Track-depth-resolved dynamics of helium excimers along 4-MeV/amu N-ion tracks in near-liquid and liquid helium

A track scope, composed of an imaging quartz fiber and a position-sensitive photon counter, was developed. Luminescence spectra, their specific intensities dL/dx, and efficiencies dL/dE were measured as functions of the helium density and of depth along the N- and He-ion tracks in dense gas and liquid helium. Observed luminescences are limited to those due to Rydberg states of excimers, ${}_{}{}^3{}_{}{}^{}$d, ${}_{}{}^3{}_{}{}^{}$D, ${}_{}{}^3{}_{}{}^{}$H, and ${}_{}{}^3{}_{}{}^{}$J, according to Herzberg’s notation. It is concluded that they are produced only by the bimolecular reaction of the lowest triplet excimer ${}_{}{}^3{}_{}{}^{}$a and that the excess energy of this reaction must be removed by the third body. Therefore, with increasing density of excitation, dL/dx and dL/dE are enhanced, but in an extreme case near the track termination, dL/dx is suppressed because the three-body reaction canno t find the third body except for the excimer ${}_{}{}^3{}_{}{}^{}$a. The appearance of an additional peak of dL/dE near the N-ion-track termination suggests the importance of additional processes such as direct excitation and charge exchange. The drastic suppression of excimer luminescences with increasing helium density is explained by instabilities of helium-excimer bubbles.