Resonance-enhanced Two-photon Ionization Research Articles

Conformer-Selective Photoelectron Circular Dichroism.

Conformational flexibility and chirality both play a key role in molecular recognition. It is therefore very useful to develop spectroscopic methods that simultaneously probe both properties. It has been theoretically predicted that photoelectron circular dichroism (PECD) should be very sensitive to conformational isomerism. However, experimental proof has been less forthcoming and only exists for a very few favorable cases. Here, we present a new PECD scheme based on resonance-enhanced two-photon ionization (RE2PI) using UV/Vis nanosecond laser excitations. The spectral resolution obtained thereby guarantees conformer-selectivity by inducing resonant conformer-specific ππ* S1←S0 transitions. We apply this experimental scheme to the study of chiral 1-indanol, which exists in two conformers linked by a ring inversion and defined by the position of the hydroxyl group, namely axial and equatorial. We show that the PECD of the equatorial and axial forms considerably differ in sign, magnitude and shape. We also discuss the influence of the total ionization energy, vibronic excitation of intermediate and final states, and relative polarization of the excitation and ionization lasers. Conformer-specificity adds a new dimension to the applications of PECD in analytical chemistry addressing now the general case of floppy systems.

Torsional Wave-Packet Dynamics in 2-Fluorobiphenyl Investigated by State-Selective Ionization-Detected Impulsive Stimulated Raman Spectroscopy.

We report the creation and observation of vibrational wave packets pertinent to torsional motion in a biphenyl derivative in its electronic ground-state manifold. Adiabatically cooled molecular samples of 2-fluorobiphenyl were irradiated by intense nonresonant ultrashort laser pulses to drive impulsive stimulated Raman excitation of torsional motion. Spectral change due to the nonadiabatic vibrational excitation is probed in a state-selective manner using resonance-enhanced two-photon ionization through the S1 ← S0 electronic transition. The coherent nature of the excitation was exemplified by adopting irradiation with a pair of pump pulses: observed signals for excited torsional levels exhibit oscillatory variations against the mutual delay between the pump pulses due to wave-packet interference. By taking the Fourier transform of the time course of the signals, energy intervals among torsional levels with v = 0-3 were determined and utilized to calibrate a density functional theory (DFT)-calculated torsional potential-energy function. Time variation of populations in the excited torsional levels was assessed experimentally by measuring integrated intensities of the corresponding transitions while scanning the delay. Early time enhancement of the population (up to ∼2 ps) and gradual degradation of coherence (within ∼20 ps) appears. To explain the observed distinctive features, we developed a four-dimensional (4D) dynamical calculation in which one-dimensional (1D) quantum-mechanical propagation of the torsional motion was followed by solving the time-dependent Schrödinger equation, whereas three-dimensional (3D) molecular rotation was tracked by classical trajectory calculations. This hybrid approach enabled us to reproduce experimental results at a reasonable computational cost and provided a deeper insight into rotational effects on vibrational wave-packet dynamics.

Ionization energies and ionization-induced structural changes in 2-phenylethylamine and its monohydrate.

We report the resonance-enhanced two-photon ionization combined with various detection approaches and quantum chemical calculations of biologically relevant neurotransmitter prototypes, the most stable conformer of 2-phenylethylamine (PEA), and its monohydrate, PEA-H2O, to reveal the possible interactions between the phenyl ring and amino group in the neutral and ionic species. Extracting the ionization energies (IEs) and appearance energy was achieved by measuring the photoionization and photodissociation efficiency curves of the PEA parent and photofragment ions, together with velocity and kinetic energy-broadened spatial map images of photoelectrons. We obtained coinciding upper bounds for the IEs for PEA and PEA-H2O of 8.63 ± 0.03 and 8.62 ± 0.04eV, within the range predicted by quantum calculations. The computed electrostatic potential maps show charge separation, corresponding to a negative charge on phenyl and a positive charge on the ethylamino side chain in the neutral PEA and its monohydrate; in the cations, the charge distributions naturally become positive. The significant changes in geometries upon ionization include switching of the amino group orientation from pyramidal to nearly planar in the monomer but not in the monohydrate, lengthening of the N-H⋯π hydrogen bond (HB) in both species, Cα-Cβ bond in the side chain of the PEA+ monomer, and the intermolecular O-H⋯N HB in PEA-H2O cations, leading to distinct exit channels.

Resonance-Enhanced Multiphoton Ionization Studies of the Lower Electronically Excited States of Flavone.

The spectroscopic and dynamics properties of flavone─the core chromophore of a wide variety of naturally occurring ultraviolet protecting filters─have been studied under supersonic beam conditions using (1 + 1') resonance-enhanced two-photon ionization spectroscopic techniques. Excitation spectra recorded under such conditions are found to differ significantly from previously reported spectra. Pump-probe studies find that intersystem crossing is the dominant decay pathway of the excited singlet manifold, in agreement with previous solution phase studies and quantum chemical predictions for the isolated molecule. Microsolvation studies on flavone-water clusters reveal that the addition of one and two water molecules leads to considerable shifts in excitation energies but that further complexation does not result in further noticeable shifts. The relaxation pathways of the electronically excited states, on the other hand, do not appear to be influenced by interactions with the solvent molecules. Finally, photoionization spectra have enabled the accurate determination of the adiabatic ionization energy to the ground state of the molecular ion─key to the antioxidant properties of flavone─as 65,415 cm-1 (8.110 eV).

Signatures of the Bromine Atom and Open-Shell Spin Coupling in the X-ray Spectrum of the Bromobenzene Cation

Tabletop X-ray spectroscopy measurements at the carbon K-edge complemented by ab initio calculations are used to investigate the influence of the bromine atom on the carbon core-valence transitions in the bromobenzene cation (BrBz+). The electronic ground state of the cation is prepared by resonance-enhanced two-photon ionization of neutral bromobenzene (BrBz) and probed by X-rays produced by high-harmonic generation (HHG). Replacing one of the hydrogen atoms in benzene with a bromine atom shifts the transition from the 1sC* orbital of the carbon atom (C*) bonded to bromine by ∼1 eV to higher energy in the X-ray spectrum compared to the other carbon atoms (C). Moreover, in BrBz+, the X-ray spectrum is dominated by two relatively intense transitions, 1sC→π* and 1sC*→σ*(C*-Br), where the second transition is enhanced relative to the neutral BrBz. In addition, a doublet peak shape for these two transitions is observed in the experiment. The 1sC→π* doublet peak shape arises due to the spin coupling of the unpaired electron in the partially vacant π orbital (from ionization) with the two other unpaired electrons resulting from the transition from the 1sC core orbital to the fully vacant π* orbitals. The 1sC*→σ* doublet peak shape results from several transitions involving σ* and vibrational C*-Br mode activations following the UV ionization, which demonstrates the impact of the C*-Br bond length on the core-valence transition as well as on the relaxation geometry of BrBz+.

Experimental/Computational Study on the Impact of Fluorine on the Structure and Noncovalent Interactions in the Monohydrated Cluster of ortho-Fluorinated 2-Phenylethylamine.

Fluorine-containing medicinal compounds frequently allow modulation of physical-chemical properties. Here, we address the effect of fluorine, near the ethylamino side chain, on conformational flexibility and noncovalent interactions (NCIs) of the selected jet-cooled monohydrated cluster of 2-(2-fluoro-phenyl)-ethylamine (2-FPEA) by mass-selected resonance-enhanced two-photon ionization and ionization-loss stimulated Raman spectroscopies. Our results show that Raman spectral signatures of the 2-FPEA-H2O cluster match the scaled harmonic vibrational Raman frequencies, resulting from density functional theory calculations of the most stable 2-FPEA gauche conformer hydrogen-bonded (HB) to water, confirming the three-dimensional cluster structure. This predicted electronic structure, together with NCI analysis, allows visualization and assessment of the attractive and repulsive interactions. The comparison of the NCIs and revealed red (O-H and N-H stretches) and blue shifts (C-H stretches and CH2 out-of-plane bends) of the cluster to other class members confirm O-H···N, N-H···π, C-H···O, and C-H···F HB formation and their contribution to structure stabilization, uncovering the potential of the approach.

Jet-cooled laser spectroscopy and solid-state vibrational circular dichroism of the cyclo-(Tyr-Phe) diketopiperazine dipeptide

The structures of the diastereomer diketopiperazine dipeptides cyclo(LTyr-LPhe) and cyclo-(DTyr-LPhe) are studied in the gas phase using resonance-enhanced two-photon ionization, IR/UV double resonance spectroscopy and quantum chemical calculation. Both diastereomers exhibit two conformations, with the tyrosine ring folded over the DKP ring and the phenylalanine ring extended outwards, or vice versa. The two diastereomers differ only slightly by the nature of weak CH···π and NH···π interactions. The crystal structure of cyclo(LTyr-LPhe) is determined by X-ray crystallography and is composed of monomers with folded tyrosine ring. The vibrational circular dichroism spectrum is interpreted by the existence of dimers in the solid state. Quantum chemical calculation shed light on the structural modifications between the gas phase and the solid state.

Photoionization Spectroscopic and Theoretical Study on the Molecular Structures of cis- and trans-3-Chlorothioanisole.

Resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra are measured for the cis- and trans-3-chlorothioanisole (3ClTA). The first electronic excitation energy (E1) and the adiabatic ionization energy (IE) of the cis-rotamer are determined to be 33 959±3 and 65 326±5 cm–1, respectively, and those of the trans-rotamer are determined to be 34102±3 and 65 471±5 cm–1, respectively. Density functional theory (DFT) calculations confirm that both the cis- and trans-rotamers of 3ClTA are stable and coexist in their respective S0, S1, and D0 states. Both rotamers adopt planar structures with cis- being slightly more stable than trans- in the respective S0, S1, and D0 states. The conformation, substitution, and isotope effects on the molecular structure, active vibrations, and electronic transition and ionization energies of 3ClTA are analyzed.

Esterification of perfluorinated carboxylic acids with bromomethyl aromatic compounds for gas chromatography combined with laser ionization mass spectrometry

Perfluorinated carboxylic acids (PFCAs) were derivatized with two types of aromatic compounds that contained a bromomethyl group, i.e., 2-(bromomethyl)naphthalene (BMN) and benzyl bromide (BB). The conditions for derivatization were optimized in terms of reaction temperature and time and the concentration of derivatizing reagent. Using these optimal conditions, the PFCAs-MN and PFCAs-B derivatives were measured by gas chromatography (GC) combined with mass spectrometry using an ultraviolet femtosecond laser (267 nm) as the ionization source. The efficiency of derivatization for PFCAs-B was higher than that for PFCAs-MN because of the smaller size of the chromophore (benzene). The ionization efficiency of PFCAs-MN, however, was better than PFCAs-B, since a larger sized chromophore (naphthalene) and then a larger molar absorptivity was preferable for resonance-enhanced two-photon ionization. Due to superior GC separation, BB was successfully used as the derivatizing agent for the trace analysis of PFCAs, with detection limits of 6.0, 8.4, and 9.5 ng/mL for perfluoroheptanoic, perfluorooctanoic, and perfluorononanoic acids, respectively. The other bromomethyl aromatic compounds were evaluated for use as a derivatization reagent in future studies.

A compact and cost-effective laser desorption source for molecular beam generation: comparison with simulations

The development of laser-desorption jet-cooling has enabled the study of low vapor pressure molecules in the gas phase. Here, we design and assemble a unique compact laser-desorption source comprising simple parts for sample movement and desorption and attach it to a home-built time-of-flight mass spectrometer for spectroscopic studies of jet-cooled biomolecules. Modeling and visualization of the molecular beam (MB) by the direct simulation Monte Carlo method provides spatial evolution and global features of the flow and new insights into source operation. The system is validated by determining operational parameters from the simulation and comparing them with the resulting resonance-enhanced two-photon ionization (R2PI) signal of tryptamine. The signal measurements in the origin band transitions region provide an R2PI spectrum resembling that previously acquired from heated samples, indicating seven conformers in the MB. Due to the potential of this source, we expect that it will reveal its feasibility through coupling to other systems and various spectroscopies.

Resonance enhanced two-photon ionization spectrum of ultracold 85Rb133Cs molecules in [formula omitted] transitions

We present resonance enhanced two-photon ionization (RETPI) spectrum between 14,500 and 15850 cm−1 for ultracold ground state 85Rb133Cs molecules. With an assistant of optical pumping from one 1070nm laser, (2)1Π1←X1Σ+ electronic transition is distinguished from (4)3Σ+←a3Σ+ and (3)3Π←a3Σ+ transitions. Some observed RETPI spectra are globally assigned to vibrational transitions from X1Σ+(v=0−5) to (2)1Π1(v=5−20). Based on these assignments, the spectroscopic constants of X1Σ+ and (2)1Π1 are simultaneously derived, including energy separation, harmonic and anharmonic constants. Then a map of Franck-Condon factors between vibrational transitions of X1Σ+(v=0−9) and (2)1Π1(v=0−20) is plotted based on these constants. Our present work would be meaningful for continuous accumulation of ultracold 85Rb133Cs molecules in the lowest vibronic state with further optical pumping.

Resonance-enhanced two-photon ionization of hydrogen atom in intense laser field investigated by Bohmian-mechanics**Project supported by Jilin Province Science and Technology Development Plan Project–Excellent Youth Talents Fund Project, China (Grant No. 20180520174JH) and the National Natural Science Foundation of China (Grant Nos. 11704145 11904050, 11774129, 11747007, and 11534004).

Resonance enhanced two-photon ionization process of hydrogen atom via the resonant laser pulse is studied by Bohmian mechanics (BM) method. By analyzing the trajectories and energies of Bohmian particles (BPs), we find that under the action of high frequency and low intensity multi-circle resonant laser pulses, the ionized BPs first absorb one photon completing the excitation, and then absorb another photon, completing the ionization after staying in the first excited state for a period of time. The analysis of work done by the forces shows that the electric field force and quantum force play a major role in the whole ionization process. At the excitation moment and in the excitation-ionization process, the effect of the quantum force is greater than that of the electric field force. Finally, we discuss the principle of work and energy for BPs, and find that the electric field force and quantum force are non-conservative forces whose work is equal to the increment of mechanical energy of the system. In addition, it is proved that the quantum potential energy actually comes from the kinetic energy of the system and the increment of kinetic energy is equal to that of the kinetic energy of the system.

Resonant and non-resonant femtosecond ionization mass spectrometry of organochlorine pesticides.

Thirteen organochlorine pesticides in a standard sample mixture were measured by gas chromatography combined with mass spectrometry using an ultraviolet femtosecond laser (267 nm) as the ionization source, and the observed mass spectra were compared with the corresponding spectra obtained using an electron ionization source. When an ultrashort optical pulse was used for ionization, molecular ions were typically produced which was preferential for reliably identifying the analytes. The ionization mechanism was studied based on three models constructed for resonance-enhanced two-photon ionization, non-resonant two-photon ionization, and non-resonant three-photon ionization. The optimal conditions for observing a molecular ion were investigated using data obtained for three pulse widths. The results suggest that two-photon ionization with minimum excess energy would be optimal for observing a molecular ion.

Formation of Al + (C 6 H 6 ) 13 : The Origin of Magic Number in Metal–Benzene Clusters Determined by the Nature of the Core

The identification of highly abundant, “magic” species in the mass spectra of clusters have proven to be valuable in nanoscience, leading to the discovery of new stable species such as fullerenes a...

R2PI and MATI spectroscopy of 3-fluoro-5-methylanisole: Combined effect of meta-substituents on molecular conformation.

The resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra of 3-fluoro-5-methylanisole (3F5MA) were recorded to explore the conformers arising from the rotation of meta-methyl group (m-CH3) and methoxy group (OCH3), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S0 and S1 states, but eclipsed in the D0 state. While for trans 3F5MA, the staggered conformer is stable only in the S1 state and the eclipsed one is stable in the S0 and D0 states. The first electronic excitation energies (E1s) of cis and trans 3F5MA were determined to be 36,709 ± 3 and 36,615 ± 3 cm-1 by the R2PI spectroscopy. Correspondingly, by the MATI spectroscopy, the adiabatic ionization energies (IEs) were measured to be 66,908 ± 5 and 66,692 ± 5 cm-1. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH3 torsions are observed for the trans conformer in the R2PI spectrum, supporting the theoretically predicated e-trans → s-trans isomerization upon excitation. The MATI spectra of the cis and trans conformers are similar. Most of the observed cationic bands are related to the m-CH3 torsion modes, revealing the s-cis → e-cis and s-trans → e-trans isomerizations upon the D0 ← S1 ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH3 group instead of the F atom. The combined effects of meta-substituents on molecular conformation and transition energies are discussed in detail. A kind of additivity of meta-substituent effects on s/e preference is qualitatively true for the S0 and S1 states but false for the D0 state. This is different from that of ortho-substituent effect, which is previously reported that the additivity of ortho-substituent effects on methyl rotation barriers in 2-fluoro-6-chlorotoluene is applicable to all the three electronic states while the additivity of meta-substituent effect on cis/trans preference is found to be true in all three electronic states of 3F5MA.

Determination of nerve agent metabolites in human urine by femtosecond laser ionization mass spectrometry using 2-(bromomethyl)naphthalene as a derivatizing reagent

Nerve agent metabolites (NAMs) derived from alkyl methyl phosphonic acids, such as ethyl methylphosphonic acid (EMPA), isopropyl methylphosphonic acid (IMPA), and pinacolyl methylphosphonic acid (PMPA), were extracted from human urine using diethyl ether as an extractant. After exchanging the diethyl ether solvent to acetonitrile, the analytes were derivatized with 2-(bromomethyl)naphthalene (BMN). The reaction products of the BMN and NAMs, i.e., MN-EMPA, MN-IMPA, and MN-PMPA, were separated by gas chromatography (GC) and measured by mass spectrometry (MS) using a femtosecond laser emitting at 267 nm as the ionization source for resonance-enhanced two-photon ionization (RE2PI). The limits of detection (LOD) were <1 ng/mL for these analytes. The use of BMN increased the volatility of the analytes for separation by GC and also increased the ionization efficiency via the RE2PI process as the result of presence of a naphthalene functional group. A two-dimensional GC-MS display can be used for comprehensive analysis of NAMs, by-products, and impurities in the sample. Then, this approach could be used to confirm the use of chemical weapons and for forensic identification.

Sub-Doppler electronic spectrum of the benzene-D2 complex.

Excitation spectrum of the benzene-D2 van der Waals complex in the vicinity of the S 1 ← S 0 60 1 vibronic transition of the monomer was recorded with sub-Doppler resolution by utilizing mass-selective two-color resonance-enhanced two-photon ionization. Contrary to the previous report on the benzene-H2 complex [M. Hayashi and Y. Ohshima, J. Phys. Chem. A 117, 9819 (2013)], both spin isomers correlating to para and ortho D2 (with rotational angular momentum j = 1 and 0, respectively) are identified by using a gas sample of normal D2. Three and two vibronic bands involving vdW-mode excitation were observed for the para and ortho species, respectively, in addition to their origin bands. Comparison of the results for the two spin isomers has allowed us to make unambiguous band assignments, and vibrational frequencies of all the three vdW modes have been determined for benzene-H2 and -D2. Among the three modes, the two-dimensional vdW twist is correlated to the hindered internal rotation of H2/D2 and the barrier for the internal rotation has been evaluated: 72 and 66 cm-1 for benzene-H2 and -D2, respectively. Vibronic-state dependence of the intermolecular distance between benzene and H2/D2 is discussed on the basis of precisely determined rotational constants. Homogenous line broadening has been identified for all the observed vibronic bands, and the corresponding upper-state lifetimes are determined to be in the range of 0.3-0.7 ns.

Analysis of the S1 ← S0 and D0 ← S1 spectra in m-bromofluorobenzene via resonance-enhanced multiphoton ionization and slow electron velocity-map imaging spectroscopy

Vibrations of m-bromofluorobenzene (m-BrFPh) in the first excited state (S1) and the cationic ground state (D0) are investigated by resonance-enhanced multiphoton ionization (REMPI) spectroscopy and slow electron velocity-map imaging (SEVI) spectroscopy, respectively. Quantum chemical calculations have been adopted to assign the observed vibrational bands, as well. The band origin of the S1 ← S0 electronic transition defines the adiabatic excitation energy to be 36,987 ± 4 cm−1 based on the resonance-enhanced one-color two-photon ionization spectrum of m-BrFPh. And adiabatic ionization potential (IP) of 73,903 ± 8 cm−1 is determined from band origin in the SEVI spectra. Meanwhile, the computed frequencies are in excellent agreement with the experimental observations and Franck-Condon simulations are performed to aid us to confidently assign major vibrational modes in S1 and D0 states. And the changes in the wavenumber of the vibrational modes upon S1 ← S0 and D0 ← S1 transitions are examined. Furthermore, the mixing of vibrational modes both between S0 & S1 and between S1 & D0 has been discussed.

Conformational preferences and isomerization upon excitation/ionization of 2-methoxypyridine and 2-N-methylaminopyridine.

Conformers from the rotations of the methyl group and the methoxy or methylamino group, namely staggered (s)/eclipsed (e)-cis/trans 2-methoxypyridine (2MOP) and 2-N-methylaminopyridine (2NMP), are studied using theoretical calculations in combination with one-color resonance-enhanced two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopies. The calculations predict that, for cis 2MOP, trans 2MOP and trans 2NMP, only the s conformers are stable in the S0, S1 and D0 states. However, for cis 2NMP, the stable conformer is staggered in the S0 state but eclipsed in the S1 and D0 states, indicating an isomerization upon the excitation or ionization from the S0 state. This is experimentally supported by the 1C-R2PI and MATI spectra of 2NMP. Due to the relative instability, the number density of trans 2MOP is too low in the sample to be detected. All the bands in the 1C-R2PI and MATI spectra of 2MOP are assigned to s-cis 2MOP. The energy differences between cis and trans conformers are derived from excitation and ionization energies, indicating another conformational isomerization: stable trans 2NMP in the S0 and S1 states but stable cis 2NMP in the D0 state. For 2MOP, the so-called syn preference previously found for the S0 state is also observed in the S1 and D0 states. The conformational preference and isomerization are discussed with natural bond orbital calculations and reduced density gradient analysis. For 2MOP, the syn preferences are mainly caused by the exchange repulsion among several σ-orbitals of the OCH3 group and the pyridine ring. While the relative stabilities of the s and e conformers of cis 2MOP and cis 2NMP are simultaneously influenced by steric repulsion and orbital interactions.

Dynamic interference in the resonance-enhanced multiphoton ionization of hydrogen atoms by short and intense laser pulses

Photoionization of the hydrogen atom by intense and short coherent laser pulses is investigated from first principles by a numerical solution of the time-dependent Schrödinger equation in the dipole-velocity gauge. The considered photon energies are resonant to the 1s→2p excitation, and the pulse intensities are high enough to induce Rabi floppings. The computed resonance-enhanced two-photon ionization spectra as well as the three-photon above threshold ionization spectra exhibit pronounced multiple-peak patterns due to dynamic interference. Fingerprints of dynamic interference can also be seen directly in the radial density of the photoelectron. The impact of the variation of the pulse intensity and photon energy on the dynamic interference is investigated, and the angular distribution of the emitted electrons is analyzed in some details. The present precise numerical results confirm our previous theoretical predictions on the two-photon ionization spectra (Demekhin and Cederbaum, 2012) made within a minimal few-level model.