Rotational assignment using phase relationships in optical–optical double resonance: The BaI C 2Π–X 2Σ+ system

We describe an optical–optical double resonance scheme in which a lower vibration-rotation level is labeled. One laser is fixed in frequency and probes the population of the labeled level via the resulting laser-induced fluorescence; a second laser is scanned in frequency through the same vibronic band excited by the probe. A double resonance signal results when the population in the labeled level is either increased or decreased by the action of the second laser. The positions and phase pattern of the double resonance spectrum reveal the J numbering of the labeled level and permit a good approximation to be made for the upper and lower state rotational constants. This information allows the J value of the labeled level to be systematically changed, permitting the spectrum to be unraveled. This technique is proven by applying it to the highly congested C–X spectrum of the BaI molecule, for which no rotational information was previously available for any of its states.