State-to-state reaction dynamics of H + D2 and the alignment and orientation of hydrogen molecules [electronic resource]
Keywords: Chemistry

This thesis details part of the ongoing experimental effort on the investigation of the dynamics of the simplest neutral bimolecular reaction system: H + D2 hydrogen exchange. A combination of molecular beam and laser-based spectroscopic techniques have been used to produce H + D2 collisions under single collision conditions, and the three-dimensional velocity distribution of state-selected HD(v', j') reaction products is captured by ion imaging. HBr and D2 are pre-mixed and co-expanded into high vacuum through a pulsed nozzle. The co-expansion produces rotationally cooled reagents with very little relative translational energy between them. The molecular beam is collimated by a skimmer before being intersected with a narrow-linewidth tunable ultraviolet laser pulse which cleaves the HBr bond, producing nearly monoenergetic H atoms which can then react with D2 molecules. Nascent HD(v', j') is state-selectively ionized via resonance-enhanced multiphoton ionization (REMPI) using radiation from another narrow-linewidth tunable ultraviolet laser pulse. This process takes place within the extraction region of a linear time-of-flight mass spectrometer fitted with an imaging detector. This configuration allows the three-dimensional velocity of individual HD(v', j') products to be recorded. Velocity distributions are analyzed and converted into center-of-mass frame scattering angle distributions or differential cross sections (DCS) using photoloc. Using these methods state-to-state differential cross sections have been measured for H + D2(v'=0, j'=0,1,2) → HD(v'=2, j'=0,3,6,9) + D for collision energies of Ec = 1.25, 1.61, and 1.97 eV. The DCSs are compared to those calculated by Foudhil Bouakline and Stuart Althorpe after being "blurred" according to our experimental conditions. The measured DCSs are in most cases in good agreement with the calculated ones, and show the expected behavior for this reaction system which takes place via a direct recoil mechanism through a collinear transition state geometry. For a given collision energy, as the HD rotational state increases the DCSs shift from being strongly peaked in the backward scattering region to being peaked toward more sideward angles. For a given HD rotational state, as the collision energy increases the DCSs narrow and become peaked at slightly larger angles. The HD(v'=2, j'=0) state produced from reactive collisions at Ec = 1.25 eV displays unique scattering behavior in that its DCS is bimodal. This suggests that there are two competing mechanisms available for this product at this collision energy. The first is the same direct recoil mechanism responsible for product formation of all other rotational state/collision energy combinations investigated. The second is an indirect mechanism opened by a barrier resonance in which the transition state is able to rotate a fraction of a cycle before breaking apart into products. The dynamics observed here are overall similar to that found in related H + D2(v'=0, j'=0,1,2) → HD(v'=1,3, j') reactive scattering studies performed by our group. Additionally, in a series of adjacent experiments, stimulated Raman pumping (SRP) with polarized light was used to prepare highly aligned and oriented samples of vibrationally-rotationally excited hydrogen molecules in a molecular beam expansion under collision-free conditions. Of particular significance, the (1,0) S(0) SRP line was used to prepare samples of H2(v=1, j=2, M=0) and H2(v=1, j=2, M=2) using linear or circularly polarized light, respectively. After excitation the degree of alignment was monitored using resonance-enhanced multiphoton ionization also with polarized light. This rotational level is not subject to hyperfine depolarization and we find that the samples produced retain their initial degree of alignment or orientation for up to 8 [Mu]s which makes them quite suitable for serving as aligned targets in H + H2 scattering experiments. The same SRP scheme was also used to produce aligned samples of HD(v=1, j=2, M=0) and D2(v=1, j=2, M=0) as well, both of which undergo hyperfine depolarization, through the coupling of the molecules' rotational angular momentum to the nuclear spin angular momenta. The result is that the degree of alignment becomes an oscillatory function of time, which again can be observed using REMPI. We compare the measured time-dependent alignment with a theoretical calculation for HD(v=0, j=2, M=0) and D2(v=0, j=2, M=0) and find the agreement to be within our experimental error. Thus we conclude that the hyperfine constants do not strongly vary with the vibrational state.