Optical–optical double resonance laser spectroscopy of BaO

BaO molecules were sequentially excited by collinearly propagating radiation from two cw lasers, an argon ion laser (488.0 and 496.5-nm lines) and a tunable (560–630 nm) rhodamine 6G dye laser. This sequential excitation technique, called optical–optical double resonance (OODR), is a special case of two-photon spectroscopy in which two visible or uv wavelength photons of different frequencies resonantly excite a molecule from an initial level (v″,J″) to a final level (v*,J*) by way of a real intermediate level (v′,J′). Two types of optical double resonance experiments were performed on BaO. The first, excitation spectroscopy, revealed 19 vibronic levels of 1Σ+ electronic symmetry between 36 490 and 38 620 cm−1 above E (X 1Σ,re). These 19 1Σ+ levels belong to two or more perturbed electronic states. No 1Π levels were found, although levels belonging to the upper state of the BaO B (1Π) –X 1Σ Parkinson bands occur in this energy region. The second type of experiment, photoluminescence spectroscopy, enabled observation of extended rotationally resolved photoluminescence progressions out of various two-photon-excited levels (v*,J*) into BaO X 1Σ (v″=0–34). Vibrational energies and rotational constants obtained from these photoluminescence progressions were used to construct an RKR potential energy curve for BaO X 1Σ+ through v″=40, from which it was shown that the X 1Σ+ and a 3Π curves cross at r=0.276 nm and E=22 750 cm−1 [above E (X 1Σ,re)]. Laser frequency jitter of less than 10−3 cm−1 resulted in large intensity fluctuations of the OODR signal, implying that the double resonance linewidth was narrower than the Doppler width of the (v′,J′) → (v*,J*) transition. This sub-Doppler width effect resulted from excitation of (v′,J′) molecules that were velocity selected relative to the laser propagation direction.