Femtosecond quantum control ofmolecular bond formationPatrick Nuernbergera,b,2, Daniel Wolperta, Horst Weissc, and Gustav Gerbera,1

aPhysikalisches Institut, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany; bInstitut für Physikalische Chemie, Universität Würzburg,Am Hubland, 97074 Würzburg, Germany; and cPolymer Research Division, BASF SE, 67056 Ludwigshafen, Germany

Edited* by Joshua Jortner, Tel Aviv University, Tel Aviv, Israel, and approved April 27, 2010 (received for review November 24, 2009)

Ultrafast lasers are versatile tools used in many scientific areas,from welding to eye surgery. They are also used to coherentlymanipulate light–matter interactions such as chemical reactions,but so far control experiments have concentrated on cleavage orrearrangement of existing molecular bonds. Here we demonstratethe synthesis of several molecular species starting from small reac-tant molecules in laser-induced catalytic surface reactions, andeven the increase of the relative reaction efficiency by feedback-optimized laser pulses. We show that the control mechanism isnontrivial and sensitive to the relative proportion of the reactants.The control experiments open up a pathway towards photocataly-sis and are relevant for research in physics, chemistry, and biologywhere light-induced bond formation is important.

femtochemistry ∣ surface science

Ever since their invention, lasers were considered the ideal toolfor microscopic control over chemical bonds, and several

seminal coherent control approaches have been developed(1–3). A very successful method to this task is femtosecond quan-tum control, where selectivity over photoinduced reactions isachieved by exploiting the coherence properties and ultrashorttime scales of femtosecond laser radiation (4–6). Combined withlearning algorithms processing experimental feedback to adap-tively find optimized pulses best suited for solving the control task(7), chemical reactions can even be controlled without a prioriknowledge about the reaction mechanisms. This scheme has beensuccessfully applied to dissociative reactions in the gas phase,first on organometallic compounds (8) and later on many othersystems. The method is not limited to gas phase experiments, asfluorescence optimizations of molecules in the liquid phase haveshown (9–12). Recently, also more complex control tasks havebeen realized, like the energy flow in large biomolecules (13) orthe quantum yield in a photoisomerization reaction (14–16). Fem-tosecond lasers have also been introduced to the field of photoas-sociation from atoms in cold traps, in both theory (17, 18) and firstexperiments (19, 20). However, the selective laser manipulation ofbond-forming reactions starting from small reactant moleculesthat may furthermore exhibit competing bond-forming reactionchannels has not been shown yet.

In this contribution, we present the realization of femtosecondlaser-assisted catalytic reactions of carbon monoxide and hydro-gen or deuterium at a metal surface and further demonstrate thatthe relative reaction efficiency can be increased by the benefits offemtosecond laser pulses tailored especially for a desired reactionoutcome. These experiments represent a first step and a reactionpath toward laser-induced catalysis of molecular systems.

Femtosecond laser sources have been employed by laserscientists to explore processes on metal surfaces as soon as theywere available. Other types of lasers have been used earlier forthis purpose, but starting from the first demonstration of intactdesorption of NO molecules from a Pd(111) single crystalinduced by femtosecond laser pulses (21), a complete field ofultrafast laser spectroscopy on metal surfaces has emergedamong the diversity of surface chemistry techniques (22, 23).Already in early time-resolved experiments (24–26) the unique

reaction pathways accessible by the short pulses and the corre-sponding nonequilibrium excitation of the substrate’s electronicsystem became apparent and revealed that femtosecond laserdesorption is not only because of a phonon-assisted heatingeffect.

Besides desorption, nuclear wave-packet dynamics of adsorbedmolecules could be manipulated (27, 28) by femtosecond lasers,and also a few chemical reactions could be induced. The mostprominent among them is the oxidation of carbon monoxidein the presence of oxygen on various surfaces (29, 30). Thepioneering work by Ertl and coworkers (30) beautifully revealedthe underlying reaction mechanism via hot substrate electrons, amechanism that also accounts for the recombinative desorptionof hydrogen under femtosecond laser irradiation, which clearlydiffers from thermal excitation (31). However, only very fewsurface reactions could be observed or assisted by femtosecondlasers, whereas complex catalytic reactions have been completelyinaccessible up to now.

We have chosen to explore the reaction of carbon monoxideand hydrogen, coadsorbed on a Pd(100) single crystal surface.When adsorbed on Pd(100), CO does not decompose in the firstlayer and adsorbs bridge-bonded with the C atom binding to themetal. If the molecular bond is broken [e.g., by electron bombard-ment (32)], the remaining C atoms can stay on the surface asbulk carbon, which lowers the adsorption energy for furtherCO adsorption (33). H2 already adsorbs dissociatively at verylow coverages and subsequently penetrates into the bulk whereit is dissolved (34). Because the maxima for thermal desorptionof hydrogen and CO are 360 (34) and 490 K (33), respectively,experiments presented in this work have been performed at290 K, so that adsorbed species should stick well to the surfaceand their mobility be still quite high. Recently, coadsorption ofcarbon monoxide and hydrogen on Pd(111) over a wide pressurerange (35) revealed that high-pressure CO structures are identi-cal to high-coverage structures in ultrahigh vacuum, but, despitediverse gas amounts and compositions, no reaction product wasobserved (35), presumably because of the high energy barriers.A femtosecond laser pulse can provide this energy, whereas spe-cially shaped pulses can steer the outcome into a desired reactionchannel.

Experimental SetupOur experiments are conducted with the setup displayed in Fig. 1.A femtosecond laser system delivers pulses that are phase-modulated in a LCD pulse shaper and then focused onto a metal

Author contributions: P.N., D.W., H.W., and G.G. designed research; P.N. and D.W.performed research; P.N., D.W., H.W., and G.G. analyzed data; and P.N., D.W., H.W.,and G.G. wrote the paper.

The authors declare no conflict of interest.

*This Direct Submission article had a prearranged editor.1To whom correspondence should be addressed. E-mail: gerber@physik.uni-wuerzburg.de.2Present address: Laboratoire d’Optique et Biosciences, Ecole Polytechnique, 91128Palaiseau, France.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.0913607107/-/DCSupplemental.

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single crystal surface in a TOF-MS. Ions produced by the laser aredetected in the TOF-MS, which is connected to a computer with alearning algorithm. All presented experiments are performed atroom temperature and under high vacuum conditions. A moredetailed description is given in Materials and Methods.

ResultsExperiments with Unshaped Pulses. When the laser beam does nothit the surface, no ions are detected at all. As soon as H2 isstreamed onto the laser-irradiated surface, three huge peaks ap-pear in the ion spectrum, attributed to Hþ, Hþ

2 , and Hþ3 (Fig. 2,

Black Line). The triatomic hydrogen molecule is unstable in theelectronic ground state, but excited and ionized states are long-lived and play an important role in molecular spectroscopy(36–40).

A totally different ion spectrum is detected if only CO entersthe chamber. Three peaks arise, which correspond to Cþ, Oþ, andCOþ, respectively (Fig. 2, Red Line). We do not know the percen-tage of desorbed CO per laser shot, but the peak heights and theamount of CO introduced into the chamber are directly corre-lated, and turning off the CO supply results in the disappearanceof these peaks.

Additional ion peaks appear when a mixture of CO and H2 isstreamed onto the surface. Not only the peaks of the respective

single mass spectra change, but also new ion signals arise withrising hydrogen concentration. New peaks at 13, 17, and29 amu show up first, followed by peaks at 14, 15, 18, and19 amu (Fig. 2, Green and Blue Lines). These peaks clearly indi-cate the formation of the ions CHþ, CHþ

2 , CHþ3 , OHþ, H2Oþ,

H3Oþ, and HCOþ. An analysis of the latter peak also indicatesthe formation of H2COþ (Fig. S1). Varying the temperature ofthe single crystal from 190 to 390 K did not lead to any furtherion peaks. No information can be derived whether methane ions(16 amu) are formed, because they would coincide with Oþ. Theproduced hydroxyl, water, and hydronium ion peaks emerge atthe expense of the Oþ signal and are not because of residualwater. By contrast, the production of methylidine, methylene,and methyl cations does not lead to a decrease of the Cþ signal,which actually grows with increasing H2 concentration. All ionsignals fade away when the gas supply is stopped, with the hydro-gen peaks persisting longest.

Single Parameter Variations. In order to further explore theobserved photochemical reactions, selected aspects of the macro-scopic experimental conditions have been changed. Hence,additional information about the underlying mechanisms andthe role of the reactant molecules, the metal surface, and thelaser characteristics is obtained.

The assignment of the observed product ions is verified byreplacing H2 with D2 (Fig. 3). The ion peaks are separated by2 amu, so that, e.g., the heavy water ion D2Oþ appears at 20 amu.The relative efficiency of the bond-forming reactions is lowercompared to H2 as can, e.g., be seen by the weak D3Oþ signaland the absence of D2COþ even for the strongest D2 excessesemployed (Fig. 3, Blue Line). This behavior can be interpretedas an indication for the reduced mobility of the heavier isotopeon the surface, and thus reactions in which more than two par-ticles have to be involved occur with much lower probability.

A central question is the role of the surface in the reactionsleading to the observed ion spectra. There is multiple evidencefor a decisive involvement of the surface, supported by additionalexperiments done with our setup and by comparison of our resultsto the literature: First, we observe the triatomic hydrogen mole-cule, whose neutral electronic ground state is unstable, in contrastto excited and ionized states. A reported mechanism for Hþ

3 for-mation is the reaction between Hþ

2 and H2 adsorbed on a metalsurface (37), providing a first indication of a surface reaction.Second, the observed Cþ increase (which may be because of bulkcarbon) under hydrogen supply is comparable to observations byDenzler et al. (31), who disclosed that laser desorption of deu-terium from a ruthenium surface is enhanced in the presenceof the lighter adsorbate hydrogen. In our case, no deuteriumis present, but the carbon monoxide should adsorb with the car-bon binding to the surface. The behavior of the Cþ signal resem-bles the situation of D2 desorption in Denzler’s experiment, with

Fig. 1. Experimental setup. The laser pulses are modulated in a LCD pulseshaper and then focused onto a Pd(100) single crystal surface located inthe interaction region of a TOF-MS. The gases are dosed with mass flow con-trollers and effusively stream via a nozzle and a skimmer onto the surface.The data are directly processed by a computer containing an evolutionaryalgorithm.

Mass-to-charge ratio [amu/e]

3.5 4.0 4.5 5.0 5.5 6.0 6.5

Time of flight [µs]

CO 4 10 4 4 4 0 0 4

H2

10 15 20 25 3031

1.0 1.5 2.0

Hydrogen peaksdemagnified 10x

CO+

CH+

OH+O+ H2O+C+

HCO+

CH2+

CH3+ H3O+ H2CO+

H3+

H2+

H+

Fig. 2. Ion spectra (vertically offset for clarity) with different amounts of COand H2 streamed onto the Pd(100) single crystal. Spectra with H2 only (Black),CO only (Red), an equal mixture of CO and H2 (Green), and an excess of H2

(Blue) are shown. The signal at early times is just shown for the H2-onlymeasurement and is demagnified by a factor of 10 to give an impressionof the hydrogen ion peaks. Gas amounts streaming through the nozzleare given in sccm (standard cubic centimeters per minute).

3.5 4.0 4.5 5.0 5.5 6.0 6.5

Time of flight [µs]

CO 4 10 4 4 4 1

D2

inte

nsity

OD+ D2O+ DCO+CD

+

D3O+

O+C+

CO+

10 15 20 25 30

Mass-to-charge ratio [amu/e]

CD+

CD+32

Fig. 3. Ion spectra (vertically offset for clarity) with different amounts ofCO and D2 streamed onto the Pd(100) single crystal. The CO amount is heldconstant, whereas the D2 amount is varied.

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the difference that the loosening of the carbon instead of the deu-terium adsorption is facilitated by additional hydrogen. Whereasthis may indicate a contribution of hydrogen dissociatively ab-sorbed beneath the surface (as do experiments with a platinumcrystal; see Fig. S2), the rise of product ions with increasinghydrogen amounts suggests the involvement of hydrogen fromthe surface. Third, when we replace carbon monoxide by carbondioxide, no CO2-related ions are observed for all providedamounts of CO2 and all applied laser intensities, indicating thatCO2 does not adsorb at all on the Pd surface at room temperatureor at least not in such a way that it could be detected analogouslyto CO. Hence, this reflects the importance of the surface and theadsorption process in the chemical reactions observed with CO.Fourth, when streaming CO and H2 on a Pt(100) single crystalsurface, methylidine, hydroxyl, and water ions are formed, butvery inefficiently. An excess of H2 leads to a shrinking of allCO-related peaks, in contrast to experiments with the Pd surface,disclosing that the surface material is essential for the initiatedcatalytic reactions (Fig. S2). Fifth, changing the linear polariza-tion of the laser, performed because signals in laser surface spec-troscopy are often polarization-sensitive (22), leads to a strongsignal variation over two orders of magnitude, evidencing a direc-tional dependence of the interaction of the light and the adsor-bate that is predetermined by the surface (Fig. S3). However,tools for in-detail analysis of the adsorbate’s structure werenot available in our experiments; therefore, the CO and hydrogencoverage rates and densities in the vicinity of the surface and alsothe relative distances of reaction partners on the surface couldnot be determined. These parameters may significantly contri-bute to identify the underlying reaction mechanism and shouldbe clarified in future studies.

To further investigate the observed reactions, pump-probeexperiments with two identical femtosecond laser pulses areperformed (Fig. S4). The ratio of the peaks does not changedrastically for different pump-probe delays, and the transiention signals resemble the cross-correlations of the two pulses—i.e., exhibit sub-200-fs dynamics. The reaction mechanism henceis not a phonon-mediated heating process (lasting typically tensof picoseconds), but possibly involves electronic excitations of theadsorbate, analogous to the laser-induced oxidation of CO that isbecause of laser-generated hot substrate electrons, giving rise tosubpicosecond reaction dynamics along a new reaction pathwayby coupling to the adsorbate (30). However, our experimentstotally failed with 400-nm laser pulses at comparable fluenceand with all possible linear laser polarizations and exhibited anabsence of any CO-related ions, an indication that the dominantmechanism may not be hot substrate electrons but rather a reso-nance-enhanced excitation and fragmentation of adsorbed COmolecules.

Another potential reaction mechanism derives from experi-ments where a beam of atomic hydrogen is impinged on a surfacepreadsorbed with alkenes or halogen atoms, leading to bondformation between the hydrogen and the adsorbate, whereasno reaction occurs with preadsorbed hydrogen atoms (41–44).Although we employ molecular rather than atomic hydrogen,the femtosecond laser initiates nonequilibrium conditions andreactive species like Hþ

3 , and possibly also energetic hydrogenatoms, are formed. Following this line, the reaction mechanismmight also comprise sequential processes or ion reactions in closevicinity to the surface.

Quantum Control Experiments. Adaptively optimized femtosecondlaser pulses have the potential to selectively manipulate the inter-actions and/or nonequlibrium conditions necessary for theobserved bond formations. To explore whether an evolutionaryalgorithm finds a pulse shape that increases the production ofions exhibiting a C-H bond, a 1∶1 mixture of CO and H2 isstreamed onto the Pd(100) surface. The goal of the experiment

is to maximize the amount of CHþ ions versus Cþ ions. Enhance-ment of about 100% compared with the unmodulated laser pulseis achieved with the optimal pulse (Fig. 4). In addition, theCHþ

2 ∕Cþ ratio is increased as well by about the same amount.A surprising feature is the strong reduction of H2Oþ

—i.e., a spe-cies having an O-H bond, with the optimal pulse, which comprisesa pulse sequence with a broad main pulse and a smaller subpulse(Fig. 4C). We have varied the pulse energy and recorded ion spec-tra with the unmodulated pulse, but neither such a reduction ofthe H2Oþ signal relative to the other peaks nor the increase inthe CHþ∕Cþ ratio could be achieved. From the experimentaldata we conclude that the optimal pulse favors the formationof ions having a C-H bond, whereas H2Oþ production is sup-pressed. The yield of COþ, HCOþ, and H2COþ relative to Cþis also greatly enhanced (Fig. 4A).

Because water formation has been reduced even though thishas not been the goal of the previous optimization, we includedthe H2Oþ yield as a parameter in the fitness function. Anotheroptimization experiment under virtually the same conditions isperformed with the chosen control goal being the maximizationof the ratio CHþ∕H2Oþ. Although performed on another dayand with a slightly different starting ratio compared to theCHþ∕Cþ optimization, the control goal has been achieved justas nicely. Fig. 5 shows that the H2Oþ peak is reduced by about50% relative to Cþ. Whereas the absolute yield for H2Oþ initiallyis larger than for CHþ, this ratio is reversed with the optimalpulse. The relative production of COþ and HCOþ is increased,whereas CHþ and CHþ

2 practically do not change relative to Cþ.Hence, with the ratio CHþ∕H2Oþ as fitness signal used here, adirect discrimination of a ion species bearing an O-H bondrelative to one bearing a C-H bond has been achieved, withthe algorithm converging faster than for the CHþ∕Cþ optimiza-tion (see Fig. 5B). A replacement of H2 with D2 in the experi-ments has led to successful optimizations with similar results.

For both optimizations, the second-harmonic frequency-resolved optical gating (SHG-FROG) traces (Figs. 4C and 5C)show a pulse sequence with a broad main pulse and a smallersubpulse, separated by ≈2 ps. In the pump-probe experiments(Fig. S4), using two identical unmodulated pulses at temporaldistances longer than one pulse width has not led to any signalat all. The energy of the incoming laser field was comparablein all the experiments; however, whereas the energy in thepump-probe experiments has been distributed evenly over thetwo pulses, the relative position and intensity of the substructuresof the optimized pulses might make the difference.

A B C

Fig. 4. (A) Ion spectra for the maximization of the ratio CHþ∕Cþ with 4 sccmCO and 4 sccm H2, obtained with unmodulated (Red) and optimal pulse(Black). For comparison, the blue curve shows the latter ion spectrum rescaledby a factor such that the Cþ peaks for unmodulated and optimal pulse match.(B) The green curve shows the development of the fitness (the average ofthe ten best individuals per generation), whereas the signal level of theunmodulated pulse is indicated by an orange line. (C) SHG-FROG trace(Top) and autocorrelation (Bottom) of the optimal pulse.

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Gas Amount Variation for Optimized Pulses. The relative proportionof the two gases can be easily changed, allowing an analysis of theoptimization effect with respect to the adsorbate composition. Asystematic variation substantiates that the optimization is notachieved via control of carbon monoxide dissociation only: Asan initial experiment, just CO is employed and the ratioCOþ∕Cþ is maximized. Afterwards, hydrogen is streamed ontothe surface. If the amount of H2 is small compared to CO, the op-timization effect of an increased ratio COþ∕Cþ is also transferredto the hydrogenated species (Fig. 6). Yet, a further rise of the H2

concentration leads to modified conditions on the surface, as, e.g.,adsorbate arrangement and electronic states change. Hence, theoptimization effect goes away when the amount ofH2 is increased,and the ratio of hydrogenated ions with the optimal pulse ap-proaches the ratio with an unmodulated pulse (Fig. 6). One canconclude that the best pulse in the absence ofH2 is not special any-more if large H2 amounts are applied, proving that the pulse isadapted to the specific conditions during the optimization. Thisconclusion has also been confirmed by the opposite procedure:Optimal pulses obtained with both gases have no effect on theshape of the mass spectrum anymore if only CO is employed.

ConclusionThe presented experiments demonstrate the feasibility of laser-induced catalytic reactions of carbon monoxide and hydrogen

on a palladium single crystal. Several product molecules havebeen synthesized, among them also species (e.g., CHþ

3 ) in whoseformation at least three of the initial particles are involved. Ourresults show that the interaction of the surface, its adsorbate, andthe femtosecond laser field occurs on an ultrafast subpicosecondtime scale, where the dominant mechanism might not be hotsubstrate electrons but possibly a resonance-enhanced process,and is sensitive to the incident laser polarization.

By applying a feedback femtosecond optimal control scheme,these reactions are manipulated and the ratio of different reac-tion channels is selectively changed. It was shown that the under-lying control mechanism is nontrivial and sensitive to the specificconditions on the surface. In contrast to previous quantum con-trol experiments, reaction channels comprising the formation ofmolecular bonds rather than the cleavage of already existingbonds are controlled, a major advance opening the door towardselective laser-induced molecular catalysis.

Materials and MethodsThe setup used in our experiment is outlined in Fig. 1. A titanium-sapphirefemtosecond regenerative amplifier delivers laser pulses with a pulse dura-tion of 80 fs and pulse energies of up to 1 mJ at a center wavelength of800 nm and at a repetition rate of 1 kHz. A LCD pulse shaper with 128 in-dependent pixels in the Fourier plane of a zero dispersion compressor is em-ployed to modify the spectral phase of the laser pulses while leaving thespectrum unchanged (for more detail, see, e.g., ref. 45). A genetic algorithmin combination with a feedback loop is established to optimize the electricfield for the desired task (fitness) by direct processing of the experimentaldata received for a certain pulse shape. Each generation comprises 60 testedpulse shapes; the 10 best ones are used to create the pulses of the nextgeneration by cloning, crossover, and mutation. The pulse shapes can becharacterized by SHG-FROG (46), where a spectrally resolved autocorrelationis used to assess the complete electric field of the pulse. The laser beam isfocused by a lens with a focal length of 40 cm into the main vacuum cham-ber onto the single crystal under an angle of roughly 15° to the surface.Only a few percent of the laser energy is employed , and the maximum in-tensity on the surface is about 1012 W∕cm2. Laser intensities are chosen suchthat the formation of metal ions is negligible. The beam is reflected by thecrystal and leaves the vacuum chamber again through another window. Thegases H2, D2, and CO have been purchased with purities of at least 99.999%,99.7%, and 99.997%, respectively, and are used as-is. Mass flow controllersespecially calibrated for the gases are employed to dose the amount of gasthat is allowed to enter the system. The two gas pipes are combined in frontof a nozzle through which the gas mixture enters the collateral vacuumchamber, where it hits a skimmer and results in a gas beam into the mainvacuum chamber. The skimmer is the only connection between the two va-cuum chambers, and the three elements nozzle, skimmer, and crystal arelocated along one line. The single crystal [Pd(100) or Pt(100), diameter10 mm, thickness 1 mm] is mounted with a slight angle of about 5° tiltedtoward the molecular beam so that the gas not only streaks over the surfacebut may actually hit it. Perpendicular to the gas beam and almost parallel tothe surface normal, there is a TOF mass spectrometer with a system of elec-trodes to accelerate ions that are produced when the laser beam interactswith the surface and its adsorbates. These ions generate an electric signal ona chevron-stacked microchannel plate assembly after flight times that aredirectly related to their mass. These signals are recorded either directlyvia a digital oscilloscope or via a time-to-digital converter after a fast pre-amplifier. The crystal, mounted on the repeller of the TOF, is set to a voltageof þ200 V. Several subsequent grounded electrodes guarantee a field-freedrift region, whereas at the detector itself the back and front plates of themicrochannel plate assembly are set to −100 and þ1;800 V, respectively,followed by a conical anode at 0 V. The whole repeller assembly includingthe single crystal can be heated, as well as cooled by a cryogenic system. Thebase pressure in the main chamber, without exposure to the two gases, is10−6 torr, whereas up to 10−4 torr are reached with the highest gasamounts employed in the experiments.

ACKNOWLEDGMENTS. H.W. appreciates constant support and encouragementof Prof. Brandstetter and Prof. Iden (BASF). We gratefully acknowledge thegroup of Prof. Gauss (Univ Mainz), Dr. Wohlleben (BASF), and Prof. Brixner(Univ Würzburg) for stimulating discussions, Dr. Niklaus and Dr. Papastatho-poulos for help in the early phase of the project, and the BASF SE for financialsupport. P.N. acknowledges financial support from the Deutsche Akademieder Naturforscher Leopoldina (BMBF-LPDS 2009-6).

A B C

Fig. 5. (A) Ion spectra for the maximization of the ratio CHþ∕H2Oþ with4 sccm CO and 4 sccm H2, obtained with unmodulated (Red) and optimalpulse (Black). For comparison, the blue curve shows the latter ion spectrumrescaled by a factor such that the Cþ peaks for unmodulated and optimalpulse match. (B) The green curve shows the development of the fitness(the average of the ten best individuals per generation), whereas the signallevel of the unmodulated pulse is indicated by an orange line. (C) SHG-FROGtrace (Top) and autocorrelation (Bottom) of the optimal pulse.

0 1 2 3 4 5 6

H2 amount [sccm]

optimized pulse

unmodulated pulse

HxCO+

CHx+x>0Σ

x>0Σ

Rat

io

Fig. 6. Variation of H2 concentration after an optimization. After maximi-zation of the ratio COþ∕Cþ with 4 sccm CO only, H2 is streamed onto thesurface. The graph shows the ratio of all species with hydrogenated CO(29–30 amu) to those with hydrogenated C (13–15 amu). The optimizationeffect fades away with increasing H2 amounts.

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