Sequential Two-photon Ionization Research Articles

Electron Correlations in Sequential Two-Photon Double Ionization of an Ar Atom

Sequential two-photon ionization is a process that is experimentally accessible due to the use of new free-electron laser sources for excitation. For the prototypical rare Ar gas atoms, a photoelectron spectrum (PES) corresponding to the second step of the sequential two-photon double ionization (2PDIII) at a photon excitation energy of 65.3 eV was studied theoretically with a focus on the consequences of electron correlations in the considered process. The calculation predicts many intense lines at low photoelectron energies, which cannot be explained on the basis of a one-electron approximation. The processes that lead to the appearance of these lines include many-electron correlations, either in the first or second step of photoionization. A significant fraction of the intensity of the low-energy part of PES is associated with the Auger decay of the excited states formed at the second step of 2PDI. The shape of the low-energy part of the 2PDIII PES is expected to be dependent on both the energy of photon excitations and the flux of the exciting beam.

Resonant two-photon ionization of atoms by twisted and plane-wave light

We study the resonant sequential two-photon ionization of neutral atoms by a combination of twisted- and plane-wave light within a fully relativistic framework. In particular, the ionization of an isotropic ensemble of neutral sodium atoms ($Z = 11$) from their ground $3^{2}S_{1/2}$ state via the $3^{2}P_{3/2}$ level is considered. We investigate in details the influence of the kinematic parameters of incoming twisted radiation on the photoelectron angular distribution and the circular dichroism. Moreover we study the influence of the geometry of the process on these quantities. This is performed by changing the propagation directions of the incoming twisted and plane-wave light. It is found that the dependence on the kinematic parameters of the twisted photon is the strongest if the plane-wave and twisted light beams are perpendicular to each other.

Coherent intense resonant laser pulses lead to interference in the time domain seen in the spectrum of the emitted particles

The dynamics of atomic levels resonantly coupled by a coherent and intense short high-frequency laser pulse is discussed and it is advocated that this dynamics is sensitively probed by measuring the spectra of the particles emitted. It is demonstrated that the time-envelope of this laser pulse gives rise to two waves emitted with a time delay with respect to each other at the rising and falling sides of the pulse, which interfere in the time domain. By computing numerically and analyzing explicitly analytically a show-case example of sequential two-photon ionization of an atom by resonant laser pulses, we argue that this dynamic interference should be a general phenomenon in the spectroscopy of strong laser fields. The emitted particles do not have to be photoelectrons. Our results allow also to interpret the already studied resonant Auger effect of an atom by intense free electron laser pulses, and also to envisage experiments in which photons are emitted.

液相における多環芳香族分子のレーザー多光子イオン化 光電流信号の解析

The laser multi-photon ionization spectra of pyrene, perylene, and naphthacene were measured in hexane. The photoionization spectra showed distinct structure and wavelengths of bands agreed roughly with those of the one-photon absorption spectra. The photocurrent signal was strong in the wavelength region where the absorption spectrum was observed as compared with that where no absorption spectrum was seen. Sequential two-photon ionization was a main process in the former region, while simultaneous two-photon ionization or three-photon ionization involving two-photon excitation in the latter region. The number of free charge carriers increased linearly with the applied electric field and was correlated with the excitation energy. The photocurrent signal was correlated with the excess excitation energy. The ionization threshold was evaluated from the photoionization spectrum in the longer wavelength region. The photoionization spectrum was simulated.

Multiple Photon High Vibrational CH Overtone Resonant and One-Photon Dissociation of Toluene Radical Cation

Photodissociation of molecular radical cations of toluenes C7H8+· (I), C6H5CD3+· (II), C7D8+· (III), cycloheptatriene C7H8+· (IV), and benzene has been studied by the method of two-pulse two-colour laser photolysis of molecules in a mass spectrometer using wavelengths 1064, 532, and 355 nm. The I ions formed due to sequential two-photon ionization of toluene molecules by the 266 nm radiation isomerize supposingly into 5-methylene-cyclohexadiene (VI). Two- and three-photon stepwise dissociation of the I and II isomers (VI and VI-d3, respectively) induced by the 1064 nm radiation has a resonance at the third overtone level of CH aliphatic bond stretching vibrations. Internal energy randomization in the two-photon dissociation is much less for VI-d3 as compared to VI. The mechanisms of isoenergetic dissociation (two- and one-photon processes induced by the 1064 and 532 nm radiations, respectively) are different: the one-photon decay via an electronically excited state proceeds considerably faster than the two-photon decomposition does in the ground state. The photodissociation cross sections have been determined. It has been shown that the intensity of CH stretching vibration overtones can be essentially higher for radical cations than for the neutral molecules.

A theoretical study of the isotope separation of Li2 by sequential two-photon ionization

The intensities of the sequential two-photon ionization of Li2 are evaluated starting from ab initio CI electronic wave functions and using orthogonalized Coulomb functions for the continuum. The rotovibrational energies and the Franck–Condon factors are computed using a spline procedure both for the fitting of the electronic potentials and for the definition of the vibrational basis set. In agreement with experimental results, a substantial isotope separation is found for the three molecular isotopes of Li2 and a wide spectrum of wavelengths useful for effective isotope enrichment is suggested. The modulation of the line intensities plays an important role in the fractionation process, while the energy matching criterion alone is not enough to establish which resonances are responsible for isotope enrichment. The importance of the Franck–Condon factor contributions to the spectrum modulation makes it possible to extend these results to other molecules.

A theoretical study of the isotope separation of Li 2 using different laser wavelengths for the excitation and ionization processes

Cross sections of the sequential two-photon ionization of Li 2 are evaluated starting from ab initio CI wavefunctions and using different laser wavelengths for the excitation and ionization processes. In agreement with experimental results and previous calculations, a substantial isotope separation is found for the three isotopes of Li 2, and a wide spectrum of wavelengths useful for effective isotope enrichment is suggested. Even in with an overcrowded spectrum, the fundamental role of Franck-Condon factors and thermal populations in defining the characteristic resonances of the fractionation process is confirmed. This result suggests a practical way to predict transitions useful for isotope enrichment.

Accurate ionization potential of Li 2 from resonant two-photon ionization

Sequential two-photon ionization of Li 2 in a supersonic beam with two pulsed tunable dye lasers yields the adiabatic ionization potential IP(Li 2) = 41475 ± 8 cm − and the dissociation energy of the ion D 0(Li + 2 2Σ + g) = 10353 ± 25 cm −1. The influence of electric fields on the measured value of the ionization potential and the appearance of numerous autoionization lines are discussed.

Autoionizing Rydberg states in Na2

Extended autoionization structure has been observed in sequential two-photon ionization of Na2 via different vibronic levels of the intermediate B 1Σu state. Partial snalysis of this structure has now been achieved. A dominant Rydberg series (quantum defect δ = 0.4) and a second, weaker series (δ = 0.57) are assigned. Convergence to many different vibronic levels of the ion core is observed, allowing a preliminary determination of ω e and ωexPde for the Na2+ 2Σ+g ground state. A difference of +49 cm−1 between the spectroscopically extrapolated ionization potential and the previously observered ionization threshold is noted and discussed.

The adiabatic ionization potentials of the alkali dimers Na 2, NaK and K 2

The adiabatic ionization potentials of three alkali dimers Na 2, Na 39K and 39, 39K 2 have been measured in supersonic alkali-metal beams using the sequential two-photon ionization method. Very accurate ionization energies and dissociation energies of the corresponding molecular ion ground states have been obtained.

Isotope fractionation in two-step photoionization of Li 2

A cw argon-ion laser beam intersects a thermal-energy lithium-beam. Sequential two-photon ionization of Li 2 is observed. Li + 2 ions are formed by most of the available laser lines. Large isotope fractionations result from this process.