Masashi Kitajima

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.

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.

Deexcitation of Ne(3P1) and Ne(3P2) in collisions with Ar, Kr, and Xe

The cross sections for the deexcitation of Ne(3P1) and Ne(3P2) by Ar, Kr, and Xe have been measured as a function of the mean collisional energy in the range of 17–38 meV or in the temperature range from 133 K to 295 K using a pulse radiolysis method combined with time-resolved optical absorption spectroscopy. The deexcitation cross sections are in the range of 18–26 A2 and 11–20 A2 for Ne(3P1) and Ne(3P2), respectively, and increase slightly with increasing the collisional energy. The absolute experimental cross sections for the Ne(3P1) deexcitation are in relatively good agreement with the total Penning ionization cross sections obtained by an optical model calculation, which shows the importance of an electron-exchange interaction in the deexcitation of Ne(3P1) in collisions with Ar, Kr, and Xe. The absolute cross section values for the deexcitation of resonant Ne(3P1), which are only slightly larger than those for the deexcitation of metastable Ne(3P2), have also supported the discussion on the deexci...

Deexcitation cross sections of Ne(3P2, 3P1, and 3P0) by molecules containing group‐IV elements

Deexcitation cross sections of Ne(3P2), Ne(3P1), and Ne(3P0) by CH4, SiH4, GeH4, CF4, and SiF4 have been measured at a mean collisional energy corresponding to room temperature (295 K) and compared systematically with those by other molecules to understand general features of the cross section values dependent on both the electronic states of excited neon atoms and the target molecules. A small difference in the cross section values between metastable atoms, Ne(3P2) and Ne(3P0), and a resonant atom, Ne(3P1), has indicated that Penning ionization by Ne(3P1) is mainly governed by an electron exchange interaction rather than a dipole–dipole interaction. A spatial electron distribution of the outermost orbital of a target molecule is especially of great importance in the determination of the absolute cross section values in the deexcitation of excited neon atoms due to an electron exchange interaction.

Deexcitation of He(2 1P) in a collision with Ne

Collisional deexcitation cross sections of He(2 1P) by Ne have been measured in a region of the mean collisional energy between 17 and 38 meV using a pulse‐radiolysis method. Measured cross sections are in a range of 20–25 A2 and increase with increasing collisional energy. The experimental cross sections are compared with the calculated ones for total and partial excitation transfer into highly excited Ne atoms. Fairly good agreement between calculated and experimental results indicates that in the deexcitation of He(2 1P) the excitation transfer into Ne*(6p3, 5s2, or 6s4) is important in the measured mean collisional energy region. It is also shown that a dipole–dipole interaction plays an important role in the excitation transfer from He(2 1P) to Ne.

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.

Doubly excited states of H2 as studied by angle-resolved electron energy loss spectroscopy in coincidence with detecting Lyman-α photons

The differential cross sections of the dissociative double excitations resulting in H(2p) formation in 80 eV electron collisions with H2 have been measured at electron scattering angles of 3, 20, 30 and 40? by means of angle-resolved electron energy loss spectroscopy in coincidence with detecting Lyman-? photons. The doubly excited states of H2 involved in this study are the optically allowed Q1?1?u(1) and Q2?1?u(1) states and the forbidden state at 26 eV, which has not been assigned so far and is temporarily referred to as the F state in this study. At a 3? electron scattering angle the differential cross section of the dissociative excitation to the forbidden F state is the largest among those for the three doubly excited states and decreases with increasing the electron scattering angle more rapidly than those for the allowed Q1?1?u(1) and Q2?1?u(1) states. The differential cross sections for the forbidden and allowed states do not differ quite so much, which may show that the forbidden F state is a dissociative state rather than an autoionizing state. The effective generalized oscillator strengths for H(2p) formation have been compared with the dipole oscillator strengths according to the limit theorem.