Pulsed molecular beams allow high-density gas samples to be cooled to low internal temperatures a... more Pulsed molecular beams allow high-density gas samples to be cooled to low internal temperatures and to produce narrow speed distributions. They are particularly useful in combination with pulsedlaser-based detection schemes and have also been used as pump pulses in pump−probe experiments with neutral matter. The mechanical response of pulsed valves and chopper wheels limits the duration of these pulses typically to about 10−100 μs. Bunch compression photolysis has been proposed as a means to produce atomic pulses shorter than 1 ns�an experimental capability that would allow new measurements to be made on chemical systems. This technique employs a spatially chirped femtosecond duration photolysis pulse that produced an ensemble of H atom photoproducts that rebunches into a short pulse downstream. To date, this technique could not produce strong enough beams to allow new experiments to be carried out. In this paper, we report production of pulsed H atom beams consistent with a 700 ps pulse duration and with sufficient intensity to carry out differentially resolved inelastic H scattering experiments from a graphene surface. We observe surprisingly narrow angular distributions for H atoms incident normal to the surface. At low incidence energies quasi-elastic scattering dominates, and at high incidence energy we observe a strongly inelastic scattering channel. These results provide the basis for future experiments where the H atoms synchronously collide with a pulsed-laser-excited surface.
We describe here the application of an inexpensive event-based/neuromorphic camera in an ion imag... more We describe here the application of an inexpensive event-based/neuromorphic camera in an ion imaging experiment operated at 1 kHz detection rate to study real-time velocityresolved kinetics of thermal desorption. Such measurements involve a single gas pulse to initiate a time-dependent desorption process and a high repetition rate laser, where each pulse of the laser is used to produce an ion image. The sequence of ion images allows the time dependence of the desorption flux to be followed in real time. In previous work where a conventional framing camera was used, the large number of megapixel-sized images required data transfer and storage rates of up to 16 GB/s. This necessitated a large onboard memory that was quickly filled and limited continuous measurement to only a few seconds. Read-out of the memory became the bottleneck to the rate of data acquisition. We show here that since most pixels in each ion image contain no data, the data rate can be dramatically reduced by using an event-based/neuromorphic camera. The data stream is thus reduced to the intensity and location information on the pixels that are lit up by each ion event together with a time-stamp indicating the arrival time of an ion at the detector. This dramatically increases the duty cycle of the method and provides insights for the execution of other high rep-rate ion imaging experiments.
Up to now, methods for measuring rates of reactions on catalysts required long measurement times ... more Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossiblecatalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10−1000 times higher. We are able to record kinetic traces of CO 2 formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions.
Up to now, methods for measuring rates of reactions on catalysts required long measurement times ... more Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossiblecatalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10−1000 times higher. We are able to record kinetic traces of CO 2 formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions.
The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as mi... more The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as microkinetic models of atmospheric chemistry provided the predictive power that led to the Montreal Protocol reversing loss of stratospheric ozone, pursuing a microkinetic approach to heterogeneous catalysis has tremendous potential for societal impact. However, the development of this approach for catalysis faces great challenges. Methods for measuring rate constants are quite limited, and the present predictive theoretical methods remain largely unvalidated. Here, we present a short Perspective on recent experimental advances in the measurement of rates of elementary reactions at surfaces that rely on a stroboscopic pump-probe concept for neutral matter. We present the principles behind successful measurement methods and discuss a recent implementation of those principles. The topic is discussed within the context of a specific but highly typical surface reaction, CO oxidation on Pt, which, despite more than 40 years of study , was only clarified after experiments with velocity-resolved kinetics became possible. This deceptively simple reaction illustrates fundamental lessons concerning the coverage dependence of activation energies, the nature of reaction mechanisms involving multiple reaction sites, the validity of transition-state theory to describe reaction rates at surfaces and the dramatic changes in reaction mechanism that are possible when studying reactions at low temperatures.
The global climate change calls for more environmentally friendly use of energy and has led to st... more The global climate change calls for more environmentally friendly use of energy and has led to stricter limits and regulations for the emissions of various greenhouse gases. Consequently, there is nowadays an increasing need for the detection of exhaust and natural gases. This need leads to an ever-growing market for gas sensors, which, at the moment, is dominated by chemical sensors. Yet, the increasing demands to also measure under harsh environmental conditions pave the way for non-invasive measurements and thus optical detection techniques. Here, we present the development of two optical detection systems using non-dispersive infrared absorption spectroscopy (NDIR). One system is intended for civilian use, capable of detecting both CO as well as CO 2 in the range of 4-5 µm. Furthermore, restrictions regarding size and economic viability are put on this sensor so it can compete with existing sensors. For CO 2 , an estimated resolution of 444 ppm is achieved, which is competitive with established sensors on the market. For CO an estimated resolution of 1401 ppm was achieved, rendering it necessary to improve this sensor to be competitive with other available sensors. The second system is used in an exhaust system and is capable of detecting CO 2 at 3.2 µm facing cross-sensitivity with H 2 O. A data analysis method is described to separate the CO 2 and H 2 O signals, revealing a time resolution of 33 µs.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights A refined data analysis procedure for a methane sensor system capable of measuring temperature and gas concentration simultaneously inside an internal combustion engine is presented. The measurement method is based on broadband infrared absorption an calculations using the HITRAN database. Validation experiments under well-defined conditions using a rapid compression machine to simulate environmental conditions inside an internal combustion engine are performed. The determination of temperature is compared to a commercially available system based on water absorption in the near infrared. Accuracy and precision of the methane sensor system in determination of air-fuel ratios are 4.6% and 1.3% respectively.
The reduction of CO 2 and other greenhouse gas emissions is an important driving force for the de... more The reduction of CO 2 and other greenhouse gas emissions is an important driving force for the development of modern engines. Especially in the transport sector, the use of alternative fuels like methane, the main component of compressed natural gas (CNG), is an applied measure to achieve this goal. This work describes the development of an optical measurement system for accurate quantification of CH 4 densities in gas flows based on broadband absorption of infrared light, i.e. non-dispersive IR absorption spectroscopy (NDIR). We demonstrate the capability of the system to achieve high time resolution as well as high measurement accuracy and precision. The optical setup of the system is designed for usage at the inlet manifold of CNG-fuelled spark ignition engines. It allows for detailed analysis of the mixture formation during single engine cycles. CH 4 densities can be determined by monitoring the infrared light attenuation around 3.3 µm caused by the ν 3 antisymmetric C-H-stretch vibration. We calculate the nonlinear relation between transmittance and CH 4 density from absorption cross sections calculated from the HITRAN database. The theoretical transmittance signals are corrected for spectral influences of the bandpass filter, the detector sensitivity, the fibre transmittance and the emission spectrum of the light source in order to calculate CH 4 densities directly from the measured transmittance. A calibration function corrects remaining differences between experimental and simulated values and improves the accuracy. We show that the sensor system is capable for determination of air-fuel ratios (λ-values) and demonstrate the dynamic quantification of a CH 4 injection into a flow channel under various flow conditions. Furthermore, we present the first measurements with a prototype probe capable of measurements inside the inlet manifold of a four-stroke spark ignition engine. We validate the detection strategy in experiments with premixed gases using a modified inlet geometry and demonstrate its application under standard engine operation with port fuel injection while varying the injection parameters.
We present quantum-state and velocity resolved experiments for molecular beam scattering of acety... more We present quantum-state and velocity resolved experiments for molecular beam scattering of acetylene (C
We measured absolute probabilities for vibrational excitation of NO ݒ( = 0) molecules in collis... more We measured absolute probabilities for vibrational excitation of NO ݒ( = 0) molecules in collisions with a Au(111) surface at an incidence energy of translation 0.4 eV and surface temperatures between 300 K and 1100 K. In addition to previously reported excitation to ݒ = 1 and ݒ = 2 , we observed excitation to ݒ = 3. The excitation probabilities exhibit an Arrhenius dependence on surface temperature, indicating that the dominant excitation mechanism is nonadiabatic coupling to electron-hole pairs. The experimental data are analyzed in terms of a recently introduced kinetic model, which was extended to include four vibrational states. We describe a sub-population decomposition of the kinetic model, which allows us examine vibrational population transfer pathways. The analysis indicates that sequential pathways (ݒ = 0 → 1 → 2 and ݒ = 0 → 1 → 2 → 3) alone cannot adequately describe production of ݒ = 2 or 3. In addition, we performed firstprinciples molecular dynamics calculations that incorporate electronically nonadiabatic dynamics via an independent electron surface hopping (IESH) algorithm, which requires as input an ab initio potential energy hypersurface (PES) and nonadiabatic coupling matrix elements; both obtained from density functional theory (DFT). While the IESH-based simulations reproduce the ݒ = 1 data well, they slightly underestimate the excitation probabilities to ݒ = 2 and they significantly underestimate those for ݒ = 3. Furthermore, this implementation of IESH appears to overestimate the importance of sequential energy transfer pathways. We make several suggestions concerning ways to improve this IESH based model.
Proceedings of the National Academy of Sciences, 2013
Significance How molecules point in space—that is, their spatial orientation—determines how they ... more Significance How molecules point in space—that is, their spatial orientation—determines how they interact with their environment. Exchange of energy, photons, and particles as well as chemical reactions are all elementary processes that depend on orientation. Electron transfer reactions are of particular interest because of their importance in a remarkably wide range of phenomena. In this work, we examine electron transfer reactions at surfaces, which control the change of oxidation state in surface chemistry, a critical factor explaining catalytic activity and selectivity. We report a strong orientation dependence for vibrational relaxation of a diatomic molecule colliding with a metal surface, an energy transfer process driven by electron transfer. These observations represent a challenge to modern theories of surface chemistry.
Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSC... more Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newlysynthesized artificial b-carotene derivative 13,13 0-diphenyl-b-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S 2 state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S 2-S 1 and S 1-S 0 * with results for b-carotene. Global analysis provides IC time constants and also time-dependent S 1 spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the 1 H NMR spectrum, a small red-shift of the S 0-S 2 (0-0) transition energy in the steady-state absorption spectrum relative to b-carotene, and almost the same S 1-S 0 * IC time constant as in b-carotene, suggesting a similar S 1-S 0 energy gap. The oscillator strength of the S 0-S 2 transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an ''S* state''. Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S 0 * superimposed on the bleach of room temperature molecules S 0. The S 0 * population is formed by IC from S 1. These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties.
We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a... more We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a Au(111) surface at incidence translational energies ranging from 0.1 to 1.2 eV. Experimentally, molecular beam-surface scattering is combined with vibrational overtone pumping and quantumstate selective detection of the recoiling molecules. Theoretically, we employ a recently developed first-principles approach, which employs an Independent Electron Surface Hopping (IESH) algorithm to model the nonadiabatic dynamics on a Newns-Anderson Hamiltonian derived from density functional theory. This approach has been successful when compared to previously reported NO/Au scattering data. The experiments presented here show that vibrational relaxation probabilities increase with incidence energy of translation. The theoretical simulations incorrectly predict high relaxation probabilities at low incidence translational energy. We show that this behavior originates from trajectories exhibiting multiple bounces at the surface, associated with deeper penetration and favored (N-down) molecular orientation, resulting in a higher average number of electronic hops and thus stronger vibrational relaxation. The experimentally observed narrow angular distributions suggest that mainly single-bounce collisions are important. Restricting the simulations by selecting only single-bounce trajectories improves agreement with experiment. The multiple bounce artifacts discovered in this work are also present in simulations employing electronic friction and even for electronically adiabatic simulations, meaning they are not a direct result of the IESH algorithm. This work demonstrates how even subtle errors in the adiabatic interaction potential, especially those that influence the interaction time of the molecule with the surface, can lead to an incorrect description of electronically nonadiabatic vibrational energy transfer in molecule-surface collisions.
Pulsed molecular beams allow high-density gas samples to be cooled to low internal temperatures a... more Pulsed molecular beams allow high-density gas samples to be cooled to low internal temperatures and to produce narrow speed distributions. They are particularly useful in combination with pulsedlaser-based detection schemes and have also been used as pump pulses in pump−probe experiments with neutral matter. The mechanical response of pulsed valves and chopper wheels limits the duration of these pulses typically to about 10−100 μs. Bunch compression photolysis has been proposed as a means to produce atomic pulses shorter than 1 ns�an experimental capability that would allow new measurements to be made on chemical systems. This technique employs a spatially chirped femtosecond duration photolysis pulse that produced an ensemble of H atom photoproducts that rebunches into a short pulse downstream. To date, this technique could not produce strong enough beams to allow new experiments to be carried out. In this paper, we report production of pulsed H atom beams consistent with a 700 ps pulse duration and with sufficient intensity to carry out differentially resolved inelastic H scattering experiments from a graphene surface. We observe surprisingly narrow angular distributions for H atoms incident normal to the surface. At low incidence energies quasi-elastic scattering dominates, and at high incidence energy we observe a strongly inelastic scattering channel. These results provide the basis for future experiments where the H atoms synchronously collide with a pulsed-laser-excited surface.
We describe here the application of an inexpensive event-based/neuromorphic camera in an ion imag... more We describe here the application of an inexpensive event-based/neuromorphic camera in an ion imaging experiment operated at 1 kHz detection rate to study real-time velocityresolved kinetics of thermal desorption. Such measurements involve a single gas pulse to initiate a time-dependent desorption process and a high repetition rate laser, where each pulse of the laser is used to produce an ion image. The sequence of ion images allows the time dependence of the desorption flux to be followed in real time. In previous work where a conventional framing camera was used, the large number of megapixel-sized images required data transfer and storage rates of up to 16 GB/s. This necessitated a large onboard memory that was quickly filled and limited continuous measurement to only a few seconds. Read-out of the memory became the bottleneck to the rate of data acquisition. We show here that since most pixels in each ion image contain no data, the data rate can be dramatically reduced by using an event-based/neuromorphic camera. The data stream is thus reduced to the intensity and location information on the pixels that are lit up by each ion event together with a time-stamp indicating the arrival time of an ion at the detector. This dramatically increases the duty cycle of the method and provides insights for the execution of other high rep-rate ion imaging experiments.
Up to now, methods for measuring rates of reactions on catalysts required long measurement times ... more Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossiblecatalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10−1000 times higher. We are able to record kinetic traces of CO 2 formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions.
Up to now, methods for measuring rates of reactions on catalysts required long measurement times ... more Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossiblecatalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10−1000 times higher. We are able to record kinetic traces of CO 2 formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions.
The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as mi... more The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as microkinetic models of atmospheric chemistry provided the predictive power that led to the Montreal Protocol reversing loss of stratospheric ozone, pursuing a microkinetic approach to heterogeneous catalysis has tremendous potential for societal impact. However, the development of this approach for catalysis faces great challenges. Methods for measuring rate constants are quite limited, and the present predictive theoretical methods remain largely unvalidated. Here, we present a short Perspective on recent experimental advances in the measurement of rates of elementary reactions at surfaces that rely on a stroboscopic pump-probe concept for neutral matter. We present the principles behind successful measurement methods and discuss a recent implementation of those principles. The topic is discussed within the context of a specific but highly typical surface reaction, CO oxidation on Pt, which, despite more than 40 years of study , was only clarified after experiments with velocity-resolved kinetics became possible. This deceptively simple reaction illustrates fundamental lessons concerning the coverage dependence of activation energies, the nature of reaction mechanisms involving multiple reaction sites, the validity of transition-state theory to describe reaction rates at surfaces and the dramatic changes in reaction mechanism that are possible when studying reactions at low temperatures.
The global climate change calls for more environmentally friendly use of energy and has led to st... more The global climate change calls for more environmentally friendly use of energy and has led to stricter limits and regulations for the emissions of various greenhouse gases. Consequently, there is nowadays an increasing need for the detection of exhaust and natural gases. This need leads to an ever-growing market for gas sensors, which, at the moment, is dominated by chemical sensors. Yet, the increasing demands to also measure under harsh environmental conditions pave the way for non-invasive measurements and thus optical detection techniques. Here, we present the development of two optical detection systems using non-dispersive infrared absorption spectroscopy (NDIR). One system is intended for civilian use, capable of detecting both CO as well as CO 2 in the range of 4-5 µm. Furthermore, restrictions regarding size and economic viability are put on this sensor so it can compete with existing sensors. For CO 2 , an estimated resolution of 444 ppm is achieved, which is competitive with established sensors on the market. For CO an estimated resolution of 1401 ppm was achieved, rendering it necessary to improve this sensor to be competitive with other available sensors. The second system is used in an exhaust system and is capable of detecting CO 2 at 3.2 µm facing cross-sensitivity with H 2 O. A data analysis method is described to separate the CO 2 and H 2 O signals, revealing a time resolution of 33 µs.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights A refined data analysis procedure for a methane sensor system capable of measuring temperature and gas concentration simultaneously inside an internal combustion engine is presented. The measurement method is based on broadband infrared absorption an calculations using the HITRAN database. Validation experiments under well-defined conditions using a rapid compression machine to simulate environmental conditions inside an internal combustion engine are performed. The determination of temperature is compared to a commercially available system based on water absorption in the near infrared. Accuracy and precision of the methane sensor system in determination of air-fuel ratios are 4.6% and 1.3% respectively.
The reduction of CO 2 and other greenhouse gas emissions is an important driving force for the de... more The reduction of CO 2 and other greenhouse gas emissions is an important driving force for the development of modern engines. Especially in the transport sector, the use of alternative fuels like methane, the main component of compressed natural gas (CNG), is an applied measure to achieve this goal. This work describes the development of an optical measurement system for accurate quantification of CH 4 densities in gas flows based on broadband absorption of infrared light, i.e. non-dispersive IR absorption spectroscopy (NDIR). We demonstrate the capability of the system to achieve high time resolution as well as high measurement accuracy and precision. The optical setup of the system is designed for usage at the inlet manifold of CNG-fuelled spark ignition engines. It allows for detailed analysis of the mixture formation during single engine cycles. CH 4 densities can be determined by monitoring the infrared light attenuation around 3.3 µm caused by the ν 3 antisymmetric C-H-stretch vibration. We calculate the nonlinear relation between transmittance and CH 4 density from absorption cross sections calculated from the HITRAN database. The theoretical transmittance signals are corrected for spectral influences of the bandpass filter, the detector sensitivity, the fibre transmittance and the emission spectrum of the light source in order to calculate CH 4 densities directly from the measured transmittance. A calibration function corrects remaining differences between experimental and simulated values and improves the accuracy. We show that the sensor system is capable for determination of air-fuel ratios (λ-values) and demonstrate the dynamic quantification of a CH 4 injection into a flow channel under various flow conditions. Furthermore, we present the first measurements with a prototype probe capable of measurements inside the inlet manifold of a four-stroke spark ignition engine. We validate the detection strategy in experiments with premixed gases using a modified inlet geometry and demonstrate its application under standard engine operation with port fuel injection while varying the injection parameters.
We present quantum-state and velocity resolved experiments for molecular beam scattering of acety... more We present quantum-state and velocity resolved experiments for molecular beam scattering of acetylene (C
We measured absolute probabilities for vibrational excitation of NO ݒ( = 0) molecules in collis... more We measured absolute probabilities for vibrational excitation of NO ݒ( = 0) molecules in collisions with a Au(111) surface at an incidence energy of translation 0.4 eV and surface temperatures between 300 K and 1100 K. In addition to previously reported excitation to ݒ = 1 and ݒ = 2 , we observed excitation to ݒ = 3. The excitation probabilities exhibit an Arrhenius dependence on surface temperature, indicating that the dominant excitation mechanism is nonadiabatic coupling to electron-hole pairs. The experimental data are analyzed in terms of a recently introduced kinetic model, which was extended to include four vibrational states. We describe a sub-population decomposition of the kinetic model, which allows us examine vibrational population transfer pathways. The analysis indicates that sequential pathways (ݒ = 0 → 1 → 2 and ݒ = 0 → 1 → 2 → 3) alone cannot adequately describe production of ݒ = 2 or 3. In addition, we performed firstprinciples molecular dynamics calculations that incorporate electronically nonadiabatic dynamics via an independent electron surface hopping (IESH) algorithm, which requires as input an ab initio potential energy hypersurface (PES) and nonadiabatic coupling matrix elements; both obtained from density functional theory (DFT). While the IESH-based simulations reproduce the ݒ = 1 data well, they slightly underestimate the excitation probabilities to ݒ = 2 and they significantly underestimate those for ݒ = 3. Furthermore, this implementation of IESH appears to overestimate the importance of sequential energy transfer pathways. We make several suggestions concerning ways to improve this IESH based model.
Proceedings of the National Academy of Sciences, 2013
Significance How molecules point in space—that is, their spatial orientation—determines how they ... more Significance How molecules point in space—that is, their spatial orientation—determines how they interact with their environment. Exchange of energy, photons, and particles as well as chemical reactions are all elementary processes that depend on orientation. Electron transfer reactions are of particular interest because of their importance in a remarkably wide range of phenomena. In this work, we examine electron transfer reactions at surfaces, which control the change of oxidation state in surface chemistry, a critical factor explaining catalytic activity and selectivity. We report a strong orientation dependence for vibrational relaxation of a diatomic molecule colliding with a metal surface, an energy transfer process driven by electron transfer. These observations represent a challenge to modern theories of surface chemistry.
Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSC... more Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newlysynthesized artificial b-carotene derivative 13,13 0-diphenyl-b-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S 2 state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S 2-S 1 and S 1-S 0 * with results for b-carotene. Global analysis provides IC time constants and also time-dependent S 1 spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the 1 H NMR spectrum, a small red-shift of the S 0-S 2 (0-0) transition energy in the steady-state absorption spectrum relative to b-carotene, and almost the same S 1-S 0 * IC time constant as in b-carotene, suggesting a similar S 1-S 0 energy gap. The oscillator strength of the S 0-S 2 transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an ''S* state''. Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S 0 * superimposed on the bleach of room temperature molecules S 0. The S 0 * population is formed by IC from S 1. These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties.
We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a... more We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a Au(111) surface at incidence translational energies ranging from 0.1 to 1.2 eV. Experimentally, molecular beam-surface scattering is combined with vibrational overtone pumping and quantumstate selective detection of the recoiling molecules. Theoretically, we employ a recently developed first-principles approach, which employs an Independent Electron Surface Hopping (IESH) algorithm to model the nonadiabatic dynamics on a Newns-Anderson Hamiltonian derived from density functional theory. This approach has been successful when compared to previously reported NO/Au scattering data. The experiments presented here show that vibrational relaxation probabilities increase with incidence energy of translation. The theoretical simulations incorrectly predict high relaxation probabilities at low incidence translational energy. We show that this behavior originates from trajectories exhibiting multiple bounces at the surface, associated with deeper penetration and favored (N-down) molecular orientation, resulting in a higher average number of electronic hops and thus stronger vibrational relaxation. The experimentally observed narrow angular distributions suggest that mainly single-bounce collisions are important. Restricting the simulations by selecting only single-bounce trajectories improves agreement with experiment. The multiple bounce artifacts discovered in this work are also present in simulations employing electronic friction and even for electronically adiabatic simulations, meaning they are not a direct result of the IESH algorithm. This work demonstrates how even subtle errors in the adiabatic interaction potential, especially those that influence the interaction time of the molecule with the surface, can lead to an incorrect description of electronically nonadiabatic vibrational energy transfer in molecule-surface collisions.
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Papers by Kai Golibrzuch