Femtosecond Multiphoton Ionization Research Articles

Preparation of Tautomer-Pure Molecular Beams by Electrostatic Deflection.

Tautomers are ubiquitous throughout chemistry and typically considered inseparable in solution. Yet (bio)chemical activity is highly tautomer-specific, with common examples being the amino and nucleic acids. While tautomers exist in an equilibrium in solution, in the cold environment of a molecular beam the barrier to tautomerization is typically much too high for interconversion, and tautomers can be considered separate species. Here we demonstrate the first separation of tautomers within a molecular beam and the production of tautomerically pure gas-phase samples. We show this for the 2-pyridone/2-hydroxypyridine system, an important structural motif in both uracil and cytosine. Spatial separation of the tautomers is achieved via electrostatic deflection in strong inhomogeneous fields. We furthermore collect tautomer-resolved photoelectron spectra using femtosecond multiphoton ionization. This paves the way for studying the structure-function-dynamic relationship on the level of individual tautomers, using approaches that typically lack the resolution to do so, such as ultrafast dynamics experiments.

High-Throughput UV Photoionization and Fragmentation of Neutral Biomolecules as a Structural Fingerprint.

We present UV photofragmentation studies of the structural isomers paracetamol, 3-Pyridinepropionic acid (3-PPIA) and (R)-(-)-2-Phenylglycine. In particular, we utilized a new laser-based thermal desorption source in combination with femtosecond multiphoton ionization at 343 nm and 257 nm. The continuous nature of our molecule source, combined with the 50 kHz repetition rate of the laser, allowed us to perform these experiments at high throughput. In particular, we present detailed laser intensity dependence studies at both wavelengths, producing 2D mass spectra with highly differential information about the underlying fragmentation processes. We show that UV photofragmentation produces highly isomer-specific mass spectra, and assign all major fragmentation pathways observed. The intensity-dependence measurements, furthermore, allowed us to evaluate the appearance intensities for each fragmentation channel, which helped to distinguish competing from consecutive fragmentation pathways.

Vaporization of Intact Neutral Biomolecules Using Laser-Based Thermal Desorption.

The production of a clean neutral molecular sample is a crucial step in many gas-phase spectroscopy and reaction dynamics experiments investigating neutral species. Unfortunately, conventional methods based on heating cannot be used with most nonvolatile biomolecules due to their thermal instability. In this paper, we demonstrate the application of laser-based thermal desorption (LBTD) to produce neutral molecular plumes of biomolecules such as dipeptides and lipids. Specifically, we report mass spectra of glycylglycine, glycyl-l-alanine, and cholesterol obtained using LBTD vaporization, followed by soft femtosecond multiphoton ionization (fs-MPI) at 400 nm. For all molecules, the signal from the intact precursor ion was observed, highlighting the softness and applicability of the LBTD and fs-MPI approach. In more detail, cholesterol underwent hardly any fragmentation. Both dipeptides fragmented significantly, although mostly through only a single channel, which we attribute to the fs-MPI process.

Control of three-dimensional electron vortices from femtosecond multiphoton ionization

We report on the creation and manipulation of three-dimensional (3D) electron vortices from femtosecond multiphoton ionization of atoms. Vortex-shaped photoelectron momentum distributions arise from the superposition of two time-delayed free-electron wave packets with different magnetic quantum numbers. In the experiment, pairs of time-delayed counter-rotating circularly polarized (CRCP) femtosecond laser pulses, generated from a polarization-shaped supercontinuum source, are used to ionize potassium atoms. The resulting 3D electron vortices are reconstructed tomographically from a set of velocity map imaging measurements. By variation of the time delay, the helicity, and the spectral bandwidth of the CRCP pulse sequence, we control the radial vortex shape. Absorption of another photon in the continuum changes the ${c}_{6}$ azimuthal symmetry of the threshold vortex into ${c}_{8}$ for above-threshold ionization. Electron vortices from nonperturbative excitation show ${c}_{4}$ azimuthal symmetry and a $\ensuremath{\pi}$-phase jump in the polar direction. Determination of the relative phase of the superposition state allows us to reconstruct a free-electron wave function.

Measurement of the density profile of pure and seeded molecular beams by femtosecond ion imaging.

Here, we report on femtosecond ion imaging experiments to measure the density profile of a pulsed supersonic molecular beam. Ion images are measured for both a molecular beam and bulk gas under identical experimental conditions via femtosecond multiphoton ionization of Xe atoms. We report the density profile of the molecular beam, and the measured absolute density is compared with theoretical calculations of the centre line beam density. Subsequently, we discuss reasons accounting for the differences between measurements and calculations and propose that strong skimmer interference is the most probable cause for the differences. Furthermore, we report on experiments measuring the centre line density of seeded supersonic beams. The femtosecond ion images show that seeding the heavy Xe atom at low relative seed fractions (1%-10%) in a light carrier gas like Ne results in strong relative enhancements of up to two orders of magnitude.

Ultrafast Dynamics of Electrons in Ammonia

Solvated electrons were first discovered in solutions of metals in liquid ammonia. The physical and chemical properties of these species have been studied extensively for many decades using an arsenal of electrochemical, spectroscopic, and theoretical techniques. Yet, in contrast to their hydrated counterpart, the ultrafast dynamics of ammoniated electrons remained completely unexplored until quite recently. Femtosecond pump-probe spectroscopy on metal-ammonia solutions and femtosecond multiphoton ionization spectroscopy on the neat ammonia solvent have provided new insights into the optical properties and the reactivities of this fascinating species. This article reviews the nature of the optical transition, which gives the metal-ammonia solutions their characteristic blue appearance, in terms of ultrafast relaxation processes involving bound and continuum excited states. The recombination processes following the injection of an electron via photoionization of the solvent are discussed in the context of the electronic structure of the liquid and the anionic defect associated with the solvated electron.

Below-Band-Gap Ionization of Liquid-to-Supercritical Ammonia: Geminate Recombination via Proton-Coupled Back Electron Transfer

Femtosecond multiphoton ionization experiments have been conducted on ammonia over a wide range of temperature (225 K ≤ T ≤ 490 K) and density (0.18 g/cm(3) ≤ ρ ≤ 0.7 g/cm(3)), thereby covering the liquid and supercritical phases. The experiments were carried out with excitation pulses having a wavelength of 400 nm, and the ionization was found to involve two photons. Therefore, the total ionization energy in this study corresponds to 6.2 eV, which is roughly 2 eV below the valence-to-conduction band gap of the fluid. The ionization generates solvated electrons, which have been detected through their characteristic near-infrared resonance, and must be facilitated through a coupling to nuclear degrees of freedom of the liquid. The recombination of the solvated electron with the geminate fragments was found to obey predominantly single-exponential kinetics with time constants between 500 fs and 1 ps. Only a very minor fraction of the photogenerated electrons is able to escape from the geminate recombination. The results indicate that the majority of electrons are injected into suitable trapping sites located between the first and second solvation shells of the initially ionized ammonia molecules. Such configurations can be considered as instantly reactive and facilitate an ultrafast barrierless electron annihilation. This process is found to exhibit a pronounced kinetic isotope effect, which indicates that the electronic decay is accompanied by the transfer of a proton. The sequence of ionization and recombination events can therefore be described appropriately as a proton-coupled electron transfer (PCET) followed by a proton-coupled back electron transfer (PCBET).

Vibrational signatures of Watson–Crick base pairing in adenine–thymine mimics

The vibrational fingerprints of hydrogen-bonding associated with the adenine-thymine (A-T) Watson-Crick (WC) base pair have been identified in an infrared study of the A-T mimics 4-aminopyrimidine-1-methylthymine (4APM-1MT) and 4-aminopyrimidine-6-methyl-4-pyrimidinone (4APM-M4PMN) in the gas-phase. The IR vibrational spectra were measured via a double resonance scheme utilizing femtosecond multiphoton ionization. The changes in the molecular structure, anharmonic vibrational parameters, and the assignment of the observed vibrational spectra in the NH/CH stretch region were investigated by carrying out high-level theoretical calculations of the anharmonic spectra. The experimental observations and theoretical calculations indicate that the hydrogen bonds associated with WC base-pairing are relatively stronger than those associated with reverse WC (rWC) base pairing. This is manifested in a more pronounced red-shift of the H-bonded vibrational modes associated with the WC as compared with the rWC base-pairing. An analysis of the factors contributing to the anharmonicity of the vibrational modes associated with H-bonding reveals that the magnitude of the off-diagonal anharmonic coupling of the H-bonded -NH2 stretch and the -NH2 bend is much smaller in WC base-pairing than in the corresponding rWC base-pairing. The chemical and biological implications of these results, especially in the context of using vibrational spectroscopy as a tool for identifying the signatures of nucleotide base vibrations is addressed.

Femtosecond Multiphoton Ionization of Pyrrole

The photoionization dynamics of pyrrole are investigated by using a photoelectron imaging method and a tunable femtosecond laser. Two-photon nonresonant ionization experiments in the wavelength range from 261 to 298 nm indicate that the cation and neutral ground states have similar structures. The main vibrational excitation in the cation ground state is the v(8) mode. Two-photon absorption at 406 nm projects neutral pyrrole into a mixed state comprising the 1B(2) valence and 3p Rydberg states. Ionization from this mixed state mainly results in the overtone excitation of vibrational mode v(8) and v(9) of the cation state. In the wavelength range from 336 to 364 nm, a mixed state comprising the 3d/4s Rydberg and the 4A(1) valence states are populated by the absorption of two photons through vibronic coupling. The partition ratio among these states varies with the excitation wavelength, resulting in dramatic changes in both kinetic energy distributions and angular distributions. As the laser wavelength becomes shorter, from 336 to 314 nm, higher excited states, 3B(2), 5A(1), 6A(1), 7B(1) and 4B(2), can be populated. Photoelectron angular distributions provide supplementary verification of assignments. Our experiments indicate that femtosecond multiphoton ionization and photoelectron imaging methods are powerful tools for investigating short-lived intermediated excited states, which cannot be detected in nanosecond experiments.

4-Aminobenzimidazole–1-Methylthymine: A Model for Investigating Hoogsteen Base-Pairing between Adenine and Thymine

We report the infrared spectrum of the 4-aminobenzimidazole-1-methylthymine (4ABI:1MT) heterodimer, detected by femtosecond multiphoton ionization. Based on calculations of both the harmonic and the anharmonic frequencies, the observed vibrational spectrum is assigned to a structure that mimics the Hoogsteen base pairing of adenine and thymine. A notable observation made in the course of this study is that there is a significant imbalance in the observed strengths of the H-bonds. While the N···H-N bond reveals a large red shift of >700 cm(-1) for the NH stretch frequency, the N-H···O bond is characterized by only a 50 cm(-1) shift. The importance of this observation in the formation of Hoogsteen duplexes by thymine-based oligonucleotides is discussed.

The structure of adenine monohydrates studied by femtosecond multiphoton ionization detected IR spectroscopy and quantum chemical calculations

We present femtosecond multiphoton ionization detected infrared spectra of jet-cooled monohydrates of adenine and 9-methyladenine. By quantum chemical vibrational analysis and comparison with available literature data we identified two isomers of adenine hydrate with one water molecule hydrogen-bonded to either the amino or the N9-H group. These two monohydrates revealed different fragmentation patterns in the ion depletion spectra, indicating isomer specific intermolecular dynamics. This different behaviour is discussed in terms of competing electronically excited state relaxation and dissociation processes.

Imaging transient species in the femtosecond A-band photodissociation of CH3I

A nonresonant femtosecond laser pulse centered at 802 nm is used to probe the real time photodissociation dynamics of CH(3)I in the A-band at 267 nm. Using multiphoton ionization with this probe laser pulse and velocity map ion imaging of CH(3)(+), we have followed the time evolution of the translational energy and spatial anisotropy of the CH(3) fragment, which in turn has permitted to image the C-I bond breaking from the initial Franck-Condon region up to the final products along the reaction coordinate. Given the temporal width of our pump and probe laser pulses (approximately 80 fs), a mechanism is proposed by which transient species are probed by simultaneous absorption of pump and probe laser pulses through intermediate Rydberg and ionic states of CH(3)I while the pump and probe pulses overlap in time. This study shows how the combination of femtosecond multiphoton ionization and ion imaging techniques provides an ideal tool to resolve in time the different stages of the bond breaking event in a polyatomic molecule.

Excited states of thiophene: ring opening as deactivation mechanism

(Time-dependent) Kohn-Sham density functional theory and a combined density functional/multi-reference configuration interaction method (DFT/MRCI) were employed to explore the ground and low-lying electronically excited states of thiophene. Spin-orbit coupling was taken into account using an efficient, nonempirical mean-field Hamiltonian. Phosphorescence lifetimes were calculated by means of spock.ci, a selecting direct multi-reference spin-orbit configuration interaction program. Throughout this paper, we use the following nomenclature: S1, S2,..., T1, T2,..., denominate electronic structures in their energetic order at the ground state minimum geometry, whereas S1, S2,..., T1, T2,..., refers to the actual order of electronic states at a given nuclear geometry. Multiple minima were found on the first excited singlet (S1) potential energy hypersurface with electronic structures S1 (piHOMO-1-->pi+piHOMO-->pi), S2 (piHOMO-->pi), and S3 (piHOMO-->sigma*) corresponding to the 2 1A1 (S1), 1 1B2 (S2), and 1 1B1 (S3) states in the vertical absorption spectrum, respectively. The S1 and S2 minimum geometries show out-of-plane deformations of the ring. The S3 electronic structure yields the global minimum on the S1 surface with an adiabatic excitation energy of merely 3.81 eV. It exhibits an asymmetric planar nuclear arrangement with one significantly elongated C-S bond. A constrained minimum energy path calculation connecting the S1 and S3 minima suggests that even low-lying vibrational levels of the S1 potential well can access the global minimum of the S1 surface. Nonradiative decay of the electronically excited singlet population to the electronic ground state via a close-by conical intersection will be fast. According to our work, this ring opening mechanism is most likely responsible for the lack of fluorescence in thiophene and the ultrafast decay of the S1 vibrational levels, as observed in time-resolved pump-probe femtosecond multiphoton ionization experiments. An alternative relaxation pathway leads from the S1 minimum via vibronic coupling to the S2 potential well followed by fast inter-system crossing to the T2 state. For an estimate of individual rate constants a quantum dynamical treatment will be required. The global minimum of the T1 surface has a chair-like nuclear conformation and corresponds to the T1 (1 3B2, piHOMO-->pi) electronic structure. Phosphorescence is weak here with a calculated radiative lifetime of 0.59 s. For the second potential well on the T1 surface with T3 (1 3B1, piHOMO-->sigma*) electronic structure, nonradiative processes are predicted to dominate the triplet decay.

Formation, detection, and stability studies of neutral vanadium sulfide clusters

Neutral vanadium sulfide clusters are generated by the reaction of seeded hydrogen sulfide in a helium carrier gas with laser ablated vanadium metal within a supersonic nozzle. The exiting clusters are expanded into a vacuum in a molecular beam and are ionized by both ultraviolet (UV) and vacuum UV (VUV) laser radiation. The generated ions are detected by a time of flight mass spectrometer. With single photon ionization (SPI) employing VUV (118 nm) radiation, sulfur rich clusters (V(m)S(n), n>m+1) and hydrogen containing clusters (V(m)S(n)H(x), x>0) are observed. With multiphoton ionization (MPI) through nanosecond UV (193 nm) radiation, these sulfur rich and hydrogen containing clusters cannot be observed, indicating severe fragmentation generated by MPI and the importance of SPI in determining the neutral vanadium sulfide cluster distribution. With MPI through femtosecond UV (226 nm) radiation, a few sulfur rich and hydrogen containing clusters are detected, but most clusters observed by SPI are still undetected even by femtosecond MPI. Density functional theory calculations are applied to optimize energies and structures of the clusters with m=1-3 and n=0-7. The experimental results are well interpreted based on the calculations. The calculated and experimental results for vanadium sulfides are compared with those of vanadium oxides in literature.

Fluorescence quenching in cyclic hydrogen-bonded complexes of 1H-pyrrolo[3,2-h]quinoline with methanol: cluster size effect

Laser induced fluorescence (LIF) excitation spectra of molecular complexes of 1H-pyrrolo[3,2-h]quinoline (PQ) with methanol (n= 1, 2, 3) as well as vibrational spectra of their electronic ground state are reported. The latter have been recorded in the mid-infrared region by fluorescence depletion (FDIR). The only PQ.methanol(n) complex clearly identified in the LIF spectrum is the triply hydrogen-bonded cyclic 1 ratio 2 aggregate. Its stoichiometry has been proven by the femtosecond multiphoton ionization detected infrared measurements [J. Am. Chem. Soc., 2006, 128, 10 000]. The structure of the 1 ratio 2 cluster is determined by means of FDIR spectroscopy in combination with ab initio and DFT calculations. No fluorescence was detected that could be attributed to the 1 ratio 1 cluster. This behaviour of the 1 ratio 1 complex is explained in terms of rapid excited state double proton transfer followed by a non-radiative relaxation. The n= 3 and heavier clusters are fluorescent. Their electronic spectra overlap, preventing the selective measurement of the FDIR spectra of individual complexes.

Detection and Structural Characterization of Clusters with Ultrashort-Lived Electronically Excited States: IR Absorption Detected by Femtosecond Multiphoton Ionization

The vibrational fingerprint of the electronically excited short-lived complex of 1-H-pyrrolo[3,2-h]quinoline:methanol was measured using femtosecond multiphoton ionization detected infrared (IR/fsMPI) spectroscopy under supersonic jet conditions. A cyclic doubly hydrogen-bonded structure of the cluster has been proven from the comparison of the measured vibrational spectrum with that calculated with density functional theory. The employed nsIR-fsUV double resonance scheme is shown to be an effective tool for structural analysis of precursors that undergo fast deactivation and/or photoreactions.

Solvation Dynamics of the Hydrated Electron Depends on Its Initial Degree of Electron Delocalization

We investigate the time scale for hydrated electron solvation as a function of the initial configuration of the electron/water system. The experiments employ various 2-pulse and 3-pulse femtosecond pulse sequences that allow for controllable preparation of the various optically excited and precursor states of the equilibrated hydrated electron. We observe that the conduction band electron, which is produced by detrapping of the hydrated electron, has the slowest time scale for electron solvation with an average solvation time constant of 400 fs. In contrast, the solvation dynamics are significantly faster for electrons that are produced in “pre-solvated” environments. These latter examples include the excited p-state of the hydrated electron and the precursor states involved in UV femtosecond multiphoton ionization of water.

Femtosecond multiphoton ionization of free and aggregated NaI via Rydberg states

Using a wave packet simulation on simplified potential curves, we investigate the vibrational dynamics of NaI in its first excited electronic state, both isolated and aggregated with a molecule of acetonitrile. The probe signal is obtained by resonant multiphoton ionization (REMPI) of the diatomic chromophore NaI via Rydberg states. The addition of acetonitrile is found to change greatly the ionization signal, suggesting strong modification of the reaction dynamics.

Dissociation pathways of sulfur dioxide clusters and mixed sulfur dioxide-water clusters

A reflectron-time-of-flight mass spectrometer (RTOFMS) coupled with a femtosecond multiphoton ionization technique is used to study the combined metastable and collision-induced dissociation of sulfur dioxide clusters and mixed sulfur dioxide-water clusters. From the dissociation patterns, the general structural arrangement of the clusters is clearly identified: (SO 2) m ·+ clusters have a SO 2 ·+ core solvated by SO 2 molecules; [SO(SO 2) n ] ·+ and [S(SO 2) n+1 ] ·+ have a structure of SO ·+ · (SO 2) n and (SO) 2 ·+ · (SO 2) n , respectively. For [HSO 2(SO 2)] + clusters, the ion core is HSO 2 +. H 3SO 3 + appears to have two isomeric structures: one HSO 2 + · H 2O, the other H 3O + · SO 2.

Lifetime of the .nu.2' = 1A~ State of ND3 Measured by Two-Color Deep-UV Femtosecond Multiphoton Ionization Spectroscopy

Lifetime of the .nu.2' = 1A~ State of ND3 Measured by Two-Color Deep-UV Femtosecond Multiphoton Ionization Spectroscopy