Vibrational energy redistribution of ethanol oligomers and dissociation of hydrogen bonds after ultrafast infrared excitation

30 January 1998

Chemical Physics Letters 283 Ž1998. 7–14

Vibrational energy redistribution of ethanol oligomers and dissociation of hydrogen bonds after ultrafast infrared excitation R. Laenen, C. Rauscher, A. Laubereau Physik-Department, Technische UniÕersitat D-85748 Garching, Germany ¨ Munchen, ¨ Received 20 October 1997; in final form 10 November 1997

Abstract 2-color infrared spectroscopy of associated ethanol molecules is carried out with subpicosecond time resolution in the solvent carbon tetrachloride Ž1.2 M. at room temperature. After excitation of the CH-vibration at 2974 cmy1 rapid population redistribution with a time constant - 0.5 ps is observed within the CH-stretching region in agreement with earlier observations, while an effective population lifetime of 12 " 2 ps is measured. In addition, amazingly fast dissociation of H-bonds occurs, the dominant component of which is governed by a time constant of approximately 2 ps and proposed to be of thermal origin. Further spectral features are discussed, e.g. energy transfer between CH- and OH-vibrations. q 1998 Elsevier Science B.V.

1. Introduction Time resolved spectroscopy in the infrared w1–5x offers detailed information on the vibrational relaxation and reorientation of molecules in the condensed phase. Especially the availability of two independently tunable infrared pulses w2,5–7x allows one to examine the pathways of vibrational dynamics. Inter- and intramolecular energy exchange can be monitored separately, due to different spectral and temporal properties. Furthermore transient bandshapes offer information on liquid structures w6x, e.g. local arrangement of H-bonds w8x. In addition polarization resolution of the probing process gives access to the reorientational motion of molecules w9x. In this work the dynamics of CH-stretching modes of ethanol oligomers are studied in the solvent carbon tetrachloride together with the OH-stretching vibration, the latter serving as a spectral probe for

the H-bonding situation. Important features as vibrational energy redistribution and dissociation of hydrogen bonds that were already studied previously with 10 ps pulses w10,11x are reinvestigated with notably increased time resolution w12x.

2. Experimental Our experimental system was described recently w13x. In short, two optical parametric oscillators ŽOPO. are excited in parallel by the second harmonic of a flashlamp-pumped, Kerr-lens mode-locked Nd:YLF laser. Two independently tunable pump and probing pulses in the spectral range 1600 to 3700 cmy1 are derived from separate optical parametric oscillator w14x ŽOPO. and amplifier ŽOPA. systems with a repetition rate of 50 Hz. Pulse parameters of the pump Žprobe. are f 2 ps Ž1 ps. duration and

0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 3 0 1 - 8

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R. Laenen et al.r Chemical Physics Letters 283 (1998) 7–14

energy up to 10 mJ Ž10 nJ. with frequency width 8 cmy1 Ž16 cmy1 . close to the Fourier transform limitation. Blocking every second pump pulse we are able to detect pump-induced, relative transmission changes lnŽTrT0 . of the sample with an accuracy of "10y3 by proper signal averaging. T and T0 here denote the measured energy transmission of the sample for the probe pulse positioned at a certain frequency n with and without excitation of the sample, respectively. Polarizer optics for the probing beam in combination with additional detectors allow the simultaneous detection of the parallel and perpendicular components of the probe transmission relative to the linear polarization of the excitation pulse. In this way the isotropic component ln Ž TrT 0 . is s wlnŽTrT0 . 5 q 2lnŽTrT0 . H xr3 and the anisotropic signal wlnŽTrT0 . 5 y lnŽTrT0 . H x are measured. The latter quantity is sensitive to not only the second-order Ž l s 2. reorientational relaxation time tor of the molecules, but also to fast energy redistribution processes or spectral relaxation. The samples were prepared from commercially available ethanol spectral grade ‘‘Uvasol’’ and carbon tetrachloride spectral grade ‘‘HPLC’’ without further purification. The absence of water in the solvent was additionally checked recording the conventional IR-spectrum. A concentration of ethanol in CCl 4 of 1.2 M was adjusted corresponding to a sample transmission of f 10% at 3330 cmy1 for our sample thickness of 0.1 mm. All measurements were performed at a low excitation level with transmission changes of only a few percent, to simplify the data analysis and also reduce heating of the sample via the deposition of infrared energy. Data were taken at room temperature.

3. Results and discussion The conventional infrared spectrum of 1.2 M ethanol dissolved in CCl 4 is depicted in Fig. 1c and Fig. 3b Ždash-dotted lines, right hand ordinate scales.. The absorption in the region between 2800 and 3000 cmy1 is attributed to the CH 2- and CH 3-stretching modes mixed by Fermi resonances with combination tones of CH-bending modes w15x. The most intense band in this region at 2974 cmy1 with a width of 21 cmy1 corresponds dominantly to the asymmetric

Fig. 1. Transient spectra of ethanol in carbon tetrachloride Ž1.2 M. at room temperature for excitation at the frequency position of the CH-stretching vibration at 2974 cmy1 Žvertical arrow. and different delay times: 0 Ža., 4 Žb. and 8 ps Žc.; experimental points, calculated solid lines. Evidence for energy redistribution in the CH-region and dissociation of hydrogen bonds is obtained. The conventional FT-IR spectrum of the same sample is shown for comparison in Žc. Ždash-dotted line, right hand ordinate scale..

CH 3-stretching vibration and will be referred to as the CH-band in the following. At higher frequencies the absorption is assigned to the OH-stretching vibration. A narrow band with width 25 cmy1 shows up at 3632 cmy1 representing ethanol monomers andror hydroxilic end groups of open oligomers with a proton acceptor function. The larger absorption band peaked at 3330 cmy1 with a broad width of approximately 210 cmy1 is attributed to the OH vibration of molecules in internal positions of moderately large Žopen and cyclic. oligomer chains. The slight asymmetry of the band with increased amplitude in the

R. Laenen et al.r Chemical Physics Letters 283 (1998) 7–14

blue part may be assigned to hydroxilic end groups with a proton donor function w16x positioned at , 3500 cmy1 with a width of about 110 cmy1 . In the extended red wing of the oligomeric OH-band a further spectral component around 3150 cmy1 Žwidth f 150 cmy1 . may be assumed to account for the measured absorption. This component could be related to internal OH-groups in longer ethanol chains as noticed in solid Ar-matrices w17x. Some results on transient infrared spectra obtained with our 1 ps probing pulses are presented in Figs. 1a–c for excitation at 2974 cmy1 Žnote vertical arrow.. The parallel component of the probe transmission change is plotted in the range 2850–3650 cmy1 for different delay times Žleft hand ordinate scales, experimental points, calculated solid curves.. The transient spectrum during the excitation process, t D s 0 ps, is depicted in Fig. 1a. A bleaching at the frequency position of 2974 cmy1 is shown, which is related to a diminished population difference between the Õ s 0 and the Õ s 1 levels of the CH 3stretching mode. The excess population of the upper level Õ s 1 can be directly monitored from the induced absorption around 2952 cmy1 and attributed to excited-state absorption ŽESA.. This band exhibits a width of 17 " 2 cmy1 . The bleaching signal at lower frequencies indicates that population changes of neighboring CH-frequencies also occur that are not directly excited by the pump pulse but are obviously due to population redistribution processes. Above 3100 cmy1 the OH-absorption changes provide evidence for a rearrangement of the H-bonding system. Cleavage of ethanol associates, into shorter pieces, is suggested by the bleaching feature peaked at 3300 cmy1 Žlack of oligomeric absorption., while the induced absorption above 3400 cmy1 may be assigned to an excess of shorter oligomers, increasing the number of OH-groups with proton a donor or acceptor function. Similar transient spectra are presented in Figs. 1b and c for t D s 4 ps and 8 ps, respectively. The amplitude increase noted in Fig. 1b compared to 1a results from the completion of the excitation process, followed by a minor decay of the CH-amplitudes in Fig. 1c, while the transient OH amplitudes are still growing until the 8 ps delay time. Corresponding signal transients are shown in Fig. 2 for three prominent probing frequencies. The mea-

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Fig. 2. Relative transmission change of the probe pulse Žisotropic signal. versus delay time for CH-pumping of ethanol molecules at 2974 cmy1 and three probing frequencies: 2974 Ža., 3330 Žb. and 3450 cmy1 Žc.; experimental points, calculated curves.

sured isotropic signal component for excitation at 2974 cmy1 is plotted versus delay time. For probing at the pump frequency Žsee Fig. 2a. the bleaching of the CH 3-mode rises within experimental time resolution to a maximum value, followed by a slow decay with a time constant of T1ŽCH. s 12 " 2 ps to a new quasi-equilibrium around t D s 50 ps. The latter notion is consistent with a transient temperature increase of approximately 1 K of the sample. The value is estimated from the relative temperature co-

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R. Laenen et al.r Chemical Physics Letters 283 (1998) 7–14

efficient of y0.008 Ky1 for the CH-band measured for our sample by conventional FT-IR spectroscopy and the simplifying assumption that the simultaneous pressure increase of the sample during the thermalization process can be disregarded. T1ŽCH. is interpreted as the effective population lifetime of the CH 3-vibration. The induced anisotropy, on the other hand, derived from the same data vanishes almost completely on a much faster time scale, corresponding to a decay time of tanis - 0.5 ps Ždata not shown.. The finding is assigned to the rapid redistribution process among the CH-stretching modes mentioned above w18x. The phenomenon is obviously too fast to be recognized in the signal rise of Fig. 2a. Tuning the probing frequency to the peak of the OH-band at 3330 cmy1 ŽFig. 2b., a different timedependence is found: the sample bleaching rises, notably delayed, with a time constant of 2.0 " 0.5 ps to a maximum around 8 ps. For later times the bleaching decreases slowly within 90 " 10 ps. In Fig. 2c, at the frequency position of 3450 cmy1 representing OH-groups with a proton donor function, e.g. ethanol dimers or trimers, we measure a delayed build-up of the induced absorption again with a time constant of tdis s 2 " 1 ps. The signal amplitude subsequently disappears with a time constant of 17 " 3 ps. Some information on the interaction between OHand CH-stretching vibrations is presented in Fig. 3. Now, the excitation pulse is positioned at 3330 cmy1 in the center of the OH-band and the transient spectrum Žparallel signal. below 3050 cmy1 is shown for two time values. At an early delay time of 2 ps ŽFig. 3a. a bleaching at f 2980 cmy1 Ždashed line. can be seen, close to the frequency position of the asymmetric CH 3-stretching mode, accompanied by an, in comparison to the bleaching component, amazingly strong induced absorption at f 2960 cmy1 Ždotted line.. Furthermore a bleaching around 2870 cmy1 is found, indicating further population changes in the CH-stretching region that may be assigned to the symmetric CH 2-stretching mode. The induced absorption above 3000 cmy1 is related to excited-state absorption from the excited OH-vibration of hydroxilic groups in internal positions of oligomer chains. At a later delay time of 8 ps Žsee Fig. 3b. the transient spectrum is similar, apart from various amplitude changes.

Fig. 3. Ža., Žb.: Same as Fig. 1, but for pumping at 3330 cmy1 in the center of the oligomeric OH-band and delay times of 0 Ža. and 4 ps Žb.; experimental points, calculated lines. Energy transfer from OH- to CH-vibrations and other features are observed. The conventional FT-IR spectrum in the CH-region is shown in Žb. Ždash-dotted curve, right hand ordinate scale.. Žc. Temperature difference spectrum of the same sample of ethanol in CCl 4 Ž1.2 M.; the absorbance difference of two conventional FT-IR spectra taken at 301 and 296 K Ž DT s 5 K. is plotted; the similarity to the transient spectra of Fig. 1 is noteworthy.

Generally speaking, after excitation of a mode A populating its Õ s 1 level, different mechanisms may contribute to an absorption change of a vibrational transition B: 1. population transfer from A to B, 2. anharmonic coupling between modes A and B so that the fundamental transition frequency of B Ž0,0 ™ 0,1. differs from the corresponding combination tone Ž1,0 ™ 1,1.,

R. Laenen et al.r Chemical Physics Letters 283 (1998) 7–14

3. population transfer to another mode C interacting in turn with mode B analogous to cases Ži. and Žii., 4. sample heating. It is proposed that interaction channels Ži., Žii. and Živ. are responsible for the bleaching and induced absorption features in the range 2950–2990 cmy1 in Fig. 3a, with mechanism Žii. obviously producing a red-shift of the monitored CH-transition. The population lifetime of the pumped OH-vibration is known to be 1.4 " 0.3 ps w12x, so that energy transfer from OH to CH 3 is restricted to a few ps. At t D s 8 ps ŽFig. 3b. the Õ s 1 population of the OH-mode has disappeared again, as also indicated by the vanishing excited state absorption around 3050 cmy1 so that mechanism Žii. can no longer contribute to the induced absorption in Fig. 3b; i.e. only excited state population of the longer-lived CH-modes and thermal effects account for the absorption changes. A careful analysis of the frequency position of the induced absorption peak at 2960 cmy1 suggests a small shift from Fig. 3a to b that may be related to the vanishing of contribution Žii. in Fig. 3b. The broad bleaching structure at frequencies below 2930 cmy1 also strongly suggests population changes Ži., i.e. depletion of the vibrational ground-state of the CH-stretching modes via energy transfer from OH to CH, while thermal effects are believed to be of minor importance in the CH-region. In Fig. 4 we present data on the temporal evolution of the relative transmission changes of the sample Žisotropic signal. for pumping of internal OHgroups at 3330 cmy1 . Probing of the CH-band at 2974 cmy1 ŽFig. 4a. we find a slightly delayed rise of the bleaching that is consistent with a superposition of mechanisms Ži. and Žii.. For larger delay time ) 8 ps the signal amplitude decays with an effective time constant of 15 " 3 ps. The agreement with the lifetime T1ŽCH. inferred from Fig. 2a indicates that a thermal contribution Živ. to the dynamics is of minor importance. A careful analysis of the signal transient suggests that a fast component with constant 2 " 1 ps is hidden in the measured time dependence around the signal maximum in Fig. 4a, consistent with a contribution of mechanism Žii.. Probing the induced absorption at 2965 cmy1 Žsee Fig. 4b. a much faster dynamics is indicated by the data: after a rapid absorption increase the signal amplitude first decays

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Fig. 4. Relative transmission change of the probe pulse Žisotropic signal. of ethanol: CCl 4 versus delay time for OH-excitation at 3330 cmy1 and two frequency positions: Ža. probing at 2974 cmy1 , the frequency position of the strong CH-band; Žb. probing of induced absorption at 2965 cmy1 ; experimental points, calculated curves. The signal transients are assigned to energy transfer OH ™ CH and anharmonic CH-frequency shifts related to the OH-population dynamics; for details see text.

with a time constant of 2 " 1 ps consistent with the population lifetime of the pumped OH-vibration, i.e. contribution of interaction channel Žii.. For t D ) 6 ps the signal seems to relax more slowly as would be expected from the contribution of excited state absorption declining with the larger lifetime of the CH 3-vibration.

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R. Laenen et al.r Chemical Physics Letters 283 (1998) 7–14

4. Interpretation The time dependencies were numerically calculated by solving a set of coupled rate equations describing a 5-level model with parallel and perpendicular components of the population numbers also including rotational diffusion for the orientational motion with a single time constant w19x. The transient spectra are calculated with a corresponding set of spectral transitions with Lorentzian ŽCH-stretching vibrations. and Gaussian ŽOH-bands. shapes. Fitting of the line positions and widths was performed using a Levenberg-Marquardt algorithm w20x. The theoretical model contains numerous parameters; nevertheless it gives some support to the physical picture presented here and is helpful for the determination of time constants from the measured signal transients. 4.1. Vibrational energy redistribution As known from previous work, excitation of a CH 3-vibration of ethanol results in a fast energy redistribution between the different CH-stretching modes w18x. Over- and combination tones of the CH-bending modes are an important relaxation channel for the excited stretching vibrations w21x. From the transient spectra, similar to Fig. 1, we are able to monitor the energy redistribution between the different CH-vibrations and the relaxation process involves the symmetric CH 2-stretching vibration, located around 2875 cmy1 . Due to the small energy mismatch of about 100 cmy1 between the excited asymmetric CH 3- and the symmetric CH 2-stretching vibration and the proposed Fermi resonance w22x between the two modes energy transfer should be fast in comparison to our time resolution. This is in agreement with the short relaxation time tanis - 0.5 ps of the induced anisotropy of the CH-band. In fact, rotation along the different axes is known to be notably slower w23x, so that the fast loss of orientational information is assigned to transfer of vibrational energy to other CH-stretching modes with different directions of the transition dipole moments. It is noteworthy in this context that we see no excited-state absorption corresponding to a vibrational population of the CH 2-mode. This could be explained by the population of several neighboring vibrations with spectral overlap of dominant bleaching and weaker contributions of induced absorption.

Our observations in context with Figs. 3 and 4 give evidence for energy transfer from the OH-mode to the CH-vibrations. A lower limit of 4 ps is inferred from the CH-amplitude changes for the transfer time constant. Assuming detailed balance, a lower limit of 24 ps is estimated for the reverse process CH ™ OH. 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 w11x. It is interesting in this context to compare with the dynamics of a single, non-hydrogen bonded ethanol molecule: for ethanol monomers in CCl 4 Ž0.05 M. an effective population lifetime of 10 " 2 ps was measured for the CH 3stretching vibration. Furthermore transfer of vibrational energy from the OH- to the asymmetric CH 3stretching mode was seen within t trans s 15 " 10 ps. The inverse process was below the detection limit consistent with detailed balance arguments for the frequency difference of 660 cmy1 between the two vibrations of the monomer w24x. 4.2. Breaking of hydrogen-bonds Previous investigations delivered detailed information on the inhomogeneous character of the OHstretching band in solution w12x. Excitation of this vibration of ethanol oligomers resulted in the dissociation of hydrogen-bonds w10,25,26x. Similar experiments with pumping of the CH 3-stretching vibration also provided evidence for bond-breaking w11x. The proposed mechanism was via energy transfer from the CH- to the OH-vibration with subsequent dissociation as for OH-excitation. Our data of Fig. 1 give additional support to the dissociation of H-bonds after CH-excitation. Comparing the maximum absorption changes in the CH-region to the bleaching around 3300 cmy1 allows one to determine the efficiency of the cleavage of oligomers. For a ratio of the integrated absorption cross-sections of CH:OH s 1:6 w27x, we estimate a large quantum efficiency of 70 " 20% for the breaking of H-bonds per absorbed IR photons. We have verified experimentally that OH-excitation of long-chained associates, particularly redshifted to positions - 3000 cmy1 does not account for the observations. To this end, a transient excita-

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tion spectrum was carried out, tuning the pump pulse in the range 2900–3000 cmy1 and monitoring the induced bleaching at 3300 cmy1 Ždata not shown.. We find a probe transmission increase that follows the absorption strength of the various CH-stretching modes and vanishes for n Pu ) 3000 cmy1 . It is concluded that residual OH-absorption in the CHstretching region cannot account for the bond-breaking after excitation at 2974 cmy1 . Energy transfer from CH- to OH-vibrations appears to be too slow to account for the fast component of the bond-breaking with time constant 2 " 1 ps, since the former process is governed by the lifetime T1ŽCH. f 12 ps, while the transfer time Ž) 24 ps. determines the yield Ž- 50%. of the relaxation channel. It is proposed that the rapid dissociation is initiated by the fast population redistribution among the CH-vibrations, leading to a rapid rearrangement of H-bonds, e.g. via local temperature changes. The redistribution process includes conversion of the vibrational energy mismatches to rotation and translation. The corresponding temperature changes are estimated to amount to several degrees under our experimental conditions, sufficient for notable bond breaking w8x. To illustrate the thermal mechanism for the dissociation H-bonds via heating of the excited molecule, conventional difference absorption spectra of our sample were measured with a commercial FT-IR spectrometer at constant ambient pressure. Examples are depicted in Fig. 3c. It is interesting to see that an increase of DT s 5 K at room temperature shifts the chemical equilibria among the ethanol oligomers with bleaching of f 0.04 around 3300 cmy1 and induced absorption at higher frequencies. The similarity with the transient spectra of Fig. 3 is striking, although some differences show up; e.g. the zero-point crossings of the spectral curves disagree. The deviations may be due to the finite rate constants of the chemical equilibria, some of them may not be able to follow the fast local temperature rise in the transient case andror the adiabatic pressure change simultaneously present. Differences are also noticed comparing the transient spectra in the OH-region after CH- and OH-excitation w12x Ždata not shown.. In the first case the bleaching around 3300 cmy1 is less pronounced resulting in a ratio of the areas of bleaching to induced absorption of f 2. In the second case a

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larger ratio of f 4 is determined. The numbers illustrate that the mechanisms for H-bond breaking are not identical for CH- and OH-pumping. It may be surmised that a second, slower component should be hidden in the amplitude growth in Figs. 2b and c, involving the population lifetime of the CH-excitation, via energy transfer to OH andror local heating with subsequent dissociation of Hbonds. Available experimental accuracy unfortunately does not allow us to determine an additional slower contribution to the H-bond dynamics.

5. Conclusions We have investigated the dynamics of associated ethanol molecules dissolved in carbon tetrachloride at room temperature after excitation of a CH-stretching vibration at 2974 cmy1 . 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 T1ŽCH. s 12 " 2 ps are observed. Almost simultaneously, dissociation of hydrogen bonds occurs, with a quantum yield of 70 " 20% and time constant 2 " 0.5 ps, as indicated by bleaching and induced absorption features in the OH-range. 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 equilibria of the H-bonding 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.

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