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Impulsive excitation of high vibrational states in I2–Xe complex on the electronic ground state
Chemical Physics Letters 491 (2010) 44–48
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Impulsive excitation of high vibrational states in I2–Xe complex on the electronic ground state Tiina Kiviniemi *, Eero Hulkko, Mika Pettersson Department of Chemistry, Nanoscience Center, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
a r t i c l e
i n f o
Article history: Received 23 February 2010 In final form 27 March 2010 Available online 31 March 2010
a b s t r a c t High vibrational states, up to m = 22, are excited and investigated on the ground electronic state of a 1:1 I2–Xe complex isolated in solid Kr using femtosecond CARS technique and spontaneous resonant Raman measurements. The results show that this system is a promising candidate for investigations of coherent control of bimolecular reactions by using vibrational wavepackets on the ground electronic state. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Femtosecond laser pulses, with their short duration and broad spectral width, allow a multitude of processes for preparation, manipulation, and investigation of molecular coherences. In addition to observing the dynamics of a system, they also open up interesting possibilities for controlling its time evolution, and even manipulating a chemical reaction. Unimolecular reactions such as dissociation are known to be, at least to some extent, controllable with ultrafast laser pulses, but controlling bimolecular reactions is still a rather demanding area in the use of light to manipulate the outcomes of reactions [1]. Whether unimolecular or bimolecular, one of the most useful coherent reaction control schemes follows the concept introduced theoretically by Tannor, Rice, and Kosloff [2,3]. In this scheme, short laser pulses are used to first generate a wavepacket that is left to evolve until a suitable conformation, or a position on the molecular potential, is reached. After the evolution period, a second laser pulse is used to excite the system on the reactive surface in a controlled way so that the desired reaction will occur. With a combination of one or several laser pulses, the evolving wavepacket can be in principle generated to any potential surface that is judged useful from the reaction point of view. In an attempt to control a bimolecular chemical reaction Potter et al. considered a harpooning reaction in gas phase between iodine molecule and xenon atom using a femtosecond pump–probe scheme [4]. The coherent vibrational motion of iodine molecule was excited on the B state, after which it was excited to the ion pair state where the reaction between iodine and xenon took place. The product yield was found to oscillate as a function of the time delay between the pump and control pulses, similarly to the wavepacket motion of the iodine molecule on the B state. However, this experiment could not distinguish between two different mechanisms; * Corresponding author. Fax: +358 (0) 14 260 4756. E-mail address: tiina.t.kiviniemi@jyu.fi (T. Kiviniemi). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.03.075
the one where the collision complex, formed between the B state iodine and a ground state xenon atom, is excited to the ion pair state leading to the reaction, and the other, where the iodine molecule alone is excited to the ion pair state prior to the reaction with xenon. As only the first of these two processes would constitute coherent control, it should be experimentally singled out to prove the existence of active control of the reaction [5]. Unfortunately, in gas phase, the proof is difficult to obtain, as the second mechanism can be estimated to completely overwhelm the signal [5]. However, it is possible to single out the reaction of the complex, if it is trapped in an environment where the diffusion of the species is restricted. In this case, the molecules can be prepared and maintained in the entrance channel complex conformation. Zadoyan and Apkarian were able to follow the bimolecular reaction between Xe+Cl and Xe in liquid Xe by interrupting the reaction with a femtosecond dump pulse [6]. By bringing the system to the neutral dissociative state the direction of the reaction was changed. Thus, this scheme represents an example of an active optical control of a bimolecular reaction. Favorable situation for bimolecular reaction control can be achieved also with matrix isolation technique, where the complex is trapped in a solid rare-gas environment. In this case, the matrix cage effect prevents the separation of the final reaction products, but the charge-transfer process can still be realized, controlled, and detected. In most cases, coherent reaction control strategies are based on the evolution of vibrational wavepackets on the excited electronic states to find a suitable configuration for optical excitation of the system to the final reactive state. However, the coherence time on the excited states is often shortened due to multiple crossings with repulsive excited states. To overcome this problem, the reaction could rather be initiated from the ground electronic state using highly excited vibrational wavepackets created by means of a CARS process, such as discussed in our earlier publication on the iodine– benzene complex [7]. In this scheme, vibrational wavepacket on the ground electronic state, created with a pump-dump part of the
T. Kiviniemi et al. / Chemical Physics Letters 491 (2010) 44–48
CARS process, is allowed to evolve until a suitable conformation for the excitation to the reactive potential surface is reached. Then, the third pulse of the CARS sequence is used to initiate a reaction via excitation to the reactive potential surface. The outcome of the reaction can thus be controlled by controlling the position and shape of the wavepacket on the ground electronic state at the moment of the final excitation. The coherence times on the ground electronic state are much longer, and allow more time for the wavepacket to reach a conformation desired for the reaction. Moreover, as the ground electronic state has no crossing repulsive states, predissociation is not a problem, unlike for higher electronic states. Such strategy allows also initiation of reactions directly on the ground electronic state if the reaction barriers are low enough. Control of vibrational dynamics by the CARS technique has been investigated for example in crystalline polydiacetylene, aiming at reaction control on the ground state potential surface [8]. In this work, we have prepared highly excited vibrational wavepackets with m0max ¼ 22, on the electronic ground state of a 1:1 iodine–xenon complex isolated in solid krypton. Isolating the complex in a low-temperature rare-gas environment allows obtaining a well-defined, isolated structure with only weak interactions with the environment and long vibrational dephasing times, making it an optimal model system for bimolecular reaction control experiments. The possibility to excite high vibrational levels on one coordinate of the complex allows to obtain information on areas of the potential energy surface that would be otherwise very difficult to probe. In addition, it opens up new reaction pathways for the coherent reaction control experiments, which are currently taking place in our laboratory. 2. Experimental The solid samples for both spontaneous resonant Raman and fsCARS measurements were prepared by depositing a gas mixture of I2/Xe/Kr = 1/2.6/2600 on a 100 lm thick sapphire window at T = 40 K in a liquid helium flow cryostat. This method produces matrices with high optical quality and an estimated complex to iodine ratio of 1/3 [9,10]. As shown previously, the samples prepared this way contain monomeric iodine molecules, and 1:1 I2–Xe complexes with most probably a linear structure [10]. The Stokes branch spontaneous resonance Raman spectrum was measured at T = 10 K, using a single-mode, frequency doubled CW Nd:YAG laser (Alphalas), whose 532 nm wavelength is resonant with iodine molecule’s B X electronic transition. The spectrum was measured with two resolutions; 1 and 5 cm1, and the frequency scale was calibrated using the vibrational frequencies of the uncomplexed iodine molecule in solid krypton, determined earlier with high accuracy from a similarly prepared sample with fs-CARS measurements [9]. The setup for the fs-CARS measurements was the same as for our earlier measurements of the I2–Xe complex, and has been described in detail in our previous publication [10]. The pulse parameters (k; Dk in nm) for the highest vibrational wavepacket measured, m = 19–22, were: pump (558; 8), dump (714; 10) and probe (620; 13). The fs-CARS signals were measured at T = 10 K, averaging 300 pulses per data point with a 20 fs step size of the probe delay line. The time delay between pump and dump pulses was adjusted so that the fs-CARS signal was maximized. The frequency domain spectrum of the fs-CARS signal was obtained by Fourier transforming the time domain signal. 3. Results and discussion The lower resolution spontaneous resonant Raman spectrum in Fig. 1a shows a long vibrational progression for both the uncom-
45
plexed and complexed iodine molecules. For the lowest vibrational states, the bands of complexed and uncomplexed iodine are not resolved, but for higher states the bands start to separate due to the different vibrational frequencies of the uncomplexed and complexed iodine. The higher resolution spectrum in Fig. 1b–j shows well-separated, narrow, instrument limited single lines, indicating a well-defined complex in a homogeneous matrix with only one kind of matrix site for each species. The progression for the 1:1 I2–Xe complex is of lower intensity due to the smaller amount of complex in the sample, but it is still visible for almost as high vibrational states, m0max ¼ 18, as for the uncomplexed iodine molecule (mmax = 20). The relative decrease in Raman intensities as a function of the vibrational state is similar for both uncomplexed and complexed iodine, indicating that the dynamics of the Raman process is similar for both species [7,11], and that the iodine molecule is only slightly perturbed upon complexation with a xenon atom. The fs-CARS signal in both time- and frequency domain for the m = 19–22 wavepacket is shown in Fig. 2. In the CARS process, the first two pulses, pump and dump, are used to create a vibrational wavepacket on the ground electronic state of the iodine molecule, and the third pulse, probe, is then used to probe the time evolution of the wavepacket. The signal thus oscillates as a function of the probe delay with the difference frequencies of the different vibrational levels excited in the wavepacket. The oscillating frequencies are either quantum beats, due to coherent excitation of different levels in the same species, or polarization beats, which are cross-terms between the signals of the two different species in the sample [7,10,12]. In a sample with no xenon, no polarization beats are detected, so the polarization beats can be reliably assigned as interference between the signals of uncomplexed iodine and I2–Xe complex [9,10]. Because the complex concentration is lower than the uncomplexed iodine concentration, the strongest signals are due to the uncomplexed iodine, and the signal from the complex is only visible in the polarization beats. The polarization beats are then used to analyze the vibrational frequencies and properties of the complex [10]. In Fig. 2, the bands are designated according to the vibrational energy level terms involved in generation of the corresponding band frequency, with primes referring to the complex. For higher vibrational levels, the dephasing times get shorter, leading to the bands overlapping in the frequency domain CARS spectrum. However, the beating frequencies can be reliably assigned to vibrational levels as high as m = 19–22, as shown in Fig. 2. The assignment agrees fully with our previous measurements on lower vibrational wavepackets, giving the same vibrational frequencies as previously for both the uncomplexed iodine and the complex [10]: xe = 211.24 ± 0.04 cm1 and xexe = 0.632 ± 0.003 cm1, and x0e ¼ 210:36 0:04 cm1 and x0e x0e ¼ 0:636 0:003 cm1 , respectively. The accuracy of the results shows, that the electronic states of both the complex and the uncomplexed iodine are well described using a Morse potential even at high vibrational energies. Overlapping of the CARS bands makes accurate determination of single bandwidths difficult, but the average dephasing rates for vibrational levels m = 19–22 can be estimated to be 0.09 ps1 for the complex, and slightly smaller, 0.08 ps1, for the uncomplexed iodine, with dephasing rate increasing as a function of the vibrational level for both species, as expected. These values also agree well with the estimation of the real, deconvoluted bandwidths from the spontaneous Raman spectrum. Such long coherence times allow for tens of round-trips of the wavepacket on the ground electronic state potential surface before dephasing. The observation of long lasting vibrational coherence of highly excited vibrational wavepackets in the complex indicates weak coupling between the complex partners. This is advantageous from
46
T. Kiviniemi et al. / Chemical Physics Letters 491 (2010) 44–48
Fig. 1. Spontaneous resonance Raman spectrum of a I2/Xe/Kr = 1/2.6/2600 sample at T = 10 K. The long vibrational progression can be seen for both the uncomplexed iodine and the I2–Xe complex. (a) The lower resolution (5 cm1) spectrum generated by combining two measurements shows the vibrational progression of iodine up to m = 20. The sharp Raman bands ride on a continuous background of hot luminescence and fluorescence [11]. (b)–(j) Higher resolution (1 cm1) Raman spectrum showing the wellseparated bands of Raman transitions to vibrational states from m = 1 up to m = 18 for both the uncomplexed iodine (higher frequency bands) and the I2–Xe complex (lower frequency bands). (b) m = 1 and 2, (c) m = 3 and 4, (d) m = 5 and 6, (e) m = 7 and 8, (f) m = 9 and 10, (g) m = 11 and 12, (h) m = 13 and 14, (i) m = 15 and 16, and (j) m = 17 and 18. The complex bands are of lower intensity, but the relative decrease of the band intensities with quantum number m is similar for both species, indicating similar dynamics for both. The intensities are not comparable between the different panels as each graph is scaled to the maximum intensity to show the features of the weaker bands.
the point of view of coherent control where long coherence times may be beneficial. For example, a longer evolution time on the ground electronic state allows filtering out possible other competing processes with shorter dephasing times. In addition, the possibility to excite high vibrational levels in a controlled and coherent manner allows the complex to reach conformations that are not possible on the lowest vibrational states, making the direct excitation of the wavepacket to the selected parts of the excited, reactive state potential possible, thus allowing control of the charge-transfer reaction in a new way. In the scheme proposed here for the I2–Xe complex, the high vibrational wavepackets on the ground electronic state of the complex are used to reach a I–I bond length long enough so that the final excitation to the charge-transfer state of the complex will lead
to a potential well minimum of a bound state rather than to the repulsive portion of the potential reachable by direct excitation from the low vibrational states [7]. Then, the evolution time of the wavepacket before the final excitation can be used to control whether the excitation leads to dissociation or not. Comparing the results shown here with our earlier results on the iodine–benzene complex [7], we can conclude, that the possibilities to use fsCARS scheme for controlling the reactions are more extensive with the iodine–xenon complex. In iodine–benzene complex it is more difficult to reach very high vibrational levels on the ground electronic state, which narrows the area of conformations which can be used as the starting point for the reaction control. With iodine–xenon complex, however, very high vibrational states can easily be reached, making it an ideal model system for the control
T. Kiviniemi et al. / Chemical Physics Letters 491 (2010) 44–48
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Fig. 2. fs-CARS spectra of a I2/Xe/Kr = 1/2.6/2600 sample for the m = 19–22 wavepacket at T = 10 K. (a) The time domain spectrum. (b)–(d) The frequency domain spectrum obtained by Fourier transforming the time domain spectrum. The frequency domain bands are designated according to the vibrational energy level terms involved in generation of the corresponding band frequency, with primes referring to the complex.
experiments using ground electronic state vibrations as the starting point.
4. Conclusions High vibrational wavepackets, with m0 = 19–22, on the ground electronic state of the 1:1 iodine–xenon complex isolated in solid krypton are observed and analyzed using both spontaneous resonant Raman and femtosecond CARS measurements. Only a small shift in the vibrational frequency upon complexation is found, and the dephasing rates are similar for both uncomplexed and complexed iodine, indicating that the iodine molecule is only slightly perturbed upon complexation with xenon. The possibility to prepare and manipulate high vibrational wavepackets in an isolated 1:1 molecular complex using femtosecond spectroscopy methods opens up interesting possibilities for controlling a bimolecular reaction using the ground electronic
state vibrations as the starting point. The weak coupling of the complex partners gives long coherence times, which can be advantageous for the reaction control scheme as it allows longer evolution times of the wavepacket before excitation to the reactive potential energy surface. Acknowledgement This work was financially supported by Academy of Finland (decision number 122620). References [1] [2] [3] [4] [5] [6]
M. Dantus, V. Lozovoy, Chem. Rev. 104 (2004) 1813. D.J. Tannor, S.A. Rice, J. Chem. Phys. 83 (1985). D.J. Tannor, R. Kosloff, S.A. Rice, J. Chem. Phys. 85 (1986) 5805. E.D. Potter, J.L. Herek, S. Pedersen, Q. Liu, A.H. Zewail, Nature 355 (1992) 66. V.A. Apkarian, J. Chem. Phys. 106 (1997) 5298. R. Zadoyan, V.A. Apkarian, Chem. Phys. Lett. 206 (1993) 475.
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[7] T. Kiviniemi, E. Hulkko, T. Kiljunen, M. Pettersson, J. Phys. Chem. A 113 (2009) 6326. [8] D. Zeidler et al., J. Chem. Phys. 116 (2002) 5231. [9] T. Kiviniemi, J. Aumanen, P. Myllyperkiö, V.A. Apkarian, M. Pettersson, J. Chem. Phys. 123 (2005) 064509.
[10] T. Kiviniemi, T. Kiljunen, M. Pettersson, J. Chem. Phys. 125 (2006) 164302. [11] J. Almy, K. Kizer, R. Zadoyan, V.A. Apkarian, J. Phys. Chem. A 104 (2000) 3508. [12] J. Faeder, I. Pinkas, G. Knopp, Y. Prior, T.J. Tannor, J. Chem. Phys. 115 (2001) 8440.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Impulsive excitation of high vibrational states in I2–Xe complex on the electronic ground state Tiina Kiviniemi *, Eero Hulkko, Mika Pettersson Department of Chemistry, Nanoscience Center, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
a r t i c l e
i n f o
Article history: Received 23 February 2010 In final form 27 March 2010 Available online 31 March 2010
a b s t r a c t High vibrational states, up to m = 22, are excited and investigated on the ground electronic state of a 1:1 I2–Xe complex isolated in solid Kr using femtosecond CARS technique and spontaneous resonant Raman measurements. The results show that this system is a promising candidate for investigations of coherent control of bimolecular reactions by using vibrational wavepackets on the ground electronic state. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Femtosecond laser pulses, with their short duration and broad spectral width, allow a multitude of processes for preparation, manipulation, and investigation of molecular coherences. In addition to observing the dynamics of a system, they also open up interesting possibilities for controlling its time evolution, and even manipulating a chemical reaction. Unimolecular reactions such as dissociation are known to be, at least to some extent, controllable with ultrafast laser pulses, but controlling bimolecular reactions is still a rather demanding area in the use of light to manipulate the outcomes of reactions [1]. Whether unimolecular or bimolecular, one of the most useful coherent reaction control schemes follows the concept introduced theoretically by Tannor, Rice, and Kosloff [2,3]. In this scheme, short laser pulses are used to first generate a wavepacket that is left to evolve until a suitable conformation, or a position on the molecular potential, is reached. After the evolution period, a second laser pulse is used to excite the system on the reactive surface in a controlled way so that the desired reaction will occur. With a combination of one or several laser pulses, the evolving wavepacket can be in principle generated to any potential surface that is judged useful from the reaction point of view. In an attempt to control a bimolecular chemical reaction Potter et al. considered a harpooning reaction in gas phase between iodine molecule and xenon atom using a femtosecond pump–probe scheme [4]. The coherent vibrational motion of iodine molecule was excited on the B state, after which it was excited to the ion pair state where the reaction between iodine and xenon took place. The product yield was found to oscillate as a function of the time delay between the pump and control pulses, similarly to the wavepacket motion of the iodine molecule on the B state. However, this experiment could not distinguish between two different mechanisms; * Corresponding author. Fax: +358 (0) 14 260 4756. E-mail address: tiina.t.kiviniemi@jyu.fi (T. Kiviniemi). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.03.075
the one where the collision complex, formed between the B state iodine and a ground state xenon atom, is excited to the ion pair state leading to the reaction, and the other, where the iodine molecule alone is excited to the ion pair state prior to the reaction with xenon. As only the first of these two processes would constitute coherent control, it should be experimentally singled out to prove the existence of active control of the reaction [5]. Unfortunately, in gas phase, the proof is difficult to obtain, as the second mechanism can be estimated to completely overwhelm the signal [5]. However, it is possible to single out the reaction of the complex, if it is trapped in an environment where the diffusion of the species is restricted. In this case, the molecules can be prepared and maintained in the entrance channel complex conformation. Zadoyan and Apkarian were able to follow the bimolecular reaction between Xe+Cl and Xe in liquid Xe by interrupting the reaction with a femtosecond dump pulse [6]. By bringing the system to the neutral dissociative state the direction of the reaction was changed. Thus, this scheme represents an example of an active optical control of a bimolecular reaction. Favorable situation for bimolecular reaction control can be achieved also with matrix isolation technique, where the complex is trapped in a solid rare-gas environment. In this case, the matrix cage effect prevents the separation of the final reaction products, but the charge-transfer process can still be realized, controlled, and detected. In most cases, coherent reaction control strategies are based on the evolution of vibrational wavepackets on the excited electronic states to find a suitable configuration for optical excitation of the system to the final reactive state. However, the coherence time on the excited states is often shortened due to multiple crossings with repulsive excited states. To overcome this problem, the reaction could rather be initiated from the ground electronic state using highly excited vibrational wavepackets created by means of a CARS process, such as discussed in our earlier publication on the iodine– benzene complex [7]. In this scheme, vibrational wavepacket on the ground electronic state, created with a pump-dump part of the
T. Kiviniemi et al. / Chemical Physics Letters 491 (2010) 44–48
CARS process, is allowed to evolve until a suitable conformation for the excitation to the reactive potential surface is reached. Then, the third pulse of the CARS sequence is used to initiate a reaction via excitation to the reactive potential surface. The outcome of the reaction can thus be controlled by controlling the position and shape of the wavepacket on the ground electronic state at the moment of the final excitation. The coherence times on the ground electronic state are much longer, and allow more time for the wavepacket to reach a conformation desired for the reaction. Moreover, as the ground electronic state has no crossing repulsive states, predissociation is not a problem, unlike for higher electronic states. Such strategy allows also initiation of reactions directly on the ground electronic state if the reaction barriers are low enough. Control of vibrational dynamics by the CARS technique has been investigated for example in crystalline polydiacetylene, aiming at reaction control on the ground state potential surface [8]. In this work, we have prepared highly excited vibrational wavepackets with m0max ¼ 22, on the electronic ground state of a 1:1 iodine–xenon complex isolated in solid krypton. Isolating the complex in a low-temperature rare-gas environment allows obtaining a well-defined, isolated structure with only weak interactions with the environment and long vibrational dephasing times, making it an optimal model system for bimolecular reaction control experiments. The possibility to excite high vibrational levels on one coordinate of the complex allows to obtain information on areas of the potential energy surface that would be otherwise very difficult to probe. In addition, it opens up new reaction pathways for the coherent reaction control experiments, which are currently taking place in our laboratory. 2. Experimental The solid samples for both spontaneous resonant Raman and fsCARS measurements were prepared by depositing a gas mixture of I2/Xe/Kr = 1/2.6/2600 on a 100 lm thick sapphire window at T = 40 K in a liquid helium flow cryostat. This method produces matrices with high optical quality and an estimated complex to iodine ratio of 1/3 [9,10]. As shown previously, the samples prepared this way contain monomeric iodine molecules, and 1:1 I2–Xe complexes with most probably a linear structure [10]. The Stokes branch spontaneous resonance Raman spectrum was measured at T = 10 K, using a single-mode, frequency doubled CW Nd:YAG laser (Alphalas), whose 532 nm wavelength is resonant with iodine molecule’s B X electronic transition. The spectrum was measured with two resolutions; 1 and 5 cm1, and the frequency scale was calibrated using the vibrational frequencies of the uncomplexed iodine molecule in solid krypton, determined earlier with high accuracy from a similarly prepared sample with fs-CARS measurements [9]. The setup for the fs-CARS measurements was the same as for our earlier measurements of the I2–Xe complex, and has been described in detail in our previous publication [10]. The pulse parameters (k; Dk in nm) for the highest vibrational wavepacket measured, m = 19–22, were: pump (558; 8), dump (714; 10) and probe (620; 13). The fs-CARS signals were measured at T = 10 K, averaging 300 pulses per data point with a 20 fs step size of the probe delay line. The time delay between pump and dump pulses was adjusted so that the fs-CARS signal was maximized. The frequency domain spectrum of the fs-CARS signal was obtained by Fourier transforming the time domain signal. 3. Results and discussion The lower resolution spontaneous resonant Raman spectrum in Fig. 1a shows a long vibrational progression for both the uncom-
45
plexed and complexed iodine molecules. For the lowest vibrational states, the bands of complexed and uncomplexed iodine are not resolved, but for higher states the bands start to separate due to the different vibrational frequencies of the uncomplexed and complexed iodine. The higher resolution spectrum in Fig. 1b–j shows well-separated, narrow, instrument limited single lines, indicating a well-defined complex in a homogeneous matrix with only one kind of matrix site for each species. The progression for the 1:1 I2–Xe complex is of lower intensity due to the smaller amount of complex in the sample, but it is still visible for almost as high vibrational states, m0max ¼ 18, as for the uncomplexed iodine molecule (mmax = 20). The relative decrease in Raman intensities as a function of the vibrational state is similar for both uncomplexed and complexed iodine, indicating that the dynamics of the Raman process is similar for both species [7,11], and that the iodine molecule is only slightly perturbed upon complexation with a xenon atom. The fs-CARS signal in both time- and frequency domain for the m = 19–22 wavepacket is shown in Fig. 2. In the CARS process, the first two pulses, pump and dump, are used to create a vibrational wavepacket on the ground electronic state of the iodine molecule, and the third pulse, probe, is then used to probe the time evolution of the wavepacket. The signal thus oscillates as a function of the probe delay with the difference frequencies of the different vibrational levels excited in the wavepacket. The oscillating frequencies are either quantum beats, due to coherent excitation of different levels in the same species, or polarization beats, which are cross-terms between the signals of the two different species in the sample [7,10,12]. In a sample with no xenon, no polarization beats are detected, so the polarization beats can be reliably assigned as interference between the signals of uncomplexed iodine and I2–Xe complex [9,10]. Because the complex concentration is lower than the uncomplexed iodine concentration, the strongest signals are due to the uncomplexed iodine, and the signal from the complex is only visible in the polarization beats. The polarization beats are then used to analyze the vibrational frequencies and properties of the complex [10]. In Fig. 2, the bands are designated according to the vibrational energy level terms involved in generation of the corresponding band frequency, with primes referring to the complex. For higher vibrational levels, the dephasing times get shorter, leading to the bands overlapping in the frequency domain CARS spectrum. However, the beating frequencies can be reliably assigned to vibrational levels as high as m = 19–22, as shown in Fig. 2. The assignment agrees fully with our previous measurements on lower vibrational wavepackets, giving the same vibrational frequencies as previously for both the uncomplexed iodine and the complex [10]: xe = 211.24 ± 0.04 cm1 and xexe = 0.632 ± 0.003 cm1, and x0e ¼ 210:36 0:04 cm1 and x0e x0e ¼ 0:636 0:003 cm1 , respectively. The accuracy of the results shows, that the electronic states of both the complex and the uncomplexed iodine are well described using a Morse potential even at high vibrational energies. Overlapping of the CARS bands makes accurate determination of single bandwidths difficult, but the average dephasing rates for vibrational levels m = 19–22 can be estimated to be 0.09 ps1 for the complex, and slightly smaller, 0.08 ps1, for the uncomplexed iodine, with dephasing rate increasing as a function of the vibrational level for both species, as expected. These values also agree well with the estimation of the real, deconvoluted bandwidths from the spontaneous Raman spectrum. Such long coherence times allow for tens of round-trips of the wavepacket on the ground electronic state potential surface before dephasing. The observation of long lasting vibrational coherence of highly excited vibrational wavepackets in the complex indicates weak coupling between the complex partners. This is advantageous from
46
T. Kiviniemi et al. / Chemical Physics Letters 491 (2010) 44–48
Fig. 1. Spontaneous resonance Raman spectrum of a I2/Xe/Kr = 1/2.6/2600 sample at T = 10 K. The long vibrational progression can be seen for both the uncomplexed iodine and the I2–Xe complex. (a) The lower resolution (5 cm1) spectrum generated by combining two measurements shows the vibrational progression of iodine up to m = 20. The sharp Raman bands ride on a continuous background of hot luminescence and fluorescence [11]. (b)–(j) Higher resolution (1 cm1) Raman spectrum showing the wellseparated bands of Raman transitions to vibrational states from m = 1 up to m = 18 for both the uncomplexed iodine (higher frequency bands) and the I2–Xe complex (lower frequency bands). (b) m = 1 and 2, (c) m = 3 and 4, (d) m = 5 and 6, (e) m = 7 and 8, (f) m = 9 and 10, (g) m = 11 and 12, (h) m = 13 and 14, (i) m = 15 and 16, and (j) m = 17 and 18. The complex bands are of lower intensity, but the relative decrease of the band intensities with quantum number m is similar for both species, indicating similar dynamics for both. The intensities are not comparable between the different panels as each graph is scaled to the maximum intensity to show the features of the weaker bands.
the point of view of coherent control where long coherence times may be beneficial. For example, a longer evolution time on the ground electronic state allows filtering out possible other competing processes with shorter dephasing times. In addition, the possibility to excite high vibrational levels in a controlled and coherent manner allows the complex to reach conformations that are not possible on the lowest vibrational states, making the direct excitation of the wavepacket to the selected parts of the excited, reactive state potential possible, thus allowing control of the charge-transfer reaction in a new way. In the scheme proposed here for the I2–Xe complex, the high vibrational wavepackets on the ground electronic state of the complex are used to reach a I–I bond length long enough so that the final excitation to the charge-transfer state of the complex will lead
to a potential well minimum of a bound state rather than to the repulsive portion of the potential reachable by direct excitation from the low vibrational states [7]. Then, the evolution time of the wavepacket before the final excitation can be used to control whether the excitation leads to dissociation or not. Comparing the results shown here with our earlier results on the iodine–benzene complex [7], we can conclude, that the possibilities to use fsCARS scheme for controlling the reactions are more extensive with the iodine–xenon complex. In iodine–benzene complex it is more difficult to reach very high vibrational levels on the ground electronic state, which narrows the area of conformations which can be used as the starting point for the reaction control. With iodine–xenon complex, however, very high vibrational states can easily be reached, making it an ideal model system for the control
T. Kiviniemi et al. / Chemical Physics Letters 491 (2010) 44–48
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Fig. 2. fs-CARS spectra of a I2/Xe/Kr = 1/2.6/2600 sample for the m = 19–22 wavepacket at T = 10 K. (a) The time domain spectrum. (b)–(d) The frequency domain spectrum obtained by Fourier transforming the time domain spectrum. The frequency domain bands are designated according to the vibrational energy level terms involved in generation of the corresponding band frequency, with primes referring to the complex.
experiments using ground electronic state vibrations as the starting point.
4. Conclusions High vibrational wavepackets, with m0 = 19–22, on the ground electronic state of the 1:1 iodine–xenon complex isolated in solid krypton are observed and analyzed using both spontaneous resonant Raman and femtosecond CARS measurements. Only a small shift in the vibrational frequency upon complexation is found, and the dephasing rates are similar for both uncomplexed and complexed iodine, indicating that the iodine molecule is only slightly perturbed upon complexation with xenon. The possibility to prepare and manipulate high vibrational wavepackets in an isolated 1:1 molecular complex using femtosecond spectroscopy methods opens up interesting possibilities for controlling a bimolecular reaction using the ground electronic
state vibrations as the starting point. The weak coupling of the complex partners gives long coherence times, which can be advantageous for the reaction control scheme as it allows longer evolution times of the wavepacket before excitation to the reactive potential energy surface. Acknowledgement This work was financially supported by Academy of Finland (decision number 122620). References [1] [2] [3] [4] [5] [6]
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