Differential cross sections have been measured for He(2 3S)+Ne at kinetic energies between 28 and 370meV. For energies above 90meV the elastic cross sections show Sttickelberg oscillations from curve crossings, which lead to the energy exchange process: He(23 S) +Ne~He(t 1S) + Ne(2p 5 4s, 3 d, 4p). Differential cross sections for this inelastic process could be measured above 200meV..A fit to the data gives the potentials for He(2 3S)+Ne and, less accurately, for He +Ne*. These results offer a simple explanation, why the exothermic pumping process of the infrared lines of the HeNe laser has a threshold of about 80 meV and a small cross section.
l. lntroductionThree different types of electronic excitation transfer can be distinguished for collisions of metastable helium atoms (He*) with groundstate atoms or molecules: I. For He* +He collisions the excitation transfer is due to the inherent symmetry of the problem and can be treated as an elastic process [1][2][3][4]. 2. For He*+Ar, Kr, Xe and any molecular collision Penning ionization can occur besides elastic scattering, because the excitation energy of the metastable helium atom, 19.8 eV for He(2 3S), is higher than the ionization energy of all atoms and molecules save Ne [5-73. 3. For He* +Ne collisions an ionizing collision is not possible at our kinetic energies due to the high ionization energy of the Ne atom (21.6eV). But transfer of the excitation energy from He* to a highly excited state of Ne can occur. This energy exchange plays a large role in HeNe gas discharges, and it is the dominant pumping process of the HeNe laser [8]. It has been shown recently by Miller and Morgner [9] that Penning ionization (excitation to a continuum) and energy transfer to highly excited states (excitation to a quasicontinuum) is not only conceptually very similar, but can be treated theoretically in a uniform manner. The principle of the HeNe laser is shown schematically in Fig. 1. He atoms are excited by electron collisions to their metastable states and transfer their energy to the excited Ne states, which are the upper laser levels. Some of the laser transitions are indicated in the figure. For isolated neon atoms roughly half of them end up in one of the two metastable states of the 2p53s configuration. The lifetimes of the metastable states 3P 0 and 3P 2 are longer than 0.8s [10 I. In a beam experiment these long living excited atoms are easily detected and can be used as a probe in the study of the energy transfer processes. Because of its importance this reaction was studied in many experiments [11][12][13][14][15]. All these experiments used discharges or afterglows which have a wide spread in collision energy, while we performed a crossed molecular beam experiment with good angle and velocity resolution. The technique of molecular beams has been improved considerably during the last years and is now an excellent tool to study collision processes. The use of nozzle beams gives the opportunity for beams in the thermal energy range with good velocity resolution and ...
