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4376 J. Chem. Phys., Vol. 108, No. 11, 15 March 1998 Communicationsplace with transform limited pulses, i.e., no chirp. However,chirped pulse excitation opens the possibility for introducingdifferent initial phases. Therefore, the shape and dynamics ofthe wave packet can be controlled by chirped pulses with thegoal of affecting the outcome of a chemical process. 7,14,15The following experiments were carried out with ahomebuilt femtosecond laser system consisting of a collidingpulse mode-locked dye laser CPM pumped by an Ar laser.The CPM output was amplified in a pulsed dye amplifier.The amplified pulses were compressed by a two-prism,double-pass pulse compressor. The laser system producedpulses with 47 fs temporal width and with the central wavelengthat 624 nm. Typical pulse energy was 0.4 mJ 30 Hzrepetition rate, but was attenuated for the experiments to 70J. In cases where 312 nm femtosecond pulses were necessary,the 624 nm beam was frequency doubled by a KDPcrystal 0.1 mm producing 7 J pulses. The pulses werethen focused into a quartz cell containing the gaseoussample. Fluorescence signals originating from the cell werecollected along the direction perpendicular to the laser beampropagation through a 0.27 m spectrometer. The laser intensityof the 312 nm beam was continuously monitored by aphotodiode.One of the compression prisms was mounted on acomputer-controlled actuator. Translation of the prism alongits bisector varied the amount of glass through which thebeam propagated and permitted the introduction of linearchirp. Because the amount of chirp was computer controlled,scans of signal intensity as a function of chirp could beachieved in a convenient and fast manner. Characterizationof the femtosecond pulses was carried out by frequency resolvedoptical gating FROG. 18 FROG measurements in ourlaboratory provided direct measurements of the frequencycomponents of each pulse as a function of time, from whichthe linear chirp was determined. A continuous chirp scale asa function of prism position was compiled using the expressionfor group velocity dispersion as a function of prismposition derived by Fork et al. 19 and simplified by Salin andBrun. 20Quartz cells containing iodine, I 2 , or methylene iodide,CH 2 I 2 Aldrich 99%, were prepared on a vacuum line, andwere degassed to less than 10 6 Torr. Iodine scavengingagents sodium thiosulphate and copper were introduced inthe CH 2 I 2 cell to ensure that the signal derived only fromnascent iodine. Experiments were carried out at room temperature21 °C with vapor pressures of 0.25 and 1.2 Torrfor I 2 and CH 2 I 2 , respectively.Spectral profiles for the 624 and 312 nm pulses measuredat zero, negative, and positive chirps were obtained inorder to experimentally confirm that the introduced chirp didnot affect the spectrum of the pulses. The intensity variationsof the 624 and 312 nm pulses as a function of chirp were alsomeasured. The intensity of the fundamental remained essentiallyconstant; whereas, a small variation in the UV intensitywas observed. This variation can be understood in terms ofthe dependence of second-harmonic generation on the peakintensity of the incoming pulses. As absolute chirp increases,the pulse width increases resulting in a reduction of peakintensity.FIG. 1. a Experimental measurement of the yield of the molecular pathwayproducing I 2 from the multiphoton dissociation of CH 2 I 2 with 624 nmlaser pulses as a function of chirp. The insert shows the relevant energeticsfor the reaction. b Same as a except for I 2 .The concerted elimination of I 2 molecules following irradiationof CH 2 I 2 is known to occur for excitation energieslarger than 9.4 eV. 21 The yield of this molecular processhas been measured to be on the order of 1%. Most otherpathways, produce atomic iodine in its ground I and spin–orbit excited states I*. The nascent I 2 molecules are formedprimarily in the D state and have been detected followingsingle and multiphoton excitation. 22 In our laboratory, wehave explored the femtosecond dynamics of the moleculardetachment process and have found it to take place in lessthan 50 fs. 23 We have also found that other halogenated alkaneswith formulas CX 2 Y 2 for XH, D, F and YCl, Br,and I undergo similar molecular detachment processes. 24Detection of the molecular product is selectively carried outby dispersing the laser-induced fluorescence and collectingonly a spectral window that contains the majority of the D→A emission.Multiphoton excitation of CH 2 I 2 with 624 nm pulsesproduces the well-known I 2 D→A emission at 342 nm.Figure 1a presents variations in the relative yield of themolecular detachment pathway as a function of linear chirp.The data have been normalized to unity at zero chirp. Notethat the maximum yield is observed for 500 fs 2 and theminimum at 2400 fs 2 . We observe a factor of 2.9 in theoverall change in the yield for this pathway as a function ofchirp. These data are contrasted with a chirp scan obtainedunder similar conditions with the 624 nm pulses on the yieldfrom three-photon excitation of I 2 vapor to yield D→X andD→A fluorescence between 320 and 345 nm. These re-
J. Chem. Phys., Vol. 108, No. 11, 15 March 1998 Communications4377FIG. 2. Experimental measurement of the yield of the molecular pathwayproducing I 2 from the multiphoton dissociation of CH 2 I 2 with 312 nm laserpulses as a function of chirp. The inserts show the relevant energetics for thereaction as well as a plot of the maximum I 2 yield enhancement recorded at2400 fs 2 chirp as a function of laser peak intensity measured at zero chirp.sults are presented in Fig. 1b and show an opposite trend tothat obtained for CH 2 I 2 dissociation. The minimum yield forthe three-photon excitation is found at 500 fs 2 but increasesby 40% as the magnitude of chirp increases. Wilsonand co-workers have investigated the chirp effect on thethree-photon absorption yield for I 2 at 550, 570, and 600nm. 15 They found the yields are significantly affected by thechirp approximately factors of 2 and 3 for 600 and 570 nmexcitation, respectively. Our results on I 2 are in very goodagreement with their findings.We have also explored the effect of chirp on the multiphotonexcitation of CH 2 I 2 with 312 nm laser pulses. Figure2 presents the yield of the molecular pathway determined bydetection of I 2 D→A fluorescence intensity as a functionof chirp. For these experiments, both the fluorescence signaland 312 nm laser intensity were recorded and averaged for500 laser shots at each chirp stop. The fluorescence intensitywas normalized against the third power of the 312 nm intensitybecause of the three-photon excitation and the inherentvariation of 312 nm intensity as a function of chirp videsupra. It is clear that increasing the chirp enhances the photodissociationyield significantly.The molecular pathway enhancement is found to be nonsymmetric,favoring positive over negative chirps. This observationimplies that the observed enhancements are not dueto pulse width effects but rather depend on the magnitudeand sign of the linear chirp. The three scans shown in Fig. 2were obtained under identical conditions except for the intentionalchanges in the pulse intensity from 0.8 to 1.610 12 W/cm 2 calculated for zero chirp pulses. The dataare shown normalized to laser pulse energy but are not correctedfor variations in peak intensity caused by pulse broadeningas a function of chirp. The effect of laser intensity onthese control experiments is shown in the insert, where themolecular pathway yield is shown to increase for positive2400 fs 2 chirp by factors of 3–25 as the zero-chirp intensityof the laser pulses in increased from 0.8 to 2.410 12 W/cm 2 , see Fig. 2 insert.We consider the differences between the dependence ofthe molecular pathway yield on laser pulse chirp for 624 and312 nm excitation to be of great interest. For 624 nm, theyield decreases with absolute chirp, while for 312 nm it increases.We have not taken into account the fact that thetemporal pulse width of the pulses increases with linearchirp. Multiphoton transitions are, in general, expected toincrease by factors proportional to the peak intensity raisedto the nth power (I n ) where n is the number of photons.Therefore, multiphoton excitation is expected to be maximizedwhen the chirp is zero. For 624 nm excitation, themaximum yield was found for a chirp of 500 fs 2 ; however,for 312 nm the maximum yield was found for a chirp of2400 fs 2 . Based on the pulse width change and the threephotonexcitation for the 312 nm case, the transition probability,proportional to (I n3 ), is expected to decrease by afactor of 27 because the pulse width triples at this chirpvalue. Therefore, the yield of I 2 production should trackwith the transition probability instead of showing the observedenhancement.The effects caused by chirp in the excitation pulses reflectcharacteristics of the potential energy surfaces and thenascent wave packet dynamics. For diatomic I 2 , Wilson andco-workers have been able to explain their observed chirpeffects based on quantum mechanical calculations that showa ‘‘wave packet following’’ effect for positive chirp. 5 Currently,the potential energy surfaces involved for CH 2 I 2 , arenot known preventing us from giving an accurate quantummechanical description of the effect. In principle, a similarwave packet following effect could be responsible given thatthe first photon transition is resonant, as in the I 2 experimentfrom Wilson’s group. 15 The order of magnitude changesdemonstrated in this communication give additional evidencethat quantum control of the yield of chemical reactionsinvolving polyatomic molecules is a promising area of research.In summary, we have found that the chirp of femtosecondpulses can have profound effects on the photophysicaland photochemical processes. For diatomic iodine, both positiveand negative chirps enhanced I 2 fluorescence signals.These findings are in good agreement with those obtained byWilson and co-workers. 15 However, for the multiphoton photodissociationof CH 2 I 2 with 624 nm, the opposite chirpingeffects with the maximum I 2 yield near chirp zero is observed.Results for 312 nm multiphoton excitation of CH 2 I 2exhibit a minimum I 2 yield near chirp zero. The yield of themolecular pathway is found to be asymmetric with respect tothe sign of the chirp and enhancements up to a factor of 25for positive chirps are observed. The amount of enhancementis also found to be highly dependent on the pulse intensities.
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