THE JOURNAL OF CHEMICAL PHYSICS 134, 174503 (2011)
Solvation dynamics through Raman spectroscopy: Hydration of Br2
and Br3 − , and solvation of Br2 in liquid bromine
Edward T. Branigan, N. Halberstadt,a) and V. A. Apkarianb)
Department of Chemistry, University of California, Irvine, California 92697-2025, USA
(Received 23 February 2011; accepted 5 April 2011; published online 3 May 2011)
Raman spectroscopy of bromine in the liquid phase and in water illustrates uncommon principles and yields insights regarding hydration. In liquid Br2 , resonant excitation over the B(3 0u + )
← X(1 g + ) valence transition at 532 nm produces a weak resonant Raman (RR) progression accompanied by a five-fold stronger non-resonant (NR) scattering. The latter is assigned to pre-resonance
with the C-state, which in turn must be strongly mixed with inter-molecular charge transfer states.
Despite the electronic resonance, RR of Br2 in water is quenched. At 532 nm, the homogeneously
broadened fundamental is observed, as in the NR case at 785 nm. The implications of the quenching of RR scattering are analyzed in a simple, semi-quantitative model, to conclude that the inertial
evolution of the Raman packet in aqueous Br2 occurs along multiple equivalent water–Br2 coordinates. In distinct contrast with hydrophilic hydration in small clusters and hydrophobic hydration
in clathrates, it is concluded that the hydration shell of bromine in water consists of dynamically
equivalent fluxional water molecules. At 405 nm, the RR progression of Br3 − is observed, accompanied by difference transitions between the breathing of the hydration shell and the symmetric stretch
of the ion. The RR scattering process in this case can be regarded as the coherent photo-induced
electron transfer to the solvent and its radiative back-transfer. © 2011 American Institute of Physics.
[doi:10.1063/1.3583477]
I. INTRODUCTION
Molecular bromine serves as a useful probe for interrogating hydration dynamics spectroscopically. The structure of H2 O:Br2 in the 1:1 complex has been established
through microwave spectroscopy and ab initio calculations.1
The lone pair of electrons on oxygen fills the σ -hole of
bromine, and although there is negligible charge transfer
between H2 O and Br2 , the binding energy is significant:
3.38 kcal/mol, determined by dipole-induced dipole and
dipole–quadrupole interactions.2 This motif of non-bonding
between a Lewis base (H2 O) and a halogen has been referred
to as the “halogen-bond,” as a weak analog of the hydrogenbond,3 or σ -hole bonding.4 In essence, in the 1:1 complex,
bromine is hydrophilic. In liquid water, Br2 is not adequately
described as hydrophilic or hydrophobic, as judged by its mild
solubility 0.605 g/mol (a mol fraction of Br2 :264H2 O).5 Only
upon freezing and clathrate hydrate formation it will behave
as a hydrophobe, as seen in its propensity to form clathrate
hydrates.6, 7 As a general rule, the solubility of small nonpolar molecules (3.5–7.5Å) greatly increases in the solid phase
by formation of clathrate hydrates.8 In the case of bromine,
the solubility increases by more than an order of magnitude: Br2 :8.62H2 O in the clathrate tetragonal structure I, and
Br2 :17H2 O in cubic structure II.7 The reduction in solubility
between the solid and liquid phase illustrates the entropic cost
a) Permanent address: Université de Toulouse, UPS, Laboratoire Collisions
Agrégats Réactivité, IRSAMC, CNRS, UMR 5589, F-31062 Toulouse,
France.
b) Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0021-9606/2011/134(17)/174503/6/$30.00
to hydrophobic hydration—pentagonal water rings being the
structure around clathrate formers.9 The hydrophobicity of
Br2 is not apparent in small clusters. In the 1:1 complex, Br2
is polarized with 5% of an electron transferred from the bound
Br atom to the free Br atom. The next water molecule hydrogen bonds to the polarized free halogen. In optimized structures of 2–8 water units, Br2 is both oxygen and hydrogen
bonded to the water network, sometimes with multiple bonds
on the same Br atom.10–12 The turnaround in the interaction
character from the 1:1 complex to bulk water makes bromine
an interesting solute for interrogating hydration. Here, we rely
on the information content in Raman spectra of Br2 and Br3 − ,
to make inferences about local structure and dynamics of
hydration.
We have previously carried out both time and frequency
domain spectroscopic studies using Br2 as a reporter of hydration structure and dynamics in amorphous ice,13 in clathrates
isolated in ice14 and in single crystal clathrates and in water.15
We have shown that Raman has the sensitivity to distinguish
between clathrate polymorphs.7 Relevant to these studies is
the Raman spectroscopy of neat liquid bromine, which was
recently reported along with a detailed analysis of its information content.16 A progression of more than 30 vibrations
can be seen when liquid bromine is excited at 405 nm. A
similar long progression is also observed in Br2 clathrate
hydrates, albeit measured at 532 nm.7 In both cases, the
implication is that the electronic coherence along the internal
coordinate of the molecule lasts long enough to prepare high
lying vibrations, and the v-dependent vibrational coherence
time is long enough to see sharp lines. A very different observation is made in liquid water. The resonant Raman (RR)
134, 174503-1
© 2011 American Institute of Physics
174503-2
Branigan, Halberstadt, and Apkarian
progressions vanish, while the non-resonant Raman line of the
fundamental remains relatively sharp and homogeneous. This
is contrasted with the aqueous tribromide ion, which shows
an RR progression with strongly distorted lines. We present
the data and expand upon what is learned about hydration
from Raman spectra, or from the quenching of RR scattering.
II. EXPERIMENTAL
Raman spectra were collected on a Renishaw Raman microscope with 405, 532, and 785 nm diode lasers. Saturated
solutions were prepared by mixing liquid bromine (purity
>99%) and nanopure water. The liquid bromine data were
collected in a cell consisting of two 250-μm thin cover slips,
sealed with Teflon grease (Krytox). The aqueous bromine
data were collected in a cuvette with its top sealed to a
250-μm thin quartz cover slip. All spectra are collected at
room temperature, under ambient conditions. The tribromide
spectra are obtained in aqueous Br2 solutions, without the addition of bromide salts—the observed ion is the equilibrium
concentration.13
III. RESULTS AND DISCUSSION
A. Pre-resonant Raman in liquid Br2 : Where the
non-resonant is more intense than the resonant
The RR spectrum of liquid bromine excited at 532 nm is
shown in Fig. 1. An intense fundamental line can be seen,
followed by a weaker progression that decays slowly with
v. The intensity ratio of the fundamental to first overtone
is 7.4:1, to be contrasted with the 1.2:1 ratio in the RR
progression seen at 405 nm.16 Clearly, scattering along two
different channels is being observed. A vibrational progression is the signature of RR, which involves evolution of the
Raman packet along the intramolecular coordinate on a real
excited electronic surface. In contrast, the observation of only
the fundamental implies non-resonant Raman, in which the
J. Chem. Phys. 134, 174503 (2011)
scattering is instantaneous (via a virtual state). The observation that the fundamental is more intense than the progression
is unusual since it implies that the non-resonant process has a
larger cross section than the resonant one. The NR scattering
must borrow intensity from a higher electronic state, i.e., must
be pre-resonant with a state which is much brighter than the
resonantly accessed state.17 The absorption spectrum of liquid
bromine, along with its decomposition, has been reported.16
At 532 nm, the resonance is with the B(3 0u + ) ← X(1 g + )
transition. At 405 nm, where B(3 0u + ) and C(1 1u ) states
equally contribute to the absorption, only RR is observed. We
may therefore conclude that the pre-resonance at 532 nm is
with the C-state. In contrast with the broad vibrational profiles
seen in Fig. 1 where by the 9th overtone the lines become too
broad to be seen above the background, the progression seen
at 405 nm is sharp, with more than 30 overtones that can be
clearly seen. The widths of Raman lines are determined by
vibrational dephasing in the ground electronic state dynamics
which is common to both excitations. However, in RR, a short
period of evolution on the excited electronic state precedes the
ground state dynamics. Evidently, the nature of the C state
reached at 405 nm, and the B state reached at 532 nm are very
different. The short-lived excited state evolution will create an
impulsive momentum kick of the solvent, dictating initial conditions for the subsequent ground state dynamics. The difference in excited state dynamics cannot be understood in terms
of the character of these states in the isolated molecule, since
both involve valence transitions in which a non-bonding π *
electron is promoted to the empty σ * orbital.18 Evidently, the
two states assume very different characters in liquid bromine.
The brightness of the pre-resonance suggests that the singlet
C-state borrows intensity by mixing with states that are only
present in the dense medium; namely, inter-molecular charge
transfer states. The collective nature of the latter would also
explain the v-dependent depolarization of the RR progression
at 405 nm, as suggested previously.16
B. Resonant Raman of aqueous tribromide
FIG. 1. Raman spectrum of liquid bromine at 532 nm. Inset: overtones enlarged to show detail.
The Raman spectrum of aqueous bromine recorded
at 405 nm is shown in Fig. 2. Although the peaks are
broad and distorted, the progression can be recognized
as the symmetric stretch of the tribromide ion. Without
decomposing the lines, the peak positions fit a progression
with harmonic frequency of ωe = 170 ± 1 cm−1 and
anharmonicity of ωe xe = 1.2 ± 0.2 cm−1 . The fundamentals
of Br3 − , both the allowed symmetric stretch, ν 1 , and the
non-allowed anti-symmetric stretch, ν 3 , have been previously
observed in water.19–21 The reported ν 1 frequencies range between 162 and 170 cm−1 and ν 3 values range between 195 and
220 cm−1 . A similar spread exists in the reported theoretical
values, due to the implemented methods.22 The prior experimental measurements were carried out in solutions of bromide salts, with variations in the observed frequencies are associated with counter-ions and their concentration. Our measurement is in a saturated solution of bromine in water, where
a possible interference may be the formation of Br5 − , which
has a known vibrational frequency of 250 cm−1 .20, 21 Based
174503-3
Br2 /Br3 − solvation dynamics
FIG. 2. Raman spectrum of the saturated Br2 water solution, at 405 nm (purple). The spectrum is dominated by the symmetric stretch progression of
Br3 − . The spectral profiles can be reproduced by decomposing each peak
into two Voigt components (lower traces, high frequency component: red,
low frequency component: blue). The shoulder near 310 cm−1 , which decomposes into a sharp peak, is assigned to the Br2 fundamental (green). The
summed fit is displayed over the experimental data (light blue).
on the clear demarcation of frequencies, the progression we
observe can safely be assigned to the symmetric stretch.
The distorted spectral profiles can be reproduced by
decomposing each peak into two Voigt components (see
Figs. 2 and 3). While the intensities of the high frequency
set decay monotonically, the low frequency set shows odd–
even variation. The shoulder on v = 2, which yields the sharp
component at 308.1 ± 1.0 cm−1 , coincides with the fundamental of neutral Br2 . Given the ∼2:1 ratio of Br2 and Br3 −
frequencies, the weak shoulder on v = 4 of Br3 − may be
suspected to belong to v = 2 of Br2 . However, the splitting
of the odd overtones, v = 3 and v = 5, cannot be explained
J. Chem. Phys. 134, 174503 (2011)
by such a coincidence. The more consistent interpretation of
the spectrum is that the overtones of the Raman active symmetric stretch of Br3 − are split in water. The high frequency
set of peaks fits to a strictly harmonic progression, ωe = 166
± 3 cm−1 , while the linear fit to the low frequency set yields
a slope of 162 ± 4 cm−1 and a negative intercept of −23
± 15 cm−1 . The decomposition suggests that we observe
a harmonic progression, with each peak accompanied by a
satellite shifted down by ∼23 cm−1 . This must arise through
motion along the solvent coordinate, i.e., vibration of the hydration shell, in which case the satellites would be assigned to
difference frequencies between cage and ion. The electron in
Br3 − is localized on the terminal atoms with a density of 0.45
on each.22 In the case of an excitation along Br3 − + ¯ω →
Br2 + Br− coordinate, the non-allowed anti-symmetric stretch
of the triatomic ion would be activated, as is well-documented
for the triiodide in polar solvents.23, 24 This may be implicated
in the prior observation of the Br3 − (ν 3 ) in water when excited
at 667.8 nm.19 Activation of only the symmetric stretch, with
strong coupling to the solvent coordinate, suggests that the
electronic resonance at 405 nm is a charge transfer to solvent
transition:25 Br3 − + ¯ω → Br3 + (H2 O)n − , and the Raman
scattering as radiative back-transfer of the electron. Such an
excitation would drive the hydration shell into motion. Difference bands in RR arise in hot-band transitions, where the
scattering occurs from a vibrationally excited initial state. In
this case, the requisite initial state is v = 1 of the solvent
cage mode, which at a vibrational energy of 23 cm−1 is fully
populated at room temperature. To appear as a well-defined
peak, the ion and its immediate cage must retain electronic
coherence during the scattering process, and vibrational coherence during the subsequent ground state evolution. With
regard to the neutral bromine molecule, we can only identify
the fundamental—the long RR progression seen in liquid Br2 ,
is completely absent in water.
C. Solvation of Br2 in water: Quenching of RR
FIG. 3. The peak positions of the Br3 − bands in the spectrum of Fig. 2.
The high frequency set (red) fits a straight line with zero intercept, implying
a harmonic series with ωe = 166 ± 3 cm−1 . The low frequency set (blue)
fits on a parallel straight line (ωe = 162 ± 4 cm−1 ), with an intercept of
−23 cm−1 . This set is assigned to difference bands between the vibrations of
the cage and the ion. The peak assigned to Br2 is shown in green.
The Raman spectrum of Br2 in liquid water, obtained
at 532 and 785 nm, is shown in Fig. 4. After background
subtraction, only the fundamental can be seen. No features in
the baseline could be attributed to either the Br2 progression,
or to Br2 –H2 O complexation—the latter search being guided
by the recently calculated spectra of the 1:1 complex.26 The
observed peak at 307.8 ± 1.0 cm−1 is red-shifted by 5%
relative to the bare molecule—a larger shift than theoretical
predictions for the 1:1 complex, of 310–311.4 cm−1 .10, 26
The compilation of the observed vibrational frequency of
Br2 in different environments is given in Table I. In the
perfectly hydrophobic hydration environment of clathrates,
the shift range is only 1%–2%. A range of site-specific
frequencies is observed for Br2 –H2 O complexes isolated in
Ar matrices, indicative of multiple structural minima that are
closely spaced in energy. The matrix stabilized complexes
must include oxygen bonded (H2 O · · · Brδ+ –Brδ− ) and
hydrogen bonded (HOH · · · Brδ− –Brδ+ ) structures, among
others. Assuming only 1:1 complexes, the largest red shift
observed in the matrix is 4.4%, shy of the values observed in
174503-4
Branigan, Halberstadt, and Apkarian
J. Chem. Phys. 134, 174503 (2011)
sistent with this, we do not see a measurable perturbation of the Raman profile of the water vibrations by the presence of bromine in the solution, despite the large vibrational
shifts predicted in the complex.11 The poor mechanical coupling between Br2 and water, yet the observation of a frequency red-shift that is larger than in the tightly bound 1:1
complex, suggests that the vibrational shift is due to dielectric
solvation. The effect can be understood as the increased polarizability of Br2 due to lowering of its ion–pair states by dielectric solvation, a model that has previously been advanced to
explain many-body systematics of vibrational shifts and interactions in halogens and hydrogen halides.27–29 The more compelling argument of the equivalence of many water molecules
in the first solvation shell can be derived from the conspicuous
absence of RR scattering in water, as we outline next.
D. Quenching of RR scattering by the bath
FIG. 4. Raman spectrum of aqueous Br2 , at 532 nm (green) and 785 nm
(red).
bulk water. Only in solids, under pressure, can the bond be
softened beyond what is observed in water.
Based on vibrational shifts it is possible to conclude that
the hydration environment in liquid water is not clathratelike. It must involve structures of the Br2 –H2 O complex with
multiple halogen and hydrogen bonds present at a time. The
Raman line profile clarifies that these are rapidly interchanging structures. The experimental profile of the fundamental
in water is Voigt, with Lorentzian and Gaussian contributions
to the width of 6 ± 1 and 11 ± 1 cm−1 at 532 nm, and 8
± 2 and 9 ± 1 cm−1 at 785 nm, respectively. The Gaussian
widths are close to the instrumental response, permitting the
conclusion that the Lorentzian components are shaped by the
hydration dynamics. The homogeneous broadening corresponds to a vibrational dephasing time of 5 ± 2 ps, much
longer than timescales of structural fluctuations in water.
To maintain such a vibrational coherence the interactions
with the hydration shell must be motionally averaged. ConTABLE I. Survey of bromine vibrational frequencies in selected hydration
environments.
moleculea
Free
TS-I clathrate hydrateb
Liquid brominec
CS-II clathrate hydrateb
H2 O:Br2 complex in matrix Ard
Aqueous solution
Solide
a
Reference 33.
Reference 7.
c
Reference 16.
d
Reference 34.
e
Reference 35.
f
Symmetry induced splitting.
b
Fundamental (cm−1 )
Shift (%)
323.2
319.6
318.6
316.1
314.3–309.1
307.8, 306b
302, 294f
0.0
1.1
1.4
2.2
2.8–4.4
4.8, 5.3
6.6, 9.0
Resonant Raman scattering is a fast process. It involves dipolar radiation during wavepacket evolution on
the electronically excited state. Vibrational intensities in
RR are determined by Franck–Condon factors between the
Raman wavefunction, Rl , and vibrational wavefunctions of
the ground electronic state, ϕg,v :17
Iv ∝ ωl ωs3 |ϕg,v |μ|Rl |2 ,
(1)
in which the Raman wavefunction is obtained by integrating
out its time evolution on the electronically excited state
∞
|Rl =
∞
ϕe (t)e
0
iωl t
dt =
e−i He t/¯ μ|ϕg (0)eiωl t dt.
(2)
0
Inserting Eq. (2) in Eq. (1) and considering the tintegrand yields the alternative picture, that the intensities
are derived from the half-Fourier transform of the correlation
function17
c(t)RR = ϕg,v (0)|μ|ϕe (t).
(3)
The time-independent Raman wavefunction Eq. (2) can
be regarded as the track of the excited state packet. According to Eq. (1) the track is all that is needed to understand intensities. The consideration allows for a simple analysis of
the effect of bath coordinates on the observable RR spectrum.
Also, the correlation function in Eq. (3) describes the absorption spectrum when v = 0. Knowledge of the absorption spectrum responsible for the RR scattering provides the auxiliary
information regarding forces on the excited state potential that
steer the Raman packet.
The evolution in Eq. (2) is controlled by the excited state
potential, Ve , which in the limited range of interest may be
defined by its local gradient:
f i qi , where f i = −(∂ V /∂qi )|g ,
Ve ({qi }) = V0 −
i
(4)
where the forces along different coordinates are the slopes of
the excited state potential at the ground state equilibrium configuration. Consider the one-dimensional evolution in the isolated molecule. In order to reach the v-th overtone, the packet
which starts as v = 0 at t = 0 must travel a distance Q(v)
174503-5
Br2 /Br3 − solvation dynamics
J. Chem. Phys. 134, 174503 (2011)
= (2v¯/μω)1/2 along the intermolecular coordinate, as given
by the classical turning points of a harmonic oscillator (v¯ω
= kQ2 /2). With the linear approximation of the excited state
in the Franck–Condon region, the time it takes to reach
the v-th overtone is Q(n) = ft2 /2μ. Vertically accessed
repulsive walls of potentials are steep, with associated force that can be extracted from the absorption
spectrum: f = E/2Q(1), where E is the half-width
at half-maximum of the absorption band (reflection
approximation).30, 31 Thus, based on spectroscopic observables (for Br2 , = 300 cm−1 , E = 2000 cm−1 for the
B←X transition)13 it can be estimated that to reach v
= 10 the wavepacket must move a distance Q(10)
= 8(v/μ)1/2 = 0.23 Å (μ in cm−1 amu units), which occurs
in τ v = 4 × 102 [μQ(v)Q(1)/E]1/2 ∼7 fs (E in cm−1 ). The
estimated scattering time of ∼1 fs/overtone is characteristic
for RR. It is difficult to disrupt such a fast process. Rather,
the quenching of RR can occur if the inertial track of the
Raman packet is not limited to the internal coordinate Q, if it
is displaced toward bath coordinates.
Let us first consider the case where the transition dipole
is localized on the molecular coordinate Q, and represent the
bath with a single solvent molecule interacting along the intermolecular orthogonal coordinate, q. Then the time correlation
(3) is separable
c(Q, q; t)RR = ϕg,v (Q, 0)|μ|ϕe (Q, t)ϕg (q, 0) | ϕe (q, t).
(5)
The chromophore correlation function is now multiplied
by the bath–bath overlap, the decay of which extinguishes
the signal. The decay is complete when the packet travels
a distance δq = (¯/2μ ω )1/2 along the q coordinate, at a
speed determined by the intermolecular force, f , on the excited surface. Since the absorption spectrum is determined
by the Fourier transform of Eq. (5) it consists of the convolution of the spectra along the individual coordinates. As
such, the acceleration on the excited state potential, f /μ ,
can be extracted from the broadening of the absorption band.
Generally, intermolecular forces are weaker and their associated states are more extended. As typical, assume ω = 0.1ω,
μ = 0.2μ, E = 0.1E, then τ /τ ∼10. The track of the
Raman packet no longer lies along Q; in the two-dimensional
q,Q plane, it climbs with a slope of τ /τ . Accordingly, the intensities of the RR progression will degrade monotonically,
with v ∼ 10 being the highest observable overtone. The progression is completely quenched when τ = τ , where only
the fundamental can be seen. This can occur for an unusually fast evolution along an orthogonal coordinate, or because
of the multiplicity of solvent coordinates. Thus, if n equivalent separable bath modes are set into motion upon electronic
excitation
c(t)RR = ϕg,v (Q, 0)|μ|ϕe (Q, t)ϕg (q, 0) | ϕe (q, t)n , (6a)
the decay of the bath-bath overlap will be accelerated
ϕg (q, 0) | ϕe (q, t)n = (1 − at 2 /2δq)n ≈ e−nat
2
= e−(t/τ ) .
2
/2δq
(6b)
The inertial decay time τ = (2μδq/nf)1/2 is shortened by
n1/2 . With the assumed values for the bath dynamics, for n
= 9, the RR progression is degraded to v = 3. The effect,
the acceleration of inertial decay due to evolution distributed
along multiple coordinates, has been previously illustrated
through explicit semi-classical many-body simulations.32
More generally, the transition dipole may be distributed
along both inter-molecular and intra-molecular coordinates,
as would be expected for a polarizable molecule interacting
with a dipolar solvent, or in the case of the tribromide ion.
Then the Raman scattering along q becomes observable. This
is nicely illustrated in the recent simulation of the RR spectrum of the 1:1 complex.26 In the complex, the inertial evolution along the Br2 –OH2 coordinate reduces the intensity of
the Br2 progression and generates a progression along the intermolecular coordinate. However, according to the ab initio
potentials f << f, the track of the Raman packet retains its
course along the Br–Br coordinate.26 The absence of such a
spectrum in the water suggests the absence of the complex
as a persistent structure. The complete quenching of the RR
progression in water requires inertial evolution along the solvent coordinate that competes with the intra-molecular force
on the B-state. The same is implied by the absorption spectrum. The B←X absorption envelope, FWHM = 3650 cm−1
in the gas phase, is broadened to 4300 and 5000 cm−1 in ice
and water, respectively.13 Assuming Gaussian profiles, the observed ∼21/2 width of the convoluted spectrum implies that
the spectral width along the bath coordinate is nearly the same
as that along the internal coordinate. Indeed, the spectrum
in ice has been reproduced assuming a single effective solvent coordinate with a steep repulsive wall.14 Based on the
ab initio potentials, we estimate that a single oxygen bonded
water molecule will only contribute ∼300 cm−1 broadening to
the absorption profile. A single water molecule cannot explain
neither the absorption nor the RR spectrum of Br2 in water —
the hydration shell must contain many (∼10) equivalent water molecules to accelerate the inertial evolution toward the
solvent. The same analysis allows conclusions about the inertial track of the Raman packet in the tribromide spectrum.
It must be displaced along the solvent cage coordinate to observe the cage motion, and must evolve with overlap along
the symmetric stretch coordinate of the ion to observe six
overtones.
IV. CONCLUSIONS
Through Raman spectroscopy of molecular bromine, we
extract insights about hydration dynamics. While this small
polarizable molecule binds strongly to water in small clusters, it is an efficient clathrate former that hydrophobically
hydrates in the solid state. Neither cluster nor clathrate structures describe its solvation in liquid water. Its homogeneous
Raman profile and the quenching of its RR response indicate
that there is no specific structure that dominates the solvation
state – the hydration environment of aqueous Br2 must consist of many time-averaged equivalent Br2 –H2 O interactions.
Its large spectral shift indicates strong electronic perturbation by dielectric solvation. We generalize the principles that
174503-6
Branigan, Halberstadt, and Apkarian
lead to the quenching of RR progressions, and offer a simple
formulation for semi-quantitative interpretation of the implied
dynamics. In contrast with the neutral dihalogen, the tribromide ion shows a progression of six vibrations along its symmetric stretch and difference bands along the solvent coordinate. The Raman scattering in this case is interpreted as
the coherent photo-induced electron transfer to the hydration
shell and its radiative back-transfer after setting the shell into
motion. The RR spectrum in this case allows the direct observation of the hydration shell. The RR spectrum of neat liquid
Br2 illustrates very different solvation dynamics. When the
electronic resonance is with isolated molecular states, a short
lived vibrational coherence is observed, while when intermolecular charge transfer states are accessed, the subsequent
ground state vibrational dynamics shows solid-like persistent
coherence.
ACKNOWLEDGMENTS
This research was initiated under an National Science
Foundation (NSF) Grant No. CHE-0404743 and completed
under NSF Grant No. CHE-0802913. N.H. acknowledges the
collaboration grant, PICS 2009, No. 4648, French National
Center for Scientific Research (CNRS).
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