Optical-optical double resonance, laser induced fluorescence, and revision of the signs of the spin-spin constants of the boron carbide (BC) free radical

The cold boron carbide free radical (BC X ⁴Σ⁻) has been produced in a pulsed discharge free jet expansion using a precursor mixture of trimethylborane in high pressure argon. High resolution laser induced fluorescence spectra have been obtained for the B ⁴Σ⁻–X ⁴Σ⁻ and E ⁴Π–X ⁴Σ⁻ band systems of both ¹¹BC and ¹⁰BC. An optical-optical double resonance (OODR) scheme was implemented to study the finer details of both band systems. This involved pumping a single rotational level of the B state with one laser and then recording the various allowed transitions from the intermediate B state to the final E state with a second laser by monitoring the subsequent E–X ultraviolet fluorescence. In this fashion, we were able to prove unambiguously that, contrary to previous studies, the spin-spin constant λ is negative in the ground state and positive in the B ⁴Σ⁻ excited state. It has been shown that λ″ < 0 is in fact expected based on a semiempirical second order perturbation theory calculation of the magnitude of the spin-spin constant. The OODR spectra have also been used to validate our assignments of the complex and badly overlapped E ⁴Π–X ⁴Σ⁻ 0-0 and 1-0 bands of ¹¹BC. The E–X 0-0 band of ¹⁰BC was found to be severely perturbed. The ground state main electron configuration is …3σ²4σ²5σ¹1π²2π⁰ and the derived bond lengths show that there is a 0.03 Å contraction in the B state, due to the promotion of an electron from the 4σ antibonding orbital to the 5σ bonding orbital. In contrast, the bond length elongates by 0.15 Å in the E state, a result of promoting an electron from the 5σ bonding orbital to the 2π antibonding orbitals