Diffuse clouds are of astronomical interest because they play a primary role in the evolution and thermodynamics of ISM.  Observation of the abundant atomic hydrogen in 21 cm has shown good signal-to-noise ratio in both emission and absorption.  Nevertheless, HI observations have a fundamental difficulty when are directed to low mass regions of the hydrogen nucleus, as the center part of a diffuse cloud:  Thermal width of hydrogen lines are the same order as the internal velocities structures of interest, so clouds components of various temperatures and central velocities are indistinguishable in the spectrum.  Molecular lines observations in principle doesnt suffer from these problems. Unlike HI, molecules generally have excitation temperature T , so that emission is very weak even from abundant species.  CO and HO are considered to be the most easily studied candidate molecules.  CO has transitions in a region of the spectrum (wavelength HO has the 18 cm emission, line convenient for absorption observations.  Observation studies provide the most sensitive means of detections of molecules with sub thermal excitation, and can give the opacity of the spectral line, which is a central issue to model the molecular region.
Studies based in the kinematic comparison of HO and HI absorption lines from diffuse clouds are useful in determining their physical conditions, especially because heavier elements provide higher velocity resolution.