We investigate the influence of continuum generation on the photofragmentation dynamics of the toluene molecule. By sending an 80-fs Ti:sapphire laser beam through a 2-mm-thick CaF2 plate we show that even moderately broadened laser frequency spectra considerably increase the degree of fragmentation, while it is nearly absent without continuum generation. The measured broadening of the frequency spectra and the known Raman-active vibrational modes of toluene afford an explanation in terms of a stimulated Raman scattering scheme, in which the molecule absorbs large amounts of vibrational energy

Kompa, K.-L.

Müller, A. M.

Uiterwaal, Cornelis J.

Wanner, J.

Witzel, B.

English

Müller, Astrid M.

Witzel, Bernd

Uiterwaal, C. J. G. J.

Wanner, Jochen

Kompa, Karl-Ludwig

Max Planck Society eDoc Server

White-light-induced fragmentation of toluene

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln C.J.G.J. Uiterwaal Publications Research Papers in Physics and Astronomy January 2002 White-Light-Induced Fragmentation of Toluene A. M. Müller Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany B. Witzel Department of Molecular and Optical Physics, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 3, 79104 Freiburg, Germany Cornelis J. Uiterwaal University of Nebraska - Lincoln, cuiterwaal2@unl.edu J. Wanner Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany K.-L. Kompa Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany Follow this and additional works at: https://digitalcommons.unl.edu/physicsuiterwaal  Part of the Physics Commons Müller, A. M.; Witzel, B.; Uiterwaal, Cornelis J.; Wanner, J.; and Kompa, K.-L., "White-Light-Induced Fragmentation of Toluene" (2002). C.J.G.J. Uiterwaal Publications. 12. https://digitalcommons.unl.edu/physicsuiterwaal/12 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in C.J.G.J. Uiterwaal Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. White-Light-Induced Fragmentation of Toluene   A. M. Müller1 *, B. Witzel2 †, C. J. G. J. Uiterwaal1 ‡,§, J. Wanner1, and K.-L. Kompa1 1Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany 2Department of Molecular and Optical Physics, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 3, 79104 Freiburg, Germany  Received 15 August 2001; published 21 December 2001 We investigate the influence of continuum generation on the photofragmentation dynamics of the toluene molecule. By sending an 80-fs Ti:sapphire laser beam through a 2-mm-thick CaF2 plate we show that even moderately broadened laser frequency spectra considerably increase the degree of fragmentation, while it is nearly absent without continuum generation. The measured broadening of the frequency spectra and the known Raman-active vibrational modes of toluene afford an explanation in terms of a stimulated Raman scattering scheme, in which the molecule absorbs large amounts of vibrational energy.  ©2002 The American Physical Society   URL: http://link.aps.org/abstract/PRL/v88/e023001  DOI: 10.1103/PhysRevLett.88.023001  PACS: 33.80.-b, 42.65.Dr   * Present address: JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309-0440.  † To whom correspondence should be addressed.Email addresses: witzel@uni-freiburg.de; cuiterwaal2@unlnotes.unl.edu  ‡  § Present address: Department of Physics and Astronomy, University of Nebraska–Lincoln, 116 Brace Lab, Lincoln, NE 68588-0111.  VOLUME 88, NUMBER 2 P H Y S I C A L R E V I E W L E T T E R S 14 JANUARY 2002White-Light-Induced Fragmentation of TolueneA. M. Müller,1,* B. Witzel,2,† C. J. G. J. Uiterwaal,1,† ,‡ J. Wanner,1 and K.-L. Kompa11Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany2Department of Molecular and Optical Physics, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 3,79104 Freiburg, Germany(Received 15 August 2001; published 21 December 2001)We investigate the influence of continuum generation on the photofragmentation dynamics of thetoluene molecule. By sending an 80-fs Ti:sapphire laser beam through a 2-mm-thick CaF2 plate weshow that even moderately broadened laser frequency spectra considerably increase the degree of frag-mentation, while it is nearly absent without continuum generation. The measured broadening of thefrequency spectra and the known Raman-active vibrational modes of toluene afford an explanation interms of a stimulated Raman scattering scheme, in which the molecule absorbs large amounts of vibra-tional energy.DOI: 10.1103/PhysRevLett.88.023001 PACS numbers: 33.80.–b, 42.65.DrIntroduction.—The study of white-light production in-duced by the interaction of an ultraintense laser pulse withcondensed optical transparent media has been the sub-ject of several recent investigations [1,2]. Applicationssuch as optical parametric amplifiers [3] for production oftunable ultrafast light pulses [4] or the use of glass-fibertechniques [5,6] for ultrashort pulse generation are nowavailable. Particularly this progress allows implementationand development of coherent control techniques, whichare important for manipulation of molecular dissociationpaths in laser-molecule ultrashort pulse interaction. Thewhite-light continuum can also be used for time-resolvedbroadband absorption and excitation spectroscopy [7], aswell as for dynamic characterization of laser-induced struc-tural transitions. The underlying process of femtosec-ond continuum generation, most probably generated byself-phase modulation [8,9], is far from being completelyunderstood. It could be shown that the circumstances forobserving white-light production are strongly related to acritical laser power threshold [10]Pcr c´0l28pn2,where c and l are, respectively, the speed of light and thewavelength of the laser light (both in vacuo); n2 is the non-linear index of refraction, defined here as n  n0 1 n2E2(n0: linear refractive index; E: peak electric field strength),not to be confused with the alternative intensity-based defi-nition of this quantity given by n  n0 1 gI. Laserscommonly used in intense-field photoionization experi-ments have powers of several GW, which is far beyondthe critical power of optical materials commonly used.CaF2, for instance, has Pcr  2.7 MW. Although anylaser beam with a power exceeding this critical value couldproduce a white-light continuum in principle, the extent towhich this happens strongly depends on the optical pathlength in the nonlinear medium and the intensity of thebeam. A good measure of the extent of self-focusing (and,hence, of continuum generation) taking place when a laserbeam travels over a distance L in the z direction througha medium with intensity-based nonlinear index g is givenby the B integral, defined as [11]B 2plZ L0gIzdz .When this dimensionless number stays below a limit of 3to 5 [11] self-focusing effects are negligible. An equiva-lent measure is the number Q, defined as [9] gILct(t  pulse duration; I  intensity). The present experi-ments are conducted under conditions where continuumgeneration starts to become noticeable as modulation anda moderate broadening of the frequency spectrum. As aconsequence, we observe a marked influence on the ion-ization and dissociation dynamics of the toluene molecule.This molecule was chosen as an example; the basic natureof the induced behavior is general and is likely to play animportant role in nearly all studies of molecular dissocia-tion processes.The experimental setup is reported in previous work[12]. A Ti:sapphire laser system (Spectra Physics) is used,delivering 80-fs, 800-nm pulses at a repetition rate of1 kHz with a maximum energy of 2.0 mJpulse (shot-to-shot stability 64%). The experimental studies reportedare performed at the fundamental wavelength. The un-focused laser beam has a Gaussian spatial profile witha FWHM diameter of 5.37 6 0.02 mm. Assuming thetemporal shape of the pulse to be Gaussian, we determinea FWHM of 80 6 1 fs, by standard autocorrelation tech-niques. Hence, the peak intensity of the unfocused beamis I0  4.2 3 1010 Wcm2. The laser beam is focusedin the first acceleration zone of a Wiley-McLaren–typetime-of-flight (TOF) mass spectrometer [13]. Experimentsare conducted with two different planoconvex, uncoatedfused silica lenses with focal lengths of 300 and 1000 mm.For beam diagnostic purposes a flipper mirror is mountedbehind the lens, which allows redirection of the beam to aCCD camera system protected with a variable attenuator.In this way the spatial profile of the focus can be analyzed023001-1 0031-90070288(2)023001(4)$20.00 © 2002 The American Physical Society 023001-1VOLUME 88, NUMBER 2 P H Y S I C A L R E V I E W L E T T E R S 14 JANUARY 2002outside the TOF for each lens independently. We use a2-mm-thick CaF2 laser flat as the entrance window of thespectrometer. This material was chosen because its largeband gap of 10.2 eV [2] makes it particularly suited tocontinuum generation. It should be noted that a multitudeof optical transparent materials often used would lead towhite-light production under our experimental conditions[1]. Depending on the focal length of the lens used, thelaser intensity at the entrance window can be chosen in therange 2 3 1010 to 3 3 1011 W cm22, correspondingto cumulative B-integral values between 0.7 and 1.5.We do not observe significant changes such as filamenta-tion in the spatial beam profile when the laser energy isvaried. A sector filter disk mounted in front of the lensenables attenuation of the beam by up to 2 orders of mag-nitude. This allows experiments with comparable laser in-tensities at focus but with different laser intensities at theentrance window. The frequency spectrum of the laserbeam is recorded by using a fiber spectrometer (OceanOptics SD2000) placed several meters behind the exitwindow (2-mm laser flat CaF2) and centered at the opti-cal axis.Each spectral measurement is an average over 1000 lasershots. Toluene vapor (Merck Uvasol, purity 99.9%) isadmitted to the vacuum chamber by means of a variableleak valve. Depending on the experimental requirements,the partial pressure was between 1028 and 1027 mbar.Under these conditions space charge effects or electronimpact ionization of rest gases do not have any influenceon the experiment.Results.—First we demonstrate the influence of the in-creasing B-integral value on ionization and dissociation oftoluene after short-pulse laser interaction. This experimentis performed with a 1000-mm lens. Mass spectra of tolueneand the corresponding wavelength spectra are taken as afunction of the intensity to which the CaF2 entrance win-dow is exposed (Fig. 1). The ratio of the intensity at fo-cus to that at the entrance window is IfIw  20. Therecorded wavelength spectra (Fig. 1a) are expected to besomewhat broader than those at focus, because the radia-tion passes through the exit window before it is analyzedby the fiber spectrometer. For better visualization the massspectra of Figs. 1b and 1c are normalized to the parent ionmass peak (Fig. 1c), and the vertical scale for the low-masspart of the spectrum (Fig. 1b) is expanded by a factor of100. Clearly, increasing the intensity not only causes spec-tral broadening of the radiation (Fig. 1a) but also leads toincreasing fragmentation (Fig. 1b). To demonstrate thismore quantitatively, we show in Fig. 1d, as an example, theincrease of the sum of the yields for masses 27 (C2H31),39 (C3H31), 51 (C4H31), and 65 (C5H51) for increasingIw . Many more peaks can be identified in Fig. 1b; peakbroadening and splitting due to kinetic energy depositionand doubly charged ions are observed as well.In principle, the increased fragmentation observed inFig. 1 could be due either to the increasing intensity orto the increasing radiation bandwidth. To shed furtherlight on the fragmentation mechanism we have carried outan experiment with two different lenses: f  300 mm(Fig. 2a) and f  1000 mm (Fig. 2b). Although the massspectrum of Fig. 2a is recorded with a higher focal inten-sity (If  1 3 1014 W cm22) than that of Fig. 2b (If 6 3 1013 W cm22), strong fragmentation is observed onlyin the latter. Obviously, this is a result of the differentFIG. 1. Mass spectra of toluene and corresponding wavelength spectra of the laser light used, recorded at different intensities at theCaF2 entrance window Iw . Panel (a): wavelength spectra for various values of Iw . Blank circles and straight dashed lines indicatecalculated Stokes and anti-Stokes cutoffs. Panel (b): mass spectra for the same Iw values as in (a), normalized to the parent ionpeak (c). The H2O1 peak (marked with an asterisk) is clipped. The vertical scale for masses below 88 mass units is enlarged by afactor of 100. Panel (d): ratio of integrated fragment peaks [sum of masses 27 (C2H31), 39 (C3H31), 51 (C4H31), and 65 (C5H51)]to integrated parent ion peak [panel (c)].023001-2 023001-2VOLUME 88, NUMBER 2 P H Y S I C A L R E V I E W L E T T E R S 14 JANUARY 2002FIG. 2. Mass spectra of toluene. Panel (a): 300-mm focallength lens; Iw  7 3 1010 W cm22, If  1 3 1014 W cm22;panel (b): 1000-mm focal length lens; Iw  3 3 1011 W cm22,If  6 3 1013 W cm22.peak intensities to which the CaF2 entrance window is ex-posed (Iw  7 3 1010 W cm22 in Fig. 2a and Iw  3 31011 W cm22 in Fig. 2b). We conclude that the increas-ing fragmentation as seen in Figs. 1b and 1d is the resultof the increasing radiation bandwidth (Fig. 1a), and not ofthe increasing intensity.Discussion.—One possible mechanism by which anincreased bandwidth can lead to increased fragmentationis a stimulated Raman scattering process. In such a sce-nario a significant number of difference frequencies areabsorbed by Raman-active modes of the neutral toluenemolecule and/or of ionized toluene. To support this view,we calculated the availability of difference frequencies.Having measured the spectral content Sv of the non-linearly distorted pulses (Fig. 1a), one can define thestrength of a specific difference frequency v0 as Dv0 RSvSv 1 v0dv. In Fig. 3, such difference fre-quency spectra Dv0 are shown for various intensities Iwat the entrance window. Clearly, the range of differencefrequencies available is significantly broadened if Iw isincreased, a particularly strong increase occurring between200 and 800 cm21. Nine vibrational modes of the neutraltoluene molecule exist in this range, eight of which areknown to be Raman active [14]. Seven of these Raman-active modes (vertical lines in Fig. 3) involve vibrationsof the carbon backbone. The strong increase of Dv0 inthis range can thus bring about strong vibrational exci-tation of the molecule, giving it enough internal energyto dissociate after ionization. Alternatively, part or all ofthe absorption of difference frequencies could take placein the ionized molecule. It is known that so-called softionization, i.e., photoionization of toluene with minimalfragmentation, takes place at 790 nm [15], 750 nm [16],and 375 nm [15]. Therefore, it seems improbable that anyof the frequencies created is responsible by itself for theincreased fragmentation. Another possible mechanismcould be stepwise multiphoton resonant ionization. Thiswould imply the availability in sufficient quantities of asmany frequencies as there are steps in such a process,including the frequency of the first electronic excitationFIG. 3. Availability of difference frequencies as derived (seetext) from the measured wavelength spectra recorded with speci-fied intensities Iw at the CaF2 entrance window (cf. Fig. 1a).[(1) Iw  27.73 1010 W cm22, (2) Iw  20.93 1010 W cm22,(3) Iw  15.23 1010 W cm22, (4) Iw  10.63 1010 W cm22,(5) Iw  7.333 1010 W cm22 d, (6) Iw  5.043 1010 W cm22,(7) Iw  3.993 1010 W cm22 d, (8) Iw  3.553 1010 W cm22].Vertical lines indicate Raman-active modes [14] of the carbonbackbone of the neutral toluene molecule between 200 and800 cm21.at 4.6 eV [17]. [The photon energy for the fundamentalwavelength (800 nm) of the Ti:sapphire laser is 1.55 eV.]Because this concept is resonant, one might argue that itcould be driven by undetectably small VIS/UV/VUV com-ponents in the frequency spectra of Fig. 1a. To clarifythis issue, we made calculations based on analytical andnumerical models for nonlinear spectral broadening [1,9].The Stokes and anti-Stokes cutoff frequencies we findare indicated as open circles in Fig. 1a and are in goodagreement with experiment. Clearly, under our experi-mental conditions the anti-Stokes wing of the continuumdoes not even reach visible frequencies, so that a stepwisemultiphoton resonant scheme can be ruled out. In our cal-culations, we find Q ø 1 [9] (Q  0.02), which confirmsthat there is only a very moderate degree of continuumgeneration with symmetric Stokes and anti-Stokes cutofffrequencies as shown in Fig. 1a.Conclusion.— In conclusion, we have shown that mod-erate amounts of nonlinear spectral broadening may al-ready lead to fundamental changes in the fragmentationbehavior of a polyatomic molecule. An explanation isafforded by a stimulated Raman process allowing the mole-cule or the molecular ion to absorb large amounts of vi-brational energy. The general nature of this phenomenonleads us to believe that it can be employed in coherent oroptimal control of fragmentation.B. W. is obliged to Max-Planck-Institut für Quantenop-tik for the possibility of participating in this work and ac-knowledges DFG (Deutsche Forschungsgemeinschaft) forsupport. The authors also thank W. Fuß and H. Schröderfor stimulating discussions. They acknowledge the techni-cal help of W. Ritt and A. Steyer.023001-3 023001-3VOLUME 88, NUMBER 2 P H Y S I C A L R E V I E W L E T T E R S 14 JANUARY 2002*Present address: JILA, University of Colorado and Na-tional Institute of Standards and Technology, Boulder, CO80309-0440.†To whom correspondence should be addressed.Email addresses: witzel@uni-freiburg.de; cuiterwaal2@unlnotes.unl.edu‡Present address: Department of Physics and Astronomy,University of Nebraska–Lincoln, 116 Brace Lab, Lincoln,NE 68588-0111.[1] A. Brodeur and S. L. Chin, J. Opt. Soc. Am. B 16, 637(1999).[2] A. Brodeur and S. L. Chin, Phys. Rev. Lett. 80, 4406 (1998).[3] K. R. Wilson and V. V. Yakovlev, J. Opt. Soc. Am. B 14,444 (1997).[4] S. Cussat-Blanc, A. Ivanov, D. Lupinski, and E. Freysz,Appl. Phys. B 70, 247 (2000).[5] A. Baltuska, Z. Wei, M. S. Pshenichnikov, D. A. Wiersma,and R. Szipocs, Appl. Phys. B 65, 175 (1997).[6] R. L. Fork, C. V. Shank, C. Hirlimann, and R. Yen, Opt.Lett. 8, 1 (1983).[7] The Supercontinuum Laser Source, edited by R. R. Alfano(Springer, New York, 1989).[8] R. R. Alfano and S. L. Shapiro, Phys. Rev. Lett. 24, 584(1970).[9] G. Yang and Y. R. Shen, Opt. Lett. 9, 510 (1984).[10] P. W. Milonni and J. H. Eberly, Lasers (Wiley, New York,1988).[11] A. E. Siegman, Lasers (University Science Books, MillValley, CA, 1986).[12] A. M. Müller, C. J. G. J. Uiterwaal, B. Witzel, J. Wanner,and K.-L. Kompa, J. Chem. Phys. 112, 9289 (2000).[13] W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum. 26, 1150(1955).[14] L. M. Sverdlov, M. A. Kovnor, and E. P. Krainov, Vibra-tional Spectra of Polyatomic Molecules (Wiley, New York,1974).[15] D. J. Smith, K. W. D. Ledingham, R. P. Singhal, H. S. Kilic,T. McCanny, A. J. Langley, P. F. Taday, and C. Kosmidis,Rapid Commun. Mass Spectrom. 12, 813 (1998).[16] K. W. D. Ledingham, D. J. Smith, R. P. Singhal, T. Mc-Canny, P. Graham, H. S. Kilic, W. X. Peng, A. J. Langley,P. F. Taday, and C. Kosmidis, J. Phys. Chem. A 103, 2952(1999).[17] UV Atlas of Organic Compounds, edited by H. H. Perkam-pus, J. Sandeman, and C. J. Timmons (Butterworths, Lon-don, 1966), Vol. II.023001-4 023001-4

White-Light-Induced Fragmentation of Toluene

Müller, A.

Uiterwaal, C.

Kompa, K.

MPG.PuRe

We investigate the influence of continuum generation on the photofragmentation dynamics of the toluene molecule. By sending an 80-fs Ti:sapphire laser beam through a 2-mm-thick CaF<sub>2</sub> plate we show that even moderately broadened laser frequency spectra considerably increase the degree of fragmentation, while it is nearly absent without continuum generation. The measured broadening of the frequency spectra and the known Raman-active vibrational modes of toluene afford an explanation in terms of a stimulated Raman scattering scheme, in which the molecule absorbs large amounts of vibrational energy

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1011&context=physicsuiterwaal

White-Light-Induced Fragmentation of Toluene

Abstract

Similar works

Full text

Available Versions

Max Planck Society eDoc Server

DigitalCommons@University of Nebraska

MPG.PuRe

MPG.PuRe