Koichiro Mitsuke

Negative‐ion mass spectrometric study of ion‐pair formation in the vacuum ultraviolet. V. CF4→F−+CF+3

Ion‐pair formation from photoexcitation of CF4 has been studied by negative‐ion mass spectrometry using synchrotron radiation in the 11–31 eV photon energy range. Negative F− ions have been observed. The appearance energy in the F− photodissociation efficiency curve is about 2 eV higher than the thermochemical threshold for the formation of the ground‐state ions F−(1Sg) and CF+3(X 1A’1). The peak features observed in the spectrum are interpreted as resulting from transitions to Rydberg states with the 1T2 symmetry. Assignments of the peaks have been made on the basis of the previous work on photoabsorption cross‐section curves of CF4. Repulsive Rydberg states converging to CF+4(X 2T1, A 2T2) give the strong and broad peaks in the F− efficiency curve. Conversion to the ion‐pair state is considered to occur through the avoided potential surface crossings along the dissociation coordinate of the CF3—F bond. In contrast, the npt2 Rydberg states (n≥4) converging to CF+4(C 2T2) show long vibrational progres...

Ion‐pair formation from saturated hydrocarbons through photoexcitation of an inner‐valence electron

Ion‐pair formation from the superexcited states of saturated hydrocarbons has been studied by negative‐ion mass spectrometry using synchrotron radiation in the 15–35 eV photon energy range. Negative ion H− has been observed from CH4, C2H6, C3H8, n‐C4H10, iso‐C4H10, and neo‐C5H12. The maximum cross section ranges from 1.6×10−21 to 1.0×10−20 cm2. Strong peaks observed in the photodissociation efficiency curve of H− are assigned as resulting from transitions to the Rydberg states formed by promotion of an electron in a carbon 2s‐type molecular orbital. In contrast, the valence‐Rydberg transitions from a carbon 2p‐type orbital have little (C2H6) or no contribution [CH4 and CmH2m+2 (3≤m≤5)] to the H− formation. This difference can be interpreted as that the latter Rydberg states have short lifetime with respect to autoionization to lower ionic states on account of relatively large overlap between two carbon 2p‐type orbitals involved in an electron exchange process.

Negative‐ion mass spectrometric study of ion‐pair formation in the vacuum ultraviolet. VI. CH3X→X−+CH+3 (X=F, Cl, Br)

Ion‐pair formation from photoexcited halomethanes, CH3X*→X−+CH+3 (X=F, Cl, Br) has been studied by measuring photodissociation efficiency curves of X− using synchrotron radiation in the 9.9–27.5 eV photon energy range. A new spectral feature is observed in each of the curves near the threshold for the removal of an na1 electron from CH3X (n=4, 6, and 8, respectively, for CH3F, CH3Cl, and CH3Br ). This feature, composed of two or three peaks in each case, is interpreted as resulting from photoexcitation to the Rydberg states converging to CH3X+(C 2A1), which then predissociate into ion pairs through avoided potential energy surface crossings. The interpretation is based on the results of the inner‐shell electron energy loss study by Brion and co‐workers and the photoabsorption study by Hochmann and co‐workers. Peak features are also observed in the X− efficiency curves near the ionization threshold for CH3X+(X 2E). The origins of these peaks are also discussed.

Molecular- and atomic-like photoionization of C60 in the extreme ultraviolet

Abstract Photoion yield spectra of C 60 in the gas phase were measured from 23 to 180 eV by synchrotron radiation. Two peaks at 26 and 34 eV and a flat area ranging 40–50 eV are newly observed in the high-energy side of the giant resonance at ∼20 eV. These features are assigned to the shape resonance on photoionization of the valence electrons of C 60 ; the ionized electron is temporarily trapped inside a centrifugal barrier. Above ∼50 eV the yield curve shows a steady decrease with increasing photon energy like the photoabsorption cross section of atomic carbon. Thus, the spectrum is interpreted as essentially determined by photoionization of the 2s orbitals of carbon atoms.

Double photoionization of C60 and C70 in the valence region

Photoion yields from gaseous fullerenes, C60 and C70, for production of singly and doubly charged ions are measured by mass spectrometry combined with tunable synchrotron radiation at hν=25–150 eV. Since the signal of triply or highly charged ions is very weak, the total photoionization yield curve can be estimated from the sum of the yields of the singly and doubly charged ions. A distinct feature appears in the resultant curve of C60 which is absent in the calculated total photoabsorption cross section previously reported. This difference is attributed to C602+ ions chiefly produced by spectator Auger ionization of the shape resonance states followed by tunneling of the trapped electron or by cascade Auger ionization. Ratios between the yields of doubly and singly charged ions for C60 and C70 are larger than unity at hν>50 eV. These ratios are quite different from those reported in the experiments using electron impact ionization.

Autoionizing resonance in photoionization from the 1πu level of acetylene

Autoionizing resonance of acetylene is studied by photoelectron spectroscopy using synchrotron radiation. Pronounced vibrational excitation in the C–H stretching mode ν1 is observed in the (1πu)−1X 2Πu band of C2H+2 at a restricted photon energy range from 12.8 to 14.1 eV. It is concluded that the 3σg→3σu autoionizing transition at ∼13.3 eV gives rise to an anomalously broad maximum in the (1πu)−1 photoionization cross section curve. The strong ν1 excitation is explained as that the equilibrium C–H bond length differs from the neutral and ionic ground states to the (3σg)−1(3σu)1 resonance state. Constant ionic state spectra for the v1=3 and 4 levels of the X 2Πu state measured over the same energy region show fine structures with regular spacings corresponding to the vibrational levels of the (3σg)−1(3σu)1 state.

Two-dimensional photoelectron spectroscopy of acetylene: Rydberg-valence interaction between the (3σg)−1(3pσu)1 and (3σg)−1(3σu)1 states

Two-dimensional photoelectron spectroscopy is performed for studying autoionization of acetylene in the Franck–Condon gap between the X 2Πu and A 2Ag states of C2H2+. The photoelectron spectrum in the photon energy range from 12.8 to 13.6 eV shows exclusive vibrational excitation of the symmetric C–H stretching mode ν1 of C2H2+(X 2Πu), which results from autoionization of the valence state (3σg)−1(3σu)1. Vibrational frequencies with anharmonicities of the ν1 and ν2 (the symmetric C–C stretch) modes are determined by a least-squares fit of the ionization energies of the observed peaks to a second order expansion. At the photon energy of 14.120 eV, autoionization of the Rydberg state (3σg)−1(3pπu)1 leads to a complicated photoelectron spectrum where probably the trans-bending mode ν4 of C2H2+(X 2Πu) as well as ν1 is excited, reflecting a substantial geometrical change during autoionization. Furthermore, a similar excitation of the ν4 mode is observed at ∼13.8 eV. An excellent agreement in positions of the v...

Superexcited states of OCS probed by using photoelectron spectroscopy for autoionizing atomic sulfur

Neutral dissociation of superexcited states of OCS has been studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 14.2–16.8 eV. A two-dimensional spectrum exhibits noticeable features which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, from Rydberg states converging to S+(2Do) to S+(4So). The precursor molecular states leading to S*+CO are considered to be multiple-electron-excited Rydberg states, OCS*(Dis), converging to OCS+ with 2Σ− and/or 2Δ symmetry. The electron signal counts due to autoionization of S* show enhancement at excitation photon energies for the Rydberg states, OCS*(RB), converging to OCS+(B 2Σ+). These results support a predissociation mechanism for the formation of S*: conversion from OCS*(RB) to OCS*(Dis). The quantum yield for the predissociation is evaluated to be ∼1% at the photon energy corresponding to the 5sσ state of OCS*(RB).

Observation of doubly excited Rydberg states of N2O by positive ion–negative ion coincidence spectroscopy

Ion‐pair formation processes of N2O are studied in detail using synchrotron radiation in the 24–34 eV photon energy range. Positive ion–negative ion coincidence (PINICO) spectroscopy makes it possible to distinguish dissociation into three bodies, O−+N++N, from dissociation into two bodies, O−+N+2. The photodissociation efficiency curves for the two processes are measured. Several resonance‐like peaks in the curves are assigned to doubly excited Rydberg states of N2O from an analysis of kinetic energy release in dissociation.

4d→4f dipole resonance of the metal atom encapsulated in a fullerene cage: Ce@C82

The yield curves for photoions from Ce@C(82) are measured by using synchrotron radiation in the photon energy range from 90 to 160 eV. Parent Ce@C(82) (z+) and fragment ions C(60) (z+) and C(70) (z+) are observed in a mass spectrum (z=1 and 2). The yield curves for doubly charged ionic species exhibit broad resonance in the photon energy region of from 120 to 140 eV which is ascribed to the 4d-->4f giant dipole resonance of the encapsulated Ce atom. The total photoabsorption cross section of Ce@C(82) was determined from partial photoionization cross sections for formation of the parent and fragment ions to be 5.3(-1.1) (+1.8) and 19.6(-3.9) (+6.5) Mb at photon energies of 110 and 130 eV, respectively.