Femtosecond Quantum Control of Molecular Bond Formation

Femtosecond quantum control of molecular bond formation

Patrick 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, reaction pathways accessible by the short pulses and the corre- from welding to eye surgery. They are also used to coherently sponding nonequilibrium excitation of the substrate’s electronic manipulate light–matter interactions such as chemical reactions, system became apparent and revealed that femtosecond laser but so far control experiments have concentrated on cleavage or desorption is not only because of a phonon-assisted heating rearrangement of existing molecular bonds. Here we demonstrate effect. the synthesis of several molecular species starting from small reac- Besides desorption, nuclear wave-packet dynamics of adsorbed tant molecules in laser-induced catalytic surface reactions, and molecules could be manipulated (27, 28) by femtosecond lasers, even the increase of the relative reaction efficiency by feedback- and also a few chemical reactions could be induced. The most optimized laser pulses. We show that the control mechanism is prominent among them is the oxidation of carbon monoxide nontrivial and sensitive to the relative proportion of the reactants. in the presence of oxygen on various surfaces (29, 30). The The control experiments open up a pathway towards photocataly- pioneering work by Ertl and coworkers (30) beautifully revealed sis and are relevant for research in physics, chemistry, and biology the underlying reaction mechanism via hot substrate electrons, a where light-induced bond formation is important. mechanism that also accounts for the recombinative desorption of hydrogen under femtosecond laser irradiation, which clearly femtochemistry ∣ surface science differs from thermal excitation (31). However, only very few surface reactions could be observed or assisted by femtosecond ver since their invention, lasers were considered the ideal tool lasers, whereas complex catalytic reactions have been completely Efor microscopic control over chemical bonds, and several inaccessible up to now. seminal coherent control approaches have been developed We have chosen to explore the reaction of carbon monoxide (1–3). A very successful method to this task is femtosecond quan- and hydrogen, coadsorbed on a Pd(100) single crystal surface. tum control, where selectivity over photoinduced reactions is When adsorbed on Pd(100), CO does not decompose in the first achieved by exploiting the coherence properties and ultrashort layer and adsorbs bridge-bonded with the C atom binding to the time scales of femtosecond laser radiation (4–6). Combined with metal. If the molecular bond is broken [e.g., by electron bombard- learning algorithms processing experimental feedback to adap- ment (32)], the remaining C atoms can stay on the surface as tively find optimized pulses best suited for solving the control task bulk carbon, which lowers the adsorption energy for further (7), chemical reactions can even be controlled without a priori CO adsorption (33). H2 already adsorbs dissociatively at very knowledge about the reaction mechanisms. This scheme has been low coverages and subsequently penetrates into the bulk where successfully applied to dissociative reactions in the gas phase, it is dissolved (34). Because the maxima for thermal desorption first on organometallic compounds (8) and later on many other of hydrogen and CO are 360 (34) and 490 K (33), respectively, systems. The method is not limited to gas phase experiments, as experiments presented in this work have been performed at fluorescence optimizations of molecules in the liquid phase have 290 K, so that adsorbed species should stick well to the surface shown (9–12). Recently, also more complex control tasks have and their mobility be still quite high. Recently, coadsorption of been realized, like the energy flow in large biomolecules (13) or carbon monoxide and hydrogen on Pd(111) over a wide pressure the quantum yield in a photoisomerization reaction (14–16). Fem- range (35) revealed that high-pressure CO structures are identi- tosecond lasers have also been introduced to the field of photoas- cal to high-coverage structures in ultrahigh vacuum, but, despite sociation from atoms in cold traps, in both theory (17, 18) and first diverse gas amounts and compositions, no reaction product was experiments (19, 20). However, the selective laser manipulation of observed (35), presumably because of the high energy barriers. bond-forming reactions starting from small reactant molecules A femtosecond laser pulse can provide this energy, whereas spe- that may furthermore exhibit competing bond-forming reaction cially shaped pulses can steer the outcome into a desired reaction channels has not been shown yet. channel. In this contribution, we present the realization of femtosecond laser-assisted catalytic reactions of carbon monoxide and hydro- Experimental Setup gen or deuterium at a metal surface and further demonstrate that Our experiments are conducted with the setup displayed in Fig. 1. the relative reaction efficiency can be increased by the benefits of A femtosecond laser system delivers pulses that are phase- femtosecond laser pulses tailored especially for a desired reaction modulated in a LCD pulse shaper and then focused onto a metal outcome. These experiments represent a first step and a reaction path toward laser-induced catalysis of molecular systems. Author contributions: P.N., D.W., H.W., and G.G. designed research; P.N. and D.W. Femtosecond laser sources have been employed by laser performed research; P.N., D.W., H.W., and G.G. analyzed data; and P.N., D.W., H.W., scientists to explore processes on metal surfaces as soon as they and G.G. wrote the paper. were available. Other types of lasers have been used earlier for The authors declare no conflict of interest. this purpose, but starting from the first demonstration of intact *This Direct Submission article had a prearranged editor. desorption of NO molecules from a Pd(111) single crystal 1To whom correspondence should be addressed. E-mail: [email protected]. induced by femtosecond laser pulses (21), a complete field of 2Present address: Laboratoire d’Optique et Biosciences, Ecole Polytechnique, 91128 ultrafast laser spectroscopy on metal surfaces has emerged Palaiseau, France. among the diversity of surface chemistry techniques (22, 23). This article contains supporting information online at www.pnas.org/lookup/suppl/ Already in early time-resolved experiments (24–26) the unique doi:10.1073/pnas.0913607107/-/DCSupplemental.

10366–10370 ∣ PNAS ∣ June 8, 2010 ∣ vol. 107 ∣ no. 23 www.pnas.org/cgi/doi/10.1073/pnas.0913607107 Downloaded by guest on September 28, 2021 single mass spectra change, but also new ion signals arise with rising hydrogen concentration. New peaks at 13, 17, and 29 amu show up first, followed by peaks at 14, 15, 18, and 19 amu (Fig. 2, Green and Blue Lines). These peaks clearly indi- þ þ þ þ þ cate the formation of the ions CH ,CH2 ,CH3 ,OH ,H2O , þ þ H3O , and HCO . An analysis of the latter peak also indicates þ the formation of H2CO (Fig. S1). Varying the temperature of the single crystal from 190 to 390 K did not lead to any further ion peaks. No information can be derived whether methane ions (16 amu) are formed, because they would coincide with Oþ. The produced hydroxyl, water, and hydronium ion peaks emerge at the expense of the Oþ signal and are not because of residual water. 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 ion Fig. 1. Experimental setup. The laser pulses are modulated in a LCD pulse shaper and then focused onto a Pd(100) single crystal surface located in signals fade away when the gas supply is stopped, with the hydro- the interaction region of a TOF-MS. The gases are dosed with mass flow con- gen peaks persisting longest. trollers and effusively stream via a nozzle and a skimmer onto the surface. The data are directly processed by a computer containing an evolutionary Single Parameter Variations. In order to further explore the algorithm. observed photochemical reactions, selected aspects of the macro- scopic experimental conditions have been changed. Hence, single crystal surface in a TOF-MS. Ions produced by the laser are additional information about the underlying mechanisms and detected in the TOF-MS, which is connected to a computer with a the role of the reactant molecules, the metal surface, and the learning algorithm. All presented experiments are performed at laser characteristics is obtained. room temperature and under high vacuum conditions. A more The assignment of the observed product ions is verified by detailed description is given in Materials and Methods. replacing H2 with D2 (Fig. 3). The ion peaks are separated by þ 2 amu, so that, e.g., the heavy water ion D2O appears at 20 amu. Results The relative efficiency of the bond-forming reactions is lower þ Experiments with Unshaped Pulses. When the laser beam does not compared to H2 as can, e.g., be seen by the weak D3O signal þ hit the surface, no ions are detected at all. As soon as H2 is and the absence of D2CO even for the strongest D2 excesses streamed onto the laser-irradiated surface, three huge peaks ap- employed (Fig. 3, Blue Line). This behavior can be interpreted þ þ þ pear in the ion spectrum, attributed to H ,H2 , and H3 (Fig. 2, as an indication for the reduced mobility of the heavier isotope Black Line). The triatomic hydrogen molecule is unstable in the on the surface, and thus reactions in which more than two par- electronic ground state, but excited and ionized states are long- ticles have to be involved occur with much lower probability.

lived and play an important role in molecular spectroscopy A central question is the role of the surface in the reactions CHEMISTRY (36–40). leading to the observed ion spectra. There is multiple evidence A totally different ion spectrum is detected if only CO enters for a decisive involvement of the surface, supported by additional the chamber. Three peaks arise, which correspond to Cþ,Oþ, and experiments done with our setup and by comparison of our results COþ, respectively (Fig. 2, Red Line). We do not know the percen- to the literature: First, we observe the triatomic hydrogen mole- tage of desorbed CO per laser shot, but the peak heights and the cule, whose neutral electronic ground state is unstable, in contrast þ amount of CO introduced into the chamber are directly corre- to excited and ionized states. A reported mechanism for H3 for- þ lated, and turning off the CO supply results in the disappearance mation is the reaction between H2 and H2 adsorbed on a metal of these peaks. surface (37), providing a first indication of a surface reaction. þ Additional ion peaks appear when a mixture of CO and H2 is Second, the observed C increase (which may be because of bulk streamed onto the surface. Not only the peaks of the respective carbon) under hydrogen supply is comparable to observations by Denzler et al. (31), who disclosed that laser desorption of deu- Mass-to-charge ratio [amu/e] terium from a ruthenium surface is enhanced in the presence 1 3 10 15 20 25 30 of the lighter adsorbate hydrogen. In our case, no deuterium + + CO H C CH + + + is present, but the carbon monoxide should adsorb with the car- 2 + O H O HCO þ 4 10 2 + CH2 + + + bon binding to the surface. The behavior of the C signal resem- H 4 4 OH + CO H CO + H O 2 4 0 CH3 3 bles the situation of D2 desorption in Denzler’s experiment, with 0 4

Hydrogen peaks Mass-to-charge ratio [amu/e] demagnified 10x + 10 15 20 25 30 H2 CO D2 + + + + + + + C CD O OD D2O DCO H3 410 + + + 4 4 CD2 CD3 + CO 4 1 D3O

1.0 1.5 2.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 intensity Time of flight [µs]

Fig. 2. Ion spectra (vertically offset for clarity) with different amounts of CO and H2 streamed onto the Pd(100) single crystal. Spectra with H2 only (Black), 3.5 4.0 4.5 5.0 5.5 6.0 6.5 CO only (Red), an equal mixture of CO and H2 (Green), and an excess of H2 Time of flight [µs] (Blue) are shown. The signal at early times is just shown for the H2-only measurement and is demagnified by a factor of 10 to give an impression Fig. 3. Ion spectra (vertically offset for clarity) with different amounts of of the hydrogen ion peaks. Gas amounts streaming through the nozzle CO and D2 streamed onto the Pd(100) single crystal. The CO amount is held are given in sccm (standard cubic centimeters per minute). constant, whereas the D2 amount is varied.

Nuernberger et al. PNAS ∣ June 8, 2010 ∣ vol. 107 ∣ no. 23 ∣ 10367 Downloaded by guest on September 28, 2021 the difference that the loosening of the carbon instead of the deu- is to maximize the amount of CHþ ions versus Cþ ions. Enhance- terium adsorption is facilitated by additional hydrogen. Whereas ment of about 100% compared with the unmodulated laser pulse this may indicate a contribution of hydrogen dissociatively ab- is achieved with the optimal pulse (Fig. 4). In addition, the þ þ sorbed beneath the surface (as do experiments with a platinum CH2 ∕C ratio is increased as well by about the same amount. þ crystal; see Fig. S2), the rise of product ions with increasing A surprising feature is the strong reduction of H2O —i.e., a spe- hydrogen amounts suggests the involvement of hydrogen from cies having an O-H bond, with the optimal pulse, which comprises the surface. Third, when we replace carbon monoxide by carbon a pulse sequence with a broad main pulse and a smaller subpulse dioxide, no CO2-related ions are observed for all provided (Fig. 4C). We have varied the pulse energy and recorded ion spec- amounts of CO2 and all applied laser intensities, indicating that tra with the unmodulated pulse, but neither such a reduction of þ CO2 does not adsorb at all on the Pd surface at room temperature the H2O signal relative to the other peaks nor the increase in or at least not in such a way that it could be detected analogously the CHþ∕Cþ ratio could be achieved. From the experimental to CO. Hence, this reflects the importance of the surface and the data we conclude that the optimal pulse favors the formation þ adsorption process in the chemical reactions observed with CO. of ions having a C-H bond, whereas H2O production is sup- þ þ þ þ Fourth, when streaming CO and H2 on a Pt(100) single crystal pressed. The yield of CO , HCO , and H2CO relative to C surface, methylidine, hydroxyl, and water ions are formed, but is also greatly enhanced (Fig. 4A). very inefficiently. An excess of H2 leads to a shrinking of all Because water formation has been reduced even though this CO-related peaks, in contrast to experiments with the Pd surface, has not been the goal of the previous optimization, we included þ disclosing that the surface material is essential for the initiated the H2O yield as a parameter in the fitness function. Another catalytic reactions (Fig. S2). Fifth, changing the linear polariza- optimization experiment under virtually the same conditions is tion of the laser, performed because signals in laser surface spec- performed with the chosen control goal being the maximization þ þ troscopy are often polarization-sensitive (22), leads to a strong of the ratio CH ∕H2O . Although performed on another day signal variation over two orders of magnitude, evidencing a direc- and with a slightly different starting ratio compared to the tional dependence of the interaction of the light and the adsor- CHþ∕Cþ optimization, the control goal has been achieved just þ bate that is predetermined by the surface (Fig. S3). However, as nicely. Fig. 5 shows that the H2O peak is reduced by about ’ þ þ tools for in-detail analysis of the adsorbate s structure were 50% relative to C . Whereas the absolute yield for H2O initially not available in our experiments; therefore, the CO and hydrogen is larger than for CHþ, this ratio is reversed with the optimal coverage rates and densities in the vicinity of the surface and also pulse. The relative production of COþ and HCOþ is increased, þ þ þ the relative distances of reaction partners on the surface could whereas CH and CH2 practically do not change relative to C . þ þ not be determined. These parameters may significantly contri- Hence, with the ratio CH ∕H2O as fitness signal used here, a bute to identify the underlying reaction mechanism and should direct discrimination of a ion species bearing an O-H bond be clarified in future studies. relative to one bearing a C-H bond has been achieved, with To further investigate the observed reactions, pump-probe the algorithm converging faster than for the CHþ∕Cþ optimiza- experiments with two identical femtosecond laser pulses are tion (see Fig. 5B). A replacement of H2 with D2 in the experi- performed (Fig. S4). The ratio of the peaks does not change ments has led to successful optimizations with similar results. drastically for different pump-probe delays, and the transient For both optimizations, the second-harmonic frequency- — ion signals resemble the cross-correlations of the two pulses resolved optical gating (SHG-FROG) traces (Figs. 4C and 5C) i.e., exhibit sub-200-fs dynamics. The reaction mechanism hence show a pulse sequence with a broad main pulse and a smaller is not a phonon-mediated heating process (lasting typically tens subpulse, separated by ≈2 ps. In the pump-probe experiments of picoseconds), but possibly involves electronic excitations of the (Fig. S4), using two identical unmodulated pulses at temporal adsorbate, analogous to the laser-induced oxidation of CO that is distances longer than one pulse width has not led to any signal because of laser-generated hot substrate electrons, giving rise to at all. The energy of the incoming laser field was comparable subpicosecond reaction dynamics along a new reaction pathway in all the experiments; however, whereas the energy in the by coupling to the adsorbate (30). However, our experiments pump-probe experiments has been distributed evenly over the totally failed with 400-nm laser pulses at comparable fluence two pulses, the relative position and intensity of the substructures and with all possible linear laser polarizations and exhibited an of the optimized pulses might make the difference. absence of any CO-related ions, an indication that the dominant mechanism may not be hot substrate electrons but rather a resonance-enhanced excitation and fragmentation of adsorbed CO AB C molecules. Another potential reaction mechanism derives from experiments where a beam of atomic hydrogen is impinged on a surface preadsorbed with alkenes or halogen atoms, leading to bond formation between the hydrogen and the adsorbate, whereas no reaction occurs with preadsorbed hydrogen atoms (41–44). Although we employ molecular rather than atomic hydrogen, the femtosecond laser initiates nonequilibrium conditions and þ reactive species like H3 , and possibly also energetic hydrogen atoms, are formed. Following this line, the reaction mechanism might also comprise sequential processes or ion reactions in close vicinity to the surface.

þ þ Quantum Control Experiments. Adaptively optimized femtosecond Fig. 4. (A) Ion spectra for the maximization of the ratio CH ∕C with 4 sccm laser pulses have the potential to selectively manipulate the inter- CO and 4 sccm H2, obtained with unmodulated (Red) and optimal pulse (Black). For comparison, the blue curve shows the latter ion spectrum rescaled actions and/or nonequlibrium conditions necessary for the þ by a factor such that the C peaks for unmodulated and optimal pulse match. observed bond formations. To explore whether an evolutionary (B) The green curve shows the development of the fitness (the average of algorithm finds a pulse shape that increases the production of the ten best individuals per generation), whereas the signal level of the ions exhibiting a C-H bond, a 1∶1 mixture of CO and H2 is unmodulated pulse is indicated by an orange line. (C) SHG-FROG trace streamed onto the Pd(100) surface. The goal of the experiment (Top) and autocorrelation (Bottom) of the optimal pulse.

10368 ∣ www.pnas.org/cgi/doi/10.1073/pnas.0913607107 Nuernberger et al. Downloaded by guest on September 28, 2021 on a palladium single crystal. Several product molecules have AB C þ been synthesized, among them also species (e.g., CH3 ) in whose formation at least three of the initial particles are involved. Our results show that the interaction of the surface, its adsorbate, and the femtosecond laser field occurs on an ultrafast subpicosecond time scale, where the dominant mechanism might not be hot substrate 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 reaction channels is selectively changed. It was shown that the underlying control mechanism is nontrivial and sensitive to the specific conditions on the surface. In contrast to previous quantum control experiments, reaction channels comprising the formation of

þ þ molecular bonds rather than the cleavage of already existing Fig. 5. (A) Ion spectra for the maximization of the ratio CH ∕H2O with bonds are controlled, a major advance opening the door toward 4 sccm CO and 4 sccm H2, obtained with unmodulated (Red) and optimal pulse (Black). For comparison, the blue curve shows the latter ion spectrum selective laser-induced molecular catalysis. rescaled by a factor such that the Cþ peaks for unmodulated and optimal pulse match. (B) The green curve shows the development of the fitness Materials and Methods (the average of the ten best individuals per generation), whereas the signal The setup used in our experiment is outlined in Fig. 1. A titanium-sapphire level of the unmodulated pulse is indicated by an orange line. (C) SHG-FROG femtosecond regenerative amplifier delivers laser pulses with a pulse dura- trace (Top) and autocorrelation (Bottom) of the optimal pulse. tion of 80 fs and pulse energies of up to 1 mJ at a center wavelength of 800 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- Gas Amount Variation for Optimized Pulses. The relative proportion ployed to modify the spectral phase of the laser pulses while leaving the of the two gases can be easily changed, allowing an analysis of the spectrum unchanged (for more detail, see, e.g., ref. 45). A genetic algorithm optimization effect with respect to the adsorbate composition. A in combination with a feedback loop is established to optimize the electric systematic variation substantiates that the optimization is not field for the desired task (fitness) by direct processing of the experimental achieved via control of carbon monoxide dissociation only: As data received for a certain pulse shape. Each generation comprises 60 tested an initial experiment, just CO is employed and the ratio pulse shapes; the 10 best ones are used to create the pulses of the next COþ∕Cþ is maximized. Afterwards, hydrogen is streamed onto generation by cloning, crossover, and mutation. The pulse shapes can be characterized by SHG-FROG (46), where a spectrally resolved autocorrelation the surface. If the amount of H2 is small compared to CO, the op- þ∕ þ is used to assess the complete electric field of the pulse. The laser beam is timization effect of an increased ratio CO C is also transferred focused by a lens with a focal length of 40 cm into the main vacuum cham- to the hydrogenated species (Fig. 6). Yet, a further rise of the H2 ber onto the single crystal under an angle of roughly 15° to the surface. concentration leads to modified conditions on the surface, as, e.g., Only a few percent of the laser energy is employed , and the maximum in- adsorbate arrangement and electronic states change. Hence, the tensity on the surface is about 1012 W∕cm2. Laser intensities are chosen such CHEMISTRY optimization effect goes away when the amount of H2 is increased, that the formation of metal ions is negligible. The beam is reflected by the and the ratio of hydrogenated ions with the optimal pulse ap- crystal and leaves the vacuum chamber again through another window. The proaches the ratio with an unmodulated pulse (Fig. 6). One can gases 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 controllers conclude that the best pulse in the absence of H2 is not special any- especially calibrated for the gases are employed to dose the amount of gas more if large H2 amounts are applied, proving that the pulse is that is allowed to enter the system. The two gas pipes are combined in front adapted to the specific conditions during the optimization. This of a nozzle through which the gas mixture enters the collateral vacuum conclusion has also been confirmed by the opposite procedure: chamber, where it hits a skimmer and results in a gas beam into the main Optimal pulses obtained with both gases have no effect on the vacuum chamber. The skimmer is the only connection between the two va- shape of the mass spectrum anymore if only CO is employed. cuum chambers, and the three elements nozzle, skimmer, and crystal are located along one line. The single crystal [Pd(100) or Pt(100), diameter Conclusion 10 mm, thickness 1 mm] is mounted with a slight angle of about 5° tilted The presented experiments demonstrate the feasibility of laser- toward the molecular beam so that the gas not only streaks over the surface but may actually hit it. Perpendicular to the gas beam and almost parallel to induced catalytic reactions of carbon monoxide and hydrogen the surface normal, there is a TOF mass spectrometer with a system of electrodes to accelerate ions that are produced when the laser beam interacts Σ + x>0 HxCO optimized pulse with the surface and its adsorbates. These ions generate an electric signal on Σ + unmodulated pulse x>0 CHx a chevron-stacked microchannel plate assembly after flight times that are directly related to their mass. These signals are recorded either directly via 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 voltage of þ200 V. Several subsequent grounded electrodes guarantee a field-free drift region, whereas at the detector itself the back and front plates of the microchannel plate assembly are set to −100 and þ1;800 V, respectively, Ratio followed by a conical anode at 0 V. The whole repeller assembly including the single crystal can be heated, as well as cooled by a cryogenic system. The base pressure in the main chamber, without exposure to the two gases, is 10−6 torr, whereas up to 10−4 torr are reached with the highest gas amounts employed in the experiments. 0123456 ACKNOWLEDGMENTS. H.W. appreciates constant support and encouragement H2 amount [sccm] of Prof. Brandstetter and Prof. Iden (BASF). We gratefully acknowledge the Fig. 6. Variation of H2 concentration after an optimization. After maximi- group of Prof. Gauss (Univ Mainz), Dr. Wohlleben (BASF), and Prof. Brixner þ þ zation of the ratio CO ∕C with 4 sccm CO only, H2 is streamed onto the (Univ Würzburg) for stimulating discussions, Dr. Niklaus and Dr. Papastatho- surface. The graph shows the ratio of all species with hydrogenated CO poulos for help in the early phase of the project, and the BASF SE for financial support. P.N. acknowledges financial support from the Deutsche Akademie (29–30 amu) to those with hydrogenated C (13–15 amu). The optimization der Naturforscher Leopoldina (BMBF-LPDS 2009-6). effect fades away with increasing H2 amounts.

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