Kosei Kameta

Photoabsorption, photoionization, and neutral-dissociation cross sections of simple hydrocarbons in the vacuum ultraviolet range

Abstract The absolute photoabsorption cross sections, σt, of CH4 and n-C4H10 have been measured in the photon energy range of the valence electrons using a double ionization chamber equipped with a metallic thin film window and synchrotron radiation as a continuous-wavelength light source. The absolute photoionization quantum yields, η, of CH4 have been also measured, from which the photoionization cross sections, σi, and neutral-dissociation cross sections, σd, are obtained. The values of σt and η of C1–C4 normal alkanes including our previous results for C2H6 and C3H8 have been compared in detail with those by the dipole-simulation method using the virtual photons. The gross features of the σt and η values of these alkanes are discussed in terms of those as functions of the photon energy and the number of C atoms in an alkane molecule.

Ionizing and nonionizing decays of superexcited acetylene molecules in the extreme-ultraviolet region

Absolute measurements of the photoionization and photodissociation cross sections of C2H2 have been made using continuum monochromatized synchrotron radiation in the 53–93 nm region. The absolute photoabsorption cross section and photoionization quantum yield of C2H2 have also been measured. The excitation spectra of C2(d 3Πg→a 3Πu), C2(C 1Πg→A 1Πu), CH(A 2Δ→X 2Π), and H(Lyman‐α) fluorescence have also been obtained. The obtained results have been compared with theoretical calculations. An unresolved problem of the spectral interpretation concentrated on the σ * and π* shape resonances has been clarified by the straightforward demonstration of the photoionization quantum yield. The ionizing and nonionizing decay processes of the superexcited C2H2 molecules have been discussed in view of the strong competition of autoionization and neutral dissociation. An overlapping nature of Rydberg states with the shape resonance is found to be important.

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.

Photoabsorption, photoionization, and neutral‐dissociation cross sections of SiF4, SiCl4, and Si(CH3)4 in the extreme‐ultraviolet range

The absolute photoabsorption cross sections (σt) and photoionization quantum yields (η) of SiF4, SiCl4, and Si(CH3)4 have been measured in the 50–100 nm range, from which we have evaluated the absolute photoionization cross sections (σi) and the total cross sections for nonionizing processes, i.e., the neutral‐dissociation cross sections (σd) following superexcitation. In addition to the broad structures in the σt curve for SiF4 around 95, 83, 78, 74.5, and 70 nm, oscillating structures which have not been found previously are revealed around 65 nm. These structures, which appear also in the present σi curve, are tentatively ascribed to the vibrational progressions of Rydberg states converging to the (4t2)−1C ionic state. Other oscillating structures in the 57–62 nm range are ascribed to the Rydberg states converging to the (5a1)−1D ionic state. Broad peaks around 95, 92, 75, and 58 nm are observed in the σt and σi curves of SiCl4. Rydberg structures converging to the 7a1−1 limit are observed in the σt ...

Absolute measurements of photoabsorption cross sections, photoionization cross sections, and photoionization quantum yields of silane in the 13–40 eV region

The absolute values of the photoabsorption cross section of silane have been measured in the energy region between 13 and 40 eV. The photoionization quantum yields have also been measured in the 13–22 eV region. Using these values, we have evaluated the absolute photoionization cross sections and the total cross sections for nonionizing processes, i.e., neutral fragmentations. The photoabsorption cross sections show a broad peak at around 14.6 eV and a vibrational structure between 16 and 18 eV. These structures are superimposed on the spectra of direct transitions to the ionization continuum. The decaying features of the superexcited states corresponding to these two bands are shown to be very different from each other. Corresponding to the broad absorption peak located at around 14.6 eV, considerable deviations from unity in the photoionization quantum yields are observed. This means that the superexcited states at this energy decay through fast neutral fragmentation rather than autoionization. On the other hand, the superexcited states, which appear as the vibrational structure between 16 and 18 eV, decay predominantly through autoionization over the neutral fragmentation.

Autoionizing‐resonance enhanced preferential photodissociation of CO2 in superexcited states

Fluorescence excitation spectra of CO2+(A 2Πu→X 2Πg), (B 2∑u+→X 2Πg), and CO(A 1Π→X 1Σ) emissions produced in the photoionization and neutral photodissociation of CO2 have been obtained in the 30–92 nm region. A strong competition between autoionization and neutral dissociation observed in the region near the ionization thresholds to form CO2+(A 2Πu) and CO2+(B 2Σu+) states clearly shows the preferential enhancement of the neutral dissociation, which is explained mainly by the intrinsic autoionization rate of the individual superexcited Rydberg states. A drastic step‐down decrease in a fluorescence excitation spectrum in the vacuum ultraviolet region at the thresholds has been ascribed to a dramatic density dilution of the superexcited states into the continuum.

Extreme ultraviolet photodissociation of O2 via the free (c 4Σ−u)3sσg state as probed by dispersed vacuum ultraviolet fluorescence

Abstract Dispersed vacuum ultraviolet (VUV) fluorescence spectra of neutral fragments formed in the extreme ultraviolet photodissociation of O 2 and their excitation spectra at the incident synchrotron radiation (SR) wavelength of 57–85 nm have been obtained using a secondary VUV monochromator to clarify the dissociation mechanism of O 2 in superexcited states. The VUV fluorescence from dissociation fragments has been observed at the lowest dissociation limits of O( n l)+O( 3 P). The dissociation of the Rydberg states converging on the O + 2 (b 4 Σ − g ) and O + 2 (B 2 Σ − g ) states has been shown to prefer dissociation limits higher than the lowest limit of O(3s 3 S)+O( 3 P) via multichannel crossing. An intensity enhancement in the O(3s 3 S→ 3 P) fluorescence excitation spectrum (FES) around the incident wavelength of 58 nm has been explained by the direct dissociation of the free (c 4 Σ − u )3sσ g state.

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)...

De‐excitation of H(2p) in a collision with a H2 molecule

The velocity assigned experiment of the collisional de‐excitation of a H(2p) atom by a H2 molecule is demonstrated by measuring the time‐dependent intensity of Lyman‐α radiation emitted in the photodissociation of H2 using a pulsed synchrotron radiation in the vacuum ultraviolet region. Observed decay rates of Lyman‐α radiation have given the rate constant or cross section for the de‐excitation of H(2p) by H2. The cross section is about 3×10−14 cm2 in the region of the collisional velocity between 4.6×105 and 12×105 cm/s, which is several times larger than that for the H(2s) de‐excitation. It has been concluded that the l‐changing collision is quite important in the de‐excitation of H(2p) by H2.

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.