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Population labeling or common lower level optical-optical double resonance spectroscopy has been applied to rotationally analyze the BaI C-X and $CO2+B~-X~(0-0)$ bands. The $C2\Pi -X2\Sigma +$ spectrum of the Bal molecule presents a challenge to small molecule spectroscopists since no state has been rotationally analyzed previously and the extremely high line density prevents assignment by a combination difference-search technique. In this work, we deseribe the analysis of the (0-0) band by population labeling in which two single-mode cw dye lasers intersect a molecular beam of BaI to generate double resonance spectra. Double resonance signals are due to the optical pumping of population either to or from a labeled level. The relative phases of these signals provide the branch assignment, while branch origins are determined by sequential double resonance scans. Thus, iterative double resonance scans allow one to converge on the assignments in a mechanical way free of pattern recognition in which the lasers work through the molecular combination differences. A variation of the above technique using pulsed lasers yielded the assignment of selected regions of the strongly perturbed $CO2+B~2\Sigma -X~\Pi (000-000)$ band. A perturbation in the c symmetry levels occurring at $J\sim 17.5$ was observed, substantially modifying the evidence regarding the vibronic character of the perturber. Supported by the Air Force Office of Scientific Research under AFOSR 81-0053.