Yohji Achiba

Photoelectron angular distribution of simple molecules at 30.4 nm photons

Abstract The asymmetry parameter (β) of angular distribution of photoelectrons has been obtained with He(I) (58.4 nm) and He(II) (30.4 nm) resonance radiation for several simple compounds (N2, O2, H2O, CO2 and CS2), in order to examine how the β value depends on the excitation energy. It has been found that on going from He(I) to He(II) radiation the energy dependence of β for the nonbonding orbitals of triatomic molecules is similar to that obtained theoretically by Manson for the corresponding atomic p shells. For N2 it has also been found that there is essentially no difference in β values between two vibrational peaks (υ′= 0 and 1) of the first ionisation band in the He(II) spectrum, whereas in the He(I) spectrum an anomalous variation is observed in the β values between the two peaks as originally indicated by Carlson. These results support the interpretation of the autoionisation mechanism suggested by Mintz and Kuppermann.

He I (584 Å) photoelectron spectra and photoionization cross sections of atomic chlorine and bromine

Abstract Well-resolved He I photoelectron spectra of atomic chlorine and bromine have been obtained without serious background electrons, giving rise to relative photoelectron intensities associated with differential photoionization cross sections at an angle of 90° with respect to the incident 584 A radiation.

Ionic photodissociation and ionic photosensitized dissociation of charge-transfer complexes

Abstract Flash spectroscopic and kinetic studies have been carried out on the charge-transfer complex of tetracyanoethylene with tetrahydrofuran in a liquid paraffin solution at room temperature. The rise and decay curves in transient electronic absorption intensity have been observed with a common rate constant, corresponding to the anion radical of tetracyanoethylene and the triplet state of the charge-transfer complex, respectively. From the kinetic analysis it has been concluded that the ionic photodissociation of this complex takes place in its lowest excited triplet state. This dissociation mechanism has also been confirmed by employing a triplet energy transfer technique with which “ionic photosensitized-dissociation” phenomenon is observed. Furthermore, a few other examples of ionic photosensitized-dissociation are demonstrated in rigid glasses with typical weak charge-transfer complexes whose photodissociation processes are well-known.

Vacuum UV photoelectron intensity of gaseous compounds: I. HeI spectra of simple compounds

Abstract A new method for determining absolute photoionization cross sections for gaseous compounds is proposed. In this method a mixture of a sample and a standard gas (N 2 ) is used in photoelectron intensity measurements so that the relative intensity of the component is obtained with respect to N 2 The relative photoelectron band area is converted to the absolute photoionization cross section on the basis of the absolute cross-section data of N 2 recently reported by Samson et al. This method has been applied to various aliphatic compounds to study the effect of alkyl substitution on photoionization cross sections of O- and N-nonbonding electrons for 584-A radiation.

Biphotonic ionic dissociation of weak charge-transfer complexes in glassy solutions at 77 K

Abstract The dependence of photoinduced radical anion formation upon exciting light intensity has been studied for glassy solutions of tetracyanobenzene (TCNB) in ether—isopentane and 2-methyl-tetrahydrofuran (MTHF) at 77 K. It has been found that in ether—isopentane glass the TCNB anion is formed biphotonically via the lowest excited triplet state of the 1:1 TCNB—ether CT complex, whereas in MTHF glass the ionic photodissociation of the TCNB—MTHF CT complex occurs monophotonically in its lowest excited singlet state.

Photoinduced radical-anion formation from a solute-solvent electron-donor-acceptor complex. Enhancement of radical-anion formation by a positive-charge scavenger in rigid glass at 77 K

Solutions of tetracyanoethylene (TCNE) in 2-methyltetrahydrofuran (MTHF) glass containing a positive-charge scavenger (benzene or toluene) have been irradiated by UV light at 77 K by exciting only the TCNE-MTHF EDA complex. The photoinduced radical-anion formation from an ion-pair state (TCNE−…MTHF+) was found to be enhanced by adding the scavengers to the EDA system. From the measured quantum yield of the trapped TCNE− species, it may be concluded that positive-charge transfer takes place from MTHF+ to a scavenger molecule when both species come into contact with each other.

Mechanism of photoinduced anion formation of pyromellitic dianhydride in n-donor solvents

Abstract The following conclusions have been drawn from flash photolysis experiments of pyromellitic dianhydride (PMDA) in various organic solvents at room temperature. (1) Short lived transients initially appearing in ether, dioxane, and tetrahydrofuran solutions are attributable to the triplet excited states of change-transfer (CT) complexes formed between PMDA and these etherial solvents. (2) Long lived transients secondarily formed in the etherial solvents are identified as the anion radical of PMDA, the anion formation taking place via the CT triplet state. (3) In acetonitrile solution, however, the anion formation occurs mainly through the excited singlet state of the CT.

HeII(304Å) photoelectron spectrum of N2O4

Abstract The 304 A HeII photoelectron (PE) spectrum of a mixture of NO 2 and N 2 O 4 in the gaseous phase has been measured in the region up to 29 eV. By subtracting the HeII spectrum of NO 2 from the mixed spectrum, the HeII spectrum due to N 2 O 4 has been deduced, indicating that there are three new PE bands in the region 20—29 eV. Relative band intensities have also been obtained from the HeII of N 2 O 4 below 16 eV, corrected for electron collecting efficiency.

Angular distribution of HeII photoelectron spectrum of H2

Abstract The angular distribution of the photoelectron spectrum from H 2 at 304 A HeII resonance radiation has newly been measured. The dependence of the asymmetry parameter β for H 2 on the vibrational state is discussed by comparison of experimental results with available theoretical results for HeI and HeII radiation.

Weak triplet-triplet absorption spectra of cyanobenzenes

Abstract Transient absorption spectra in the visible region have been measured for benzonitrile, o -, m - and p -dicyanobenzene, and 1,2,4,5-tetracyanobenzene in ether-isopentane-ethanol glass at 77 K by means of a flash spectroscopy technique with a sample cell of long path length. Weak transient absorptions decaying at the same rates as the corresponding phosphorescences have been detected in the visible region (400–600 nm), and are attributed to Tue5f8T transitions which may be associated with local excitations within the benzene ring.