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We measured transient reflectivity of single-crystalline and polycrystalline gold thin films in the wavelength region of 450–700 nm pumped by a 400 nm femtosecond pulse. The observed reflectivity changes were reproduced well by a reflectivity model that we developed based on the joint-density-of-states calculated from the band structure of gold and the Drude model. By a combination use of the reflectivity model and the two-temperature model, we evaluated the temporal evolution of the electron and phonon temperatures from the observed reflectivity changes. Based on the resultant electron and phonon temperatures, we derived the electron–phonon coupling factors, which were consistent with the literature values, in the wide range of the absorbed energy density.

Time domain Ramsey-type interferometry is useful for investigating spectroscopic information of quantum states in atoms and molecules. The energy range of the quantum states to be observed with this scheme has now reached more than 20 eV by resolving the interference fringes with a period of a few hundred attoseconds. This attosecond Ramsey-type interferometry requires the irradiation of a coherent pair of extreme ultraviolet (XUV) light pulses, while all the methods used to deliver the coherent XUV pulse pair until now have relied on the division of the source of an XUV pulse in two before the generation. In this paper, we report on a novel technique to perform attosecond Ramsey-type interferometry by splitting an XUV high-order harmonic (HH) pulse of a sub-20 fs laser pulse after its generation. By virtue of the postgeneration splitting of the HH pulse, we demonstrated that the optical interference emerging at the complete temporal overlap of the HH pulse pair seamlessly continued to the Ramsey-type electronic interference in a helium atom. This technique is applicable for studying the femtosecond dephasing dynamics of electronic wavepackets and exploring the ultrafast evolution of a cationic system entangled with an ionized electron with sub-20 fs resolution.

Among currently available optical spectroscopic methods, Raman spectroscopy has versatile application to investigation of dynamical processes of molecules leading to chemical changes in the gas and liquid phases. However, it is still a challenge to realize an ideal standoff coherent Raman spectrometer with which both high temporal resolution and high-frequency resolution can be achieved, so that one can remotely probe chemical species in real time with high temporal resolution while monitoring the populations in their respective rovibronic levels in the frequency domain with sufficiently high spectral resolution. In the present study, we construct an air-laser-based Raman spectrometer, in which near-infrared femtosecond (fs) laser pulses at 800 nm and cavity-free picosecond N2+ air-laser pulses at 391 nm generated by the filamentation induced by the fs laser pulses are simultaneously used, enabling us to generate a hybrid ps/fs laser source at a desired standoff position for standoff surveillance of chemical and biochemical species. With this prototype Raman spectrometer, we demonstrate that the temporal evolution of the electronic, vibrational, and rotational states of N2+ and the coupling processes of the rovibrational wave packet of N2 molecules can be probed.

We developed an apparatus for measuring kinetic energy and two-dimensional angular distributions of femtosecond laser-assisted electron scattering (LAES) signals with a high detection efficiency, consisting of a photocathode-type ultrashort pulsed electron gun, a gas injection nozzle, an angle-resolved time-of-flight analyzer, and a time-and-position sensitive electron detector. We also established an analysis method for obtaining the kinetic energy and two-dimensional angular distributions of scattered electrons from raw data of their flight times and the detected positions at the detector recorded using the newly developed apparatus. From the measurement of the LAES processes of Ar atoms in a femtosecond near-infrared intense laser field, we obtained a two-dimensional angular distribution image of the LAES signals and showed that the detection efficiency of the LAES signals was raised by a factor of 40 compared with that achieved before in 2010.

Attosecond laser pulses allow to explore the ultrafast atomic or molecular processes occurring in chemical reactions. Here the authors utilize a sequence of two XUV pump-probe pulses to control the dissociation dynamics of the hydrogen molecular ion.

We generate unidirectional coherent radiations at 391 and 428 nm corresponding to the B^{2}Σ_{u}^{+}-X^{2}Σ_{g}^{+} (0,0) and (0,1) emission of N_{2}^{+} by irradiating neutral N_{2} with an elliptically modulated near-IR ultrashort laser field generated using a quarter-wave plate (QWP) with the different orders, and reveal that the lasing intensity at 391 nm becomes maximum at a specific orientation of the fast axis of QWP with respect to the polarization direction of the input laser field. We reproduce this observation by numerical calculations and show that the lasing intensity can be optimized by adjusting the temporal separation between the two polarization components of the elliptically modulated laser pulse and their relative amplitudes so that the inverted population between the B^{2}Σ_{u}^{+} and X^{2}Σ_{g}^{+} states can be achieved efficiently and the population in the X^{2}Σ_{g}^{+}(v^{″}=0) level is further depleted in the latter part of the laser pulse by the A^{2}Π_{u}-X^{2}Σ_{g}^{+} transition.

The recent progress in experimental studies of laser-assisted electron scattering (LAES) induced by ultrashort intense laser fields is reviewed. After a brief survey of the theoretical backgrounds of the LAES process and earlier LAES experiments started in the 1970s, new concepts of optical gating and optical streaking for the LAES processes, which can be realized by LAES experiments using ultrashort intense laser pulses, are discussed. A new experimental setup designed for measurements of LAES induced by ultrashort intense laser fields is described. The experimental results of the energy spectra, angular distributions, and laser polarization dependence of the LAES signals are presented with the results of the numerical simulations. A light-dressing effect that appeared in the recorded LAES signals is also shown with the results of the numerical calculations. In addition, as applications of the LAES process, laser-assisted electron diffraction and THz-wave-assisted electron diffraction, both of which have been developed for the determination of instantaneous geometrical structure of molecules, are introduced.

We experimentally investigate the dependence of the fragmentation behavior of the ethylene dication on the intensity and duration of the laser pulses that initiate the fragmentation dynamics by strong-field double ionization. Using coincidence momentum imaging for the detection of ionic fragments, we disentangle the different contributions of ionization from lower-valence orbitals and field-driven excitation dynamics to the population of certain dissociative excited ionic states that are connected to one of several possible fragmentation pathways towards a given set of fragment ions. We find that the excitation probability to a particular excited state and therewith the outcome of the fragmentation reaction strongly depend on the parameters of the laser pulse. This, in turn, opens up new possibilities for controlling the outcome of fragmentation reactions of polyatomic molecules in that it may allow one to selectively enhance or suppress individual fragmentation channels, which was not possible in previous attempts of controlling fragmentation processes of polyatomic molecules with strong laser fields.

We investigate lasing action in aligned nitrogen molecular ions (N_{2}^{+}) produced in an intense laser field. We find that, besides the population inversion between the B^{2}Σ_{u}^{+}-X^{2}Σ_{g}^{+} states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state (v=0) of the excited electronic B^{2}Σ_{u}^{+} state has been created in N_{2}^{+}. The rotational coherence can faithfully encode its characteristics into the amplified seed pulses, enabling reconstruction of rotational wave packets of molecules in a single-shot detection manner from the frequency-resolved laser spectrum. Our results suggest that the air laser can potentially provide a promising tool for remote characterization of coherent molecular rotational wave packets.

We review the basics of quantum computation and provide an overview of the quantum computers currently in operation. We introduce the error mitigation methods and the methods for simulation of the time-dependent dynamics of quantum systems using quantum computers. As an example of the application of a quantum computer, we discuss our recent attempt to simulate the time-dependent spin state populations in a three-site spin chain described by the Heisenberg XXX Hamiltonian using IBM’s superconducting-qubit type quantum computer ibm_kawasaki.We show that accurate time-dependent populations can be obtained after the error mitigation by the Clifford data regression and the Pauli twirling. [ABSTRACT FROM AUTHOR]

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 derWaals dimer [60]fullerene (C60) toC120 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 preexponential factor and the Arrhenius activation energy--the 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". [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this chapter, we introduce our recent studies on themechanisms responsible for the optical amplification in N2+ induced by irradiating nitrogen molecules N2 with intense laser fields. We demonstrate that the lasing intensities of N2+ at 391.4 nm, corresponding to the B²Σu+ (V = 0)-X²Σg+ (V" = 0) transition, can be strongly modulated by manipulating the polarization state of the driven laser field, which is ascribed to the post-ionization coupling between the ground X²Σg+ state and the first excited A²Πu state of N2+. By using pump-probe methods, we show direct evidences for the optical transition between the X²Σg+ and A²Πu states of N2+ in intense laser field, based on which we show the optimization of N2+ lasing by designing intense laser fields. [ABSTRACT FROM AUTHOR]

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 ±hω, 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. [ABSTRACT FROM AUTHOR]

When intense femtosecond laser pulses are focused in air, unidirectional and coherent radiation called air-lasing is generated, originating from the population inversion processes between an electronically excited state and the electronic ground state of N2 and N2+. In 2015, it was shown experimentally that the air-lasing at 391 nm, corresponding to the B²Σu + (v = 0) - X²Σg + (v = 0) emission of N2+, is generated in an sub-10 fs near-IR laser field (Xu et al, Nat Commun 6:8347, 2015 [6]). The behavior that N2+ generated in the pulse is exposed to the intense laser field immediately after its birth, is considered critical in leading to the population inversion between the B and X states. This chapter includes the demonstrations that the sudden exposure to an intense laser field could result in efficient population transition in a two-level system at off-resonance (Zhang et al, J Phys B: At Mol Opt Phys 50:185603, 2017 [32]), and that population inversion can be achieved in N2+by using sudden turn-on pulse (Zhang et al, J Phys B: At Mol Opt Phys 52:055401, 2019 [33]). This scenario of the population transfer to the excited state is expected to be universal and can be applied to an interpretation of population inversion of any kind of atomic and molecular ions created in a pulsed intense laser field. [ABSTRACT FROM AUTHOR]

The dissociative ionization of ethanol C[sub 2]H[sub 5]OH in an intense laser field is investigated with a chirped laser pulse. From the sensitive dependence of the relative yields of the fragment ions on the absolute values of the linear chirp rate, it is shown that the light-dressed potential-energy surface (LDPES) at the singly charged stage governs the nuclear dynamics, and that the nuclear wave packet flow into the breaking of either of the C–C and C–O chemical bonds could be characterized by the holding time t[sub hold] during which the LDPESs are maintained. It is also understood in term of the holding time that the enhanced ionization into the doubly charged stage followed by the Coulomb explosion at C–C or C–O proceeds when the nuclear wave packet at the singly charged stage reaches the critical distance for the further ionization. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]