where B and D are the equilibrium rotational and centrifugal distortion constants (in cm -1 ) for the ground state of HC 3 . Plotting ln(Peak area/(ν J (J ′ + J ′′ + 1))) as a function of E J will create a linear "Boltzmann plot" with a slope of -1/(k B T rot ), allowing the rotational temperature to be extracted. Figure shows a Boltzmann plot constructed from several of the measured transitions, with the red dashed line showing the linear fit. The calculated rotational temperature was found to be 21 ± 4 K, which shows that the rotational temperature of the molecule is-within error-thermalized to the buffer-gas cell temperature and in thermal equilibrium. Having confirmed that our HC 3 N has been suitably cooled, we can begin assigning molecular constants and line centers. The 2ν 1 transition of HC 3 N was initially simulated in PGOPHER (Western 2017) using a previously published band center ) and groundstate rotational constants . This simulation was then refined using high-resolution data obtained in this work, using PGOPHER the latter to assign J states to each transition. A comparison between the lower-resolution survey spectrum and the simulated PGOPHER spectrum is shown in Figure . The data are normalized, and the simulation results are shown using the refined rotational constants. The fit resulted in a 2ν 1 band origin of 6552.275 ± 0.0026 cm -1 and rotational constants B = 0.15130 ± 3.8 × 10 -5 cm -1 and D = 3.762 × 10 -7 ± 1.587 × 10 -7 cm -1 for the vibrational excited state. Table contains the line center and rotational state assignment for each measured transition.