Detailed H(n=2) density measurements in a magnetized hydrogen plasma jet

We present detailed spatial density measurements of H(n = 2), i.e. the first excited state of atomic hydrogen, in a hydrogen plasma expansion that is weakly magnetized. At a specific distance from the source of the expansion a sharp transition from a red light emitting plasma (dominated by H-alpha emission) to a blue light emitting plasma (dominated by H-beta and H-gamma emission) occurs. Molecular processes such as dissociative recombination and processes with negative ions are suspected to be key in the understanding of the distinct emissions observed in the two different plasma regions. The relevance of the presented work is to underline these molecular processes in atomic regimes of hydrogen-containing plasmas. The first excited state density, n = 2, is determined with tunable diode laser absorption spectroscopy on the Balmer-alpha transition to investigate how important molecular processes such as dissociative recombination are in the plasma. The density of n = 2 is 1 x 10(17) m(-3) close to the plasma source and decreases gradually along the plasma column to 10(14) m(-3) at 20 cm from the plasma source exit. The presented results show that the theoretical possibility to generate a stable hydrogen laser in the visible light, i.e. population inversion of n = 3 with respect to n = 2, is not obtained due to the population of n = 2 by dissociative recombination of H-3(+) ions. The presence of molecular processes in the plasma is further evidenced with the use of a collisional radiative model.