Doublet-Quartet Intersystem Crossing in Negative Molecular Ions with an Abnormally Long Lifetime

摘要

Formation of negative molecular ions in the gas phase is often studied by negative ion mass spectrometry with resonant electron capture (REC) by molecules [1] in combination with other methods [2-9]. This necessitates deep insight into the regularities of the REC processes. However, there are problems in the field. One of them is an anomalously long (on the order of microseconds) lifetime (tau) (relative to the electron autodetachment) of molecular ions formed by polyatomic molecules at electron energies (E_(el)) of 1-2 eV. Generally, only ions formed in the ground state (~(GM)M~-) at E_(el) close to zero have x values on the order of microseconds. For the ions formed in the gas phase at E_(el) > 0 eV, tau values on the order of 10~(-8)-10~(-9) (is are characteristic since such ions, being involved in single collisions, cannot dissipate the energy excess obtained with the additional electron. Parabenzoquinone (PBQ) is a classical example of this anomaly where negative molecular ions with x = 30 mu s are observed at E_(el) = 1.35 eV [10-12]. Various hypotheses have been put forward in order to explain this phenomenon [10-12], but no one of these hypotheses has received wide acceptance. For this reason, one more hypothesis has been proposed in the present work. In our opinion, the effect of abnormally long x's is associated with the transformation of the doublet ions (~D M~-) forming at E_(el) > 0 eV in the REC process into quartet ions (~QM~- ) with three unpaired electrons with parallel spins and, correspondingly, with a multiplicity of four (M = 4). Such a ~QM~-ion is similar to the doublet ion formed by the intershell resonance mechanism, which also has three unpaired electrons, but with anti-parallel spins [8, 13]. The transformation of the "initial" ~DM~- ion to the ~QM~- ion is a result of intersystem crossing, i.e., the radiationless transition of the system from the potential energy surface (PES) of the former to the PES of the latter, with an electron spin flip ( ) at the intersection point of the PESs: ~DM~-? -> ~QM~-. The main feature of the ~QM~- ion is that it is unable to rapidly decay to the ground state molecule (~(GS)M~0) even if the molecule lies lower on the energy scale than the ion because this process also involves spin flip: ~QM~- ? -> ~(GS)M~0 + e. As a result, the electron autodetachment is delayed in time. This mechanism is analogous to that of phosphorescence; therefore, the autodetachment delay should have the same microsecond time scale.