High-order harmonic generation by a bicircular field, which consists of two coplanar counter-rotating circularly polarized fields of frequency $r\omega$ and $s\omega$ ($r$ and $s$ are integers), is investigated for a polyatomic molecule. This field possesses dynamical symmetry, which can be adjusted to the symmetry of the molecular Hamiltonian and used to investigate the molecular symmetry. For polyatomic molecules having the $C_{r+s}$ symmetry, only the harmonics $n=q(r+s)\pm r,q=1,2,...,$ are emitted having the ellipticity ${\varepsilon }_n=\pm 1$. We illustrate this using the example of the planar molecules ${\mathrm{BH}}_3$ and ${\mathrm{BF}}_3$, which obey the $C_3$ symmetry. We show that for the ${\mathrm{BF}}_3$ molecule, similarly to atoms with a $p$ ground state, there is a strong asymmetry in the emission of high harmonics with opposite helicities. This asymmetry depends on the molecular orientation.

• Received 5 July 2016

DOI:https://doi.org/10.1103/PhysRevA.94.033419