VUV resonance radiation trapping in an effusive He beam

We have characterized experimentally the trapping of resonance radiation in an effusive He beam produced by a needle injector. In order to take into account the influence of secondary effects, such as collisions, on the trapping process, the measurements were carried out by investigating the line intensity dependence on the atomic density of transitions 3(1)P --> 2(1)S (lambda = 501.6 nm), and 3(1)D --> 2(1)P (lambda = 667.8 nm) in the visible and 3(1)P --> 1(1)S (lambda = 53.7 nm) and 2(1)P --> 1(1)S (lambda = 58.4 nm) in the vacuum ultraviolet region of the helium spectrum. The comparison between visible and vacuum ultraviolet spectra allowed us to characterize the excitation process and to evaluate the contribution of collisional effects. We have calculated the escape factor, g, as a function of the atomic density and of the distance from the needle injector, i.e. along the beam path. The results were also compared with the Holstein and Phelps approach describing the trapping process in terms of an effective radius, rho. Our results show that in the case of an effusive beam, the effective radius is of the order of the effusive beam radius, namely rho(b) = 0.70 +/- 0.04 mm. RI Berardi, Vincenzo/H-4550-2011