Near-Resonant Electronic Energy Transfer from Argon to Hydrogen

The argon sensitized fluorescence of molecular hydrogen has been observed. Relative cross sections have been determined for transitions of the type $${\mathrm{H}}_2{\mathrm{(X\hspace{0.17em}}}^1{\Sigma }_g^{+}\text{,\hspace{0.17em} v=0,\hspace{0.17em}J) +}{\mathrm{Ar}}^{*}\to {\mathrm{H}}_2{\mathrm{(B\hspace{0.17em}}}^1{\Sigma }_u^{+}\mathrm{,\hspace{0.17em}\; v\prime ,\hspace{0.17em}J\prime )\; +}\mathrm{Ar}\mathrm{+\; \Delta \; E,}$$ where Ar^* is either Ar $\text{4s[1}\frac{1}{2}{]}_{\text{J=1}}^0$ or $\mathrm{Ar}\text{\hspace{0.17em}4s′[}\frac{1}{2}{]}_{\text{J=1}}^0$ and the hydrogen molecules, H₂, D₂, and HD are initially thermally distributed among J states at $\text{300 ± 30°}\mathrm{K}$ . Calculations indicate that the largest cross sections, on the order of gas kinetic or larger, are due primarily to long range dipole‐dipole interactions. These cross sections are for transitions for which $\text{|\hspace{0.17em} Δ E\hspace{0.17em} |\hspace{0.17em} 〈 100\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ when $\text{Δ J ≡ J′ − J=± 1.}$ The long range dipolar calculations establish an absolute magnitude scale for the cross sections. A curve crossing mechanism is possible for negative ΔE and provides a plausible explanation for the remaining levels excited. Generally, only one vibrational level of the hydrogen molecule is excited by each argon state. This contrasts sharply with many previous sensitized excitation experiments in which most of the energetically accessible energy levels are excited.