While the amount of the dust, gas, and ice in dense interstellar clouds is large enough to block out most of the ultraviolet (UV) radiation field found in the diffuse interstellar medium (DISM), low levels of radiation still exist in these clouds. Some of the radiation, especially nearer the surfaces of the clouds, is just the 'tail end' of the attenuated diffuse ISM UV radiation field. Even deep within the clouds there is a low level of background UV radiation produced by the interaction of cosmic rays with the dust, gas, and ice of the cloud. Finally, since protostars are frequently very energetic objects, the UV radiation field can be quite intense in the vicinity of forming stars.

The presence of this radiation has important chemical implications. The temperatures in dense interstellar clouds are very low (typically only 10-50 K (-263 to -223 C, about -440 to -370 ^oF). Chemical reactions are essentially precluded at these temperature because of molecular energy barriers. However, the presence of ionizing radiation can drive a rich photochemistry in these environments because it can break chemical bonds, create highly reactive ions and radicals, and create 'hot' molecular fragments despite the low temperatures.

We now know that much of the material in dense molecular clouds exists in the form of mixed molecular ices frozen out onto more refractory silicate and carbonaceous dust grains (for more information about this, click here). These ices consist largely of simple molecules like H₂O, CH₃OH, CO, CO₂O, NH₃, etc. These ices should be exposed to significant levels of UV radiation and the result should be the destruction of some compounds and the formation of others.

Is there evidence that photoprocessing is occuring in dense clouds in the interstellare medium? Yes! While the materials in these clouds block out visible radiation, they don't block out infrared radiation. At infrared frequencies, the chemical compounds that are present selectively absorb specific wavelengths of light that depend on their molecular bonding and composition (for a more detailed explanation of this, click here). The positions of the resulting infrared absorption features can be measured using special infrared telescopes. Comparisons between the infrared absorption bands produced by the dust and ice in the dense clouds and laboratory standards can then be used to constrain the composition of the dust (for more details about how this is done, click here).

These comparisons have detected the presence of materials that can be best explained as the products of photochemistry. An example of this is shown below. This figure shows an infrared absorption feature seen superimposed on the infrared emission being produced by the protostar W33A. This feature is presumably produced by material that exists in the dense cloud along the line-of-sight between us and the protostar. The identity of the material responsible for this feature is currently somewhat controversial, although most people believe the band is due to a C-N stretching vibration of some sort (for example the CN stretch of a nitrile or isonitrile). For this reason the feature is frequently referred to as the "XCN" band.

One of the interesting things about this band is that the molecule that produces it, whatever its actual identity, is rapidly produced when 10 K interstellar ice analogs are irradiated by UV radiation. The fact that this feature is generally seen along lines-of-sight towards protostars, i.e., environments with high radiation fields, rather than in the general cloud medium is consistent with the interpretation that the carrier of this band is the result of photochemsitry.

Interstellar photochemistry of this type may result in the production of a suite of complex organic materials, some of which may have important implications for the origin of life. This carrier of the "XCN" band may also be related to the production of hexamethylenetetramine (HMT; C₆H₁₂N₄) in the interstellar medium. HMT may be an important precursor to amino acids.

For more detailed information on the UV radiation processing of the materials in dense interstellar molecular clouds, see:

Sandford, S. A., Allamandola, L. J., & Bernstein, M. P. (1997). The Composition and Ultraviolet and Thermal Processing of Interstellar Ices. In From Star Dust to Planetesimals, Astron. Soc. Pac. Conf. Ser., Vol. 122, Y. J. Pendleton & A. G. G. M. Tielens (eds.), (ASP: San Francisco), pp. 201-213.

Bernstein, M. P., Sandford, S. A., & Allamandola, L. J. (1997). The Infrared Spectra of Nitriles and Related Compounds Frozen in Ar and H₂O. Astrophys. J. 476, 932-942.

Allamandola, L. J., Bernstein, M. P., & Sandford, S. A. (1997). Photochemical evolution of interstellar/precometary organic material. In Astronomical and Biochemical Origins and the Search for Life in the Universe, C.B. Cosmovici, S. Bowyer, & D. Werthimer (eds.), Proc. 5th International Conf. on Bioastronomy, IAU Coll. #161, Capri, 1-5 July 1996, (Editrice Compositori: Bologna), pp. 23-47.

Sandford, S. A. (1996). The Inventory of Interstellar Materials Available for the Formation of the Solar System. Meteoritics and Planetary Science 31, 449-476.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Chang, S., & Scharberg, M. A. (1995). Organic Compounds Produced by Photolysis of Realistic Interstellar and Cometary Ice Analogs Containing Methanol. Astrophys. J. 454, 327-344.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., & Chang, S. (1994). Infrared Spectrum of Matrix-Isolated Hexamethylenetetramine in Ar and H2O at Cryogenic Temperatures. J. Phys. Chem. 98, 12206-12210.

Tegler, S. C., Weintraub, D. A., Allamandola, L. J., Sandford, S. A., Rettig, T. W., & Campins, H. (1993). Detection of the 2165 cm-1 (4.619 µm) XCN Band in the Spectrum of L1551 IRS 5. Astrophys. J. 411, 260-265.