Near-infrared photoabsorption by C(60) dianions in a storage ring

We present a detailed study of the electronic structure and the stability of C(60) dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least similar to 1.5 eV. For C(60)(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C(60)(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C(60) to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11 500 cm(-1) to 13 500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C(60)(-) may also be a transition to a cone state

K-shell ionization of low-Z elements in ion-solid collisions and applicability of the local plasma approximation

K-shell vacancy production cross sections are measured using X-ray technique, in collisions of highly charged fluorine ions with various solid targets such as, Cl, K, Fe and Cu, at energies from 50 to 110 MeV. The experimental data is compared with an ab initio model based on local plasma approximation (LPA) and the usually employed ECPSSR. A detailed comparison with the LPA model is presented as a function of generalized perturbation strength.Fil: Kadhane, U.. Tata Institute of Fundamental Research; IndiaFil: Kumar, Ajay. Tata Institute of Fundamental Research; IndiaFil: Montanari, Claudia Carmen. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Tribedi, L. C.. Tata Institute of Fundamental Research; Indi

Thermal properties of clusters and molecules - Experiments on evaporation, thermionic emission, and radiative cooling

This thesis presents experiments performed on clusters and molecules, where the three channels of unimolecular decay have been studied. Evaporation from protonated and negatively charged water cluster have yielded size dependent heat capacities, where the smallest sizes with fewer than $21$ molecules show a heat capacity similar to bulk ice whereas clusters with molecules between $21$ and $300$ have a heat capacity in between that of ice and liquid water. The increase in heat capacity per added molecule in the cluster indicates that the intramolecular degrees of freedom are frozen at the temperatures in the experiment (T$\approx\!\!\!160$~K). Experiments on small mixed water-ammonia clusters resulted in relative evaporation fractions for sizes between a total of three to eleven molecules, and $16$ molecules. The clusters were found to evaporate predominantly water molecules except for clusters containing six or more ammonia molecules. Relative evaporation rates for D$_2$O, HDO, and H$_2$O were measured for NH$_4^+$(H$_2$O)$_4$ with zero to six deuteriums interchanged with the hydrogens. The relative rates were found to be $1:0.71:0.56$. Absolute timedependent cooling rates for hot C$_{60}^-$ were obtained in an electrostatic storage ring with single photon absorption experiment. The cooling of the molecule could be divided into a thermionic emission part and a radiative part, where the crossover between the two occurred at $5$~ms, after which radiation was shown to be the dominant cooling channel. The spontaneous decay profiles were used to extract decay parameters of the large organic anion zink phthalocyanine (ZnPc). Numerical simulations of the decay process show good agreement with measurements, using parameters derived from an analytical approximation also used for fullerenes. Photoabsorption experiments were performed on the much smaller C$_5^-$, showing the presence of strong radiative cooling. The cooling rate was determined by the dependence of the photoinduced neutralization yield vs. photon energy and laser firing time

Electron-Capture–Induced Dissociation of Protoporphyrin IX Ions

Electron-capture induced dissociation of protoporphyrin cations and anions has been studied. The cations captured two electrons in two successive collisions and were converted to the corresponding even-electron anions. About one fifth of the ions lost a hydrogen atom to become radical anions but otherwise very little fragmentation was observed. The anions captured an electron to become dianions. No hydrogen loss occurred, and the only fragmentation channel observed was loss of CO2H, to give a doubly charged carbanion. Our results indicate that protoporphyrin ions are very efficient in accommodating one or even two electrons in the lowest unoccupied molecular orbital of the porphyrin macrocycle, and that electron capture induces only limited dissociation. (J Am Soc Mass Spectrom 2008, 19, 809–813

Near-infrared photoabsorption by C-60 dianions in a storage ring

We present a detailed study of the electronic structure and the stability of C-60 dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least similar to 1.5 eV. For C-60(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C-60(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C-60 to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11 500 cm(-1) to 13 500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C-60(-) may also be a transition to a cone state