Study of Nb2O y (y = 2-5) anion and neutral clusters using anion photoelectron spectroscopy and density functional theory calculations

Vibrationally resolved photoelectron spectra of MgO- and ZnO- have been recorded at several photon energies under varied experimental conditions. Peaks in these highly structured spectra have been assigned to photodetachment transitions from the MgO- and ZnO- ground state (X 2Σ+) to vibrational progressions in the ground and several low lying neutral excited states. In addition, a high-temperature MgO- spectrum shows spectral features due to photodetachment from an excited electronic state of the MgO- anion, which has been assigned to an A2Π anionic state. From the MgO- spectra, the electron affinity of the MgO ground state (XΣ+) is determined to be 1.630 (0.025) eV. Four electronic excited states of MgO, a3Π, A1Π, b3Σ+, and B1Σ+, were found to lie 2510, 3390, 8390, and 20 000 cm-1 above the X1Σ+ neutral ground state, respectively. The excited MgO- AΠ anion state was found to lie 4791 cm-1 above the MgO- X2Σ+ anion ground state. The photoelectron spectra of ZnO-, presented here at higher photon energies, extend a previous photoelectron study by Fancher et al. to the first two excited neutral states, aΠ and A1Π, which have been found to lie 2460 and 4960 cm-1 above the X1Σ+ ground state, respectively. From Franck−Condon analyses of the well-resolved vibrational progressions for each electronic transition, equilibrium internuclear distances and fundamental vibrational frequencies of the MgO and ZnO neutral electronic states were determined. Moreover, because the sources employed produced vibrationally hot anions, the bond length and vibrational frequencies of both the MgO- ground and excited states were found from the vibrational hot band transitions.

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A matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometer was developed which uses a novel reflectron composed of a grounded cylinder and an adjustable endcap electrode to provide high-order kinetic energy focusing for a miniaturized mass analyzer. The nearly quadratic potential form of the reflecting field focuses ions desorbed from a source of very small dimensions formed by placing the sample probe within the centered hole of the coaxial dual channel plate detector. At the same time, the depth of the reflectron can be adjusted to accommodate a short drift length between the source/detector and the reflectron. For larger drift lengths, in particular to allow the addition of an XY stage for the analysis of sample arrays, endcap reflectron focusing can be combined with time-delayed ion extraction to achieve good mass resolution. The instrument has been used for the analysis of peptides digested with trypsin or carboxypeptidase, and also small DNA oligomers.