Includes bibliographical references (pages 033001-4).We demonstrate the creation and measurement of shaped multimode vibrational wave packets with overtone and combination mode excitation in CCl4. Excitation of wave packets through nonresonant impulsive stimulated Raman scattering allows for coherent control of molecular vibrations without passing through an electronic resonance. This technique is therefore very general and can be implemented in a large class of molecular gases and liquids at STP, which were previously inaccessible because their resonances are in the VUV

Bartels, R. A.

Kapteyn, H. C.

Leone, S. R.

Murnane, M. M.

Weinacht, T. C.

English

We demonstrate the creation and measurement of shaped multimode vibrational wave packets with overtone and combination mode excitation in CCl4. Excitation of wave packets through nonresonant impulsive stimulated Raman scattering allows for coherent control of molecular vibrations without passing through an electronic resonance. This technique is therefore very general and can be implemented in a large class of molecular gases and liquids at STP, which were previously inaccessible because their resonances are in the VUV

Murnane, M. M., author

Kapteyn, H. C., author

Leone, S. R., author

Weinacht, T. C., author

Bartels, R. A., author

American Physical Society, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

VOLUME 88, NUMBER 3 P H Y S I C A L R E V I E W L E T T E R S 21 JANUARY 200203300Nonresonant Control of Multimode Molecular Wave Packets at Room TemperatureR. A. Bartels,* T. C. Weinacht, S. R. Leone, H. C. Kapteyn, and M. M. MurnaneJILA, University of Colorado and National Institutes of Technology, Boulder, Colorado 80309(Received 1 June 2001; published 4 January 2002)We demonstrate the creation and measurement of shaped multimode vibrational wave packets withovertone and combination mode excitation in CCl4. Excitation of wave packets through nonresonant im-pulsive stimulated Raman scattering allows for coherent control of molecular vibrations without passingthrough an electronic resonance. This technique is therefore very general and can be implemented ina large class of molecular gases and liquids at STP, which were previously inaccessible because theirresonances are in the VUV.DOI: 10.1103/PhysRevLett.88.033001 PACS numbers: 33.80.–b, 42.65.DrWith the advent of ultrafast lasers capable of generatingpulses that are short in comparison to molecular timescales, there has been considerable interest in creating andmeasuring vibrational wave packets. Early experimentsgenerated wave packets in excited electronic states anddemonstrated control over the position of these wave pack-ets as a function of time [1]. Recent experiments [2] haveincluded nuclear motion in the ground electronic state,which is directly relevant to controlling and monitoringmany chemical reactions. These experiments focused oncontrolling motion in a single normal mode of the system.Extending control and measurement of vibrational motionto several modes including overtones is an exciting pros-pect for several types of studies, including mode couplingand bond selective chemistry [3,4]. Furthermore, most pastwork on ground state vibrations relied on transferringpopulation through an excited electronic state (resonant Ra-man). In this Letter we demonstrate the creation and mea-surement of shaped multimode vibrational wave packets inCCl4 with strong overtone excitation, implying a signifi-cant vibrational amplitude. We also control which modesand which mode combinations are excited. A learning algo-rithm is used to automate the control process. Past workin CCl4 used sequences of five pulses to excite vibrationalmodes to investigate intramode coupling and anharmonic-ity, although no overtone or combination modes wereobserved [5].The vibrational wave packets in this work are excitedwith a single pulse far detuned from electronic resonances,achieving nonresonant control of the amplitude of modesin the wave packet, as shown in earlier solid state im-pulsive stimulated Raman scattering (ISRS) experiments[6,7]. Because it is nonresonant, it can be generalized toany molecular gas or liquid with Raman-active modes thatcan be excited impulsively, and it is not limited by the needto access resonances in the VUV. This approach works athigh pressures and temperatures, and thus may facilitatelaser control of chemical reactions with macroscopic prod-uct yields. In particular, since changes in chemical bond-ing will involve a superposition of several modes, controlof multimode wave packets may lead to selective controlover reactions.1-1 0031-90070288(3)033001(4)$20.00Excitation of Raman-active modes involves two pho-tons with a frequency difference equal to the vibrationalenergy. If the duration of a driving laser pulse tp is shorterthan the vibrational period tn  2pVn, the bandwidthof the laser pulse DvL exceeds the vibrational frequency,and the excitation can be considered impulsive. DuringISRS, frequency pairs with a difference of Vn within thebandwidth of a single laser pulse act as pump and Stokesphotons. For appropriate phasing of pairs within the band-width, strong excitation results. In a simple classical model,the displacement from equilibrium of the nuclear coordi-nate Q for a given mode n in the ISRS limit can be de-scribed by [8] dQ2dt2 1 2gdQdt 1 V2nQ 12N dadQ 0E2L, whereg is the coherence decay rate, Qn is the vibrational fre-quency, N is the number density of molecules, dadQ0is the change in polarizability with intermolecular distanceQ, and ELt 12 ALte2ivLt 1 c.c. is the laser field.Expanding the laser field as E2Lt 14 2ALtALt 1A2Le2i2vLt 1 A2L ei2vLt gives a “dc” term correspondingto the intensity profile It, and two second harmonicterms. The nuclei, which have too much inertia to re-spond to the fundamental (or second harmonic) field, areeffectively driven by It. Thus, the strength of excitationof a vibrational mode with frequency V is proportional tothe power spectral density DV of the intensity profileat that vibrational frequency, where DV R`2` dv 3aLvaLV 2 v exp2ifv 2 fV 2 v andAv  F At  aLv exp2ifv. Based on thissimple analysis, a pulse with spectral phase described bya periodic function at the molecular vibrational frequency(e.g., a train of pulses separated by the vibrational period[6,8]) should be as efficient as a transform-limited pulsein exciting vibrations [6,8,9]. [Note: to maximize DV,fv 2 fV 2 v  const.] Later in the text, we com-pare the excitation of Raman modes with intense transform-limited pulses to that of low-intensity chirped Ramanexcitation and observe the breakdown of the weak excita-tion limit described by this analysis.For our experiments, we used 15 fs (660 cm21 FWHMbandwidth) laser pulses generated by an amplified Ti:sap-phire laser system, at a 1 kHz repetition frequency, andwith an energy of 1 mJ [10]. The pulses from our laser© 2002 The American Physical Society 033001-1VOLUME 88, NUMBER 3 P H Y S I C A L R E V I E W L E T T E R S 21 JANUARY 2002were shaped using two separate techniques. The laser isequipped with a deformable mirror pulse shaper, which al-ters the spectral phase of the output pulses. Alternatively,a pulse pair generated by a Michelson interferometer canbe used for somewhat less flexible control. This pulse paircan be either transform limited or “chirped” by adjustinga pair of gratings in the pulse compressor. By adjustingthe relative delay of the chirped or compressed pulse pair,we control which modes in the wave packet are excited.A small portion of the pump pulse (10%) is used as aprobe of the Raman coherence. This probe pulse is fre-quency doubled in a long nonlinear crystal to generate anarrow bandwidth (narrower than the separation betweenmodes in CCl4) probe (	1 THz, 450 fs, 400 nm) that iswell separated spectrally from the pump.The pump and probe pulses are focused into a 30-cm-long, 450-mm-diameter hollow-core fiber filled with amolecular gas or vapor from a molecular liquid at pres-sures of 100–900 torr [11,12]. The pump pulse excitesa vibrational coherence in the molecules, which is de-tected by the probe as a time-varying index of refraction.This results in the appearance of FM sidebands on theprobe spectrum, separated by the vibrational frequency.Monitoring the intensity of a given sideband provides ameasure of the vibrational excitation of the mode labeledby the sideband.Figure 1 shows the results of selective excitation of vi-brational modes in CCl4 using impulsive chirped Ramanexcitation. For this experiment, the pulse was first chirpedin time (to 83 fs, with a chirp rate of 0.1125 THzfs)prior to entering the Michelson interferometer. The in-stantaneous frequency [vi  dwtdt  2bt, wherecR  bp is the chirp rate as defined by wt  vLt 1 bt2]of the pulse sweeps linearly in time, producing a constantfrequency difference (Dn) between the two pulses fromthe interferometer for nonzero delays. The interferencebetween the two pulses produces a beat note with a periodof 1Dn in the intensity profile described by Itt Iint 1 Iint 2 t 1 2pIintIint 2 t cosvLt 1 bt2 22btt, where It is the intensity profile, t is the delaybetween the pulses, and Dn  cRt. Changing the delaytunes the frequency difference and the period of the inten-sity modulations, allowing for mode selective excitationof the molecule. The chirped pulses were injected into thehollow-core fiber filled with a room-temperature vaporpressure (80 torr) of CCl4. The three Raman modesunder control are n1, n2, and n4, whose frequencies are13.77 (459 cm21), 6.54 (218 cm21), and 9.42 (314 cm21)THz, respectively. Figure 1(a) shows the probe spectrumas a function of interferometer delay. As the frequencydifference in the pump matches the vibrational frequencyof each mode in the molecule, that mode is driven effi-ciently while the other modes remain relatively unaffected.Figure 1(b) shows lineouts of the scattered probe inten-sity at frequencies corresponding to each of the fundamen-tal modes. For a fixed chirp rate (pulse length), the optimal033001-2Interferometer Delay (fs)-200 -100 100 200013.6 THz9.9 THz6.4THzFrequency (THz)015-15Scattered Intensity (a.u.)ν1ν4ν2(b)(a)FIG. 1. Probe spectrum of CCl4 when excited by two chirpedpulses. The shading in the figure is proportional to the scatteredlight intensity. The chirped pulse is still sufficiently short todrive some ISRS excitation near time zero. (a) Probe spectrumvs delay between pump pulses. (b) Lineouts at vibrational fre-quencies n1, n4 , and n2.delay for mode n1 is given by t1  V1cR . The optimaltime delays for modes n1, n4, and n2 appear at 122, 88,and 58 fs, implying frequencies of 13.6, 9.9, and 6.5 THz,respectively. This technique also works well in othermolecules such as SF6 [12]. The data show that vibra-tional modes within the laser bandwidth can be separatelycontrolled with a properly constructed It.Excitation of Raman modes in CCl4 exhibits differentbehavior when driven by intense, transform-limited pulses.Figure 2 shows the probe spectrum obtained after exci-tation of CCl4 with two time-delayed transform-limitedpulses. Excitation with transform-limited pulses can leadto overtone excitation, as shown in Fig. 3(b). This can alsobe seen by performing a 1D Fourier transform of the datain Fig. 2(a) to produce a data set that can be comparedwith that of Fig. 1(a). Figure 3(a) shows such a transformin which various modes and mode combinations can beseen. The two transform-limited pulses excite not only thenormal modes of the system but also overtones and com-bination modes not observed in the chirped pulse data, asshown in Figs. 2 and 3.The excitation mechanism is best understood in comple-mentary domains for the two experiments. For transform-limited pulses, the first pulse creates a wave packet thatbegins to evolve. The second pulse then generates another033001-2VOLUME 88, NUMBER 3 P H Y S I C A L R E V I E W L E T T E R S 21 JANUARY 2002FIG. 2. Probe spectrum of CCl4 excited by two transform-limited pulses. (a) Probe spectrum vs time delay between pumppulses. (b) Lineouts at frequencies corresponding to n1, n4, andn1 2 n4.wave packet, which interferes with the first, enhancing orsuppressing modes in the wave packet depending on the de-lay between pulses. The case of chirped excitation pulsesis best understood in the frequency domain. A chirpedbroadband pulse can be thought of as a comb of 109phase-locked quasi-cw lasers spread out in time (reducingthe peak intensity by spreading energy in time), tuningfrom red to blue. The second time-delayed pulse causesinterference between two spectrally shifted combs, equiva-lent to detuning a cw laser by an amount proportional tothe delay time and the chirp rate. For a purely linear re-sponse, we expect the same behavior for both cases. Thedifferences we observe indicate a nonlinear response of themolecular system to an intense transform-limited pulse.The light scattered at the frequency 2n1 in Fig. 3(a)shows two local maxima, only one of which lies on theline of slope 1. The maximum occurring with a pulse peri-odicity of 1n1 cannot be unambiguously assigned to lightscatter from the overtone as it may include light scatteredtwice from n1. The second spot marked is scattered to afrequency 2n1 and has a periodicity of 12n1, provid-ing unambiguous evidence for overtone excitation. Fur-ther evidence for overtone excitation is revealed by ob-serving the dephasing and rephasing of the wave pack-ets due to the anharmonic potential. Figure 3(b) showsthe results of sweeping the probe delay while monitoringlight scattered to 13.77 THz in CCl4. A 10.6 6 0.7 psbeat period is observed, corresponding to an anharmonic-ity (2vexe) of 0.0948 6 0.006 THz (3.16 6 0.2 cm21).033001-3FIG. 3. (a) Data from Fig. 2, where the temporal data at eachscattered frequency is Fourier transformed. The shaded regionsindicate the power spectral density. (b) Scattered probe light at13.77 THz in CCl4 shows beating due to overtone excitation.The fast-Fourier transform (FFT) of the time delay scan in theinset shows excitation in both the first and second overtones.Fourier transforming the time scan reveals evidence forhigher overtone excitation [Fig. 3(b) inset]. The depthof modulation, due to the level of overtone excitation, isseen to decay faster than the fundamental vibration co-herence (estimated to be 18 ps). Detailed knowledge ofthe vibrational potential should allow for determinationof the eigenstate populations [13]. We also observe evi-dence for overtone excitation in CO2 for the asymmetricstretch mode n1 (41.64 THz). We measure rephasing witha period of 1.47 6 0.1 ps, implying a 0.681 6 0.093 THz(22.7 6 3.1 cm21) anharmonicity, in agreement with pub-lished values [14]. An alternate technique can measure theanharmonicity of IR active modes in organic molecules byobserving the beating of vibrational echo signals initiatedby pairs of IR free electron laser pulses [15]. In contrast,our technique measures the anharmonicity of Raman ac-tive modes with nonresonant ISRS excitation.The lineouts of Fig. 2(b) demonstrate our ability to ex-cite a combination mode that is a coherent sum of twodifferent normal modes. By adjusting the delay betweenpump pulses to 1n1 2 n4, we can excite vibrational mo-tion that is a combination of the n1 and n4 modes of the033001-3VOLUME 88, NUMBER 3 P H Y S I C A L R E V I E W L E T T E R S 21 JANUARY 2002 Select 9.42 THz Select 13.77 THz7 8 9 10 11 12 13 14 15 16101520250.00.20.405Scattered Energy (a.u.)Spectral Intensity (a.u.)Frequency (THz)(a)(b)FIG. 4. Learning control and selective excitation of two Raman-active modes in CCl4. (a) Enhancement of either the 13.77 THzmode (black, optimizing n1 vs n4) or the 9.42 THz mode(hatched, optimizing n4 vs n1). (b) Power spectrum of theoptimized pulse intensity profile.molecule [lowest lineout in Fig. 2(b), at position A], whereno individual excitation of modes n1 or n4 is observed.This indicates that the probe does not simply acquire modu-lations at the difference frequency through optical mixingof the two normal mode frequencies, but rather that it ismodulated by the motion of the molecule in the combina-tion mode. Position B marks a delay between pump pulsesfor which the excitation of both normal modes is maxi-mum, but where there is no excitation of the combinationmode. Excitation of such combination modes is an impor-tant step toward bond selective excitation, since reactioncoordinates do not always correspond to normal modes ofmolecules.Finally, we also demonstrate an alternate pulse shapingscheme based on a deformable mirror pulse shaper [16], to-gether with a feedback loop [17] that incorporates a learn-ing algorithm [18]. This approach allows for more generalshaping of a multimode wave packet because the pulseshaper is programmable and can be guided by the learningalgorithm. For example, exciting CCl4 with a single, shortpulse coherently excites all three Raman-active vibrationalmodes (n1, n2, n4) simultaneously. Tailoring the intensityprofile allows us to control the excitation of each vibra-tional mode independently. In this experiment, an evo-lutionary algorithm was used to direct the pulse shaper toexcite a specific mode (n1) while suppressing others by ad-justing the relative spectral phases in the pulse. The blackbar in Fig. 4(a) shows that the relative energy scatteredinto n1 is over 10 times larger than for n4. The same trend033001-4is reflected in the power spectrum of the intensity profileat the vibrational frequencies [Fig. 4(b)]. The reverse ex-periment (optimizing n4 vs n1) was also performed. Thedegree of control shown here is limited by the resolu-tion of the pulse shaper. Although an algorithm is not,in principle, required for this technique, it is very conve-nient because it compensates for any experimental nonide-alities and automates the tailoring of populations (modeamplitudes) in the wave packet by selecting which modesparticipate.In conclusion, we have demonstrated the ability to create“custom” multimode vibrational wave packets in molecu-lar systems at STP. Significant vibrational amplitude isachieved as evidenced by strong overtone excitation. Inaddition to creating shaped molecular wave packets, wemeasure and observe dynamics using temporally resolvednonlinear spectroscopy. These techniques are general andcan be applied to many transparent molecular gases, or liq-uids with high vapor pressures at room temperature.We acknowledge useful discussions with David Nesbitt,David Jonas, and Paul Corkum. The authors gratefully ac-knowledge support for this work from the National ScienceFoundation.*Author to whom correspondence should be addressed.Email address: bartels@colorado.edu[1] B. Kohler et al., Phys. Rev. Lett. 74, 3360 (1995).[2] G. Knopp et al., J. Raman Spectrosc. 31, 51 (2000); V. Lo-zovoy et al., ibid.31, 41 (2000); T. Hornung et al., Chem.Phys. Lett. 326, 445 (2000); M. Karavitis et al., J. Chem.Phys. 114, 4131 (2001); E. Hertz et al., Phys. Rev. A 61,033816 (2000).[3] F. Crim, Acc. Chem. Res. 32, 877 (1999).[4] R. Zare, Science 279, 1875 (1998).[5] A. Tokmakoff et al., Phys. Rev. Lett. 79, 2702 (1997).[6] L. Dhar, J. Rogers, and K. A. Nelson, Chem. Rev. 94, 157(1994).[7] A. M. Weiner et al., Science 247, 1317 (1990).[8] Y. X. Yan, E. B. Gamble, Jr., and K. A. Nelson, J. Chem.Phys. 83, 5391 (1985).[9] D. Meshulach and Y. Silberberg, Phys. Rev. A 60, 1287(1999).[10] E. Zeek et al., Opt. Lett. 25, 587 (2000).[11] A. Nazarkin et al., Phys. Rev. Lett. 83, 2560 (1999).[12] T. Weinacht et al., Chem. Phys. Lett. 344, 333 (2001).[13] V. Romero-Rochín et al., J. Chem. Phys. 111, 3559 (1999).[14] Q. Meng, Y. Zheng, and S. Ding, Int. J. Quantum Chem.81, 154 (2001).[15] K. D. Rector et al., J. Chem. Phys. 106, 10 027 (1997).[16] E. Zeek et al., Opt. Lett. 24, 493 (1999).[17] R. Bartels et al., Nature (London) 406, 164 (2000).[18] R. S. Judson and H. Rabitz, Phys. Rev. Lett. 68, 1500(1992).033001-4

Nonresonant control of multimode molecular wave packets at room temperature

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Nonresonant control of multimode molecular wave packets at room temperature

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