Recombination lines observed toward the pPN CRL 618 (histogram) and synthetic line profiles (pink) from our model in Table 4 (Sect. 5.1). The bottom and top X-axis represent V_LSR (km s^-1) and frequency (MHz). The frequency of He and C α-transitions, some of them detected, are also indicated. The many additional features observed in the spectrum, with emission and absorption profile components, are molecular transitions produced in the C-rich molecular envelope of CRL 618. Some of the mm-RRLs observed are partially blended with molecular lines.