Ultrafast Photoionization Research Articles

Ultrafast resonance-enhanced multiphoton ionization in the azabenzenes: Pyridine, pyridazine, pyrimidine, and pyrazine

We report on the ultrafast photoionization of pyridine, pyridazine, pyrimidine, and pyrazine. These four molecules represent a systematic series of perturbations into the structure of a benzene ring which explores the substitution of a C-H entity with a nitrogen atom, creating a heterocyclic structure. Data are recorded under intense-field, single-molecule conditions. The pulses (50 fs, 800 nm) are focused into the molecular vapor, and ion mass spectra are recorded for intensities of ~10(13) W/cm(2) to ~10(15) W/cm(2). We measure ion yields in the absence of the focal volume effect without the need for deconvolution of the data. For all targets, stable singly- and doubly-charged parent ions (C(6-n)H(6-n)N(n)(+(+))) are observed with features suggesting resonance-enhanced ionization. From the intensity dependence of the ion yield, we infer that excitation occurs both through (1)ππ* transitions (remnants of the benzene structure) and through (1)nπ* transitions, the latter being a result of Rydberg-like excitations of the lone pair electrons of the nitrogen atoms. Stability against intense-field fragmentation is also discussed.

Ultrafast photoionization dynamics at high laser intensities in the xuv regime

We study the ionization dynamics in the soft-x-ray regime for high intensities and short pulses for excitations near the ionization threshold. Using a one-dimensional helium atom model, we compare exact numerical solutions with time-dependent Hartree-Fock results in order to identify the role of electron-electron correlations. At moderate intensities but still in the x-ray and short-pulse regime, we find that the Hartree-Fock theory reproduces well the dynamics of the ground-state occupation, while at high intensities strong correlation effects occur for excitations close to the threshold. From their characteristic momentum distributions, we can identify contributions to the double ionization from sequential three-photon and nonsequential or sequential two-photon processes. At elevated intensities these contributions deviate from their usual intensity scaling due to saturation effects, even though the total double-ionization probability stays below 10%. Furthermore, analysis of the time evolution of the momentum distribution reveals signatures of the energy-time uncertainty which indicate a coherent regime of the dynamics.

Extending electron orbital precession to the molecular case: Use of orbital alignment for observation of wavepacket dynamics

The complexity of ultrafast molecular photoionization presents an obstacle to the modelling of pump-probe experiments. Here, a simple optimized model of atomic rubidium is combined with a molecular dynamics model to predict quantitatively the results of a pump-probe experiment in which long range rubidium dimers are first excited, then ionized after a variable delay. The method is illustrated by the outline of two proposed feasible experiments and the calculation of their outcomes. Both of these proposals use Feshbach 87Rb2 molecules. We show that long-range molecular pump-probe experiments should observe spin-orbit precession given a suitable pump-pulse, and that the associated high-frequency beat signal in the ionization probability decays after a few tens of picoseconds. If the molecule was to be excited to only a single fine structure state state, then a low-frequency oscillation in the internuclear separation would be detectable through the timedependent ionization cross section, giving a mechanism that would enable observation of coherent vibrational motion in this molecule.

Dependence of carbon fragments from methane in strong and ultrastrong elliptically polarized laser fields

We present the ellipticity dependence of the ultrafast photoionization for Cn+ fragments from methane. The study extends from the strong field (C+, C2+) at 1014 W cm−2 to the ultrastrong field (C5+) at 1018 W cm−2. The measurements show that C+ and C2+ ionization have limited sensitivity to the field polarization. As the laser intensity and corresponding degree of ionization increase (C4+, C5+), the dependence on the field polarization increases. Comparison to a semi-classical field ionization model shows that the ellipticity dependence of the relative ion yield for higher charge states comes from the field dependence of tunnelling ionization rather than nonsequential ionization due to rescattering. A movement from a molecule-like response to an atom-like response with the increase in intensity is observed.

Simulation of time resolved photoelectron spectra with Stieltjes imaging illustrated on ultrafast internal conversion in pyrazine

We present an approach for the simulation of time resolved photoelectron spectra based on the combination of the ab initio nonadiabatic molecular dynamics "on the fly" with the Stieltjes imaging method utilizing discrete neutral states above the ionization limit for the approximate description of the ionization continuum. Our approach has been implemented in the framework of the time-dependent density functional theory and has been applied to interrogate the ultrafast internal conversion between the S(2) and S(1) states in pyrazine. The simulations reveal that, parallel to the S(2)-->S(1) internal conversion, a change in the dominant ionization process (S(2)-->D(1) versus S(1)-->D(0)) occurs on the time scale of 20 fs such that no significant change in the photoelectron kinetic energy distribution is observed. The presented results are in full agreement with the experimental results presented in the accompanying paper [Suzuki et al., J. Chem. Phys. 132, 174302 (2010)] and provide an insight into the interplay between the nonradiative relaxation and the photoionization process in pyrazine as reflected in the time resolved photoelectron spectrum. Our approach represents a general tool for the investigation of ultrafast photoionization processes in complex systems and thus can be used to investigate the ultrafast femtochemistry of complex molecular systems including all degrees of freedom.

Femtosecond Laser Photoionization Time-of-Flight Mass Spectrometry of Nitro-aromatic Explosives and Explosives Related Compounds

The ultrafast laser-induced photoionization and photodissociation processes of the nitroaromatic containing explosive and explosive related compounds (ERCs) nitrobenzene (NB), 1,3-dinitrobenzene (DNB), m-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) have been investigated at three laser wavelengths and power densities using a time-of-flight mass spectrometer. Examination of the mass spectra of these compounds reveals the enhanced formation of the molecular ion [M(+)] when ultraviolet (332 nm) and visible (495 nm) light is used relative to infrared (795 nm) radiation. In addition, at 795 nm and a power density of 3.5 x 10(14) W/cm(2), the presence of a competition between multiphoton ionization (MPI) and Coulomb explosion (CE) channels is revealed by peak shape analysis, and is thought to be operative under these conditions for all of the molecules investigated.

Competition between ultrafast relaxation and photoionization in excited prefluorescent states of tryptophan and indole

The quantum yield of photoionization of TrpH and IndH from the nonrelaxed prefluorescent state S* increases with the temperature decrease. This effect is attributed to the competition between temperature independent ionization and ultrafast thermal relaxation S* --> S1. The rate constant of the relaxation does not depend on the solvent and on the presence of the amino acid side chain: the temperature dependences of photoionization quantum yield, obtained for TrpH and IndH in different solvents, practically coincide. The activation energy for the relaxation rate constant Er approximately 4.5 kJ/mol probably corresponds to intramolecular process or to the formation of the vibronically excited transient complex between photoexcited molecule and solvent molecules.

Motivation and development of ultrafast laser-based accelerator techniques for chemical physics research

The products of radiation induced chemical reactions are determined by rapid primary processes such as energy transfer, thermalization and solvation. Ultrafast photoionization experiments on liquid water demonstrate that these initial events occur on time scales <5ps and involve a complicated interplay between electronic relaxation and vibrational energy redistribution. These experiments also show that the chemical processes originating from ionizing radiation are unique and cannot be reproduced by laser photons alone. Due to the lack of a suitable femtosecond source of ionizing radiation, knowledge of the primary processes in radiation chemistry remains poor. To address this issue a 20TW laser system has been constructed to obtain subpicosecond electron pulses with energies in the 1–10MeV range. In addition to the production of femtosecond electron pulses, future efforts will be directed towards using this laser for accelerating heavier particles such as protons and generating hard X-rays.

Ultrafast dissociation dynamics of ketones at 195 nm

The photodissociation dynamics of the 3s Rydberg state of three ketones (CH 3CO–R, R=C 2H 5, C 3H 7, and iso-C 4H 9) and the ensuing dissociation of the nascent acetyl radical following 195 nm excitation were investigated by ultrafast photoionization spectroscopy. The 3s state the lifetimes of these ketones are similar (2.5–2.9 ps), though lifetimes of the acetyl radical range from 8.6 ps for CH 3CO–C 2H 5, 15 ps for CH 3CO–C 3H 7, to 23 ps for CH 3CO–( iso-C 4H 9), which suggests that for larger R more vibrational degrees of freedom compete for the excess energy so that less energy is partitioned into the internal energy of the acetyl radical.

Quantum dynamics simulation of the ultrafast photoionization of Li2

The Li2 species offers an ideal system to compare experimental pump/probe ultrafast photoionization with quantum dynamical calculations on well characterized potential energy surfaces. The present work utilizes the best available potential energy surfaces and appropriate quantum dynamical methods to analyze the photoionization and dynamics of a wave packet prepared in the E 1Σg+ shelf state of lithium dimer. A direct comparison between calculated (ab initio) and measured quantum dynamics is made for signals obtained with different laser pulse shapes, intensities, and chirps, and the validity of the theoretical model is considered, as well as the applicability and failure of perturbation theory. The results illustrate the high sensitivity of the time-dependent pump/probe ionization transient signals to the detailed modeling of both the pump and probe stages. They also show some of the inadequacies of the current potential surfaces and dipole moment matrix elements of lithium dimer.

Ketone excited state lifetimes measured by deep UV ultrafast photoionization spectroscopy

Ultrafast photoionization spectroscopy has been used to measure excited state lifetimes for several ketones excited near 193 nm. The decay times for aliphatic ketones are between 2 and 9 ps, which are consistent with 3s Rydberg state predissociation and are similar to previously reported results for acetone. The lifetimes of cyclic ketones are somewhat longer than for acyclic ones, which is attributed to predissociation promoted by internal rotation for the latter. The aromatic ketone acetophenone dissociates more quickly, probably because of rapid intersystem crossing and/or interaction with a valence state that lies in this spectral region as a result of delocalization.

Methylsulfonyl Radical Unimolecular Dissociation Studied by Deep-UV Ultrafast Photoionization Spectroscopy

The secondary dissociation dynamics of the methylsulfonyl radical following the photodissociation near 193 nm of methylsulfonyl chloride and methylsulfonyl ethanol has been studied using femtosecond mass-resolved photoionization spectroscopy. The primary dissociation is instrument-limited (<200 fs) for both precursors. The methylsulfonyl unimolecular dissociation is measured to have a lifetime of 0.34 ps with complete secondary dissociation following methylsulfonyl chloride photolysis. For the case of methylsulfonyl ethanol, the measured lifetime is 1.1 ps and ∼40% of the radical remains undissociated for the 18 ps duration of the experiment. We have successfully modeled the experimental results assuming an impulsive dissociation process to yield the internal energy distributions in the primary photoproducts and RRKM calculations based on structures and energetics of the methylsulfonyl radical obtained from the literature.

Ultrafast photodissociation studies of acetyl cyanide and acetic acid and unimolecular decomposition rates of the acetyl radical products

Unimolecular decomposition rates for acetyl radical following the photodissociation of acetyl cyanide and acetic acid near 193 nm have been studied using ultrafast mass-resolved photoionization spectroscopy. In both cases, the parent decays with an instrumentally limited lifetime, while the acetyl radical behaves in a manner consistent with an RRKM mechanism, in contrast to our previous results on acetone. It is necessary to convolute the population distribution with the microcanonical RRKM rates in order to achieve this agreement. We have also undertaken an ab initio study of the excited states of acetyl cyanide to clarify the assignments of these states. The state excited at 193 nm arises from a π→π* transition with a calculated transition velocity dipole moment oriented at an angle of 57° with respect to the C–C≡N bond, resulting in an anisotropy parameter of −0.22. This is in reasonable agreement with the previous data of North et al. [J. Phys. Chem. A 101, 9224 (1997)]. The apparent RRKM behavior of the acetyl radical formed by the photodissociation of acetic acid and acetyl cyanide indicates that acetyl radical produced by the photodissociation of acetone at 193 nm may exhibit “extrinsic non-RRKM” effects, i.e., dynamic bottlenecks or mode specific effects.

Femtosecond dynamics of NaI ionization and dissociative ionization

The femtosecond dynamics of NaI two-body (NaI+hν→NaI++e−) and three-body (NaI+hν→Na++I+e−) break-up is investigated by quantum mechanical simulations. A first femtosecond laser pulse creates an excited molecular wave packet, and an additional ultrafast photoionization step monitors its subsequent time evolution. The theoretical results are compared with available experimental data. The branching ratio between these two competing fragmentation channels is extracted and an experimental scenario is proposed to control the outcome of this reactive half-collision.