Resonance-enhanced Multiphoton Ionization Research Articles

The role of the intermediate state in angle-resolved photoelectron studies using (2 + 1) resonance-enhanced multiphoton ionization of the chiral terpenes, α-pinene and 3-carene

Photoelectron angular distributions (PADs), ranging up to the maximum 6th order Legendre polynomial term set by the Yang theorem, have been recorded for the (2 + 1) resonance enhanced multiphoton ionization (REMPI) of two terpene isomers, 3-carene and α-pinene, employing femtosecond lasers and electron velocity map imaging detection. PAD measurements made with coincident photoion detection allow ion fragmentation effects to be assessed. Using circular polarization and enantiomerically pure samples the PAD measurements are extended to include chiral (odd) Legendre polynomial terms, and these are analysed and discussed as multiphoton photoelectron circular dichroism (MP-PECD). Comparisons are also made with single photon (synchrotron radiation) PECD of these compounds. Although for a given compound a common final cation state is reached, pronounced differences are observed between PECD and MP-PECD, and between the alternative identified REMPI intermediate states in the case of MP-PECD.

Trapping, Cooling, and Photodissociation Analysis of State-Selected H2+ Ions Produced by ( 3+1 ) Multiphoton Ionization

We report on the production of cold, state-selected H$_2^+$ molecular ions in a linear RF trap. The ions are produced by (3+1) resonance-enhanced multi-photon ionisation (REMPI) of H$_2$, and sympathetically cooled by laser-cooled Be$^+$ ions. After demonstrating and characterizing the REMPI process, we use photodissociation by a deep UV laser at 213~nm to verify the high vibrational purity of the produced H$_2^+$ ion samples. Moreover, the large difference between the photodissociation efficiencies of ions created in the $v=0$ and $v=1$ levels provides a way to detect a $v=0 \to 1$ transition. These results pave the way towards high-resolution vibrational spectroscopy of H$_2^+$ for fundamental metrology applications.

Experimental and theoretical study on p-chlorofluorobenzene in the S, S1 and D states

The geometric structures and vibration frequencies of para-chlorofluorobenzene (p-ClFPh) in the first excited state of neutral and ground state of cation were investigated by resonance-enhanced multiphoton ionization and slow electron velocity-map imaging. The infrared spectrum of S0 state and absorption spectrum for S1 ← S0 transition in p-ClFPh were also recorded. Based on the one-color resonant two-photon ionization spectrum and two-color resonant two-photon ionization spectrum, we obtained the adiabatic excited-state energy of p-ClFPh as 36302±4 cm−1. In the two-color resonant two-photon ionization slow electron velocity-map imagin spectra, the accurate adiabatic ionization potential of p-ClFPh was extrapolated as 72937±8 cm−1 via threshold ionization measurement. In addition, Franck-Condon simulation was performed to help us confidently ascertain the main vibrational modes in the S1 and D0 states. Furthermore, the mixing of vibrational modes between S0 → S1 and S1 → D0 has been analyzed.

Photon‐Stimulated Desorption of MgS as a Potential Source of Sulfur in Mercury's Exosphere

AbstractMercury has a relatively high sulfur content on its surface, and a signal consistent with ionized atomic sulfur (S+) was observed by the fast ion plasma spectrometer (FIPS) instrument on the MESSENGER spacecraft. To help confirm this assignment and to better constrain the sources of exospheric sulfur at Mercury, 193 nm photon‐stimulated desorption (PSD) of neutral sulfur atoms (S0) from MgS substrates was studied using resonance‐enhanced multiphoton ionization (REMPI) spectroscopy and time‐of‐flight (TOF) mass spectrometry. Though the PSD process is inherently nonthermal, the measured velocities of ejected S0 were fit using flux‐weighted Maxwellian distributions with translation energies <E> expressed as translational “temperatures” Ttrans = <E>/μkB. A bimodal distribution consisting of both thermal (Ttrans = 300 K) and suprathermal (Ttrans > 1,000 K) components in roughly a 2:1 ratio was found to best fit the data. The PSD cross‐section was measured to be approximately 4 × 10−22 cm2 and, together with the velocity distributions, was used to calculate the PSD source rate of S0 into the exosphere of Mercury. Exosphere simulations using the calculated rates demonstrate that PSD is likely the primary source of S0 in Mercury's exosphere at low (<1,000 km) altitudes.

Experimental Approach to Physicochemical Hydrogen Processes on Cosmic Ice Dust

Hydrogen is the most abundant element in the universe, and thus atomic and molecular hydrogen play important roles in chemical evolution in space. In particular, the physicochemical processes of hydrogen on cosmic dust, such as the diffusion of H atoms and nuclear spin conversion of H2 molecules at low temperatures, are recognized to significantly influence the subsequent chemical evolution. However, it is not easy to track the hydrogen on the surface by conventional experiments. We have recently succeeded in applying a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods to detect hydrogen on cosmic dust analogues. In this paper, we present a brief review of our recent experiments for elucidating the behavior of hydrogen on water ice, pure solid carbon monoxide, and diamond-like carbon as cosmic dust analogue surfaces.

Mode-specific vibrational predissociation dynamics of (HCl)2 via the free and bound HCl stretch overtones.

Velocity-map ion imaging has been used to study the vibrational predissociation dynamics of the HCl dimer following infrared (IR) excitation in the HCl stretch overtone region near 1.77 Å. HCl monomer predissociation products were detected state-selectively using 2 + 1 resonance-enhanced multiphoton ionization spectroscopy. The IR action spectrum shows the free HCl stretch (2ν1), the bound HCl stretch (2ν2), and a combination band involving the intermolecular van der Waals stretching mode (2ν2 + ν4). Fragment speed distributions extracted from ion images obtained for a range of HCl(v = 0, 1; J) levels following vibrational excitation on the 2ν1 and 2ν2 bands yield the correlated product pair distributions. All product pairs comprise HCl(v = 1) + HCl(v = 0) and show a strong propensity to minimize the recoil kinetic energy. Highly non-statistical and mode-dependent HCl product rotational distributions are observed, in contrast to that observed following stretch fundamental excitation. Predissociation lifetimes are also mode-dependent: excitation of the free HCl leads to τVP = 13 ± 1 ns, while the bound stretch has a shorter lifetime τVP ≤ 6 ns. The dimer dissociation energy determined from energy conservation (D0 = 397 ± 7 cm-1) is slightly smaller than the previously reported values. The results are discussed in the context of previous observations for (HF)2 and (HCl)2 after excitation of HX stretch fundamentals and models for vibrational predissociation.

Smart Online Coffee Roasting Process Control: Modelling Coffee Roast Degree and Brew Antioxidant Capacity for Real-Time Prediction by Resonance-Enhanced Multi-Photon Ionization Mass Spectrometric (REMPI-TOFMS) Monitoring of Roast Gases.

Process control with high time resolution is essential to maintain high product quality in coffee roasting. However, analytical techniques for quality assurance or measurements of desired coffee properties are often labor-intensive and can only be conducted after dropping the coffee beans. Resonance-enhanced multi-photon ionization time-of-flight mass spectrometry (REMPI-TOFMS) at 248 nm and 266 nm was applied to analyze the composition of the roast gas from small-scale Arabica coffee roasting. Coffee beans were dropped after different roasting times, ground and analyzed by Colorette to obtain the roast degree. Additionally, the antioxidant capacity of the coffee brew was determined by Folin–Ciocalteu (FC) assay. Models for the prediction of Colorette and FC values from REMPI mass spectra were constructed by partial least squares (PLS) regression. REMPI-TOFMS enables the prediction of Colorette values with a root-mean-square error in prediction (RMSEP) below 5 for both wavelengths. FC values could be predicted using REMPI at 248 nm with an RMSEP of 80.3 gallic acid equivalents (GA-eq) mg L−1, while REMPI at 266 nm resulted in RMSEP of 151 GA-eq mg L−1. Finally, the prediction of Colorette and FC value at 5 s time resolution were demonstrated with online measurements.

Effect of meta substitution of methyl group on 2-hydroxypyridine: Spectroscopic investigation

We have reported here the detailed investigation of the effect of methyl group substitution on the meta-position of the 2-hydroxypyridine molecule. Resonance enhanced multiphoton ionization (REMPI), FT-IR and Raman spectroscopic techniques have been used for the experimental study of the molecules. Ab initio calculations were used for theoretical investigations of the molecules. The origin band of the molecules 3-methyl-2-hydroxypyridine (3M2HP) and 5-methyl-2-hydroxypyridine (5M2HP) was observed at 33830 and 34105 cm–1 in their REMPI spectroscopy, and the bands assigned as a ππ* transition state. The vibronic coupling of nπ* and ππ* transition states took place in 3M2HP, thus some low intense bands near the origin band of the molecule were observed in the REMPI spectrum. However, there was no such kind of bands in 5M2HP. The π*–σ* hyperconjugation is responsible for the conformational change of the methyl group in 3M2HP upon excitation (S0 → S1).

Coherent microwave scattering from xenon resonance-enhanced multiphoton ionization-initiated plasma in air

Here we present the experimental and computational study of resonance-enhanced multiphoton ionization (REMPI) of xenon and subsequent avalanche ionization of air. Xenon was excited from the ground state to the excited 6p state (89162cm−1) by two photons at 224.3 nm. The third photon at 224.3 nm subsequently produced ionization of xenon in air. The seed electrons from the ionization served as the medium to further absorb the laser pulse for the rotational and vibrational excitation and avalanche ionization of O2 and N2. Plasma chemistry of O2 and N2 in air was included in the model. The results are useful for understanding REMPI-initiated plasma in air and possibly new diagnostics tools based on REMPI-initiated plasma emissions.

Photodissociation branching ratios of 12C16O from 108000 cm−1 to 113200 cm-1 measured by two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method: Observation of channels for producing O(1D)

The photoabsorption and photodissociation of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region is one of the most important photochemical processes in the interstellar medium, thus it has attracted numerous experimental and theoretical studies. Here, we employed the two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method to measure the relative branching ratios [C(3P0)+O(1D)]/ {[C(3P0)+O(3P)]+ [C(3P0)+O(1D)]} and [C(3P2)+O(1D)]/ {[C(3P2)+O(3P)]+[C(3P2)+O(1D)]} in the VUV photoexcitation energy range of 108000−113200 cm−1. Here, one tunable VUV laser beam is used to excite CO to specific rovibronic states, and a second independently tunable VUV laser beam is used to state-selectively ionize C(3P0) and C(3P2) for detection. State-selective photoionization through the 1VUV+1UV/visible resonance-enhanced multiphoton ionization scheme has greatly enhanced the detection sensitivity, which makes many new weak absorption bands observable in the current study. The branching ratio measurement shows that the spin-forbidden channels C(3P0)+O(1D) and C(3P2)+O(1D) only open at several discrete narrow energy windows. This might be caused by certain accidental resonance-enhanced spin-orbit interactions between the directly excited Rydberg states and valence states of triplet type which finally dissociate into the spin-forbidden channels.

Dynamic quantum state holography

We present a pulse-shaper-based holographic technique for the time-resolved and phase-sensitive observation of ultrafast quantum dynamics. The technique combines bichromatic white light polarization pulse shaping with the tomographic reconstruction of photoelectron wave packets. The physical scheme is based on the interference of a probe wave packet from $N+1$ resonance-enhanced multiphoton ionization via the target states and a reference wave packet from $M+1$ multiphoton ionization of the ground state. To create the wave packets, we employ carrier-envelope phase stable bichromatic ($M\ensuremath{\omega}$:$N\ensuremath{\omega}$) pump-probe pulse sequences. The scheme is demonstrated on femtosecond Rydberg wave-packet dynamics in potassium atoms using corotating circularly polarized ($2\ensuremath{\omega}$:$3\ensuremath{\omega}$) pulse sequences. The interference of continuum states with different angular momenta yields a crescent-shaped photoelectron wave packet rotating in the laser polarization plane due to the interplay of the optical phase and the accumulated quantum phase. Carrier-envelope phase control of the rotation provides access to the photoelectron asymmetry, enabling background-free detection of the crescent's angular motion which maps the bound-electron dynamics.

Femtochemistry under scrutiny: Clocking state-resolved channels in the photodissociation of CH3I in the A-band.

Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH3I in the A-band is re-examined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion (VMI) imaging as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D nonadiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A 104, 1009 (2000)]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product. In addition, high level ab initio electronic structure calculations have been carried out to describe the CH3 3pz Rydberg state involved in the (2 + 1) REMPI probing process, as a function of the carbon-iodine (C-I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the nonadiabatic crossing through the conical intersection present in the first absorption band. The vibrationally state resolved clocking times observed experimentally can be rationalized when the (2 + 1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e., multiphoton ionization photoelectron spectroscopy, soft X-ray inner-shell photoelectron spectroscopy, VUV single-photon ionization, and XUV core-to-valence transient absorption spectroscopy, have been able to provide quantum state-resolved (vibrational) clocking times. More experiments would be needed to disentangle the fine details in the clocking times and dissociation dynamics arising from the detection of specific quantum-states of the molecular fragments.

An experimental and computational study of the effect of aqueous solution on the multiphoton ionisation photoelectron spectrum of phenol.

We revisit the photoelectron spectroscopy of aqueous phenol in an effort to improve our understanding of the impact of inhomogeneous broadening and inelastic scattering on solution-phase photoelectron spectra. Following resonance-enhanced multiphoton ionisation via the 11ππ* and 11πσ* states of phenol, we observe 11ππ*-D0/D1 ionisation and competing direct S0-D0/D1 ionisation. Following resonance-enhanced multiphoton ionisation via the 21ππ* state, we observe the signature of solvated electrons. By comparing the photoelectron spectra of aqueous phenol with those of gas-phase phenol, we find that inelastic scattering results in peak shifts with similar values to those that have been observed in photoelectron spectra of solvated electrons, highlighting the need for a robust way of deconvoluting the effect of inelastic scattering from liquid-phase photoelectron spectra. We also present a computational strategy for calculating vertical ionisation energies using a quantum-mechanics/effective fragmentation potential (QM/EFP) approach, in which we find that optimising the configurations obtained from molecular dynamics simulations and using the [phenol·(H2O)5]QM[(H2O)n≥250]EFP (B3LYP/aug-cc-pvdz) method gives good agreement with experiment.

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.