  Hydrocarbon dust is one of the dominant components of interstellar dust,
which mainly consists of polycyclic aromatic hydrocarbons and aliphatic
hydrocarbons. While hydrocarbon dust is thought to be processed in interstellar
radiation fields or shocks, detailed processing mechanisms are not completely
understood yet. We investigate the processing of hydrocarbon dust by analyzing
the relation between the luminosities emitted by hydrocarbon dust and the total
infrared luminosities $(L_{\mathrm{IR}})$ for 138 star-forming galaxies at
redshift $z\ <\ 0.3$. Using near-infrared 2.5-5 $\mathrm{\mu m}$ spectra
obtained with AKARI, we derived the luminosities of the aromatic hydrocarbon
feature at 3.3 $\mathrm{\mu m}$ ($L_\mathrm{aromatic}$) and the aliphatic
hydrocarbon feature at 3.4-3.6 $\mathrm{\mu m}$ ($L_\mathrm{aliphatic}$). We
also derived $L_\mathrm{IR}$ and the radiation field strength by modeling the
spectral energy distributions of the 138 galaxies with AKARI, WISE and IRAS
photometry data. We find that galaxies with higher $L_\mathrm{IR}$ tend to
exhibit lower $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratios. Furthermore,
we find that there is an anti-correlation between
$L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratios and the radiation field
strength, and also that the galaxies with low
$L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratios are dominated by merger
galaxies. These results support that hydrocarbon dust is processed through
photodissociation in strong radiation fields and/or shocks during merging
processes of galaxies; the $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ ratio is
likely to decrease in such harsh interstellar conditions since the aliphatic
bonds are known to be chemically weaker than the aromatic bonds.
