The (D^{\prime} 2\left({ }^{3} P_{2}\right) \rightarrow A^{\prime} 2\left({ }^{3} \Pi\right)) transition in IBr occurs with maximum intensity near (3850 \AA) when mixtures of (\mathrm{I}{2}) and (\mathrm{Br}{2}) in (\mathrm{Ar}) are excited by a Tesla coil. Spectra are recorded photographically for (\mathrm{I}^{\mathrm{I}} \mathrm{Br}) and (\mathrm{I}^{\mathrm{B}}{ }^{1} \mathrm{Br}) separately, and (24 v^{\prime}-v^{\prime \prime}) bands spanning (v^{\prime}=0-5) and (v^{\prime \prime}=11-17) are rotationally analyzed, yielding (\sim 3000) line assignments. Together with existing high-quality data covering the high-v', low-v" region, these data are analyzed through a deperturbation model which assumes that minor anomalies in the (D^{\prime})-state rotational constants are the result of weak interactions with the nearby (\beta 1\left({ }^{3} P_{2}\right)) state. This fit pins down the absolute energies of both states for the first time, yielding (T_{\mathrm{e}^{\prime}}=39457(4) \mathrm{cm}^{-1}, T_{\mathrm{e}^{\prime \prime}}=11437(4) \mathrm{cm}^{-1}). Other primary spectroscopic constants (\left(\mathrm{cm}^{-1}\right)) are (\Delta T_{\mathrm{c}}=28019.22, \omega_{\mathrm{c}}^{\prime}=123.064, \omega_{\mathrm{c}} x_{\mathrm{c}}^{\prime}=0.2812, B_{\mathrm{c}}^{\prime}=0.02860, \omega_{\mathrm{c}}^{\prime \prime}=147.631, \omega_{\mathrm{c}} x_{\mathrm{c}}^{\prime \prime}) (=1.260, B_{c}^{\prime \prime}=0.04290, R_{c}^{\prime}=3.4806 \AA, R_{\mathrm{e}}^{\prime \prime}=2.842 \AA). @ 1994 Academic Press. Inc.