Bimolecular, single collision reaction of ground and metastable excited states of titanium with O2, NO2, and N2O: Confirmation of D(TiO)

Abstract

A beam-gas arrangement has been used to study the chemiluminescent emission which results when a thermal beam of titanium atoms (2200–2650 K) intersects a tenuous atmosphere (10−4 to 10−5 torr) of thermal O2, NO2, and N2O molecules (300 K). The visible chemiluminescence extends from 7000 to ∼4000 Å for the titanium–O2 reaction and 7000 to ∼3000 Å for the titanium–NO2 and titanium–N2O reactions. The emission which characterizes the Ti–O2 reaction corresponds predominantly to the B 3Π–X 3Δ and C 3Δ–X 3Δ band systems of titanium monoxide. Some weak emission corresponding to the c 1φ–a 1Δ band system is also observed. The emission from the Ti–NO2 and Ti–N2O reactions is considerably more complicated; however, both reactions are characterized by strong B 3Π–X 3Δ and C 3Δ–X 3Δ emission features. The Ti–N2O reaction also displays strong emission from the D–X 3Δ TiO band system. A detailed analysis of the emission spectrum resulting from the Ti–O2 reaction is presented. Although emission is observed from the seven lowest vibrational levels of the C 3Δ state, only the emission from 0⩽v′⩽4 is the result of a bimolecular process. The v′=5,6 levels are formed in a process second order in oxygen molecules. The temperature dependence of the chemiluminescent spectrum is analyzed. From this analysis, we determine a slope heat for the process which produces the titanium atom whose reaction with O2 leads to the observed chemiluminescence. This slope heat is used to deduce the activation energy for the possible processes which lead to TiO C 3Δ chemiluminescence. We combine this analysis with a comparative study of the chemiluminescence resulting from the Ti–O2 and Ti–N2O reactions and conclude that emission from the Ti–O2 reaction is due overwhelmingly to a metathesis involving excited state titanium atoms. Further analysis demonstrates that the reaction of Ti(5F) excited states is the most likely process leading to the observed C 3Δ–X 3Δ emission. Combining the results of temperature and pressure dependence measurements on the Ti–O2(TiO) chemiluminescent spectrum, we deduce a lower bound of 159.8±3 kcal/mole for the dissociation energy of ground state titanium monoxide. This lower bound is in close agreement with recent mass spectrometric studies. The present study is the first to demonstrate the importance of temperature calibration in single collision chemiluminescent studies and demonstrates the implications of such measurements for accurate mechanistic and thermochemical determinations. Ramifications for previous studies are discussed. Using available spectroscopic data, we generate an RKR potential for the first ten levels of the C 3Δ state of TiO and deduce Franck–Condon factors for the C 3Δ–X 3Δ transition. Based upon these results, a semiquantitative indication of relative (v′) vibrational level populations is presented for the Ti–O2 reaction.