Akiyoshi Hishikawa

Coincidence imaging of Coulomb explosion of CS2 in intense laser fields

Abstract The coincidence imaging technique is applied to direct determination of the momentum vectors of all the fragment ions produced through every event of the Coulomb explosion of a single molecular ion, CS 2 z+ (z=2–4) formed in intense laser fields (∼60 fs, 0.36×10 15 W / cm 2 ). The molecular structure of CS23+ just before the Coulomb explosion, CS23+→S++C++S+, is reconstructed from the measured momentum vectors of the fragment ions. The mean C–S bond length of 〈r〉=2.5 A and the azimuthally averaged S–C–S bond angle, 〈γ〉=145°, thus determined indicate that structural deformation occurs to a large extent in the intense laser fields.

Experimental and theoretical exploration of photodissociation of SO2 via the C̃1B2 state: identification of the dissociation pathway

Abstract The photodissociation reaction of SO2 via the C1B2 state, SO 2 ( C 1 B 2 ) → SO ( 3 Σ − ) + O ( 3 P ) was investigated by experimental and theoretical approaches cooperatively to clarify its dissociation mechanism. We measured the laser induced fluorescence (LIF) spectrum of the C-X band in the short UV wavelength region (210-200 nm) under jet-cooled conditions. The fluorescence quantum yields and the dissociation rates of individual vibronic levels were determined in the 220-200-nm region using (i) the LIF spectrum measured in the present study, (ii) that measured previously by Yamanouchi et al. (J. Mol. Struct. 352/353 (1995) 541) in the longer wavelength region above 210 nm, and (iii) the high-resolution absorption spectrum measured by Freeman et al. (Planet. Space. Sci. 32 (1984) 1125). The dissociation rates were also derived in the 210-200-nm region from the broadening of the rotational lines of the C-X vibronic transitions. It was found that the dissociation rates determined through two different procedures were consistent with each other, and that the rate increases almost exponentially as an excess energy above the dissociation threshold increases though there is a certain fluctuation of the dissociation rates reflecting a mode specificity. We also performed theoretical ab initio calculations to derive potential energy surfaces (PESs) of the electronic ground states and low-lying electronically excited states of SO2 within the MCSCF and MRCI levels. The theoretical calculations showed that (i) the PES of the 21A′ state (the C1B2 state in C2v symmetry), correlating with the SO ( 1 Δ) + O ( 1 D ) asymptote, crosses with the repulsive singlet (31A′) state, correlating with the SO ( 3 Σ − ) + O ( 3 P ) asymptote, to form a pseudo-seam, (ii) the crossing pseudo-seam of these two PESs is located near the equilibrium bent angles for the X and C states along the energy contour of ∼9700 cm−1 measured from the SO ( 3 Σ − ) + O ( 3 P ) dissociation limit, and (iii) the crossing seam between the 21A′ (C1B2) and repulsive 23A′ states is located in a lower energy region than the singlet seam; at ∼6700 cm−1 measured from the SO ( 3 Σ − ) + O ( 3 P ) dissociation limit. On the basis of the above experimental and theoretical results together with the previous experimental evidence, we propose that (i) the photodissociation reaction via the C state proceeds mainly through the vibronic mixing between the C state vibronic levels with the quasi-bound dissociation continuum of the electronic ground X1A1 state, and (ii) the additional dissociation channels may be open through the crossing seam with the repulsive singlet (31A′) state and that with the repulsive triplet (23A′) state in their narrow crossing energy regions.

Compact XFEL and AMO sciences: SACLA and SCSS

The concept, design and performance of Japans compact free-electron laser (FEL) facilities, the SPring-8 Compact SASE Source test accelerator (SCSS) and SPring-8 Angstrom Compact free electron LAser (SACLA), and their applications in mainly atomic, molecular and optical science are reviewed. At SCSS, intense, ultrafast FEL pulses at extreme ultraviolet (EUV) wavelengths have been utilized for investigating various multi-photon processes in atoms, molecules and clusters by means of ion and electron spectroscopy. The quantum optical effect superfluorescence has been observed with EUV excitation. A pump?probe technique combining FEL pulses with near infrared laser pulses has been realized to study the ultrafast dynamics of atoms, molecules and clusters in the sub-picosecond regime. At SACLA, deep inner-shell multi-photon ionization by intense x-ray FEL pulses has been investigated. The development of seeded FEL sources for producing transversely and temporally coherent light, as well as the expected impact on advanced science are discussed.

Ionization and fragmentation dynamics of benzene in intense laser fields by tandem mass spectroscopy

Using a tandem time-of-flight mass spectrometer, benzene cations produced by the resonantly enhanced multiphoton ionization are mass separated and are exposed to intense laser fields (∼2×1016 W/cm2) at λ∼790 and 395 nm with the pulse duration of ∼50 fs. Comparing the yields of the product ions with those obtained from neutral benzene molecules, the ionization and dissociation dynamics of benzene in intense laser fields is investigated. At λ∼790 nm, the formation of parent benzene ions is a dominant process irrespective of the initial charge states, i.e., major products obtained when starting from neutral benzene are benzene cations and dications and those obtained when starting from benzene cation are benzene dications. On the other hand, at λ∼395 nm, the fragmentation processes to produce C4Hi+(i=2–4) and C3Hj+(j=1–3) dominate over further ionization to the benzene dication for both cases starting from neutral benzene and benzene cation, indicating the population trapping occurs by the efficient confinem...

Sequential three-body Coulomb explosion of CS2 in intense laser fields appearing in momentum correlation map

Abstract The three-body Coulomb explosion process of CS 2 in intense laser fields ( 60 fs , 0.36×10 15 W / cm 2 ), CS 2 3+ →S + +C + +S + , is studied by the triple coincidence momentum imaging of the atomic fragment ions. The set of the observed momentum vectors of the fragment ions determined for respective explosion events exhibits a clear trajectory originating from the rotational motion of an intermediate CS 2+ ion, formed in a sequential explosion process of CS 2 3+ prior to the complete three-body fragmentation. The contribution of the sequential pathway to the overall three-body explosion process is estimated to be 17(1)%.

Mass-resolved two-dimensional momentum imaging of the Coulomb explosion of N2 and SO2 in an intense laser field

Abstract The Coulomb explosion reactions of N 2 and SO 2 caused by the irradiation of intense laser light were investigated by high-resolution mass spectroscopy. For N 2 , from the least-squares fit to the split mass patterns for the N + , N 2+ and N 3+ channels, kinetic energy releases of six explosion pathways were determined. By rotating the laser polarization direction with respect to the detection axis, angular distributions for the singly and multiply charged atomic ion fragments were obtained and were converted to mass-resolved two-dimensional momentum imaging (MRMI) maps. This MRMI method was found to be useful to correlate ion fragments produced after the Coulomb explosion. MRMI was also applied to the Coulomb explosion of SO 2 .

Acetylene-vinylidene isomerization in ultrashort intense laser fields studied by triple ion-coincidence momentum imaging

The isomerization of acetylene via hydrogen migration in intense laser fields (8 x 10(14) W/cm2) has been investigated by coincidence momentum imaging of the three-body Coulomb explosion process, C2H2 (3+)-->H+ + C+ + CH+. When ultrashort (9 fs) laser pulses are used, the angle between the momenta of C+ and H+ fragments exhibits a sharp distribution peaked at a small angle ( approximately 20 degrees ), showing that the hydrogen atom remains near the original carbon site in the acetylene configuration. On the other hand, a significantly broad distribution extending to larger momentum angles ( approximately 120 degrees ) is observed when the pulse duration is increased to 35 fs, indicating that the ultrafast isomerization to vinylidene is induced in the longer laser pulse.

Visualizing hydrogen atoms migrating in acetylene dication by time-resolved three-body and four-body Coulomb explosion imaging

The visualization of ultrafast isomerization of deuterated acetylene dication (C(2)D(2)(2+)) is demonstrated by time-resolved Coulomb explosion imaging with sub-10 fs intense laser pulses (9 fs, 0.13 PW cm(-2), 800 nm). The Coulomb explosion imaging monitoring the three-body explosion process, C(2)D(2)(3+)→ D(+) + C(+) + CD(+), as a function of the delay between the pump and probe pulses revealed that the migration of a deuterium atom proceeds in a recurrent manner; One of the deuterium atoms first shifts from one carbon site to the other in a short timescale (∼90 fs), and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation. Correlated motion of the two deuterium atoms associated with the hydrogen migration and structural deformation to non-planar geometry are identified by the time-resolved four-body Coulomb explosion imaging, C(2)D(2)(4+)→ D(+) + C(+) + C(+) + D(+).

Extraction of molecular dynamics in intense laser fields from mass-resolved momentum imaging maps: application to Coulomb explosion of NO

The nuclear dynamics of NO in intense laser fields (~1.4 PW cm-2 ) is studied on the basis of the momentum-scaled time-of-flight spectra and mass-resolved momentum imaging maps of the atomic fragment ions, Np + and Oq + (p ,q = 1-3), produced from the (p ,q ) Coulomb explosion pathways of NO, i.e. NO(p +q )+ Np + +Oq + . A procedure to extract nuclear dynamics from the momentum maps is proposed by taking account of the effect of the finite detector size. The resultant nine (p ,q ) single-pathway components show that (a) the distributions of the N-O internuclear distance of NOz + just before the Coulomb explosion exhibit significant broadening (~1 A); (b) the distance at which the corresponding distribution takes a maximum increases from 1.68 to 2.34 A as z increases from 2 to 6 with a small amount of suppression at odd z (z = 3,5); and (c) the atomic fragment ions exhibit narrower angular distributions for a larger total charge z ( = p +q ) of NOz + .

Ultrafast structural deformation of NO2 in intense laser fields studied by mass-resolved momentum imaging

The ultrafast structural deformation of NO2 in an intense laser field (1.0 PW/cm2) is studied by mass-resolved momentum imaging (MRMI) of the Op+ and Nq+ (p,q=1–3) fragment ions produced from NO2z+ through the Coulomb explosion processes, NO2z+→Op++Nq++Or+ (z=p+q+r). The N–O distance just before the Coulomb explosion is elongated significantly from that in the electronic ground state, and it monotonically increases from 1.7 to 2.1 A as z increases from 4 to 9. The ∠O–N–O bond angle increases toward a linear configuration as a function of z, which is interpreted in terms of the formation of the light-dressed potential energy surfaces. The two-body fragmentation pathways to produce NO+ and NO2+ are also investigated by the MRMI measurements to derive the extent of the asymmetrical bond elongation of one of the two N–O bonds.