The Fourier transform infrared spectrum of monoisotopic (\mathrm{SC}^{80} \mathrm{Se}) has been investigated in the (\nu_{2}, \nu_{3}, 2 \nu_{2}, 2 \nu_{3}), and (\nu_{1}) regions with a resolution between 3 and (4 \times 10^{-3} \mathrm{~cm}^{-1}). In addition, the millimeter-wave spectrum has been studied in the region 150 to (320 \mathrm{GHz}), and ground and (v_{2}=) 1 excited state transitions have been measured. Ground state constants, (B_{0}=2043.2854(4) \mathrm{MHz}) and (D_{0}=146.53(5) \mathrm{Hz}), have been determined from a merge of millimeter-wave data and ground state combination differences spanning (J) values up to 77 and 143 , respectively. The band centers (\nu_{2}=352.341075(9) \mathrm{cm}^{-1}) and (\nu_{3}=505.48006(5) \mathrm{cm}^{-1}) have been determined. The rovibrational parameters of numerous overtone and combination levels (\left(v_{1} v_{2}^{l_{2}} v_{3}\right)=02^{0} 0.02^{2} 0,03^{1} 0,03^{3} 0), (04^{\circ} 0,04^{2} 0,00^{\circ} 2), and (00^{\circ} 3) have been obtained from polynomial analyses whose standard deviations ranged from 0.7 to (3.5 \times 10^{-4} \mathrm{~cm}^{-1}). The (10^{0} 0) level, (\nu_{\text {eff }} 1435.840 \mathrm{~cm}^{-1}), is anharmonically perturbed by the (04^{\circ} 0) level, with an avoided crossing at (J=55), and (W_{12222}=0.96309(1) \mathrm{cm}^{-1}). Transitions to both the upper (\left(E^{+}\right))and lower (\left(E^{-}\right))sublevels of the dyad were observed for (1 \leqslant) (J^{\prime} \leqslant 117) and (4 \leqslant J^{\prime} \leqslant 171), respectively, and the deperturbed wavenumbers (v_{1}=1435.54276(2)) and (4 \nu_{2}^{0}=1432.72500(3) \mathrm{cm}^{-1}) were derived. Furthermore, a local crossing of the (E^{-})and (04^{2} 0) levels involving (l)-type resonance was observed at (J=91). C 1995 Academic Press, Inc.