Clusters in Isentropically Expanding Nitric Oxide and Their Effect on the Chemiluminous NO–O Reaction

Recently a hypothesis was forwarded that molecular clusters are responsible for the enhanced light‐emission rate in the reaction of adiabatically expanded nitric oxide and atomic oxygen. Clusters with masses up to 600 amu have now been detected by mass‐spectrometric sampling of expanding NO jets. The fraction of dimers formed is strongly dependent on $p_0$ , the reservoir pressure, and $d$ , the orifice diameter. Less than 1% dimers were detected at $p_0\text{d\hspace{0.17em}=\hspace{0.17em}10}\mathrm{torr}·\mathrm{cm}$ ; at $p_0\text{d\hspace{0.17em}=\hspace{0.17em}35}$ , the dimer concentration was 5%. When nitrogen is added to dilute NO, larger values of $p_{0}d$ are required to detect dimers. Addition of SF₆ reduces the dimer concentration substantially, because the lower specific heat ratio of the mixture results in higher free‐jet temperatures. Photometric measurements of the NO–O headglow show a direct relationship between radiance and dimer concentration in the expanding NO jet, confirming the above hypothesis. Intense headglows are obtained only for $p_0\text{d\hspace{0.17em}≥\hspace{0.17em}35}$ , and in 50% SF₆–50% NO jets, the headglow radiance is decreased significantly. Between 1 to 10 dimer–atomic‐oxygen collisions yield an electronically excited NO₂*, which subsequently radiates. The spectral intensity distribution of the headglow is similar to the normal NO–O chemiluminescence, indicating that the same excited state is involved. The onset of headglow radiation is within 100–200 Å of the normal afterglow onset; therefore, the dimer bond energy is probably less than 3 kcal mole⁻¹.