Kanekazu Seki

Photofragmentation of mono‐ and dichloroethylenes: Translational energy measurements of recoiling Cl and HCl fragments

Translational energy (Et ) spectra of Cl and HCl fragments from vinylchloride, trans‐dichloroethylene, cis‐dichloroethylene, and 1,1’‐dichloroethylene have been measured for the π*←π excitation at 193 nm. Et distribution and angular dependence of the Cl fragment indicate that the two‐center dissociation occurs in a time faster than a rotation period and the recoiling organic radical (the counter fragment) is highly vibrationally excited. In dichloroethylenes, the presence of a second channel producing Cl atoms has been confirmed and attributed to the dissociation from the lower (n,σ*) state through a (π,σ*) state. The Et distribution of HCl fragments is nonstatistical and found to converge to null population at an energy less than half of the total available energy. This convergence point coincides with the value of the local available energy for the elimination reaction (activation energy —ΔH0reaction ). The yield of HCl molecules relative to Cl atoms is estimated to be ∼1.1 for vinylchloride. High effic...

Infrared diode laser kinetic spectroscopy of the CCH radical ν3 band

Infrared diode laser kinetic spectroscopy was applied to the photodecomposition of acetylene at 193 nm to observe the ν3 (C–C stretching) band of the CCH radical. More than one hundred absorption lines were observed between 1750 and 1900 cm−1, but none of them could have been assigned to the ν3 band. This is probably due to CCH molecules being excited to high‐lying vibrational states by the excess energy of the photolysis. In fact, when either hydrogen or deuterium was added to acetylene as a buffer gas, new absorption lines appeared and were assigned to the ν3 fundamental band. The band origin which was derived from the observed spectrum is 1840.5711(4) cm−1, in agreement with the result of a previous matrix study. The spin–rotation interaction constant was found to decrease by as much as 30% upon ν3 excitation, presumably because of mixing with the lowest excited electronic state.

Room-temperature growth of AlN thin films by laser ablation

Excimer laser ablation of compressed AlN powder has been used to grow thin AlN films at room temperature on a variety of substrates. The films have a band gap of 6.15 eV as measured by UV absorption. Examination with a scanning electron microscope and an optical microscope shows that the films are smooth. The IR spectrum has an absorption characteristic of AlN. Growth rates are extremely rapid, exceeding 70 nm/min.

Reaction rates of the CN radical with diacetylene and dicyanoacetylene

Abstract Rates of CN reactions with diacetylene (DA) and dicyanoacetylene (DCA) have been measured at room temperature from the decay of the CN radical at various sample pressures by laser induced fluorescence. The rate constants are (4.2 ± 0.2) × 10 −10 and (5.4 ± 0.2) × 10 −13 in units of cm 3 molec −1 s −1 for DA and DCA, respectively, which follow the general pattern of CN reactions with hydrocarbons and nitriles. In the lower temperature circumstellar envelopes and Titans atmosphere, the CN reaction with C 4 H 2 may be more important, while the reaction with C 4 N 2 may become less important.

Photochemistry of acetylene at 193 nm: Two pathways for diacetylene formation

Nanosecond time‐resolved absorption spectra of diacetylene were observed in the 193 nm excitation of acetylene to the A 1Au(v’3=10) state. The formation of diacetylene was confirmed by comparing it with the spectrum of pure diacetylene. Time behavior of the diacetylene spectrum showed that there are two pathways for diacetylene formation. The first one, which finishes within a laser pulse width of 20 ns (FWHM), is attributed to the reaction of C2H radical. The second one is a slow process of diacetylene formation continuing up to ∼1.5 μs at 50 Torr. This slow process is probably the reaction via an excited triplet vinylidene radical. A Lindemann type unimolecular reaction scheme is introduced to explain the results.

Counterion and position-dependent methylation effects on the excited states of zinc(II) complexes with 1,10-phenanthroline

Abstract Time-resolved electron paramagnetic resonance and luminescence spectra have been observed for the lowest excited states of 1,10-phenanthroline (phen), 2-methyl-1,10-phenanthroline (2-Mephen), 2,9-dimethyl-1,10-phenanthroline (2,9-Me 2 phen), 4,7-Me 2 phen, 5,6-Me 2 phen and their Zn 2+ complexes in rigid solutions at 77 K. The sublevel preferentially populated by intersystem crossing (ISC) is the out-of-plane sublevel in Zn-2-Mephen-Cl 2 and Zn-2,9-Me 2 phen-Cl 2 . The observed anisotropy in the ISC of these Zn 2+ complexes is anomalous for the ligand-localized 3 ππ * state. The luminescence spectra and fluorescence lifetimes indicate that the increment of the ISC rate for the out-of-plane sublevel is responsible for the anomalous anisotropy in the ISC of these Zn 2+ complexes.

Optical and time-resolved electron paramagnetic resonance studies of the excited states of para-methylcinnamic acid and para-methylcinnamate anion

Time-resolved electron paramagnetic resonance (EPR), fluorescence, and phosphorescence spectra have been observed for the excited states oftrans-p-methylcinnamic acid (p-MeCA) andtrans-p-methylcinnamate anion (p-MeCA−) in rigid organic glasses at 77 K. With a stretched-polymer-film technique, we assigned the resonance fields in the time-resolved EPR spectra of the lowest excited triplet (T1) states ofp-MeCA andp-MeCA−. From the analysis of these spectra we concluded that the deviation from planar structure in the T1 state is small inp-MeCA andp-MeCA− and the direction of C=O iss-cis to the ethylenic C=C bond inp-MeCA. The deprotonation appears to have little effect on the zero-field splitting (ZFS) parameters. The ZFS parameters were calculated theoretically using semiempirical molecular orbitals. The sublevel preferentially populated by intersystem crossing (ISC) is Ty in bothp-MeCA andp-MeCA−. However, following the deprotonation ofp-MeCA, (Py−Pz)/(Px−Pz) changes from 5.7 inp-MeCA to 2.1 inp-MeCA− (Pi are relative populating rates; thex andy axes are close to the long and short in-plane molecular axes, respectively, andz-axis is perpendicular to the molecular plane). The fluorescence lifetimes indicate that the deprotonation has little effect on the sum of the three ISC rate constants for the three T1 sublevels. A decrease in acidity ofp-MeCA upon excitation has been observed.

Optical and time-resolved EPR studies of the excited states of azastilbenes and their protonated cations

The effects of protonation on the excited states oftrans-3-styrylpyridine (StP) andtrans-4,4′-dipyridylethylene (DPE) have been studied through measurements of the time-resolved electron paramagnetic resonance (EPR), ultraviolet absorption, and fluorescence spectra in methanol-water mixtures at 77 K. The assignment of the transient EPR signals was carried out with the aid of the stretched poly(vinyl alcohol) films method. From the analysis of these spectra it is concluded that the single protonation appears to have little effect on the zero-field splitting parameters and the anisotropy in the sublevel populating rates of the lowest excited triplet (T1) states of StP and DPE. However, the decay rate constants of the fluorescent states decrease and fluorescence quantum yields increase on single protonation. These experimental results suggest that the single protonation causes a decrease in the intersystem crossing (ISC) rates for the three T1 sublevels. These results are explained in terms of the vibronic mixing between the1nπ* and1ππ* states in the lowest excited singlet state. The assignment of StP to the specified conformer was carried out through the analysis of the anisotropic ISC processes.

Time-resolved electron paramagnetic resonance study of the lowest excited triplet states of trans-cinnamic acid and trans-cinnamate anion

Abstract Time-resolved and steady-state electron paramagnetic resonance spectra have been observed for the lowest excited triplet (T 1 ) states of trans-cinnamic acid (RCOOH) and trans-cinnamate anion (RCOO − ) in methanol-water and in stretched poly(vinyl alcohol) films at 77 K. The zero-field splitting parameters | D | and | E | in the T 1 state slightly increase on deprotonation. The sublevel preferentially populated by intersystem crossing in T y in RCOOH, while the in-plane T x and T y sublevels are prefentially populated in RCOO − (the x and y axes are close to the long and short in-plane molecular axis, respectively).

Laser-induced chemical vapor deposition of aluminum from trimethylaluminum: wavelength and temperature dependence

Abstract Aluminum films were produced by laser-induced chemical vapor deposition (LICVD) from trimethylaluminum (TMA) using KrF (248 nm) and ArF (193 nm) excimer lasers. Film growth was measured at substrate temperatures between 20 and 150°C. It is demonstrated that the deposition rate is governed by the mole fraction of gas phase TMA monomer in the region near the heated susceptor. At 248 nm, growth from the monomer was more than seven times as efficient as that from the dimer. LICVD from monomeric TMA at 248 nm was 3.5 times more efficient than at 193 nm. These experimental results suggest an improved LICVD method for aluminum deposition and deposition of aluminum containing materials such as GaAlAs.