The electronic excitation-function of the metastable 23S-level of helium1)

Edward S. Fry, W. Williams

Beams of metastable helium produced by electron bombardment generally contain atoms in both the 21S and 23S states. It is important in many experiments to distinguish between these states, or at least to know the population ratio. A beam of 23S and ground state helium atoms has been produced by electron bombardment and subsequent resonant quenching of 21S atoms with 21S−21P 2 μ radiation. This radiation mixes the 21S and 21P states, the latter decaying primarily to the 11S state by emission of 584 Å radiation. The quenching has been studied as a function of beam atom velocity. The results indicate that beams of metastable helium containing less than 1% 21S atoms can be produced.

E. H. S. Burhop

I. P. Bogdanova, V. D. Marusin

D. Vrinceanu, H. R. Sadeghpour

Cross sections and rate constants for total elastic, diffusion and transfer of metastability for collisions between He(11S)–He(23S) and He(11S)–He(23P) atoms over a wide range of energies and temperatures are presented. The rate constant for spin metastability excitation in He(11S)–He(23P) collision is several orders of magnitude larger than that for He(11S)–He(23S) collision at cold temperatures, due to attractive 3Πg potential interaction.

Гостев Валерий Анатольевич, D. V. Elakhovskii, A. D. Khakhaev

Deqian Yuan

Research in optically induced transitions between dierent atomic levels has a long history. For transitions between states driven by a coherent optical eld, the theoretical eciency could be ideally high as 100% but there could be many factors preventing this. In the three state helium atom excitation process, i.e. 23S! 33P! nL , the stimulated emission from intermediate state makes it hard to achieve ecient population transfer to the nal state through an intuitive excitation order. One technique to achieve a higher eciency is Stimulated Raman Adiabatic Passage (STIRAP) which is being studied and under research in our lab. Unlike traditional three level excitation processes, STIRAP actually uses a counter intuitive pulsed laser beams timing arrangement. The excitation objects are metastable helium atoms traveling in a vacuum system with a longitudinal velocity of 1070 m/s. We are using a 389 nm UV laser to connect the 23S and the 33P state and a frequency tunable 790 nm iii IR laser to connect the 33P state and the dierent Rydberg states. A third 1083 nm wavelength laser beam drives the 23S ! 23P transition to transversely separate the residual metastable atoms and the Rydberg atoms for eciency measurements. The data is taken by a stainless steel detector in the vacuum system. As the Rydberg atoms will get ionized by blackbody radiation under room temperature, we can utilize this for their detection. An ion detector sitting on the eld plate is capable to collect the ion signals of the Rydberg atoms for detection. So far the whole system has not been ready for data collection and measurement, so here we are using data and results from previous theses for discussions. The highest transition frequency that has ever been achieved in our lab is around 70% after corrections. | 144 pages