Keiichi Yokoyama

Discrimination of product isomers in the photodissociation of propyne and allene at 193 nm

The photodissociation dynamics of propyne and allene are investigated in two molecular beam/photodissociation instruments, one using electron impact ionization and the other using tunable vacuum ultraviolet (VUV) light to photoionize the photoproducts. The primary dissociation channels for both reactants are C3H3+H and C3H2+H2. Measurement of the photoionization efficiency curves on the VUV instrument shows that the C3H3 product from propyne is the propynyl (CH3CC) radical, whereas the C3H3 product from allene is the propargyl (CH2CCH) radical. The dominant C3H2 product from both reactants is the propadienylidene (H2CCC) radical. We also observe a small amount of secondary C3H2 product from photodissociation of the C3H3 radicals in both cases.

Ab initio study on the ground and low-lying excited states of cesium iodide (CsI)

Potential energy curves (PECs) for the ground and low-lying excited states of the cesium iodide (CsI) molecule have been calculated using the internally contracted multireference configuration interaction calculation with single and double excitation method with the relativistic pseudopotentials. PECs for seven Lambda-S states, X 1Sigma+, 2 1Sigma+, 3Sigma+, 1Pi, and 3Pi are first calculated and then those for 13 Omega states are obtained by diagonalizing the matrix of the electronic Hamiltonian H(el) plus the effective one-electron spin-orbit (SO) Hamiltonian H(SO). Spectroscopic constants for the calculated ground X 0+-state PEC with the Davidson correction are found to agree well with the experiment. Transition dipole moments (TDMs) between X 0 and the other Omega states are also obtained and the TDM between X 0+ and A 0+ is predicted to be the largest and that between X 0+ and B 0+ is the second largest around the equilibrium internuclear distance. The TDMs between X 0+ and the Omega=1 states are estimated to be nonzero, but they are notably small as compared with those between the 0+ states. Finally, vibrational levels of the X 0+ PEC for the two isotopic analogs, (133)CsI and (135)CsI, are numerically obtained to investigate the isotope effect on the vibrational-level shift. It has been found that the maximized available isotope shift is approximately 30 cm(-1) around nu=136.

Ab initio MRSDCI study on the low-lying electronic states of the lithium chloride molecule (LiCl)

Potential energy curves (PECs) for the low-lying states of the lithium chloride molecule (LiCl) have been calculated using the internally contracted multireference single- and double-excitation configuration interaction (MRSDCI) method with the aug-cc-PVnZ (AVnZ) and aug-cc-PCVnZ (ACVnZ) basis sets, where n = T, Q, and 5. First, we calculate PECs for 7 spin-orbit (SO)-free Λ-S states, X(1)Σ(+), A(1)Σ(+), (3)Σ(+), (1)Π, and (3)Π, and then obtain PECs for 13 SO Ω states, X0(+), A0(+), B0(+), 0(-)(I), 0(-)(II), 1(I), 1(II), 1(III), and 2, by diagonalizing the matrix of the electronic Hamiltonian plus the Breit-Pauli SO Hamiltonian. The MRSDCI calculations not including core orbital correlation through the single and double excitations are also performed with the AV5Z and ACV5Z basis sets. The Davidson corrections (Q0) are added to both the Λ-S and Ω state energies. Vibrational eigenstates for the obtained X(1)Σ(+) and X0(+) PECs are calculated by solving the time-independent Schrödinger equation with the grid method. Thus, the effects of basis set, core orbital correlation, and the Davidson correction on the X(1)Σ(+) and X0(+) PECs of LiCl are investigated by comparing the spectroscopic constants calculated from the PECs with one another and with experiment. It is confirmed that to accurately predict the spectroscopic constants we need to include core-electron correlation in the CI expansion and use the basis sets designed to describe core-valence correlation, i.e., ACVnZ. The SO PECs presented in this paper will be of help in the future study of diatomic alkali halide dynamics.

Quantum control study of multilevel effect on ultrafast isotope-selective vibrational excitations

Quantum optimal control calculations have been carried out for isotope-selective vibrational excitations of the cesium iodide (CsI) molecule on the ground-state potential energy curve. Considering a gaseous isotopic mixture of (133)CsI and (135)CsI, the initial state is set to the condition that both (133)CsI and (135)CsI are in the vibrational ground level (v=0) and the target state is that (133)CsI is in the v=0 level while (135)CsI in the first-excited level (v=1). We find that, using the density-matrix formalism, perfect isotope-selective excitations for multilevel systems including more than ten lowest vibrational states can be completed in much shorter time scales than those for two-level systems. It is likely that this multilevel effect comes from the large isotope shifts in the vibrational levels of v>1. To check the reliability of the calculation we also carry out optimal control calculations based on the conventional wave-packet formalism, where the wave-function amplitude is temporally propagated on the grid points in real space, and obtain almost the same results as those with the density-matrix formalism.

Numerical Study on Quantum Walks Implemented on Cascade Rotational Transitions in a Diatomic Molecule

We propose an implementation scheme for the continuous-time quantum walk using a diatomic molecule and an optical frequency comb. We show an analogy between the quantum walk and the cascade rotational transitions induced by the optical frequency comb whose frequency peaks are tuned to the pure rotational transitions in the molecule. A strategy to compensate for the centrifugal distortion of the real molecule is also demonstrated.

INFRARED MULTIPHOTON DISSOCIATION OF CBRF2CHCLF, CBRF2CHBRF, AND CBRCLFCBRF2 IN A MOLECULAR BEAM

Dynamics and mechanisms of infrared multiphoton dissociation of CBrF2CHClF, CBrF2CHBrF, and CBrClFCBrF2 have been studied using a photofragmentation translational spectroscopy. All molecules dissociated through C–Br bond rupture reactions. At high laser fluence, the halogenated ethyl radicals produced by the primary dissociation reactions dissociated through carbon–halogen bond ruptures. Center‐of‐mass product translational energy distributions for the C–Br and C–Cl bond ruptures of all halogenated ethanes and ethyl radicals studied are essentially consistent with those calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This indicates that there exists essentially no exit channel barrier on the potential energy surface for the C–Br or C–Cl bond rupture of the halogenated ethanes and ethyl radicals.

An Analytic Formula for Describing the Transient Rotational Dynamics of Diatomic Molecules in an Optical Frequency Comb

We derive an analytic expression for evaluating the transient rotational dynamics for diatomic molecules in an optical frequency comb which is tuned to induce a series of pure rotational transitions. The formulation is made with the matrix spectral decomposition technique. The derived transient probability amplitude can be separated into a Bessel function of the first kind and a summation of other oscillating terms. The time dependence of the obtained probability is shown as a step function. We confirm that the evaluated probabilities are consistent with the numerical computations.

Isotopically Selective Infrared Multiphoton Dissociation of 2,3-Dihydropyran

Oxygen isotopic selectivity on infrared multiphoton dissociation of 2,3-dihydropyran has been studied by the examination of the effects of excitation frequency, laser fluence, and gas pressure on the dissociation probability of 2,3-dihydropyran and isotopic composition of products. Oxygen-18 was enriched in a dissociation product: 2-propenal. The enrichment factor of 18O and the dissociation probability were measured at a laser frequency between 1033.5 and 1057.3 cm-1, the laser fluence of 2.2-2.3 J/cm2, and the 2,3-dihydropyran pressure of 0.27 kPa. The dissociation probability decreases as the laser frequency being detuned from the absorption peak of 2,3-dihydropyran around 1081 cm-1. On the other hand, the enrichment factor increases with detuning the frequency. The enrichment factor of 18O increases with increasing the 2,3-dihydropyran pressure at the laser fluence of 2.7 J/cm2 or less and the laser frequency of 1033.5 cm-1, whereas the yield of 2-propenal decreases with increasing the pressure. A very high enrichment factor of 751 was obtained by the irradiation of 0.53 kPa of 2,3-dihydropyran at 2.1 J/cm2. Collisional effect of vibrationally excited molecules with ambient molecules on isotopic selectivity is discussed on the basis of a rate equation model including a collisional vibrational de-excitation process.

Quantum optimal control for the full ensemble of randomly oriented molecules having different field-free Hamiltonians

We have presented the optimal control theory formulation to calculate optimal fields that can control the full ensemble of randomly oriented molecules having different field-free Hamiltonians. The theory is applied to the fifty-fifty mixture of randomly oriented (133)CsI and (135)CsI isotopomers and an optimal field is sought to achieve isotope-selective vibrational excitations with high efficiency. Rotational motion is frozen and two total times (Ts) of electric field duration, 460,000 and 920,000 a.u. (11.1 and 22.2 ps), are chosen in the present calculation. As a result, the final yields for T = 460,000 and 920,000 a.u. are calculated to be 0.706 and 0.815, respectively. The relatively high final yield obtained for T = 920,000 a.u. strongly suggests that a single laser pulse can control the full ensemble of randomly oriented non-identical molecules. The result is quite encouraging in terms of the application to isotope-separation processes.

Theoretical study of the adsorption of Cs, Cs+, I, I−, and CsI on C60 fullerene

ABSTRACT Theoretical investigation for the adsorption of the cesium atom (Cs), the cesium iodide molecule (CsI), the iodine atom (I), the cesium cation (Cs+), and the iodide anion (I−) onto the surface of a single fullerene molecule (C60) are reported. A hybrid exchange–correlation functional using the Coulomb-attenuating method (CAM-B3LYP) is employed. The adsorption energies, i.e., the opposite of enthalpy change through adsorption, are calculated to be 143, 12, 9, 46, and 49 kJ mol−1 for Cs, CsI, I, Cs+, and I−, respectively. The equilibrium constant for Cs is calculated to be 7×103 atm−1 at the temperature of 1000 K and is seven orders of magnitude higher than that for CsI, indicating that the C60 molecule adsorb the Cs atom highly selectively against the CsI molecule.