Rotation-Vibration Energy Transfer in Collisions between OH(A 2Σ+) and Ar and N2

Abstract

Collisional relaxation of $\mathrm{OH}{\mathrm{(A}}^2{\Sigma }^{+}\mathrm{,\; \upsilon \prime ,\; K\prime )}$ in high rotational levels of $\text{υ′\hspace{0.17em}=\hspace{0.17em}0}$ and 1 has been investigated with respect to transitions from rotational levels in $\text{υ′\hspace{0.17em}=\hspace{0.17em}0}$ to levels in $\text{υ′\hspace{0.17em}=\hspace{0.17em}1}$ . Initial nonequilibrium rotational distributions of $\mathrm{OH}{\mathrm{(A}}^2{\Sigma }^{+})$ in $\text{υ′\hspace{0.17em}=\hspace{0.17em}0}$ and 1 were produced by monochromatic photodissociation of H₂O with the radiation of a krypton resonance lamp at 1236 and 1165 Å. The effect of added foreign gases (Ar and N₂) on the population of individual levels in $\text{υ′\hspace{0.17em}=\hspace{0.17em}0}$ and 1 has been studied under steady‐state conditions by observing the emission intensities of individual lines in the (0, 0) and (1, 1) bands of the $\mathrm{OH}{\mathrm{(A}}^2{\Sigma }^{+}{\mathrm{\to X}}^2\mathrm{\Pi )}$ transition. The essential observation was made on the population of the rotational level $\text{K′\hspace{0.17em}=\hspace{0.17em}15}$ in $\text{υ′\hspace{0.17em}=\hspace{0.17em}1}$ . The population of this level increased significantly in the presence of Ar and N₂ beyond the initial population produced from H₂O alone. In comparison, the population of adjacent levels remained relatively unchanged or decreased when foreign gas was added. The effect on the $\text{(υ′\hspace{0.17em}=\hspace{0.17em}1, K′\hspace{0.17em}=\hspace{0.17em}15)}$ level is attributed to the collisional transfer process $$\mathrm{OH}{\mathrm{(A}}^2{\Sigma }^{+}\text{, υ′\hspace{0.17em}=\hspace{0.17em}0, K′\hspace{0.17em}=\hspace{0.17em}20)\hspace{0.17em}+\hspace{0.17em}}{\mathrm{Ar,\; N}}_2\mathrm{\hspace{0.17em}\to \hspace{0.17em}}\mathrm{OH}{\mathrm{(A}}^2{\Sigma }^{+}\text{, υ′\hspace{0.17em}=\hspace{0.17em}1, K′\hspace{0.17em}=\hspace{0.17em}15)\hspace{0.17em}+\hspace{0.17em}}{\mathrm{Ar,\; N}}_2\mathrm{\hspace{0.17em}+\hspace{0.17em}\Delta E,}$$ where the energy difference, $\text{ΔE\hspace{0.17em}=\hspace{0.17em}27}{\mathrm{cm}}^{\text{−1}}$ , between the two levels is small compared to the heat bath energy, $\text{kT\hspace{0.17em}=\hspace{0.17em}250}{\mathrm{cm}}^{\text{−1}}$ . For this process and its reverse, a rate constant of the order of 10⁻¹¹ cm³ molecule⁻¹·sec⁻¹ has been derived. According to angular‐momentum conservation, the process involves, in the case of Ar as collision partner, an increase in the impact parameter which is estimated to be about 0.3 Å.