I present measurements of optical emission-line ratios from long-slit spectra of 14 star-forming dwarf galaxies. All members of the sample have a prominent component of diffuse warm ionized gas (DIG). Spectra of the DIG are characterized by high [O I]/H alpha intensity ratios in addition to high [O II]/[O III], [S II]/H alpha, [N II]/H alpha, and low [O III]/H beta ratios relative to the H II regions. Measurements from similar to 350 spectra extracted at spatial intervals of similar to 20-150 pc show that this spectral change is gradual and continuous; hence, the boundary between H Ir regions and DIG is not well defined spectroscopically. In diagnostic line-ratio diagrams, the spectral transition advances along a narrow track that is distinctly different from the well-established H II region excitation sequence. The line ratios progress toward the region populated by weak-[O I] Low-Ionization Nuclear Emission-Line Regions. I show that this H II-DIG transition sequence is driven primarily by a decrease in the relative density of ionizing photons to atoms, i.e., the ionization parameter. The radial gradient in the ionization parameter is consistent with the dilution of radiation from a centralized source. This result suggests that the dominant excitation mechanism of the DIG is photoionization by massive stars associated with the main star-forming regions. In several of the galaxies, however, an additional excitation mechanism is required to explain the moderately high [O III]/H beta at very high [S II]/H alpha and the sharp increase in [O I]/H alpha with only a moderate increase in [O II]/[O III]. I show that these line ratios can be explained by a contribution to the emission from shock-excited gas. The inferred shock speeds are between 60 km s(-1) and 100 km s(-1); the line ratios in the regions with the lowest surface brightness require the largest relative contribution from shocks, up to 30%-50% of the emission. The prevalence of a secondary emission component from shocked gas is difficult to quantify. Where the metallicity is very low and/or the shock speed is less than or similar to 60 km s(-1), the line ratios alone cannot distinguish emission from shocked gas from that from dilute H in regions. Among the galaxies, differences in the DIG line ratios can be largely attributed to the galactic metallicity. At similar metallicity, however, spectral differences are related to the ionization parameter and ionization state of the parent H II regions. This result provides direct evidence that star formation affects the structure of the interstellar medium on a global scale. I find that much of the observed variation in H II region ionization parameters is caused by substantial differences in their filling factors of warm ionized clouds. Based on these results, I suggest that the DIG spectra reflect the porosity of the star-forming regions.
