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II. TIME-RESOLVED IR SPECTROSCOPY OF STRONGLY ASSOCIATED LIQUIDS

II. TIME-RESOLVED IR SPECTROSCOPY OF STRONGLY ASSOCIATED LIQUIDS

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a powerful spectroscopic tool that may provide an improved basis of

understanding. This is because of its photo-physical hole-burning potential

to reveal inhomogeneous broadening mechanisms, on the one hand, and

its experimental time resolution, on the other hand, so that vibrational and

structural dynamics can be directly followed as a function of time. One of

our main interests in this context is the investigation of hydrogen bonds

in condensed matter that play a major role in nature. The H-bridge bond

vibrations themselves show up in FIR at frequencies below ³400 cm 1 ,

which is difficult to access for time-resolved pump-probe spectroscopy. On

the other hand, the OH-stretching mode of the bonded hydroxylic group

in the 3000 1 region is known from conventional spectroscopy to be a

sensitive indicator of the hydrogen-bonding situation, which is more readily

accessible. In fact, it was demonstrated that the vibration can serve as a

spectral probe with ultrashort time resolution (62). Qualitatively speaking,

the OH-stretching vibration distinctly shifts towards lower frequencies for

stronger H bonds, while the IR absorption cross section correspondingly

increases by a factor up to an order of magnitude (1). This particular

feature makes IR studies of the OH stretch particularly sensitive to structural

changes as compared to Raman scattering or other vibrational modes, where

the amplitude effect is practically missing.

B. General Considerations



In conventional spectroscopy, inhomogeneous broadening of a vibrational

band is difficult to verify. Spectral hole burning can provide direct evidence

for a band substructure and reveal subcomponents hidden by a featureless

band contour. Exploiting the spectral and temporal resolution of pumpprobe experiments in the OH-stretching region, structural and dynamical

processes of H-bonded systems can be studied. The inherent problem

of relating measured spectral shifts to a local structural picture may be

tackled by computer simulations of the liquid ensemble. The method is, of

course, limited by the uncertainty principle; a compromise between spectral

and time resolution requirements has to be found for proper experimental

investigations.

The principle of the experiment is illustrated by Fig. 12. On the lefthand side the absorption band of an inhomogeneously broadened transition,

e.g., the OH-stretching band in different H-bonded local environments, is

depicted schematically. Structures with different OH-O bond angles and/or

O-O bond lengths show up in the spectrum with different OH frequencies; linear bonds and shorter bond length correspond to larger red shifts



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Figure 12 The principle of a pump-probe experiment performed on an inhomogeneously broadened OH-stretching band is depicted. On the left-hand side three structural components with decreasing H-bond strength and corresponding increasing

OH frequency are shown. The corresponding energy level scheme describing the

pump-probe experiment on the illustrated OH band is seen on the right-hand side.

The pump pulse excites predominantly one structure. Determination of the sample

transmission with an independently tunable probe pulse yields the temporal evolution of the structural components via spectral hole burning and structural relaxation

with time constant s . In addition, the vibrational lifetime T1 and in special cases

phase relaxation with time constant T2 can be determined.



of the OH frequency. Three spectral subcomponents with a certain quasihomogeneous width via dephasing processes are explicitly shown in the

figure (thin lines). The total band contour (thick line) results from the

superposition of the subspecies. A second mechanism that also effects the

OH frequency position via the local H-bonding situation will be discussed

below. Structural relaxation of the molecular environment corresponds to

fluctuations of the individual OH frequencies (spectral relaxation) and is

described by a time constant s . With respect to the experimental situation the process is assumed to be sufficiently slow compared to the inverse

quasi-homogeneous width of a subcomponent. The IR pump pulse with

a spectral width notably smaller than the total band resonantly interacts

only with a fraction of the OH groups. In this way a subensemble is



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selected according to its frequency position and promoted from the ground

state to an excited vibrational level. A situation is depicted where the

structural components located at the peak of the OH band are predominantly excited.

The corresponding energy level scheme is shown on the righthand side of Fig. 12. Arrows denote the pump process (thick arrow)

and possible probing transitions (thin arrows) between the v D 0 ! v D 1

transitions (ground state bleaching) and v D 1 ! v D 2 transitions (excited

state absorption). The induced changes of the sample transmission are

measured by the help of a weak, independently tunable probe pulse. Probing

transitions are indicated by thin vertical arrows. Spectral relaxation is

indicated in the level scheme and appears as an energy transfer process,

although the mechanism is a shift of excited individual OH groups to

a different frequency position. The process has to be distinguished from

energy migration among neighboring groups. The time constant of spectral

relaxation may be determined from the temporal evolution of the ground

state population, e.g., the missing molecules in the vibrational ground level

leading to a transient sample bleaching.

The population and reorientational dynamics are not indicated in

the figure, but may be also derived from the pump-probe measurements.

The population lifetime T1 in the first excited level can be inferred from

the excited state absorption monitoring the v D 1 ! v D 2 transition. The

molecular reorientation becomes experimentally accessible, introducing

polarization resolution in the probing step. For known structural and

population dynamics the temporal evolution of the width of the observed

spectral hole also provides information on the dephasing time T2 of the

vibrational transition (63).

For a quantitative treatment the molecular ensemble is described by an

n-level system with random orientational distribution and a corresponding

set of rate equations that implies short lifetimes of the induced transition

dipole moments compared to the experimental pulse duration T2 − tp .

Each level represents a spectral subemsemble of molecules in the vibrational

ground state v D 0 or the first excited state v D 1 of the OH-stretching

vibration and possible further intermediate states involved in the relaxation pathway, depending on the respective situation. Additional levels,

i.e., the terminating states of the probing transitions, need not be explicitly taken into account because of the negligible populations generated by

weak probing pulses. The coherent coupling artefact for pump and probe

pulses at the same frequency position is included by additional terms. The

coupled rate equations and the propagation equations for the incident pulses



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are solved numerically. The polarization dependence of the probe transmission is computed by integration over the orientational distributions, while

the reorientational motion is treated in the limit of rotational diffusion

(64). In a more general approach coherent effects can be also included;

the finite T2 plays an important role for the time evolution of spectral

holes (63).

As a simple example a three-level system with slow reorientation is

considered. States (0) and (1) are directly coupled to the excitation pulse,

while the intermediate level (2) is populated in the relaxation path of the

excess population N1 of the upper level on its way back to the ground state.

The temporal evolution of the population numbers and the pump intensity

IPu is given by:

∂N0

ncε0

N2

01

D

IPu

N0 N1 C

∂t

2

h¯ ω01

T20

ncε0

N1

∂N1

01

D

IPu

N0 N1

∂t

2

h¯ ω01

T12

N1

∂N2

N2

D

∂t

T12

T20



n∂



C

IPu D

∂y c ∂t



IPu



01



N0



N1



13

14

15

16



with 01 being the absorption cross section, Tij the relaxation time constants

i, j D 0–2 , and n the refractive index.

Numerical solutions of equations like the ones shown above for the

probe transmission may be fitted to the measured signal transients yielding

the relevant relaxation rates. Measured transient spectra are interpreted by

comparison with calculated spectra for a set of probing transitions with

assumed Lorentzian (spectral holes) and Gaussian lineshape (other spectral

components), consistent with an n-level model describing the time evolution, and proper convolution with the spectral profile of the probe pulses of

approximately Gaussian shape. In this way the finite instrumental resolution

is taken into account and true spectral parameters of the physical system

deduced from the experiment. Fitting to the experimental data is performed

using a Levenberg-Marquardt algorithm (65).

C. Experimental Aspects



In order to obtain new information on the structural and vibrational

relaxation of molecular systems with subpicosecond time resolution, we



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have developed a suitable laser system for two-color IR pump-probe

spectroscopy with pulses close to the Fourier limitation and adjustable in the

range 0.4–2.6 ps, derived from synchronously pumped optical parametric

oscillators. For the measurement of spectral holes with widths below

³30 cm 1 , a frequency width of 20 cm 1 of the pulses is available in

our experiments. The instrumental parameters may be compared with other

setups based on commercially available Ti:sapphire lasers with intense IR

pulses (13–15) of shorter duration, down to 100 fs, but at the expense

of spectral resolution due to a pulse width around 100 cm 1 of optical

parametric amplifier devices. The latter value is not appropriate for the

investigated spectral holes of widths below 50 cm 1 .

We start with a specially designed, Kerr-lens mode-locked Nd:YLF

laser (12,66) for the synchronous pumping of two optical parametric oscillators (OPOs) in parallel (repetition rate 50 Hz). The OPOs are operated at

variable pump intensities slightly above threshold up to the strong saturation regime. This allows for a tunable pulse duration of the fed-back idler

component in the OPO in the range of 2.6 ps to 260 fs. Single-pulse selection, frequency down-conversion, and amplification of the OPO outputs

are carried out in subsequent optical parametric amplifier stages equipped

with AgGaS2 crystals for an extended range in the IR. Two synchronized pulses with independent, automatic tuning ranges are accomplished.

Frequency setting using computer-controlled stepping motors requires a

fraction of a second. This makes the setup a real spectrometer for timeresolved investigations.

Working in the short-pulse regime, for example, excitation and

probing pulses of ½450 fs duration and spectral width Ä35 cm 1 are

generated in the range 1600–3700 cm 1 (12). The pulse energy amounts

to Ä10 µJ (pump) and ³10 nJ (probe). For long-pulse conditions, on the

other hand, pump (probe) pulses of 2 ps (1 ps) of spectral width 8 cm 1

16 cm 1 are available, providing superior spectral resolution. The data

discussed in the following mostly refer to these longer pulses if use of the

sub-ps pulses is not explicitly stated.

The noncollinear pump-probe experiment is depicted schematically

in Fig. 13. The linearly polarized (P3) pump pulse is focused (L1) into

the sample producing induced transmission changes. The polarization of

the probe beam is adjusted to 45° relative to the pump with a half-wave

plate /2 and a Glan polarizer (P1). By the help of an analyzer (P2)

simultaneous detection of the parallel jj and perpendicular ? components of the energy transmission T , tD of the probe through the sample

is installed. For blocked excitation (chopper, Ch) the sample transmission



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Figure 13 Schematic of the setup of the pump-probe experiment with polarization

resolution for the probing of the induced change in sample transmission. /2:

half-wave plate; P1–P3: polarizers; L1–L4: lenses; D1–D5: detectors; Ch: chopper;

VD: optical delay line. The sample is permanently moved in a plane perpendicular

to the beams in order to avoid accumulative thermal effects.



T0

is measured. The resulting relative transmission changes ln T/T0 jj,?

for variable probe frequency and probe delay time tD (VD) are used in

the following as the relevant signal quantities, from which the following

quantities are derived, as demonstrated by Graener et al. (11):

The isotropic signal amplitude,

ln T/T0 is D ln T/T0 jj C 2 ln T/T0 ? /3

The anisotropic signal ln T/T0 anis D ln T/T0 jj ln T/T0

The induced dichroism ln T/T0 anis /2 ln T/T0 is



?



The isotropic signal delivers (rotation-free) information on the

temporal evolution of the population numbers of the investigated vibrational

transition(s). The induced dichroism is governed by the time constant

or (second-order reorientational correlation time, 1 D 2) and possibly

population redistribution that may contribute to the loss of induced

optical anisotropy. The zero-setting of the delay time scale (maximum

overlap between pump and probing pulses) is determined by a two-photon

absorption technique in independent measurements with an accuracy of

better than š0.2 ps (67).



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Two-color pump-probe absorption spectroscopy is carried out with

moderate pump energies producing small depletions of the vibrational

ground state of only a few percent in order to avoid secondary excitation

steps and minimize the temperature increase of the sample due to the

deposited pump energy.

D. Alcohols in Solutions



In this section we will discuss results of transient IR spectroscopy of

different alcohols in solution in a wide concentration range from almost

monomeric alcohol samples to strongly associated oligomers. In order to

investigate the influence of hydrogen bonds on the dynamical properties

of the molecules, we present first a discussion of the data on the vibrational and reorientational dynamics of the OH mode of isolated molecules

in the solvent.

1. Monomers in an Apolar Solution

Early time-resolved experiments in the infrared by Heilweil and coworkers

were performed on an alcohol in apolar solvents (68). Since the laser

system utilized in these investigations delivered pulses of several tens

of picosecond duration, a problem was to deconvolute the instrumental

response function from the measured transients. A further complication for

the interpretation of the data arose from the fact that one-color measurements were carried out; parts of the same pulse were used for excitation and

probing. In such experiments it is difficult to separate the coherent coupling

artefact and to distinguish the recovery of the ground state from the population decay of the first excited state. The latter point is important if longerlived intermediates are involved in the relaxation pathway. It is more advantageous to conduct two-color measurements where the vibrational population lifetime can be deduced from the transient excited state absorption

monitoring the v D 1 ! v D 2 transition. In addition, the effect of reorientational dynamics has to be removed from the data in general by the help of

polarization resolution, measuring the isotropic signal transient (11,69). The

technique dwells on the common anharmonic frequency shift of vibrational

modes that is quite pronounced for the OH stretch ³200 cm 1 , comparing

the v D 1 ! v D 2 transition with the fundamental mode frequency. Timeresolved spectroscopy on binary systems in the liquid phase have recently

been reported by the group of Heilweil (70) demonstrating conservation of

vibrational excitation during chemical reactions.



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In the following we present data on the simple alcohols methanol

(Me), ethanol (Eth), decanol (De), and 2,2-dimethyl-3-ethyl-3-pentanol

(DMEP) in dilute solutions of CCl4 . Methanol is additionally investigated

at low concentrations in the solvents C2 Cl4 , C4 Cl6 , and C5 Cl6 providing

environments with differently shaped molecules (71). Here we are interested

in the lifetime and the relaxation channels of the excited OH-stretching

mode of monomers as well as the reorientational dynamics of the OH

group. A concentration of 0.25 M for the DMEP and of 0.05 M for the other

alcohols in the respective solvents was adjusted corresponding to a sample

transmission of ³50% at the respective peak position of the OH stretch.

Some typical results are presented in Fig. 14 for ethanol in CCl4 .

On the left-hand side the conventional (a) and time-resolved (b) spectra



Figure 14 Conventional (a) and isotropic transient (b) spectra of a 0.05 M

mixture of ethanol and CCl4 . The bleaching of the time-resolved spectrum shown

at a delay time of 0 ps (b) is related to ground state depopulation while the

corresponding excited-state absorption is red-shifted by 170 cm 1 . Time-dependent

measurements taken within the maximum of the respective components are shown

in (c) and (d), as denoted in the figure.



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are shown; the latter represents the rotation-free isotropic component at

zero-delay time (optimum overlap between pump and probing pulse) and

pumping in the maximum of the OH vibration. The different abscissa scales

should be noted. The conventional absorption (a) is dominated by the

OH mode with a maximum at 3636 cm 1 and the CH2,3 -stretching modes

including CH-bending overtones in the range of 2800–3100 cm 1 . In the

blue wing of the former a weak band is assigned to a combination tone

between a CH stretch and a CO- or COH-stretching mode (72) that plays

a role in the relaxation of the OH vibration. The extended red wing of

the OH band suggests the presence of a small number of ethanol dimers

positioned around 3500 cm 1 .

The time-resolved spectrum (Fig. 14b) consists of a bleaching feature

of the sample at the OH frequency resulting from the depopulation of

the vibrational ground state and induced (excited-state) absorption (ESA),

which is red-shifted because of the vibrational anharmonicity. From the

smaller spectral width of the sample bleaching 17 cm 1 with respect to

the conventional absorption band (width 29 cm 1 ), one concludes that spectral hole burning occurs in the OH band of monomeric ethanol, since the

reorientational motion is too slow to account for the difference. The inhomogeneous character is due to solvent-solute interaction and is probably

related to different preferred constellations of the OH group relative to the

ethyl group.

Some time-dependent data are depicted in Fig. 14c,d. From a comparison of model computations with the measured temporal evolution of the

induced bleaching (isotropic component, Fig. 14c) as well as the ESA

(Fig. 14d) directly related to excited state population, we infer a lifetime of the OH-stretching mode of T1 D 8 š 1 ps for the dissolved ethanol

monomers. From the anisotropic signal component measured in the same

experimental runs we determine the reorientation time to be or D 2 š

0.5 ps at room temperature.

The results for various alcohol solutions are compiled in Table 2.

The data refer to room temperature except for methanol in C5 Cl6 , which

was measured at 333 K. A few spectroscopic parameters are shown in the

first four lines. There is evidence for inhomogeneous broadening of the

monomeric OH-stretching vibration for the various investigated alcohols

with a width of the observed spectral holes as small as 50% of the conventional absorption band (DMEP). The anharmonic shift of the OH mode

is indicated in the table to amount to 170 cm 1 , independent of solute

or solvent within experimental accuracy. The large anharmonicity of the

monomeric alcohols is noteworthy. The population lifetime T1 of the mode



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displays little dependence on the number of CH groups of the alcohol and

on the apolar solvents leading to numbers of 8–10 ps. The data suggest

that intramolecular energy redistribution is dominant; in fact, the solvent

molecules do not not offer suitable accepting modes for vibrational energy

transfer of the OH vibration. The relaxation pathway was studied in detail in

the case of monomeric ethanol (73). From time-delayed bleaching at CHstretching frequencies intramolecular relaxation via these vibrations was

inferred. Theoretical arguments based on Fermi resonance coupling suggest

energy transfer to the adjacent combination tone of CH-, CO-stretching

modes that decays, in turn (in part), to the CH stretch. Considering the

frequency spacing and amplitude of the combination band relative to the

OH absorption, an OH lifetime of 8 ps is estimated (74) in perfect agreement with the measured value. The slightly longer values of the ground

state recovery time T10 in Table 2 are consistent with the multiple step

energy decay via CH to the ground state.

The reorientational motion of the OH group is found to proceed in

the mixture with CCl4 at room temperature with time constants of 2–4 ps

(see last line of Table 2). With increasing size of the solute molecule, the

reorientation seems to slow down. For the different mixtures, the largest

or ³ 7 ps occurs for the most viscous liquid C5 Cl6 .

2. Ethanol Oligomers in Solution: Spectral Holes and Vibrational

Lifetime Shortening

Equipped with information on the dynamical properties of the OH-stretching

mode of monomers, we are now in a position to tackle the role of H-bridge

bonds for the structure and dynamics of alcohols. The first time-resolved

investigations of this kind were performed by Laubereau and coworkers

(62,75). A mixture of ethanol and CCl4 was investigated by two-color

IR spectroscopy with 11 ps pulses. From the time evolution of the spectra

clear evidence was obtained that after excitation of the OH mode of ethanol

molecules with internal position in oligomer chains, ultrafast breaking of

H bonds occurs, followed by partial reassociation of the broken oligomers.

The measurements showed a shift of the chemical equilibrium towards

higher temperatures after OH excitation, while the available time resolution was not sufficient to study several details of the dynamics including the

possibility of spectral hole burning of the OH band in the liquid. The latter

was found in these early studies for a hydrogen-bonded polymer film (76).

With improved laser systems (12–15) further experiments were

conducted several years later. Woutersen et al. (77) reported one-color

investigations with 200 fs pulses of a 0.4 M mixture of ethanol and



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Table 2



Experimental Results on Transient Spectroscopy of Monomeric, Simple Alcohols in Different Solvents



š1 cm 1

 c š1 cm

 h cm 1

 12 cm 1

1

c – 12 cm

T10 (ps)

T1 (ps)

or (ps)

c



1



Me:CCl4



Me:C2 Cl4



Me:C4 Cl6



Me:C5 Cl6



De:CCl4



DMEP:CCl4



Eth:CCl4



3644

22

15 š 3

20 š 3

168 š 3

12 š 2

9š1

2 š 0.5



3643

21

15 š 3

20 š 3

170 š 3

10 š 1

10 š 1

2 š 0.5



3639

22

19 š 3

19 š 3

170 š 3

13 š 2

8š1

2.5 š 0.5



3639

24

15 š 2

30 š 3

168 š 5

10 š 2

8š3

7š3



3638

17

16 š 3

30 š 3

171 š 3

9š1

9š1

4š1



3625

20

11 š 2

15 š 2

170 š 3

8š1

8š1

4š1



3636

29

17 š 2

29 š 4

168 š 5

8š1

8š2

2 š 0.5



All data are taken at room temperature besides the one shown for methanol diluted in C5 Cl6 (T D 333 K). c : peak position of the OH stretch;

 h ,  12 : spectral width of the transient hole and the excited-state absorption, respectively; anharmonic shift c

12 ; T1 , T10 , or : time

constants for the vibrational lifetime of the OH, ground state filling and reorientation, respectively.



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