Lifetime measurements and quantum-defect theory treatment of the k 3Πu- state of hydrogen molecule

The experimental and theoretical lifetimes for rovibronic k (3)Pi(u)(-) states of H-2 and D-2 isotopomers have been investigated over a wide range of vibrational v(') and rotational N-' quantum numbers. Lifetimes have been measured by a delayed coincidence method, combined with direct electron-impact excitation of the ground state molecules and single photon counting techniques to detect induced fluorescence to the a (3)Sigma(g)(+) state. Pronounced pressure-dependence of the experimental lifetimes was observed and properly taken into account. The pure radiative lifetimes of the k (3)Pi(u)(-) states were estimated using theoretical transition dipole moments responsible for the visible k (3)Pi(u)-->a (3)Sigma(g)(+) transition plus infrared emission on the higher-lying (3)Lambda(g) states belonging to the 3s,d (3)Lambda(g) complex. Both the predissociative and autoionization decay rates were predicted by the Fermi-Golden rule based on radial coupling matrix elements for the k (3)Pi(u)similar toc (3)Pi(u) and k (3)Pi(u)similar toX(2)Sigma(g)(+)(H-2(+)) pairs of interacting states, respectively. The required electronic matrix elements as a function of internuclear distance R were derived in the framework of quantum-defect theory modified to allow explicit consideration of regular radial coupling effects. The relevant quantum-defect functions of all states treated were extracted from published highly accurate Born-Oppenheimer potential curves. Both the total theoretical radiative, predissociative plus autoionization rates and the calculated rovibronic term values agree well with their experimental counterparts. The N-'-dependence of the experimental and theoretical lifetimes is found to be negligible for both isotopomers while a pronounced v(')-dependence is observed for a H-2 isotopomer. The vibrational predissociation is very weak comparing with radiative decay for both isotopomers whereas the autoionization rate is comparable with the radiative ones for the H-2 v'greater than or equal to4 levels though it is still negligible for the D-2 v(')less than or equal to6 levels. (C) 2003 American Institute of Physics.