Quantum-resolved dynamics of excited states. Part 4.—Radiative and predissociative lifetimes of IF B3Π(0+)

Abstract

The kinetics of decay of excited IF B³Π(0⁺) molecules in defined ro-vibrational states (v′, J′) have been determined under collision-free conditions. Laser-induced fluorescence was used to directly determine the dynamics of excited IF(B); the laser bandwidth was 1 pm, using excitation wavelengths between 440 and 530 nm.

Collision-free lifetimes τ₀ were measured for all stable states of IF(B). The mean values τ₀(1σ) varied monotonically with v′ from (8.80 ± 0.89)µs (6 J′ 0) for v′= 9, down to (6.96 ± 0.47)µs (45 J′ 5) for v′= 0. The mean value of the electric dipole moment |R_e|² for the B–X transition was calculated to be (0.101 ± 0.010) D².

As for BrF(B), the higher-energy vibrational levels of IF(B) showed predissociation. This predissociation of IF in the states v′= 8, J′ 52 and v′= 9, J′ 7 was observed as a shortening of lifetime. All accessible rotational states in the (8,J′) and (9,J′) manifolds above the onset of predissociation had similar lifetimes near 1 µs. However, the (10,J′) manifold showed a rotationally dependent predissociation, with a nearly linear dependence of 1/τ₀ upon J′(J′+ 1). The magnitudes of the predissociation energies in the (8,J′) and (9,J′) manifolds indicate that the predissociating state is a shallow, bound state [Herzberg case 1(b)]. Narrow limits for the (previously uncertain) dissociation energy of IF X¹Σ⁺ can thus be obtained: D⁰₀(IF)=(22 333 ± 2) cm^–1.

The low-lying states of IF are considered in relation to the present and previous results. It is concluded that the predissociation of the (8,J′) and (9,J′) states is probably due to interaction of the B state with a weakly-bound 0⁺ state, designated here as C³Π(0⁺), which dissociates diabatically to I²P+ F²P_½ atoms. Lifetime shortening is ascribed to interaction at long r-values of the bound C³Π(0⁺) state with a repulsive 0⁺ state, that correlates with I²P+ F²P atoms. The strong predissociation in IF(B) found previously at higher energies [in the (11, 45) and higher states] is due to direct interaction of the B state and the repulsive 0⁺ state. Similarities with BrF are indicated.