Shuntaro Machida

PHOTOABSORPTION PHOTOIONIZATION, AND NEUTRAL-DISSOCIATION CROSS SECTIONS OF C2H6 AND C3H8 IN THE EXTREME-UV REGION

Abstract The absolute photoabsorption cross sections (σt) and photoionization quantum yields (η) of alkanes (C2H6 and C3H8) have been measured in the 50 – 120 nm region using synchrotron radiation as a light source and an ionization chamber combined with metallic thin film windows. We have evaluated the absolute photoionization cross sections (σi) and the total cross sections for non-ionizing processes, i.e., the neutral-dissociation cross sections (σd) following superexcitation. Several superexcited states have been revealed in these spectra.

Extreme‐ultraviolet photodissociation of N2O in superexcited states

We demonstrate the observation of neutral dissociation, which provides the possibility of a spectroscopy of highly lying superexcited states. The yield spectrum of the undispersed fluorescence radiation of wavelength λf in the region 113≤λf≤180 nm from excited neutral fragments in the photodissociation of N2O is presented in the region of excitation photon wavelength λex in the region 30≤λex≤111 nm (photon energy region 41.3–11.2 eV). We show the evidence of selective or preferential neutral dissociation in the decay of superexcited N2O in competition with autoionization; in particular, the evidence for an important role of neutral dissociation of superexcited N2O followed by the production of ionic fragments. We also show the neutral dissociation of superexcited states with the character of double‐holed doubly excited states located for 30≤λex≤60 nm. The aspect of superexcitation (bound channel of electron promotion) in the region of inner‐valence excitation is discussed in relation with free (continuum)...

Dissociative single and double photoionization with excitation between 37 and 69 eV in N2

Dissociative single and double photoionization of N2 as a function of the exciting-photon energy has been investigated using monochromatized synchrotron radiation in the exciting-photon energy range between 37 and 69 eV by dispersed VUV fluorescence spectroscopy (80 nm fl 200 nm). Relative partial emission cross sections for N I and N II fluorescence have been recorded as a function of the exciting-photon energy. There is strong evidence that the precursors of dissociative single ionization with one of the fragments being excited are the N2 + 2 g -1 2 g + state with its closely lying correlation states only. An approximately linear excess energy dependence of the cross sections for the molecular double photoionization into the N2 2+ D 1 u + state has been observed in the excitation spectrum of D 1 u + X 1 g + fluorescence in N2 2+ close to its threshold. Evidence for dissociative double photoionization with excitation has been found.

Photoabsorption, photoionization and neutral-dissociation cross sections of cyclopropane in the photon energy range of 10-24 eV

We measure the photoabsorption cross sections and photoionization quantum yields of cyclopropane in the photon energy range of 10-24 eV by means of a double ionization chamber equipped with a window and synchrotron radiation as a continuous-wavelength light source. The photoionization cross sections and neutral-dissociation cross sections are derived from them. It is verified that our values are more correct and comprehensive than those reported before. Several superexcited states, valence-excited states and Rydberg states, are found in the curves of cross section against photon energy mentioned above, and it turns out that they are classified into three groups according to the features of the competition between neutral dissociation and autoionization.

Photodissociation of CO: partial cross sections for neutral dissociative excitation

Dispersed vacuum-ultraviolet fluorescence from neutral dissociation fragments in the wavelength range from 100 - 200 nm has been measured after photodissociation of CO as a function of the exciting photon energy between 18.5 and 29 eV. The decay channels of neutral dissociative excitation of the formed CO Rydberg states converging to the D and C states have been analysed. The vibrationally excited levels of the and Rydberg states preferentially dissociate into the fragments . Relative cross sections for the observed lines of and have been obtained. For the lines at 193, 167.5 and 156 nm estimations for absolute cross sections are presented. Evidence for superexcited states has been found in the exciting photon energy range between 24 and 27.5 eV.

Threshold behaviour of neutral photodissociation with excitation of CO probed by dispersed fragment VUV fluorescence

Dispersed VUV fluorescence emitted from photodissociation fragments in the wavelength range between 90 and 200 nm has been recorded following the neutral photodissociation of CO using monochromatized synchrotron radiation for exciting-photon energies close to the respective dissociation thresholds. Qualitatively different energy dependencies of the fluorescence intensities have been observed close to the thresholds. Atomic carbon fluorescence at wavelengths of 119, 128, 132.9, and 146 nm has been observed for the first time after neutral photodissociation with excitation of CO.

CO Rydberg series converging to the D and C states observed by VUV-fluorescence spectroscopy

The undispersed and dispersed vacuum ultraviolet fluorescence ( - 200 nm) after neutral photodissociation excitation of CO has been measured as a function of the exciting-photon energy in the region between 18.5 and 36 eV. The target-gas pressure dependence of the fluorescence intensities for energies below 23 eV was observed to be linear. For exciting-photon energies above 23 eV photoelectron impact excitation of neutral CO from the ground to the CO(A ) state causes a nonlinear pressure dependence of the undispersed fluorescence intensity. CO Rydberg states converging to the D and C states have been assigned on the basis of the recorded spectra. Previous assignments of the (3s,4s,3p,4p) and (3s,4s) Rydberg series could be supported and the identification of their vibrational levels have been considerably extended. Two new progressions ((5s,6s)) have been assigned.

One-electron and multi-electron transitions observed in the excitation function of the dissociative photoionization excitation of

The excitation function of the dissociative photoionization excitation ( DIE) of has been measured with the detection technique of coincidence between photoion and a fluorescence photon emitted from an excited atom N* using extreme- UV synchrotron radiation ( SR) as an excitation source in the energy region of 35 - 65 eV. The axis of the detection system was set at two angles, parallel or perpendicular with respect to the major axis of the elliptically polarized SR, in the present measurement. The coincidence spectra obtained in this energy region show at least two qualitatively different structures originating from two different DIE processes. The major components of the DIE in the present energy region are ascribed to the dissociation of the state which is strongly coupled with correlation states in the symmetry. In addition, DIE from other precursor states in the symmetry or dissociative photo-double ionization excitation ( DDIE) was also observed.

Coincidence measurements of the dissociative photoionization excitation of N2

The excitation function of the dissociative photoionization excitation of N2 has been measured with the detection technique of coincidence between photoion and fluorescence photon using extreme-UV synchrotron radiation as an excitation source. Coincidence spectra obtained at the excitation photon energy in the region of 36.5-44.3 eV show at least two components of the dissociative photoionization excitation, one of which is ascribed to the dissociation of the 2 sigma g-12 Sigma g+ state.

Dissociative photoionization of O2 in the vuv region studied by photoion kinetic energy spectroscopy

Abstract Photoion kinetic energy spectra of O+ produced by the dissociative photoionization of O2 have been measured as a function of photon wavelength in the vacuum-uv region. which have shown both the non-Franck-Condon population of the vibrational O2+ (B2 ∑g−) states near the ionization thresholds via both resonance-assisted photoionization and direct ionization.