Simultaneous measurement of rotational and translational relaxation by sub-Doppler optical–optical double resonance spectroscopy: BaO(A 1Σ+)–Ar and BaO(A1Σ+)–CO2

Sub-Doppler optical–optical double resonance (OODR) spectroscopy is used to correlate BaO(A1Σ+) rotational and translational relaxation induced by collisions with Ar or CO2 in a 0.3–2 Torr Ba+CO2+Ar flame. A selected rovibronic BaO A 1Σ+←X 1Σ+ transition is pumped by the first of two frequency stabilized, 1 MHz bandwidth, cw dye lasers. Collision-induced level-to-level processes in the A 1Σ+ state are monitored with the second laser by excitation of C 1Σ+←A1Σ+ probe transitions and detection of C 1Σ+→X 1Σ+ ultraviolet fluorescence. The dependence of J-changing collision rates on the initial A 1Σ+ (v=1) rotational level, Jo, is monitored by independent preparation of Jo=0 and Jo=15. For Ar, ‖ΔJ‖=1–20 collision rates are found to be independent of Jo and well represented by the power law, where ΔEr is the change in BaO (A 1Σ+) rotational energy in cm−1; ε=ΔEr for Jo<J and ε=0 for Jo≳J. For BaO∼CO2 collisions a single power law is inadequate to represent Jo=0 and Jo=15 data: Total BaO–Ar J-changing collision cross sections are: 60±15 Å2 for Jo=15 and 40±10 Å2 for Jo=0. For BaO–CO2, total J-changing collision cross sections are: 230±40 Å2 for Jo=15 and 50±20 Å2 for Jo=0. ΔJ≠ 0 OODR line shapes, broadened by velocity changing collisions, sample the center of mass (cm) scattering angle (ψ) distribution. For ΔJ=±1 transitions, the cm scattering angle distribution is characterized by <cosψ≳=0.81±0.05 or <ψ≳=36°±5° for BaO–Ar and <cosψ≳=0.989±0.01 or <ψ≳=8.5°±36° for BaO∼CO2 collisions. Significantly smaller angle scattering by CO2 (vs Ar) is manifested by a dramatic line narrowing upon additon of excess CO2, at constant Ar partial pressure. A small monotonic increase in <ψ≳ with ‖ΔJ‖ is found, which confirms the expectation that small angle scattering and concomitant small changes in J result from long-range collisions, whereas wide angle cm scattering with large ΔJ results from the short range part of the intermolecular potential.