C2N2 photodissociation at 1576 Å. I. CN(A 2Π) radiative lifetimes, nascent vibrational distribution, and C2N2 quenching

Abstract

Photodissociation of C2N2 at the 1576 Å F2 laser line generates CN(A 2Π) up to the thermodynamic limit of v=5, with a bimodal vibrational distribution peaking at v=0 and v=2. Radiative lifetimes for the six observed levels have been obtained by extrapolating time decays to zero pressure, and the data show rapidly decreasing lifetimes with increasing v, in accord with recent theoretical predictions, and contrary to previous experimental determinations. The range of lifetimes observed is 4.3–8.5 μs. Quenching of these levels by the parent molecule shows a factor of 12 increase in rate coefficient in going from v=0 to v=5, correlated to the energy gap between the A 2Π state and the closest lower X 2Σ+ level, suggesting that the quenching mechanism is one involving cross relaxation between CN(A) and CN(X).

EINSTEIN A COEFFICIENTS AND OSCILLATOR STRENGTHS FOR THE A 2 Π- X 2 Σ + (RED) AND B 2 Σ + -X 2 Σ

“…Although the calculated values agree well with the values obtained from several theoretical calculations for the A 2 Π state, they were significantly larger than the most recent experimental lifetimes (Taherian & Slanger 1984;Lu et al 1992). Also, the experimental lifetimes of the A 2 Π state measured by different groups do not agree with each other, as can be seen in Table 5.…”

“…Also, the experimental lifetimes of the A 2 Π state measured by different groups do not agree with each other, as can be seen in Table 5. Among the experimental values, the lifetimes reported most recently by Taherian & Slanger (1984) and Lu et al (1992) have better agreement. Lifetimes of 8.5 ± 0.05, 8.02 ± 0.6, 6.67 ± 0.5, 5.5 ± 0.5, and 4.70 ± 0.2 μs have been reported for v = 0, 1, 2, 3, and 4, respectively, by Taherian & Slanger (1984).…”

“…On the other hand, Snedden & Lambert (1982) have reported much higher values ranging from 14.2 μs for v = 0, to 5.2 μs for v = 10, based on an analysis of the solar spectrum. However, the most recent experimental values of Taherian & Slanger (1984;6.67 ± 0.60 μs for v = 2 to 4.3 ± 0.85 μs for v = 5) and Lu et al (1992;6.96 ± 0.3 μs for v = 2 to 3.38 ± 0.2 μs for v = 5) show better agreement, at least for the lower vibrational levels. In contrast, the experimental lifetimes of the B 2 Σ + state obtained in different studies agree reasonably well with each other (Nishi et al 1982;Duric et al 1978;Jackson 1974;Luk & Bersohn 1973).…”

“…There have been many experimental studies of the lifetimes of the A 2 Π state (Jeunehomme 1965;Katayama et al 1979;Snedden & Lambert 1982;Nishi et al 1982;Duric et al 1978;Taherian & Slanger 1984;Lu et al 1992;Huang et al 1993;Halpern et al 1996) and the B 2 Σ + state (Nishi et al 1982;Duric et al 1978;Jackson 1974;Luk & Bersohn 1973) over the past four decades. It has been noted that the experimental lifetimes of the A 2 Π state reported by different groups show poor agreement with each other.…”

Photodissociation Dynamics of Small Molecules

“…It might be expected that this predissociation would be a strong function of the excess available energy. This ratio has been determined by measuring the change in the CN X state concentration as a function of time (162). The A/X ratio is then the ratio of final CN concentration divided by the initial CN concentration minus one.…”