The effect of particle-hole excitations of the Fermi sea on the zero-phonon line (ZPL) of the optical spectrum of a chromophore in a He-3 droplet is considered. Due to the finite value of the density of states, the contribution of the low energy particle-hole excitations in the optical spectrum diverges at T -> 0. A non-perturbative theory is proposed that takes explicitly these excitations into account. It is shown that the ZPL takes a finite homogeneous width and a strong asymmetry, in agreement with the experimental observation. (C) 2012 Elsevier B. V. All rights reserved.