Photoionization Pathways Research Articles

Photoabsorption of Microhydrated Naphthalene and Its Cyano-Substituted Derivatives: Probing Prereactive Models for Photodissociation in Molecular Clouds.

We investigate the photoionization pathways of naphthalene, 1-cyanonaphthalene, and 2-cyanonaphthalene upon complexation with the water dimer, aiming to understand the photodissociation process under conditions of the interstellar medium (ISM). We analyze the intermolecular bonding pattern, equilibrium rotational properties, energy complexation, far-IR spectra, and ionic trends of the possible photoproducts using dispersion-corrected density functional theory (DFT-D) and time-dependent DFT (TD-DFT). For the different configurations, we evaluate the possible charge-transfer (CT) excitations near the photoionization limit. Our results indicate that, in high-radiation regions of the ISM (>8.0 eV), CT excitations occur from localized occupied molecular orbitals (MOs) in the aromatic molecules to mixed unoccupied MOs in the complexes, favoring cationic aromatic species in these conditions. We notice that the photoabsorption spectra depend on the type of intermolecular interaction (H-bonds or O-H···π bonds) in the complexes, as well as the presence and position (1 or 2) of the cyano-functional group in naphthalene. For hydrated naphthalene, the O-H···π complexes assume a more relevant role for photodissociation. In the case of the cyano-substituted derivatives, the H-bonded structures are more favorable to be considered as prereactive models. However, the cyano group at position 2 indicates that CT excitations toward the water dimer are more likely to occur.

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde.

We studied the threshold photoionization and dissociative ionization of para-, meta-, and ortho-anisaldehyde by photoelectron photoion coincidence spectroscopy in the 8.20-19.00 eV photon energy range. Vertical ionization energies by equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) calculations reproduce the photoelectron spectral features in all three isomers. The dissociative photoionization (DPI) pathways of para- and meta-anisaldehyde are similar and differ markedly from those of ortho-anisaldehyde. In the para and meta isomers, the lowest-energy DPI channel corresponds to hydrogen atom loss to form the C8H7O2+ fragment at m/z 135, which undergoes sequential dissociation processes at higher energies, such as carbon monoxide loss to C7H7O+ (m/z 107) and further, sequential CH3, CH2O, and CH2CO losses to produce C6H4O+ (m/z 92), C6H5+ (m/z 77), and C5H5+ (m/z 65), respectively. Carbon monoxide loss from the parent ions, yielding C7H8O+ (m/z 108), is a subordinate dissociation channel parallel to H atom loss. At higher energies, it also gives rise to sequential formaldehyde (CH2O) loss to produce C6H6+ (m/z 78). In the ortho-anisaldehyde cation, the vicinity of the aldehyde and methoxy groups opens up low-energy hydrogen-transfer processes, which allow for seven fragmentation channels to compete effectively with the H- and CO-loss channels. Thus, the fragmentation mechanism changes considerably, thanks to the steric interaction of the substituents. Functional group interactions, in particular H transfer pathways, must therefore be considered when predicting the isomer-specific unimolecular decomposition mechanism of cationic and neutral species, as well as mass spectra for isomers.

Observation of Rabi dynamics with a short-wavelength free-electron laser

Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field1. They are ubiquitous in physics with applications in different areas such as photonics2, nano-electronics3, electron microscopy4 and quantum information5. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light–matter interactions within the rotating-wave approximation6. Although intense nanometre-wavelength light sources have been available for more than a decade7–9, Rabi dynamics at such short wavelengths has not been directly observed. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser10 can drive Rabi dynamics between the ground state and an excited state in helium atoms. The measured photoelectron signal reveals an Autler–Townes doublet and an avoided crossing, phenomena that are both fundamental to coherent atom–field interactions11. Using an analytical model derived from perturbation theory on top of the Rabi model, we find that the ultrafast build-up of the doublet structure carries the signature of a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond12 and few-femtosecond13 extreme-ultraviolet pulses, our results unfold opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.

Complete characterization of attosecond photoelectron wave packets

Development of attosecond laser technology allows us to measure electron dynamics such as photoionization delays between different species or electronic states. In general, the measured photoionization phase is a mixture of the spectral phase of the extreme ultraviolet (XUV) pulse and the atomic phases inherent to the optical transitions. Hence, it is difficult to disentangle these phases independently. Here we separate these phases by using an XUV attosecond pulse train containing both even and odd harmonic orders, generated by an 800- and 400 nm laser pulse, in the presence of the infrared 800-nm pulse. We measure the photoelectron angular distributions as a function of two independently controlled delays, the XUV-IR and the 800--400 nm delays, with attosecond time resolution. We analyze the photoelectron angular distributions to determine the relative amplitudes and phases of each angular momentum component. Using an in situ technique, we determine the phases of the harmonic orders and thereby completely determine the atomic phases. Using the obtained atomic phases and amplitudes, we reconstruct the real and imaginary parts of the continuum wave functions associated with three individual photoionization pathways.

Investigation of dissociative photoionization pathways for orientation-selected molecules by phase-controlled laser fields

Intense (5.0 × 1012 W/cm2) nanosecond phase-controlled laser fields consisting of fundamental and second-harmonic light induce orientation-selective molecular tunneling ionization in a randomly orientated molecular ensemble in a gas phase. The selection of oriented molecules enables one to elucidate dissociative photoionization pathways while eliminating loss of information due to orientational averaging. Here, we have investigated the dissociative ionization of hydrofluorocarbon molecules induced by phase-controlled two-color laser fields. From the phase-dependent behavior of photofragment emission from orientation-selected molecules, dissociation pathways were elucidated experimentally. Bond dissociation energies obtained by quantum chemical calculations support the identified dissociation pathways.

Photoionization of KCs Molecule: Origin of the Structured Continuum?

We report the experimental observation of photoionization bands of the KCs molecule in the deep ultraviolet spectral region between 200 and 420 nm. We discuss the origin of observed photoionization bands as stemming from the absorption from the ground state of the KCs molecule to the excited states of KCs+ molecule for which we used existing potential curves of the KCs+ molecule. An alternative explanation relies on the absorption from the ground state of the KCs molecule to the doubly excited states of the KCs** molecule, situated above the lowest molecular state of KCs+. The relevant potential curves of KCs** are not known yet, but all those KCs** potential curves are certainly autoionizing. However, these two photoionization pathways may interfere resulting in a special interference structured continuum, which is observed as complex bands.

Extended x-ray emission times of clusters in intense x-ray pulses

Recently, intensity correlation of x-ray fluorescence, based on the principle introduced by Hanbury Brown and Twiss [R. Hanbury Brown and R. Q. Twiss, Nature (London) 177, 27 (1956)], has been proposed for high-resolution imaging of a three-dimensional arrangement of atoms. To explore the applicability of this fluorescence approach, we theoretically investigate fluorescence dynamics of nonperiodic systems subject to x-ray free electron laser (XFEL) pulses over a range of fluences from the linear to nonlinear x-ray absorption regimes as a function of system size, from a single atom to a cluster of 149 171 atoms. Fluorescence dynamics in the nonlinear x-ray regime differs from that in the weak x-ray field in that intense x-ray pulses interrupt the fluorescence dynamics by multiphoton absorption creating a dense electron environment within the sample on a femtosecond timescale. In large systems, the presence of both recombination and photoionization pathways gives rise to an enhanced $K\ensuremath{\alpha}$ and $K{\ensuremath{\alpha}}^{h}$ emission yield and an extended emission time beyond the lifetime of the core-excited states. Our analysis suggests that, in an intense x-ray pulse, the $K{\ensuremath{\alpha}}^{h}$ emission line can be a good candidate for fluorescence imaging as it has a higher yield and an emission time that is short relative to the x-ray-induced lattice distortion time.

Recovery of High-Energy Photoelectron Circular Dichroism through Fano Interference.

It is commonly accepted that the magnitude of a photoelectron circular dichroism (PECD) is governed by the ability of an outgoing photoelectron wave packet to probe the chiral asymmetry of a molecule. To be able to accumulate this characteristic asymmetry while escaping the chiral ion, photoelectrons need to have relatively small kinetic energies of up to a few tens of electron volts. Here, we demonstrate a substantial PECD for very fast photoelectrons above 500eV kinetic energy released from methyloxirane by a participator resonant Auger decay of its lowermost O 1s excitation. This effect emerges as a result of the Fano interference between the direct and resonant photoionization pathways, notwithstanding that their individual effects are negligibly small. The resulting dichroic parameter has an anomalous dispersion: It changes its sign across the resonance, which can be considered as an analogue of the Cotton effect in the x-ray regime.

Photoionization and dissociation study of 2-methyl-2-propen-1-ol: Experimental and theoretical insights

The photoionization and dissociation of 2-methyl-2-propen-1-ol (MPO) have been investigated by using molecular beam experimental apparatus with tunable vacuum ultraviolet synchrotron radiation in the photon energy region of 8.0 $-$ 15.5 eV. The photoionization efficiency (PIE) curves for molecule ion and fragment ions: C $_4$ H $_8$ O $^+$ , C $_4$ H $_7$ O $^+$ , C $_3$ H $_5$ O $^+$ , C $_4$ H $_7$ $^+$ , C $_4$ H $_6$ $^+$ , C $_4$ H $_5$ $^+$ , C $_2$ H $_4$ O $^+$ , C $_2$ H $_3$ O $^+$ , C $_3$ H $_6$ $^+$ , C $_3$ H $_5$ $^+$ , C $_3$ H $_3$ $^+$ , CH $_3$ O $^+$ , CHO $^+$ have been measured, and the ionization energy (IE) and the appearance energies (AEs) of the fragment ions have been obtained. The stable species and the first order saddle points have been calculated on the CCSD(T)/cc-pvTZ//B3LYP/6-31+G(d, p) level. With combination of theoretical and experimental results, the dissociative photoionization pathways of 2-methyl-2-propen-1-ol are proposed. Hydrogen migrations within the molecule are the dominant processes in most of the fragmentation pathways of MPO.

Laser-induced tunneling, the Kapitza effective potential, and the limits of perturbative nonlinear optics.

The canonical framework of optical physics connects the validity of perturbative nonlinear optics to the smallness of the optical driver field E compared to a characteristic field Eat that acts on electrons in an atom, a molecule, or a crystal lattice. However, in a vast area of strong-field optical science, the borderline between perturbative and nonperturbative nonlinear optics is defined as γ ~1, where γ is the Keldysh parameter. Not only is this criterion frequency-dependent, in a stark contrast with E/Eat ~1, but it also often dictates much weaker fields at which the perturbative treatment is still valid, leading to a dramatic shrinkage in the convergence radius of perturbation-theory expansions. Here, we identify the physics behind the gap between the E/Eat ~1 and γ ~1 conditions as the limits of perturbative nonlinear optics. We argue that, while the criterion E/Eat << 1 sets a universal upper-bound limit on the validity of perturbative nonlinear optics and its central concept of nonlinear-optical susceptibilities, optical nonlinearities related to photoionization pathways become nonperturbative in much weaker optical fields, with the limits of a perturbative treatment defined by the Keldysh parameter γ rather than the E/Eat ratio.

Quantum Control of Photoelectron Circular Dichroism.

We demonstrate coherent control over the photoelectron circular dichroism in randomly oriented chiral molecules, based on quantum interference between multiple photoionization pathways. To significantly enhance the chiral signature, we use a finite manifold of indistinguishable (1+1^{'}) resonantly enhanced multiphoton ionization pathways interfering at a common photoelectron energy but probing different intermediate states. We show that this coherent control mechanism maximizes the number of molecular states that constructively contribute to the dichroism at an optimal photoelectron energy and thus outperforms other schemes, including interference between opposite-parity pathways driven by bichromatic (ω, 2ω) fields as well as sequential pump-probe ionization.

Control of H_{2} Dissociative Ionization in the Nonlinear Regime Using Vacuum Ultraviolet Free-Electron Laser Pulses.

The role of the nuclear degrees of freedom in nonlinear two-photon single ionization of H_{2} molecules interacting with short and intense vacuum ultraviolet pulses is investigated, both experimentally and theoretically, by selecting single resonant vibronic intermediate neutral states. This high selectivity relies on the narrow bandwidth and tunability of the pulses generated at the FERMI free-electron laser. A sustained enhancement of dissociative ionization, which even exceeds nondissociative ionization, is observed and controlled as one selects progressively higher vibronic states. With the help of abinitio calculations for increasing pulse durations, the photoelectron and ion energy spectra obtained with velocity map imaging allow us to identify new photoionization pathways. With pulses of the order of 100fs, the experiment probes a timescale that lies between that of ultrafast dynamical processes and that of steady state excitations.

Publisher’s Note: “Photoionization pathways and thresholds in generation of Lyman-α radiation by resonant four-wave mixing in Kr-Ar mixture” [AIP Advances 6, 095018 (2016)

Publisher’s Note: “Photoionization pathways and thresholds in generation of Lyman-α radiation by resonant four-wave mixing in Kr-Ar mixture” [AIP Advances 6, 095018 (2016)

Photoionization pathways and thresholds in generation of Lyman-α radiation by resonant four-wave mixing in Kr-Ar mixture

We develop a set of analytical approximations for the estimation of the combined effect of various photoionization processes involved in the resonant four-wave mixing generation of ns pulsed Lyman-α (L-α) radiation by using 212.556 nm and 820-845 nm laser radiation pulses in Kr-Ar mixture: (i) multi-photon ionization, (ii) step-wise (2+1)-photon ionization via the resonant 2-photon excitation of Kr followed by 1-photon ionization and (iii) laser-induced avalanche ionization produced by generated free electrons. Developed expressions validated by order of magnitude estimations and available experimental data allow us to identify the area for the operation under high input laser intensities avoiding the onset of full-scale discharge, loss of efficiency and inhibition of generated L-α radiation. Calculations made reveal an opportunity for scaling up the output energy of the experimentally generated pulsed L-α radiation without significant enhancement of photoionization.

Effects of Hydration on the Photoionization Threshold Energy of DNA Components and on the Activation Barriers for Guanine Methylation by Dimethyl Sulfate

The evaluation of nucleotide photoionization threshold energies provides useful information for understanding mechanisms of carcinogenesis. A comparison of aqueous photoionization energies with gas-phase ionization potentials indicates that hydration lowers these energies by 3.31 to 3.83 eV for the nucleobases, 2.92 to 3.11 eV for the nucleosides and alters the ionization events for the nucleotides. Nucleotides in gas-phase are ionized at the phosphate moiety but hydration favors base ionization, which is consistent with experimental data indicating that at 77 K in aqueous perchlorate glasses, the primary photoionization pathway involves base ionization followed by deprotonation. The gas-phase ionization potentials were calculated using the B3LYP/6-31+G∗ density functional method. Solvation energy calculations were performed employing the SM8 continuum solvation model. The photoionization energies in aqueous solution are estimated to be 4.78, 5.08, and 4.96, and 5.26 eV for 5’dGMP-, 5’dCMP-, 5’dAMP-, and 5’dTMP-, respectively. For the test molecules indole and tryptophan the aqueous photoionization energies (4.26 and 4.18 eV, respectively) are in agreement with the experimental values (4.35 and 4.45 eV). The SN2 transition states of the methylation reaction of guanine with dimethyl sulfate, the vapors of which may cause severe inflammation and necrosis of the eyes, mouth, and respiratory tract of humans and tumors in the nasal passages, lungs, and thorax of animals, were examined in the gas- and aqueous- phases at N7, N3 and O6 sites employing the same theoretical approach. Results show that the transition states in water are influenced by steric interactions, and the activation barriers are lower by 4.0 to 11.0 kcal/mol than in the gas-phase.

Theoretical Investigations on Laser-Assisted Depletion of Gd152 Isotope From Natural Gadolinium

We have proposed laser-assisted depletion of Gd152 isotope from a natural isotopic mixture of Gd to enhance its functional efficiency as a burnable poison. Theoretical investigations on laser-assisted depletion of Gd152 isotope from natural gadolinium have been carried out for two-color resonant three-color photoionization pathways using density matrix formalism. Calculations have been carried out using a density matrix formalism to optimize conditions for high ionization efficiency without much sacrifice in the isotopic selectivity. Optimum conditions for good isotopic selectivity of Gd152 without significant sacrifice in the ion yield have been identified. Under appropriate conditions, all the 17 photoionization schemes are found to be useful for the laser-assisted separation of Gd152 isotopes which can be used for reactor applications. The effect of source, laser, and atom parameters on isotopic selectivity and ionization efficiency has been investigated. Among the photoionization schemes investigated, one of the photoionization scheme has been investigated in detail. Under optimized conditions, this photoionization scheme has resulted in high ionization efficiency (&gt;50%) and high isotopic selectivity (1.2×104).

Measurements of excited-state-to-excited-state transition probabilities and photoionization cross-sections using laser-induced fluorescence and photoionization signals

Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic samarium using Nd:YAG-pumped dye lasers. Two-color, three-photon photoionization and two-color fluorescence signals were recorded simultaneously as a function of the second-step laser power for two photoionization pathways. The density matrix formalism has been employed to analyze these signals. Two-color laser-induced fluorescence signal depends on the laser powers used for the first and second-step transitions as well as the first and second-step transition probability whereas two-color, three-photon photoionization signal depends on the third-step transition cross-section at the second-step laser wavelength along with the laser powers and transition probability for the first and second-step transitions. Two-color laser-induced fluorescence was used to measure the second-step transition probability. The second-step transition probability obtained was used to infer the photoionization cross-section. Thus, the methodology combining two-color, three-photon photoionization and two-color fluorescence signals in a single experiment has been established for the first time to measure the second-step transition probability as well as the photoionization cross-section.

Optical selectivity calculations of ^90Sr isotope in double optical resonance photoionization schemes

Seven new schemes have been theoretically studied using the density-matrix approach for the ns2(1S0)→nsnp(3P01 or 1P01)→nsms(1S0) and the ns2(1S0)→nsnp(3P01 or 1P01)→np2(1S0 or 1D2) double optical resonance photoionization pathways to establish the possibility of selective ionization of the rare strontium 90Sr isotope for continuous wave laser excitation. The double-resonance excitation is followed by photoionization using a 488 nm photon. Density-matrix equations for the double-resonance ionization have been numerically integrated by incorporating the effects of Doppler broadening, velocity-dependent interaction times, time varying Rabi frequencies, and laser bandwidths. All the optical selectivity calculations were carried out on a high-speed parallel processing machine. Theoretical computations on five previously studied photoionization pathways have also been performed and tabulated for the sake of completeness. The conditions for obtaining optimum optical selectivities have been evaluated. This study results in seven new photoionization pathways. The optical selectivities are a few orders higher compared to the previously studied photoionization schemes for five out of the seven schemes studied. The remaining two schemes are slightly better than the previously studied schemes.

VUV photophysics and dissociative photoionization of pyrimidine, purine, imidazole and benzimidazole in the 7–18 eV photon energy range

Photoionization mass spectrometry is used to study ionization processes and fragmentation pathways of four prebiotic species, pyrimidine, purine, imidazole and benzimidazole, in the 7–18 eV photon energy region, with synchrotron radiation as excitation source. These molecules are possible precursors of the nucleic acid bases that occur in DNA and RNA. Ionization energies and ion appearance energies are reported. They are compared with electron impact and other studies and are discussed in terms of the electronic and nuclear structures of these species and their cations. The ion appearance energies, in conjunction with thermochemical data, were used to propose dissociative photoionization pathways, principally involving loss of HCN molecules in each of the four species. Astrophysical implications of the results concern the prospects for observation and survival of these molecules in the interstellar medium, in comets and in meteorites. Suggestions are made concerning suitable sites for radioastronomical searches for these purines and pyrimidines.

Photoelectron Imaging of Helium Droplets Doped with Xe and Kr Atoms

Helium droplets doped with Xe and Kr atoms were photoionized by using VUV synchrotron radiation from the Advanced Light Source and the resulting photoelectron images were measured. A wide range of He droplet sizes, photon energies, and dopant pick-up conditions was investigated. Significant ionization of dopants was observed at 21.6 eV, the absorption maximum of 2p (1)P1 electronic excited state of He droplets, indicating an indirect ionization mechanism via excitation transfer. The photoelectron images and spectra reveal multiple photoionization mechanisms and pathways for the photoelectrons to escape the droplet. Specifically, they show sets of sharp peaks assigned to two mechanisms for Penning ionization of the dopant by He* in which the photoelectrons leave the droplet with no detectable energy loss, a broad, intense feature representing electrons that undergo significant energy loss, and a small amount of ultraslow electrons that may result from electron trapping at the droplet surface. The droplet-size dependence of the broad, intense feature suggests the development of the conduction band edge in the largest droplets seen here ((N) approximately 250,000).