Anomalously long-lived molecular negative ions of duroquinone.

The problem under investigation here is establishment of mechanisms of the resonant electron capture by molecules, using the example of duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone). A solution is important because it will provide new insights into the fundamental physical laws and widespread applications in various fields like molecular nanoelectronics, touched upon herein too. Resonant electron capture (REC) in duroquinone was studied with negative ion mass spectrometry of the REC as the main method, and UV absorption and the photoelectron spectroscopy as the auxiliary ones. The latter were used to study the electronic structures of the various neutral molecular states that are the parent ones for the negative molecular ions formed by electron attachment to the molecules. B3LYP/6-311 + G(d,p) calculations were widely used throughout the study. As a result, an intensive peak of the negative molecular ions with anomalously high lifetime (200 microseconds) was registered at the attached electron energy of 1.8 eV. The ions were determined to be quartets delaying the electron autodetachment because of spin prohibition and appearing via inter-system crossing from the negative molecular ion doublets produced in the core-excited Feshbach resonances. Finally, the pattern of the REC in duroquinone was obtained for the energy region of 1-4 eV which is presented by shape resonances, core-excited Feshbach resonances and by mechanisms little-known for molecules of inter-shell resonances and the formation of ion quartets. The latter were proposed to be related to the negative differential resistance in molecular nanoelectronics.

Electron capture negative ion mass spectra of some typical matrix-assisted laser desorption/ionization matrices.

A series of seven typical matrix-assisted laser desorption/ionization (MALDI) matrices has been investigated by means of electron capture negative ion mass spectrometry (ECNI-MS). It has been shown that the most effective matrices form deprotonated negative ions predominantly in the low-energy region. Relative dissociative cross sections have been measured for all molecules under investigation. The relative integrated abundance of [M - H](-) ion formation in the series changes by four orders of magnitude. It has been shown that 2,5-DHB (gentisic acid), one of the most effective MALDI matrices, has maximal relative intensity of [M - H](-) formation at the energy approximately equal 0.8 eV. This result is in accordance with a finding of Frankevich and Zenobi [Book of Abstracts, Workshop-school "Mass spectrometry in chemical physics, bio-physics and environmental sciences", Zvenigorod, Russia, April, 25-26, 2002, p. 40] that a probable origin of negative ions in MALDI is the process of low-energy (0.5-1 eV) dissociative electron capture by matrix molecules.

Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones.

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI-MS). 1Cl-AQ and 2Br-AQ display dramatically steep positive temperature dependencies of Hal(-) ion abundance in the low electron energy region. Molecular NI intensity decreases rapidly with increasing temperature in the case of 1I-AQ. In the case of 2Br-AQ, a metastable NI peak (m/z 22.9) corresponding to the process BrAQ(-) --> Br(-) + AQ(0) was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br-AQ(-) is at least approximately 25 micros at the energy approximately 0.67 eV in the low-temperature spectrum (T approximately 80 degrees C), and at the energy approximately 0.13 eV in the hot spectrum (T approximately 290 degrees C). Together with the observed temperature dependence of the 2Br-AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl-AQ. There are three consecutive stages in the process of molecular NI dissociation of Cl- and Br-substituted AQ, namely, electron capture into the empty pi-orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic pi-state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the pi-term into the sigma-term, so-called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I-AQ, an effect of inversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so-called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI-MS, but also in other experimental investigations using low-energy electron-molecule and ion-molecule collisions.