Core extraction for measuring state-to-state differential cross sections of bimolecular reactions

Abstract

We describe a method we call core extraction for measuring the speed distributions of products from photoinitiated bimolecular reactions for the purpose of determining state-to-state differential cross sections. Core extraction is demonstrated by determination of the state-to-state differential cross section for the reaction ClϩCH 4 ͑ 3 ϭ1͒→HCl͑ϭ1, Jϭ1͒ϩCH 3. The method of core extraction measures three-dimensional projections of the velocity distribution using a time-of-flight mass spectrometer equipped with a mask to reject off-axis scattered products. This three-dimensional projection is then converted to a state-to-state differential cross section via simple transformations. Competition between instrumental resolution and signal in core extraction is discussed, and the behavior of our system is checked with simple velocity distributions that result from photodissociation of Cl 2. Core extraction is compared with other methods for the measurement of state-resolved differential cross sections.

Product rotational polarization: Stereodynamics of the reaction Cl(2P3/2)+CD4(v=0,j=0)→DCl(v′=0,j′=1)+CD3

“…Furthermore, the angular distribution describing the relative orientation of vectors k, kЈ, and JЈ in space may be termed the k-kЈ -JЈ distribution: the correlations which characterize it are some interesting double and triple vector correlations. 15 The experimental techniques of core extraction combined with resonance-enhanced multiphoton ionization ͑REMPI͒ 16 and polarized Doppler-resolved laser spectroscopy have expanded the laser ''pump-probe'' technique from the scalar to the vectorial arena to provide a key general method, which is sensitive to both the linear and rotational angular momentum distributions among the scattered products of reactive collisions. These techniques motivate the interest to measure features of the k-kЈ -JЈ distribution in probing the product state selective dynamics of photoninitiated bimolecular reactions and have been successfully exploited by a number of groups to determine product stateselective differential cross sections with high precision.…”

“…These techniques motivate the interest to measure features of the k-kЈ -JЈ distribution in probing the product state selective dynamics of photoninitiated bimolecular reactions and have been successfully exploited by a number of groups to determine product stateselective differential cross sections with high precision. 16,17 Measurements have been performed either under thermal conditions 18,19 or, with potentially higher collision energy and angular resolution, under jet cooled expansion conditions. [20][21][22] The enhanced reagent number densities achieved using these techniques, compared with those typically obtained in conventional crossed-molecular beam methods, have now enabled full state-to-state differential cross sections to be measured for the ClϩCH 4 reaction.…”

“…[20][21][22] The enhanced reagent number densities achieved using these techniques, compared with those typically obtained in conventional crossed-molecular beam methods, have now enabled full state-to-state differential cross sections to be measured for the ClϩCH 4 reaction. [16][17][23][24][25] Blais and Truhlar 26 and Han et al 27 have studied the alignment of the product rotational angular momentum as a function of scattering angle using quasiclassical-trajectory Legendre moment method and provided many new and valuable results. Recently, Zare and co-workers 28 have applied the experimental technique of core extraction combined with resonance-enhanced multiphoton ionization ͑REMPI͒ with a polarized laser beam to probe the angular-momentum alignment of the DCl product of the reaction of Cl with CD 4 .…”

Anatomy of an Elementary Chemical Reaction

“…The AB molecules are ionized by tuning the probe laser to excite a specific vibrational-rotational level (v′,J′) and the packet of ions continues to expand as a result of the initial velocities imparted to the AB molecules from the reaction. The whole ion packet is forced along a flight tube by means of an electric field, and a core is taken through the ion packet by use of a mask placed before the detector (28). The method of ionization, by absorption of 3 photons, is shown schematically in (b).…”

An investigation of polarized atomic photofragments using the ion imaging technique

“…The principle of this technique has been described in detail by Syage [16] and by Simpson et al [118]. In this method, only those photofragments with recoil velocity directed (nearly) parallel to the ion flight axis are detected, providing a near one-to-one correspondence between recoil velocity and ion flight time.…”