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Ethanol Oligomers in Solution: Spectral Holes and Vibrational Lifetime Shortening

Ethanol Oligomers in Solution: Spectral Holes and Vibrational Lifetime Shortening

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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.



Copyright © 2001 by Taylor & Francis Group, LLC



CCl4 . The authors found a faster predissociation process with time

constants of 250 fs Pu D 3225 cm 1 to 900 fs Pu D 3450 cm 1 . For

the subsequent reassociation a time constant of 15 ps was measured. From

subsequent investigations of the probe transmission with perpendicular

polarization, the authors inferred a fast delocalization of the deposited

vibrationally energy along the oligomer chain confirming the findings of

Ref. 78.

A more detailed picture was derived from two-color spectroscopy

with 2 ps pump and 1 ps probing pulses (78,79). The probing of the sample

transmission in the whole frequency range of the fundamental v D 0 !

v D 1 and excited state v D 1 ! v D 2 OH transition provides much more

spectroscopic insight and is discussed in the following.

The transient spectra for a 0.17 mixture of ethanol in CCl4 at room

temperature are presented in Fig. 15 for three different delay times of

2 ps (top), 0 ps (middle), and 11 ps (bottom) (78). The conventional

absorption spectrum depicted in Fig. 15f (dash-dotted curve) exhibits two

peaks in the investigated spectral region: one related to monomeric ethanol

molecules and/or species with proton acceptor function in open oligomer

chains 3636 cm 1 , and another involving OH groups in internal oligomer

positions with proton donor and acceptor function. The band maximum

occurs at 3340 cm 1 , where the excitation pulse is positioned (vertical

arrows). The slight asymmetry of the band with a shoulder in the blue

wing may be assigned to hydroxilic end groups with proton donor function (80) positioned at '3500 cm 1 (width ³110 cm 1 ). In the extended

red wing of the oligomeric OH band a further spectral component around

3150 cm 1 (width ³150 cm 1 ) may be assumed and related to internal OH

groups in long ethanol chains as seen in solid Ar matrices (81).

In order to compare primary dynamics with secondary relaxation

steps, we depict on the left-hand side of Fig. 15 the “anisotropic” spectra

(a–c), which consist mainly of spectral components with the same linear

polarization as directly induced by the pump pulse. On the right-hand side

of the figure the corresponding “isotropic” spectra (d–f) are shown. In the

latter spectral components can notably contribute that result from a relaxation process, where the initially orientation of the OH transition dipole is

(partially) lost.

Transient hole burning is clearly indicated by the data for the

anisotropic component of the probe transmission change (a, b) with a

halfwidth (true FWHM) of 45 š 5 cm 1 of the Lorentzian shaped hole

at early delay times, tD Ä 0 (a). For the isotropic component of the probing

signal, the narrow hole (dotted lines in d, e) is superimposed on a broader



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Figure 15 The evolution of the transient spectra is shown here for a 0.17 M

mixture of ethanol and CCl4 and the anisotropic (a–c) and isotropic (d–f) components. With increasing delay time from 2 ps (a,d) to 0 ps (b,e) and finally 11 ps

(c,f), clearly a short-living spectral hole (a, dotted line) as well as a completely

isotropic Gaussian component (d, dashed line) is noticed.



component (dashed lines) of approximately Gaussian shape as suggested

by a numerical decomposition of the measured transmission changes and

very similar to the findings at higher concentration (79). The latter feature

obviously is of almost isotropic character, independent of the polarization

of the excitation process, and thus a secondary phenomenon involving some

relaxation process. The same conclusion holds for the induced absorption



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in the blue part of the probe spectra above 3400 cm 1 of Fig. 15d and e.

Some dynamics obviously proceed on a time scale comparable to or shorter

than the duration of the pump pulse of 2 ps. The induced absorption in

the red part of the oligomer absorption, on the other hand, displays some

polarization dependence, suggesting it to be (at least in part) a primary

feature of the population changes of the pump process. At tD D 0 (b and

e) the broad absorption increase at 3140 cm 1 with width of 160 cm 1 is

readily observed and attributed to excited-state absorption from the first

excited level of the OH groups in internal positions. At a later delay time

of 11 ps the narrow bleaching feature (spectral hole) has disappeared while

the broad bleaching component at 3320 cm 1 of the oligomer band with

approximately Gaussian shape (width 130 š 10 cm 1 ) has survived and

even can be seen in the anisotropic spectrum (Fig. 15c and f, respectively).

Part of the induced absorption at higher frequencies is also still existing.

The dynamics derived from the evolution of different spectral components is interpreted by the help of the five-level scheme shown in Fig. 16

(78). Starting from the ground state (0), the excited vibrational level (1)

of an oligomeric subensemble is populated by the pumping process with

fast spectral redistribution among the neighboring OH groups of the same

chain represented by level 2. In addition to population decay, breaking of

hydrogen bonds follows decribed by level 3. The final slow rearrangement

of the chemical equilibrium is described by a long-lived modified ground



Figure 16 Simplified energy level scheme to account for the dynamics after vibrational excitation of an oligomeric subensemble of H-bonded ethanol molecules.

While the thick arrow marks excitation, the thin arrows refer to the probing positions and dashed lines denote the different possibilities of relaxation. For details

see text.



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state (level 4). The five-level scheme is based on various pieces of experimental observations and can account for the measured spectral dynamics

in the investigated frequency range.

First, transient hole burning in the ³220 cm 1 broad ethanol band is

observed and accounted for by the excited molecular subensemble (1). The

measured hole width of 45 š 5 cm 1 may be compared with the transient

hole width of the OH dimer band of the amorphous polymer matrix PVB,

where a value of approximately 100 cm 1 was found at room temperature

and smaller values at lower temperature (76). The smaller width determined

here for the ethanol sample may be explained by the more pronounced

motional narrowing in the liquid phase. Furthermore, this hole width is

comparable to the recently published value for higher ethanol concentration

(2.4 M) (82). The lifetime of the spectral hole of hole D 1.2 š 0.2 ps agrees

with the number found in previous investigations for a concentration of

1.2 M (79). hole may be governed by three processes: population decay

to the vibrational ground state, energy migration to neighboring ethanol

molecules with shifted frequency positions, and structural relaxation of the

excited OH groups also generating frequency shifts (79).

Subsequently fast spectral redistribution to level 2 is indicated by the

short lifetime of the spectral hole and the early observation of the 130 cm 1

broad oligomer bleaching in the isotropic part of the transient spectrum. The

bleaching component located at 3320 cm 1 is present already at a delay

time of 2 ps. From time-dependent measurements taken at frequencies

>3400 cm 1 (data not shown), a time constant of mig < 1 ps is inferred

for this process. A lower limit of the proposed energy migration time is

set by the transient hole width of about 0.2 ps, assuming other dephasing

processes to be negligible.

On the other hand, structural relaxation involving reorientation of the

OH groups with corresponding changes of the bond angles of the H bridges

(and of the OH frequencies) is found to be slow with a time constant of

8 ps as inferred from the anisotropic signal component of time-dependent

measurements taken at D Pu D 3340 cm 1 . The reorientational motion

cannot account for the observed rapid spectral changes; the fast hole relaxation involves obviously molecules in the v D 1 state. It is concluded that

energy migration, i.e., near-resonant transfer of vibrational quanta of the OH

stretching mode, explains the spectral features. The larger bandwidth of the

excited state absorption (ESA) of 160 š 15 cm 1 relative to the hole width

may be also explained by the migration along the oligomer chains. This

conclusion is supported by the fact that the band area, which is comprised

by the ESA in the isotropic spectrum, exceeds the one of the hole by a



Copyright © 2001 by Taylor & Francis Group, LLC



factor of almost 3 (see Fig. 15d–f). A substantial part of the isotropic ESA

has to be related to the broad oligomer bleaching of similar bandwidth. It

is proposed that the same process accounts for the rapid loss of orientational information, i.e., the isotropic character of the oligomer bleaching

and fragments produced by bond breaking. As a consequence the broad

bleaching component at early delay times is interpreted in part as a manifestation of vibrationally excited ethanol molecules at different positions in

the hydrogen-bonded chain, populated by energy migration. The remaining

part of the transient oligomer bleaching is attributed to a depletion of the

vibrational ground state, as will be discussed in the following.

The excitation of the internal OH vibrations is accompanied by a redshifted ESA around 3120 cm 1 representing a direct measure of the population of the v D 1 level. The ESA disappears with a time constant of T1 D

1.7 š 0.3 ps. The same value is measured for the isotropic and anisotropic

signal transients, suggesting a minor contribution of reorientational motion

only. Because of the large width of the ESA band, structural relaxation

and migration of the vibrational quanta are not resolved and T1 obviously

represents the effective population lifetime of the oligomeric OH groups. As

compared to the monomeric alcohol discussed above, a significant lifetime

shortening is noticed for internal OH groups in oligomer chains; a decrease

from 8 ps (see above) to 1.7 ps for a 0.17 M ethanol:CCl4 mixture and

1.4 ps for a 1.2 M ethanol:CCl4 sample is found (83).

The next step following spectral hole relaxation (i.e., energy migration along the oligomer chain) is breaking of a hydrogen bond producing

thereby dimers (84) positioned at 3500 cm 1 or trimers (81) at 3450 cm 1

(level 3) in the OH ground state. The mechanism contributes to the population decay of the excited vibrational state. The generation of the smaller

species is indicated by the time-delayed appearance of induced absorption at

the respective frequencies. The delayed bond breaking is also inferred from

the induced absorption at 3633 cm 1 , the frequency position of monomers

or OH groups with proton acceptor function. It is further proposed that

the delayed weak bleaching at 3140 cm 1 is directly related to the bond

breaking (Fig. 15f): the frequency position refers to internal OH groups in

long chains (N > 5). Pumping a suitable hydroxylic group of one such long

chain at 3340 cm 1 , with subsequent bond breaking, a shorter chain results

lacking absorption at 3140 cm 1 , as observed experimentally. According

to this mechanism the delayed bleaching at 3140 cm 1 should build up

as rapid as the induced absorption of the fragments, consistent with the

observations. The equal values (within measuring accuracy) for the subsequent decay times of approximately 20 ps for the dynamics at 3140, 3340



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(isotropic component), and 3633 cm 1 also gives some support to this physical picture.

Finally, reassociation of the broken oligomers occurs with time

constants between 9 ps (dimers) and 14 ps (trimers) to a new thermal

quasi-equilibrium (level 4) with lifetimes in the nanosecond region. The

system does not return to the initial state (0) as a direct consequence of

the deposited energy of the excitation process. The resulting temperature

increase of the sample is small but produces measurable effects and is

estimated from the long-lived amplitude of the signal transients to be

below 1 K.

Similar experiments have also been performed on a higher concentrated mixture of 1.2 M ethanol and CCl4 . Again spectral holes could be

identified from the transient spectra with a width of 25 cm 1 and lifetime of

³1 ps. From transient spectra taken for different excitation frequencies in

the OH band, evidence for a faster hole relaxation with increasing red shift

(bond strength) is inferred, which is accompanied by the differences in the

temporal evolution of the isotropic Gaussian component related to level 2.

Recalling the situation for monomeric ethanol, the question arises

in what respect the CH-stretching modes involved in the OH relaxation

(see above) may influence the structural dynamics due to H-bond breaking.

Transient spectra tackling this question are shown in Fig. 17. Data are taken

for a 1.2 M mixture of ethanol with CCl4 at room temperature and excitation

in the CH-stretching region at 2974 cm 1 (note vertical arrow) (85).

The conventional infrared spectrum of the sample is depicted in

Fig. 17c (dash-dotted line, righthand ordinate scale). The absorption in

the region between 2800 and 3000 cm 1 originates from CH2 - and CH3 stretching vibrations mixed by Fermi resonance to overtones of CH-bending

modes (86). The most intense band at 2974 cm 1 with a width of 21 cm 1

is assigned to the asymmetrical CH3 -stretching vibration, which will be

referred to here as the CH vibration. The absorption at higher frequencies

above 3050 cm 1 is attributed to the OH-stretching vibration.

The parallel component of the probe transmission change is plotted

in Fig. 17 in the range 2850–3650 cm 1 for different delay times (lefthand ordinate scales, experimental points, calculated solid curves). The

transient spectrum during the excitation process, tD D 0 ps, is depicted in

Fig. 17a. A bleaching at the frequency position of 2974 cm 1 is shown

because of the excitation of the CH vibration. The excess population

of the upper level v D 1 can be directly monitored from the induced

absorption around 2952 cm 1 and is attributed to excited-state absorption

(width of 17 š 2 cm 1 ). The bleaching signal at lower frequencies indicates



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Figure 17 Transient spectra of a 1.2 M ethanol and CCl4 mixture taken at room

temperature and excitation within the CH-stretching modes at 2974 cm 1 . The data

are shown at three different delay times of 0 ps (a), 4 ps (b), and 8 ps (c). The

conventional absorption of the sample is shown for comparison in (c), right-hand

ordinate scale and dash-dotted line. Measured data of the parallel signal; calculated

lines.



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population changes of other CH transitions that are not directly excited

by the pump pulse but are obviously due to population redistribution

processes. Above 3100 cm 1 the measured OH-absorption changes provide

evidence for a rearrangement of the H-bonding system. Since excited state

absorption around 3100 cm 1 is not found, population of the excited OH

level is of minor importance. Cleavage of ethanol associates to shorter

pieces is suggested by the bleaching feature peaking at 3300 cm 1 (lack

of oligomeric absorption), while the induced absorption above 3400 cm 1

is to be assigned to an excess of shorter oligomers, increasing the number

of OH groups with proton donor or acceptor function. Similar transient

spectra are presented in Figs. 17b and c for tD D 4 ps and 8 ps, respectively

(note different abscissa scales). The CH-amplitude increase in Fig. 17b

compared to Fig. 17a results from the completion of the excitation process,

followed by a minor decay of the CH amplitude in Fig. 17c, while the

transient OH amplitudes are still growing until 8 ps delay time. Evidence

for rapid energy redistribution in the CH-stretching region with a time

constant <0.5 ps and an effective population decay of the CH modes

with time constant T10 CH D 12 š 2 ps are observed from time-dependent

measurements taken with probing and pump pulse resonant to the respective

absorptions (data not shown).

Three different contributions to the dynamics in Fig. 17 can be identified (85):

1.

2.



3.



Population transfer from CH to OH

Anharmonic coupling between CH- and OH-stretching modes so

that the fundamental transition frequency of CH 0, 0 ! 0, 1

differs from the corresponding combination tone 1, 0 ! 1, 1

Thermalization processes



The three interaction channels are responsible for the bleaching and

induced absorption features in the range 2950–2990 cm 1 in Fig. 17a,

with mechanism 2 obviously producing a small red shift of the monitored CH-transition. The population lifetime of the excited OH vibration

is known to be 1.4 š 0.3 ps (79) so that energy transfer from CH3 to

OH cannot populate the upper OH level notably. There is evidence for

energy transfer between the OH and CH vibrations. For OH excitation

the measured CH-amplitude changes suggest a lower limit of 4 ps for the

transfer time constant. Assuming detailed balance, a lower limit of 24 ps

is estimated for the reverse process CH ! OH in Fig. 17. From previous

spectroscopic experiments on associated ethanol an energy transfer time of

12 ps was reported for OH ! CH and 60 ps for the reverse process (75).



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It is interesting in this context to compare the dynamics of a nonbonded

ethanol molecule: for ethanol monomers in CCl4 (0.05 M) an effective

population lifetime of 10 š 2 ps was measured for the CH3 -stretching vibration. Furthermore, energy transfer from the OH- to the asymmetrical CH3 stretching mode was seen with trans D 15 š 10 ps. The inverse process was

below the detection limit, consistent with detailed balance arguments for

the frequency difference of 660 cm 1 of the two vibrations in the monomer

case (73).

Almost simultaneously dissociation of hydrogen bonds occurs, with a

quantum yield of 70 š 20% and time constant 2 š 0.5 ps, as indicated by

the bleaching and induced absorption features in the OH range in Fig. 17.

A thermal mechanism may account for the rapid bond breaking, involving

the energy redistribution process of the CH vibrations and implying large

rate constants for the rearrangement of the chemical equilibrium of the Hbonding system. The transient changes after CH pumping are similar but

not identical to those after the OH excitation in the center of the oligomer

band, alluding to differences of the dissociation mechanism.

To account for the measured conventional and transient spectra, we

have developed a structural model for associated ethanol at room temperature starting from the following asumptions (87):

1.



2.



3.



4.



In the liquid phase at medium to high concentrations, we expect

ethanol to arrange to open chains with the number of H-bonded

molecules denoted by L.

For each associate of length L the absorption frequency of the

OH-stretching mode depends on the position n in the chain,

i.e., on the H-bond strength. In accordance with the literature,

we assume end groups with proton acceptor function to show

up in the spectrum like ethanol monomers with absorption at

3633 cm 1 independent on the chain length. The frequency position of the proton donor end group of 3500 cm 1 is kept constant

in the same way.

For the shortest and longest chains that occur in the computation,

we choose the OH frequency of the internal groups in the middle

position and interpolate the frequency position for the other Ls.

The absorption frequencies of internal molecules L n are

derived from assumptions 2 and 3 above by cubic interpolation

with three fixed values (end groups of the chain and

middle position); alternatively equal frequencies for all internal

molecules are considered.



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5.



6.



7.



The proton acceptor end group exhibits a Lorentzian line shape

with width 25 cm 1 , while all H-bonded molecules are assumed

to experience local disorder (different bond angles and O-O

lengths) leading to inhomogeneous broadening with Gaussian

shape and width of 100 cm 1 . The inhomogeneous character of

the OH band of associated ethanol was verified by transient hole

burning (79).

The value of the absorption cross section is fixed at three positions

in the chain (end positions and in the middle), with a parabolic

dependency on position for the other groups within each associate.

We take a Gaussian distribution of chain lengths with a full-width

half maximum L and peaking at Lp to account for the expected

disorder in the liquid phase at room temperature.



The conventional OH-absorption spectra of ethanol:CCl4 in the range

0.05 M up to the neat liquid can be perfectly fitted using the model

assumptions above. The ratio of absorption cross sections is determined

to be 3633 : 3500 : 3330 D 1:2:9. From the experimental concentration

dependence the most probable length of the oligomer chains is inferred

to be proportional to log c 2 , where c denotes the ethanol concentration,

yielding numbers of 1, 2-3, 5, and 11–16, respectively, for c D 0.05 M,

0.17 M, 1.2 M, and the neat liquid, respectively. The model also accounts

for the time-resolved spectra if some additional assumption on the breaking

of the oligomer after vibrational excitation is made. A semiquantitative

understanding of the formation of hydrogen bonds is provided, emphasizing

the importance of open chains for tetramers and larger oligomers, while for

trimers the cyclic configuration seems to prevail (lack of OH end groups

in the transient spectrum after bond breaking). For details the reader is

referred to the original work (87).

3. Fully Associated Ethanol in Isotopic Mixtures

It is interesting to study the structural properties of the neat hydrogenbonded liquid without the perturbing influence of apolar molecules in

mixtures. As an example in this direction the dynamics of ethanol in the

isotopic mixture are investigated. Data will be presented on ethanol-d6

samples containing 1 vol% (diluted) or 50 vol% (concentrated) of protonic

ethanol (88). In contrast to the apolar CCl4 environment, additional H bonds

between the ethanol molecules and their environment can be formed representing new species with modified properties.



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