Reaction of Cr Atoms with O2at Low Pressures: Observation of New Chemiluminescence Bands from CrO2*

Abstract

Ground and low-lying electronic states of Cr atoms in the gas phase were generated from photolysis of <TEX>$Cr(CO)_6$</TEX> vapor in He or Ar using an unfocussed weak UV laser pulse and their reactions with <TEX>$O_2$</TEX> and <TEX>$N_2O$</TEX> were studied. When 0.5-1.0 Torr of <TEX>$Cr(CO)_6$</TEX> /<TEX>$O_2$</TEX> /He or Ar mixtures were photolyzed using 295-300 nm laser pulses, broadband chemiluminescence peaked at ~420 and ~500 nm, respectively, was observed in addition to the atomic emissions from <TEX>$z^7P^{\circ}$</TEX>, <TEX>$z^5P^{\circ}$</TEX>, and <TEX>$y^7P^{\circ}$</TEX> states of Cr atoms. When <TEX>$N_2O$</TEX> was used instead of <TEX>$O_2$</TEX>, no chemiluminescence was observed. The chemiluminescence intensities as well as the LIF intensities for those three low-lying electronic states (<TEX>$a^7S_3,\;a^5S_2\;and\;a^5D_J$</TEX>) showed second-order dependence on the photolysis laser power. Also, the chemiluminescence intensities were first-order in <TEX>$O_2$</TEX> pressure, but the presence of excess Ar showed a strong inhibition effect on them. Based on the experimental results, the chemiluminecent species in this work is attributed to <TEX>$CrO_2^*$</TEX> generated from hot ground state Cr atoms with <TEX>$O_2$</TEX>. The apparent radiative lifetimes of the chemiluminescent species and collisional quenching rate constants by <TEX>$O_2$</TEX> and Ar also were investigated.