Pub Date : 2020-12-08DOI: 10.1016/J.MOLLIQ.2021.116770
Ra'ul Fuentes-Azcatl
{"title":"Flexible model of water based on the dielectric and electromagnetic spectrum properties : TIP4P/$epsilon$ Flex.","authors":"Ra'ul Fuentes-Azcatl","doi":"10.1016/J.MOLLIQ.2021.116770","DOIUrl":"https://doi.org/10.1016/J.MOLLIQ.2021.116770","url":null,"abstract":"","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"55 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-12-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83644685","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-11-30DOI: 10.26434/chemrxiv.13308458
F. Stender, Keisuke Obata, Max Baumung, F. Abdi, M. Risch
Generator-collector experiments offer insights into the mechanisms of electrochemical reactions by correlating the product and generator currents. Most commonly, these experiments are performed using a rotating ring-disk electrode (RRDE). We developed a double electrode flow cell (DEFC) with exchangeable generator and detector electrodes where the electrode width equals the channel width. Commonalities and differences between the RRDE and DEFC are discussed based on analytical solutions, numerical simulations and measurements of the ferri-/ferrocyanide redox couple on Pt electrodes in a potassium chloride electrolyte. The analytical solutions agree with the measurements using electrode widths of 5 and 2 mm. Yet, we find an unexpected dependence on the exponent of the width so that wider electrodes cannot be analysed using the conventional analytical solution. In contrast, all the investigated electrodes show a collection efficiency of close to 35.4% above a minimum rotation speed or flow rate, where the narrowest electrode is most accurate at the cost of precision and the widest electrode the least accurate but most precise. Our DEFC with exchangeable electrodes is an attractive alternative to commercial RRDEs due to the flexibility to optimize the electrode materials and geometry for the desired reaction.
{"title":"Characterization of a Modular Flow Cell System for Electrocatalytic Experiments and Comparison to a Commercial RRDE System","authors":"F. Stender, Keisuke Obata, Max Baumung, F. Abdi, M. Risch","doi":"10.26434/chemrxiv.13308458","DOIUrl":"https://doi.org/10.26434/chemrxiv.13308458","url":null,"abstract":"Generator-collector\u0000experiments offer insights into the mechanisms of electrochemical reactions by\u0000correlating the product and generator currents. Most commonly, these\u0000experiments are performed using a rotating ring-disk electrode (RRDE). We\u0000developed a double electrode flow cell (DEFC) with exchangeable generator and\u0000detector electrodes where the electrode width equals the channel width.\u0000Commonalities and differences between the RRDE and DEFC are discussed based on\u0000analytical solutions, numerical simulations and measurements of the\u0000ferri-/ferrocyanide redox couple on Pt electrodes in a potassium chloride\u0000electrolyte. The analytical solutions agree with the measurements using\u0000electrode widths of 5 and 2 mm. Yet, we find an unexpected dependence on the\u0000exponent of the width so that wider electrodes cannot be analysed using the\u0000conventional analytical solution. In contrast, all the investigated electrodes\u0000show a collection efficiency of close to 35.4% above a minimum rotation speed\u0000or flow rate, where the narrowest electrode is most accurate at the cost of\u0000precision and the widest electrode the least accurate but most precise. Our\u0000DEFC with exchangeable electrodes is an attractive alternative to commercial RRDEs\u0000due to the flexibility to optimize the electrode materials and geometry for the\u0000desired reaction.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"61 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80945722","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Emma Lumiaro, M. Todorovi'c, T. Kurtén, H. Vehkamäki, P. Rinke
Abstract. The formation, properties and lifetime of secondary organic aerosols in the atmosphere are largely determined by gas-particle partitioning coefficients of the participating organic vapours. Since these coefficients are often difficult to measure and to compute, we developed a machine learning model to predict them given molecular structure as input. Our data-driven approach is based on the dataset by Wang et al. (Atmos. Chem. Phys., 17, 7529 (2017)), who computed the partitioning coefficients and saturation vapour pressures of 3414 atmospheric oxidation products from the master chemical mechanism using the COSMOtherm program. We trained a kernel ridge regression (KRR) machine learning model on the saturation vapour pressure (Psat), and on two equilibrium partitioning coefficients: between a water-insoluble organic matter phase and the gas phase (KWIOM/G), and between an infinitely dilute solution with pure water and the gas phase (KW/G). For the input representation of the atomic structure of each organic molecule to the machine, we tested different descriptors. We find that the many-body tensor representation (MBTR) works best for our application, but the topological fingerprint (TopFP) approach is almost as good, and is significantly more cost effective. Our best machine learning model (KRR with a Gaussian kernel + MBTR) predicts Psat and KWIOM/G to within 0.3 logarithmic units and KW/G to within 0.4 logarithmic units of the original COSMOtherm calculations. This is equal or better than the typical accuracy of COSMOtherm predictions compared to experimental data (where available). We then applied our machine learning model to a dataset of 35,383 molecules that we generated based on a carbon 10 backbone functionalized with 0 to 6 carboxyl, carbonyl or hydroxyl groups to evaluate its performance for polyfunctional compounds with potentially low Psat. The resulting saturation vapor pressure and partitioning coefficient distributions were physico-chemically reasonable, and the volatility predictions for the most highly oxidized compounds were in qualitative agreement with experimentally inferred volatilities of atmospheric oxidation products with similar elemental composition.
{"title":"Predicting Gas-Particle Partitioning Coefficients of Atmospheric\u0000Molecules with Machine Learning","authors":"Emma Lumiaro, M. Todorovi'c, T. Kurtén, H. Vehkamäki, P. Rinke","doi":"10.5194/ACP-2020-1258","DOIUrl":"https://doi.org/10.5194/ACP-2020-1258","url":null,"abstract":"Abstract. The formation, properties and lifetime of secondary organic aerosols in the atmosphere are largely determined by gas-particle partitioning coefficients of the participating organic vapours. Since these coefficients are often difficult to measure and to compute, we developed a machine learning model to predict them given molecular structure as input. Our data-driven approach is based on the dataset by Wang et al. (Atmos. Chem. Phys., 17, 7529 (2017)), who computed the partitioning coefficients and saturation vapour pressures of 3414 atmospheric oxidation products from the master chemical mechanism using the COSMOtherm program. We trained a kernel ridge regression (KRR) machine learning model on the saturation vapour pressure (Psat), and on two equilibrium partitioning coefficients: between a water-insoluble organic matter phase and the gas phase (KWIOM/G), and between an infinitely dilute solution with pure water and the gas phase (KW/G). For the input representation of the atomic structure of each organic molecule to the machine, we tested different descriptors. We find that the many-body tensor representation (MBTR) works best for our application, but the topological fingerprint (TopFP) approach is almost as good, and is significantly more cost effective. Our best machine learning model (KRR with a Gaussian kernel + MBTR) predicts Psat and KWIOM/G to within 0.3 logarithmic units and KW/G to within 0.4 logarithmic units of the original COSMOtherm calculations. This is equal or better than the typical accuracy of COSMOtherm predictions compared to experimental data (where available). We then applied our machine learning model to a dataset of 35,383 molecules that we generated based on a carbon 10 backbone functionalized with 0 to 6 carboxyl, carbonyl or hydroxyl groups to evaluate its performance for polyfunctional compounds with potentially low Psat. The resulting saturation vapor pressure and partitioning coefficient distributions were physico-chemically reasonable, and the volatility predictions for the most highly oxidized compounds were in qualitative agreement with experimentally inferred volatilities of atmospheric oxidation products with similar elemental composition.\u0000","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"221 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79877273","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Dupuy, M. Haubner, B. Henrist, J. Fillion, V. Baglin
Electron-stimulated desorption (ESD) of cryosorbed molecules on surfaces is a process of relevance to fields as varied as vacuum dynamics in accelerators and astrochemistry. While desorption from such molecular systems induced by keV electrons and fast ions has been extensively studied, the sub-keV electron regime is comparatively little known. We measured and quantified electron-stimulated desorption from molecular ice systems (layers of N$_2$, CO, CO$_2$, Ar and H$_2$O/D$_2$O condensed at cryogenic temperatures) in the 150-2000 eV electron energy range. In this regime stopping power is no longer sufficient to explain the electron energy dependence of ESD yields. We introduce the notion of desorption-relevant depth, which characterizes the transition between two energy deposition regimes near the surface. We then apply this notion to the different systems, showing how ESD in the sub-keV regime can for example reveal the differences in species diffusion in crystalline and porous amorphous CO$_2$ ices.
表面冷冻吸附分子的电子刺激解吸(ESD)是一个与加速器真空动力学和天体化学等领域相关的过程。虽然由keV电子和快离子诱导的分子系统的脱附已经被广泛研究，但亚keV电子的机制相对来说鲜为人知。在150 ~ 2000 eV的电子能范围内，对分子冰体系(N$_2$， CO$_2$， CO$_2$， Ar $和H$_2$O/D$_2$O)的电子激发解吸进行了测量和量化。在这种情况下，停止功率不再足以解释ESD产量的电子能量依赖性。我们引入了解吸相关深度的概念，它表征了地表附近两种能量沉积机制之间的转变。然后，我们将这一概念应用于不同的系统，展示了亚kev状态下的ESD如何揭示晶体和多孔非晶CO$_2$冰中物质扩散的差异。
{"title":"Electron-stimulated desorption from molecular ices in the 0.15–2 keV regime","authors":"R. Dupuy, M. Haubner, B. Henrist, J. Fillion, V. Baglin","doi":"10.1063/5.0021832","DOIUrl":"https://doi.org/10.1063/5.0021832","url":null,"abstract":"Electron-stimulated desorption (ESD) of cryosorbed molecules on surfaces is a process of relevance to fields as varied as vacuum dynamics in accelerators and astrochemistry. While desorption from such molecular systems induced by keV electrons and fast ions has been extensively studied, the sub-keV electron regime is comparatively little known. We measured and quantified electron-stimulated desorption from molecular ice systems (layers of N$_2$, CO, CO$_2$, Ar and H$_2$O/D$_2$O condensed at cryogenic temperatures) in the 150-2000 eV electron energy range. In this regime stopping power is no longer sufficient to explain the electron energy dependence of ESD yields. We introduce the notion of desorption-relevant depth, which characterizes the transition between two energy deposition regimes near the surface. We then apply this notion to the different systems, showing how ESD in the sub-keV regime can for example reveal the differences in species diffusion in crystalline and porous amorphous CO$_2$ ices.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"6 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"72779204","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We present the synthesis and characterization of (15-crown-5)BiI$_3$ (1) and (15-crown-5)BiI$_3$*0.5TIE (2), a halogen bonded adduct with tetraiodoethylene (TIE), a typical halogen bond donor. Our results show that crown ether complexes of main group metal halides can be employed as halogen bond acceptors for the synthesis of new supramolecular aggregates and highlight the significant interaction between the two building blocks.
{"title":"(15‐crown‐5)BiI\u0000 3\u0000 as a Building Block for Halogen Bonded Supramolecular Aggregates","authors":"Bettina Wagner, Johanna Heine","doi":"10.1002/zaac.202000422","DOIUrl":"https://doi.org/10.1002/zaac.202000422","url":null,"abstract":"We present the synthesis and characterization of (15-crown-5)BiI$_3$ (1) and (15-crown-5)BiI$_3$*0.5TIE (2), a halogen bonded adduct with tetraiodoethylene (TIE), a typical halogen bond donor. Our results show that crown ether complexes of main group metal halides can be employed as halogen bond acceptors for the synthesis of new supramolecular aggregates and highlight the significant interaction between the two building blocks.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"6 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83595976","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-09-25DOI: 10.1103/PHYSREVMATERIALS.5.013803
Chenyang Li, T. Nilson, Liang Cao, Tim Mueller
Kinetic Monte Carlo models parameterized by first principles calculations are widely used to simulate atomic diffusion. However, accurately predicting the activation energies for diffusion in substitutional alloys remains challenging due to the wide variety of local environments that may exist around the diffusing atom. We address this challenge using a cluster expansion model that explicitly includes a sublattice of sites representing transition states and assess its accuracy in comparison with other models, such as the broken bond model and a model related to Marcus theory, by modeling vacancy-mediated diffusion in Pt-Ni nanoparticles. We find that the prediction error of the cluster expansion is similar to that of other models for small training sets, but with larger training sets the cluster expansion has a significantly lower prediction error than the other models with comparable execution speed. Of the simpler models, the model related to Marcus theory yields predictions of nanoparticle evolution that are most similar to those of the cluster expansion, and a weighted average of the two approaches has the lowest prediction error for activation energies across all training set sizes.
{"title":"Predicting activation energies for vacancy-mediated diffusion in alloys using a transition-state cluster expansion","authors":"Chenyang Li, T. Nilson, Liang Cao, Tim Mueller","doi":"10.1103/PHYSREVMATERIALS.5.013803","DOIUrl":"https://doi.org/10.1103/PHYSREVMATERIALS.5.013803","url":null,"abstract":"Kinetic Monte Carlo models parameterized by first principles calculations are widely used to simulate atomic diffusion. However, accurately predicting the activation energies for diffusion in substitutional alloys remains challenging due to the wide variety of local environments that may exist around the diffusing atom. We address this challenge using a cluster expansion model that explicitly includes a sublattice of sites representing transition states and assess its accuracy in comparison with other models, such as the broken bond model and a model related to Marcus theory, by modeling vacancy-mediated diffusion in Pt-Ni nanoparticles. We find that the prediction error of the cluster expansion is similar to that of other models for small training sets, but with larger training sets the cluster expansion has a significantly lower prediction error than the other models with comparable execution speed. Of the simpler models, the model related to Marcus theory yields predictions of nanoparticle evolution that are most similar to those of the cluster expansion, and a weighted average of the two approaches has the lowest prediction error for activation energies across all training set sizes.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"14 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-09-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87298843","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-09-18DOI: 10.1103/physreva.102.052806
K. Strasburger
The energies of the lowest $^2P_u$, $^4P_g$ and $2D_g$ states of the boron atom are calculated with $mu$hartree accuracy, in the basis of symmetrized, explicitly correlated Gaussian lobe functions. Finite nuclear mass and scalar relativistic corrections are taken into account. This study contributes to the problem of the energy differences between doublet and quartet states of boron, which have not been measured to date. It is found that the $^2P_urightarrow^4P_g$ excitation energy, recommended in the Atomic Spectra Database, appears underestimated by more than 300~cm$^{-1}$.
{"title":"Energy difference between the lowest doublet and quartet states of the boron atom","authors":"K. Strasburger","doi":"10.1103/physreva.102.052806","DOIUrl":"https://doi.org/10.1103/physreva.102.052806","url":null,"abstract":"The energies of the lowest $^2P_u$, $^4P_g$ and $2D_g$ states of the boron atom are calculated with $mu$hartree accuracy, in the basis of symmetrized, explicitly correlated Gaussian lobe functions. Finite nuclear mass and scalar relativistic corrections are taken into account. This study contributes to the problem of the energy differences between doublet and quartet states of boron, which have not been measured to date. It is found that the $^2P_urightarrow^4P_g$ excitation energy, recommended in the Atomic Spectra Database, appears underestimated by more than 300~cm$^{-1}$.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"2 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-09-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80897731","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-09-02DOI: 10.1063/1674-0068/cjcp2107119
Zhendong Li
In (relativistic) electronic structure methods, the quaternion matrix eigenvalue problem and the linear response (Bethe-Salpeter) eigenvalue problem for excitation energies are two frequently encountered structured eigenvalue problems. While the former problem was thoroughly studied, the later problem in its most general form, namely, the complex case without assuming the positive definiteness of the electronic Hessian, is not fully understood. In view of their very similar mathematical structures, we examined these two problems from a unified point of view. We showed that the identification of Lie group structures for their eigenvectors provides a framework to design diagonalization algorithms as well as numerical optimizations techniques on the corresponding manifolds. By using the same reduction algorithm for the quaternion matrix eigenvalue problem, we provided a necessary and sufficient condition to characterize the different scenarios, where the eigenvalues of the original linear response eigenvalue problem are real, purely imaginary, or complex. The result can be viewed as a natural generalization of the well-known condition for the real matrix case.
{"title":"Structured eigenvalue problems in electronic structure methods from a unified perspective","authors":"Zhendong Li","doi":"10.1063/1674-0068/cjcp2107119","DOIUrl":"https://doi.org/10.1063/1674-0068/cjcp2107119","url":null,"abstract":"In (relativistic) electronic structure methods, the quaternion matrix eigenvalue problem and the linear response (Bethe-Salpeter) eigenvalue problem for excitation energies are two frequently encountered structured eigenvalue problems. While the former problem was thoroughly studied, the later problem in its most general form, namely, the complex case without assuming the positive definiteness of the electronic Hessian, is not fully understood. In view of their very similar mathematical structures, we examined these two problems from a unified point of view. We showed that the identification of Lie group structures for their eigenvectors provides a framework to design diagonalization algorithms as well as numerical optimizations techniques on the corresponding manifolds. By using the same reduction algorithm for the quaternion matrix eigenvalue problem, we provided a necessary and sufficient condition to characterize the different scenarios, where the eigenvalues of the original linear response eigenvalue problem are real, purely imaginary, or complex. The result can be viewed as a natural generalization of the well-known condition for the real matrix case.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"7 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-09-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84209395","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-31DOI: 10.1021/acscatal.0c03773.s001
D. Borodin, K. Golibrzuch, M. Schwarzer, Jan Fingerhut, Georgios Skoulatakis, D. Schwarzer, T. Seelemann, T. Kitsopoulos, A. Wodtke
Up to now, the methods available for measuring the rate constants of reactions taking place on heterogeneous catalysts require that the catalyst be stable over long measurement times. But catalyst are often non-stationary, they may become activated under reaction conditions or become poisoned through use. It is therefore desirable to develop methods with high data acquisition rates for kinetics, so that transient rates can be measured on non-stationary catalysts. 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 molecular beam pulses incident at the surface and the product formation rate is observed by a sequence of laser pulses at a high repetition rate. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the method using a 10 Hz pulsed CO molecular beam pulse train to initiate CO desorption from Pd(332) - desorbing CO is detected every millisecond by non-resonant multiphoton ionization using a 1-kHz Ti:Sapphire laser. 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 also apply this method to CO oxidation on Pd(332) - we record kinetic traces of CO$_2$ formation while a CO beam titrates oxygen atoms from an O-saturated surface. This provides the reaction rate as a function of O-coverage in a single experiment. We exploit this to produce controlled yet inhomogeneously mixed reactant samples for measurements of reaction rates under diffusion-controlled conditions.
{"title":"Measuring transient reaction rates from non-stationary catalysts","authors":"D. Borodin, K. Golibrzuch, M. Schwarzer, Jan Fingerhut, Georgios Skoulatakis, D. Schwarzer, T. Seelemann, T. Kitsopoulos, A. Wodtke","doi":"10.1021/acscatal.0c03773.s001","DOIUrl":"https://doi.org/10.1021/acscatal.0c03773.s001","url":null,"abstract":"Up to now, the methods available for measuring the rate constants of reactions taking place on heterogeneous catalysts require that the catalyst be stable over long measurement times. But catalyst are often non-stationary, they may become activated under reaction conditions or become poisoned through use. It is therefore desirable to develop methods with high data acquisition rates for kinetics, so that transient rates can be measured on non-stationary catalysts. 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 molecular beam pulses incident at the surface and the product formation rate is observed by a sequence of laser pulses at a high repetition rate. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the method using a 10 Hz pulsed CO molecular beam pulse train to initiate CO desorption from Pd(332) - desorbing CO is detected every millisecond by non-resonant multiphoton ionization using a 1-kHz Ti:Sapphire laser. 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 also apply this method to CO oxidation on Pd(332) - we record kinetic traces of CO$_2$ formation while a CO beam titrates oxygen atoms from an O-saturated surface. This provides the reaction rate as a function of O-coverage in a single experiment. We exploit this to produce controlled yet inhomogeneously mixed reactant samples for measurements of reaction rates under diffusion-controlled conditions.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"45 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81872127","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-06DOI: 10.1103/physreva.102.053115
V. Hanus, S. Kangaparambil, S. Larimian, Martin Dorner-Kirchner, Xinhua Xie, A. Baltuvska, Markus Kitzler-Zeiler
We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of N$_2$-N$_2$ and N$_2$-O$_2$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighbouring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.
{"title":"Exploring photoelectron angular distributions emitted from molecular dimers by two delayed intense laser pulses","authors":"V. Hanus, S. Kangaparambil, S. Larimian, Martin Dorner-Kirchner, Xinhua Xie, A. Baltuvska, Markus Kitzler-Zeiler","doi":"10.1103/physreva.102.053115","DOIUrl":"https://doi.org/10.1103/physreva.102.053115","url":null,"abstract":"We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of N$_2$-N$_2$ and N$_2$-O$_2$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighbouring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.","PeriodicalId":8439,"journal":{"name":"arXiv: Chemical Physics","volume":"355 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-08-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77752675","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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