Department of Chemistry, University of Puerto Rico, Rio Piedras, P.O. Box 23346, San Juan, Puerto Rico 00931-3346.
Dynamics of the IR emission induced by excitation of the acetylene molecule using the (3(2)K(a) (0,1,2),A (1)A(u)<--4(1)l(a) (1),X (1)Sigma(g) (+)) transition was investigated. The observed IR emission was assigned to transitions between the ground-state vibrational levels. Acetylene fluorescence quenching induced by external electric and magnetic fields acting upon the system prepared using the (3(4)K(a) (1),A (1)A(u)<--0(0)l(a) (0),X (1)Sigma(g) (+)) excitation was also studied. External electric field creates an additional radiationless pathway to the ground-state levels, coupling levels of the A (1)A(u) excited state to the quasiresonant levels of the X (1)Sigma(g) (+) ground state. The level density of the ground state in the vicinity of the excited state is very high, thus the electric-field-induced transition is irreversible, with the rate constant described by the Fermi rule. Magnetic field alters the decay profile without changing the fluorescence quantum yield in collisionless conditions. IR emission from the CCH transient was detected, and was also affected by the external electric and magnetic fields. Acetylene predissociation was demonstrated to proceed by the direct S(1)-->S(0) mechanism. The results were explained using the previously developed theoretical approach, yielding values of the relevant model parameters.