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

Abstract

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, J_o, is monitored by independent preparation of J_o=0 and J_o=15. For Ar, ‖ΔJ‖=1–20 collision rates are found to be independent of J_o and well represented by the power law, where ΔE_r is the change in BaO (A ¹Σ⁺) rotational energy in cm⁻¹; ε=ΔE_r for J_oJ_o≳J. For BaO∼CO₂ collisions a single power law is inadequate to represent J_o=0 and J_o=15 data: Total BaO–Ar J‐changing collision cross sections are: 60±15 Ų for J_o=15 and 40±10 Ų for J_o=0. For BaO–CO₂, total J‐changing collision cross sections are: 230±40 Ų for J_o=15 and 50±20 Ų for J_o=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 3_6° 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.