State-selective dissociative double ionization of CH3I and CH2I2 via I 4d core-hole states studied by multi-electron-ion coincidence spectroscopy.

The dissociative double ionization of CH3I and CH2I2 irradiated with extreme ultraviolet light at hv = 100 eV is investigated by multi-electron-ion coincidence spectroscopy using a magnetic bottle type electron spectrometer. The spin-orbit state-resolved Auger electron spectra for the I 4d core-hole states, (I 4d3/2)-1 and (I 4d5/2)-1, provide clear identifications of electronic states of CH3I2+ and CH2I22+. The dominant ion species produced after the double ionization correlate with the Auger electron energy, showing that different fragmentation pathways are open depending on the electronic states populated by the Auger decay.

Exploring the ultrafast and isomer-dependent photodissociation of iodothiophenes site-selective ionization.

C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy.

Wave packet dynamics and control in excited states of molecular nitrogen.

Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4-9) and a range of vibrational and rotational quantum numbers.

Asymmetric Dissociative Tunneling Ionization of Tetrafluoromethane in - 2 Intense Laser Fields.

Dissociative ionization of tetrafluoromethane (CF) in linearly polarized -2 ultrashort intense laser fields (1.4 × 10 W/cm, 800 and 400 nm) has been investigated by three-dimensional momentum ion imaging. The spatial distribution of produced by CF → + F + e exhibited a clear asymmetry with respect to the laser polarization direction.

Post-Ionization Dynamics of the Polar Molecule OCS in Asymmetric Laser Fields.

We have investigated the dissociation mechanisms of the prototypical heavy polar molecule OCS into the two break-up channels of the dication, OCS → O + CS and OC + S, in phase-locked two-color intense laser fields. The branching ratio of the breaking of the C-O and C-S bonds followed a pronounced 2-oscillation with a modulation depth of 11%, depending on the relative phase of the two-color laser fields. The fragment ejection direction of both break-up channels reflects the anisotropy of the tunneling ionization rate, following a 2-periodicity, as well.

Electronic relaxation and dissociation dynamics in formaldehyde: pump wavelength dependence.

The effect of the incident UV pump wavelength on the subsequent excited state dynamics, electronic relaxation, and ultimate dissociation of formaldehyde is studied using first principles simulation and Coulomb explosion imaging (CEI) experiments. Transitions in a vibronic progression in the ← absorption band are systematically prepared using a tunable UV source which generates pulses centered at 304, 314, 329, and 337 nm. We find, both simulation and experimental results, that the rate of excited state decay and subsequent dissociation displays a prominent dependence on which vibronic transition in the absorption band is prepared by the pump.

Association Reaction of Gaseous CH in Femtosecond Laser Filaments Studied by Time-of-Flight Mass Spectrometry.

Association reactions by femtosecond laser filamentation in gaseous CH were studied by time-of-flight mass spectrometry of neutral reaction products. Direct sampling from the reaction cell to a mass spectrometer via a differential pumping stage allowed the identification of various hydrocarbon molecules C H with = 3-7 and = 4-7, which includes species not observed in the previous studies. It was found that products containing three and four carbon atoms dominate the mass spectrum with smaller yields for higher-mass species, suggesting that carbon chain growth proceeds through the reaction with CH in the reaction cell.

Capturing roaming molecular fragments in real time.

Since the discovery of roaming as an alternative molecular dissociation pathway in formaldehyde (HCO), it has been indirectly observed in numerous molecules. The phenomenon describes a frustrated dissociation with fragments roaming at relatively large interatomic distances rather than following conventional transition-state dissociation; incipient radicals from the parent molecule self-react to form molecular products. Roaming has been identified spectroscopically through static product channel-resolved measurements, but not in real-time observations of the roaming fragment itself.

Characterization of soft X-ray FEL pulse duration with two-color photoelectron spectroscopy.

The pulse duration of soft X-ray free-electron laser (FEL) pulses of SACLA BL1 (0.2-0.3 nC per bunch, 0.

Probing Rydberg-Rydberg interactions in N by ultrafast EUV-NIR photoelectron spectroscopy.

The ultrafast dynamics of molecular nitrogen (N) just below the ionization threshold has been investigated by time-resolved photoelectron spectroscopy using a single harmonic centered at hν = 15.38 eV. The evolution of the Rydberg wavepacket launched by the ultrashort EUV pulse is probed by a time-delayed femtosecond NIR laser pulse.

Selective bond breaking of CO in phase-locked two-color intense laser fields: laser field intensity dependence.

Selective bond breaking of CO in phase-locked ω-2ω two-color intense laser fields (λ = 800 nm and 400 nm, total field intensity I ∼ 10 W cm) has been investigated by coincidence momentum imaging. The CO and O fragment ions produced by two-body Coulomb explosion, CO → CO + O, exhibit asymmetric distributions along the laser polarization direction, showing that one of the two equivalent C-O bonds is selectively broken by the laser fields. At a field intensity higher than 2 × 10 W cm, the largest fragment asymmetry is observed when the relative phase ϕ between the ω and 2ω laser fields is ∼0 and π.

Single-order laser high harmonics in XUV for ultrafast photoelectron spectroscopy of molecular wavepacket dynamics.

We present applications of extreme ultraviolet (XUV) single-order laser harmonics to gas-phase ultrafast photoelectron spectroscopy. Ultrashort XUV pulses at 80 nm are obtained as the 5th order harmonics of the fundamental laser at 400 nm by using Xe or Kr as the nonlinear medium and separated from other harmonic orders by using an indium foil. The single-order laser harmonics is applied for real-time probing of vibrational wavepacket dynamics of I molecules in the bound and dissociating low-lying electronic states and electronic-vibrational wavepacket dynamics of highly excited Rydberg N molecules.

Imaging Electronic Excitation of NO by Ultrafast Laser Tunneling Ionization.

Tunneling-ionization imaging of photoexcitation of NO has been demonstrated by using few-cycle near-infrared intense laser pulses (8 fs, 800 nm, 1.1×10^{14}  W/cm^{2}). The ion image of N^{+} fragment ions produced by dissociative ionization of NO in the ground state, NO (X^{2}Π,2π)→NO^{+}+e^{-}→N^{+}+O+e^{-}, exhibits a characteristic momentum distribution peaked at 45° with respect to the laser polarization direction.

A magnetic-bottle multi-electron-ion coincidence spectrometer.

A novel multi-electron-ion coincidence spectrometer developed on the basis of a 1.5 m-long magnetic-bottle electron spectrometer is presented. Electrons are guided by an inhomogeneous magnetic field to a detector at the end of the flight tube, while a set of optics is used to extract counterpart ions to the same detector, by a pulsed inhomogeneous electric field.

Time-resolved EUV photoelectron spectroscopy of dissociating I2 by laser harmonics at 80 nm.

Generation of single-order laser harmonics in extreme ultraviolet (EUV) and the application to the time-resolved photoelectron spectroscopy of I(2) are demonstrated. The EUV pulses at 80 nm were generated from Kr as the 5th order harmonics of intense 400 nm laser pulses and then separated from other harmonic orders by a thin indium foil. The pump-probe photoelectron spectroscopy of I(2) in the B (3)Π(0(u)(+)) and B" (1)Π(1(u)) states excited by visible laser pulses at 490 nm showed a rapid increase in the yield of atomic iodine (~400 fs), reflecting the dissociation dynamics evolving simultaneously in the two excited states.

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.

Multiple explosion pathways of the deuterated benzene trication in 9-fs intense laser fields.

The fragmentation of deuterated benzene (C6D6) in ultrashort intense laser fields (9 fs, 1 x 10(15) W/cm2) is studied by the ion-coincidence momentum imaging technique. Five two-body and eight three-body Coulomb explosion pathways from the trication (C6D6(3+)), associated with the deprotonation and ring-opening reactions, are identified. It is found from the fragment momentum correlation that all the observed three-body explosion processes proceed sequentially via the two-body Coulomb explosion forming molecular dications, C(m)D(n)(2+), with (m,n) = (6,5), (5,5), (5,4), (4,4), (4,3), and (3,3), which further dissociate into pairs of monocations.

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 recurrently migrating hydrogen in acetylene dication by intense ultrashort laser pulses.

We demonstrate the visualization of ultrafast hydrogen migration in deuterated acetylene dication (C2D2{2+}) by employing the pump-probe Coulomb explosion imaging with sub-10-fs intense laser pulses (9 fs, 0.13 PW/cm{2}, 800 nm). It is shown, from the temporal evolution of the momenta of the fragment ions produced by the three-body explosion, C2D2{3+}-->D{+} + C{+} + CD{+}, that the migration proceeds in a recurrent manner: the deuterium atom first shifts from one carbon site to the other in a short time scale (approximately 90 fs) and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation.

Dalitz plot analysis of Coulomb exploding O3 in ultrashort intense laser fields.

The three-body Coulomb explosion of O3, O3(3+)-->O++O++O+, in ultrashort intense laser fields (2x10(15) W/cm2) is studied with two different pulse durations (9 and 40 fs) by the coincidence momentum imaging method. In addition to a decrease in the total kinetic energy release, a broadening in the Dalitz plot distribution [Philos. Mag.

Electronic and nuclear responses of fixed-in-space H2S to ultrashort intense laser fields.

The Coulomb explosion dynamics of H2S, H2S3+-->H+ +S+ + H+, in ultrashort intense laser fields (12 fs, approximately 2 x 10(14) W/cm2) is studied by the coincidence momentum imaging of the three fragment ions. Different electronic and nuclear responses are identified depending on the direction of laser polarization epsilon in the molecular frame. The dependence can be interpreted in terms of the electronic and bonding characters of charge transfer states of H2S coupled to the electronic ground state.

Concerted and sequential Coulomb explosion processes of N2O in intense laser fields by coincidence momentum imaging.

The Coulomb explosion dynamics of N2O in intense laser fields (800 nm, 60 fs, approximately 0.16 PWcm2) is studied by the coincidence momentum imaging method. From the momentum correlation maps obtained for the three-body fragmentation pathway, N2O3+-->N++N++O+, the ultrafast structural deformation dynamics of N2O prior to the Coulomb explosion is extracted.

Probing the ultrafast nuclear motion in CS2 2+ in intense laser fields.

The temporal evolution of the nuclear wave packet of CS2 2+ formed in an intense laser field (60 fs, 0.13 PW/cm2) is traced in real time by the pump-and-probe technique combined with coincidence momentum imaging of the Coulomb explosion process, CS2 3+-->S+ + C+ + S+. The momentum correlations among the fragment ions obtained as a function of the pump-probe time delay between 133 fs to 3 ps reveal that the nuclear wave packet in CS2 2+ evolves not only along the anti-symmetric stretching coordinate to yield S+ and CS+ but also along the symmetric stretching coordinate leading to the simultaneous breaking of the two C-S bonds.