a b s t r a c tEmission spectra of bismuth monohydride (BiH) radicals excited by energy transfer from O 2 (a 1 D g ) or NF(a 1 D) or by a DC discharge in a mixture of bismuth vapor and hydrogen in a fast-flow system were studied in the wavenumber range from 4000 to 25 000 cm À1 with a Fourier-transform spectrometer.The X 2 1 ? X 1 0 + transition between the spin components of the X 3 R À ground state was remeasured at high spectral resolution and signal/noise. Six bands of the Dv = 0 and +1 sequences near 4950 and 6600 cm À1 have been observed. The low-J lines of these bands are split into up to 10 hfs components due to magnetic hyperfine structure splitting of the X 2 1 levels. In addition to the previously observed electric dipole lines, weak magnetic dipole lines show up in the Dv = 0 but not in the Dv = +1 bands. The intensities of the Dv = +1 bands are about a factor of 10 higher than expected from FranckCondon factors indicating that the electric dipole transition moment l X2-X1 shows a strong dependence on internuclear distance. Near 5450 cm À1 , the 0-0 and 1-1 bands of the transition a2 ? X 2 1 from the hitherto unknown first excited state a 1 D(a2) to the upper component of the ground state have been observed. The lines in these bands show the magnetic hfs splitting of both the a2 and X 2 1 states. In the visible range near 21 300 and 16 300 cm À1 , the 0-0 and 1-1 bands of the b0 + ? X 1 0 + system and the 0-0 band of the b0 + ? X 2 1 transition likewise have been measured at high resolution. The lines of the latter band also show the hfs splitting of the X 2 1 state. Least-squares fits have yielded rotational and vibrational constants of the X 1 0 + , X 2 1, a2, and b0 + states as well as hfs parameters of the X 2 1 and a2 states and the electronic energy of the hitherto unknown a 1 D(a2) level.