ratio can result from SNR shocks [*interacting with the ISM*]{} by the combined effects of ‘shock excitation’ and the enhanced gas-phase iron abundance. First, since the ionization potential of iron atom is only 7.9 eV, FUV photons from the hot shocked gas can penetrate far downstream to maintain the ionization state of Fe$^+$ where H atoms are primarily neutral [@mck84; @hol89b; @oli89]. Therefore, lines are emitted mainly in gas with a low degree of ionization at $T=10^3$–$10^4$ K. This partly explains the observed high ratio of 164 to lines, but not all. Shock model calculations showed that the ratio is $\sim 1$ if the gas-phase iron abundance is depleted as in normal ISM. According to [@hol89b], the ratio is $\sim 1.5$ for shocks at velocities 80–150  propagating into a molecular gas of $\nh=10^3$ cm$^{-3}$ with iron depletion $\deltafe\equiv{\rm [Fe/H]/[Fe/H]_\odot}=0.03$ where \[Fe/H\]$_\odot$=$3.5\times 10^{-5}$. [@mck84] presented the results on atomic shock calculations including grain destruction: for a 100  shock propagating into atomic gas of $\nh=10$ and 100 cm$^{-3}$, \[Fe II\] 1.2567 $\mu$m/=2.7 and 3.7 with $\deltafe=0.53-0.58$ in the far downstream. If we use 0.033 as the ratio of to  line intensities which corresponds to a Case B nebula at 5,000 K [@ost89], 