Resonance-enhanced Multiphoton Ionization Research Articles

Ultrafast Protonation of an Amide: Photoionization-Induced Proton Transfer in Phenol-Dimethylformamide Complex Cation.

In this study we explore the photoionization-induced proton transfer (PT) dynamics in the hydrogen-bonded complex of phenol (PhOH) and a simple amide, dimethylformamide (DMF). Neutral PhOH-DMF complexes produced in a supersonic expansion are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via its S1 state, and the subsequent PT dynamics occurring in the [PhOH-DMF]+ cation is probed by delayed pulses that lead to ion fragmentation. The experiments and density functional theory calculations reveal that the photoionization-induced PT proceeds in two consecutive steps of very different time scales. Upon femtosecond ionization the [PhOH-DMF]+ cation is initially prepared with a non-PT geometry close to that of the dominant neutral complex. The ionic system then rapidly relaxes into a configuration possessing both non-PT and PT characteristics in ∼0.5 ps. This partial-PT intermediate then undergoes a much slower barrier crossing in ∼25 ps to a more stable structure in which PT is more complete. The slow isomerization step not only corresponds to PT but also to a hydrogen-bonding site switching. The present study simulates a scenario of suddenly bringing a strong acid to the close vicinity of an amide to watch how protonation occurs. Our results suggest that the initial protonation of a peptide-like unit in acid-induced protein processes requires a relaxation time of ∼0.5 ps, which must be taken into account in complete descriptions of protein dynamics.

Using Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry to Quantitatively Analyze the Creaming of an Emulsion.

In this study, we used a quantitative analytical method to indicate creaming behavior in an emulsion. An oil-in-water emulsion was directly measured by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry, and the time profiles of the peak areas of an oil component, styrene, were obtained at heights of 1, 2, and 3 cm from the bottom of a sample that had a height of 4 cm. All time profiles roughly indicated that the signal intensity increased once, then decreased, and finally settled. Moreover, we proposed a fitting equation for the time profiles by subtracting two sigmoid functions, whereby the degree of the signal increases at the initial stage, the degree of the signal decreases after the increase, and the times for continuing the higher signal intensities were all longer as the monitoring positions were raised. This method would surely provide useful information about emulsions that undergo creaming behavior.

Development and Optimization of an External-Membrane Introduction Photoionization Mass Spectrometer for the Fast Analysis of (Polycyclic)Aromatic Compounds in Environmental and Process Waters.

The development of sensitive analytical techniques for the real-time detection of aromatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs) is of high importance, because of their impact on human health and the environment. A promising approach, regarding to direct determination of (P)AHs in aqueous samples, is resonance-enhanced multiphoton ionization (REMPI) coupled to external-membrane introduction mass spectrometry (eMIMS). In eMIMS, analytes are extracted from the water phase into the gas phase, which is supplied to the MS by using an external semipermeable membrane setup. As a result, no laborious enrichment techniques are needed. With REMPI, ions are formed by the subsequent absorption of two photons via an excited molecular state. The unique ionization scheme of REMPI provides selective and sensitive detection of (P)AHs. When combining the capabilities of REMPI and MIMS, direct measurements of sub-μg/L concentrations of small (polycyclic)aromatic compounds are feasible. In this study, we present an external sheet membrane probe (ESMP) for the determination of selected (polycyclic)aromatic species in water samples by using REMPI time-of-flight mass spectrometry (REMPI-TOFMS). This inlet design shows promising results with respect to the direct analysis of (P)AHs in aquatic environments. With this early stage system, concentrations down to tens of ng/L for selected small (polycyclic)aromatic compounds are accessible within minutes without any sample preparation.

Description of Steam Cracker Fouling and Coking Residues by Thermal Analysis-Photoionization Mass Spectrometry

Two organic fouling samples obtained from downstream the cracking oven (DS) and from upstream the hot zone (US) of a steam cracker facility were characterized. For this purpose, a simultaneous thermal analyzer coupled to a photoionization mass spectrometer (STA-PI-MS) and a thermal desorption/pyrolysis gas chromatograph (TD/Py-GC-EI-MS) were used. Mass loss and differential scanning calorimetry information revealed the degradation of the materials beginning at 130 °C with two distinct maxima for US and one for DS (230–330 °C) as well as broad signals (330–500 °C) for both. Structural motives of different polymeric-like structures were assigned based on PI-MS of the effluent and separately conducted TD/Py-GC-EI-MS. The advantage of soft photoionization over hard ionization techniques such as electron ionization is the considerable reduction of fragmentation, yielding higher abundancies of molecular ions. Thus, even though complex samples are studied, evolving constituents can often be easily tracked in a time-resolved manner (1 Hz). While single photon ionization (SPI, 118 nm = 10.5 eV) ionizes most organic molecules, resonance-enhanced multiphoton ionization (REMPI, 2 × 266 nm = 9.4 eV) selectively addresses aromatic species. Differentiation of polymeric-like structures was achieved by exploiting this selectivity (SPI vs REMPI) and comparison of molecular patterns with GC-EI-MS data, which supports the identification of compounds by providing fragmentation patterns and chemical information based on retention time. US shows high inorganic content (∼50%) and more diversity in its organic part, as indicated by four types of patterns: polyethylene-like, Diels–Alder-like, polythioether/polysulfide-like, and polystyrene-like motives. In contrast, DS exhibits almost only signals of Diels–Alder-like and polystyrene-like structures and contains a less inorganic material (∼23%). Additionally, first attempts to quantify the Diels–Alder content by STA-SPI-MS were successfully conducted.

Quantum State-Resolved Characterization of a Magnetically Focused Beam of ortho-H2O.

Magnetic focusing of a molecular beam formed from a rotationally cooled supersonic jet of H2O seeded in argon is shown to yield water vapor highly enriched in the ortho-H2O nuclear spin isomer (NSI). Rotationally resolved resonance-enhanced multiphoton ionization time-of-flight mass spectrometry demonstrates that this methodology enables the preparation of a beam of water molecules enriched to >98% in the ortho-H2O NSI, that is, having an ortho-to-para ratio in excess of 50:1. The flux and quantum-state purity achieved through the methodology described herein could enable heterogeneous chemistry applications including the preparation ofnuclear spin-polarized water adlayers, making nuclear magnetic resonance investigations amenable to surface science studies, as well as laboratory astrophysics investigations of NSI interconversion mechanisms and rates in ice and at its surface.

Origin of Three Fold Torsional Potential in Methylated 2-Hydroxypyridine

In this paper, the detailed study of the methylated 2-hydroxypyridine molecule has been carried out using resonance-enhanced multiphoton ionization (REMPI) spectroscopic technique. However, the theoretical investigations have been performed using ab initio calculations. The origin band of the molecules 4-methyl-2-hydroxypyridine (4M2HP) and 6-methyl-2-hydroxypyridine (6M2HP) was observed at 34987 cm-1 and 35405 cm-1 in their REMPI spectrum and the bands assigned as ππ* transition state. The vibronic coupling of the nπ* and ππ* transition states took place in 4M2HP, thus some low intense bands near the original band of the molecule were observed in the spectrum. However, no such kind of bands observed in 6M2HP. The π*–σ* hyperconjugation is responsible for the conformational change of the methyl group in 4M2HP upon excitation (S0→S1).

Resonance-enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry for Evaluating Emulsion Inversion via Temperature Change.

This study used resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) to evaluate the phase inversion that is driven via temperature change. A change in temperature prompts phase inversion in an oil-in-water (O/W) emulsion that uses a nonionic surfactant as an emulsifier. In this study, an O/W emulsion was prepared containing Triton X-100 as an emulsifier. To promote emulsion inversion, we heated only the outlet of a capillary column that was used for sample introduction during REMPI-TOFMS. When the emulsion was continuously measured at 40°C, several intense spikes could be detected on a time profile for the analyte, toluene, constructed by extracting the peak areas from a series of obtained mass spectra. This indicated the presence of toluene droplets in the O/W emulsion. No intense spikes appeared at 80°C, however, which suggested the shrinkage or even disappearance of the oil droplets following phase inversion. Using this technique, an emulsion can be measured without affecting the influence of vaporization even at elevated temperatures, which surely is a serious concern when using other analytical techniques. Therefore, this technique would be quite useful for evaluating the phase inversion of an emulsion at elevated temperatures.

Observation of Rainbows in the Rotationally Inelastic Scattering of NO with CH4

Using a combination of velocity-map imaging and resonance-enhanced multiphoton ionization detection with crossed molecular beam scattering, the dynamics of rotational energy transfer have been examined for NO in collisions with CH4 at a mean collision energy of 700 cm-1. The images of NO scattered into individual rotational (jNO') and spin-orbit (Ω) levels typically exhibit a single broad maximum that gradually shifts from the forward to the backward scattering direction with increasing rotational excitation (i.e., larger ΔjNO). The rotational rainbow angles calculated with a two-dimensional hard ellipse model show reasonable agreement with the observed angles corresponding to the maxima in the differential cross sections extracted from the images for higher ΔjNO transitions, but there are clear discrepancies for lower ΔjNO (in particular, final rotational levels with jNO' = 7.5 and 8.5). The sharply forward scattered angular distributions for these lower ΔjNO transitions better agree with the predictions of an L-type rainbow model. The more highly rotationally excited NO appears to coincide with low rotational excitation of the co-product CH4, indicating a degree of rotational product-pair anticorrelation in this bimolecular scattering.

Range of multiple filamentation of a terawatt-power large-aperture KrF laser beam in atmospheric air

The range of multiple low-ionized filaments, either stochastically distributed or regularly arranged, produced in atmospheric air by terawatt-power picosecond pulses of a KrF laser was investigated for a nearly collimated beam of 60 mm in diameter. Filamentation patterns, energy attenuation, and spectra evolution were monitored along a 100 m pass in the laboratory air. Resonance-enhanced multiphoton (2+1) ionization of water vapor naturally contained in the atmosphere and stimulated rotational Raman scattering in nitrogen are identified as the main features of the filamentation mechanism, while a self-induced loss of laser radiation coherence is the main factor limiting a multiple-filamentation range.

Ionization-Induced π → H Site Switching in Resorcinol–Arn (n = 1 and 2) Clusters Probed by Infrared Spectroscopy

Infrared (IR) spectra of resorcinol (Rs)-Arn clusters (n = 1 and 2) have been measured in the neutral and cationic ground states (S0 and D0) by IR dip and resonance-enhanced multiphoton ionization (REMPI)-IR spectroscopy. The OH stretching vibrations in S0 keep their frequency regardless of the number of Ar atoms and the conformation of the OH groups in Rs (rotamers RsI and RsII), demonstrating that the Ar atoms are attached to the aromatic π-ring (π-bound structure) in S0. In the D0 state, the IR spectra of Rs+-Arn reflect the difference in the Rs conformations (RsI+ and RsII+). For n = 1, the IR spectra of both rotamers are almost the same as those of the corresponding monomer cations, indicating that Ar ligands essentially remain π-bonded after ionization. In contrast, the IR spectra of Rs+-Ar2 show hydrogen-bonded and free OH stretching vibrations, demonstrating that for a significant fraction of the clusters, the Ar atoms migrate from the π-bound site to the OH groups. The ionization-induced π → H migration yields are not unity for both rotamers RsI+-Ar2 and RsII+-Ar2. This result is in sharp contrast to phenol+-Ar2, in which one of the Ar atoms migrates to the OH site with 100% yield. The mechanism leading to the nonunity yield in Rs+-Ar2 is discussed in terms of the number of OH binding sites and Franck-Condon factors. The ionization excess energy dependence of the IR spectra of Rs+-Ar2 and its Rs+-Ar fragments is discussed in terms of the Ar binding energies estimated from the photoionization and photodissociation efficiency spectra.

Conformational Study of the Jet-Cooled Diketopiperazine Peptide Cyclo Tyrosyl-Prolyl.

The conformational landscape of the diketopiperazine (DKP) dipeptide built on tyrosine and proline, namely, cyclo Tyr-Pro, is studied by combining resonance-enhanced multiphoton ionization, double resonance infrared ultraviolet (IR-UV) spectroscopy, and quantum chemical calculations. Despite the geometrical constraints due the two aliphatic rings, DKP and proline, cyclo Tyr-Pro is a flexible molecule. For both diastereoisomers, cyclo LTyr-LPro and cyclo LTyr-DPro, two structural families coexist under supersonic jet conditions. In the most stable conformation, the aromatic tyrosine substituent is folded over the DKP ring (g+ geometry of the aromatic ring) as it is in the solid state. The other structure is completely extended (g- geometry of the aromatic ring) and resembles that proposed for the vapor phase. IR-UV results are not sufficient for unambiguous assignment of the observed spectra to either folded or extended conformations and the simulation of the vibronic pattern of the S0-S1 transition is necessary. Still, the comparison between IR-UV results and anharmonic calculations allows explanation of the minor structural differences between cyclo LTyr-LPro and cyclo LTyr-DPro in terms of different NH···π and CH···π interactions.

Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H2 Formation and Desorption

Abstract The interactions of atomic and molecular hydrogen with bare interstellar dust grain surfaces are important for understanding H2 formation at relatively high temperatures (>20 K). We investigate the diffusion of physisorbed H atoms and the desorption energetics of H2 molecules on an amorphous diamond-like carbon (DLC) surface. From temperature-programmed desorption experiments with a resonance-enhanced multiphoton ionization (REMPI) method for H2 detection, the H2 coverage-dependent activation energies for H2 desorption are determined. The activation energies decrease with increasing H2 coverage and are centered at 30 meV with a narrow distribution. Using a combination of photostimulated desorption and REMPI methods, the time variations of the surface number density of H2 following atomic and molecular hydrogen depositions are studied. From these measurements, we show that H2 formation on a DLC surface is quite efficient, even at 20 K. A significant kinetic isotope effect for H2 and D2 recombination reactions suggests that H-atom diffusion on a DLC surface is mediated by quantum mechanical tunneling. In astrophysically relevant conditions, H2 recombination due to physisorbed H-atoms is unlikely to occur at 20 K, suggesting that chemisorbed H atoms might play a role in H2 formation at relatively high temperatures.

Stereodynamic Imaging of Bromine Atomic Photofragments Eliminated from 1-Bromo-2-methylbutane Oriented via Hexapole State Selector.

Both single-laser and two-laser experiments were conducted to look into the ion-imaging of Br*(2P1/2) and Br(2P3/2) photofragmented from 1-bromo-2-methylbutane in the range 232-240 nm via a detection scheme of (2+1) resonance-enhanced multiphoton ionization. The angular analysis of these photofragment distributions yields the anisotropy parameter β = 1.88 ± 0.06 for the Br* excited state which arises from a parallel transition, while β = 0.63 ± 0.09 for the Br ground state indicates the contribution from both a perpendicular transition and a non-adiabatic transition. When a hexapole coupled with an orienting field was implemented, the parent molecules are spatially oriented to yield an orientation efficiency |⟨cosθ⟩| of 0.15. Besides the χ angle between the recoil velocity v and the transition dipole moment μ, orienting molecules allows for the evaluation of the angle α between v and the permanent molecular dipole moment d. The angular analysis of Br* photofragment distribution yields χ = 11.5° and α in the range from 160° to 180° with weak dependency. In the two-laser experiments, the angular anisotropy of Br photofragment distribution was found to be smaller (0.38 ± 0.10) when the photolysis wavelength was red-shifted to 240 nm, suggesting the increasing contributions from perpendicular transitions.

Impact of photochemical ageing on Polycyclic Aromatic Hydrocarbons (PAH) and oxygenated PAH (Oxy-PAH/OH-PAH) in logwood stove emissions

The combustion of spruce logwood in a modern residential stove was found to emit polycyclic aromatic hydrocarbons (PAH) and oxygenated polycyclic aromatic hydrocarbons (OPAH) with emission factors of 404 μg MJ−1 of 35 analysed PAH, 317 μg MJ−1 of 11 analysed Oxy-PAH and 12.5 μg MJ−1 of 5 analysed OH-PAH, most of which are known as potential mutagens and carcinogens. Photochemical ageing in an oxidation flow reactor (OFR) degraded particle-bound PAH, which was also reflected in declining PAH toxicity equivalent (PAH-TEQ) values by 45 to 80% per equivalent day of photochemical ageing in the atmosphere. OPAH concentrations decreased less than PAH concentrations during photochemical ageing, supposedly due to their secondary formation, while 1-hydroxynaphthalene, 1,5-dihydroxynaphthalene and 1,8-naphthalaldehydic acid were significantly increased after ageing. Furthermore, secondary organic aerosol (SOA) formation and aromatic compounds not included in targeted analysis were investigated by thermal-optical carbon analysis (TOCA) hyphenate to resonance-enhanced multi-photon ionisation time-of-flight mass spectrometry (REMPI-TOFMS).The commonly used PAH-source indicators phenanthrene/anthracene, fluoranthene/pyrene, retene/chrysene, and indeno[cd]pyrene/benzo[ghi]perylene remained stable during photochemical ageing, enabling identification of wood combustion emissions in ambient air. On the other hand, benz[a]pyrene/benz[e]pyrene and benz[a]anthracene/chrysene were found to decrease with increasing photochemical age. Retene/chrysene was not a proper classifier for the wood combustion emissions of this study, possibly due to more efficient combustion than in open wood burning, from which this diagnostic ratio was initially derived.This study motivates in-depth investigation of degradation kinetics of particle-bound species on different combustion aerosol as well as the consequences of photochemical ageing on toxicity and identification of wood combustion emissions in ambient air.

Radar resonance-enhanced multiphoton ionization for measurement of atomic oxygen in non-equilibrium pulsed plasmas

In this work, coherent microwave Rayleigh scattering for the detection of resonance-enhanced multiphoton ionization (Radar REMPI) was used for the measurement of atomic oxygen in a nonequilibrium pulsed plasma discharge. Specifically, the detection of atomic oxygen is demonstrated in a pulsed discharge at 100 Torr in a molecular oxygen and helium gas mixture. REMPI was conducted using Nd:YAG pumped dye radiation at 618.9 nm mixed with the Nd:YAG 355 nm radiation, generating 225.6 nm laser light, of which two photons match the energy of the 2P33p(3P) electronically excited intermediate state of atomic oxygen. A third photon from the laser source then readily ionizes the atom in the 2 + 1 photon REMPI scheme. The Radar REMPI results are compared to two-photon absorption laser-induced fluorescence measurements from the literature, which shows reasonable agreement.

Highly Efficient Detection and Separation of Chiral Molecules through Shortcuts to Adiabaticity.

A highly efficient method for optical or microwave detection and separation of left- and right-handed chiral molecules is proposed. The method utilizes a closed-loop three-state system in which the population dynamics depends on the phases of the three couplings. Because of the different signs of the coupling between two of the states for the opposite chiralities the population dynamics is chirality dependent. By using the "shortcuts to adiabaticity" concept applied to the stimulated Raman adiabatic passage technique, one can achieve 100% contrast between the two enantiomers in the population of a particular state. It can be probed by light-induced fluorescence for large ensembles or through resonantly enhanced multiphoton ionization for single molecules.

Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm.

Roaming dynamics is observed in the photodissociation of formic acid (HCOOH) at 230 nm by using the slice imaging method. In combination with rotational state selective (2 + 1) resonance-enhanced multiphoton ionization of the CO fragments, the speed distributions of the CO fragments exhibit a low recoil velocity at low rotational levels of J = 9 and 20, while the velocity distributions of CO at high rotational levels of J = 30 and 48 show a relatively large recoil velocity. The experimental results indicate that the roaming of OH radical should be related with the formation of CO + H2O channel at the present photolysis energy. Unlike the roaming pathways occurring in H2CO that can be described by loose flat potential, our CO speed distribution analysis suggests the presence of a "tight" flat potential in the roaming dynamics of HCOOH molecules.

Behaviour of radicals on interstellar dust analogues

AbstractSurface reactions of radicals play important roles in the formation of complex molecules on interstellar dust grains. Under interstellar conditions, because the coverage of adsorbates on dust is significantly low, surface reactions are often limited by precedent processes, namely, the adsorption and diffusion of reactants. Therefore, to appropriately incorporate dust surface reactions into chemical models, information on the adsorption and diffusion of radicals is crucial. However, it is not easy to follow the behaviour of radicals on surfaces by conventional experimental methods. To monitor radicals on interstellar dust analogues, we have recently succeeded in applying a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods. In this paper, we briefly review our recent experiments for clarifying radical behaviour on water ice, pure solid CO and diamond-like carbon.

Multistage Zeeman deceleration of NH XΣ−3 radicals

We report on the Zeeman deceleration of ground-state NH radicals, using a decelerator that consists of 100 pulsed solenoids and 100 permanent hexapoles. Packets of state-selected NH ($X\,^3\Sigma^-$, N=0, J=1) radicals are produced with final velocities ranging between 510 m/s and 150 m/s. The velocity distributions of the packets of NH exiting the Zeeman decelerator are probed using velocity map imaging detection. We present a new 1+2' resonance-enhanced multiphoton ionization scheme for NH, that allows for velocity map imaging detection under ion recoil-free conditions. The packets of Zeeman-decelerated NH radicals, in combination with the new detection scheme, offer interesting prospects for the use of this important radical in high-resolution crossed-beam scattering experiments.

Visible and ultraviolet laser spectroscopy of ThF

The molecular ion ThF+ is the species to be used in the next generation of search for the electron’s Electric Dipole Moment (eEDM) at JILA. The measurement requires creating molecular ions in the eEDM sensitive state, the rovibronic ground state 3Δ1,v+=0,J+=1. Survey spectroscopy of neutral ThF is required to identify an appropriate intermediate state for a Resonance Enhanced Multi-Photon Ionization (REMPI) scheme that will create ions in the required state. We perform broadband survey spectroscopy (from 13,000 to 44,000 cm−1) of ThF using both Laser Induced Fluorescence (LIF) and 1+1′ REMPI spectroscopy. We observe and assign 345 previously unreported vibronic bands of ThF. We demonstrate 30% efficiency in the production of ThF+ ions in the eEDM sensitive state using the Ω=3/2 [32.85] intermediate state. In addition, we propose a method to increase the aforementioned efficiency to ∼100% by using vibrational autoionization via core-nonpenetrating Rydberg states, and discuss theoretical and experimental challenges. Finally, we also report 83 vibronic bands of an impurity species, ThO.