The quantitative analysis of enhancement of high-order harmonics in two-color intense laser fields

The time-dependent Schr?dinger equation of the interaction of laser pulse with He⁺ is solved by using the asymptotic boundary condition and symplectic algorithm in fundamental laser-field and two-color laser fields. We find that the conversion efficiency of high-order harmonic generation (HHG) is higher in the two-color laser fields than in the fundamental laser field, especially for the combination of ω ₀ − 19ω ₀. To explain these phenomena, the ionization, the average distance, the probability of first excited sate, and the transition probability are calculated. We give the qualitative and quantitative analysis for the enhancement of conversion efficiency of HHG.     

Role of higher excited electronic states on high harmonic generation in H2(+)--a time-independent Hermitian Floquet approach

We have theoretically studied the role of high-lying molecular electronic states on the high harmonic generation (HHG) in H(2)(+) within the framework of a time-independent Hermitian nonperturbative three-dimensional Floquet technique for continuous wave monochromatic lasers of intensities of 2.59 × 10(13), 4.0 × 10(13), and 5.6 × 10(13) W∕cm(2), and wavelengths of 1064, 532, and 355 nm. To evaluate the HHG spectra, the resonance Floquet quasienergy and the Fourier components of the Floquet state corresponding to the initial vibrational-rotational level v = 0, J = 0 have been computed by solving the time-independent close-coupled Schro?dinger equation following the Floquet method. The calculations include seven molecular electronic states in the basis set expansion of the Floquet state. The electronic states considered, apart from the two lowest 1sσ(g) and 2pσ(u) states, are 2pπ(u), 2sσ(g), 3pσ(u), 3dσ(g), and 4fσ(u). All the concerned higher excited molecular electronic states asymptotically degenerate into the atomic state H(2 l) with l = 0, 1. The computations reveal signature of significant oscillations in the HHG spectra due to the interference effect of the higher molecular electronic states for all the considered laser intensities and wavelengths. We have attempted to explain, without invoking any ionization, the dynamics of HHG in H(2)(+) within the framework of electronic transitions due to the electric dipole moments and the nuclear motions on the field coupled ground, the first and the higher excited electronic states of this one-electron molecular ion.     

Spectroscopic Characterization of Open-Shell Cations: Emission and Laser Excitation spectra of Rotationally Cooled CH3(CC)2X+, X = Cl,Br

Gas phase emission and laser excitation spectra of the òE?X?²E (Σ = +½, ?½) transition of rotationally/vibrationally cooled 1-chloro- and 1-bromo-1,3-pentadiyne cations have been obtained. The emission was excited by electron impact on a seeded helium supersonic free jet and the fluorescence by laser excitation of cations produced by Penning ionization and collisional relaxation. From these two sets of data the origin bands of the spin-orbit systems are located and for the bromo species this leads to better values of the spin-orbit splitttings in the two electronic states and of the first adiabatic ionization energy. The vibrational frequencies of many of the fundamentals of these cations in the X?²E and òE states have been obtained to within ±2 cm^?1.     

Difluorocarbene Studied with Threshold Photoelectron Spectroscopy (TPES): Measurement of the First Adiabatic Ionization Energy (AIE) of CF2

The first photoelectron band of difluorocarbene CF₂, has been studied by threshold photoelectron (TPE) spectroscopy. CF₂ was prepared by microwave discharge of a flowing mixture of hexafluoropropene, C₃F₆, and argon. A vibrationally resolved band was observed in which at least twenty‐two components were observed. In the first PE band of CF₂, the adiabatic ionization energy differs significantly from the vertical ionization energy because, for the ionization CF₂⁺ (X?²A₁)+e^? ← CF₂ (X?¹A₁), there is an increase in the FCF bond angle (by ≈20°) and a decrease in the C? F bond length (by ≈0.7 Å). The adiabatic component was not observed in the experimental TPE spectrum. However, on comparing this spectrum with an ab initio/Franck–Condon simulation of this band, using results from high‐level ab initio calculations, the structure associated with the vibrational components could be assigned. This led to alignment of the experimental TPE spectrum and the computed Franck–Condon envelope, and a determination of the first adiabatic ionization energy of CF₂ as (11.362±0.005) eV. From the assignment of the vibrational structure, values were obtained for the harmonic and fundamental frequencies of the symmetric stretching mode (ν₁′) and symmetric bending mode (ν₂′) in CF₂⁺ (X?²A₁).     

Role of multiphoton excitation and two-electron effects in high harmonic generation of H2: A TDDFT calculation

Using two different TDDFT methods, we study the role of electronically excited states and two-electron dynamics in high harmonic generation (HHG) of H₂. The two methods produce slightly different electronic structures as reflected in the calculated ionization potentials. They nevertheless give similar HHG spectra. The difference between the two methods increases with the laser intensity, while their predictions remain qualitatively consistent.Our results suggest that two-electron dynamics can extend the HHG cutoff. Specifics of such extension depends on the internuclear distance and the laser intensity. We propose an ion excitation plus tunneling ionization mechanism to explain these extensions.The involvement of excited states is further revealed when we analyze each harmonic as a function of the internuclear distance. We see resonant peaks that are due to multiphoton excitation. These peaks exist above the ionization threshold as well.     

Sub‐cycle Dynamics of High Harmonic Generation of Ne Atoms Excited by Attosecond Pulses and Driven by Near‐infrared Laser Fields: A Self‐Interaction‐Free TDDFT Approach

In the framework of the self‐interaction‐free time‐dependent density‐functional theory (TDDFT), we have performed three‐dimensional ab initio calculations of Ne atoms in near‐infrared (NIR) laser fields subject to excitation by a single extreme ultraviolet (XUV) attosecond pulse (SAP). The TDDFT equations are solved accurately and efficiently by means of the time‐dependent generalized pseudo spectral (TDGPS) method. We have explored the transient dynamical behavior of the sub‐cycle high harmonic generation (HHG) for transitions from the excited states to the ground state and found oscillation structures with respect to the time delay between the SAP and NIR fields. We investigate the harmonic emission spectrum from singly excited state 2p3s, 2p4s, 2p3d, 2p5s, 2p4d and 2p6s, 2p5d and the virtual states 2p3p‐, 2p4f‐ and 2p4p+ as the function of time delay. We explore the sub‐cycle Stark shift phenomenon in NIR fields and its influence on the photon emission process. Our analysis reveals several novel features of the sub‐cycle transient HHG dynamics and spectra, the quantum interference pattern between different multiphoton excitation pathways, and we identify the mechanisms responsible for the observed peak splitting in the photon emission spectra.     

N‐Methyl‐trimethylacetamide in Thin Films Displays Infrared Spectra of π‐Helices,with Visible Static and Dynamic Growth Phases,and then a β‐Sheet

The simplest (minimal) peptide model is HCONHCH₃. An increase in the π‐helix content with increased substitution in the acyl portion suggested the examination of N‐methyl‐trimethylacetamide) (NMT). NMT displays spectra, in which there is evolution of a set of helices defined by their amide I maxima near 1686 (3₁₀), 1655 (first π), and, most importantly, at 1637 cm^?1 (π). Expanded thin‐film infrared spectroscopy (XTFIS) shows pauses or slow stages, which are identified as static phases followed by dynamic phases with the incremental gain or loss of a helix turn. In addition, absorbance at 1637 cm^?1 suddenly increases at 82.1 s (30 % over 0.3 s), indicating a phase change and crystallization of the π‐helix, along with a coincidental decrease in the absorbance for the first π‐helix. A sharp peak occurs at the maximum of the phase change at 82.5 s, representing a pure NMT π‐helix. The spectra then undergo a decreasing general absorption loss over 150 s, with the π‐helix evolving further to an antiparallel β‐sheet fragment. The spectral quality arises from the immobilization of polar molecules on polar surfaces. The crystal structure is that of an antiparallel β‐sheet.     

IR-MALDI ion mobility spectrometry: physical source characterization and application as HPLC detector

Infrared matrix-assisted laser dispersion and ionization (IR-MALDI) in combination with ion mobility (IM) spectrometry enables the direct analysis of biomolecules in aqueous solution. The release of ions directly from an aqueous solution is based on a phase explosion, induced by the absorption of an IR laser pulse, which disperses the liquid as vapor, nano- and micro-droplets. The ionization process is characterized initially by a broad spatial distribution of the ions, which is a result of complex fluid dynamics and desolvation kinetics. These processes have a profound effect on the shape and width of the peaks in the IM spectra. In this work, the transport of ions by the phase explosion-induced shockwave could be studied independently from the transport by the electric field. The shockwave-induced mean velocities of the ions at different time scales were determined through IM spectrometry and shadowgraphy. The results show a deceleration of the ions from 118 m?s^?1 at a distance of 400 μm from the liquid surface to 7.1 m?s^?1 at a distance of 10 mm, which is caused by a pile-up effect. Furthermore, the desolvation kinetics were investigated and a first-order desolvation constant of 325?±?50 s^?1 was obtained. In the second part, the IR-MALDI-IM spectrometer is used as an HPLC detector for the two-dimensional separation of a pesticide mixture.     

Liquid‐crystalline Metallomacrocycles Composed of Bis(hydroxypyridono)toluene Ligand and Cu2+ Ions

A series of π‐extended chelating scaffolds incorporating two hydroxypyridone moieties were synthesized. X‐ray crystallographic analysis revealed that a bis(hydroxypyridono)toluene ligand possessed a unique π‐extended structure and exhibited efficient phase segregation from the aliphatic chains attached at the heterocyclic nitrogen. The bis‐bidentate ligand formed a metal‐coordination‐induced macrocycle with Cu²⁺ ions. During the complexation, a spectral change in the visible region was induced. Furthermore, the successful development of a liquid crystal of the metallomacrocycle with appropriate side chains was achieved. Examples of liquid‐crystalline macrocycles formed via metal‐mediated self‐assembly are still rare. Among them, the macrocycle described in this paper showed an obvious hexagonal columnar phase reflecting the three‐fold symmetric planar structure of the mesogenic metal complex.     

Multi-photon ionization in the mass spectrometry of polyatomic molecules: Cross sections

Multi-photon ionization (MPI) with tunable visible/UV laser light is shown to be a sensitive tool for analysis of traces in gas mixtures when combined with a mass spectrometer. Mass spectra of six different organic molecules, obtained with low intensity laser light, are presented and demonstrate the facility of ionization without fragmentation (soft ionization) under proper experimental conditions. Quantitative values for the cross sections for both two photon steps are obtained from the measured intensity dependence and the absolute ion numbers. Such quantitative data help in the evaluation and definition of this new ionization technique in mass spectrometry. Efficiencies of ionization for some molecules are as high as 25% leading to 10⁶ ions in a single pulse from the dye laser (1 kW). Detectability as low as 2 parts in 10⁹ is thus predicted.     

A π‐stacked and conjugated hybrid based on poly(N‐vinylcarbazole) postfunctionalized with terfluorene for stable deep‐blue hole‐transporting materials

The combination of π‐stacked with π‐conjugated building blocks offers an essential strategy to construct multifunctional organic semiconductors (MOSs) with the unique optoelectrical properties. Covalent hybrids can efficiently avoid the intrinsic phase‐separation defects in corresponding blend system. In this contribution, poly(vinylcarbazole) tethered with terfluorene, PVK‐TF, as a double‐channeled π‐stacked and π‐conjugated hybrid (SCH), has been constructed via Friedal‐Crafts click postmodification (FCCP). The chemical structure and optoelectrical property were determined by GPC, UV–vis, PL, TGA, DSC, and CV. Its PL spectra in the annealing thin film at N₂ atmosphere without low‐energy emission bands centered at the 530 nm indicates that no π‐stacks between carbazole and TF or among TFs dominate the whole condensed phase, which is in agreement with the intrachain T‐shaped π‐pitched motifs in molecular modeling simulation due to steric hindrance effect in complicated diarylfluorenes (CDAFs). A supporting prototype stable deep‐blue PLED was successfully obtained with an Internationale de l'Eclairage (CIE) coordinates of (0.20, 0.10) and a width at half maximum (FWHM) of about 60 nm at high current density of 100 mA/cm² (35 V). Deep‐blue PVK‐TF is a promising MOS for hole‐transporting and host materials. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5221–5229, 2009     

The photoassociation and photodissociation in laser‐assisted collision reaction H + Cl+

The collision reaction H + Cl⁺ assisted by the ultra‐short laser pulse is investigated using the time‐dependent quantum wave packet method. The probability of dissociation depends on the yield ratio of association product HCl⁺. The greater the laser frequency is, the lower the vibrational level of HCl⁺ is. With lowering laser frequency, the probabilities of photoassociation and photodissociation increase, and the ratio of products H⁺ + Cl(²P⁰) to H(²S) + Cl⁺(¹D) also increases. The kinetic energy spectra of the dissociated fragments at low frequency are wider than those at high frequency. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Gold(III) π‐Allyl Complexes

Gold(III) π‐complexes have been authenticated recently with alkenes, alkynes, and arenes. The key importance of Pd^II π‐allyl complexes in organometallic chemistry (Tsuji–Trost reaction) prompted us to explore gold(III) π‐allyl complexes, which have remained elusive so far. The (P,C)Au^III(allyl) and (methallyl) complexes 3 and 3′ were readily prepared and isolated as thermally and air‐stable solids. Spectroscopic and crystallographic analyses combined with detailed DFT calculations support tight quasi‐symmetric η³‐coordination of the allyl moiety. The π‐allyl gold(III) complexes are activated towards nucleophilic additions, as substantiated with β‐diketo enolates.     

Offset‐compensated and zero‐quantum suppressed ROESY provides accurate 1H–1H distances in small to medium‐sized molecules

A robust version of the off‐resonance ROESY pulse scheme is suggested for the measurement of proton–proton distances or slow chemical exchange in small to medium‐sized molecules. The method implements adiabatic ramps to establish a pair of opposite frequency off‐resonance spin lock fields – with optionally randomized duration – and adiabatic inversion pulses with simultaneous gradients for efficient zero‐quantum suppression. The amended pulse sequence yields pure absorption cross‐peaks and works safely for small to medium‐sized molecules. The applicability of the method has been demonstrated using small, rigid molecules (strychnine and codeine) and was also applied for a cyclic peptide and a small protein. We found that the pure phase cross‐peaks of the new ROESY version are beneficial for distance measurements. The one‐dimensional (selective) version of the new method is also powerful for measuring selected pair‐wise interactions and distance determination. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis of Triply Fused Porphyrin‐Nanographene Conjugates

Two unprecedented porphyrin fused nanographene molecules, 1 and 2 , have been synthesized by the Scholl reaction from tailor‐made precursors based on benzo[m]tetraphene‐substituted porphyrins. The chemical structures were validated by a combination of high‐resolution matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (HR MALDI‐TOF MS), IR and Raman spectroscopy, and scanning tunnelling microscopy (STM). The UV‐vis‐near infrared absorption spectroscopy of 1 and 2 demonstrated broad and largely red‐shifted absorption spectra extending up to 1000 and 1400 nm, respectively, marking the significant extension of the π‐conjugated systems.     

On Metal-Atom Clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules

The investigation of electronic and structural parameters of metal molecules as a function of size may be decisive for understanding and control of heterogeneous catalysis with finely divided metals. Metal-atom clusters can be prepared by several methods, most of which yield a molecular mixture only. Expansion of an atomic vapour into vacuum through a supersonic nozzle creates a complex cluster-spectrum which has been investigated by photoionization using a mass-spectrometer as selective detector. Broad band single photoionization ( PI. ) yields the abundances and ionization thresholds for Na_x(x?16), K_x(x?12) and Na_xK_y(x+y?6). Rough indications about other deactivation channels of excited alkali-molecules are obtained from an analysis of the photoionization efficiency (PIE.) curves. Two-photon PIE. curves with narrow-band (laser) light sources give an accurate photoionization threshold value and detailed information on ionization processes. Two-photon ionization spectra via a real intermediate state reached by laser excitation are equivalent to normal absorption spectra, if the exciteation step is controlled by the true transition probabilities. By investigation of the power dependence of the ion current of Na as a function of the wavelength of the tunable cw dye laser (excitation) and of the ionizing Ar⁺- or Kr⁺-laser, conditions under which true spectra are obtained have been clarified, in good agreement with a photon-kinetic model of the processes involved. Vibronic and rovibronic spectra of several transitions in Na₂ and K₂ have been measured. Similarly a spectrum of Na₃ has been determined mass-selectively. The scope of the new method for an absorption spectroscopy in molecular beams is discussed.     

High-order Harmonic Generation of Aligned Acetylene in Elliptically Polarized Strong Laser Fields?

We perform an experimental study on high-order harmonic generation (HHG) of aligned acetylene molecules induced by a 35-fs 800-nm strong laser field, by using a home-built HHG spectrometer. It is observed that the molecular HHG probability declines with increasing the laser ellipticity, which is in consistence with the deduction from the well-known tunneling-plus-rescattering scenario. By introducing a weak femtosecond laser pulse to nonadiabatically align the molecules, we investigated the molecular orbital effect on the HHG in both linearly and elliptically polarized driving laser fields. The results show that the harmonic intensity is maximum for the molecular axis aligned perpendicularly to the laser electric field. It indicates that both the highest occupied molecular orbitals (HOMO) and HOMO-1 contribute to the strong-field HHG of acetylene molecules. Our study should pave the way for understanding the interaction of molecules with ultrafast strong laser fields.     

Highly Pure Synthesis,Spectral Assignments,and Two‐Photon Properties of Cruciform Porphyrin Pentamers Fused with Benzene Units

Tetrameric porphyrin formation of 2‐hydroxymethylpyrrole fused with porphyrins through a bicyclo[2.2.2]octadiene unit gave bicyclo[2.2.2]octadiene‐fused porphyrin pentamers. Thermal conversion of the pentamers gave fully π‐conjugated cruciform porphyrin pentamers fused with benzene units in quantitative yields. UV/Vis spectra of fully π‐conjugated porphyrin pentamers showed one very strong Q absorption and were quite different from those of usual porphyrins. From TD‐DFT calculations, the HOMO level is 0.49 eV higher than the HOMO?1 level. The LUMO and LUMO+1 levels are very close and are lower by more than 0.27 eV than those of other unoccupied MOs. The strong Q absorption was interpreted as two mutually orthogonal single‐electron transitions (683 nm: 86 %, HOMO→LUMO; 680 nm: 86 %, HOMO→LUMO+1). The two‐photon absorption (TPA) cross section value (σ⁽²⁾) of the benzene‐fused porphyrin pentamer was estimated to be 3900 GM at 1500 nm, which is strongly correlated with a cruciform molecular structure with multidirectional π‐conjugation pathways.     

How different is pyrimidine as a core component of DNA base from its diazine isomers: A DFT study?

Recent photoemission spectroscopic (X‐ray photoemission spectra) study revealed less dramatic chemical changes for pyrimidine (PyM, 1, 3‐diazine) with in its ionization potential. Present systematic study using density functional theory calculations shows that PyM is indeed quite different from its diazine isomers (PyD, 1, 2‐diazine and PyA, 1, 4‐diazine). It is discovered that the most stable isomer PyM is relaxed from C_2V to C₁ point symmetry with a total electronic energy deduction of ?15.86 kcal.mol^?1. Although not substantial, PyM has the smallest molecule shape (electronic spatial extent) and the largest HOMO‐LUMO energy gap of 5.65 eV; only one absorption band in the region of 200–300 nm of the UV‐Vis spectrum but three clusters of chemical shift in the carbon and hydrogen NMR spectra. The energy decomposition analyses revealed that the interaction energy (ΔE_Int) of PyM is preferred over PyA by 4.08 kcal.mol^?1 and over PyD by 22.32 kcal.mol^?1, with the preferred N? C? N bond revealed by graph theory.     

A Nonaromatic Thiophene‐Fused Heptalene and Its Aromatic Dianion

Heptalene, a nonaromatic, bicyclic 12 π‐electron system with a twisted structure, is of great interest with regard to its potential Hückel aromaticity in the two‐electron oxidized or reduced forms. The synthesis of thiophene‐fused heptalene 5 from the reductive transannular cyclization of bisdehydro[12]annulene 4 , and its solid‐state structure, which was confirmed by X‐ray crystallographic analysis, is presented. Chemical reduction of 5 readily generated the corresponding dianion, which was successfully isolated as [(K[2.2.2]cryptand)⁺]₂ 5 ^2?. The X‐ray crystallographic analysis of the dianion revealed a shallower saddle structure for the heptalene moiety and a lesser degree of bond alternation relative to 5 . ¹H NMR spectroscopy exposed the effect of a diamagnetic ring current on dianion 5 ^2?, which was corroborated by nucleus‐independent chemical shift (NICS) calculations. These results demonstrate that the heptalene dianion, containing 14 π‐electrons, does indeed exhibit pronounced degrees of Hückel aromaticity.