Jae Kyu Song

Photoelectron spectroscopy of naphthalene cluster anions

Mass spectrometric and anion photoelectron spectroscopic studies of homogeneous naphthalene cluster anions, (Nph)n=2–7−, were conducted to characterize the nature of their anionic cores. The smallest stable species in this series was found to be the naphthalene dimer anion. The vertical detachment energies of naphthalene clusters, n=2–7, were determined and found to increase smoothly with cluster size. By extrapolation, the vertical detachment energy of the isolated naphthalene molecule was found to be −0.18 eV, in agreement with its adiabatic electron affinity value from literature. The strong similarity between the spectral profiles of (Nph)2− and (Nph)1−(H2O)1 implied that (Nph)2− possesses a solvated monomeric anion core. All of the naphthalene cluster anions studied here were interpreted as having monomer anion cores.

Photoelectron spectroscopy and ab initio study of mixed cluster anions of [(CO2)1–3(Pyridine)1–6]−: Formation of a covalently bonded anion core of (C5H5N–CO2)−

Anions of mixed cluster between carbon dioxide (CO2) and pyridine (C5H5N, denoted by Py), each possessing a negative adiabatic electron affinity, were studied. The minimum size cluster anion was the 1:1 complex of [(CO2)1(Py)1]−, which displayed highest mass spectral intensity in the series. It was shown by photoelectron spectroscopy that [(CO2)1(Py)1]− is not a typically expected ion–molecule complex of the type (CO2)1−(Py)1 or (CO2)1(Py)1−. Ab initio calculation in the UHF/6-311++G** level revealed that [(CO2)1(Py)1]− is indeed a totally new anion of the type (C5H5N-CO2)− with a planar C2v structure, bound by an intermolecular bond of a strongly covalent character. The calculation predicted 1.24 eV for the vertical detachment energy of this anion, which is in reasonably good agreement with the measured value of 1.46 eV by photoelectron spectroscopy. A Mulliken analysis showed that the excess negative charge is fully delocalized throughout the entire molecular frame. Further calculation on intramolecular...

Anion clusters of anthracene, Ann−(n=1–16)

We studied the anion clusters of anthracene, Ann− (n=1–16), by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. The magic numbers observed at n=5 and 13 indicated formation of the half-filled and completely-filled first solvation shell, respectively. We found that autodetachment could occur via a short-lived excited state of the anion, producing autodetached electrons at a nearly constant kinetic energy, irrespective of the photon energy. Three distinct forms of anion core previously proposed were confirmed that are monomeric, dimeric, and trimeric in nature. As the clusters grow in size from the monomer, the character of the anion core undergoes multiple switching until the first solvation shell is half-filled. Between the half-filled and completelyfilled first solvation shell, the coexistence of the monomeric and dimeric anion cores was observed at certain cluster sizes, most notably at n=8, 10, and 11. Only the monomeric form of anion core was observed once the first solvati...

Femtosecond photoelectron imaging on pyrazine: Spectroscopy of 3s and 3p Rydberg states

Two-color and one-color resonance enhanced multiphoton ionization photoelectron spectroscopies (REMPI–PES) have been applied to Rydberg states (n=3) of jet-cooled pyrazine. The 3s and 3p members of Rydberg series converging to the ground state (n−1) of the cation and the 3s member of a Rydberg series converging to an excited state of the cation (π−1) were observed. The photoelectron angular distributions (PADs) measured via the 3s(n−1) state drastically differed for the two-color and one-color REMPI experiments. This behavior is ascribed to different molecular axis alignments created by the two-photon excitation schemes. The PADs were also used to discriminate between the different Rydberg series.

PHOTOELECTRON SPECTROSCOPY OF PYRIDINE CLUSTER ANIONS, (PY)N-(N=4-13)

Photoelectron spectroscopy was carried out for mass-selected anion clusters of pyridine (C5H5N=Py) up to (Py)13−. The smallest anion cluster observed was (Py)4−, which exhibited two distinctly different photoelectron bands arising from dipole-bound and valence electron states. A mixed cluster of [(Py)3(H2O)1]− displayed similar features. No dipole-bound state was observed in the larger clusters of neat pyridine, (Py)5–13−, which were interpreted as solvated clusters of pyridine molecular anion, Py−(Py)4–12. Threshold electron binding energies were measured as the upper limit value of adiabatic electron affinities. They increased monotonically from 0.33 eV for the cluster size of n=4 to 1.02u200aeV for n=13. But their incremental change showed a large drop at n=8, as did the incremental change in vertical detachment energy, which was viewed as due to the completion of the first solvation shell at n=7. The energetics of anion solvation suggested nearly pure electrostatic interactions at play. A boundary was dra...

Photoelectron spectroscopy of pyrazine anion clusters

We studied the energetics and character of electron binding in the pyrazine anion clusters by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. The mass distribution showed that the minimum number of molecules in a neat cluster of pyrazine to form an anion was two, with a single pyrazine molecule incapable of accommodating an excess electron. On the other hand, even the addition of a very weak solvent such as Ar sufficed to bring the affinity level of pyrazine below the vacuum level. Photoelectron spectra of some pyrazine-containing anion clusters, (Pz)1−–Arn (n=1–10) and (Pz)1−–S1 (S=pyrazine, benzene, and water), were obtained. A vibrational progression was observed in the photoelectron spectra of (Pz)1−–Arn. The electron affinity of pyrazine was determined to be −0.01±0.01u2009eV from extrapolation. A small drop in incremental electron affinity was observed from (Pz)1−–Ar4 to (Pz)1−–Ar5, indicating closure of the first solvation shell by four Ar atoms. The pyrazine dimer anion wa...

Simultaneous observation of dipole-bound and valence electron states in pyridine tetramer anion

Coexistence of dipole-bound and valence electron states was observed by photoelectron spectroscopy in pyridine tetramer anion despite the fact that pyridine itself does not bind electrons by either mechanism. The photoelectron yield for the two states showed distinctly different laser polarization dependence. The fact that these two states have rather disparate thermodynamic stabilities suggests that there may exist an energy barrier separating the two forms in configuration space of the cluster.

Photoelectron spectroscopy of s-triazine anion clusters: Polarization-induced electron binding in aza-aromatic molecule

Photoelectron spectroscopy was carried out for the mass-selected cluster anions of s-triazine molecule, Tzn−u2009(n=1–6). The mass spectrum and vibrationally resolved photoelectron spectrum of Tz− showed that unlike pyridine and pyrazine, Tz binds an electron and thus becomes the first molecule in the azabenzene series with a positive electron affinity (0.03 eV). This indicates that the local charge polarization in the aromatic ring by the three nitrogen atoms is large enough to facilitate electron binding to a homologue of benzene. A Jahn–Teller distortion was proposed to explain the vibrational progressions of the photoelectron spectrum of Tz−. A series of Ar-solvated clusters of Tz−, Tz−⋅Armu2009(m=1–7), have been also studied. Their photoelectron spectra showed a drop in the incremental electron binding energy when going from m=4 to 5, indicating the closure of a solvation shell with four Ar atoms. In the mass abundance spectrum of Tzn−, a distinctly high intensity for Tz2− indicated its exceptional stability...

The naphthalene-benzene anion: Anion complex of aromatic hydrocarbons with the smallest electron affinity

The mixed cluster anions between naphthalene and benzene were studied by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. We found clear evidence for a stable anion of the 1:1 complex, [(Np)1(Bz)1]−, which has the smallest electron affinity measured so far for complexes of aromatic hydrocarbons. The photoelectron spectrum of [(Np)1(Bz)1]− was identical in shape with those of (Np)2− and [(Np)1(H2O)1]−, implying the presence of a common ion core, (Np)−, in these systems. Thus [(Np)1(Bz)1]− is viewed to be (Np)1−(Bz)1, where Bz stabilizes the core anion, (Np)−, as a neutral solvent. The geometry of (Np)1−(Bz)1 was calculated to be T-shaped, with a strong character for the π-hydrogen bonding. An adiabatic electron affinity of 0.01 eV was determined from the measured vertical detachment energy of 0.03 eV and the theoretical estimate of the energy difference between the two neutral structures involved.

Photoelectron spectroscopy of pyrene anion clusters : Autodetachment via excited states of anion and intermolecular interactions in anion clusters

This study examined the anion clusters of pyrene (Py) by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. The photoelectron spectra of Py(n)(-) (n=1-4) were obtained at various photon energies. A change in photodetachment wavelength resulted in a large change in the relative intensities of vibrational progression in the photoelectron spectra. It is proposed that the observed modulation of the Franck-Condon factors by the different photon energies reflects autodetachment via the excited states of anion. The photoelectron spectra of Py(n)(-) at 355 nm showed a broad band structure between the S(0) and T(1) states, which is also due to the autodetachment via a Feshbach resonance state. The photoelectron spectra of Py(2)(-) suggest the presence of a unique dimeric interaction between the two pyrene moieties, whereas the spectral features of Py(3)(-) are similar to those of Py(1)(-). The stable structures of Py(2)(-) and Py(3)(-) obtained by density functional theory calculations support the experimental findings, where different intermolecular interactions govern the stabilization of these two species.