Removal rates for the collisional quenching of various vibronic levels of ground state NCO by simple molecules (N2, O2, NO, CO2, N2O, and SO2)
Authors: Fernández José A. , Puyuelo Pilar , Husain David , Sánchez Rayo Marı́a N. , Castaño Fernando 
Published: July 2002

The collisional behavior of NCO[X̃(0,n,0)] in specific vibronic states in the gas phase has been investigated in the time-domain by laser-induced fluorescence (LIF) on transitions within the system NCO(Ã 2Σ+–X̃ 2Π). The NCO radical was generated by the infrared multiphoton dissociation (IRMPD) of phenyl isocyanate (PhNCO) by means of a TEA-CO2 laser operating on the 9R24 line at λ=9.25 μm with subsequent monitoring of the vibronic levels of the X̃ state, characterized by Renner–Teller coupling, in the presence of N2, O2, NO, CO2, N2O, SO2, and PhNCO itself. The states probed were as follows: (0010)2Π3/2, (0010)2Π1/2, (0100)μ2Σ+, (0120)2Δ5/2, (0120)2Δ3/2, (0210)μ2Π3/2,1/2, (0230)2Φ7/2, and (0230)2Φ5/2. Various pairs of spin–orbit states were found to be tightly coupled kinetically. Thus, the time-evolution of the pairs of vibronic states (0010)2Π3/2 and (0010)2Π1/2; (0120)2Δ5/2 and (0120)2Δ3/2; (0230)2Φ7/2 and (0230)2Φ5/2 were found to be equal, yielding an effective local equilibrium within these spin–orbit components within experimental error. Further, states such as NCO(0100) and NCO(0120) were characterized by relatively long decay profiles in the presence of molecules such as CO2 and O2 where the contribution of rotational quenching to the overall decay process could be neglected. By contrast, NCO(0210) and NCO(0230) were removed on significantly faster time scales on collision with SO2. In the absence of extensive information required for solving the large set of coupled differential kinetic equations, albeit reduced in number of those states strongly coupled kinetically, such as a detailed knowledge of the nascent state distributions in NCO following IRMPD, not necessarily Boltzmann in character, the vibronic states were taken to behave independently as the most practical approach to this study. Absolute second-order rate data for the collisional quenching of NCO in the vibronic states (0010), (0100), (0120), (0210), and (0230) by the above molecular species are reported. No clear selection rules are apparent except for the low propensity rule ΔK=2 within the same vibronic state, i.e., μ 2Σ+(0100)–2Δ5/2(0120) and Π43/2,1/2(0210)–2Φ7/2(0230). This is presumed to reflect the role in the collisional interaction of the oscillating dipole in the vibronic state, facilitating ΔK=1, whereas ΔK=2 would involve the quadrupole which is smaller. It is found that the data for (V–V) and (V–T) energy transfer correlate best with the attractive part of the potential curves between the collision partners using the established Parmenter–Seaver plots, yielding well depths [(εMM/kB)1/2] for the vibronic states NCO[μ 2Σ+(0100), ∂25/2(0120), (0210), and Φ47/2(0230)], significantly larger than those of the closed shell collision partners and equal within experimental error. The data are also considered in terms of a multipolar attractive force model involving a collision complex where a sensible correlation is found between the computed and observed collision cross sections for O2, N2, CO2, N2O, and SO2 assuming no change in the multipoles with vibrational state.