Laser-induced fluorescence studies: the B–X transition of Cl2. Part 3.—Collisional energy transfer rates in the B3Π(0 +u) state of Cl2

Abstract

Resolved ro-vibrational states (v′, J′) in the B³Π(O⁺_u) manifolds of ³⁵Cl³⁵Cl and ³⁵Cl³⁷Cl were excited by pulses of 10 ns duration from a 1 pm bandwidth dye laser. States whose energies are less than 500 cm^–1 below the strong predissociation in Cl₂(B) undergo rapid collisional depletion by upward vibrational energy transfer into the unstable part of the Cl₂(B) manifold. Rate constants for vibrational transfer in Cl₂(B) may therefore be determined from lifetime measurements under conditions where the number of collisions is carefully controlled, by using pressures ⩽5 mTorr. The state-to-state rate constants k(v′, Δv) for collisions with Cl₂(X) of Cl₂(B, u′), to form Cl₂[B,(v′+Δv)], were determined as follows (cm³ molecule^–1 s^–1, 1σ, 298 K): k(12, 1)=(1.5 ± 0.2)× 10^–10, k(11, 2)=(1.2 ± 0.2)× 10^–10, k(10, 3)=(5.6 ± 1.0)× 10^–11.

The rate of electronic self-quenching of Cl₂(B) was found to be low. Therefore, at higher chlorine pressures (50 > pCl₂ > 5 mTorr), multiple collisions occur within one lifetime of the excited B state. The initially-formed vibrational distribution is drastically modified by efficient vibrational transfer and the observed fluorescence decay rate is critically dependent on the wavelength envelope of the spectral response of the fluorescence detector. Spurious results, therefore, can readily be obtained if the effects of vibrational transfer are ignored.

At still higher chlorine pressures, pCl₂ > 50 mTorr, vibrational equilibrium in the B state is achieved after a few µs. Fluorescence decay rates thereafter are independent of vibrational transfer and their pressure dependence give a value for the rate constant of electronic self-quenching, k_Q=(6 ± 1)× 10^–12 cm³ molecule^–1 s^–1.