Reika Kanya

Determination of dipole moment change on the electronic excitation of isolated Coumarin 153 by pendular-state spectroscopy

Abstract The change (Δ μ ) in the permanent dipole moment on the S 1 ←S 0 electronic excitation is determined for Coumarin 153 (C153) in a molecular beam from the spectral change induced by applying a strong dc electric field up to 200 kV/cm. The comparison of the observed fluorescence excitation spectra under various external fields with the corresponding simulations for a pendular-state molecule yields Δ μ=7.1±0.4 D for both the syn and anti conformers of C153. Previous experimental results on Δ μ in the condensed phase, as well as the theoretical predictions reported in the literature, are discussed on the basis of the present value under an isolated condition.

Pendular-state spectroscopy of the S1–S0 electronic transition of 9-cyanoanthracene

Fluorescence excitation spectra of the S(1)-S(0) origin band of 9-cyanoanthracene have been observed under a uniform electric field up to 200 kV/cm to explore pendular-state spectrum of an asymmetric-top molecule close to the strong field limit. The observed spectra exhibit distinct evolution of the band contour as a function of the applied electric field, which are much different from each other for different excitation configurations. An approximate method suitable for spectrum simulations of large asymmetric-top molecules in a pendular condition is developed for the analysis of the experimental results. The comparison of the observed and simulated spectra shows that the spectra are well ascribed in terms of the pendular-state selection rules, which have recently been derived from theoretical consideration of the pendular-limit representation of energy levels and spectra.

Hydrogen scrambling in ethane induced by intense laser fields: statistical analysis of coincidence events.

Two-body Coulomb explosion processes of ethane (CH(3)CH(3)) and its isotopomers (CD(3)CD(3) and CH(3)CD(3)) induced by an intense laser field (800 nm, 1.0 × 10(14) W/cm(2)) with three different pulse durations (40 fs, 80 fs, and 120 fs) are investigated by a coincidence momentum imaging method. On the basis of statistical treatment of the coincidence data, the contributions from false coincidence events are estimated and the relative yields of the decomposition pathways are determined with sufficiently small uncertainties. The branching ratios of the two body decomposition pathways of CH(3)CD(3) from which triatomic hydrogen molecular ions (H(3)(+), H(2)D(+), HD(2)(+), D(3)(+)) are ejected show that protons and deuterons within CH(3)CD(3) are scrambled almost statistically prior to the ejection of a triatomic hydrogen molecular ion. The branching ratios were estimated by statistical Rice-Ramsperger-Kassel-Marcus calculations by assuming a transition state with a hindered-rotation of a diatomic hydrogen moiety. The hydrogen scrambling dynamics followed by the two body decomposition processes are discussed also by using the anisotropies in the ejection directions of the fragment ions and the kinetic energy distribution of the two body decomposition pathways.

Laser-assisted electron diffraction for femtosecond molecular imaging

We report the observation of laser-assisted electron diffraction (LAED) through the collision of 1 keV electrons with gas-phase CCl4 molecules in a femtosecond near-infrared laser field. In the angular distribution of the scattered electrons with the energy shifts of ±ℏω, we observed clear diffraction patterns which reflect the geometrical structure of the molecules at the moment of laser irradiation. Our results demonstrate that ultrafast nuclear dynamics of molecules can be probed by LAED with the high temporal (<10 fs) and spatial (∼0.01 Å) resolutions.

Apparatus for laser-assisted electron scattering in femtosecond intense laser fields

An apparatus for observation of laser-assisted electron scattering (LAES) in femtosecond intense laser fields was developed. The unique apparatus has three essential components, i.e., a photocathode-type ultrashort pulsed-electron gun, a toroidal-type electron energy analyzer enabling simultaneous detection of energy and angular distributions of scattered electrons with high efficiency, and a high repetition-rate data acquisition system combined with a high power 5 kHz Ti:sapphire laser system. These advantages make extremely weak femtosecond-LAES signals distinguishable from the huge elastic scattering signals. A precise method for securing a spatial overlap between three beams, that is, an atomic beam, an electron beam, and a laser beam, and synchronization between the electron and laser pulses is described. As a demonstration of this apparatus, an electron energy spectrum of the LAES signals with 1.4 × 10(12) W/cm(2), 795 nm, 50 fs laser pulses was observed, and the detection limit and further improvements of the apparatus are examined.

Light-Dressing Effect in Laser-Assisted Elastic Electron Scattering by Xe

The light-dressing effect in Xe atoms was identified in laser-assisted elastic electron scattering (LAES) signals. In the angular distribution of LAES signals with energy shifts of ±ℏω recorded by the scattering of 1 keV electrons by Xe in an intense nonresonant laser field, a peak profile appeared at small scattering angles (<0.5°). This peak was interpreted as evidence of the light dressing of Xe atoms induced by an intense laser field on the basis of a numerical simulation in which the light-dressing effect is included.

Decomposition of cyclohexane ion induced by intense femtosecond laser fields by ion-trap time-of-flight mass spectrometry

Decomposition of cyclohexane cations induced by intense femtosecond laser fields at the wavelength of 800 nm is investigated by ion-trap time-of-flight mass spectrometry in which cyclohexane cations C6H12 (+) stored in an ion trap are irradiated with intense femtosecond laser pulses and the generated fragment ions are recorded by time-of-flight mass spectrometry. The various fragment ion species, C5Hn (+) (n = 7, 9), C4Hn (+) (n = 5-8), C3Hn (+) (n = 3-7), C2Hn (+) (n = 2-6), and CH3 (+), identified in the mass spectra show that decomposition of C6H12 (+) proceeds efficiently by the photo-irradiation. From the laser intensity dependences of the yields of the fragment ion species, the numbers of photons required for producing the respective fragment ions are estimated.

Numerical simulations of time-domain interferometric soft X-ray microscope with broadband high-order harmonic light sources

A scheme for achieving high spatial resolution in soft X-ray microscopy with coherent broadband light sources is proposed, in which the chromatic aberration at a Fresnel zone plate lens can be canceled out by introducing time-delayed double pulses as an input light and by the Fourier transformation of recorded images with respect to the time delay. Numerical simulations of microscope images show that the spatial resolution of the proposed method is determined only by specifications of zone plates even when a broadband soft X-ray is used as a light source.

Electronic spectrum of the anthracene–ammonia complex

Fluorescence excitation and ultraviolet–ultraviolet hole burning spectra of jet-cooled anthracene–ammonia clusters were observed. The S1–S0 origin of the 1∶1 complex shows doublet bands, which were assigned to those in the ground and the first excited states of the internal rotation associated with the ortho and para hydrogen nuclear spin states of NH3, respectively. The ammonia molecule is located above the centre of the aromatic plane with a centre-of-mass distance of 3.6 A, which is typical of a π hydrogen bonded aromatic complex. The rotational contour of the internal rotation excited state was analyzed with an effective Hamiltonian including the coupling between the internal and end-over-end rotations, to show that the internal rotation angular momentum is partially quenched because of the potential anisotropy for rotation around the axis that connects the centres of mass of ammonia and anthracene.

Laser-Assisted Electron Scattering and Diffraction for Ultrafast Imaging of Atoms and Molecules

We report recent progress in experimental studies on laser-assisted electron scattering (LAES) processes induced by ultrashort laser pulses. Observation of LAES processes induced by femtosecond laser pulses is reported. As an application of this femtosecond-LAES process, a new method of time-resolved gas electron diffraction for determining instantaneous geometrical structures of molecules with high precision is proposed, and its feasibility is demonstrated by the measurements of LAES signals from CCl4 molecules.