The 248 nm photodissociation of KI: Determination of the branching ratio of K(4 2P J) doublets in the presence of Ar, H2, and N2

Abstract

By means of a three-level kinetic model developed previously, the nascent fine-structure branching ratios of K 4 2PJ doublets following photodissociation of KI at 248 nm can be determined accurately in the presence of foreign gases. With the forward and reverse Ar-induced collisional fine-structure mixing cross sections of 15 and 30 Å2 reported by Lijnse, and our measurement of Ar quenching cross section 0.81±0.08 Å2, the nascent branching ratio of the K 4 2P3/2 component is determined to be 0.611±0.002. Analogously, with the N2-induced collisional mixing cross sections of 100 and 190 Å2 reported by Lijnse and Hornman, and our measurement of N2 quenching cross section 18±2 Å2, the branching ratio of K 4 2P3/2 is determined to be 0.608±0.002. The agreement between these values confirms reliability of the kinetic model. However, a lack of confirmity is found in the presence of H2 quencher, using the collisional mixing cross sections of 53 and 75 Å2 reported by McGills and Krause, and our quenching cross section measurement of 2.8±0.3 Å2. The discrepancy is caused by the unsuitable adoption of mixing cross sections. Using the average branching ratio 0.610 determined from the cases of Ar and N2, the H2-induced collisional mixing cross sections can be evaluated to be 51±4 and 90±7 Å2. The obtained quenching cross sections by collisions with foreign gases are consistent with those reported elsewhere except for the case of Ar. The discrepancy of our Ar measurement of 0.81±0.08 Å2 from a reported value <0.07 Å2 does not cause a significant difference in the branching ratio determination; it is because the magnitude of the mixing cross sections are more than ten times larger than the relevant quenching cross sections, such that the dependence on the collisional quenching becomes insignificant.