hydrocarbon with 19 carbon atoms will have its emission line of optimal fre- 

 quency reduced in intensity by a factor 300 from a linear hydrocarbon with 

 3 carbon atoms, assuming the species have the same dipole moment and the 

 same abundance. 



Although these considerations suggest that observing more complex mole- 

 cules in the interstellar medium via microwave and millimeter studies will be 

 difficult, it is possible that other wavelength regions may provide a better 

 method of detecting these molecules. Studies in the infrared region can be con- 

 veniently divided into low-resolution and high-resolution investigations. High- 

 resolution studies can be used to study gas-phase interstellar molecules in both 

 absorption and emission. Low-resolution studies can be used for both the 

 gaseous and condensed phases. Emission studies in the infrared require a warm 

 or nonthermal (e.g., shocked) portion of an interstellar cloud, whereas absorp- 

 tion studies require a suitable background continuum source. At first glance it 

 would appear that infrared observations avoid the severe problems encountered 

 in the microwave and millimeter ranges caused by the dilution of rotational 

 states of complex molecules since the density of vibrational levels of complex 

 molecules is quite low even for very complex species, at least at low excitation 

 energies. This apparent advantage versus rotation cannot be realized in high- 

 resolution studies because gas-phase molecules rotate as well as vibrate and a 

 high-resolution infrared transition involves a change in the rotational, as well as 

 the vibrational, quantum state. Low-resolution infrared studies avoid this prob- 

 lem of rotational fine structure, but at the expense of facile identification of the 

 emitting or absorbing species. To minimize the rotational fine structure, it would 

 be desirable to study molecules in absorption at the lowest possible excitation 

 temperatures and densities in order to minimize the excitation of rotational 

 levels. Unfortunately, most theories of interstellar chemistry do not predict 

 significant abundances of complex molecules in low-density, diffuse clouds. In 

 addition, the Doppler broadening caused by large-scale rotation within the 

 clouds may blend the closely spaced lines together and make them unresolvable 

 even with ultrahigh-resolution infrared spectroscopy. However, high-resolution 

 infrared studies will be beneficial for smaller emitting or absorbing species in the 

 gas phase once the practical difficulties of this type of astronomy are overcome 

 by space observatories such as SI RTF. 



It would thus seem that infrared studies at lower resolution offer our best 

 hope for complex molecule observation. However, the difficulty of identifying 

 the carrier of a low-resolution spectrum can be great. Consider the broad, 

 3.4-jiim feature which has been explained by varieties of organic matter on dust 

 surfaces, but has also been claimed to be due to bacteria. Or consider the tenta- 

 tive, but hardly unambiguous, identification of gaseous polycyclic aromatic 

 hydrocarbons via broad features at 6.2 and 7.7 ^m. Perhaps the best that can be 

 hoped for is the functional group analysis used by organic chemists who observe 



62 



