  We report here on a survey of carbon recombination lines (RLs) near 8.5 GHz
toward 17 ultra-compact \HII regions (\UCHII s). Carbon RLs are detected in 11
directions, indicating the presence of dense photodissociation regions (PDRs)
associated with the \UCHII s. In this paper, we show that the carbon RLs
provide important, complementary information on the kinematics and physical
properties of the ambient medium near \UCHII s. Non-LTE models for the carbon
line forming region are developed, assuming that the PDRs surround the \UCHII
s, and we constrained the model parameters by multi-frequency RL data. Modeling
shows that carbon RL emission near 8.5 GHz is dominated by stimulated emission
and hence we preferentially observe the PDR material that is in front of the
\UCHII continuum. We find that the relative motion between ionized gas and the
associated PDR is about half that estimated earlier, and has an RMS velocity
difference of 3.3 \kms. Our models also give estimates for the PDR density and
pressure. We found that the neutral density of PDRs is typically $>$ 5 $\times$
10$^5$ \cmthree and \UCHII s can be embedded in regions with high ambient
pressure. Our results are consistent with a pressure confined \HII region model
where the stars are moving relative to the cloud core. Other models cannot be
ruled out, however. Interestingly, in most cases, the PDR pressure is an order
of magnitude larger than the pressure of the ionized gas. Further investigation
is needed to understand this large pressure difference.
