  High-harmonic spectroscopy driven by circularly-polarized laser pulses and
their counter-rotating second harmonic is a new branch of attosecond science
which currently lacks quantitative interpretations. We extend this technique to
the mid-infrared regime and record detailed high-harmonic spectra of several
rare-gas atoms. These results are compared with the solution of the
Schrodinger equation in three dimensions and calculations based on the
strong-field approximation that incorporate accurate scattering-wave
recombination matrix elements. A quantum-orbit analysis of these results
provides a transparent interpretation of the measured intensity ratios of
symmetry-allowed neighboring harmonics in terms of (i) a set of propensity
rules related to the angular momentum of the atomic orbitals, (ii)
atom-specific matrix elements related to their electronic structure and (iii)
the interference of the emissions associated with electrons in orbitals co- or
counter-rotating with the laser fields. These results provide the foundation
for a quantitative understanding of bi-circular high-harmonic spectroscopy.
