Laser-induced fluorescence measurements with a pulsed dye laser and measurements of near-infrared chemiluminescence spectra with a high-resolution Fourier-transform spectrometer have been used to study electronic states and transitions of diatomic and triatomic radicals in the gas phase. The aim of the LIF studies of the diatomic molecules NI, SbH (SbD), and BiH (BiD) was to supplement the spectroscopic information on the X³Σ-(X₁0⁺, X₂1) ground states and the low-lying b¹Σ +(b0⁺) states of the hydrides and halides of group Va atoms which were the subject of parallel detailed FT emission studies of M. Beutel. The LIF technique allowed first observation of the b¹Σ ⁺(b0⁺) ↔ X³Σ-(X₁0⁺, X₂1) electronic spectra of NI, SbH, and SbD which hitherto have not been found in emission studies. Analyses of high-resolution LIF excitation spectra yielded first accurate vibrational and rotational constants of the b¹Σ +(b0⁺) states of these species. For the first time ever in a b ↔ X transition of a group V hydride, due to the high sensitivity of the LIF technique, in the b0⁺ ↔ X₂1 subsystem of SbH and SbD weak magnetic dipole lines were observed in addition to the much stronger electric dipole lines. Time-resolved measurements of the fluorescence decays allowed determination of the radiative lifetimes of rovibronic levels of the b¹Σ + states of NI, SbH (SbD), and BiH (BiD) and of rate constants for collisional quenching of the states by rare gas atoms and some simple molecules.
The main topic of the present work was the first study of the low-lying electronic states of the hitherto unknown triatomic radicals BiOH (BiOD) using LIF in the near-infrared (NIR) and visible region and Fourier-transform emission spectroscopy (FT) in the NIR region. In good agreement with calculated theoretical data of Buenker and co-workers, parallel transitions between four stable electronic states , , , and have been observed. Spectra of the transitions and contain bands with Δ Kₐ = 0 subbands, whereas spectra of the transition and of the fine structure transition contain bands with both Δ Kₐ = 0 and Δ Kₐ = ± 1 subbands. The latter are attributed to hybrid bands of a nearly symmetric top molecule.
Rotational analysis of the 000- 000 band of a high-resolution FT-emission spectrum of the fine structure transition near 6200 cm-1 including 431 line positions of five subbands with Δ Kₐ = ± 1 allowed determination of the rotational constants A, B and C of the asymmetric rotor molecule. Using these constants, a computer simulation of the central region of the band involving the P-branch heads of the Δ Kₐ = 0 subbands allowed vibrational assignment of these overlapping subbands.
The 000- 000 band of the next higher transition near 11300 cm-1, measured in LIF and FT spectra, shows Δ Kₐ = 0 subbands with heads in the P-branches. The subband wavenumbers cannot be fitted with the symmetric rotor formula. It is concluded that the Kₐ structure of the state is severely perturbed by Renner-Teller interaction with the not yet observed state. Assuming an interaction matrix element of -2AΛ Kₐ, a good fit of the subband structure is obtained.
In LIF studies of the state near 22800 cm-1, excitation spectra of the and transitions have been measured. Whereas the bands of the first transition show parallel type structure with Δ Kₐ = 0 subbands only, those of the transition are hybrid bands with Δ Kₐ = 0 and Δ Kₐ = ± 1 subbands of similar intensity. As the state mixes
with the ground state, this explains the appearance of weak Δ Kₐ = ± 1 subbands in the transition. Furthermore, when pumping the state, dispersed fluorescence spectra of transitions to all three lower-lying states , , and could be observed confirming the identification and assignment of the states and transitions. Time-resolved measurements of the decay of the fluorescence from the state allowed determination of the radiative lifetime and of rate constants for quenching by some added gases.
In addition to BiOH(D), the heavier species BiSH(D), BiSeH(D), and BiTeH(D) have also been studied by means of FT and LIF measurements. The transitions have been observed for BiSH(D) and BiSeH(D), and the transitions have been found for all eight species. Because of a strong increase of superpositions in the subband structures with increase of the mass of the molecules, analyses of the band structures were impossible for the heavier molecules.