Hydrogen Atom Rydberg Tagging Time-of-Flight Crossed Molecular Beam Apparatus

This chapter focuses on hydrogen atom Rydberg tagging time-of-flight (HRTOF) crossed molecular beam apparatus, which was used for the research described in this thesis. In Sect. 2.1, I will introduce the general knowledge of molecular beam and crossed mol

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Hydrogen Atom Rydberg Tagging Time-of-Flight Crossed Molecular Beam Apparatus

Hydrogen is the most abundant and most important element in the universe. The hydrogen is the simplest atom and also the lightest and composed of a proton and an electron. Therefore, hydrogen atom is intensively studied. In nature, hydrogencontaining compounds are very important. Photochemical and chemical reactions of the hydrogen-containing compound are important subjects in the molecular reaction dynamics. For instance, photodissociation of water H2O ? hm ? OH ? H [8, 39], photodissociation of formaldehyde CH2O ? CHO ? H [34, 38], the simplest bimolecular chemical reaction in nature H ? H2 ? H2 ? H [1, 7, 11], and the famous reaction F ? H2 ? HF ? H [18]. This chapter focuses on hydrogen atom Rydberg tagging time-of-flight (HRTOF) crossed molecular beam apparatus, which was used for the research described in this thesis. In Sect. 2.1, I will introduce the general knowledge of molecular beam and crossed molecular beam and some history; HRTOF technique is described in Sect. 2.2; the vacuum system, detection and acquirement system as well as the resolution of the apparatus are described in detail in Sect. 2.3.

2.1 Molecular Beam and Crossed Molecular Beam Techniques 2.1.1 Molecular Beam Technique Duniyer carried out the first molecular beam experiment in 1911. By that time, high-speed vacuum pump had just been invented, so a good vacuum could be achieved to avoid collisions between molecular beam and background gases. Otto Stern was the first scientist who carried systematic research on molecular beams. He measured the Maxwell–Boltzmann distribution of the atom speed in the silver atom beam. Isidor Rabi created a new era of molecular beam, by carrying out magnetic resonance studies using molecular beam techniques [12]. Figure 2.1 shows the history of the development of molecular beam technique. Z. Ren, State-to-State Dynamical Research in the F?H2 Reaction System, Springer Theses, DOI: 10.1007/978-3-642-39756-1_2,  Springer-Verlag Berlin Heidelberg 2014

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2 Hydrogen Atom Rydberg Tagging Time-of-Flight

Fig. 2.1 Development history of molecular beam technique and other close techniques, cited from the Nobel Lecture of Herschbach in 1986 [12]

The earliest molecular beam source is a small hole at one side of a container containing reactants, where molecules flow effusively. The molecular mean free path of the beam source (ko) is much larger than the size (D) of the furnace orifice, so that the molecules outflow without collision. This is the so-called effusive source or diffusion source. This is the method Stern used to measure the silver atom speed at the very beginning. There is no collision between molecules, so the molecular speed distribution is the Boltzmann distribution at the corresponding reservoir temperature. Gas molecular flow will be generated if condensable reactants pass through heating container. The advantage of effusive source is that it is applicable to a lot of material, and its structure is simple and easy to