We used the Space Telescope Imaging Spectrograph ( STIS ) with its smallest entrance aperture ( 0 \farcs 03 wide slit ) and highest resolution echelle gratings ( E140H and E230H ) to record the interstellar absorption features for 10 different multiplets of neutral carbon at a resolving power of \lambda / \Delta \lambda = 200 , 000 in the UV spectra of 21 early-type stars .
Our objective was to measure the amount of C I in each of its three fine-structure levels of the ground electronic state , so that we could determine the thermal pressures in the absorbing gas and how much they vary in different regions .
Our observations are principally along directions out to several kpc in the Galactic plane near longitudes \ell = 120 \arcdeg and 300 \arcdeg , with the more distant stars penetrating nearby portions of the Perseus and Sagittarius-Carina arms of the Galaxy .
We devised a special analysis technique to decipher the overlapping absorption features in the different multiplets , each with different arrangements of the closely spaced transitions .
In order to derive internally consistent results for all multiplets , we found that we had to modify the relative transition f - values in a way that made generally weak transitions stronger than amounts indicated in the current literature .

We compared our measured relative populations of the excited fine-structure levels to those expected from equilibria calculated with collisional rate constants for various densities , temperatures and compositions .
The median thermal pressure for our entire sample was p / k = 2240 { cm } ^ { -3 } K , or slightly higher if the representative temperatures of the material are much above or below a most favorable temperature of 40K for the excitation of the first excited level at a given pressure .
For gas that is moving outside the range of radial velocities permitted by differential Galactic rotation between us and the targets , about 15 % of the C I indicates a thermal pressure p / k > 5000 { cm } ^ { -3 } K. For gas within the allowed velocities , this fraction is only 1.5 % .
This contrast reveals a relationship between pressure enhancements and the kinematics of the gas .

Regardless of velocity , we usually can register the presence of a very small proportion of the gas that seems to be at p / k \gtrsim 10 ^ { 5 } { cm } ^ { -3 } K. We interpret these ubiquitous wisps of high pressure material to arise either from small-scale density enhancements created by converging flows in a turbulent medium or warm turbulent boundary layers on the surfaces of dense clouds moving through an intercloud medium .
For turbulent compression , our C I excitations indicate that the barytropic index \gamma _ { eff } \gtrsim 0.90 must apply if the unperturbed gas starts out with representative densities and temperatures n = 10 { cm } ^ { -3 } and T = 100 K. This value for \gamma _ { eff } is larger than that expected for interstellar material that remains in thermal equilibrium after it is compressed from the same initial n and T .
However , if regions of enhanced pressure reach a size smaller than \sim 0.01 pc , the dynamical time starts to become shorter than the cooling time , and \gamma _ { eff } should start to approach the adiabatic value c _ { p } / c _ { v } = 5 / 3 .

Some of the excited C I may arise from the target stars ’ H II regions or by the effects of optical pumping from the sub-millimeter line radiation emitted by them .
We argue that these contributions are small , and our comparisons of the velocities of strongly excited C I to those of excited Si II seem to support this outlook .

For 6 stars in the survey , absorption features from interstellar excited O I could be detected at velocities slightly shifted from the persistent features of telluric origin .
These O I* and O I** features were especially strong in the spectra of HD 93843 and HD 210839 , the same stars that show exceptionally large C I excitations .

In appendices of this paper , we present evidence that ( 1 ) the wavelength resolving power of STIS in the mode we used is indeed about 200,000 , and ( 2 ) the telluric O I* and O I** features exhibit some evidence for macroscopic motions , since their broadenings are in excess of that expected for thermal Doppler broadening at an exospheric temperature T = 1000 K .