We present spectroscopic observations of diffuse ionized gas ( DIG ) in M33 near the H ii \ > region NGC 604 .
We present the first detection of [ O i ] \ > \lambda 6300 \ > in the DIG of M33 , one of the critical lines for distinguishing photo- from shock ionization models .
We measure [ O i ] /H \alpha \ > in the range of 0.04 to 0.10 and an increase in this ratio with decreasing emission measure .
Our measurements of [ S ii ] /H \alpha \ > and [ N ii ] /H \alpha \ > also rise with decreasing emission measure , while our [ O iii ] /H \beta \ > measurements remain fairly constant .
We have one tentative detection of He i \ > in the region of brightest emission measure , with a ratio of He i/H \alpha = 0.033 \pm 0.019 , indicating that the helium is at least partially ionized .
We compare our observed emission line ratios to photoionization models and find that field star ionization models do not fit our data well .
Leaky H ii \ > region models are consistent with our data , without the need to invoke additional ionization mechanisms to fit our [ O i ] \ > or [ O iii ] \ > measurements .
The closest large H ii \ > region is NGC 604 and is therefore a likely candidate for the source of the ionizing photons for the gas in this region .