(2+1) resonance-enhanced ionization spectroscopy of a state-selected beam of OH radicals

A state-selected beam of hydroxyl radicals is generated using a pulsed discharge source and hexapole field. The OH radicals are characterized by resonance-enhanced multiphoton ionization (REMPI) spectroscopy via the nested D 2Sigma- and 3 2Sigma- Rydberg states. Simplified spectra are observed from the selected |MJ|=3/2 component of the upper Lambda-doublet level of the lowest rotational state (J=32) in ground (v"=0) and excited (v"=1-3) vibrational levels of the OH X 2Pi3/2 state. Two-photon transitions are observed to the D 2Sigma-(v'=0-3) and 3 2Sigma-(v'=0,1) vibronic levels, extending previous studies to higher vibrational levels of the Rydberg states. Spectroscopic constants are derived for the Rydberg states and compared with prior experimental studies. Complementary first-principle theoretical studies of the properties of the D 2Sigma- and 3 2Sigma- Rydberg states [see M. P. J. van der Loo and G. C. Groenenboom, J. Chem. Phys. 123, 074310 (2005), following paper] are used to interpret the experimental findings and examine the utility of the (2+1) REMPI scheme for sensitive detection of OH radicals.     

Two-photon ionization of Li2: isotopic separation and determination of IP(Li2) and De(Li+2)

Complete isotope separation is achieved by two-photon ionization of Li₂ by a single mode Ar⁺ laser. With the use of two Ar⁺ lasers, the ionization potential of Li₂ is found to be 5.174 ± 0.013 eV, and the dissociation energy D_e(Li⁺₂) to be 1.274 ± 0.019 eV.     

Coherent control and time-dependent density functional theory: towards creation of wave packets by ultrashort laser pulses

Explicitly time-dependent density functional theory (TDDFT) is a formally exact theory, which can treat very large systems. However, in practice it is used almost exclusively in the adiabatic approximation and with standard ground state functionals. Therefore, if combined with coherent control theory, it is not clear which control tasks can be achieved reliably, and how this depends on the functionals. In this paper, we continue earlier work in order to establish rules that answer these questions. Specifically, we look at the creation of wave packets by ultrashort laser pulses that contain several excited states. We find that (i) adiabatic TDDFT only works if the system is not driven too far from the ground state, (ii) the permanent dipole moments involved should not differ too much, and (iii) these results are independent of the functional used. Additionally, we find an artifact that produces fluence-dependent excitation energies.     

Control of Li2 wave packet dynamics by modification of the quantum mechanical amplitude of a single state

Sequences of pulses with different spectra are used to control rotational wave packet dynamics in Li(2) by exploiting quantum interference phenomena. Wave packet superpositions are excited in a two-step resonant Raman process by two different pulses. Interferences between individual states shared by both wave packets can be used to enhance or destroy specific components of a superposition by varying the time delay between the pulses and/or the relative phase within the pulses. Elimination of selected quantum beats is achieved by greater than 94% for each case. A simple, yet effective, method for generating different color phase-locked pairs of laser pulses in a liquid-crystal pulse shaper setup without the need for interferometric stabilization schemes is described. The ability to manipulate single states of a superposition is an important advancement for intuitive control schemes and provides a potential new approach for initialization schemes in the field of quantum information.     

Carbonyl vibrational wave packet circulation in Mn2(CO)10 driven by ultrashort polarized laser pulses

The excitation of the degenerate E(1) carbonyl stretching vibrations in dimanganese decacarbonyl is shown to trigger wave packet circulation in the subspace of these two modes. On the time scale of about 5 ps, intramolecular anharmonic couplings do not cause appreciable disturbance, even under conditions where the two E(1) modes are excited by up to about two vibrational quanta each. The compactness of the circulating wave packet is shown to depend strongly on the excitation conditions, such as pulse duration and field strength. Numerical results for the solution of the seven-dimensional vibrational Schro?dinger equation are obtained for a density functional theory based potential energy surface and using the multi-configuration time-dependent Hartree method.     

Stark-assisted population control of coherent CS(2) 4f and 5p Rydberg wave packets studied by femtosecond time-resolved photoelectron spectroscopy

A two-color (3+1(')) pump-probe scheme is employed to investigate Rydberg wave packet dynamics in carbon disulfide (CS(2) (*)). The state superpositions are created within the 4f and 5p Rydberg manifolds by three photons of the 400 nm pump pulse, and their temporal evolution is monitored with femtosecond time-resolved photoelectron spectroscopy using an 800 nm ionizing probe pulse. The coherent behavior of the non-stationary superpositions are observed through wavepacket revivals upon ionization to either the upper (12) or lower (32) spin-orbit components of CS(2) (+). The results show clearly that the composition of the wavepacket can be efficiently controlled by the power density of the excitation pulse over a range from 500 GWcm(2) to 10 TWcm(2). The results are consistent with the anticipated ac-Stark shift for 400 nm light and demonstrate an effective method for population control in molecular systems. Moreover, it is shown that Rydberg wavepackets can be formed in CS(2) with excitation power densities up to 10 TWcm(2) without significant fragmentation. The exponential 1e population decay (T(1)) of specific excited Rydberg states are recovered by analysis of the coherent part of the signal. The dissociation lifetimes of these states are typically 1.5 ps. However, a region exhibiting a more rapid decay ( approximately 800 fs) is observed for states residing in the energy range of 74 450-74 550 cm(-1), suggestive of an enhanced surface crossing in this region.     

In situ analysis of soybeans and nuts by probe electrospray ionization mass spectrometry

The probe electrospray ionization (PESI) is an ESI‐based ionization technique that generates electrospray from the tip of a solid metal needle. In the present work, we describe the PESI mass spectra obtained by in situ measurement of soybeans and several nuts (peanuts, walnuts, cashew nuts, macadamia nuts and almonds) using different solid needles as sampling probes. It was found that PESI‐MS is a valuable approach for in situ lipid analysis of these seeds. The phospholipid and triacylglycerol PESI spectra of different nuts and soybean were compared by principal component analysis (PCA). PCA shows significant differences among the data of each family of seeds. Methanolic extracts of nuts and soybean were exposed to air and sunlight for several days. PESI mass spectra were recorded before and after the treatment. Along the aging of the oil (rancidification), the formation of oxidated species with variable number of hydroperoxide groups could be observed in the PESI spectra. The relative intensity of oxidated triacylglycerols signals increased with days of exposition. Monitoring sensitivity of PESI‐MS was high. This method provides a fast, simple and sensitive technique for the analysis (detection and characterization) of lipids in seed tissue and degree of oxidation of the oil samples. Copyright © 2015 John Wiley & Sons, Ltd.     

Calculation of the rate constant for state-selected recombination of H+O2(nu) as a function of temperature and pressure

Classical trajectory calculations using the MERCURY/VENUS code have been carried out on the H+O(2) reactive system using the DMBE-IV potential energy surface. The vibrational quantum number and the temperature were selected over the ranges nu=0 to 15, and T=300 to 10 000 K, respectively. All other variables were averaged. Rate constants were determined for the energy transfer process, H+O(2)(nu)-->H+O(2)(nu(")), for the bimolecular exchange process, H+O(2)(nu)-->OH(nu('))+O, and for the dissociative process, H+O(2)(nu)-->H+O+O. The dissociative process appears to be a mere extension of the process of transferring large amounts of energy. State-to-state rate constants are given for the exchange reaction, and they are in reasonable agreement with previous results, while the energy transfer and dissociative rate constants have never been reported previously. The lifetime distributions of the HO(2) complex, calculated as a function of v and temperature, were used as a basis for determining the relative contributions of various vibrational states of O(2) to the thermal rate coefficients for recombination at various pressures. This novel approach, based on the complex's ability to survive until it collides in a secondary process with an inert gas, is used here for the first time. Complete falloff curves for the recombination of H+O(2) are also calculated over a wide range of temperatures and pressures. The combination of the two separate studies results in pressure- and temperature-dependent rate constants for H+O(2)(nu)(+Ar) right arrow over left arrow HO(2)(+Ar). It is found that, unlike the exchange reaction, vibrational and rotational-translational energy are liabilities in promoting recombination.     

Distinguishing between relaxation pathways by combining dissociative ionization pump probe spectroscopy and ab initio calculations: a case study of cytosine

We present a general method for tracking molecular relaxation along different pathways from an excited state down to the ground state. We follow the excited state dynamics of cytosine pumped near the S(0)-S(1) resonance using ultrafast laser pulses in the deep ultraviolet and probed with strong field near infrared pulses which ionize and dissociate the molecules. The fragment ions are detected via time of flight mass spectroscopy as a function of pump probe delay and probe pulse intensity. Our measurements reveal that different molecular fragments show different timescales, indicating that there are multiple relaxation pathways down to the ground state. We interpret our measurements with the help of ab initio electronic structure calculations of both the neutral molecule and the molecular cation for different conformations en route to relaxation back down to the ground state. Our measurements and calculations show passage through two seams of conical intersections between ground and excited states and demonstrate the ability of dissociative ionization pump probe measurements in conjunction with ab initio electronic structure calculations to track molecular relaxation through multiple pathways.     

Development of a force field for Li(2)SiF(6)

A force field has been developed for Li(2)SiF(6) for subsequent use in Molecular Dynamics (MD) simulations involving Li(+) and SiF(2-) (6) ions in a polymer electrolyte host. Both ab initio calculations and available empirical data have been used. The force field has been verified in simulations of the crystal structure of Li(2)SiF(6) in two different space groups: P321 and P3(-)m1. The use of MD simulation to assess the correct space group for Li(2)SiF(6) shows that it is probably P321.     

Detection of protein from detergent solutions by probe electrospray ionization mass spectrometry (PESI-MS)

Detergents are necessarily used for different extraction protocols of proteins from biological cells or tissues. After the extraction, elimination of detergent is necessary for the better performance of electrospray ionization mass spectrometry (ESI-MS). Elimination of detergents is laborious and time-consuming, and also sample loss may be unavoidable. Probe electrospray ionization (PESI) developed in our laboratory has been found to be tolerant to the presence of salts and buffers in sample solutions. In this report, it was examined whether PESI is applicable to the sample solutions that contain high-concentration of detergents. It was found that PESI is highly tolerant to the presence of sodium dodecyl sulphate, cetyl trimethylamminium bromide, Triton X100 and 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate compared with conventional ESI and nanoESI. Therefore, PESI can be a potential analytical tool for direct analysis of protein extracts and digests containing high-concentration detergents.     

Dynamics of quantum wave packets in complex molecules traced by 2D coherent electronic correlation spectroscopy

Electronic and nuclear molecular wavepackets are a clear manifestation of the wavelike properties of matter at the very heart of quantum mechanics. In this work we demonstrate how electronic two-dimensional spectroscopy (2D) serves as a highly evolved tool for the simultaneous investigation of both phenomena. In further analysis and theoretical treatments, 2D spectra form an ideal basis for the discussion of electronic decoherence, vibrational relaxation and electron-phonon coupling.     

Cross sections for the ionization continuum of H2 in the 15.3-17.2 eV energy range

The absolute values for the cross sections of the ionization continuum of H2 had been measured at low energy (15.3-17.2 eV) Both absorption and ionization spectra were simultaneously recorded with linear response and absolute values, using a 10 m normal incidence monochromator installed on a synchrotron undulator beamline. The vibrational thresholds can be clearly seen as steps in the experimental data. The agreement between experimental values and theoretical ones is very good except around the first and second vibrational thresholds where the experimental data exceed the theoretical ones.     

Molecular frame photoelectron angular distributions for H2 ionization by single and trains of attosecond XUV laser pulses

The delayed autoionization of H₂ doubly excited states into channels of different inversion symmetry gerade and ungerade is investigated by using pulses of attosecond duration (isolated or packed in trains), linearly polarized along the molecular axis. It has been shown in previous work, by using XUV laser pulses with durations of 4 fs or longer, that the molecular frame photoelectron angular distributions (MFPAD) associated with the dissociative channel H⁺ + H(n?) are not symmetric with respect to the inversion center of the molecule. In contrast, the MFPADs become symmetric for shorter fs pulses. Here we show that, although this is still the case for pulses of attosecond duration, the combination of two of these pulses with a controlled time delay may still lead to asymmetric MFPADs. From the analysis of the time evolution of the calculated MFPADs, we propose a way to elucidate autoionization lifetimes of molecular resonant states. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2462–2471, 2010     

Revealing the ionization ability of binding site I of human serum albumin using 2-(2'-hydroxyphenyl)benzoxazole as a pH sensitive probe

The ability of site I of human serum albumin (HSA) to bind medium sized molecules is important for the distribution, metabolism, and efficacy of many drugs. Herein, we show that this binding site has the ionization ability that may alter the drug structure during the process of its delivery. We reveal this ability by employing 2-(2'-hydroxyphenyl)benzoxazole (HBO) as a pH sensitive probe. Binding of HBO in site I is studied here at physiological pH 7.2 using steady-state and lifetime spectroscopic measurements, molecular docking and molecular dynamics (MD) simulation methods. The complex photophysics of HBO and the unique fluorescence signature of its anionic form indicate that, upon binding with HSA, the molecule exists in equilibrium between the anionic and the syn-keto forms. The position of HBO inside the binding site was determined experimentally by measuring the fluorescence quenching of W214, the sole tryptophan residue in HSA. The ionization degree of HBO inside the binding site was estimated to be close to the ionization degree of HBO in an aqueous solution of pH 10. This was concluded by comparing the fluorescence behavior of bound HBO to that of HBO in different solvents and in aqueous solutions of different pH values. Molecular docking and MD simulations show that HBO binds in site I close to W214, confirming the experimental results, and pinpoint the dominant role of hydrophobic interactions in the binding site. The formation of the anionic form is proposed to be due to through-space interaction between the OH group of HBO and both R222 and I290 with a binding mode similar to that of warfarin in site I. Comparison of the results with those of HBO mixed with key amino acids in solution indicates the importance of through-space interaction in the formation of the anion, similar to enzymatic reactions.     

Direct probe electrospray (and nanospray) ionization mass spectrometry of neat ionic liquids

Electrospray ionization mass spectrometry of neat ionic liquids does not require continuous sample injection and the presence of a molecular solvent facilitates analysis of the ionic liquid itself and dissolved analytes.     

Dynamical symmetry analysis of ionization and harmonic generation of atoms in bichromatic laser pulses

Recently the effect of the relative phase ? in a high‐intensity (～10¹⁴ W/cm²) two‐color (bichromatic) CW laser with frequencies ω and 2ω on the high‐order harmonic generation (HHG) was studied within the framework of the non‐Hermitian quantum mechanics (NHQM) [Phys Rev A 2004, 69, 043404/1]. Here we emphasize the study of symmetries in bichromatic HHG spectra within the framework of the conventional Hermitian QM, and in particular by taking the duration of the laser pulse into consideration (an effect that has not been included in the non‐Hermitian studies due to the time asymmetry problem in NHQM). The phase dependence of HHG and intense‐field ionization probability in a 1D Xe atom with symmetric field‐free potential and symmetric initial wave function were studied numerically and analytically. From simulations based on a single‐particle response it can be seen that the HHG spectra is symmetric with respect to inversion in the relative phase between the two colors ? only if ionization is forbidden in the system and the laser pulse is an adiabatic one. The HHG spectra is symmetric with respect to a π‐shift in ? whenever the laser pulse is an adiabatic one, either for bound or open (ionized) systems. The ionization probability is symmetric both to inversion or π‐shift in ?; the component probabilities (right‐ and left‐ionization probabilities) have the same ?‐dependence, up to a shift of π. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Synthesis and structure of the Li13 cage [[[O-P(mu-NtBu)]2Li2]3(LiCl)6Li(Cl/OnBu)0.5(thf)7], containing a [O-P(mu-NtBu)]2(2-) dianion

The hydrolysis of [ClP(mu-NtBu)]2 with H2O-Et3N in thf, followed by in situ lithiation with nBuLi gives the Li13 cage [[[O-P(mu-NtBu)]2Li2]3(LiCl)6Li(Cl/OnBu)0.5(thf)7], containing a [O-P(mu-NtBu)]2(2-) dianion that is isoelectronic with ligands of the type [(RN)P(mu-NR)]2(2-).     

Lithium-ion Mobility in Li6B18(Li3N) and Li Vacancy Tuning in the Solid Solution Li6B18(Li3N)1−x(Li2O)x

All-solid-state batteries are promising candidates for safe energy-storage systems due to non-flammable solid electrolytes and the possibility to use metallic lithium as an anode. Thus, there is a challenge to design new solid electrolytes and to understand the principles of ion conduction on an atomic scale. We report on a new concept for compounds with high lithium ion mobility based on a rigid open-framework boron structure. The host–guest structure Li₆B₁₈(Li₃N) comprises large hexagonal pores filled with Li₇N] strands that represent a perfect cutout from the structure of α-Li₃N. Variable-temperature ⁷Li NMR spectroscopy reveals a very high Li mobility in the template phase with a remarkably low activation energy below 19 kJ mol⁻¹ and thus much lower than pristine Li₃N. The formation of the solid solution of Li₆B₁₈(Li₃N) and Li₆B₁₈(Li₂O) over the complete compositional range allows the tuning of lithium defects in the template structure that is not possible for pristine Li₃N and Li₂O.