We have carried out an in-depth study of the peripheral region of the molecular cloud L1204/S140 , where the far-ultraviolet radiation and the density are relatively low .
Our observations test theories of photon-dominated regions ( PDRs ) in a regime that has been little explored .
Knowledge of such regions will also help to test theories of photoionization-regulated star formation .
[ C II ] 158 µm and [ O I ] 63 µm lines are detected by ISO at all 16 positions along a 1-dimensional cut in right ascension .
Emission from H _ { 2 } rotational transitions J = 2 \rightarrow 0 and J = 3 \rightarrow 1 , at 28 and 17 µm , was also detected at several positions .
The [ C II ] , [ O I ] , and H _ { 2 } intensities along the cut show much less spatial variation than do the rotational lines of ^ { 12 } CO and other CO isotopes .
The average [ C II ] and [ O I ] intensities and their ratio are consistent with models of PDRs with low far-ultraviolet radiation ( G _ { 0 } ) and density .
The best-fitting model has \mbox { $G _ { 0 } $ } \sim 15 and density , n \sim \mbox { $ 10 ^ { 3 } $ } cm ^ { -3 } .
Standard PDR models underpredict the intensity in the H _ { 2 } rotational lines by up to an order of magnitude .
This problem has also been seen in bright PDRs and attributed to factors , such as geometry and gas-grain drift , that should be much less important in the regime studied here .
The fact that we see the same problem in our data suggests that more fundamental solutions , such as higher H _ { 2 } formation rates , are needed .
Also , in this regime of low density and small line width , the [ O I ] line is sensitive to the radiative transfer and geometry .
Using the ionization structure of the models , a quantitative analysis of timescales for ambipolar diffusion in the peripheral regions of the S140 cloud is consistent with a theory of photoionization-regulated star formation .
Observations of [ C II ] in other galaxies differ both from those of high G _ { 0 } PDRs in our galaxy and from the low G _ { 0 } regions we have studied .
The extragalactic results are not easily reproduced with mixtures of high and low G _ { 0 } regions .