A study of the collisional quenching of Ca(4s4p( 3P J)) and Ca(4s3d( 1D 2)) with N 2O by time-resolved atomic emission and time-resolved molecular chemiluminescence from CaO

Abstract

We present a detailed investigation of the collisional behaviour of the optically metastable calcium atoms, Ca(4s4p( 3P J )) (1.888 eV) and Ca(4s3d( 1D 2)) (2.709 eV), with N 2O. These metastable states were generated directly by pulsed dye-laser excitation at λ = 657.3 and 457.5 nm respectively, and monitored photoelectrically in the time-resolved mode by boxcar integration using the forbidden atomic emission at these resonance wavelengths. The removal of the two atomic states by N 2O has been characterised across the temperature range 725–1100 K, yielding the Arrhenius forms: k(Ca(4 3P J ) + N 2O) = (3.5 +8.5 −2.5) × 10 −10 exp(−36.1 ± 11.6 kJ mol −1/ RT) cm 3 molecule −1 s −1; k(Ca(4 1D 2) + N 2O) = (3.8 +2.9 −1.7) × 10 −10 exp(−17.5 ± 3.7 kJ mol −1 / RT) cm 3 molecule −1 s −1. At elevated temperatures, the atomic decay profiles included the effects of components due to the collisional removal of Ca(4 3P J ) and Ca(4 1D 2) by O 2 resulting from the thermal decomposition of N 2O, which were quantified. The main objective of the present paper concerns the accompanying time-resolved study of molecular chemiluminescence from CaO, involving the monitoring of some forty transitions in different electronic and vibronic states, and characterising the time-resolved molecular emission profiles and relating them quantitatively to parameters describing the atomic emission profiles. The following groups of chemiluminescence transitions were monitored: the CaO “orange arc band” system (nine transitions); the CaO “green arc band” system (two transitions); the CaO(A′ 1ΠX 1Σ +, ν′ = 8–15) band system (twenty-one transitions); the CaO(A 1Σ +X 1Σ +, Δν = −2 to −9 (eight transitions). The combination of the time dependences of the molecular and atomic emission profiles enabled the production of the emitting molecular states to be ascribed either specifically to direct formation from chemical reaction between Ca(4 3P J ) and Ca(4 1D 2) with N 2O or to EE transfer between these two atomic states and CaO(X 1Σ +). The different routes to specific product states of CaO are presented in detail and are found to be in accord with the energetic accessibility of these states and with a correlation diagram connecting the states of Ca + N 2O and CaO + N 2 constructed on the basis of C s symmetry on collision and using the weak spin-orbit coupling approximation. For the case of EE transfer, measurement of the vibrational temperature across ν′ = 8–15 in CaO(A′ 1Π) indicates some degree of vibrational excitation accompanying these processes. Intensity measurements corrected for photomultiplier gain, frequency and Franck-Condon factor yielded estimates of relative populations in the emitting electronic states resulting from these two general routes. Product states arising from direct reaction were found to be in accord with symmetry arguments. Finally, the variation of such corrected molecular emission intensity measurements with temperature yielded results in broad agreement with thermodynamic data.