Yasumasa Hikosaka

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 Xu20092Πu and Au20092Ag 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+(Xu20092Πu), 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+(Xu20092Πu) 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+(Bu20092Σ+). 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).

Spectator- and participant-like behavior of a Rydberg electron on predissociation of superexcited states of OCS

Predissociation of superexcited states of OCS is studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 15–16.5 eV. A two-dimensional photoelectron spectrum exhibits two kinds of characteristic patterns both of which are ascribed to autoionization of sulfur atoms. This superexcited atom S* is produced by predissociation of a Rydberg state OCS*(RB) converging to OCS+(Bu20092Σ+). The pattern of the first kind results from predissociation processes in which the effective principal quantum number n of the Rydberg electron is almost conserved. This suggests that the Rydberg electron behaves as a spectator because of its negligibly weak interaction with the ion core (spectator predissociation). On the contrary, n of S* does not accord with that of OCS*(RB) in the pattern of the second kind, indicating that the Rydberg electron participates directly in the electron exchange mechanism controlling conversion from OCS*(RB) to a predissociating state (participant...

Autoionization of NO in an excited valence state affected by perturbations from valence-Rydberg mixing

A two-dimensional photoelectron spectrum of NO has been measured in the photon energy range of 10.7–13.4eV. The spectrum exhibits many salient features due to autoionization. The vibrational distribution for NO+ (X1∑+) is simulated by means of Franck-Condon analysis, using a harmonic or anharmonic potential energy curve for the autoionizing state. The vibrational levels of this state are substantially perturbed by the Rydberg states converging to NO+ (X1∑+).

Formation of autoionizing atomic nitrogen from superexcited states of nitric oxide

Photodissociation of NO followed by autoionization of an N atom has been studied by two‐dimensional photoelectron spectroscopy using synchrotron radiation. In addition to the bands due to molecular ionization, the two‐dimensional spectrum shows several characteristic patterns in the photon energy range of 21.5–27 eV which result from autoionization of the Rydberg states converging to N+(1De) into the ionic ground state N+(3Pe). The electronic states of the counter atomic oxygen can be determined from the lower onset energies of these patterns to be 3Pe and/or 1De. Discussion is made about the transition region for the photodissociation reaction, especially being focused on the nature of the primary molecular states and their dynamical aspects including competitive molecular autoionization.

Laser photoionization of polarized Ar atoms produced by excitation with synchrotron radiation

The laser-synchrotron radiation combination technique has recently been incorporated into an apparatus for two-dimensional photoelectron spectroscopy of atoms and molecules in order to investigate photoionization dynamics of polarized atoms. Ground state Ar atoms are excited with linearly polarized synchrotron radiation to Rydberg states lying below the first ionization potential. The aligned atoms thus formed are ionized by irradiation of a laser which is also linearly polarized. Photoelectrons emitted in the direction of the electric vector of the synchrotron radiation are sampled and energy analysed. The photoelectron angular distribution is measured with respect to the electric vector of the laser. Expressions which correlate the asymmetric coefficients for the angular distribution with theoretical dynamic parameters involving transition dipole matrix elements are derived. The anisotropy of the present angular distribution can be reasonably explained, assuming that the matrix elements and phase shift differences are essentially independent of the total angular momentum quantum number of the final state and that the spin-orbit interaction in the continuous spectrum is small.

Superexcitation and subsequent decay of triatomic molecules studied by two-dimensional photoelectron spectroscopy

Abstract Photoionization and photodissociation processes of SO2 and CS2 in vacuum UV are studied by using two-dimensional photoelectron spectroscopy with a monochromatized synchrotron radiation source. The principal focus is on the mechanisms of autoionization and neutral dissociation of superexcited states. Photoelectron spectra of SO2 exhibit characteristic peaks at the electron kinetic energy below 1.8 eV which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, into the ground S+( 4 S° ) state. These S* atoms are in the singlet Rydberg states converging to S+(2D°). The precursor molecular states, SO2*, are considered to be multiple-electron excited Rydberg states lying at the photon energy above ∼22 eV. The onset of the photoelectron yield due to the atomic autoionization accords with that expected from the thermochemical threshold for the formation of S* through three-body dissociation SO2*→S*+O+O. The two-dimensional photoelectron spectrum of CS2 provides tangible evidence for the formation of a dipole-forbidden Rydberg state (6σg)−1(3dσg)11Σg+ at the photon energy of 14.88 eV which autoionizes into the v3=1 vibrational state of the antisymmetric stretch ν3 mode of CS2+ (X2Πg,Ω, Ω=1/2 and 3/2). This Rydberg state is expected to borrow substantial oscillator strength from the (6σg)−1(5pσu)11Σu+ state through vibronic coupling involving the ν3 vibration.

Autoionization and neutral dissociation of superexcited HI studied by two-dimensional photoelectron spectroscopy.

Two-dimensional photoelectron spectroscopy of hydrogen iodide (HI) has been performed in the photon energy region of 11.10-14.85 eV, in order to investigate dynamical properties on autoionization and neutral dissociation of Rydberg states HI*(RA) converging to HI+(A 2Sigma1/2(+)). A two-dimensional photoelectron spectrum exhibits strong vibrational excitation of HI+(X 2Pi) over a photon energy region from approximately 12 to 13.7 eV, which is attributable to the autoionizing feature of the 5 dpi HI*(RA) state. A noticeable set of stripes in the photon energy region of 13.5-14.5 eV is assigned as resulting from autoionization of the atomic Rydberg states of I* converging to I+ (3P0 or 3P1). The formation of I* is understood in terms of predissociation of multiple HI*(RA) states by way of the repulsive Rydberg potential curves converging to HI+(4Pi1/2).

Detection of Neutral Species in the MALDI Plume Using Femtosecond Laser Ionization: Quantitative Analysis of MALDI-MS Signals Based on a Semiequilibrium Proton Transfer Model.

We investigated neutral species in the matrix-assisted laser desorption and ionization (MALDI) plume using femtosecond laser ionization spectrometry with simultaneous measurement of the standard MALDI spectrum of the identical MALDI event induced by pulsed UV laser irradiation. The ratio of neutral species in the plume [A]p/[M]p (A = phenylalanine (Phe) or alanine (Ala), M = 2,5-dihydroxybenzoic acid (DHB)) was confirmed to be the same as that of the sample mixture in the range of [A]0/[M]0 = 4 × 10-4-1, indicating the validity of the widely adopted approximation [A]p/[M]p = [A]0/[M]0 in the reaction quotient of the proton transfer reaction MH+ + A ⇄ M + AH+. An effective parameter representing the extent of thermal equilibrium in the thermal proton transfer model is introduced for the first time. Numerical simulation based on this semiequilibrium model successfully reproduced variations of MALDI signal intensities AH+ and MH+ with two parameters: the fraction of ionized matrix a ≤ 10-5 and an effective temperature T = 1200 and 1100 K for Phe/DHB and Ala/DHB systems, respectively. These values show good agreement with those determined previously by different experimental approaches. The extent of thermal equilibrium was determined to be 95% and 98% for Phe/DHB and Ala/DHB systems, respectively, suggesting that the proton transfer reactions almost proceed to their thermal equilibrium.