Transient emitting species in phosphorus chemiluminescence

Abstract

The phosphorescence of phosphorus oxidation is the oldest and the best known chemiluminescing reaction, but a definitive spectroscopic study of this classic system has been lacking. In this paper we report the results of an investigation of the oxidation of P4 vapor under atmospheric conditions, with added H2O or D2O vapor. In spectral studies of small horizontal rectangular cross sections of the phosphorus flame, the relative ratios of emission from the transient species remained the same regardless of the cross section chosen, which permits accurate spectral analysis of the chemiluminescence. We have analyzed the visible and ultraviolet spectrum of the reaction, consisting of discrete band structure in the 228.8–272.1 nm region and a broad continuum onsetting at 335 nm and extending to 800 nm and longer, upon which are superimposed a number of weak bands from 450–650 nm. Discrete band emissions enabled the identification of the bands at 228.8–272.1 nm with the PO γ-system transitions, PO(? 2Σ+) →PO(X 2Π). Spectral changes arising from the substitution of D2O vapor for H2O vapor in the reaction led to the assignment of the weak bands at 450–650 nm to HPO (or DPO), ? (1A″) →? (1A′). The main band emission in the visible region, the broad continuum which cannot be identified with any simple electronic transition, exhibits the kinetic and spectral characteristics of an excimer, an excited state dimer with only one member of the molecular pair being electronically excited. In the case of phosphorus chemiluminescence the dimer is (PO*⋅⋅⋅PO) and the equilibrium reaction for its formation is (PO*⋅⋅⋅PO) ?PO(4Π)+PO(X 2Π) ?PO(B 2Σ+)+PO(X 2Π) The existence of the (PO) *2 excimer has been confirmed by affecting the dynamics of this equilibrium through dilution with buffer gas and through thermal dissociation of the excimer, resulting in the appearance of the formerly quenched (0,0) transition of the PO β emission in the spectrum. Our investigation has eliminated the possibility that the visible continuum arises from either PO2 or HOPO. Enough information has been extracted from temperature dependent studies to construct the approximate shapes of the potential energy surfaces of the ground (Δ?g=35±200 cm−1) and the first excited state (Δ?ex=846±200 cm−1) of the (PO)2* excimer.