Formation of Metastable Triplet Acetylene from the Ã(1Au) State Near the Dissociation Threshold

Fluorescence lifetimes of the Ã1Πu state of C3

The fluorescence lifetimes of 115 vibrational levels of the Ã1Π(u) state of C3 have been measured under supersonic molecular beam conditions. Of these, ninety-one are Π(u) vibronic levels, for which the lifetimes lie in the range 190-700 ns. The lifetimes of those Π(u) levels where only the bending vibration is excited lie in the range 190-235 ns. There is very little variation with bending quantum number, and the lifetimes of the two orbital components of the 1Π(u) state are essentially the same. When ν1 and ν3 are excited, the lifetimes become longer and/or reach a maximum for levels with v1 + v3 ~ 4. Excitation of the bending vibration in addition to the stretching vibrations shortens the lifetime slightly. Several of the levels show double-exponential decays. Another 23 levels, of Σ(u)+ vibronic symmetry, mostly have lifetimes that are longer than 300 ns. Interaction with nearby "dark" electronic states, such as B1Σ(u)-, B'1Δ(u), C1Π(g), and b3Π(g), is proposed to account for the observed lifetime lengthening. A particularly clear instance of such an interaction is the long lifetime (914 ns) of a perturbing Σ(u)+ level at 30,181 cm(-1), which is confirmed as belonging to the perturbing B'1Δ(u) state. A single level of Δ(u) symmetry at 29,170 cm(-1), which perturbs one of the Π(u) levels, is shown to belong to the à state.

Excited-state dynamics of acetylene excited to individual rotational level of the V04K01 subband

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.