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We report observations of the dust cloud L1204 with the Onsala 25-m telescopein the 6 cm (1$_{11}-1_{10}$) transition of \htco. The observed region includesthe  S140 H${\alpha}$ arc. This spectral line is seen here in absorption againstthe cosmic microwave background, indicating the presence of widespread warmmolecular gas at intermediate densities. Overall, the distributions of H$_2$COand CO (taken from the literature) are fairly similar, though significantdifferences exist at small scales. Most notably, while the CO peak is nearlycoincident with the S140 H${\alpha}$ arc, the maximum H$_2$CO absorption isclearly separated from it by a full 10$'$ beam ($\sim$ 3 pc). We argue thatthese differences result from differing abundances and excitation requirements.The CO(1-0) line is more optically thick and more biased towards warm gas thanthe H$_2$CO 6 cm line. On the other hand, formaldehyde is more easilyphotodissociated and is, therefore, a poorer tracer of the molecular gaslocated immediately behind Photon Dominated Regions.

A Comparison between Anomalous 6-cm H$_2$CO Absorption and CO(1-0) Emission in the L1204/S140