Keiji Nakashima

Dissociative recombination of H+2, HD+, and D+2 by collisions with slow electrons

Using the multichannel quantum defect theory (MQDT), dissociative recombination of H+2, HD+, and D+2 via the two‐electron excited superexcited state 1Σg(2pσu)2 is studied for each specified initial vibrational state vi (≲4) of molecular ions at low electron energies 0.02≲E≲1.0 eV. The necessary basic physical quantities, electronic coupling V(R) and quantum defect as a function of internuclear distance R, are obtained from our previous scattering calculations. As a by‐product we have also obtained the singlet scattering d‐wave partial cross sections for the various vibrational transitions of molecular ions by electron impact. It is found that the Franck–Condon like factor between vibrational state and dissociative continuum is most decisive in characterizing the overall magnitude and energy dependence of the cross sections. Even the vibrational transitions occur predominantly via the dissociative superexcited state. This suggests that the preionization mechanism of the states of the 1Σg symmetry is very d...

Autoionization mechanism of the NO molecule: Calculation of quantum defect and theoretical analysis of multiphoton ionization experiment

Quantum defects of lλ‐Rydberg states (l=s,p,d, λ=σ,π,δ) of NO were calculated by SCF and CI methods at various internuclear distances. Employing the multichannel quantum defect theory (MQDT), the recently observed resonantly enhanced multiphoton ionization (REMPI) photoelectron spectra were analyzed with use of the calculated quantum defects. Analysis was made particularly to the autoionization of vibrationally excited Rydberg states of σ and δ symmetry. It is concluded that the autoionization is dominated by the two‐step electronic mechanism in which dissociative superexcited state plays a role of intermediate state. Potential curves and electronic coupling strengths of the relevant dissociative states (B’ 2Δ and I 2∑) were estimated from this analysis. This provides us with useful information to investigate various dynamic processes involving these superexcited states. Importance of dissociative superexcited state is emphasized in reference to the various dynamic processes.

Angular dependence of Doppler profiles of atomic emission produced in electron-molecule collisions: Estimation of anisotropy parameters

The angular dependence of Doppler profiles of atomic fluorescence produced in electron impact dissociation of molecules was simulated in consideration of the effect of the anisotropy of dissociation and the ‘‘polarization’’ in magnetic sublevel. The asymmetry parameter b and the polarization of the electric vector of emission Jp are key parameters of Doppler profiles for the excited atom of known translational energy distribution. The difference of two Doppler profiles taken at 90° and 45°, which is denoted as angular difference Doppler profile, is shown to be useful to estimate these two key parameters.

Angular difference Doppler profiles of the excited hydrogen atom produced in eCH4 collisions and dissociation dynamics of methane

Abstract The Doppler profiles of the Balmer-β lines of H*(n = 4) produced in eCH4 collisions were measured at a resolution of 0.0065 nm. The angular difference Doppler profiles, the difference of two profiles taken at 90° and 45° (55°) with respect to the electron beam, were calculated. Component 1 (the peak of the translational energy distribution at 3 eV and the threshold energy at 21.6 eV) showed a clear anisotropy; the asymmetry parameter b is 0.24 and the anisotropy of atomic emission Jp is 0. The asymmetry parameter shows that the dissociation is slow with respect to the rotation, and/or that the dissociation proceeds toward different directions with respect to the transition dipole moment. These finding that non-spherical Rydberg states converging to either the A2A1 state or the (1t−22 state of CH+4 are intermediate states for the formation of component 1.

Symmetry of the dissociation of hydrogen: angular dependence of the Doppler profile of the excited hydrogen atom (n=4) produced in e-H2 collisions

Abstract The angular dependence of the Doppler profile of the Balmer-β line indicates that the asymmetry parameter, b , is positive and the polarization of the electric vector, J p , is 0.7 ± 0.1 for the formation of H ∗ ( n =4) from H 2 . Thus, H ∗ ( n =4) is produced in a parallel transition, and the transition moment of H ∗ lies along the dissociation axis. This result suggests that the intermediate states for the fast and anisotropic H ∗ ( n =4) atoms should be of the type 1 Σ + u (2pσ u )( n /σ g ).

Laser two-photon ionization spectrometry and photoionization threshold of perylene on the surface of water

Abstract There are a few bands in the two-photon ionization spectrum (wavelength dependence of the photoionization charge) of perylene on the water surface in the 340–440 nm region. The photoionization threshold of perylene on water was 5.95 ± 0.10 eV. The polarization energy of the positive ion ( P + ) was determined as −0.95 eV, which was almost identical with that in trimethylpentane and was about half as much as the calculated value. The effective dielectric constant of water should be much smaller for surface photoionization.

Fano plots for the slow and fast groups of excited hydrogen atoms produced in e-H2 collisions

Abstract Excited hydrogen atoms ( n = 3, 4, 5) produced in the electron-impact dissociation of hydrogen molecules have two groups; slow and fast. The Balmer α, β, and γ lines, obtained at high optical resolution, have been separated into two groups and the emission cross sections of each group have been determined for electron energies of 30–2000 eV. Their Fano plots indicate that the slow group is produced through optically allowed transitions and that the fast group is produced through optically forbidden transitions.

Translational energy distribution and asymmetry parameter of the excited hydrogen atom produced in e-C2H2 collisions: Dissociation dynamics of acetylene

Dissociative excitation of acetylene induced in e‐C2H2 collisions was investigated by measuring the Doppler profile and its angular dependence from H*(n=4) at a resolution of 0.0065 nm. The translational energy distribution and the angular difference Doppler profile were calculated. The Balmer line was unpolarized. There are three major dissociation processes for H*(n=4) from C2H2; their threshold energies and translational energy distributions are (1) 20.8, 0–2 eV, (2) ∼30, 3–5 eV, and (3) 35–40, 6–8 eV, respectively. Component 1 showed a clear anisotropy: the asymmetry parameter b is 0.5±0.1 and the anisotropy of atomic emission Jp is 0. Thus, component 1 should be produced by predissociation through Rydberg states converging to the (2σu)−1 state. The decrease of b from the theoretical maximum is due to arbitrary direction of the momentum transfer vector and a relatively long lifetime of the intermediate excited states. Components 2 and 3 are also anisotropic and b>0.

Distributions over the 4s, 4p and 4d sublevels and hydrogen atom emission cross sections following e-H2 collisions

Abstract The azimuthal quantum number (s, p, d) distributions of the excited hydrogen atom ( n = 4) produced in electron-impact dissociation of H 2 were measured by a time-resolved single-photon counting method. Relative emission intensities at 19–32 eV were approximately I (4s): I (4p): I (4d) = 31:14:53, and those at 50–100 eV were 21:15:62; the cascade effect was small. The absolute emission cross sections (in units of 10 −20 cm 2 ) of three sublevels (s, p, d) were calculated: σ(4s) = 3.8, σ(4p) = 2.7 and σ(4d) = 12.3 at 100 eV. This distribution is consistent with a model in which the dissociation proceeds axially in the Σ and Π symmetries and the azimuthal quantum number distribution within each symmetry is statistical.

Dependence of the laser two-photon ionization signal of anthracene on the electron mobility and the excess energy in non-polar solvents

Abstract The photocurrent and photoionization charge of anthracene in various non-polar solvents have been measured under the thirdharmonics of a Nd-YAG laser excitation. They are quadratically proportional to the laser pulse energy, though those of water are cubically proportional to them. Anthracene exhibited a larger signal in those solvents in which both the electron mobility and the excess energy are large. Such solvents offered a higher sensitivity for the photoionization technique. Degassing showed only a minor effect. The detection limits were 14 ng/dm 3 (ppt) in hexane and 3.0 ng/dm 3 (ppt) in 2,2,4-trimethylpentane.