The researchers from Institute of Modern Physics, Chinese Academy of Sciences (IMP) investigated the electronic states of arsenic carbide (AsC) free radical together with Professor Dennis J Clouthier from University of Kentucky, USA.

The previously unknown AsC free radical has been identified in the gas phase through a combination of laser-induced fluorescence (LIF), single vibronic level emission, and stimulated emission pumping (SEP) spectroscopy in a supersonic expansion. High resolution studies of the B−X 0–0 band by LIF and the B−A 0–0 band by SEP spectroscopy enabled a determination of the molecular structures in the three states. The AsC ground state is ^2_∑+ with an r_⁰ bond length of 1.680 Å and a vibrational frequency of ω_e = 1022 cm^-1. Two excited electronic states have been identified: A ²∏_i with T₀ = 5586.9 cm^-1 and B ²∑+ with T₀ = 26066.0 cm^-1.

The A state bond length is 0.095 Å longer than that of the ground state, due to promotion of an electron from the bonding 4π orbitals to the 10 σ  HOMO which has substantial antibonding character, similar to the A states of CN and CP. The B state of AsC, which has a much longer bond length than the ground state, appears to be similar to the B state of CP which has been shown to be predominantly due to the promotion of an electron from the bonding π orbitals to the antibonding π* orbitals. In contrast, the B state of CN is formed by promotion of an electron from the strongly antibonding 4σ* orbital to the 5σ orbital which has both bonding and antibonding character, resulting in a slight decrease in the bond length.

The work has been published in J. Chem. Phys. 135, 054309(2011). The article can be linked as follows:：http://jcp.aip.org/resource/1/jcpsa6/v135/i5/p054309_s1?isAuthorized=no&

Fig. 1: The calculated (B3LYP/aug-cc-pV5Z) molecular orbitals of AsC and the orbital occupancy for the ground electronic state (Imaged by IMP).

Fig. 2: A portion of the high resolution LIF spectrum of the B ²∑+ - X ²∑+ 0-0 band of As¹²C. The top trace is the experimental spectrum and the bottom trace is spectrum calculated from the constants obtained from the rotational analysis (Imaged by IMP).