changes of the electronic transition moment
with the motion of the nuclei, one can express the electronic
transition moment in the form of a Taylor expansion in normal
coordinates while retaining only the first two terms: Re ≈ Re0
+ Σi ReiQi where Rei ) ∂Re/∂Qi. Inserting this expression into

5434 J. Phys. Chem. A, Vol. 106, No. 22, 2002
the above equation, one obtains
Lorenz and Bondybey
TABLE 1: Adiabatic Energies of the Low CuBr2 Electronic
States (cm-1
)
gas phasea
gas phaseb
neonc
theoryd
0
Rev ) V′| Re|V′′ ) Re0 V′ | V′′ + Σi Rei V′ | Qi|V′′
X2Πg (3/2)
A2Πg (1/2)
B2Σg+ (1/2)
C2∆g (3/2)
D2∆g (5/2)
E2Πu
0
1050
2200
7650
9350
10 550
11 800
0
1900
3100
0
1820
Depending on the symmetry of the Qi mode, the second term
may be nonzero and result in nontotally symmetric vibrations
appearing in the spectrum. Particularly interesting is the case
where Re0 and the first term are zero, that is, the transition is
forbidden, and only bands deriving their intensity from the
second integral are observed. Transitions of this type are
characterized by the absence of the true 0-0 origin and
occurrence of progressions in the totally symmetric vibrations
built upon “false origins” involving odd quanta of vibrations
belonging to a symmetry species that makes the second integral
nonzero. A typical example is the A1B2u-X1A1g transition of
benzene, where one observes in absorption as well as in emission
progressions in the totally symmetric V1 vibration combined with
1 quantum of the e2g V6 mode. A similar situation can also occur
for copper dihalides, where the g-g electronic transitions are
symmetry-forbidden but weak bands may appear because of
vibronic interactions involving the V2 Πu bending vibration.
As noted above, an analysis of the spectrum suggests that
the fluorescence occurs from a state located at about 8700 cm-1
into the ground state and into a low-lying state at around 1800
cm-1. In our recent study of CuCl2, we have concluded21 that
the fluorescence originates from the lower component of the
2∆g ligand field, and a similar interpretation seems reasonable
1609
7322
8700
8756
10 500
11 500
12 800
11 212
10 408
F2Πu
a State energies as reported by Wang et al.24 b Our values derived
from our reevaluation of Wang et al.’s data24 as explained in the text.
cNeon matrix values from this work. d Theoretical results consisting
of CCSD(T) calculations neglecting spin-orbit splitting.24
TABLE 2: Vibrational Constants of 63Cu79Br2 and 63Cu35Cl2
in Solid Neon (cm-1
)
T0
V1
V2
x1,1
x2,2
x1,2
CuBr2
59.09 -0.24
X2Πg (3/2)
0
226.94
2.37
0.63
A2Πg (1/2) 1757.4 226.29
0.09
CuCl2
96.97 -0.52
103.56 0.04 -0.38
X2Πg (3/2)
A2Πg (1/2)
0
178
370.55
350.1
4.48 -0.16
a value of 66.4 cm-1 for the “promoting” asymmetric πu mode,
in adequate agreement with theory. This result also explains
the presence of another weaker progression originating at 8542.7
cm-1, which is 146.9 cm-1 below the 8689.6 cm-1 band. This
band is another false origin, with 3 quanta of the bending
vibration, 3V2′ being the promoting level.
-
for the isovalent CuBr2. The CuBr2 photodetachment spectra
of Wang et al.24 exhibit two broad peaks in this region, which
they label C and D. They assign them, by analogy to similar B
and C peaks detected in the spectrum of the isovalent CuCl2
molecule, to the two spin-orbit components of a 2∆g state and
report values of 7650 and 9350 cm-1 for the vertical detachment
energies of the 2∆g (5/2) and 2∆g (3/2) components, respectively.
When we apply the same treatment we have used for the E
state, that is, we find the photodetachment onset for each of
the bands observed in the Wang et al.24 spectrum, and then use
these onsets rather than the band maxima of the broad bands to
derive the adiabatic energies, we obtain values of 8700 and
10 500 cm-1 for the C and D states, respectively. The former
value is again in satisfactory accord with the matrix observation.
The most consistent and reasonable interpretation of the neon
matrix observations appears to be that the initially excited E2Πu
state relaxes nonradiatively into the slightly lower-lying 2∆g state
and ends up in the vibrationless level of its lower component,
the C state of Wang et al.24 as shown schematically in the
energy-level diagram presented on the right-hand side of Figure
As noted above, the V1 progressions can be followed at least
up to V1′′ ) 6, with the higher members (V1′′ g 3) exhibiting a
fully resolved bromine isotopic structure. The calculation of the
isotopic factors Fi and the isotopic shifts in linear centrosym-
metric molecules such as CuBr2, where there is just one vibration
of each symmetry species, is quite straightforward. In principle,
bands involving the bending vibration should exhibit a 65Cu-
63Cu isotopic structure, with F2(65Cu) ) 0.989. Unfortunately,
in view of the relatively large ∼2 cm-1 line widths, the expected
splitting of about 0.7 cm-1 is too small to be clearly resolved.
The least-squares fit of all the observed bands using the
appropriate isotopic relationships gives a mean-square deviation
of 0.16 cm-1. The bending V2 vibration exhibits a “negative
anharmonicity”, x22 ) 2.37 cm-1, which is reasonable because
the bending potential must be symmetric and cannot contain
cubic or higher-order odd terms. We list the vibrational constants
of 63Cu79Br2 derived from the least-squares fits in Table 2, where
they are compared with the corresponding values reported for
63Cu35Cl2.21 Using the listed constants, the observed bands can
be computed to within the experimental accuracy.
2
5. This state then emits into the Πg ground state, with the
higher-energy fluorescence in Figure 3 corresponding to the
transition into the X2Πg (3/2) ground state and the weaker
emission in the InSb detector range (Figure 4) terminating in
the higher A2Πg (1/2) component. Table 1 gives a summary of
the reported energies24 of all the relevant states of CuBr2 along
with our revised values and compares them with the theoretical
and neon matrix data.
The second fluorescent transition observed with an origin at
6870 cm-1 is then again assigned to emission from the
vibrationless level of the C2∆g state into the higher spin-orbit
component of the ground state, A2Πg. The emission is undoubt-
edly again due to vibronic coupling, but because the signal-to-
noise ratio with the indium antimonide detector is considerably
lower, neither the true origin nor the progression involving 3V2′′
are observed in this case. The totally symmetric vibrational
frequency is very similar to that in the lower component, but
interestingly, although V1 of the higher state exhibits a small
negative anharmonicity, that of the ground state has a regular
anharmonicity. This result could be explained by interactions
and “repulsion” between the two states in the matrix.
Vibrational Structure of the CuBr2 2Πg Ground State. By
accepting the interpretation that the symmetry forbidden transi-
tion occurs because of Herzberg-Teller vibronic coupling, the
strongest bands have to be assigned to a progression of the type
10n 20 , with the first band at 8689.6 cm-1 corresponding to the
1
n ) 0 false origin. This interpretation is then supported by the
observation of a very weak “true” 0-0 origin at 8756.0 cm-1
,
which is 66.4 cm-1 above the false one. Although in the gas
phase this transition would be rigorously forbidden, it appears
weakly because of interactions with the solid medium, yielding

Electronic and Vibrational Structure of CuBr2
J. Phys. Chem. A, Vol. 106, No. 22, 2002 5435
Figure 6. Three-dimensional equal intensity contour plot, with the excitation wavenumber on the ordinate and the emission wavenumber on the
abscissa. Individual transitions appear as islands in this plot; different sites are diagonally shifted. Emission in the region of the 10 201 transition is
4
shown; note the horizontally clearly separated bromine isotopic triplets. The separation in the vertical direction is much smaller because the laser
was scanned near the V′ ) 1 upper-state level.
The observed ground-state vibrational frequencies appear to
be in acceptable agreement with our B3LYP density functional
calculations as well as with the recent computational results of
Wang et al.24 The similar vibrational structure of the two states
is consistent with their assignment to two components when
while the laser was tuned approximately to the V′)1 level of
the E2Πu state. One can clearly identify the Br2 triplet near
11 387 cm-1 with the isotopic components separated horizon-
tally by nearly 5 cm-1, as expected for emission into the V1′′ )
4 level. The splitting in the vertical direction is much smaller
(∼1 cm-1) because the V1′ ) 1 level is excited. This triplet
pattern is repeated ∼35 and again ∼64 cm-1 higher in energy.
Some of these observations are clearly due to site effects, but
in view of the limited excitation range available for study, the
details of this structure are not fully understood.
2
one neglects the spin-orbit coupling of the same Πg ground
state. Also, the 1820 cm-1 value does not appear unreasonable
for the splitting of the two spin-orbit components in the heavy
CuBr2 molecule.
Site Effects and Matrix Shifts. It is often observed in
matrices for both atoms and molecules that although transitions
between states arising from the same electronic configuration
appear sharp those involving configurational changes are
broadened and exhibit larger inhomogeneous broadening and
extensive phonon sidebands.31 Also, in the present case, one
can see by comparing the spectrum in Figure 1 with those in
Figures 3 and 4 that although the bands due to the C2∆g-A,
X2Πg transitions between the ligand field states, which all
formally arise from the d9 Cu2+ configuration, appear sharp the
interconfiguration E2Πu-X2Πg bands (with the E state being
derived from d10 Cu+) are broad and the vibrational progression
appears irregular. Usually, such interconfiguration transitions
also exhibit much larger medium shifts from gas to matrix or
from one matrix material to another.31 Therefore, although the
matrix values for the transitions between the different ligand
field states should be within 1-2% of the free-molecule values,
a somewhat larger medium shift might be expected for the
E2Πu-X2Πg spectrum.
The limited matrix data also does not provide any clear clues
for the causes of the irregularity of the E2Πu vibrational
progression seen in Figure 1. The presence of Fermi resonances
between the V1 stretching vibration and the overtones of the V2
bending vibration, resulting in unresolved “Fermi Polyads” could
be one explanation. There is also no clear experimental proof
that the charge-transfer upper state could not be bent, in which
case progressions in the bending vibration could also be
involved. Finally, interactions, perhaps matrix-induced, between
the two E and F components of the 2Πu excited state could lead
to irregularities in the vibrational structure. Clearly, a high-
resolution gas-phase study would be helpful in providing a
detailed understanding of the excited E2Πu state properties.
Summary
Absorption and laser-induced fluorescence spectra of copper
dibromide in solid neon were investigated. An absorption due
to the E2Πu-X2Πg transition with an origin near 11 212 cm-1
is observed. When the E2Πu state arising from Cu+ with the
closed-shell d10 configuration is excited, it relaxes nonradiatively
to lower-lying states, the so-called ligand field states of CuBr2,
which can be formally represented as a d9 Cu2+ ion interacting
with two bromide anions, and it populates the C2∆g state. Two
transitions from this state are observed in the emission that
terminates in the X2Πg (3/2) ground state and in the A2Πg (1/2)
spin-orbit component that is about 1820 cm-1 higher in energy.
These symmetry-forbidden transitions are made possible by
vibronic Herzberg-Teller coupling with the Πu V2 vibration
The involvement of inhomogeneous broadening is clearly
demonstrated by the laser excitation spectra and by the ability
to excite individual sites selectively. In such cases, one can often
get additional insight from 3-D plots where the emission
wavenumber is on the abscissa, the excitation wavenumber is
on the ordinate, and the lines connect points of equal emission
intensity. Each vibronic transition appears in such a plot as a
separate “island”. Unfortunately, we could record such spectra
only in a fairly narrow spectral region. Figure 6 displays a
0
0
section of the emission around the C2∆g-X2Πg 14 31 band

5436 J. Phys. Chem. A, Vol. 106, No. 22, 2002
Lorenz and Bondybey
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resolved, and a least-squares fitting of the observed bands yields
the vibrational frequencies and anharmonicities of the states
involved. The results for both CuBr2 and CuCl2 are tabulated
and summarized and are compared both with theory and with
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corresponding anions.
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