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Roaming dynamics in real time Roaming is distinct from conventional reaction channels because of the unusual geometries that chemical systems use to bypass the minimum energy pathway. It is a relatively new phenomenon that is usually determined in experiments through spectroscopic characterization of the roaming products. Using a combination of time-resolved Coulomb explosion imaging and quasiclassical trajectory analysis, Endo et al. report real-time observation of individual fragments of the prototypical reaction of deuterated formaldehyde (D 2 CO) dissociation as they roam on ultrafast time scales. They show that roaming not only occurs several orders of magnitude earlier than previously expected but also that it can terminate in a radical (D + DCO) rather than the well-known molecular (D 2 + CO) product channel. Science , this issue p. 1072

Molecules irradiated with intense laser pulses (field intensity ∼1015 W/cm2) exhibit a variety of characteristic processes, such as tunneling ionization, electron rescattering, high-order harmonics...

Electron-ion coincidence spectroscopy is powerful to elucidate tunneling ionization dynamics, but applicable only to molecules susceptible to dissociation. Here we introduce a novel scheme with additional pulses, which has been successfully applied to O2 HOMO-1.

Ultrafast dynamics of highly excited Rydberg states of ${\mathrm{Xe}}^{+}$ is investigated by time-resolved shot-by-shot photoelectron spectroscopy with EUV--free-electron-laser pump (40.8 eV) and near-infrared probe (1.56 eV) pulses. Sorting the single-shot spectra in the order of relative delay between the two pulses allows for investigating ultrafast decays of intermediate states participating in the double ionization of Xe. Observed photoelectron spectra show a biexponential decay within \ensuremath{\sim}100 fs and \ensuremath{\sim}300 ps in addition to constant signals lasting longer than 1 ns. These ultrafast decays reflect the lifetimes of autoionizing Rydberg states converging to electronically excited states of ${\mathrm{Xe}}^{2+}$. The results demonstrate that time-resolved shot-by-shot EUV--free-electron-laser photoelectron spectroscopywith a synchronized optical laser provides a powerful tool for investigating ultrafast electron emission processes.

We have used the FERMI free-electron laser to perform time-resolved photoelectron imaging experiments on a complex group of resonances near 15.38 eV in the absorption spectrum of molecular nitrogen, N2, under jet-cooled conditions. The new data complement and extend the earlier work of Fushitani et al. [Opt. Express 27, 19702–19711 (2019)], who recorded time-resolved photoelectron spectra for this same group of resonances. Time-dependent oscillations are observed in both the photoelectron yields and the photoelectron angular distributions, providing insight into the interactions among the resonant intermediate states. In addition, for most states, we observe an exponential decay of the photoelectron yield that depends on the ionic final state. This observation can be rationalized by the different lifetimes for the intermediate states contributing to a particular ionization channel. Although there are nine resonances within the group, we show that by detecting individual photoelectron final states and their angular dependence, we can identify and differentiate quantum pathways within this complex system.

Using time-resolved Coulomb explosion imaging (CEI) in an ultraviolet (UV) pump near infrared (NIR) probe experiment, we directly image the different dissociation dynamics in the formaldehyde molecule. Different pathways are distinguished from each other, despite their statistical nature. To extract such dynamics, hidden in a statistical background, calls for an elimination of noise. In our approach, we take advantage of CEI being a quasi-background-free technique., International Conference of Computational Methods in Sciences and Engineering ICCMSE 2020, April 29 – May 3, 2020, Crete, Greece (Virtual), Series: AIP Conference Proceedings

Ion-coincidence momentum imaging of Coulomb explosion of formaldehyde (H2CO) in femtosecond intense laser fields (800 nm, 1.3 × 1015 W/cm2) is performed with two different laser pulse durations (7 fs and 35 fs). In the 7-fs laser fields, the full fragmentation pathway from H2CO4+ → H+ + H+ + C+ + O+ is identified. The angles between the fragment momenta are well reproduced by a simple Coulomb explosion model from the geometry of neutral formaldehyde, showing that the molecular structure is virtually frozen along the bending coordinates during the multiple ionization. Three-body Coulomb explosion pathways from triply charged states, H2CO3+ → H+ + H+ + CO+ and H+ + CH+ + O+, are observed in both the 7-fs and 35-fs laser fields. Significant changes in the momentum angle distribution and asymmetric energy partitioning between two H+ ions are observed in the 35-fs case, which are attributed to structural deformation prior to the Coulomb explosion in the longer pulse.

The pulse duration of soft X-ray free-electron laser (FEL) pulses of SACLA BL1 (0.2–0.3 nC per bunch, 0.5–0.8 MeV) were characterized by photoelectron sideband measurements. The intensity of the He 1 s−1 photoelectron sidebands generated by a near-infrared femtosecond laser was measured as a function of the time delay between the two pulses using an arrival time monitor. From the width of the cross-correlation trace thus derived, the FEL pulse duration was evaluated to be 28 ± 5 fs full width at half-maximum in the photon energy range between 40 eV and 120 eV.

“Roamers” are directly observed in the prototypical roaming reaction in the formaldehyde molecule. Despite their statistical nature, roaming is well discriminated from the radical- and molecular dissociation channels, using Coulomb explosion imaging and theoretical modeling., International Conference on Ultrafast Phenomena 2020, Nov. 16-19, 2020, Washington, D.C.

Dissociative tunneling ionization of nitric oxide (NO) in linearly polarized phase-locked two-color femtosecond intense laser fields (45 fs, $\ensuremath{\lambda}=800$ and 400 nm, total field intensity $I=1\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$) has been studied by three-dimensional ion momentum imaging. The ${\mathrm{N}}^{+}$ fragment produced by the dissociative ionization, $\mathrm{NO}\ensuremath{\rightarrow}\mathrm{N}{\mathrm{O}}^{+}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{N}}^{+}+\mathrm{O}+{e}^{\ensuremath{-}}$, exhibits a butterflylike momentum distribution peaked at finite angles with respect to the laser polarization direction. In addition, a clear dependence on the relative phase between the two laser fields is observed, showing that the tunneling ionization occurs efficiently when the electric field points from N to O. For the highest kinetic energy component, the observed orientation dependence is well explained with theoretical calculations by the weak-field asymptotic theory for the 2\ensuremath{\pi} highest occupied molecular orbital (HOMO). On the other hand, the peak angle shifts toward the laser polarization direction as the kinetic energy decreases, indicating that pathways other than direct ionization from the HOMO contribute to the dissociative ionization.

The ultrafast dynamics of molecular nitrogen (N

Coulomb explosion of CO 2 , CO 2 → CO 2 2 + + 2e − → CO + + O + + 2e − , in phase-locked two-color ultrashort intense laser fields (800 nm and 400 nm, ∼10 14 W/cm 2 ) has been investigated by coincidence momentum imaging. The momentum images of the O + and CO + fragments show asymmetric distributions along the laser polarization direction depending on the relative phase of the two-color laser fields. The fragment asymmetry becomes most prominent (∼4%) at the relative phases providing the largest asymmetry of the electric field amplitude, with the O + fragments preferentially ejected to the stronger field side. The mechanism of the asymmetric bond breaking of the two equivalent C O bonds is discussed in terms of the nuclear dynamics in CO 2 2 + .

Two-photon Rabi oscillations are observed in He on a timescale of 10 fs by utilizing the strong-field phenomenon known as Freeman resonance. The importance of ponderomotive shifts of both the Rydberg states and the ionization limit is highlighted. Coherent light–matter interaction provides powerful methods for manipulating quantum systems1,2,3. Rabi oscillation is one such process. As it enables complete population transfer to a target state, it is thus routinely exploited in a variety of applications in photonics, notably quantum information processing4,5. The extension of coherent control techniques to the multiphoton regime offers wider applicability, and access to highly excited or dipole-forbidden transition states. However, the multiphoton Rabi process is often disrupted by other competing nonlinear effects such as the a.c. Stark shift, especially at the high laser-field intensities necessary to achieve ultrafast Rabi oscillations6. Here we demonstrate a new route to drive two-photon Rabi oscillations on timescales as short as tens of femtoseconds, by utilizing the strong-field phenomenon known as Freeman resonance7. The scenario is not specific to atomic helium as investigated in the present study, but broadly applicable to other systems, thus opening new prospects for the ultrafast manipulation of Rydberg states8.

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. On the other hand, a broad distribution centered at ∼0° appears when the A^{2}Σ^{+} (3sσ) excited state is prepared as the initial state by deep-UV photoexcitation. The observed angular distributions are in good agreement with the corresponding theoretical tunneling ionization yields, showing that the fragment anisotropy reflects changes of the highest-occupied molecular orbital by photoexcitation.

Photoelectron spectroscopy of isolated atoms and molecules using single-order high-harmonics of Ti:Sapphire laser pulses (800 nm, 12 fs/30 fs) is demonstrated. Dielectric multilayer mirrors, SiC/Mg and Mo/Si, are used to isolate the 27th (42 eV) and 59th (91 eV) order harmonics, respectively. The obtained harmonics are characterized by valence and inner-shell photoelectron spectroscopy of Xe. The applications to two-color two-photon ionization of He and pump-probe spectroscopy of ultrafast photodissociation of Br2, Br2(C1Πu) → Br(2P3/2) + Br(2P3/2), are presented.

The ν3 and ν4 vibrational transitions of CH3 and CD3 isolated in solid parahydrogen were studied by high-resolution infrared spectroscopy. The radicals were produced by in situ UV photolysis of methyl iodides trapped in solid parahydrogen. The observed spectra showed clear rotational fine structures, which were subjected to further splitting due to the electrostatic field of solid parahydrogen. The molecular constants and crystal field parameters of the radicals in solid parahydrogen were determined by analyzing the spectral structures of the ν3 and ν4 transitions of CD3 by the crystal field theory. The rotational constants of the CD3 radical were found to be only a few percent smaller than those in the gas phase. The determined crystal field parameters indicated significant quantum effects in the intermolecular interactions between the radicals and hydrogen molecules in solid parahydrogen.

Vibronic progressions are programmed into a pulse shaper which converts them via the inherent Fourier transformation into a train of femtosecond pulses in time domain for chromophore excitation. Double pulse results agree with phase-sensitive wave packet superposition from a Michelson interferometer which delivers coherence times with high reliability. Spectral resolution of 1 nm and a spacing of around 4 nm within the 20 nm envelope centered at 590 nm delivers a train of seven phase-controlled 40 fs subpulses separated by 250 fs. Combs adjusted to the zero phonon lines (ZPL) and phonon sidebands (PSB) of the B state vibronic progression are reproduced in the chromophore for a coherent subpulse accumulation. B state ZPL wave packet dynamics dominates in pump-probe spectra due to its coherence despite an overwhelming but incoherent A state contribution in absorption. PSB comb accumulation is also phase sensitive and demonstrates coherence within several 100 matrix degrees of freedom in the vicinity.

An electron–ion coincidence momentum imaging apparatus has been developed and applied to dissociative ionization of CS 2 in ultrashort intense laser fields. Photoelectron images of CS 2 in 35 fs intense laser fields (2 × 10 13 W/cm 2 , 800 nm, linearly polarized) recorded in coincidence with the parent ion, CS 2 + , show clear concentric ring patterns due to the above-threshold ionization (ATI) process. On the other hand, broad structureless distributions elongated along the direction of the laser polarization are observed in the coincidence electron images for the CS + and S + fragment ions. The difference in the electron images indicates that the dissociative ionization does not proceed sequentially by the formation and photodissociation of CS 2 + in intense laser fields.

Femtosecond pump–probe 2D (wavelength and delay) optical Kerr effect (OKE) spectroscopy is used to measure laser induced birefringence in parahydrogen and normal hydrogen crystals at 4.2 K. At the probe center wavelength the OKE signal reveals underdamped sinusoidal modulations at frequencies corresponding to the Raman-active transverse optical phonon mode of solid parahydrogen (36.8 cm−1) and normal hydrogen (38.5 cm−1), respectively. For parahydrogen crystals, at small shifts away from the probe center wavelength the OKE signal displays high frequency oscillations (355 cm−1) which correspond to J = 2 rotons with a 94 fs time period.

Photochemical reactions of molecules in solid orthodeuterium (o-D 2 ) have been studied by high-resolution infrared spectroscopy and compared with previous results obtained in solid parahydrogen (p-H 2 ). Ultraviolet photolysis of CD 3 I molecules in solid o-D 2 yielded CD 3 radicals and iodine atoms efficiently, which indicates a small cage effect in solid o-D 2 , as in the case of solid p-H 2 . The Fourier transform infrared spectrum of the v 3 vibrational band of CD 3 showed a rotational structure with additional splitting due to crystal field interactions. The magnetic dipole transition ( 2 P 1/2 - 2 P 3/2 ) of the I atom isolated in solid o-D 2 was observed together with a strong rotational satellite of deuterium molecules through the electron-roton coupling in solid hydrogen. The tunneling reaction between CD 3 and D 2 was not observed in a time scale of a few days, which gives the upper limit of the tunneling reaction rate of 10 -8 s -1 at 4.2 K.

We perform time resolved pump-probe spectroscopy on small halogen molecules ClF, Cl2, Br2, and I2 embedded in rare gas solids (RGS). We find that dissociation, angular depolarization, and the decoherence of the molecule is strongly influenced by the cage structure. The well ordered crystalline environment facilitates the modelling of the experimental angular distribution of the molecular axis after the collision with the rare gas cage. The observation of many subsequent vibrational wave packet oscillations allows the construction of anharmonic potentials and indicate a long vibrational coherence time. We control the vibrational wave packet revivals, thereby gaining information about the vibrational decoherence. The coherence times are remarkable larger when compared to the liquid or high pressure gas phase. This fact is attributed to the highly symmetric molecular environment of the RGS. The decoherence and energy relaxation data agree well with a perturbative model for moderate vibrational excitation and follow a classical model in the strong excitation limit. Furthermore, a wave packet interferometry scheme is applied to deduce electronic coherence times. The positions of those cage atoms, excited by the molecular electronic transitions are modulated by long living coherent phonons of the RGS, which we can probe via the molecular charge transfer states.

The ortho-to-para nuclear spin ratio of cyclopropenylidene (cyclic C3H2) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four ortho lines (J = 212-101, 312-303, 321-312, and 330-321) and four para lines (J = 202-111, 211-202, 220-211, and 322-313) of cyclic C3H2 in six cores in TMC-1. A statistical equilibrium analysis of the observed intensities revealed apparent correlation between the ortho-to-para ratio of cyclic C3H2 and chemical evolution of the molecular cloud; the ortho-to-para ratio in younger cores is less than 2, while that in older cores is close to the statistical ratio of 3. A simple chemical model calculation indicates that the correlation is ascribed to the conservation of nuclear spin angular momenta in chemical reactions relevant to the production, destruction, and reproduction of cyclic C3H2. Since cyclic C3H2 has been observed in a variety of sources, the ortho-to-para ratio of cyclic C3H2 may be a useful probe of chemical evolution of molecular clouds.

Solid parahydrogen, known as a quantum crystal, is a novel matrix for the study of physical and chemical processes of molecules at low temperatures by high-resolution infrared spectroscopy. Ro-vibrational motion of molecules embedded in solid parahydrogen is well quantized on account of the weak interaction in the crystal. In addition, molecules are almost free from the cage effect because of the softness of the quantum crystal, so that a variety of chemical reactions could be observed for molecules in solid parahydrogen at liquid He temperatures. In this article, we survey our recent studies on photodissociation of methyl radicals and subsequent reactions in solid parahydrogen, and discuss (1) the nuclear spin selection rule in chemical reactions and (2) pure tunnelling reactions obtained from the analysis of the present system. Contents PAGE 1. Introduction 534 2. Solid parahydrogen as a matrix 535 3. Experiments 536 4. Photochemistry of methyl iodide 537 5. Nuclear spin selection rule in chemical ...

Despite several suggestions concerning the existence of negatively charged molecules and negative atomic ions in molecular clouds, few attempts have been made to search for negative ions in molecular clouds. In the present study, we extensively searched for three negative molecular ions (CCH−, NCO−, and NCS−) in a dark cloud, L134N, using the Nobeyama 45-m radio telescope. The three negative ions are molecules whose rotational constants have been reported based on laboratory experiments. After a long accumulation, a trace amount of an unidentified emission line was detected at a frequency close to the theoretical prediction of the J = 1–0 transition of CCH−. If this unidentified line is attributable to CCH−, the observed emission intensity would provide an estimation of the column density of CCH− in L134N to be 1.0× 1011 cm−2, which corresponds to a fractional abundance of 5× 10−12 relative to hydrogen. However, our recent observation using the IRAM 30-m telescope did not reproduce the J = 1–0 signal, nor detect any trace of the J = 2–1 transition. Thus, the identification of CCH− in L134N is not yet confirmed. As for other ions, signals of neither NCO− nor NCS− were detected in L134N. Although we also searched for the three negative ions in a translucent cloud, CB228, and a star-forming region, SgrB2, no signals were detected. The upper limit abundances of the ions in these clouds are discussed.

Tunneling chemical reactions between deuterated methyl radicals and the hydrogen molecule in a parahydrogen crystal have been studied by Fourier transform infrared spectroscopy. The tunneling rates of the reactions R + H2 --RH + H (R = CD3,CD2H,CDH2) in the vibrational ground state were determined directly from the temporal change in the intensity of the rovibrational absorption bands of the reactants and products in each reaction in solid parahydrogen observed at 5 K. The tunneling rate of each reaction was found to differ definitely depending upon the degree of deuteration in the methyl radicals. The tunneling rates were determined to be 3.3 x 10(-6) s(-1), 2.0 x 10(-6) s(-1), and 1.0 x 10(-6) s(-1) for the systems of CD3, CD2H, and CDH2, respectively. Conversely, the tunneling reaction between a CH3 radical and the hydrogen molecule did not proceed within a week's time. The upper limit of the tunneling rate of the reaction of the CH3 radical was estimated to be 8 x 10(-8) s(-1).

Solid parahydrogen provides a novel matrix for isolation spectroscopy of atoms and molecules. Ro-vibrational motion of molecules embedded in solid parahydrogen is well quantized on account of the weak interactions in the crystal and of the softness of the crystal being characteristic of quantum crystals. Most of the observed spectral linewidths are one or two orders of magnitude sharper than those observed in conventional rare gas matrices. The sharp linewidths make the parahydrogen crystal an excellent matrix for the study of ro-vibrational states and dynamics of dopant molecules in the condensed phase by high-resolution spectroscopy. In this article, we have summarized the most fundamental part of our study, that is, the theory of ro-vibrational states of dopant molecules in the crystal, which is necessary for the quantitative analysis of high-resolution infrared spectra. We also discuss what we have learned from the analysis of high-resolution infrared spectra in solid parahydrogen. These include perturbations to rotational motion of dopant molecules, reduction of rotational constants, vibrational dephasing and relaxation. Outstanding questions to be solved are also discussed herein.

Pure vibrational overtone transitions of solid parahydrogen were studied using high-resolution laser spectroscopy. Extremely narrow spectral linewidth (~20 MHz) allowed us to observe rich spectral structure that originates in subtle intermolecular interactions in the crystal. It was found that anisotropy of the distribution of zero-point lattice vibration of hydrogen molecules perturbs the energy levels of the vibrationally excited states significantly. Large amplitude of the zero-point lattice vibration being an intrinsic nature of quantum solids, was directly observed from the present high-resolution spectroscopy. The first observation of a pure vibrational overtone transition of solid orthodeuterium is also discussed.

Diffusion of hydrogen atoms in solid parahydrogen was investigated using high-resolution infrared spectroscopy. Hydrogen atoms were produced as by-products of a photoinduced reaction of nitric oxides embedded in solid parahydrogen. The diffusion of the hydrogen atoms is mainly terminated by the reaction of H + NO → HNO. The diffusion rate determined from the increase of the intensity of rotation-vibration transitions of HNO molecules was found to be two orders of magnitude larger than that determined by the self-recombination reaction of H + H → H₂ in pure parahydrogen crystals.

Three-body Coulomb explosion of formaldehyde (H2CO) in intense 7- and 35-fs laser fields (1.3 × 1015 W/cm2) has been investigated by using ion-coincidence momentum imaging technique. Two types of explosion pathways from the triply charged state, H2CO3+ → (i) H+ + H+ + CO+ and (ii) H+ + CH+ + O+, have been identified. It is shown from the momentum correlation of the fragment ions of pathway (i), that the geometrical structure of the molecule is essentially frozen along the H-C-H bending coordinate for the 7-fs case. On the other hand, for a longer pulse duration (35 fs), structural deformation along the C-H stretching and H-C-H bending coordinates is identified, which is ascribed to the nuclear dynamics in the dication states populated within the laser pulse duration.

Photolysis of a methyl radical CH3 in solid parahydrogen produces a methane molecule CH4 via the reaction between an intermediate singlet methylene 1CH2 and a parahydrogen molecule H2. Conservation of nuclear spin during the reaction has been investigated by the intensity distribution of the rotation-vibration spectrum of methane produced by the reaction. It was found that the population of each nuclear spin state of methane just after the reaction was different from that of the statistical ratio, which indicates that a nuclear spin selection rule does exist in the reaction. However, the observed population was significantly different from the theoretically predicted ratio. The discrepancy between the experiment and the theory may indicate a breakdown of the nuclear spin conservation during the reaction, if the reaction mechanism in solid parahydrogen is the same as in the gas phase.

Alkyl iodides were photolyzed in solid parahydrogen at about 5 K. The first photolysis with 253.7 nm photons yielded two products by reactions, RI +hν (253.7 nm )→ R free + I free and ( R – I ), the latter being a complex between the radical R and the iodine atom I . Subsequent photolysis with 193 nm photons activated the radical moiety in both products to induce a reaction, R + H 2 +hν (193 nm )→ RH + H , which gave the products RHfree and (RH– I ). The radical R and the alkane RH were characterized by the mid-IR absorption while the iodine atoms in I free , and ( R – I ) and (RH– I ) were identified by the near IR absorption of the magnetic dipole transition of the atom.

A multiphoton ionization process of He in intense extreme ultraviolet free-electron laser fields is investigated in a photon energy range of $h\mathit{v}=20.1\ensuremath{-}20.8\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ using shot-by-shot photoelectron spectroscopy. A photoelectron peak resulting from sequential double ionization of He due to five-photon absorption is observed around $h\mathit{v}=20.4\phantom{\rule{0.16em}{0ex}}$. Shot-by-shot analysis of photoelectron spectra shows that two-photon resonance to the $\mathrm{H}{\mathrm{e}}^{+}(2s{\phantom{\rule{0.16em}{0ex}}}^{2}S)$ state from the ${\mathrm{He}}^{+}$ ground state is responsible for the substantial enhancement of five-photon double ionization. Effective cross sections for two-photon absorption between the $1s$ and $2s$ states of ${\mathrm{He}}^{+}$ and for one-photon absorption from the ${\mathrm{He}}^{+}$ $2s$ state are obtained.

Photolysis of methyl iodide in solid parahydrogen (p-H2) at about 5 K is studied with ultraviolet light at 253.7 and 184.9 nm. It is found that the light at 253.7 nm produces only methyl radical, whereas the light at 184.9 nm yields both methyl radical and methane. The mechanism of the formation of the photoproducts is elucidated by analyzing the temporal behavior of the observed vibrational absorption. It is concluded that methyl radical in the ground state does not react with p-H2 molecules appreciably but that the radical in the electronic excited state of B(2A1′), accessible by reabsorption of 184.9 nm photons by the radical, decomposes to a singlet methylene CH2 a(1A1) and a hydrogen atom (2S) and that the singlet methylene reacts with a p-H2 molecule to give methane.

Time-resolved photoelectron spectroscopy of N2 Rydberg using single-order high harmonics at 80 nm reveals ultrafast wavepacket dynamics (~3 fs) of both Rydberg electron and the corresponding N2+ core as the beating oscillation (~ 300 fs).

Ultrafast three-body Coulomb explosion of formaldehyde (H2CO) in intense laser fields has been studied. The pulse duration dependence of the Newton plot of the fragment ions revealed the ultrafast molecular dynamics in the dication states.

Single-shot photoelectron spectroscopy of nonresonant two-color two-photon ionization of He has been carried out using an intense free-electron laser (FEL) at 59.7 nm and a femtosecond UV laser at 268 nm. By shot-by-shot analysis of photoelectron spectra, the cross section for nonresonant two-color ionization of He is determined to be ${\ensuremath{\sigma}}^{(2)}=4.1(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}52}$ cm${}^{4}$ s. The cross-correlation trace derived from the nonresonant signals shows that the temporal resolution of the system is 0.36(3) ps, which is mainly governed by the timing jitter between the FEL and UV laser pulses.

Shot-by-shot photoelectron spectroscopy has been performed to study the three-photon double ionization of Ar in intense extreme-ultraviolet free-electron laser fields over a photon energy range of 19.6--22.0 eV. It is found that the double ionization to Ar${}^{2+}$ 3${p}^{\ensuremath{-}2}$ proceeds sequentially via the formation of singly charged Ar${}^{+}$ 3${p}^{\ensuremath{-}1}$ states and is enhanced around photon energies of 20.5 and 21.5 eV. Two types of resonance are identified in the two-photon ionization of the Ar${}^{+}$ 3${p}^{\ensuremath{-}1}$ states: (i) resonances to intermediate states at the one-photon energy level and (ii) those to autoionizing states above the Ar${}^{+}$ ionization threshold.

Nonlinear processes of rare gas atoms in intense extreme ultraviolet (EUV) free-electron laser (FEL) fields have been investigated by photoelectron spectroscopy. Simultaneous recording of fluctuating laser spectra from self-amplified spontaneous emission (SASE) FEL and nonlinear responses of target atoms provided a deeper understanding on the multiphoton processes in (i) double ionization of Ar and (ii) double excitation of He, in intense EUV laser fields. Results on two-color time-resolved experiment are also presented.

We report the results of our preliminary investigations into the suitability of condensed-phase parahydrogen as a Raman-shifting medium for infrared cavity ringdown laser absorption spectroscopy. We have observed the conversion of ∼10-ns pulses of 532-nm radiation into first-, second-, and third-order vibrational Stokes radiation in bulk liquid and solid parahydrogen after a single 11-cm pass. Unexpectedly, we find that liquid H2 yields more efficient conversion than solid H2 with certain focal geometries, and that in the case of the solid, a collimated or loosely focused pump geometry is more efficient than a tight focus.

Ultrafast hydrogen migration in deuterated acetylene dication (C\({}_{2}{\mathrm{D}}_{2}^{2+}\)) is studied by time-resolved four-body Coulomb explosion imaging, C2D\({}_{2}^{4+}\, \rightarrow \,\,{\mathrm{D}}^{+} +{ \mathrm{C}}^{+} +{ \mathrm{C}}^{+} +{ \mathrm{D}}^{+}\), using a pair of few-cycle intense laser pulses (9 fs, 1. 3 ×1014 W/cm2). Momentum correlation of the D + ions produced by the full fragmentation process shows that (1) motions of the two deuterium atoms are strongly correlated during the isomerization and (2) the molecular structure deforms to non-planar geometries.

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields ($\ensuremath{\sim}24.1\text{ }\text{ }\mathrm{eV}$, $\ensuremath{\sim}5\text{ }\text{ }\mathrm{TW}/{\mathrm{cm}}^{2}$) is presented. Resonances to the doubly excited states converging to the ${\mathrm{He}}^{+}$ $N=3$ level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schr\"odinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.

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(+).

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ∼ 21 eV, ∼ 5 TW/cm²), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e⁻ and Ar²(+) + 2hν→Ar(+) + e⁻. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).