Yoshihiro Yamakita

Raman spectra of polycyclic aromatic hydrocarbons. Comparison of calculated Raman intensity distributions with observed spectra for naphthalene, anthracene, pyrene, and perylene

Abstract Theoretical calculations of Raman spectra have been performed for naphthalene, anthracene, pyrene, and perylene. Normalcoordinate calculations have been carried out by ab initio Hartree-Fock (HF) theory, density functional (DF) theory, and a simple force field model ( MO 8 model). Raman intensities have been evaluated for each normal coordinate by the finite field method using polarizability derivatives calculated with the wave functions from HF and DF theories. The calculated vibrational frequencies show good agreement with observed ones within ∼10 cm−1 at the BLYP 6-31 G ∗ level. Normal coordinates obtained by the MO 8 model generate better intensity distributions than those calculated at the BLYP 6-31 G ∗ level; the average deviation factor of intensity ratios, which is defined as an invariant index for discrepancies of intensity distributions between calculation and experiment, is found to be 1.35–1.97 for MO 8 normal coordinates, whereas it is 1.58–5.57 for BLYP normal coordinates.

Deflection and deceleration of hydrogen Rydberg molecules in inhomogeneous electric fields

Hydrogen molecules are excited in a molecular beam to Rydberg states around n=17-18 and are exposed to the inhomogeneous electric field of an electric dipole. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H2 molecules in the inhomogeneous electric field. The trajectories of the molecules are monitored using ion-imaging and time of flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole, respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k and of the duration of the applied field. It is also shown that a large deflection is observed when populating the (17d2)1 state at zero field and switching the dipole field on after a delay. With the dipole mounted perpendicular to the beam direction, the molecules are either accelerated or decelerated as they move towards the dipole. The Rydberg states are found to survive for over 100 micros after the dipole field is switched off before being ionized at the detector and the time of flight is measured. A greater percentage change in kinetic energy is achieved by initial seeding of the beam in helium or neon followed by inhomogeneous field deceleration/acceleration. Molecular dynamics trajectory simulations are presented highlighting the extent to which the trajectories can be predicted based on the known Stark map. The spectroscopy of the populated states is discussed in detail and it is established that the N+=2, J=1, MJ=0 states populated here have a special stability with respect to decay by predissociation.

Controlling the motion of hydrogen molecules

The deflection and deceleration of neutral hydrogen molecules in the presence of an inhomogeneous electric field is demonstrated. The molecules are laser-excited in a molecular beam to selected Rydberg–Stark states in the field of an electric dipole for a fixed period, and their trajectories are monitored using ion imaging and time-of-flight measurement. The Rydberg states (n � 17) show remarkably long lifetimes (>100 ls) and these results point to a novel method for trapping and cooling non-polar, non-magnetic molecules. 2003 Elsevier Science B.V. All rights reserved.

Molecular vibrations of [n]oligoacenes (n=2-5 and 10) and phonon dispersion relations of polyacene

As model compounds for nanosize carbon clusters, the phonon dispersion curves of polyacene are constructed based on density functional theory calculations for [n]oligoacenes (n=2-5, 10, and 15). Complete vibrational assignments are given for the observed Fourier-transform infrared and Raman spectra of [n]oligoacenes (n=2-5). Raman intensity distributions by the 1064-nm excitation are well reproduced by the polarizability-approximation calculations for naphthalene and anthracene, whereas several bands of naphthacene and pentacene at 1700-1100 cm(-1) are calculated to be enhanced by the resonance Raman effect. It is found from vibronic calculations that the coupled a(g) modes between the Kekulé deformation and joint CC stretching give rise to the Raman enhancements of the Franck-Condon type, and that the b(3g) mode corresponding to the graphite G mode is enhanced by vibronic coupling between the (1)L(a)((1)B(1u)) and (1)B(b)((1)B(2u)) states. The phonon dispersion curves of polyacene provide a uniform foundation for understanding molecular vibrations of the oligoacenes in terms of the phase difference. The mode correlated with the defect-sensitive D mode of the bulk carbon networks is also found for the present one-dimensional system.

Large Raman-scattering activities for the low-frequency modes of substituted benzenes: Induced polarizability and stereo-specific ring-substituent interactions

The large nonresonant Raman-scattering activities of the out-of-plane bending and torsional modes of monosubstituted benzene analogs are studied by low-frequency Raman experiments and B3LYP6-31++G(d,p) calculations. Electronic interactions between the sigma orbitals of the substituent and the pi orbitals of the ring are found to enhance the Raman activities, depending on the substituent and its conformation. In the case of tert-butylbenzene [C6H5C(CH3)3] and trimethylphenylsilane [C6H5Si(CH3)3], three single bonds which are linked to the alpha atom of the substituent have low rotational barriers around the joint bond. Nearly free rotation of the substituents leads to a significant probability for one of the single bonds to occupy a conformation close to the vertical configuration with respect to the ring at room temperature. The resultant sigma-pi electronic interaction gives rise to the large Raman activities. In contrast, those possessing a single bond in a coplanar (or nearly coplanar) configuration at the most stable equilibrium state, i.e., anisole (C6H5OCH3), thioanisole (C6H5SCH3), and N-methylaniline (C6H5NHCH3), display no prominent Raman bands for the low-frequency vibrational modes. In these molecules, the sigma-pi conjugation does not take place due to the orthogonal orientation of the orbitals. Strong conformational dependence of the sigma-pi Raman enhancement is clearly obtained for the metastable vertical conformer of thioanisole, for which Raman activities are one-order magnitude greater than those of the coplanar conformer.

A highly sensitive electron spectrometer for crossed-beam collisional ionization: A retarding-type magnetic bottle analyzer and its application to collision-energy resolved Penning ionization electron spectroscopy

A highly sensitive electron energy analyzer which utilizes a “magnetic bottle” combined with a retarding electrostatic field has been developed for Penning ionization electron spectroscopy. A beam of metastable rare-gas atoms is crossed with a continuous supersonic sample beam in the source region of the analyzer. The emitted electrons are collected by an inhomogeneous magnetic field (the magnetic bottle effect) with a high efficiency of nearly 4π solid angle, which is more than 103 times higher than that of a conventional hemispherical analyzer. The kinetic energy of electrons is analyzed by scanning the retarding field in a flight tube of the analyzer in the presence of a weak magnetic field. The velocity of the metastable atoms can also be resolved by a time-of-flight method in the present instrument. Examples of Penning ionization electron energy spectra as a function of collision energy are presented for Ar and N2 with metastable He*(2 3S) atoms. This instrument has opened the possibility for extensi...

Photoionization and density functional study of clusters of alkali metal atoms solvated with acetonitrile molecules, M(CH3CN)n (M=Li and Na)

Abstract Photoionization mass spectroscopy of clusters of alkali metal atoms solvated with acetonitrile molecules, M(CH 3 CN) n (M=Li and Na), has shown that their ionization potentials decrease from n =1 to 3, while they increase for n ⩾4. Calculations based on density functional theory are also made for these clusters and their cations with n =1–3. The nature of the anomalous n dependence of the ionization potential is discussed in relation to the stabilization of neutral clusters due to dipole–dipole interaction between acetonitrile molecules.

Penning ionization of [2,2]-paracyclophane by collision with metastable He*() atoms

Abstract Penning ionization of [2,2]-paracyclophane (PC) with metastable He*( 2 3 S ) atoms has been studied by means of simultaneous detection of collision energy and electron energy. Negative collision energy dependences of partial Penning ionization cross-sections have been observed in the range of 115–283 meV. This indicates the presence of an attractive potential well around PC. Assignments of Penning ionization electron and HeI ultraviolet photoelectron spectra have been made by outer valence Greens function calculations. Molecular-orbital and model-potential calculations have also been performed, and the characteristic features of electron density and interaction potential are discussed in comparison with the case of benzene.

COLLISION ENERGY RESOLVED PENNING IONIZATION ELECTRON SPECTRA OF POLYCYCLIC AROMATIC HYDROCARBONS

Abstract Penning ionization of naphthalene and anthracene with He*(23S) atoms has been studied by means of simultaneous detection of collision energy and electron energy. The anisotropy of interaction potentials between He*(23S) atoms and the target molecules has been found on the basis of the collision energy dependences of partial Penning ionization cross-sections. Based on the analyses of collision energy dependences and electron correlation calculations, assignments of ultraviolet photoelectron spectra are discussed.

Strong Raman activities of low frequency vibrational modes in alkylbenzenes : conformation specific σ-π interactions between alkyl chain and benzene ring

Abstract Non-resonant Raman intensities for alkylbenzenes were studied based on FT-Raman spectra and density functional calculations. Strong bands were found for a bending mode and a torsion mode in a range of 19–217 cm −1 , where no strong bands were observed for benzene and alkanes. In a staggered form of ethylbenzene and a gauche form of n-propylbenzene, non-totally symmetric torsion modes of 19 and 38 cm −1 gave remarkably enhanced Raman intensities much stronger than ring-breathing modes. Strong Raman activities of low frequency modes in alkylbenzenes can be ascribed to stereo-specific σ–π interactions between alkyl chain and benzene ring.