Motomichi Tashiro

Double-core-hole spectroscopy for chemical analysis with an intense X-ray femtosecond laser

Theory predicts that double-core-hole (DCH) spectroscopy can provide a new powerful means of differentiating between similar chemical systems with a sensitivity not hitherto possible. Although DCH ionization on a single site in molecules was recently measured with double- and single-photon absorption, double-core holes with single vacancies on two different sites, allowing unambiguous chemical analysis, have remained elusive. Here we report that direct observation of double-core holes with single vacancies on two different sites produced via sequential two-photon absorption, using short, intense X-ray pulses from the Linac Coherent Light Source free-electron laser and compare it with theoretical modeling. The observation of DCH states, which exhibit a unique signature, and agreement with theory proves the feasibility of the method. Our findings exploit the ultrashort pulse duration of the free-electron laser to eject two core electrons on a time scale comparable to that of Auger decay and demonstrate possible future X-ray control of physical inner-shell processes.

Molecular double core-hole electron spectroscopy for chemical analysis

We explore the potential of double core hole electron spectroscopy for chemical analysis in terms of x-ray two-photon photoelectron spectroscopy. The creation of deep single and double core vacancies induces significant reorganization of valence electrons. The corresponding relaxation energies and the interatomic relaxation energies are evaluated by complete active space self-consistent field (CASSCF) calculations. We propose a method on how to experimentally extract these quantities by the measurement of single ionization potentials (IPs) and double core hole ionization potentials (DIPs). The influence of the chemical environment on these DIPs is also discussed for states with two holes at the same atomic site and states with two holes at two different atomic sites. Electron density difference between the ground and double core hole states clearly shows the relaxations accompanying the double core hole ionization. The effect is also compared to the sensitivity of single core hole IPs arising in single co...

The effect of spin-orbit coupling in complex forming O(3P) + O2 collisions

The effect of spin–orbit coupling on O(3P)+O2(3Σg−) collisions is investigated for J=0 using time-dependent wave packets. The probability of forming O3 complexes, which is important for understanding the atom exchange reaction mechanism, is calculated in two different ways. The first approach follows the standard treatment in that only the reactive ground electronic state is included. In the second approach all 27 states correlating with O(3P)+O2(3Σg−) and the nonadiabatic transitions induced by spin–orbit coupling are taken into account; all the excited electronic states are repulsive and thus do not lead to complex formation if nonadiabatic transitions are neglected. The required nine diabatic potential energy surfaces (not including spin–orbit coupling) for the electronic states 1 sA′, 2 sA′, and sA″ (s=1, 3, and 5) are constructed by high-level electronic structure calculations in the asymptotic O+O2 channel with the O2 bond length being fixed. The two sets of calculations show that spin–orbit couplin...

Self-Regulation of Star Formation in Low-Metallicity Clouds

We investigate the process of self-regulated star formation via the photodissociation of hydrogen molecules in low-metallicity clouds. We evaluate the scale of the influence region for a massive star in low-metallicity gas clouds whose temperatures are between 102 and 104 K. A single O star can photodissociate H2 in the whole of the host cloud. If the metallicity is less than about 10-2.5 of the solar metallicity, the depletion of coolants in the host cloud is very serious, so that the cloud cannot cool in a free-fall time, and subsequent star formation is almost quenched. On the other hand, if the metallicity is greater than about 10-1.5 of the solar metallicity, star formation regulation via photodissociation is not efficient. The typical metallicity when this transition occurs is ~10-2 of the solar metallicity. This indicates that stars do not form efficiently before the metallicity becomes larger than about 10-2 of the solar metallicity, and we consider that this value is the lower limit of the metallicity of luminous objects such as galaxies.

Auger decay of molecular double core-hole state

We report on theoretical Auger electron kinetic energy distribution originated from sequential two-step Auger decays of molecular double core-hole (DCH) state, using CH(4), NH(3), and H(2)CO molecules as representative examples. For CH(4) and NH(3) molecules, the DCH state has an empty 1s inner-shell orbital and its Auger spectrum has two well-separated components. One is originated from the 1st Auger transition from the DCH state to the triply ionized states with one core hole and two valence holes (CVV states) and the other is originated from the 2nd Auger transition from the CVV states to quadruply valence ionized (VVVV) states. Our result on the NH(3) Auger spectrum is consistent with the experimental spectrum of the DCH Auger decay observed recently [J. H. D. Eland, M. Tashiro, P. Linusson, M. Ehara, K. Ueda, and R. Feifel, Phys. Rev. Lett. 105, 213005 (2010)]. In contrast to CH(4) and NH(3) molecules, H(2)CO has four different DCH states with C1s(-2), O1s(-2), and C1s(-1)O1s(-1) (singlet and triplet) configurations, and its Auger spectrum has more complicated structure compared to the Auger spectra of CH(4) and NH(3) molecules. In the H(2)CO Auger spectra, the C1s(-1)O1s(-1) DCH → CVV Auger spectrum and the CVV → VVVV Auger spectrum overlap each other, which suggests that isolation of these Auger components may be difficult in experiment. The C1s(-2) and O1s(-2) DCH → CVV Auger components are separated from the other components in the H(2)CO Auger spectra and can be observed in experiment. Two-dimensional Auger spectrum, representing a probability of finding two Auger electrons at specific pair of energies, may be obtained by four-electron coincidence detection technique in experiment. Our calculation shows that this two-dimensional spectrum is useful in understanding contributions of CVV and VVVV states to the Auger decay of molecular DCH states.

The Huggins band of ozone: Unambiguous electronic and vibrational assignment

The Huggins band of ozone is investigated by means of exact dynamics calculations using a new (diabatic) potential energy surface for the (1)B(2) state. The remarkable agreement with the measured spectrum strongly suggests that the Huggins band is due to the two C(s) potential wells of the (1)B(2) state. The vibrational assignment, based on the nodal structure of wave functions, supports the most recent experimental assignment.

Theoretical molecular double-core-hole spectroscopy of nucleobases.

Double-core-hole (DCH) spectra have been investigated for pyrimidine, purine, the RNA/DNA nucleobases, and formamide, using the density functional theory (DFT) method. DCH spectra of formamide were also examined by the complete-active-space self-consistent-field (CASSCF) method. All possible single- and two-site DCH (ssDCH and tsDCH) states of the nucleobases were calculated. The generalized relaxation energy and interatomic generalized relaxation energy were evaluated from the energy differences between ssDCH and single-core-hole (SCH) states and between tsDCH and SCH states, respectively. The generalized relaxation energy is correlated to natural bond orbital charge, whereas the interatomic generalized relaxation energy is correlated to the interatomic distance between the core holes at two sites. The present analysis using DCH spectroscopy demonstrates that the method is useful for the chemical analysis of large molecular systems.

R-matrix calculation of electron collisions with electronically excited O~2 molecules (8 pages)

Low-energy electron collisions with

N1s and O1s double ionization of the NO and N2O molecules

{\mathrm{O}}_{2}

Quantum dynamics study on predissociation of H3 Rydberg states: Importance of indirect mechanism

molecules are studied using the fixed-bond