Search

Molecular ions are relatively cold in the E region ionosphere; however, they can upwell to the magnetosphere during geomagnetically active times. Resonance between electrostatic ion cyclotron (EIC) waves is a potential pathway to energize molecular ions. In this work, the E region ionospheric plasma was modeled in the laboratory, and EIC waves were excited by a nonuniform field‐aligned current. The EIC wave was excited even when the ion neutral collision frequency is much higher than the ion cyclotron frequency, and the fundamental frequency was observed to be below the ion cyclotron frequency. In addition, the wave dispersion of the collisional EIC wave was calculated, which shows a consistent trend with experimental results as the collisions increasing. Therefore, this work suggests that EIC waves can be excited in the E region ionospheric‐like plasma, which can support the explanation of the energization of molecular ions in the E region ionosphere. Plain Language Summary: Spacecraft observed the presence of molecular ions in the topside ionosphere (600–1,000 km) and the magnetosphere during geomagnetically active times, and the potential mechanisms responsible for the acceleration of these molecular ions are still not fully understood. The resonance between electrostatic ion cyclotron (EIC) waves and molecular ions is a potential pathway to energize the molecular ions. However, most previous works on EIC waves were studied in collisionless plasma similar to the magnetospheric plasma, which is different from the E region ionospheric plasma. A typical characteristic of E region ionospheric plasma is the partially ionized effect, which introduces new physical processes that do not occur in collisionless plasmas. In this work, strong collisions that occurred in the E region ionosphere were simulated in the laboratory, and the current‐driven electrostatic ion cyclotron waves were observed in the modeled ionosphere. It is found that electrostatic ion cyclotron waves can be generated in the E region ionospheric‐like plasma. This work provides solid experimental evidence that electrostatic ion cyclotron instability can exist in weakly ionized plasmas of the E region ionosphere, and can be applied to explain the transverse ion heating of bulk ions in the bottomside ionosphere. Key Points: The E region ionospheric‐like collisional plasma was modeled in the laboratoryEIC waves were excited by field aligned current in the ionospheric‐like plasmaThis work can be applied to explain the energization of molecular ions in the E region ionosphere [ABSTRACT FROM AUTHOR]

Abstract Molecular ions are relatively cold in the E region ionosphere; however, they can upwell to the magnetosphere during geomagnetically active times. Resonance between electrostatic ion cyclotron (EIC) waves is a potential pathway to energize molecular ions. In this work, the E region ionospheric plasma was modeled in the laboratory, and EIC waves were excited by a nonuniform field‐aligned current. The EIC wave was excited even when the ion neutral collision frequency is much higher than the ion cyclotron frequency, and the fundamental frequency was observed to be below the ion cyclotron frequency. In addition, the wave dispersion of the collisional EIC wave was calculated, which shows a consistent trend with experimental results as the collisions increasing. Therefore, this work suggests that EIC waves can be excited in the E region ionospheric‐like plasma, which can support the explanation of the energization of molecular ions in the E region ionosphere.

Molecular hydrogen ions in weakly bound states close to the first dissociation threshold are attractive quantum sensors for measuring the proton-to-electron mass ratio and hyperfine-induced ortho-para mixing. The experimental accuracy of previous spectroscopic studies relying on fast ion beams could be improved by using state-of-the-art ion trap setups. With the electric-dipole moment vanishing in H 2 + and preventing fast spontaneous emission, radiative lifetimes of the order of weeks are found. We include the effect of black-body radiation that can lead to photodissociation and rovibronic state redistribution to obtain effective lifetimes for trapped ion experiments. Rate coefficients for bound-bound and bound-continuum processes were calculated using adiabatic nuclear wave functions and nonadiabatic energies, including relativistic and radiative corrections. Effective lifetimes for the weakly bound states were obtained by solving a rate equation model and lifetimes in the range of 4–523 and >215 ms were found at room temperature and liquid nitrogen temperature, respectively. Black-body induced photodissociation was identified as the lifetime-limiting effect, which guarantees the purity of state-selectively generated molecular ion ensembles. The role of hyperfine-induced g/u-mixing, which allows pure rovibrational transitions, was found to be negligible. [ABSTRACT FROM AUTHOR]

Molecular hydrogen ions in weakly bound states close to the first dissociation threshold are attractive quantum sensors for measuring the proton-to-electron mass ratio and hyperfine-induced ortho-para mixing. The experimental accuracy of previous spectroscopic studies relying on fast ion beams could be improved by using state-of-the-art ion trap setups. With the electric-dipole moment vanishing in H 2 + and preventing fast spontaneous emission, radiative lifetimes of the order of weeks are found. We include the effect of black-body radiation that can lead to photodissociation and rovibronic state redistribution to obtain effective lifetimes for trapped ion experiments. Rate coefficients for bound-bound and bound-continuum processes were calculated using adiabatic nuclear wave functions and nonadiabatic energies, including relativistic and radiative corrections. Effective lifetimes for the weakly bound states were obtained by solving a rate equation model and lifetimes in the range of 4–523 and >215 ms were found at room temperature and liquid nitrogen temperature, respectively. Black-body induced photodissociation was identified as the lifetime-limiting effect, which guarantees the purity of state-selectively generated molecular ion ensembles. The role of hyperfine-induced g/u-mixing, which allows pure rovibrational transitions, was found to be negligible. [ABSTRACT FROM AUTHOR]

The behavior of a series of previously unknown N-(5-aminothiophen-2-yl)- and N-[2-(methyl-sulfanyl)-1,3-thiazol-5-yl]isothioureas under electron impact ionization (70 eV) has been studied for the first time. 2-Thienylisothioureas give rise to a fairly stable molecular ion (Irel 11–25%), whereas no molecular ion peak is present in the mass spectra of 1,3-thiazolylisothioureas. A common fragmentation pathway of the molecular ions of 2-thienyl- and 1,3-thiazolylisothioureas involves cleavage of the C–N bond in the isothiourea fragment with charge localization on the imino nitrogen atom to give [R3SC≡NR2]+ ion (Irel 34–100%); furthermore, [M – R3SC=NR2]+ ion with charge localization on the amino nitrogen atom is formed from thienyl derivatives and is the most abundant (Irel 91–100%). The mass spectra of 1,3-thiazolylisothioureas also showed [M – MeSCN]+· and [MeSCS]+ ion peaks resulting from decomposition of the thiazole ring in the molecular ion. In addition, unlike 2-thienylisothioureas, the molecular ions of 1,3-thiazolyl analogs underwent Chet–N bond cleavage with charge localization on the thiazole fragment. [ABSTRACT FROM AUTHOR]

Atmospheric pressure field desorption (APFD) mass spectrometry (MS) has recently been explored as a new contribution to the field of ambient desorption/ionization (ADI). Depending on the selected polarity applied to the field emitter, ionic and polar analytes were demonstrated to deliver positive as well as negative ions. Whereas this recent study solely reported on the formation of even-electron ions of either polarity, the present work on APFD-MS demonstrates the abundant formation of positive molecular ions, M+•, from polycyclic aromatic compounds. Molecular ions were formed on and desorbed from standard 13-µm activated tungsten wire emitters at atmospheric pressure. The commercial field emitters were positioned at about 2 mm distance in front of the atmospheric pressure interface of a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer and the entrance electrode of the interface was set to –4.5 to –5.5 kV with respect to the emitter. Emitter-disrupting electric discharges did normally not occur under these conditions. The electric field strengths achieved at the dendritic microneedles were sufficient to allow for the abundant formation of M+• ions of various polycyclic aromatic compounds such as benzo[a]pyrene, anthracene, fluoranthene, 1,1,4,4-tetraphenyl-butadiene, and 1-aza-[6]helicene. In case of the extremely basic 1-aza-[6]helicene protonation strongly competed with molecular ion formation and tended to suppress the field ionization process. All molecular ion compositions were assured by accurate mass-based formula assignments. [ABSTRACT FROM AUTHOR]

The electron ionization mass spectra of previously unknown N-(5-aminothiophen-2-yl)thioureas obtained in one preparative step from propargylamines and isothiocyanates were studied for the first time, and the main fragmentation patterns of these compounds were revealed. All the thienylthioureas studied form a molecular ion (Irel 7–61%), whose common fragmentation pathway {except for N-[5-(diethylamino)thiophen-2-yl]-N,N'-diphenylthiourea} is the cleavage of the R1N–C(=S) bond to form an intense [M – R2NHCS]+ ion peak (Irel 35–85%). The main fragmentation pathway of the molecular ions of N-[5-(diethylamino)thiophen-2-yl]-N,N'-diphenylthiourea is associated with the opening of the thiophene ring by the C2–S and C4–C5 bonds. N-[5-(Pyrrolidin-1-yl)thiophen-2-yl]- and N-[5-(piperidin-1-yl)thiophen-2-yl]thioureas characteristically undergo intense specific rearrangements that reveal themselves in the occurrence of unexpected primary fragmentation pathways of the molecular ions, the main of which is the elimination of an N,N'-dimethylcarbodiimide molecule. [ABSTRACT FROM AUTHOR]

Recently, the upper bounds on the static time-reversal (T) and spatial parity (P)-violating electron electric dipole moment (eEDM) and dimensionless constant, characterizing the strength of the T , P -violating scalar–pseudoscalar nucleus–electron interaction, have been updated in the JILA experiment using the HfF+ cations. We considered two other sources of the T , P -violation in HfF+–axion-like-particle (ALP)-mediated scalar–pseudoscalar electron–electron and nucleus–electron interactions. To estimate the magnitude of effects, induced by such interactions in HfF+ we have developed and applied a method which implies the direct use of the ab initio relativistic coupled cluster theory to calculate molecular parameters that characterize the interactions. Using these parameters, we showed that an order of magnitude updated laboratory constraints on the ALP-mediated electron–electron and nucleus–electron interactions can be derived from the experimental data on T , P -violating effects in HfF+ for a wide range of ALP masses. [ABSTRACT FROM AUTHOR]

Precision spectroscopy has been the driving force for progress of our physical understanding and still is a promising tool for the investigation of new physics. Molecules offer transitions which allow tests that are not possible in atomic systems. However, usually precision spectroscopy of molecules is challenging due to the lack of cycling transitions for state preparation and state detection. For molecular ions, this obstacle can be overcome by quantum logic spectroscopy, where dissipation for state preparation and detection is provided by a co-trapped atomic ion exploiting the shared eigenstates of motion. Here, we propose a full quantum logic spectroscopy scheme for molecular oxygen ions and theoretically investigate the feasibility of quantum logic-assisted state detection and preparation. Furthermore, we provide coupling rates for a direct single-photon quadrupole excitation of a vibrational overtone transition that can serve as a sensitive reference for tests of a possible variation of the proton-to-electron mass ratio.

The importance of the symmetric as well as non-symmetric ion-atom absorption processes in broad spectral region within a model of solar photosphere and also within the corresponding sunspot models, has been previously emphasized. This motivated us to study the photo-association, charge-exchange and photo-dissociation absorption processes in the cases of non-symmetric and symmetric systems involving hydrogen and potassium atoms, ions, molecules and molecular-ions. We have calculated the data for molecules and molecular state characterizations needed for current computation. We determined the partial and average cross-sections and corresponding spectral absorption rate coefficients, as the functions of wavelengths, and temperatures (100 nm ⩽ λ ⩽ 1000 nm, 400 K ⩽ T ⩽ 10 000 K). Our calculations are performed for conditions that exist in different layers of weakly ionized stellar atmospheres. The obtained data are potentially of high impact and may have broad further applications, e.g., for modelling of laboratory plasmas and various astrophysical objects including geocosmical ones. [ABSTRACT FROM AUTHOR]

In this study, the processes of photo-dissociation in the cases of symmetric and strongly non-symmetric systems which involve hydrogen and potassium are investigated. The aim of this study was to calculate data for those small molecules and molecular state characterizations as well as to obtain the corresponding partial and average cross-sections. Moreover, the corresponding spectral absorption rate coefficients for symmetric and strongly non-symmetric cases i.e. for K 2 + and KH+ cases are given in tables as a function of temperatures and wavelengths allowing potential modeling. The current research and presented results can be of interest for astrophysical plasma research and laboratory plasma investigation. [ABSTRACT FROM AUTHOR]

In this manuscript, the chemi-ionization (CI) processes in atom - Rydberg atom collisions are examined. We obtained the cross-sections, as well as rate coefficients for CI processes in potassium and sodium collisions for principal quantum numbers 5 ⩽ n ⩽ 25 in the 500 K ⩽ T ⩽ 1500 K temperature region. The presented data could be useful for the improvement of analysis and modelling of weakly ionized layers of different atmospheres and cosmic objects where these and other CI processes could change excited state populations and the free electron density, consequently affecting the optical characteristics. [ABSTRACT FROM AUTHOR]

Previously unknown methyl N-(3-substituted thiophen-2-yl)carbamimidothioates were synthesized in one preparative step from hetero-substituted allenes (methoxy-, 1-ethoxyethoxy-, and 1H-pyrrol-1-ylallenes) and aliphatic, cycloaliphatic, and aromatic isothiocyanates, and their behavior under electron impact (70 eV) and chemical ionization (reactant gas methane) was studied for the first time. Unstable molecular ion derived from the title compounds under electron impact decomposes mainly via C–N bond cleavage in the carbamimido-thioate fragment with charge localization on the imide nitrogen atom. A similar fragmentation pathway is observed in the chemical ionization mass spectra of these compounds. Characteristic but low-intense ion peaks in the electron impact mass spectra of methyl N-(3-methoxythiophen-2-yl)carbamimidothioates become predominating in their chemical ionization mass spectra. The base peak in the chemical ionization mass spectra of methyl N-[3-(1H-pyrrol-1-yl)thiophen-2-yl]carbamimidothioates is that of the [M – SMe]+ ion. The most abundant ion in the chemical ionization mass spectrum of methyl N-[3-(1-ethoxyethoxy)thiophen-2-yl]-carbamimidothioate was generated by successive elimination of ethoxyethene and methanethiol molecules from the [M + H]+ ion. [ABSTRACT FROM AUTHOR]

The electronic states of ArXe+ and KrXe+ molecular ions are theoretical studied. Electronic terms and oscillator strengths of transitions from the ground term are calculated using the multireference quantum chemical approach. Relative efficiencies of transitions are determined. The results are compared with existing theoretical data. [ABSTRACT FROM AUTHOR]

We discuss newly measured rotational transitions of CO+ ( X 2 Σ + ) in its vibrational ground state v = 0, in particular the fine-structure components of the N = 0−1 and N = 1−2 transitions. We employed a rotational action spectroscopic technique in a cryogenic ion trap for the measurements. The recorded low-temperature and high-resolution spectra of CO+ show resolved Zeeman splittings caused by the Earth's magnetic field. The highly accurate experimental transition frequencies and derived molecular constants agree with previous measurements and improve the spectroscopic parameters for this known interstellar molecular ion. [ABSTRACT FROM AUTHOR]

The vibrational predissociation spectra of the CN–H2 and CN–D2 complexes are measured in regions between 450 and 3100 cm − 1 in an ion trap at different temperatures at the free electron lasers for infrared experiments (FELIX) Laboratory. Strong differences between the vibrational spectra of the two para and ortho nuclear spin isomers (CN–(p-H2) or CN–(o-H2) and CN–(p-D2) or CN–(o-D2)) are detected. In the case of CN–H2 we could assign the observed bands with the help of an accurate quantum calculation. The spectrum is dominated by the first overtone of the hindered H2 rotation in combination with the hindered CN rotation of the CN–(o-H2) nuclear spin isomer. The energetically higher-lying CN–(p-H2) isomer was not observed due to efficient ortho-para ligand exchange. The C–N stretching mode was recorded at 2040(1) cm − 1 . For the CN–D2 system several vibrational modes of both spin isomers overlap in their low-lying intermonomer streching and bending region, but the main observed bands are attributed to the more stable CN–(p-D2) spin isomer. The D–D stretching vibration was found to be at 2898(1) cm − 1 . [ABSTRACT FROM AUTHOR]

The vibrational predissociation spectra of the CN–H2 and CN–D2 complexes are measured in regions between 450 and 3100 cm − 1 in an ion trap at different temperatures at the free electron lasers for infrared experiments (FELIX) Laboratory. Strong differences between the vibrational spectra of the two para and ortho nuclear spin isomers (CN–(p-H2) or CN–(o-H2) and CN–(p-D2) or CN–(o-D2)) are detected. In the case of CN–H2 we could assign the observed bands with the help of an accurate quantum calculation. The spectrum is dominated by the first overtone of the hindered H2 rotation in combination with the hindered CN rotation of the CN–(o-H2) nuclear spin isomer. The energetically higher-lying CN–(p-H2) isomer was not observed due to efficient ortho-para ligand exchange. The C–N stretching mode was recorded at 2040(1) cm − 1 . For the CN–D2 system several vibrational modes of both spin isomers overlap in their low-lying intermonomer streching and bending region, but the main observed bands are attributed to the more stable CN–(p-D2) spin isomer. The D–D stretching vibration was found to be at 2898(1) cm − 1 . [ABSTRACT FROM AUTHOR]

Recently, the upper bounds on the static time-reversal (\({\mathcal{T}}\)) and spatial parity (\({\mathcal{P}}\))-violating electron electric dipole moment (eEDM) and dimensionless constant, characterizing the strength of the \({\mathcal{T}}\),\({\mathcal{P}}\)-violating scalar–pseudoscalar nucleus–electron interaction, have been updated in the JILA experiment using the HfF+ cations. We considered two other sources of the \({\mathcal{T}}\),\({\mathcal{P}}\)-violation in HfF+–axion-like-particle (ALP)-mediated scalar–pseudoscalar electron–electron and nucleus–electron interactions. To estimate the magnitude of effects, induced by such interactions in HfF+ we have developed and applied a method which implies the direct use of the ab initio relativistic coupled cluster theory to calculate molecular parameters that characterize the interactions. Using these parameters, we showed that an order of magnitude updated laboratory constraints on the ALP-mediated electron–electron and nucleus–electron interactions can be derived from the experimental data on \({\mathcal{T}}\),\({\mathcal{P}}\)-violating effects in HfF+ for a wide range of ALP masses.

Quantification of the composition of binary mixtures in secondary ion mass spectrometry (SIMS) is required in the analyses of technological materials from organic electronics to drug delivery systems. In some instances, it is found that there is a linear dependence between the composition, expressed as a ratio of component volumes, and the secondary ion intensities, expressed as a ratio of intensities of ions from each component. However, this ideal relationship fails in the presence of matrix effects and linearity is observed only over small compositional ranges, particularly in the dilute limits. In this paper, we assess an empirical method, which introduces a power law dependence between the intensity ratio and the volume fraction ratio. A previously published physical model of the organic matrix effect is employed to test the limits of the method and a mixed system of 3,3′‐bis(9‐carbazolyl) biphenyl and tris(2‐phenylpyridinato)iridium (III) is used to demonstrate the method. This paper introduces a two‐point calibration, which determines both the exponent in the power law and the sensitivity factor for the conversion of ion intensity ratio into volume fraction ratio. We demonstrate that this provides significantly improved accuracy, compared with a one‐point calibration, over a wide compositional range in SIMS quantification and with a weak dependence on matrix effects. Because the method enables the use of clearly identifiable secondary ions for quantitative purposes and mitigates commonly observed matrix effects in organic materials, the two‐point calibration method could be of significant benefit to SIMS analysts. [ABSTRACT FROM AUTHOR]

Ionospheric molecular ions, such as NO+, N2+ and O2+, are gravitationally bound, and are expected to undergo recombination to form a pair of neutral atoms, due to short dissociative recombination lifetime. Therefore, they are expected to be relatively cold in the Earth’s atmosphere, compared with light ions such as H+ and He+, or even heavier ions such as N+ or O+. However, several spacecraft missions observed their presence in the high-altitude ionosphere and the magnetosphere, predominantly during the geomagnetically active times. This hints to the possibility that molecular ions have the ability to acquire sufficient energy in a very short time, and can be used as tracers of mass differentiated vertical transport to understand the mechanisms responsible for “fast ionospheric outflow.” In this letter, we review the observational data sets that reported on the abundances of molecular ions in the Earth’s magnetosphere-ionosphere system, starting from their first observations by the Sputnik III mission, to the current Arase (ERG) satellite and Enhanced Polar Outflow Probe (e-POP) missions. The available data suggests that molecular ions are quite abundant in the lower atmosphere at all times, but are only seen in the high-altitude ionosphere and magnetosphere during the times of increased geomagnetic activity.

The fragmentation of 5-[(cyanomethyl)sulfanyl]- and 5-[(1,3-dioxolan-2-ylmethyl)sulfanyl]-1H-pyrrol-2-amines under electron impact (70 eV) and chemical ionization with methane as reactant gas has been studied for the first time. Electron impact ionization of 5-[(cyanomethyl)sulfanyl]-substituted pyrroles generates low-stability molecular ions {M+·, Irel 4–22%; cf. Irel 16–69% for 5-[(1,3-dioxolan-2-ylmethyl)sulfanyl] analogs} whose primary fragmentation involves cleavage of the C–S bond with expulsion of NCCH2 radical. Other fragmentation pathways of the molecular ions include formation of [M – CHR4]+· ions (R4 = CN) and substituent decomposition products. The mass spectra of 5-[(1,3-dioxolan-2-ylmethyl)sulfanyl]pyrroles characteristically show [M – SCH2R4]+ ion peak. The most intense ion peaks in the chemical ionization mass spectra of 5-[(cyanomethyl)sulfanyl] derivatives are those of M+· (Irel 18–69%) and [M + H]+ (Irel 34–100%). Both 5-[(cyanomethyl)sulfanyl]- and 5-[(1,3-dioxolan-2-ylmethyl)sulfanyl]pyrroles decompose mainly through cleavage of the CH2–S and C5–S bonds with the formation of stable [M – CH2R4]+ (Irel 92–100%), [M + H – CH2R4]+· (Irel 22–75%), and [M + H – SCH2R4]+· ions (Irel 6–25%). [ABSTRACT FROM AUTHOR]

Specific features of the cross sections and rate constants of the electron-impact dissociative excitation of molecular ions are studied in a wide range of gas and electron temperatures. It is found that an increase in the gas temperature results in a significant change in the dependence of the cross sections on the incident electron energy. The efficiencies of dissociative recombination with Rydberg state population and the dissociative excitation of ions are compared. [ABSTRACT FROM AUTHOR]

Abstract Molecular ions in the magnetosphere can be a tracer of fast ion outflows from the deep ionosphere. Statistical properties of molecular ions (O2+/NO+/N2+) in the ring current are investigated based on ion composition measurements (

Fragmentation of 1-alkyl(cycloalkyl, alkoxyalkyl)-5-[(alkyl, allyl, benzyl)sulfanyl]-1H-pyrrol-2-amines under electron impact (70 eV) and chemical ionization (with methane as reactant gas) has been studied for the first time. All the examined compounds under electron impact generated the molecular ion [M]+· (Irel 5–90%) whose primary fragmentation involved mainly cleavage of the C–S bond in the SR4 substituent with elimination of R4 radical (except for 1-isopropyl- and 1-cycloalkyl-1H-pyrrol-2-amines). The main pathway of the decomposition of 1-isopropyl(cycloalkyl)-N,N-dimethyl-5-[(methyl, allyl)sulfanyl]-1H-pyrrol-2-amines was cleavage of the C–N bond with both elimination of the R1 radical from the pyrrole nitrogen atom and fragmentation of the pyrrole ring through elimination of imine (or aziridine) molecule and formation of odd-electron [M – NR1]+· radical cation. Chemical ionization of the title compounds involved protonation, charge exchange, and electrophilic addition processes and elimination of Me (Et) and SMe (SEt) radicals from the [M + H]+ ions. [ABSTRACT FROM AUTHOR]

I present a novel scheme for producing cold (magnetically trappable) atomic hydrogen, based on threshold photodissociation of the BaH ^+ molecular ion. BaH ^+ can be sympathetically cooled using laser cooled Ba ^+ in an ion trap, before it is photodissociated on the single photon A ^1 Σ ^+ ← X ^1 Σ ^+ transition. The small mass ratio between Ba ^+ and BaH ^+ ensures a strong overlap within the ion trap for sympathetic cooling, while the large mass ratio between BaH ^+ and H means that the released hydrogen can be up to 139 times colder than the parent molecular ions. I examine the hydrogen production rate, and describe how the trap dynamics and photodissociation laser detuning influence the achievable energies. The low infrastructure costs and the ion trap nature of the scheme make it suitable for loading hydrogen into an antihydrogen experiment. This would support a direct matter–antimatter comparison, which could provide important clues as to why our Universe contains so little antimatter.

The features of the photodissociation spectra of molecular ions are studied in a wide range of gas temperatures. It is found that an increase in the gas temperature leads to a significant change in the frequency dependence of the photodissociation cross sections. A simple physical explanation of the temperature dependence of the cross sections is given. [ABSTRACT FROM AUTHOR]

Polarizable force fields are emerging as a more accurate alternative to additive force fields in terms of modeling and simulations of a variety of chemicals including biomolecules. Explicit treatment of induced polarization in charged species such as phosphates and sulfates offers the potential for achieving an improved atomistic understanding of the physical forces driving their interactions with their environments. To help achieve this, in this study we present balanced Drude polarizable force field parameters for molecular ions including phosphates, sulfates, sulfamates, and oxides. Better balance was primarily achieved in the relative values of minimum interaction energies and distances of the anionic model compounds with water at the Drude and quantum mechanical (QM) model chemistries. Parametrization involved reoptimizing available parameters as well as extending the force field to new molecules with the goal of achieving self-consistency with respect to the Lennard-Jones and electrostatic parameters targeting QM and experimental hydration free energies. The resulting force field parameters achieve consistent treatment across the studied anions, facilitating more balanced simulations of biomolecules and small organic molecules in the context of the classical Drude polarizable force field. [ABSTRACT FROM AUTHOR]

The formation of nanoparticles under the irradiation of a thin metallic gold film by accelerated atomic and molecular ions is demonstrated. The obtained structures are used to form porous silicon by the metal-assisted chemical etching. The size of the gold nanoparticles and structure of porous silicon greatly depend on the type of incident particles and their fluence. A local increase in the density of energy released at the target surface under molecular ion bombardment significantly reduces the doses required to form the desired film morphologies and spread of nanoparticles over the surface and simultaneously makes a weaker radiative impact on the substrate. The shape of the fluorescence and fluorescence-excitation spectra of porous silicon obtained from the irradiated structures is independent of the irradiation parameters, but changes with the etching-solution concentration. [ABSTRACT FROM AUTHOR]

We produced aniline cations (c –C 6 H 5 NH 2 +) and their dehydro- (H-loss) cations inside helium droplets by electron ionization and observed their mass-selected vibrational spectra in the N H stretching region. We observed vibrational bands at m/q = 93 which were identified as aniline cations. These bands showed only a few cm−1 shifts from the symmetric and anti-symmetric N H stretching bands for the amino (NH 2) group in the gas phase. For the H-loss cation at mass m/q = 92, the agreement of the observed N H band frequency with the result of DFT calculations suggests several candidate species, including a seven-membered ring structure, 4-monodehydro azepinylium (c –C 6 NH 6 +). A new reaction pathway to this cation was discussed by considering large excess energy in the ionization process. [Display omitted] • Aniline cations were produced in cold helium nanodroplets by electron ionization. • Both symmetric and anti-symmetric N–H stretching vibrational bands were observed. • H-loss fragment cations were found at m/q = 92 with a new N–H vibrational peak. • DFT calculations suggest candidate structures at 92 including a seven-membered ring. [ABSTRACT FROM AUTHOR]

For more than two decades, encapsulation in superfluid helium nanodroplets has served as a reliable technique for probing the structure and dynamics of molecules and clusters at a low temperature of ≈0.37 K. Due to weak interactions between molecules and the host liquid helium, good spectral resolution can usually be achieved, making helium droplets an ideal matrix for spectroscopy in a wide spectral range from infrared to ultraviolet. Furthermore, rotational structure in the spectra of small molecules provides a unique probe for interactions with the superfluid on an atomic scale. This review presents a summary of results and a discussion of recent experimental developments in helium droplet spectroscopy with the emphasis laid on infrared studies. Initially, studies focused on single molecules and have been expanded to larger species, such as metal-molecular clusters, biomolecules, free radicals, ions, and proteins.