  High-resolution mass spectra of helium droplets doped with C$_{60}$ and
formic acid (FA) are ionized by electrons. Positive ion mass spectra reveal
cluster ions [(C$_{60}$)$_p$FA$_n$]$^+$ together with their hydrogenated and
dehydrogenated counterparts. Also observed are ions containing one or more
water (W) molecules. The abundance distributions of these ions reveal several
interesting features: i) [(C$_{60}$)$_p$FA$_n$]$^+$ ions are more abundant than
hydrogenated [(C$_{60}$)$_p$FA$_n$H]$^+$ ions even though the opposite is true
in the absence of C$_{60}$ (i.e. if $p$ = 0); ii) although [C$_{60}$FA]$^+$ is
the most abundant ion containing a single C$_{60}$, multiple C$_{60}$ suppress
the [(C$_{60}$)$_p$FA]$^+$ signal; iii) an enhanced stability of
[(C$_{60}$)$_p$W$_1$FA$_5$H]$^+$ and [(C$_{60}$)$_p$W$_2$FA$_6$H]$^+$ mirrors
that of [W$_1$FA$_5$H]$^+$ and [W$_2$FA$_6$H]$^+$, respectively. On the other
hand, the enhanced stability of [C$_{60}$FA$_6$H]$^+$ finds no parallel in the
stability pattern of [FA$_n$H]$^+$ or FA$_n$$^+$. Negative ion mass spectra
indicate a propensity for non-dissociated [(C$_{60}$)$_p$FA$_n$]$^-$ anions if
$p \geq 1$ which contrasts with the dominance of dehydrogenated [FA$_n$-H]$^-$
anions.
