C. Leach, A. Tsekouras, P. Vaccaro, R. Zare, Daqing Zhao
By choosing a kinematically constrained bimolecular reaction of the type H + H′L → HH′+ L, where H and H′ are heavy atoms and L is a light atom, the orbital angular momentum, L, of the reagents appears almost exclusively as HH′ product rotation, J. It follows that the magnitude of J is given by |L|=µvrelb, where µ is the reduced mass of the reagent collision partners, vrel is their relative velocity and b is the impact parameter, i.e. the distance of closest approach if the reagents moved in undeflected straight-line paths. Hence, by measuring both the product rotational distribution and the relative velocity distribution of the reagents, it is possible to deduce the range of impact parameters resulting in specific vibrational levels of the HH′ product. A prototype system is Ba + HI → BaI + H. Experimental results for BaI (v= 0) are presented for this kinematically constrained reaction showing that the range of b is restricted, peaking at or near the energetic cut-off.
{"title":"Indirect information on reactive transition states from conservation of angular momentum","authors":"C. Leach, A. Tsekouras, P. Vaccaro, R. Zare, Daqing Zhao","doi":"10.1039/DC9919100183","DOIUrl":"https://doi.org/10.1039/DC9919100183","url":null,"abstract":"By choosing a kinematically constrained bimolecular reaction of the type H + H′L → HH′+ L, where H and H′ are heavy atoms and L is a light atom, the orbital angular momentum, L, of the reagents appears almost exclusively as HH′ product rotation, J. It follows that the magnitude of J is given by |L|=µvrelb, where µ is the reduced mass of the reagent collision partners, vrel is their relative velocity and b is the impact parameter, i.e. the distance of closest approach if the reagents moved in undeflected straight-line paths. Hence, by measuring both the product rotational distribution and the relative velocity distribution of the reagents, it is possible to deduce the range of impact parameters resulting in specific vibrational levels of the HH′ product. A prototype system is Ba + HI → BaI + H. Experimental results for BaI (v= 0) are presented for this kinematically constrained reaction showing that the range of b is restricted, peaking at or near the energetic cut-off.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"15 1","pages":"183-190"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82340447","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}
L. Schnieder, K. Seekamp-Rahn, F. Liedeker, H. Steuwe, K. Welge
Despite its fundamental importance as the prototypical bimolecular reaction, the hydrogen exchange reaction still remains a challenging and open problem, both experimentally and theoretically. Theory has now developed to a stage much superior to that of experiment. Nowhere is this more true than, for example, in the determination of differential scattering cross-sections, state-to-state specific with respect to the vibrational and rotational degrees of freedom of the molecular products. In this paper we describe a new experimental approach to such measurements, and present first results from crossed-beam studies of the H + D2 reaction (at relative translational energies of 1.29 and 0.54 eV) using the novel technique of hydrogen Rydberg atom time-of-flight spectroscopy to monitor the velocity and angular distributions of the D atom product.
{"title":"Hydrogen exchange reaction H + D2 in crossed beams","authors":"L. Schnieder, K. Seekamp-Rahn, F. Liedeker, H. Steuwe, K. Welge","doi":"10.1039/DC9919100259","DOIUrl":"https://doi.org/10.1039/DC9919100259","url":null,"abstract":"Despite its fundamental importance as the prototypical bimolecular reaction, the hydrogen exchange reaction still remains a challenging and open problem, both experimentally and theoretically. Theory has now developed to a stage much superior to that of experiment. Nowhere is this more true than, for example, in the determination of differential scattering cross-sections, state-to-state specific with respect to the vibrational and rotational degrees of freedom of the molecular products. In this paper we describe a new experimental approach to such measurements, and present first results from crossed-beam studies of the H + D2 reaction (at relative translational energies of 1.29 and 0.54 eV) using the novel technique of hydrogen Rydberg atom time-of-flight spectroscopy to monitor the velocity and angular distributions of the D atom product.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"39 1","pages":"259-269"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84436743","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 have performed calculations for H2 molecules being scattered from a metal surface. The emphasis of the work is aimed at understanding the role of rotational energy on the dissociative adsorption reaction. The time-dependent wavepacket formalism has been employed on a vibrationally adiabatic PES which explicitly contains two rotational degrees of freedom and one translational. The PES is activated (ca. 0.25–1.5 eV) and the value of the barrier depends strongly on the angle that the molecular bond axis makes with the surface. This PES enables a scattering event to be modelled which corresponds to a normally incident beam of hydrogen in the ground vibrational state and the J, mJ rotational state. We have found that the translational energy dependence of the dissociation probability of para- and normal hydrogen is approximately the same. This reflects the fact that the quantum of rotational energy is rather small in comparison to the activation barrier height. More interestingly it has been found that the form chosen for the PES gives rise to effective angular discrimination whereby molecules having only particular orientations will dissociate. This, in turn, gives rise to a characteristic rotational distribution which may be used to explore the symmetry of the dissociative transition state.
{"title":"Probing the transition state in dissociative adsorption","authors":"S. Holloway, X. Chang","doi":"10.1039/DC9919100425","DOIUrl":"https://doi.org/10.1039/DC9919100425","url":null,"abstract":"We have performed calculations for H2 molecules being scattered from a metal surface. The emphasis of the work is aimed at understanding the role of rotational energy on the dissociative adsorption reaction. The time-dependent wavepacket formalism has been employed on a vibrationally adiabatic PES which explicitly contains two rotational degrees of freedom and one translational. The PES is activated (ca. 0.25–1.5 eV) and the value of the barrier depends strongly on the angle that the molecular bond axis makes with the surface. This PES enables a scattering event to be modelled which corresponds to a normally incident beam of hydrogen in the ground vibrational state and the J, mJ rotational state. We have found that the translational energy dependence of the dissociation probability of para- and normal hydrogen is approximately the same. This reflects the fact that the quantum of rotational energy is rather small in comparison to the activation barrier height. More interestingly it has been found that the form chosen for the PES gives rise to effective angular discrimination whereby molecules having only particular orientations will dissociate. This, in turn, gives rise to a characteristic rotational distribution which may be used to explore the symmetry of the dissociative transition state.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"14 1","pages":"425-436"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81047002","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}
A model is presented to explain quantitatively the Λ doublet propensities observed by King, Stephenson, Foy and Casassa in the infrared multiphoton and vibrational overtone decomposition of HN3 to yield NH(a 1Δ) fragments. This Franck–Condon-type model involves the projection of the electronic vibrational wavefunction of the HN⋯NN system at the transition state onto the electronic-rotational wavefunction of the free NH rotor. Ab initio calculations are used to estimate the angular extent of the torsional degrees of freedom which subsequently become converted to rotations of the NH fragment. This model deals explicitly with the two-electron character of the 1Δ state. The experimentally observed ratio of Δ(A′) to Δ(A″)Λ doublet populations vs. the rotational quantum number J of the NH fragment, as well as the observed average rotational excitation of the NH products, are well predicted. In contrast, the observed v, J correlation is not as well predicted. The model presented here represents an advance over those previously developed to explain Λ doublet propensities in that it is based on properties of the specific system under consideration.
{"title":"Potential-energy surface control of the NH product state distribution in the decomposition reaction HN3(X 1A′)→ NH(a 1Δ)+ N2(X 1Σ+g)","authors":"M. Alexander, P. Dagdigian, H. Werner","doi":"10.1039/DC9919100319","DOIUrl":"https://doi.org/10.1039/DC9919100319","url":null,"abstract":"A model is presented to explain quantitatively the Λ doublet propensities observed by King, Stephenson, Foy and Casassa in the infrared multiphoton and vibrational overtone decomposition of HN3 to yield NH(a 1Δ) fragments. This Franck–Condon-type model involves the projection of the electronic vibrational wavefunction of the HN⋯NN system at the transition state onto the electronic-rotational wavefunction of the free NH rotor. Ab initio calculations are used to estimate the angular extent of the torsional degrees of freedom which subsequently become converted to rotations of the NH fragment. This model deals explicitly with the two-electron character of the 1Δ state. The experimentally observed ratio of Δ(A′) to Δ(A″)Λ doublet populations vs. the rotational quantum number J of the NH fragment, as well as the observed average rotational excitation of the NH products, are well predicted. In contrast, the observed v, J correlation is not as well predicted. The model presented here represents an advance over those previously developed to explain Λ doublet propensities in that it is based on properties of the specific system under consideration.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"35 1","pages":"319-335"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83093533","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}
This article presents the progress made in probing femtosecond transition–state dynamics of elementary reactions. Experiments demonstrating the dynamics in systems characterized by a transition region and by a saddle-point transition state are reported, and comparison with theory is made.
{"title":"Femtosecond transition-state dynamics","authors":"A. Zewail","doi":"10.1039/DC9919100207","DOIUrl":"https://doi.org/10.1039/DC9919100207","url":null,"abstract":"This article presents the progress made in probing femtosecond transition–state dynamics of elementary reactions. Experiments demonstrating the dynamics in systems characterized by a transition region and by a saddle-point transition state are reported, and comparison with theory is made.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"237 1","pages":"207-237"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75894667","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}
Two, quite different, experimental studies have been carried out on the reaction between OH and CO at low total pressures. In the first, rate constants have been determined at 82 and 106 K. At the lower temperature, measurements were made at 2 and 5 Torr total pressure, yielding k=(1.0 ± 0.12)× 10–13 cm3 molecule–1 s–1 and (0.91 ± 0.1)× 10–13 cm3 molecule–1 s–1, respectively. At 106 K and 4 Torr, k=(0.98 ± 0.08)× 10–13 cm3 molecule–1 s–1. Theoretical considerations show that the reaction must be in its low-pressure limit, yielding H + CO2, and that the vibrational ground-state adiabatic barrier to formation of HOCO must be <200 cm–1, significantly lower than estimated previously.In the second series of experiments, a tunable diode laser has been used to observe transient absorptions on transitions in the ν3 infrared bands of the CO2 product of the reaction, when it is initiated by flash photolysis at room temperature. There is no excitation of the ν3 mode, and the overall vibrational distribution corresponds to an averaged vibrational energy yield of only 6%. It is concluded that energy is released largely as repulsion following passage through a transition state in which the OCO angle is ca. 171° and the O—C, C—O bond distances are very similar to those in isolated CO2.
{"title":"Energy and structure of the transition states in the reaction OH + CO → H + CO2","authors":"M. Frost, P. Sharkey, Ian W. M. Smith","doi":"10.1039/DC9919100305","DOIUrl":"https://doi.org/10.1039/DC9919100305","url":null,"abstract":"Two, quite different, experimental studies have been carried out on the reaction between OH and CO at low total pressures. In the first, rate constants have been determined at 82 and 106 K. At the lower temperature, measurements were made at 2 and 5 Torr total pressure, yielding k=(1.0 ± 0.12)× 10–13 cm3 molecule–1 s–1 and (0.91 ± 0.1)× 10–13 cm3 molecule–1 s–1, respectively. At 106 K and 4 Torr, k=(0.98 ± 0.08)× 10–13 cm3 molecule–1 s–1. Theoretical considerations show that the reaction must be in its low-pressure limit, yielding H + CO2, and that the vibrational ground-state adiabatic barrier to formation of HOCO must be <200 cm–1, significantly lower than estimated previously.In the second series of experiments, a tunable diode laser has been used to observe transient absorptions on transitions in the ν3 infrared bands of the CO2 product of the reaction, when it is initiated by flash photolysis at room temperature. There is no excitation of the ν3 mode, and the overall vibrational distribution corresponds to an averaged vibrational energy yield of only 6%. It is concluded that energy is released largely as repulsion following passage through a transition state in which the OCO angle is ca. 171° and the O—C, C—O bond distances are very similar to those in isolated CO2.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"20 1","pages":"305-317"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"72998491","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}
The technique of two-colour sub-Doppler circular dichroism (SDCD) is described and a four-vector correlation molecular dynamics experiment is demonstrated. Results are given for elastic and inelastic collisional processes between Li2 and the rare gases Ar and Xe. The technique is of wide applicability and could be used to study the dynamics of elastic, inealstic and reactive collisions.
{"title":"Two-colour sub-Doppler circular dichroism: a four-vector correlation molecular dynamics experiment for inelastic and reactive collisions","authors":"T. Collins, A. J. McCaffery, M. Wynn","doi":"10.1039/DC9919100091","DOIUrl":"https://doi.org/10.1039/DC9919100091","url":null,"abstract":"The technique of two-colour sub-Doppler circular dichroism (SDCD) is described and a four-vector correlation molecular dynamics experiment is demonstrated. Results are given for elastic and inelastic collisional processes between Li2 and the rare gases Ar and Xe. The technique is of wide applicability and could be used to study the dynamics of elastic, inealstic and reactive collisions.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"68 1","pages":"91-96"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81112691","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}
{"title":"Professor R. B. Bernstein: an appreciation","authors":"A. Buckingham","doi":"10.1039/DC9919100001","DOIUrl":"https://doi.org/10.1039/DC9919100001","url":null,"abstract":"","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"63 1","pages":"1-4"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81087051","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}
S. Dixon-Warren, E. Jensen, J. Polanyi, G. Xu, Shihe Yang, H. Zeng
In part 1 of this paper we report direct evidence for the process of charge-transfer photodissociation (CT/PDIS), namely the formation of negatively charged photofragments from a halide adsorbed on a metal substrate. The system was CCl4/Ag(111). The observation was that 1 µJ cm–2 of laser radiation with photon energies in excess of a threshold hν0≈ 5 eV, approximately equal to the work function of the substrate, gave rise to a readily detectable flux of Cl– leaving the surface, attributable to a harpooning process CCl4/Ag(111)+hν→ CCl–4/Ag(111)→ Cl–(g)+ CCl3/Ag(111). In part 2 the cross-section, σ(CT/PDIS), is reported over the range 193–700 nm incident wavelength for charge-transfer photodissociation of HI and HBr adsorbed on 2–3 monolayers of K deposited on Ag(110) or LiF(001). The observed shift in threshold photon energy, Δhν0, to higher energy in going from HI(ads) to HBr(ads) was in the direction expected for dissociative electron attachment (DA). There was evidence of a significant tunnelling regime for CT/PDIS of HI/Ag but not for HBr/Ag, a finding that can be understood in terms of DA.
{"title":"Photochemistry of adsorbed molecules. Part 10.—Harpooning a fixed target: charge transfer from Ag or K substrates to halide adsorbates","authors":"S. Dixon-Warren, E. Jensen, J. Polanyi, G. Xu, Shihe Yang, H. Zeng","doi":"10.1039/DC9919100451","DOIUrl":"https://doi.org/10.1039/DC9919100451","url":null,"abstract":"In part 1 of this paper we report direct evidence for the process of charge-transfer photodissociation (CT/PDIS), namely the formation of negatively charged photofragments from a halide adsorbed on a metal substrate. The system was CCl4/Ag(111). The observation was that 1 µJ cm–2 of laser radiation with photon energies in excess of a threshold hν0≈ 5 eV, approximately equal to the work function of the substrate, gave rise to a readily detectable flux of Cl– leaving the surface, attributable to a harpooning process CCl4/Ag(111)+hν→ CCl–4/Ag(111)→ Cl–(g)+ CCl3/Ag(111). In part 2 the cross-section, σ(CT/PDIS), is reported over the range 193–700 nm incident wavelength for charge-transfer photodissociation of HI and HBr adsorbed on 2–3 monolayers of K deposited on Ag(110) or LiF(001). The observed shift in threshold photon energy, Δhν0, to higher energy in going from HI(ads) to HBr(ads) was in the direction expected for dissociative electron attachment (DA). There was evidence of a significant tunnelling regime for CT/PDIS of HI/Ag but not for HBr/Ag, a finding that can be understood in terms of DA.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"18 1","pages":"451-463"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82383566","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}
W. Stwalley, P. Kleiber, K. Sando, A. Lyyra, Li Li, S. Ananthamurthy, S. Bililign, He Wang, J. X. Wang, V. Zafiropulos
Atomic line broadening has traditionally emphasized resonance broadening by like atoms and ‘inert perturber’ broadening by rare gases and hydrogen. Such methods are ideal for qualitative and quantitative understanding of reactive transition states, including especially non-adiabatic interactions and polarization, orientation and alignment effects. Experiments at Iowa include a variety of such studies with alkali-metal and alkaline-earth metal atoms, e.g. diatomic photodissociation (including state-selected photodissociation through quasibound resonances) and reactive transition-state absorption. In each case theoretical information is available concerning the relevant potential-energy curves (or surfaces) and their couplings, and there are approximate dynamical theories (e.g. orbital locking) to be tested. A summary of recent experimental results and theoretical comparisons emphasizing diatomic photodissociation and its relation to transition state absorption will be presented.
{"title":"Metal–metal and metal–hydrogen reactive transition states","authors":"W. Stwalley, P. Kleiber, K. Sando, A. Lyyra, Li Li, S. Ananthamurthy, S. Bililign, He Wang, J. X. Wang, V. Zafiropulos","doi":"10.1039/DC9919100097","DOIUrl":"https://doi.org/10.1039/DC9919100097","url":null,"abstract":"Atomic line broadening has traditionally emphasized resonance broadening by like atoms and ‘inert perturber’ broadening by rare gases and hydrogen. Such methods are ideal for qualitative and quantitative understanding of reactive transition states, including especially non-adiabatic interactions and polarization, orientation and alignment effects. Experiments at Iowa include a variety of such studies with alkali-metal and alkaline-earth metal atoms, e.g. diatomic photodissociation (including state-selected photodissociation through quasibound resonances) and reactive transition-state absorption. In each case theoretical information is available concerning the relevant potential-energy curves (or surfaces) and their couplings, and there are approximate dynamical theories (e.g. orbital locking) to be tested. A summary of recent experimental results and theoretical comparisons emphasizing diatomic photodissociation and its relation to transition state absorption will be presented.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"131 1","pages":"97-110"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91370019","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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