Collisional quenching of excited electronic species and the Parmenter et al. correlation.
The model of collision-induced deactivation of excited species proposed by Parmenter and co-workers has been examined in its application to the collisional quenching of PH(2)(A(2)A(1)). The correlation between the quenching efficiency and the molecular properties of the quenchers is seen to be closely obeyed only by rare gases and two molecules, H(2) and CH(4). The other molecular quenchers, conversely, show only a very loose trend of variation with the quantity epsilon(M-M), well depth of the potential between two molecules of the quencher M, which constitutes the basis for application of the Parmenter et al. model. This apparent limited validity of the Parmenter et al. correlation has been ascribed to the fact that the combining rule applied to the well depth epsilon(PH(2)*-M) of the collision complex PH(2)*-M to express this quantity as (epsilon(PH(2)*-PH(2)*))(1/2)(epsilon(M-M))(1/2) is applicable only to rare gases and not to most of the other quenchers. A procedure to find the characteristics of rare gas-like molecules with quenching efficiencies equal to those of the real quenchers has been proposed. A plot of the quenching data vs (epsilon(Mh-Mh)/k)(1/2) of these hypothetical molecules M(h) with the exception of NO, PH(3), NH(3), and H(2)S shows data points lying on the rare gas line. This indicates that the Parmenter et al. correlation is again verified and that the quenching mechanism of the corresponding real quenchers is the same as that of the rare gases. For NO, PH(3), NH(3), and H(2)S it has been demonstrated that chemical reactions should play an important role in the removal of PH(2)(A(2)A(1)), and the relative parts of physical and chemical quenchings have been evaluated. It has also been possible to determine the otherwise inaccesible quantity epsilon(PH(2)*-M), i.e., the well depth of the collision complexes, present in the original equation of the Parmenter et al. model. By plotting the quenching data vs epsilon(PH(2)*-M)/k, it can be seen again that the Parmenter et al. model is followed by all the quenchers treated, except the above-mentioned molecules.
Nguyen Xuan
Chieu
C
Istituto di Metodologie Inorganiche e dei Plasmi, Consiglio Nazionale delle Ricerche, Area della Ricerca di Roma 1, Via Salaria Km 29,300, 00016 Monterotondo Scalo (Roma), Italy. Chieu.NguyenXuan@imip.cnr.it