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Vibrational energy dynamics of water studied with ultrafast Stokes and anti-Stokes Raman spectroscopy
Chemical Physics Letters 397 (2004) 40–45 www.elsevier.com/locate/cplett
Vibrational energy dynamics of water studied with ultrafast Stokes and anti-Stokes Raman spectroscopy Zhaohui Wang, Yoonsoo Pang, Dana D. Dlott
*
School of Chemical Sciences, University of Illinois at Urbana-Champaign, Box 01-6 CLSL, 600 S. Mathews Avenue, Urbana, IL 61801, USA Received 17 May 2004; in final form 18 August 2004 Available online 12 September 2004
Abstract The transient Stokes Raman spectroscopy method is introduced to study the dynamics of OH-stretching vibrations in water excited by ultrashort infrared pulses. The combination of Stokes and anti-Stokes Raman probing allows the the absorption and emission contributions to be measured separately. Experiments with 3400 cm1 pumping of OH-stretching of HOD solute in D2O solvent are reported. The Stokes Raman method is used to study the delay between the excited-state decay and the groundstate recovery, the vibrational Stokes shift, and the generation of weakened hydrogen bonding due to heat released by vibrational relaxation. 2004 Elsevier B.V. All rights reserved.
1. Introduction In this Letter, we introduce transient Stokes Raman spectroscopy to study vibrational energy relaxation (VR) of water. VR of water and the HOD solute in D2O has been the subject of numerous recent theoretical [1–4] and experimental papers [5–14]. In experiments, a mid-IR pulse is used to excite a portion of the inhomogeneously broadened OH stretch mOH to the v = 1 state. Both IR [5–10] and anti-Stokes Raman spectroscopies [11–14] have now been used to probe the VR. The mOH dynamics of water have proven to be extremely complicated, and despite numerous previous studies questions still remain. Three pertinent issues are addressed here. (1) The mOH lifetime of HOD/D2O pumped near the absorption maximum is well known [8–10,12,15] to be T1 0.9 ps, but neither the quantum yields for the various intermediate states excited by mOH decay nor the de-
*
Corresponding author. Fax: +1 2172443186. E-mail address: [email protected] (D.D. Dlott).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.073
tails of the pathways back to the ground state [1–4,12– 14] are known with certainty. Our anti-Stokes Raman measurements showed that parent mOH decay generates dOH bending excitations (1640 cm1) in water [13,14] and mOD stretching (2500 cm1) and dHOD (1450 cm1) and dD2O (1200 cm1) bending excitations [12,13] in HOD/D2O, but these observed intermediates account for only a part of the 3400 cm1 mOH energy. Other likely intermediates such as torsions (450–950 cm1) or hydrogen bond stretches (180 cm1) have not been directly observed. With the combination of anti-Stokes and transient Stokes Raman methods, we can individually measure the excited-state decay and the ground-state recovery. Unseen intermediate states cause the ground-state recovery to lag behind the excited-state decay [16,17]. (2) The vibrational Stokes shift [18–20] of mOH is the shift between the v = 0 absorption and the v = 1 emission. (To avoid confusion with Stokes Raman spectroscopy, this effect will be consistently termed the Ôvibrational Stokes shiftÕ.) mOH excitation is expected to be followed by a contraction of the hydrogen bond to its equilibrium position in the excited state, leading
Z. Wang et al. / Chemical Physics Letters 397 (2004) 40–45
to a dynamic emission redshift [18]. Although IR probe experiments [18] and theoretical calculations [19] had apparently agreed on a vibrational Stokes redshift in the 55–70 cm1 range, our recent study [20] showed that the redshift at shorter times was pump frequency dependent, and as time increased (t > 1 ps) the redshift became a blueshift. In that work, the Stokes shift was found by comparing the peak of the excited-state emission from anti-Stokes scattering to the peak of the equilibrium Stokes Raman spectrum. Prof. Mukamel suggested to us that, since the mOH transition is inhomogeneously broadened and since the excited-state decay rate depends on frequency, it might make more sense to compare the excited-state spectrum to the nonequilibrium spectrum of the ground-state hole produced by the pump pulse. The ground-state hole cannot be seen by anti-Stokes probing, but it can be seen with transient Stokes Raman probing. (3) Hydrogen-bond weakening. The pump pulse can cause structural evolution toward states with weaker hydrogen bonding in two distinct ways. With vibrational pre-dissociation [3,21], the decay of mOH excited states would involve breaking or weakening a hydrogen bond. In addition, the decay back to the ground-state dumps heat into the bath, which leads to thermally induced hydrogen-bond weakening in the mOH ground state [22,23]. The former process weakens only hydrogen bonds associated with the excited molecule; the latter process can weaken any hydrogen bond in the system. (Technically in HOD/D2O it is deuterium bonds that are weakened.) The generation of mOH ground-state molecules again cannot be observed in the anti-Stokes spectrum, but in the transient Stokes spectrum it results in the formation of holes and anti-holes in the mOH spectrum. Owing to the general correspondence between vibrational blueshift and weaker hydrogen bonding [24,25], the anti-holes are blueshifted from the holes. In the rest of this Letter, we first compare IR, antiStokes and Stokes probing methods. We then present preliminary results with both Stokes and anti-Stokes probing, using 3400 cm1 pumping of mOH in HOD/ D2O. We discuss the implications for the problems of intermediate states on the return pathway back to the ground state, the vibrational Stokes shifts and hydrogen bond dynamics.
2. Raman and IR probing For this limited discussion, we will use the simplified level diagram [23,26] in Fig. 1. The mid-IR pump xIR is assumed to generate only mOH fundamental excitations. We are primarily concerned with population dynamics and spectral diffusion. Coherent effects that appear when the pump and probe pulses are nearly time-coincident are regarded as artifacts [27], although the coher-
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Fig. 1. Energy level diagram for mOH excitations with one intermediate state on the pathway back to the ground state. The notation |ijæ means i-quanta of stretch and j-quanta of bend.
ence dynamics of water [28–30] is itself an active field. In this model, when the parent mOH stretch decays with time constant T1, it populates the bending vibration dOH which is an intermediate state on the pathway back to the ground state. The mOH decay may produce one or two quanta of bending excitation [1–4,12–14]. The intermediate-state dOH decays to the ground state [23] with time constant seq. In reality mOH decay involves multiple pathways that compete with the dOH channel [12,14], but the extension to multiple pathways is straightforward. The notation |ijæ denotes i quanta of excitation in the parent mOH and j quanta of excitation in the daughter dOH. Most experiments on water use IR probe methods [4,28]. IR probe transmission is sensitive to both the absorption and the induced emission. In the harmonic approximation, upon mOH excitation the probe transmission is increased due to the bleach of the |00æ ! |10æ ground-state absorption and the induced emission from the |10æ ! |00æ transition. The probe transmission is reduced due to excited-state |10æ ! |20æ absorption. These two effects exactly cancel, so there is no transient signal at all. Similarly, in the harmonic approximation stretchto-bend relaxation also produces no signal, whether the probe pulse is in the stretching region or in the bending region. In the stretching region both the induced emission and the excited-state absorption disappear with time constant T1 to be replaced by a new |01æ ! |11æ excited-state absorption that exactly cancels the ground-state bleach. With a bending region probe, the ground-state bleach, induced emission and excited-state absorption once again exactly cancel. In actual water, the mOH anharmonicity is large, 250 cm1. The excited-state absorption is significantly redshifted away from the ground-state absorption and the induced emission. For this reason, the lifetime T1 is most often measured using redshifted pulses to probe a region where the excited-state absorption predominates. The anharmonic shifts of the intermediate-state transitions such as |01æ ! |02æ and |01æ ! |11æ
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are much smaller, apparently small enough that so far IR experiments have not directly observed intermediatestate populations. Anti-Stokes and Stokes Raman scattering by a probe pulse xL result in a segregation of the emission and absorption processes [31,32], so even in the harmonic approximation all these cancellations do not occur. The simpler case is anti-Stokes probing, which sees only excited states against a nominally dark background. The mOH excitations created by the pump pulse are observed at xL + mOH, due to the |10æ ! |00æ transition. Stretchto-bend relaxation causes the mOH signal to lose intensity with time constant T1, with an accompanying build up of the dOH signal at xL + dOH. Subsequently this dOH signal decays with time constant seq. Using a multichannel spectrograph, the stretch and bend populations can be observed simultaneously [13]. Whether stretch-tobend excitation produces one or two quanta of dOH excitation cannot be directly determined in the harmonic approximation, since the |02æ ! |01æ and |01æ ! |00æ transitions are spectroscopically coincident, but given an anharmonic shift this determination becomes possible. The transient Stokes probe is more complicated since the signals reflect changes in both ground-state and excited-state absorptions. The data are more difficult to obtain, since the signal is a pump-induced change on a significantly larger ambient Stokes scattering background. The pump pulse bleaches the |00æ ! |10æ ground-state absorption and creates a |10æ ! |20æ excited-state absorption. In the harmonic approximation these transitions are coincident at xL mOH, but the excited-state absorption is twice as strong as the bleach. Thus, exciting mOH in the harmonic approximation does not result in complete signal cancellation as with IR probing, but instead causes a net increase of the Stokes signal at xL mOH. Stretch-to-bend relaxation causes the excited-state |10æ ! |20æ absorption to disappear, to be replaced by a weaker |01æ ! |11æ absorption that exactly cancels the ground-state bleach, so the stretching signal at xL mOH vanishes with time constant T1. Meanwhile, the dOH excitations cause the Stokes signal at xL dOH to increase with time constant T1 as the result of an |01æ ! |02æ excited-state absorption that is stronger than the |00æ ! |01æ bleaching. This dOH excited-state absorption may also include the |02æ ! |03æ transition if v = 2 excitations are generated. Bend decay with time constant seq causes the dOH signal at xL dOH to disappear, but it has no effect on the mOH signal at xL mOH (ground-state hole) since the |01æ ! |11æ absorption is replaced by an identical |00æ ! |10æ absorption. With anharmonicity the mOH excitedstate absorption is significantly redshifted from the ground-state bleach, but the intermediate-state transitions are not shifted very far. So as with IR, the mOH lifetime T1 can be measured from the excited-state
absorption decay, but it is difficult to directly observe effects of intermediate-state populations. Although in the harmonic approximation creating a vibrational excitation leads to no signal with IR probing, this is not the case with Raman probing. A vibrational excitation causes both the anti-Stokes and the Stokes signals to increase.
3. Experimental The apparatus [26] and experimental technique [13,33,34] have been described previously. The pump is an IR pulse at 3400 cm1 and the probe a 532 nm visible pulse. The IR pulse causes a bulk temperature jump DT = 8 K. The apparatus response in the temporal and spectral domains is determined from measurements of the coherent artifact [13] generated by nonlinear light scattering (NLS). This response is a good fit to Gaussian functions with FWHM 1.1 ps and 35 cm1 [11]. The sample consisted of D2O with 10% HOD. This 10% concentration is about the highest HOD concentration possible before excitation hopping and H2O impurities become a problem [12,20]. For transient Stokes experiments, a series of 2 min acquisitions were performed with the delay at the indicated value and at a negative time value (probe precedes pump). The signal-background-signal cycle was repeated and averaged eight times for each displayed spectrum.
4. Results 4.1. Features of the Stokes difference spectrum Fig. 2 compares the HOD/D2O equilibrium Stokes Raman spectrum to the transient Stokes and anti-Stokes spectra, with 3400 cm1 pumping at a delay of t = 1 ps. NLS artifacts at xL + xIR in the anti-Stokes spectrum and at xL xIR in the Stokes spectrum have been removed by subtraction, as discussed previously [11]. In the equilibrium Stokes spectrum, the mOH signal is due to HOD solute on top of a small contribution from H2O impurity, and the more intense mOD signal is due to D2O solvent on top of a small contribution from HOD [13]. The anti-Stokes signal indicates the instantaneous populations of excited states. By t = 1 ps, the pumped mOH excitations have decayed considerably, and a portion (10%) of these decay processes have generated mOD intermediates. These mOD intermediates are believed to be predominantly [13] mOD(D2O), generated by the mOH(HOD) ! mOD(HOD) ! mOD(D2O) pathway. The Stokes signal is complicated, with negative and positive-going features labeled in Fig. 2a–e. These features are due in part to pump-induced changes in the
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Fig. 2. Comparison of the HOD/D2O equilibrium Stokes spectrum with the transient anti-Stokes and transient Stokes spectra. These transient spectra are obtained with 3400 cm1 pumping and a time delay of 1 ps. Features in the transient Stokes spectrum labeled (a), (b), (c) and (d) are the mOH hole and anti-hole, and the mOD hole and antihole. Feature (e) is the mOH excited-state absorption.
ground-state and excited-state absorptions as discussed in Section 2, and in part they are ground-state holes and blueshifted anti-holes created by hydrogen bond weakening. Feature (a) is the mOH ground-state hole and feature (b) is the mOH ground-state anti-hole. Feature (c) is the mOD ground-state hole and feature (d) is the mOD ground-state anti-hole. Feature (e) is the mOH excited-state absorption. 4.2. Time dependence of Stokes and anti-Stokes spectra Fig. 3 shows time-dependent anti-Stokes and Stokes spectra. By t = 3 ps, almost all the mOH and mOD excited states have disappeared, and the remaining features are
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Fig. 4. The mOH excited-state decay determined from anti-Stokes measurements of the excited-state emission and from transient Stokes measurements of the excited-state absorption both give a 0.9 ps lifetime.
long-lived holes and anti-holes. These are observed for more than 100 ps, and we believe they persist until the 100 ls time scale [13] of the DT decay. Fig. 4 shows the mOH excited-state decay kinetics. The anti-Stokes data are the amplitude of the mOH excitedstate emission peak. The Stokes data are the amplitudes of the |10æ ! |20æ excited-state absorption (feature (e) in Fig. 2), near 3160 cm1 where the ground-state hole contribution is minimal. The anti-Stokes signal builds up with the apparatus time response and decays with T1 = 0.9(±1) ps, which is in agreement with IR measurements from other labs [8–10,15]. The Stokes data have a worse signal-to-noise ratio, but within experimental error give exactly the same T1. Fig. 5a–d gives the time dependence of the Stokes transient data at the peaks and nulls of the features labeled a–d in Fig. 1. The data for feature (a) were derived from the signal at 3380 cm1 to avoid the NLS artifact at 3400 cm1. These rises and decays are compared to a theoretical curve f(t) = [1 exp(t/T1)] that represents an exponential build-up with time constant T1 = 0.9 ps.
5. Discussion
Fig. 3. Transient anti-Stokes and transient Stokes spectra of HOD/ D2O with 3400 cm1 pumping. The vertical dashed lines at the minimum of the mOH hole are used to illustrate the vibrational Stokes shift, which is a redshift at shorter time and a blueshift at longer times.
Part of the complexity of the transient Stokes data in Figs. 3 and 5 stems from the mixing of excited-state features with ground-state holes and anti-holes. The blueshifted transients at 3590 and 2590 cm1 in Fig. 5b, d should rather faithfully represent the anti-hole formation dynamics, since there is hardly any excited-state contribution. However, both 3380 and 2380 cm1 transients in Fig. 5a, c evidence both effects. Recalling from Section 2 that a pump pulse causes a Stokes signal increase near the pump frequency, the shorter-time signal
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Fig. 5. Time dependence of portions of the transient Stokes spectrum. Panels a–d correspond to the holes and anti-holes a–d in Fig. 2. The calculated smooth curves illustrate an exponential build-up with the 0.9 ps mOH excited-state lifetime.
increase in Fig. 5a is attributed to mOH excitations and the subsequent decrease to the hole created by hydrogen-bond weakening.
fects of bending excitations caused by mOH decay, so our preliminary Stokes transient data give us little reason to believe in other significantly populated, long-lived (T1 P 0.5 ps) intermediate states.
5.1. Excited-state decay versus ground-state recovery We know from previous anti-Stokes experiments [12] that with 3400 cm1 pumping, the generation by mOH decay of observable intermediate-states mOD, dHOD and dD2O is small (estimated at 10–15%). These intermediate states have lifetimes in the 1–3 ps range [12]. On this basis, ground-state recovery might be expected to be marginally slower than the 0.9 ps excited-state decay. However, there could be other intermediate states that have not or cannot be observed, that could further slow down the ground-state recovery. These might include torsions in the 450–950 cm1 region and hydrogen bond stretches near 180 cm1. The Stokes transients contain new but problematic information about the ground-state recovery. The recovery of the ground-state hole in Fig. 5a is at present too complicated to analyze with sufficient detail. Even if we could isolate the excited-state contribution from the ground-state contribution, all likely intermediate states including dHOD have mOH absorptions close by. The rise of the anti-holes in Fig. 5b, d is sensitive to the total heat build-up resulting from mOH relaxation, even from intermediate states that are spectroscopically invisible. The complication here is that the thermally induced weakening of the hydrogen bonding may not be instantaneous, so the signal build-up might lag behind the heat build-up. Fig. 5 definitely shows that the rise of the weaker hydrogen-bond species is a bit slower than the 0.9 ps excited-state decay. These transients also appear to be nonexponential in time. Much of the time lag in the ground state recovery can be attributed to the known ef-
5.2. Vibrational Stokes shift Using the data in Fig. 3, we can compare the mOH excited-state emission from anti-Stokes data to the mOH ground-state hole from transient Stokes data. There is a problem in determining the frequency of the hole minimum. As Fig. 3 shows, at shorter delay times the redshifted excited-state absorption (e) pulls the hole minimum to the blue, and at longer delay times the rise of the blueshifted anti-hole (b) pulls the minimum to the red. Nevertheless, these effects do not appear large enough to preclude a qualitative assessment of our results. At shorter delay times near t = 0, the vibrational Stokes shift is still a redshift in the 50–70 cm1 range, and at longer delay times it is still a blueshift, which agrees with our previous results [20]. 5.3. Hydrogen bond weakening Hydrogen bond weakening can occur by vibrational pre-dissociation or by heat build-up. Vibrational predissociation would result in a rise of the mOH anti-hole that tracked the mOH decay, whereas thermal bond breaking would lag behind the mOH decay by an amount that depended on the frequencies and lifetimes of the intermediate states. Our anti-hole rise data in Fig. 5b, d do not indicate the presence of a fast transient, so at present we have no evidence for vibrational pre-dissociation. The rise of the anti-holes is fast enough to show that the time constant for thermal bond breaking is fast compared to the mOH lifetime.
Z. Wang et al. / Chemical Physics Letters 397 (2004) 40–45
6. Summary and conclusions We have for the first time used the transient Stokes method to study vibrational dynamics in HOD/D2O, which is a useful complement to IR and anti-Stokes methods. The IR method is sensitive to both absorption and emission processes, but the Raman methods segregate the absorption and emission contributions. Thus whereas in the harmonic approximation producing a vibrational excited state gives no signal with an IR probe, the same excitation results in an increase of both the anti-Stokes and Stokes signal at the Raman shift of the excited state. The Stokes signal is harder to interpret than the anti-Stokes signal, since the latter contains information only from excited state populations, while the former contains both ground-state and excited-state population dynamics. The preliminary Stokes data we have obtained with a pump frequency of 3400 cm1 have a somewhat worse signal-to-noise ratio than the anti-Stokes signals, because we have to subtract the ambient Stokes background. Nevertheless, these results help clarify several important issues. We clearly see transients that lag behind the mOH excited-state decay. However, the time lag is consistent with what is known about the populations of the bending vibrations, so thus far we have been unable to detect the effects of unseen intermediate states as a result of the delay they might induce in the ground-state recovery. Using the ground-state hole as a reference for the vibrational Stokes shift measurement resulted in no qualitative change in our results and conclusions [20]. No evidence has been seen for vibrational pre-dissociation, and thermal hydrogen-bond weakening was seen to be faster than the excited-state decay. These initial results are promising and point to the need for more investigations. In particular we need to investigate the pump frequency and concentration dependence. We know the VR pathways and intermediate states are pump-frequency dependent. Redshifted pump pulses result in faster decay lifetimes and higher quantum yields for bending vibrations [12,14]. The vibrational predissociation study could be made more sensitive by decreasing the HOD concentration, which would minimize the thermal effects that create the long-lived holes and anti-holes without affecting the fast antihole generation that would signal pre-dissociation.
search Contract F49620-03-1-0032 and by Army Research Office Contract DAAD19-00-1-0036. We thank Prof. S. Mukamel for suggesting the use of transient Stokes measurements.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
Acknowledgements This material is based on work supported by the National Science Foundation under Award No. DMR-0096466, by Air Force Office of Scientific Re-
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Vibrational energy dynamics of water studied with ultrafast Stokes and anti-Stokes Raman spectroscopy Zhaohui Wang, Yoonsoo Pang, Dana D. Dlott
*
School of Chemical Sciences, University of Illinois at Urbana-Champaign, Box 01-6 CLSL, 600 S. Mathews Avenue, Urbana, IL 61801, USA Received 17 May 2004; in final form 18 August 2004 Available online 12 September 2004
Abstract The transient Stokes Raman spectroscopy method is introduced to study the dynamics of OH-stretching vibrations in water excited by ultrashort infrared pulses. The combination of Stokes and anti-Stokes Raman probing allows the the absorption and emission contributions to be measured separately. Experiments with 3400 cm1 pumping of OH-stretching of HOD solute in D2O solvent are reported. The Stokes Raman method is used to study the delay between the excited-state decay and the groundstate recovery, the vibrational Stokes shift, and the generation of weakened hydrogen bonding due to heat released by vibrational relaxation. 2004 Elsevier B.V. All rights reserved.
1. Introduction In this Letter, we introduce transient Stokes Raman spectroscopy to study vibrational energy relaxation (VR) of water. VR of water and the HOD solute in D2O has been the subject of numerous recent theoretical [1–4] and experimental papers [5–14]. In experiments, a mid-IR pulse is used to excite a portion of the inhomogeneously broadened OH stretch mOH to the v = 1 state. Both IR [5–10] and anti-Stokes Raman spectroscopies [11–14] have now been used to probe the VR. The mOH dynamics of water have proven to be extremely complicated, and despite numerous previous studies questions still remain. Three pertinent issues are addressed here. (1) The mOH lifetime of HOD/D2O pumped near the absorption maximum is well known [8–10,12,15] to be T1 0.9 ps, but neither the quantum yields for the various intermediate states excited by mOH decay nor the de-
*
Corresponding author. Fax: +1 2172443186. E-mail address: [email protected] (D.D. Dlott).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.073
tails of the pathways back to the ground state [1–4,12– 14] are known with certainty. Our anti-Stokes Raman measurements showed that parent mOH decay generates dOH bending excitations (1640 cm1) in water [13,14] and mOD stretching (2500 cm1) and dHOD (1450 cm1) and dD2O (1200 cm1) bending excitations [12,13] in HOD/D2O, but these observed intermediates account for only a part of the 3400 cm1 mOH energy. Other likely intermediates such as torsions (450–950 cm1) or hydrogen bond stretches (180 cm1) have not been directly observed. With the combination of anti-Stokes and transient Stokes Raman methods, we can individually measure the excited-state decay and the ground-state recovery. Unseen intermediate states cause the ground-state recovery to lag behind the excited-state decay [16,17]. (2) The vibrational Stokes shift [18–20] of mOH is the shift between the v = 0 absorption and the v = 1 emission. (To avoid confusion with Stokes Raman spectroscopy, this effect will be consistently termed the Ôvibrational Stokes shiftÕ.) mOH excitation is expected to be followed by a contraction of the hydrogen bond to its equilibrium position in the excited state, leading
Z. Wang et al. / Chemical Physics Letters 397 (2004) 40–45
to a dynamic emission redshift [18]. Although IR probe experiments [18] and theoretical calculations [19] had apparently agreed on a vibrational Stokes redshift in the 55–70 cm1 range, our recent study [20] showed that the redshift at shorter times was pump frequency dependent, and as time increased (t > 1 ps) the redshift became a blueshift. In that work, the Stokes shift was found by comparing the peak of the excited-state emission from anti-Stokes scattering to the peak of the equilibrium Stokes Raman spectrum. Prof. Mukamel suggested to us that, since the mOH transition is inhomogeneously broadened and since the excited-state decay rate depends on frequency, it might make more sense to compare the excited-state spectrum to the nonequilibrium spectrum of the ground-state hole produced by the pump pulse. The ground-state hole cannot be seen by anti-Stokes probing, but it can be seen with transient Stokes Raman probing. (3) Hydrogen-bond weakening. The pump pulse can cause structural evolution toward states with weaker hydrogen bonding in two distinct ways. With vibrational pre-dissociation [3,21], the decay of mOH excited states would involve breaking or weakening a hydrogen bond. In addition, the decay back to the ground-state dumps heat into the bath, which leads to thermally induced hydrogen-bond weakening in the mOH ground state [22,23]. The former process weakens only hydrogen bonds associated with the excited molecule; the latter process can weaken any hydrogen bond in the system. (Technically in HOD/D2O it is deuterium bonds that are weakened.) The generation of mOH ground-state molecules again cannot be observed in the anti-Stokes spectrum, but in the transient Stokes spectrum it results in the formation of holes and anti-holes in the mOH spectrum. Owing to the general correspondence between vibrational blueshift and weaker hydrogen bonding [24,25], the anti-holes are blueshifted from the holes. In the rest of this Letter, we first compare IR, antiStokes and Stokes probing methods. We then present preliminary results with both Stokes and anti-Stokes probing, using 3400 cm1 pumping of mOH in HOD/ D2O. We discuss the implications for the problems of intermediate states on the return pathway back to the ground state, the vibrational Stokes shifts and hydrogen bond dynamics.
2. Raman and IR probing For this limited discussion, we will use the simplified level diagram [23,26] in Fig. 1. The mid-IR pump xIR is assumed to generate only mOH fundamental excitations. We are primarily concerned with population dynamics and spectral diffusion. Coherent effects that appear when the pump and probe pulses are nearly time-coincident are regarded as artifacts [27], although the coher-
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Fig. 1. Energy level diagram for mOH excitations with one intermediate state on the pathway back to the ground state. The notation |ijæ means i-quanta of stretch and j-quanta of bend.
ence dynamics of water [28–30] is itself an active field. In this model, when the parent mOH stretch decays with time constant T1, it populates the bending vibration dOH which is an intermediate state on the pathway back to the ground state. The mOH decay may produce one or two quanta of bending excitation [1–4,12–14]. The intermediate-state dOH decays to the ground state [23] with time constant seq. In reality mOH decay involves multiple pathways that compete with the dOH channel [12,14], but the extension to multiple pathways is straightforward. The notation |ijæ denotes i quanta of excitation in the parent mOH and j quanta of excitation in the daughter dOH. Most experiments on water use IR probe methods [4,28]. IR probe transmission is sensitive to both the absorption and the induced emission. In the harmonic approximation, upon mOH excitation the probe transmission is increased due to the bleach of the |00æ ! |10æ ground-state absorption and the induced emission from the |10æ ! |00æ transition. The probe transmission is reduced due to excited-state |10æ ! |20æ absorption. These two effects exactly cancel, so there is no transient signal at all. Similarly, in the harmonic approximation stretchto-bend relaxation also produces no signal, whether the probe pulse is in the stretching region or in the bending region. In the stretching region both the induced emission and the excited-state absorption disappear with time constant T1 to be replaced by a new |01æ ! |11æ excited-state absorption that exactly cancels the ground-state bleach. With a bending region probe, the ground-state bleach, induced emission and excited-state absorption once again exactly cancel. In actual water, the mOH anharmonicity is large, 250 cm1. The excited-state absorption is significantly redshifted away from the ground-state absorption and the induced emission. For this reason, the lifetime T1 is most often measured using redshifted pulses to probe a region where the excited-state absorption predominates. The anharmonic shifts of the intermediate-state transitions such as |01æ ! |02æ and |01æ ! |11æ
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are much smaller, apparently small enough that so far IR experiments have not directly observed intermediatestate populations. Anti-Stokes and Stokes Raman scattering by a probe pulse xL result in a segregation of the emission and absorption processes [31,32], so even in the harmonic approximation all these cancellations do not occur. The simpler case is anti-Stokes probing, which sees only excited states against a nominally dark background. The mOH excitations created by the pump pulse are observed at xL + mOH, due to the |10æ ! |00æ transition. Stretchto-bend relaxation causes the mOH signal to lose intensity with time constant T1, with an accompanying build up of the dOH signal at xL + dOH. Subsequently this dOH signal decays with time constant seq. Using a multichannel spectrograph, the stretch and bend populations can be observed simultaneously [13]. Whether stretch-tobend excitation produces one or two quanta of dOH excitation cannot be directly determined in the harmonic approximation, since the |02æ ! |01æ and |01æ ! |00æ transitions are spectroscopically coincident, but given an anharmonic shift this determination becomes possible. The transient Stokes probe is more complicated since the signals reflect changes in both ground-state and excited-state absorptions. The data are more difficult to obtain, since the signal is a pump-induced change on a significantly larger ambient Stokes scattering background. The pump pulse bleaches the |00æ ! |10æ ground-state absorption and creates a |10æ ! |20æ excited-state absorption. In the harmonic approximation these transitions are coincident at xL mOH, but the excited-state absorption is twice as strong as the bleach. Thus, exciting mOH in the harmonic approximation does not result in complete signal cancellation as with IR probing, but instead causes a net increase of the Stokes signal at xL mOH. Stretch-to-bend relaxation causes the excited-state |10æ ! |20æ absorption to disappear, to be replaced by a weaker |01æ ! |11æ absorption that exactly cancels the ground-state bleach, so the stretching signal at xL mOH vanishes with time constant T1. Meanwhile, the dOH excitations cause the Stokes signal at xL dOH to increase with time constant T1 as the result of an |01æ ! |02æ excited-state absorption that is stronger than the |00æ ! |01æ bleaching. This dOH excited-state absorption may also include the |02æ ! |03æ transition if v = 2 excitations are generated. Bend decay with time constant seq causes the dOH signal at xL dOH to disappear, but it has no effect on the mOH signal at xL mOH (ground-state hole) since the |01æ ! |11æ absorption is replaced by an identical |00æ ! |10æ absorption. With anharmonicity the mOH excitedstate absorption is significantly redshifted from the ground-state bleach, but the intermediate-state transitions are not shifted very far. So as with IR, the mOH lifetime T1 can be measured from the excited-state
absorption decay, but it is difficult to directly observe effects of intermediate-state populations. Although in the harmonic approximation creating a vibrational excitation leads to no signal with IR probing, this is not the case with Raman probing. A vibrational excitation causes both the anti-Stokes and the Stokes signals to increase.
3. Experimental The apparatus [26] and experimental technique [13,33,34] have been described previously. The pump is an IR pulse at 3400 cm1 and the probe a 532 nm visible pulse. The IR pulse causes a bulk temperature jump DT = 8 K. The apparatus response in the temporal and spectral domains is determined from measurements of the coherent artifact [13] generated by nonlinear light scattering (NLS). This response is a good fit to Gaussian functions with FWHM 1.1 ps and 35 cm1 [11]. The sample consisted of D2O with 10% HOD. This 10% concentration is about the highest HOD concentration possible before excitation hopping and H2O impurities become a problem [12,20]. For transient Stokes experiments, a series of 2 min acquisitions were performed with the delay at the indicated value and at a negative time value (probe precedes pump). The signal-background-signal cycle was repeated and averaged eight times for each displayed spectrum.
4. Results 4.1. Features of the Stokes difference spectrum Fig. 2 compares the HOD/D2O equilibrium Stokes Raman spectrum to the transient Stokes and anti-Stokes spectra, with 3400 cm1 pumping at a delay of t = 1 ps. NLS artifacts at xL + xIR in the anti-Stokes spectrum and at xL xIR in the Stokes spectrum have been removed by subtraction, as discussed previously [11]. In the equilibrium Stokes spectrum, the mOH signal is due to HOD solute on top of a small contribution from H2O impurity, and the more intense mOD signal is due to D2O solvent on top of a small contribution from HOD [13]. The anti-Stokes signal indicates the instantaneous populations of excited states. By t = 1 ps, the pumped mOH excitations have decayed considerably, and a portion (10%) of these decay processes have generated mOD intermediates. These mOD intermediates are believed to be predominantly [13] mOD(D2O), generated by the mOH(HOD) ! mOD(HOD) ! mOD(D2O) pathway. The Stokes signal is complicated, with negative and positive-going features labeled in Fig. 2a–e. These features are due in part to pump-induced changes in the
Z. Wang et al. / Chemical Physics Letters 397 (2004) 40–45
Fig. 2. Comparison of the HOD/D2O equilibrium Stokes spectrum with the transient anti-Stokes and transient Stokes spectra. These transient spectra are obtained with 3400 cm1 pumping and a time delay of 1 ps. Features in the transient Stokes spectrum labeled (a), (b), (c) and (d) are the mOH hole and anti-hole, and the mOD hole and antihole. Feature (e) is the mOH excited-state absorption.
ground-state and excited-state absorptions as discussed in Section 2, and in part they are ground-state holes and blueshifted anti-holes created by hydrogen bond weakening. Feature (a) is the mOH ground-state hole and feature (b) is the mOH ground-state anti-hole. Feature (c) is the mOD ground-state hole and feature (d) is the mOD ground-state anti-hole. Feature (e) is the mOH excited-state absorption. 4.2. Time dependence of Stokes and anti-Stokes spectra Fig. 3 shows time-dependent anti-Stokes and Stokes spectra. By t = 3 ps, almost all the mOH and mOD excited states have disappeared, and the remaining features are
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Fig. 4. The mOH excited-state decay determined from anti-Stokes measurements of the excited-state emission and from transient Stokes measurements of the excited-state absorption both give a 0.9 ps lifetime.
long-lived holes and anti-holes. These are observed for more than 100 ps, and we believe they persist until the 100 ls time scale [13] of the DT decay. Fig. 4 shows the mOH excited-state decay kinetics. The anti-Stokes data are the amplitude of the mOH excitedstate emission peak. The Stokes data are the amplitudes of the |10æ ! |20æ excited-state absorption (feature (e) in Fig. 2), near 3160 cm1 where the ground-state hole contribution is minimal. The anti-Stokes signal builds up with the apparatus time response and decays with T1 = 0.9(±1) ps, which is in agreement with IR measurements from other labs [8–10,15]. The Stokes data have a worse signal-to-noise ratio, but within experimental error give exactly the same T1. Fig. 5a–d gives the time dependence of the Stokes transient data at the peaks and nulls of the features labeled a–d in Fig. 1. The data for feature (a) were derived from the signal at 3380 cm1 to avoid the NLS artifact at 3400 cm1. These rises and decays are compared to a theoretical curve f(t) = [1 exp(t/T1)] that represents an exponential build-up with time constant T1 = 0.9 ps.
5. Discussion
Fig. 3. Transient anti-Stokes and transient Stokes spectra of HOD/ D2O with 3400 cm1 pumping. The vertical dashed lines at the minimum of the mOH hole are used to illustrate the vibrational Stokes shift, which is a redshift at shorter time and a blueshift at longer times.
Part of the complexity of the transient Stokes data in Figs. 3 and 5 stems from the mixing of excited-state features with ground-state holes and anti-holes. The blueshifted transients at 3590 and 2590 cm1 in Fig. 5b, d should rather faithfully represent the anti-hole formation dynamics, since there is hardly any excited-state contribution. However, both 3380 and 2380 cm1 transients in Fig. 5a, c evidence both effects. Recalling from Section 2 that a pump pulse causes a Stokes signal increase near the pump frequency, the shorter-time signal
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Fig. 5. Time dependence of portions of the transient Stokes spectrum. Panels a–d correspond to the holes and anti-holes a–d in Fig. 2. The calculated smooth curves illustrate an exponential build-up with the 0.9 ps mOH excited-state lifetime.
increase in Fig. 5a is attributed to mOH excitations and the subsequent decrease to the hole created by hydrogen-bond weakening.
fects of bending excitations caused by mOH decay, so our preliminary Stokes transient data give us little reason to believe in other significantly populated, long-lived (T1 P 0.5 ps) intermediate states.
5.1. Excited-state decay versus ground-state recovery We know from previous anti-Stokes experiments [12] that with 3400 cm1 pumping, the generation by mOH decay of observable intermediate-states mOD, dHOD and dD2O is small (estimated at 10–15%). These intermediate states have lifetimes in the 1–3 ps range [12]. On this basis, ground-state recovery might be expected to be marginally slower than the 0.9 ps excited-state decay. However, there could be other intermediate states that have not or cannot be observed, that could further slow down the ground-state recovery. These might include torsions in the 450–950 cm1 region and hydrogen bond stretches near 180 cm1. The Stokes transients contain new but problematic information about the ground-state recovery. The recovery of the ground-state hole in Fig. 5a is at present too complicated to analyze with sufficient detail. Even if we could isolate the excited-state contribution from the ground-state contribution, all likely intermediate states including dHOD have mOH absorptions close by. The rise of the anti-holes in Fig. 5b, d is sensitive to the total heat build-up resulting from mOH relaxation, even from intermediate states that are spectroscopically invisible. The complication here is that the thermally induced weakening of the hydrogen bonding may not be instantaneous, so the signal build-up might lag behind the heat build-up. Fig. 5 definitely shows that the rise of the weaker hydrogen-bond species is a bit slower than the 0.9 ps excited-state decay. These transients also appear to be nonexponential in time. Much of the time lag in the ground state recovery can be attributed to the known ef-
5.2. Vibrational Stokes shift Using the data in Fig. 3, we can compare the mOH excited-state emission from anti-Stokes data to the mOH ground-state hole from transient Stokes data. There is a problem in determining the frequency of the hole minimum. As Fig. 3 shows, at shorter delay times the redshifted excited-state absorption (e) pulls the hole minimum to the blue, and at longer delay times the rise of the blueshifted anti-hole (b) pulls the minimum to the red. Nevertheless, these effects do not appear large enough to preclude a qualitative assessment of our results. At shorter delay times near t = 0, the vibrational Stokes shift is still a redshift in the 50–70 cm1 range, and at longer delay times it is still a blueshift, which agrees with our previous results [20]. 5.3. Hydrogen bond weakening Hydrogen bond weakening can occur by vibrational pre-dissociation or by heat build-up. Vibrational predissociation would result in a rise of the mOH anti-hole that tracked the mOH decay, whereas thermal bond breaking would lag behind the mOH decay by an amount that depended on the frequencies and lifetimes of the intermediate states. Our anti-hole rise data in Fig. 5b, d do not indicate the presence of a fast transient, so at present we have no evidence for vibrational pre-dissociation. The rise of the anti-holes is fast enough to show that the time constant for thermal bond breaking is fast compared to the mOH lifetime.
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6. Summary and conclusions We have for the first time used the transient Stokes method to study vibrational dynamics in HOD/D2O, which is a useful complement to IR and anti-Stokes methods. The IR method is sensitive to both absorption and emission processes, but the Raman methods segregate the absorption and emission contributions. Thus whereas in the harmonic approximation producing a vibrational excited state gives no signal with an IR probe, the same excitation results in an increase of both the anti-Stokes and Stokes signal at the Raman shift of the excited state. The Stokes signal is harder to interpret than the anti-Stokes signal, since the latter contains information only from excited state populations, while the former contains both ground-state and excited-state population dynamics. The preliminary Stokes data we have obtained with a pump frequency of 3400 cm1 have a somewhat worse signal-to-noise ratio than the anti-Stokes signals, because we have to subtract the ambient Stokes background. Nevertheless, these results help clarify several important issues. We clearly see transients that lag behind the mOH excited-state decay. However, the time lag is consistent with what is known about the populations of the bending vibrations, so thus far we have been unable to detect the effects of unseen intermediate states as a result of the delay they might induce in the ground-state recovery. Using the ground-state hole as a reference for the vibrational Stokes shift measurement resulted in no qualitative change in our results and conclusions [20]. No evidence has been seen for vibrational pre-dissociation, and thermal hydrogen-bond weakening was seen to be faster than the excited-state decay. These initial results are promising and point to the need for more investigations. In particular we need to investigate the pump frequency and concentration dependence. We know the VR pathways and intermediate states are pump-frequency dependent. Redshifted pump pulses result in faster decay lifetimes and higher quantum yields for bending vibrations [12,14]. The vibrational predissociation study could be made more sensitive by decreasing the HOD concentration, which would minimize the thermal effects that create the long-lived holes and anti-holes without affecting the fast antihole generation that would signal pre-dissociation.
search Contract F49620-03-1-0032 and by Army Research Office Contract DAAD19-00-1-0036. We thank Prof. S. Mukamel for suggesting the use of transient Stokes measurements.
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Acknowledgements This material is based on work supported by the National Science Foundation under Award No. DMR-0096466, by Air Force Office of Scientific Re-
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