Melting entropy of crystals determined by electron-beam-induced configurational disordering.

Upon melting, the molecules in a crystal explore numerous configurations, reflecting an increase in disorder. The molar entropy of disorder can be defined by Boltzmann's formula ΔSd = Rln(Wd), where Wd is the increase in the number of microscopic states, so far inaccessible experimentally. We found that the Arrhenius frequency factor A of the electron diffraction signal decay provides Wd through an experimental equation A = AINTWd, where AINT is an inelastic scattering cross section. The method connects Clausius and Boltzmann experimentally and supplements the Clausius approach, being applicable to a femtogram quantity of thermally unstable and biomolecular crystals. The data also showed that crystal disordering and crystallization of melt are reciprocal, both governed by the entropy change but manifesting in opposite directions.

Quantum computing of Hückel molecular orbitals of π-electron systems.

In order to demonstrate an applicability of quantum computing to fundamental electronic structure problems of molecules, we describe the Hückel Hamiltonian matrix in terms of quantum gates and obtain the orbital energies of fundamental π-electron molecules (C2H4, C3H4, C4H4, C4H6, and C6H6) using a superconducting-qubit-type quantum computer (ibm_kawasaki) with a post-selection error mitigation method. We show that the orbital energies are obtained with sufficiently high accuracy and small uncertainties and that characteristic features of the electronic structure of the π-electron molecules can be extracted by quantum computing in a straightforward manner.

Ionization and electron excitation of C60 in a carbon nanotube: A variable temperature/voltage transmission electron microscopic study.

There is increasing attention to chemical applications of transmission electron microscopy, which is often plagued by radiation damage. The damage in organic matter predominantly occurs via radiolysis. Although radiolysis is highly important, previous studies on radiolysis have largely been descriptive and qualitative, lacking in such fundamental information as the product structure, the influence of the energy of the electrons, and the reaction kinetics. We need a chemically well-defined system to obtain such data and have chosen as a model a variable-temperature and variable-voltage (VT/VV) study of the [2 + 2] dimerization of a van der Waals dimer [60]fullerene (C60) to C120 in a carbon nanotube (CNT), as studied for several hundred individual reaction events at atomic resolution. We report here the identification of five reaction pathways that serve as mechanistic models of radiolysis damage. Two of them occur via a radical cation of the specimen generated by specimen ionization, and three involve singlet or triplet excited states of the specimen, as initiated by electron excitation of the CNT, followed by energy transfer to the specimen. The [2 + 2] product was identified by measuring the distance between the two C60 moieties, and the mechanisms were distinguished by the pre-exponential factor and the Arrhenius activation energy­the standard protocol of chemical kinetic studies. The results illustrate the importance of VT/VV kinetic analysis in the studies of radiation damage and show that chemical ionization and electron excitation are inseparable, but different, mechanisms of radiation damage, which has so far been classified loosely under the single term "ionization."

Single and sequential double ionization of NO radical in intense laser fields.

We examine the dependences of the single and double ionization probabilities of NO radical on the angle between the NO axis and the laser polarization direction in an intense laser field (790 nm, 100 fs, 1-10 × 1014 W/cm2) and show that the double ionization is enhanced when the NO axis is parallel to the laser polarization direction. We reveal that the angular dependence of the sequential double ionization probability is determined by the shape of the 5σ orbital of NO+ from which the second photoelectron is emitted in the ionization from NO+ to NO2+. We also reveal that the fast oscillation in the probability of the tunnel ionization of NO originating from a coherent superposition of the two spin-orbit components in the electronic ground X2Π state is described well based on the molecular Ammosov-Delone-Krainov (MO-ADK) theory in which the time evolution of the electron density distribution of the 2π orbital is taken into account.

Photodissociation of [Ar-N2]+ induced by near-IR femtosecond laser fields by ion-trap time-of-flight mass spectrometry.

Photodissociation of [Ar-N2]+ induced by a near-IR (800 nm) femtosecond laser pulse is investigated using ion-trap time-of-flight mass spectrometry. The intra-complex charge transfer proceeding in the course of the decomposition of the electronically excited Ar+(2P3/2)⋯N2(X1Σg +), prepared by the photoexcitation of the electronic ground Ar(1S0)⋯N2 +(X2Σg +), is probed by the ion yields of Ar+ and N2 +. The yield ratio γ of N2 + with respect to the sum of the yields of Ar+ and N2 + is determined to be γ = 0.62, which is much larger than γ ∼ 0.2 determined before when the photodissociation is induced by a nano-second laser pulse in the shorter wavelength region between 270 and 650 nm. This enhancement of γ at 800 nm and the dependence of γ on the excitation wavelength are interpreted by numerical simulations, in which the adiabatic population transfer from Ar+(2P3/2)⋯N2(X1Σg +) to Ar(1S0)⋯N2 +(X2Σg +) at the avoided crossings is accompanied by the vibrational excitation in the N2 +(X2Σg +) moiety followed by the intra-complex vibrational energy transfer from the N2 +(X2Σg +) moiety to the intra-complex vibrational mode leading to the dissociation.

Extremely enhanced N2+ lasing in a filamentary plasma grating in ambient air.

Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by three to four orders of magnitude in ambient air for the self-seeded N2+ lasing at 428 nm, assigned to the B2Σu+(ν'=0) and X2Σg+(ν''=1) emission, by modulating the spatiotemporal overlap of ultrashort near-infrared control-pump pulses in a filamentary plasma grating; meanwhile, the spontaneous emission from the same transition is only enhanced by three to four times. We find that this enhancement is sensitive to the relative polarization and interference time of the two pulses, and reveal that the formation of the plasma grating induces different population variations in the B2Σu+(ν'=0) and X2Σg+(ν''=1) levels, resulting in an enormous population inversion between the two levels, thereby a higher gain for the giant enhancement of N2+ lasing in ambient air.

Switching Competition between Electron and Energy Transfers in Porphyrin-Fullerene Dyads.

Porphyrin-fullerene dyads were intensively studied as molecular donor-acceptor systems providing efficient photoinduced charge separation (CS). A practical advantage of the dyads is the possibility to tune its CS process by the porphyrin periphery modification, which allows one to optimize the dyad for particular applications. However, this tuning process is typically composed of a series of trial stages involving the development of complex synthetic schemes. To address the issue, we synthesized a series of dyads with properties switching between electron and energy transfer in both polar (benzonitrile) and nonpolar (toluene) media and developed a computation procedure with sufficient reliability by which we can predict the CS properties of the dyad in different media and design new dyads. The dyads photochemistry was established by conducting ultrafast transient absorption studies in toluene, anisole, and benzonitrile. The most crucial step in computational modeling was to establish a procedure for correction of the electronic-state energies obtained by DFT so that the effects of the electron correlation and the long-range interactions are properly incorporated. We also carried out standard electrochemical measurements and show that our computation approach predicts better thermodynamics of the dyads in different solvents.

Giant Enhancement of Air Lasing by Complete Population Inversion in N_{2}

A fine manipulation of population transfer among molecular quantum levels is a key technology for control of molecular processes. When a light field intensity is increased to the TW-PW cm^{-2} level, it becomes possible to transfer a population to specific excited levels through nonlinear light-molecule interaction, but it has been a challenge to control the extent of the population transfer. We deplete the population in the X^{2}Σ_{g}^{+}(v=0) state of N_{2}^{+} almost completely by focusing a dual-color (800 nm and 1.6 µm) intense femtosecond laser pulse in a nitrogen gas, and make the intensity of N_{2}^{+} lasing at 391 nm enhanced by 5-6 orders of magnitude. By solving a time-dependent Schrödinger equation describing the population transfer among the three lowest electronic states of N_{2}^{+}, we reveal that the X^{2}Σ_{g}^{+}(v=0) population is depleted by the vibrational Raman excitation followed by the electronic excitation A^{2}Π_{u}(v=2,3,4)←X^{2}Σ_{g}^{+}(v=1)←X^{2}Σ_{g}^{+}(v=0), resulting in the excessive population inversion between the B^{2}Σ_{u}^{+}(v=0) and X^{2}Σ_{g}^{+}(v=0) states. Our results offer a promising route to efficient population transfer among vibrational and electronic levels of molecules by a precisely designed intense laser field.

Generalized Phase Sensitivity of Directional Bond Breaking in the Laser-Molecule Interaction.

We establish a generalized picture of the phase sensitivity of laser-induced directional bond breaking using the H_{2} molecule as the example. We show that the well-known proton ejection anisotropy measured with few-cycle pulses as a function of their carrier-envelope phases arises as an amplitude modulation of an intrinsic anisotropy that is sensitive to the laser phase at the ionization time and determined by the molecule's electronic structure. Our work furthermore reveals a strong electron-proton correlation that may open up a new approach to experimentally accessing the laser-sub-cycle intramolecular electron dynamics also in larger molecules.

Surface processing of PMMA and metal nano-particle resist by sub-micrometer focusing of coherent extreme ultraviolet high-order harmonics pulses.

We demonstrate sub-micrometer processing of two kinds of thin films, polymethyl methacrylate (PMMA) and metal nano-particle resist, by focusing high-order harmonics of near-IR femtosecond laser pulses in the extreme ultraviolet (XUV) wavelength region (27.2-34.3 nm) on the thin film samples using an ellipsoidal focusing mirror. The ablation threshold fluences for the PMMA sample and the metal nano-particle resist per XUV pulse obtained by the accumulation of 200 XUV pulses were determined to be 0.42mJ/cm2 and 0.17mJ/cm2, respectively. The diameters (FWHM) of a hole created by the ablation on the PMMA film at the focus were 0.67 µm and 0.44 µm along the horizontal direction and the vertical direction, respectively. The fluence dependence of the Raman microscope spectra of the processed holes on the PMMA sample showed that the chemical modification, in which C=C double bonds are formed associated with the scission of the PMMA polymer chains, is achieved by the irradiation of the XUV pulses.

Absolute carrier-envelope-phase dependences of single and double ionization of methanol in a near-IR few-cycle laser field.

We investigate the carrier-envelope phase (CEP) dependences of the single and double ionization processes of methanol (CH3OH) in an intense near-IR few-cycle laser field (2.1 × 1014 W/cm2) by the asymmetry in the ejection direction of CH3 + for the non-hydrogen migration channels and CH2 + for the hydrogen migration channels created through the C-O bond breaking after the ionization. Based on the absolute CEP values at the laser-molecule interaction point, calibrated by the method using intense few-cycle circularly polarized laser pulses [Fukahori et al., Phys. Rev. A 95, 053410-1-053410-14 (2017)], we confirm that methanol cations are produced by tunnel ionization and methanol dications are produced by the recollisional double ionization. We obtain the phase offset for the double ionization accompanying no hydrogen migration to be 1.85π as the absolute CEP at which the extent of the asymmetry becomes maximum. We interpret the phase shift of 0.85π from the phase offset of 1.0π for the tunnel ionization, estimated by a tunnel ionization model incorporating the chemical bond asymmetry, as the corresponding time delay associated with the electron recollisional ionization. The positive phase shift of 0.13π for the single ionization in the non-hydrogen migration channel is interpreted as the additional time (165 as) with which a methanol cation can be excited electronically prior to the decomposition. The additional phase shift of 0.22π for the single ionization in the hydrogen migration channel is interpreted as the additional time (280 as) required for a methanol cation to be excited electronically leading to the hydrogen migration prior to the decomposition.

Rotational, Vibrational, and Electronic Modulations in N_{2}

We investigate lasing of a N_{2} gas induced by intense few-cycle near-IR laser pulses. By the pump-probe measurements, we reveal that the intensity of the B^{2}Σ_{u}^{+}-X^{2}Σ_{g}^{+} lasing emission of N_{2}^{+} oscillates at high (0.3-0.5 PHz), medium (50-75 THz), and low (∼3 THz) frequencies, corresponding to the energy separations between the rovibrational levels of the A^{2}Π_{u} and X^{2}Σ_{g}^{+} states. By solving the time-dependent Schrödinger equation, we reproduce the oscillations in the three different frequency ranges and show that the coherent population transfer among the three electronic states of N_{2}^{+} creates the population inversion between the B^{2}Σ_{u}^{+} and X^{2}Σ_{g}^{+} states, resulting in the lasing at 391 nm.

Significant Enhancement of N_{2}

We show that the intensity of self-seeded N_{2}^{+} lasing at 391 nm, assigned to the B^{2}Σ_{u}^{+}(v^{'}=0)→X^{2}Σ_{g}^{+}(v^{''}=0) emission, is enhanced by 2 orders of magnitude by modulating in time the polarization of an intense ultrashort near-IR (40 fs, 800 nm) laser pulse with which N_{2} is irradiated. We find that this dramatic enhancement of the 391 nm lasing is sensitive to the temporal variation of the polarization state within the laser pulse while the intensity of the spontaneous fluorescence emission at 391 nm is kept constant when the polarization state varies. We conclude that a postionization multiple-state coupling, through which the population can be transferred from the X^{2}Σ_{g}^{+} state of N_{2}^{+} to the first electronically excited A^{2}Π_{u} state, leads to the depletion of the population in the X^{2}Σ_{g}^{+} state, and consequently, to the population inversion between the X^{2}Σ_{g}^{+} state and the B^{2}Σ_{u}^{+} state.

Strong-Field Fourier Transform Vibrational Spectroscopy of D_{2}

The photoionization of D_{2} and dissociation of the resultant D_{2}^{+} are monitored by pump-probe measurements using intense near-infrared few-cycle laser pulses. The yields of D_{2}^{+} and D^{+} recorded up to the pump-probe delay time of 527 ps exhibit oscillatory structures reflecting the motion of the created vibrational wave packet of D_{2}^{+}, and the Fourier transform of the data in time domain reveals the vibrational level separations with uncertainties of 0.0002-0.0097 cm^{-1}, showing a potential application of the strong-field pump-probe measurements to high-resolution spectroscopy of molecular ions.

Third-harmonic generation and scattering in combustion flames using a femtosecond laser filament.

Coherent radiation in the ultraviolent (UV) range has high potential applicability to the diagnosis of the formation processes of soot in combustion because of the high scattering efficiency in the UV wavelength region, even though the UV light is lost largely by the absorption within the combustion flames. We show that the third harmonic (TH) of a Ti:sapphire 800 nm femtosecond laser is generated in a laser-induced filament in a combustion flame and that the conversion efficiency of the TH varies sensitively by the ellipticity of the driver laser pulse but does not vary so much by the choice of alkanol species introduced as fuel for the combustion flames. We also find that the TH recorded from the side direction of the filament is the Rayleigh scattering of the TH by soot nanoparticles within the flame and that the intensity of the TH varies depending on the fuel species as well as on the position of the laser filament within the flame. Our results show that a remote and in situ measurement of distributions of soot nanoparticles in a combustion flame can be achieved by Rayleigh scattering spectroscopy of the TH generated by a femtosecond-laser-induced filament in the combustion flame.

Rovibrational Resonances in H2He.

The nuclear dynamics of the metastable H2He+ complex is explored by symmetry considerations and angular momentum addition rules as well as by accurate quantum chemical computations with complex coordinate scaling, complex absorbing potential, and stabilization techniques. About 200 long-lived rovibrational resonance states of the complex are characterized and selected long-lived states are analyzed in detail. The stabilization mechanism of these long-lived resonance states is discussed on the basis of probability density plots of the wave functions. Overlaps of wave functions derived by a reduced-dimensional model with the full-dimensional wave functions reveal dissociation pathways for the long-lived resonance states and allow the calculation of their branching ratios.

Direct Microscopic Analysis of Individual C60 Dimerization Events: Kinetics and Mechanisms.

Modern transition state theory states that the statistical behavior of a chemical reaction is the sum of individual chemical events that occur randomly. Statistical analysis of each event for individual molecules in a three-dimensional space however is practically impossible. We report here that kinetics and mechanisms of chemical reactions can be investigated by using a one-dimensional system where reaction events can be observed in situ and counted one by one using variable-temperature (VT) atomic-resolution transmission electron microscopy (TEM). We thereby provide direct proof that the ensemble behavior of random events conforms to the Rice-Ramsperger-Kassel-Marcus theory, as illustrated for [2 + 2] cycloaddition of C60 molecules in carbon nanotubes (CNTs). This method gives kinetic and structural information for different types of reactions occurring simultaneously in the microscopic view field, suggesting that the VT-TEM opens a new dimension of chemical kinetics research on molecules and their assemblies in their excited and ionized states. The study carried out at 393-493 K showed that pristine CNT primarily acts as a singlet sensitizer of the cycloaddition reaction that takes place with an activation energy of 33.5 ± 6.8 kJ/mol. On the other hand, CNT suffers electron damage of the conjugated system at 103-203 K and promotes a reactive radical cation path that takes place with an activation energy of only 1.9 ± 0.7 kJ/mol. The pre-exponential factor of the Arrhenius plot gave us further mechanistic insights.

Parametric amplification of 100 fs mid-infrared pulses in ZnGeP2 driven by a Ho:YAG chirped-pulse amplifier.

We report on the parametric generation of 100 fs sub-6-cycle 40 µJ pulses with the center wavelength at 5.2 µm using a 1 ps 2.1 µm pump laser and a dispersion management scheme based on bulk material. Our optically synchronized amplifier chain consists of a Ho:YAG chirped-pulse amplifier and white-light-seeded optical parametric amplifiers providing simultaneous passive carrier-envelope phase locking of three ultrashort longwave pulses at the pump, signal, and idler wavelengths corresponding, respectively, to 2.1, 3.5, and 5.2 µm. We also demonstrate bandwidth enhancement and efficient control over nonlinear spectral phase in the regime of cascaded χ2 nonlinearity in ZnGeP2.

Sub-10-fs control of dissociation pathways in the hydrogen molecular ion with a few-pulse attosecond pulse train.

The control of the electronic states of a hydrogen molecular ion by photoexcitation is considerably difficult because it requires multiple sub-10 fs light pulses in the extreme ultraviolet (XUV) wavelength region with a sufficiently high intensity. Here, we demonstrate the control of the dissociation pathway originating from the 2pσu electronic state against that originating from the 2pπu electronic state in a hydrogen molecular ion by using a pair of attosecond pulse trains in the XUV wavelength region with a train-envelope duration of ∼4 fs. The switching time from the peak to the valley in the oscillation caused by the vibrational wavepacket motion in the 1sσg ground electronic state is only 8 fs. This result can be classified as the fastest control, to the best of our knowledge, of a molecular reaction in the simplest molecule on the basis of the XUV-pump and XUV-probe scheme.

Decomposition of the configuration-interaction coefficients in the multiconfiguration time-dependent Hartree-Fock method.

An approximate implementation of the multiconfiguration time-dependent Hartree-Fock method is proposed, in which the matrix of configuration-interaction coefficients is decomposed into a product of matrices of smaller dimension. The applicability of this method in which all the configurations are kept in the expansion of the wave function, while the configuration-interaction coefficients are approximately calculated, is discussed by showing the results on three model systems: a one-dimensional model of a beryllium atom, a one-dimensional model of a carbon atom, and a one-dimensional model of a chain of four hydrogen atoms. The time-dependent electronic dynamics induced by a few-cycle, long-wavelength laser pulse is found to be well described at a lower computational cost compared to the standard multiconfiguration time-dependent Hartree-Fock treatment. Drawbacks of the method are also discussed.