Using the multichannel quantum defect theory (MQDT), dissociative recombination ofH 2 +, HD + , and D 2 + via the two-electron excited superexcited state 1 ~g (2pa u ) 2 is studied for each specified initial vibrational state V; ( S 4) of molecular ions at low electron energies 0.02 SES 1.0 eV. The necessary basic physical quantities, electronic coupling VCR) and quantum defect as a function of internuclear distance R, are obtained from our previous scattering calculations. As a by-product we have also obtained the singlet scattering d-wave partial cross sections for the various vibrational transitions of molecular ions by electron impact. It is found that the Franck-Condon like factor between vibrational state and dissociative continuum is most decisive in characterizing the overall magnitude and energy dependence of the cross sections. Even the vibrational transitions occur predominantly via the dissociative superexcited state. This suggests that the preionization mechanism of the states of the 1 ~g symmetry is very different from that of 1 ~u and 1 flu in the vicinity of ionization threshold. [n -/LA (R)]where n is the principal quantum number and E; (R) is the 726

Rotational effects on the dissociative recombination (OR) of the system t e are investigated using multichannel quanhlm defect theory (MQDT). An explicit MQDT formula is obtained to show the rotational effeu. The cross section calculated with the formula has a resonance struchue induced by the rotation. Its magnitude sensitively depends on the i n i w rotational state in some cases. The mechanism of the rotational effect is discasred in two pam:the direct (open channel) mntribudon and the indirect (closed c h e l ) one. The mtational motion affects the vibronic interaction and the configuration interaction between the initial and the final dissociative state. The rotational effect on the rate constant is also discussed. The resonance struchue induced by the rotation appears even after averaging with an initial rotational distribution which may be appropriate to experimental conditions. The resulting smcture, however, is not necessarily in agreement with experiment Due to the rotational e&cG the DR turns out to depend on the nuclear spin, especially at low temperatureS.

The dissociative recombination (DR) of HeH + is studied using the multichannel quantum defect theory (MQDT) of molecules. The MQDT is extended to include the dissociative Rydberg states. The idea of a "closed dissociative channel" is introduced for a precise description. The calculated DR cross section sensitively depends upon rotational motion, which enhances the DR at a collision energy lower than 0.2 eV. Calculations in the present study reproduced the DR rate coefficient measured by two facilities of storage rings (CRYRING and TARN II). A great change of the adiabatic quantum defect with internuclear distance induced a large DR cross section in the noncrossing system. This mechanism is an alternative to the electronic resonance of the crossing existing system.

The dissociative recombinations of

Dissociative recombination afCH* is studied by using the multichannel quantum defect theory (MQDT). The relevant electronic states were calculated by adopting the self-consistent field (SCF) and the configuration mixing ( C M ) methods. Particular care was taken to represent correctly the diffuse character of the excited states, especially of the two-electron excited states. The dissociative state coupled with the initial channel was found to cross the lowest potential of CH* near the left turning point of the first excited vibrational state. We took into account both the p n and d n partial waves ofthe incident electron in the MQDT treatment. For this purpose. we modified the MQDT formulation by Giuiti so as to fully describe the angular parts. The rate constant obtained is 1 . 1 2 ~ IO-'cm's-' at 120 K, which is about one third of the experimental value of Mitchell andMcGowan. The contribution of the d a wave was found to be comparable to that of the D n wave.