The Production and Characterization of an Intense Hyperthermal Beam of H₃ Molecules and of H Atoms

Citation

Garvey, James F. (1985) The Production and Characterization of an Intense Hyperthermal Beam of H₃ Molecules and of H Atoms. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/r4n1-8762. https://resolver.caltech.edu/CaltechTHESIS:02132012-111043915

Abstract

The simplest bimolecular reaction involving neutral reagents is the reaction

H + H₂ → H₂ + H.

This system, because of its simplicity and fundamental significance in chemical dynamics, has been the subject of extensive accurate ab initio theoretical quantum mechanical calculations and the results obtained have been used to test approximate theories of reaction dynamics. However, this system has proved difficult to study experimentally due to its high activation barrier (~ .33 eV) and its small reactive cross section (~ 1 x 10^-16 cm². Because of these factors, state-to-state reaction dynamics have only become feasible recently.

In order to study this reaction in a crossed molecular beam experiment, a means of generating an intense beam of H atoms is essential. This thesis documents the design and operation of a hydrogen arc discharge for generating such a beam. The method consists of using a high power arc discharge to create a high temperature (~ 12,000 K) plasma in which H₂ molecules can be dissociated into atoms. By using an arc source of the type developed by Kunth, a stable, intense, hyperthermal H atom beam has been successfully produced. Section 2 of this thesis discusses the design and operation of the apparatus and Section 3 of the thesis discusses the characterization of the hydrogen atoms within the beam. The laboratory energy distribution function of these atoms was determined approximately, and spans the range from 0.5 eV to about 12 eV. The total intensity of this beam is of the order of ~10²² atoms/sterad/sec. With such an intense and energetic beam, a wealth of chemical dynamical experiments may now become feasible.

In the course of developing this intense beam of hydrogen atoms, the source was found to also produce metastable H₃ molecules. This is the first direct, unambiguous observation of such a specie in a molecular beam, and is discussed in Section 4. Translational energy analysis of this H₃ molecule indicates an energy distribution similar to that of the H atoms, suggesting that the lifetime of H₃ is of the order of 40 µsec or longer. We also observed emission spectra from the Rydberg states of H₃ identified previously by Herzberg. This is spectroscopic evidence for the presence of this neutral molecule in our beam. The only know state of H₃ capable of having the long lifetime observed is 2p ²A"₂, the second excited state of this species.

In addition, we detected this molecule by a variety of other independent techniques. It has long been known that alkali metal atoms with low ionization potential will ionize upon collision with a metal surface having a high work function. The 2p ²A"₂ state of H₃ has an IP of ~ 3.7 eV and is expected to behave in a similar way. As a result, surface ionization of metastable ₃ has been observed for for a variety of metals and is reported in this thesis. From that low IP one would also expect that the metastable H₃ should be photoionizable using an appropriate light source. In this thesis we report the generation of H⁺₃ through irradiation of the beam with the light from a high intensity mercury lamp. Lastly, since the metastable H₃ is in a Rydberg state, it would be expected to exhibit a large total scattering cross section due to the diffuse nature of the Rydberg orbital. Such cross sections were measured by the attenuation of the H₃ beam as it passed through a gas cell.

Section 5 describes the first observation of the electronic spectrum of WH. This emission spectrum was due to WH formed by the presence of the tungsten anode and cathode which are heated too close to its melting point by the arc discharge. Analysis of this spectrum has given rotational constants and bond distances for the electronic states of the mono-hydride involved in the observed transisiton. Due to relativistic terms in the potential, the theoretical calculation of WH has proved difficult to date. Recently a reliable potential for W has been generated such that our experimental bond distances will provide an important empirical check for any further theoretical calculations performed on this system. Likewise, the arc source may be employed in the future as a means of generating new metal hydride emission spectra.

Appendix A of the thesis details the design and construction of the inhomogeneous magnet to serve in the future as a velocity selector for the hyperthermal hydrogen beam in our crossed beam experiment. Using such a Stern-Gerlach magnet as a velocity selector, the dynamics of the H + H₂ reaction can be probed as a function of translational energy of the reactants. Appendix B grew out of an interesting Ch 227 project and will now become a theoretical paper. Exact quantum mechanical calculations of the collinear reaction Be + FH (ν = 0, 1) have been performed and the effects of reagent translational and vibrational excitation on reaction probabilities and product state distributions are examined. These quantum mechanical results are compared with those of quasi-classical trajectory calculations reported previously.

Thesis Files