Photodetachment and photofragmentation studies of semiconductor cluster anions

Delayed photodissociation of the tin cluster Sn22. Phys Chem Chem Phys 2024;26:1105-1112. [PMID: 38098439 DOI: 10.1039/d3cp04476a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Millisecond-delayed photodissociation of gas-phase Sn22- clusters stored in a Penning trap is investigated as a function of excitation energy. Sn15- is the only significant charged fragment, indicative of the break-off of neutral heptamers. Fits of the time-resolved fragmentation require a distribution of decay constants, caused by the finite width of the internal energy distribution of the cluster ensemble prior to photoexcitation. A lower limit for the dissociation energy for the loss of Sn7 is determined to be 2.1(1) eV, a factor of two above literature quantum chemical calculations.

Energetic degeneracy and electronic structures of germanium trimers doped with titanium

Geometries and electronic structures of germanium trimer clusters doped with titanium TiGe₃ ^-/0 were studied making use of the complete active space self-consistent field followed by second-order perturbation theory explicitly correlated coupled cluster singles and doubles method with perturbative triples corrections CCSD(T)-F12 and Tao-Perdew-Staroverov-Scuseria methods. Two electronic states (²A' and ²A″) of the anion (pyramid shape) were determined to be nearly degenerate and energetically competing for the anionic ground state of TiGe₃ ^-. These two anionic states are believed to be concurrently populated in the experiment and induce six observed anion photoelectron bands. Total 14 electronic transitions starting from the ²A' and ²A″ states were assigned to five out of six visible bands in the experimental anion photoelectron spectrum of TiGe₃ ^-. Each band was proven to be caused by multiple one-electron detachments from two populated anionic states. The last experimental band with the highest detachment energy is believed to be the result of various inner one-electron removals.

Thermal radiation and fragmentation pathways of photo-excited silicon clusters

The fragmentation of laser heated silicon clusters was studied by time-of-flight mass spectrometry. For Si(n)(+) (n = 5-19, 21), the lowest energy fragmentation pathways were identified as the metastable decay channel occurring after the primary acceleration of the ions. The radiative cooling of laser excited Si(n)(+) (n = 5-9, 11, and 13) was quantified via its quenching effect on the amount of metastable fragmentation. The quenching varied strongly with cluster size, from no observable amount for Si7(+) to a cooling constant of 3 ⋅ 10(5) s(-1) for Si13(+). In addition, based on the observed fragmentation channels, the ionization energies and the relative binding energies of the clusters were partially ordered, and several ionization energies have been bracketed more precisely.

Atomic scale dynamics of ultrasmall germanium clusters

Starting from the gas phase, small clusters can be produced and deposited with huge flexibility with regard to composition, materials choice and cluster size. Despite many advances in experimental characterization, a detailed morphology of such clusters is still lacking. Here we present an atomic scale observation as well as the dynamical behaviour of ultrasmall germanium clusters. Using quantitative scanning transmission electron microscopy in combination with ab initio calculations, we are able to characterize the transition between different equilibrium geometries of a germanium cluster consisting of less than 25 atoms. Seven-membered rings, trigonal prisms and some smaller subunits are identified as possible building blocks that stabilize the structure.

Molecular anions

The experimental and theoretical study of molecular anions has undergone explosive growth over the past 40 years. Advances in techniques used to generate anions in appreciable numbers as well as new ion-storage, ion-optics, and laser spectroscopic tools have been key on the experimental front. Theoretical developments on the electronic structure and molecular dynamics fronts now allow one to achieve higher accuracy and to study electronically metastable states, thus bringing theory in close collaboration with experiment in this field. In this article, many of the experimental and theoretical challenges specific to studying molecular anions are discussed. Results from many research groups on several classes of molecular anions are overviewed, and both literature citations and active (in online html and pdf versions) links to numerous contributing scientists' Web sites are provided. Specific focus is made on the following families of anions: dipole-bound, zwitterion-bound, double-Rydberg, multiply charged, metastable, cluster-based, and biological anions. In discussing each kind of anion, emphasis is placed on the structural, energetic, spectroscopic, and chemical-reactivity characteristics that make these anions novel, interesting, and important.

Dynamics of Clusters Initiated by Photon and Surface Impact

Clusters of atoms/molecules show dynamics characteristic of the method of excitation. Two contrasted processes are discussed: (1) electronic excitation via single-photon absorption and (2) impulsive excitation of nuclear motions by surface impact. Process 1 is exemplified by photodissociation dynamics of size-selected metal cluster ions. The electronic energy is converted most likely to vibrational energy of internal modes; dissociation follows via statistical mechanism to produce energetically favored fragments. Exceptionally, a silver cluster ion, Ag4(+), is shown to undergo nonstatistical dissociation along the potential-energy surface of the excited state. Energy partitioning to translational and vibrational modes of fragments is analyzed as well as bond dissociation energies. Furthermore, the spectrum of the photodissociation yield provides electronic and geometrical structures of a cluster with the aid of ab initio calculations; manganese, Mn(N)(+), and chromium, Cr(N)(+), cluster ions are discussed, where the importance of magnetic interactions is manifested. On the other hand, momentum transfer upon surface impact plays a role in process 2. An impulsive mechanical force triggers extraordinary chemical processes distinct from those initiated by atomic collision as well as photoexcitation. Experiments on aluminum, Al(N)(-), silicon, SiN(-), and solvated, I(2)(-)(CO2)(N), cluster anions provide evidence for reactions proceeding under extremely high temperatures, such as pickup of surface atoms, annealing of products, and mechanical splitting of chemical bonds. In addition, a model experiment to visualize and time-resolve the cluster impact process is performed by using a micrometer-sized liquid droplet. Multiphoton absorption initiates superheating of the droplet surface followed by a shock wave and disintegration into a number of small fragments (shattering). These studies further reveal how the nature of chemical bonds influences the dynamics of clusters.

Jahn-Teller distortion geometries and electronic structures of Ga3Ge, GaGe3, and their ions

Computational studies on the geometric, electronic, and spectroscopic properties of electronic states are presented for GaGe3, Ga3Ge, and their ions using the CASSCF and DFT(B3LYP)/CCSD(T)/QCISD(T) techniques. The optimized geometries of electronic states, vibrational frequencies, energy separations, Mulliken populations, ionization energies, electron affinities, binding energies are obtained. The anion photoelectron spectra of GaGe3- and Ga3Ge- are predicted. In addition, the results from the low-lying states of GaGe3 and Ga3Ge are compared with those of GaSi3 and Ga3Si. Results demonstrate that the equilibrium geometries of most electronic states of GaGe3, Ga3Ge, and their ions exhibit Jahn-Teller distortion. It is interesting to find that there is not charge transfer between Ga and Ge atoms upon an electron attachment to the ground states of GaGe3 and Ga3Ge, while the amount of charge transfer between Ga and Ge atoms significantly increases upon an electron ionization from the ground state of Ga3Ge.

Theoretical study of electronic structures and spectroscopic properties of Ga3Sn, GaSn3, and their ions

Ground and excited states of mixed gallium stannide tetramers (Ga3Sn, Ga3Sn+, Ga3Sn-, GaSn3, GaSn3+, and GaSn3-) are investigated employing the complete active space self-consistent-field (CASSCF), density function theory (DFT), and the coupled-cluster single and double substitution (including triple excitations) (CCSD(T)) methods. The ground states of Ga3Sn, Ga3Sn+, and Ga3Sn- are found to be the 2A1, 3B1, and 1A1 states in C2v symmetry with a planar quadrilateral geometry, respectively. The ground states of GaSn3 and GaSn3- is predicted to be the 2A1 and 1A1 states in C2v point group with a planar quadrilateral structure, respectively, while the ground state of GaSn3+ is the 1A1 state with ideal triangular pyramid C3v geometry. Equilibrium geometries, vibrational frequencies, binding energies, electron affinities, ionization energies, and other properties of Ga3Sn and GaSn3 are computed and discussed. The anion photoelectron spectra of Ga3Sn- and GaSn3- are also predicted. It is interesting to find that the amount of charge transfer between Ga and Sn2 atoms in the 1A1 state of GaSn3+ greatly increases upon electron ionization from the 2A1 state of GaSn3, which may be caused by large geometry change. On the other hand, the results of the low-lying states of Ga3Sn and GaSn3 are compared with those of Ga3Si and GaSi3.

Theoretical study of structure and photoelectron spectroscopy of In(x)P(y)- and In(x)P(y) (x + y < or = 6) clusters

The geometries and vibrational frequencies of In(x)P(y)- and In(x)P(y) are investigated by hybrid B3LYP functional for x + y < or = 6 and CCSD(T) method for x + y < or = 3. As for the small clusters having two to three atoms, the geometrical and electronic structures and vibrational frequencies at the B3LYP level are in good agreement with those at the CCSD(T) level. Among the most stable structures of In(x)P(y)- and In(x)P(y) (x + y < or = 6) clusters the P-rich clusters are more stable than In-rich clusters. Moreover, we found that those P atoms in In(x)P(y)- and In(x)P(y) (x + y < or = 6) clusters prefer to form a P-P bond, triangle, quadrangle, and pentagon for y = 2, 3, 4, and 5, respectively. Also, the vertical detachment energies of In(x)P(y)- (x + y < or = 6) and electron affinities of In(x)P(y) (x + y < or = 6) clusters obtained by B3LYP are in good agreement with the experimental values available. Theoretically, we show that the electron affinity of In3P3 is very low because, as observed in the experiment, there is a formation of a new P-P bond after an electron is lost from In(3)P(3)-, and we find that the similar phenomena exhibit in In2P4(-) cluster as well.

Formation and stability of charged amino acid clusters and the role of chirality

Amino acid clusters have been studied by several groups and most notably magic number clusters and chiral recognition have been reported. In this work, we have studied the formation of amino acid clusters by electrospray ionization (ESI) and their stability by high-energy collision-induced dissociation (CID). Appearance sizes were determined for multiply charged clusters where the charge is either due to protons or to sodium ions. Finally, we conclude that chiral selectivity plays an important role in cluster formation but seems to be of minor importance for the fragmentation of mixed clusters.

Electronic states of Ga(3)Si, GaSi(3), and their ions

The equilibrium geometries and electronic states of Ga(3)Si, GaSi(3), and their ions are investigated using the complete active space self-consistent-field (CASSCF) and DFT(B3LYP)/CCSD(T) techniques. The (2)B(1), (3)B(1), and (1)A(1) states in C(2v) symmetry with a planar quadrilateral geometry are found to be the ground states of Ga(3)Si, Ga(3)Si(+), and Ga(3)Si(-), respectively. On the other hand, the ground states of GaSi(3), GaSi(3)(-) are also predicted to undergo Jahn-Teller distortion to the (2)A'' and (1)A' states in C(s) with a distorted triangular pyramid geometry, respectively, whereas that of GaSi(3)(+) is found to be the (1)A(1) state in C(3v) with symmetric triangular pyramid structure. Binding energies, electron affinities, ionization energies of Ga(3)Si and GaSi(3) are computed at the CCSD(T)/QCISD(T) level and discussed.

Chemical Bonding in Si52- and NaSi5- via Photoelectron Spectroscopy and ab Initio Calculations

Photoelectron spectroscopy and ab initio calculations are used to investigate the electronic structure and chemical bonding of Si5(-) and Si5(2-) in NaSi5(-). Photoelectron spectra of Si5(-) and NaSi5(-) are obtained at several photon energies and are compared with theoretical calculations at four different levels of theory, TD-B3LYP, R(U)OVGF, UCCSD(T), and EOM-CCSD(T), all with 6-311+G(2df) basis sets. Excellent agreement is observed between experiment and theory, confirming the obtained ground-state structures for Si5(-) and Si5(2-), which are both found to be trigonal bipyramid with D3h symmetry at several levels of theory. Chemical bonding in Si5, Si5(-), and Si5(2-) is analyzed using NPA, molecular orbitals, ELF, and NICS indices. The bonding in Si5(2-) is compared with that in the isoelectronic and isostructural B5H5(2-) species, but they are found to differ due to the involvement of electron densities, which are supposed to be lone pairs in the skeletal bonding in Si5(2-).

Spectroscopic properties of gallium arsenide tetramers: Ga2As2, Ga2As2+ and Ga2As2-

Spectroscopic properties of the low-lying electronic states of Ga2As2 and its ions are studied using the complete active-space self-consistent field (CASSCF) and density function theory (DFT) followed by the coupled-cluster single and double substitutions (including triple excitations) (CCSD(T)) calculations. The stability of low-lying electronic states is examined by computing vibrational frequency. The energies of the ground states and a number of excited electronic states have been computed to predict the spectra of Ga2As2 and its ions. The ionization energy, electronic affinity, and atomization energy are estimated at the CCSD(T)/6-311+G(d) level and compared with the available experimental results.

Search for global minimum geometries for medium sized germanium clusters: Ge12–Ge20

We have performed an unbiased search for the global minimum geometries of small-to-medium sized germanium clusters Gen(12< or =n< or =18) as well as a biased search (using seeding method) for Gen(17< or =n< or =20). We employed the basin-hopping algorithm coupled with the plane-wave pseudopotential density functional calculations. For each size, we started the unbiased search with using several structurally very different initial clusters, or we started the biased search with three different seeds. Irrespective of the initial structures of clusters we found that the obtained lowest-energy clusters of the size n=12-16 and 18 are the same. Among them, the predicted global minima of Gen(12< or =n< or =16) are identical to those reported previously [Shvartsburg et al., Phys. Rev. Lett. 83, 167 (1999)]. For n=17-20, we have identified two or three nearly isoenergetic low-lying isomers (for each size) that compete for the global minimum. Nearly all the low-lying clusters in the size range of 12< or =n< or =20 contain the tri-caped trigonal prism motif and are all prolate in geometry, in agreement with the experiment.