VUV RESONANCE RADIATION TRAPPING IN AN EFFUSIVE HE BEAM

We have characterized experimentally the trapping of resonance radiati on in an effusive He beam produced by a needle injector. In order to t ake into account the influence of secondary effects, such as collision s, on the trapping process, the measurements were carried out by inves tigating the line intensity dependence on the atomic density of transi tions 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 o f the helium spectrum. The comparison between visible and vacuum ultra violet spectra allowed us to characterize the excitation process and t o 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 resul ts were also compared with the Holstein and Phelps approach describing the trapping process in terms of an effective radius, rho. Our result s show that in the case of an effusive beam, the effective radius is o f the order of the effusive beam radius, namely rho(b) = 0.70 +/- 0.04 mm.