Direct Multiphoton Ionization Research Articles

Theory of multiphoton photoemission disclosing excited states in conduction band of individual TiO2 nanoparticles* *Project supported by the National Natural Science Foundation of China (Grant Nos. 91850109, 11474040, 61605017, and 61775021) and the “111” Project of China (Grant No. D17017).

A theory of multiphoton photoemission is derived to explain the experimentally observed monotonic decrease with the wavelength in the electron yield of TiO2 nanoparticles (NPs) by as large as four orders of magnitude. It is found that the fitting parameter corresponds to the energy position of Ti3d eg and t2g states, and the derived theory is a novel diagnostic of excited states in the conduction band, very importantly, applicable to individual NPs. The difference between four-photon slope NPs and three-photon slope NPs is attributed to the difference in defect density. The success of the theory in solving the puzzling result shows that thermal emission from high-lying levels may dominate over direct multiphoton ionization in solids when the photon number larger than four is required.

Ionization Channels of the Molecular Ion H+2 in Intense Laser Field

Ionization channels of the molecular ion H+2 for various initial vibrational states in intense laser field (80 fs, 800 nm, I = 6.8 × 1013 W/cm2) are theoretically investigated by numerically solving the time-dependent Schrödinger equation. The results confirm that the channels largely depend on the selection of initial vibrational states by analyzing the variations of peak locations in the nuclear initial kinetic-energy-release spectra. Furthermore, the selection of the ionization channels is sensitive to the wavelength of the laser pulse. In addition, time-dependent competition between direct multi-photon ionization and charge-resonance-enhanced ionization are is discussed.

Multiple ionization under strong XUV to X-ray radiation

We review the main aspects of multiple photoionization processes in atoms exposed to intense, short wavelength radiation. The main focus is the theoretical framework for the description of such processes as well as the conditions under which direct multiphoton multiple ionization processes can dominate over the sequential ones. We discuss in detail the mechanisms available in different wavelength ranges from the infrared to the hard X-rays. The effect of field fluctuations, present at this stage in all SASE free-electron-laser (FEL) facilities, as well as the effect of the interaction volume integration, are also discussed.

Route to direct multiphoton multiple ionization

We address the concept of direct multiphoton multiple ionization in atoms exposed to intense, short wavelength radiation and explore the conditions under which such processes dominate over the sequential. Their contribution is shown to be quite robust, even under intensity fluctuations and interaction volume integration, and reasonable agreement with experimental data is also found.

Multiphoton Ca2+ production occurring before the onset of Ca+ saturation: is it a fingerprint of direct double ionization?

Singly and doubly charged Ca ions have been produced by multiphoton excitation of Ca vapour with 5 ns, low intensity (⩽3 × 1011 W cm−2) dye-laser pulses in the 675–685 nm wavelength range, including the four-photon 4s2 1S0 → 4p2 1S0 excitation. The intensity and wavelength dependence of the ion yields was recorded as well as the fluorescence emission from excited states of the Ca ion in an effort to identify the excitation pathways leading to single and double ionization. Unambiguous evidence for the absorption of at least two photons above the first ionization threshold was recorded, in agreement with earlier results for Mg and Sr obtained under similar conditions. However, certain characteristics of the process differ significantly from those of the earlier results, despite the apparent similarity in both the atomic structure and the excitation scheme. The most striking and unexpected finding is that for a certain wavelength, the Ca2+ yield is observable well before the saturation intensity of Ca+ and, moreover, it grows with intensity and saturates in parallel with the Ca+ yield. Possible mechanisms behind this outcome are discussed in detail as well as their implications for the multiphoton multiple ionization of complex atoms. Our tentative conclusion is that the occurrence of doubly charged ion production before the singly charged ion saturation should not be considered as ‘synonymous’ with direct (or non-sequential) multiphoton double ionization.

Momentum-map-imaging photoelectron spectroscopy of fullerenes with femtosecond laser pulses

Photoelectron spectra of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, ionized with ultrashort laser pulses, are measured with a momentum-map-imaging electron spectrometer. Above the photon energy, 1.6 eV, the spectra are essentially structureless and well described by Boltzmann distributions, with temperatures on the order of ${10}^{4}$ K. This result is similar to previous results for ${\mathrm{C}}_{60}$ using a time-of-flight electron spectrometer, which are confirmed in this study. Comparisons of electron energy distributions recorded for identical laser intensities but for different pulse durations demonstrate unambiguously that a significant fraction of the electrons are emitted in quasithermal processes and results argue strongly against a field ionization or direct multiphoton ionization mechanism. For electron energies above the photon energy, which account for about half the intensity, the whole signal is consistent with this quasithermal emission.

Photoionization in combined ultra short XUV and infrared laser pulses

Multi-photon-double ionization of xenon by Ti:Sapphire laser pulses combined with their 25th harmonic has been studied by means of a momentum imaging spectrometer. The determination of the momenta of the emitted photoelectron pair and of its energy and angular correlation gives insight into the various mechanisms leading to double ionization. Although the conditions for non sequential direct multi-photon double ionization are met in the experiment, it is found that two-step sequential processes prevail.

Ion Kinetic Energy Distributions and Mechanisms of Pulsed Laser Ablation on Al

A detailed investigation on the ion kinetic energy distributions of ions ejected in the nanosecond pulsed laser ablation of aluminum is reported. For laser fluences just over threshold, the emerging ions fit shifted neat Maxwell−Boltzmann-Coulomb (MBC) distributions. For fluences higher than ∼1.3 J/cm2, the Al+ distributions split into two MBC contributions peaked at different energies. It is demonstrated that the observed Al+ ion distribution has two components, one fast, correlated with the direct multiphoton laser ionization, and the other slow, associated with electron−Al0 collisions in the solid. A similar behavior is observed at higher fluences for all Al ion distributions indicating that the electron-impact ionization of Al rate constants is faster than that of recombination and other possible collision channels. In addition, the linear relationship between the Coulomb velocities and the ion charges and the behavior of Coulomb energy of the ions versus the laser fluence suggest the appearance of an...

Nonlinear photoionization in the soft X-ray regime

At the new Free-electron LASer in Hamburg FLASH, we have studied photon–matter interaction by means of ion time-of-flight spectroscopy on gases in the soft X-ray regime. Emphasis was laid on the quantitative investigation of non-linear effects upon photoionization by the highly intense soft X-ray laser pulses. In the photon energy range from 38 to 93 eV, we have observed nonlinearities due to space-charge effects, target depletion, and sequential and direct multi-photon ionization at irradiance levels of up to 1016 W cm−2. The work is related to the development of photon diagnostic tools that are based on gas-phase photoionization for current and future X-ray laser facilities but might be of significance for any experiment at fourth generation light sources.

Ionization of molecular nitrogen by radiation from a channel surface discharge

Ionization of atmospheric-pressure nitrogen by nearly blackbody radiation from a channel surface discharge is investigated. By analyzing the data from measurements of the current of an electrostatic electron detector, the ionization rate of nitrogen and the time evolution of the electron density are determined. It is shown that the electron density reaches its maximum ∼5 μs after the irradiation pulse. The results obtained indicate that the most probable mechanism for the observed nitrogen ionization is the formation of an ensemble of nitrogen molecules in metastable states and their subsequent collisional ionization, rather than direct one-photon or multiphoton ionization.

Threshold photoionization in time-of-flight mass spectrometry.

Multi-photon excitation in a time-of-flight mass spectrometer (TOF-MS) is shown to lead to threshold ions with defined internal energy. A powerful technique for the production of threshold ions is based on the excitation of high long-lived Rydberg states embedded in the ionization continuum. The Rydberg molecules are separated with suitable separation techniques from ions produced by a direct multi-photon ionization process. Finally, the ionization of the Rydberg molecules in a delayed pulsed electric field leads to threshold ions. This work reviews several separation techniques, and reports on applications of threshold ionization for investigation of the structure, energetics, and dynamics of neutral molecules, molecular cations, and cluster cations.

Logic gates using high Rydberg states.

Connected logic gates can be operated on the levels of one molecule by making use of the special properties of high Rydberg states. Explicit experimental results for the NO molecule are provided as an example. A number of other options, including that of several gates concatenated so as to operate as a full adder, are discussed. Specific properties of high Rydberg states that are used are: their autoionization is delayed so that they can be distinguished from direct multiphoton ionization, during their long life such states also can decay by energy transfer to the molecular core in a way that can be controlled by the judicious application of very weak external electrical fields, and the Rydberg states can be detected by the application of an ionizing electrical field. The combination of two (or three) color photons with and without external weak fields allows the construction of quite elaborate logic circuit diagrams and shows that taking advantage of the different intramolecular dynamics of levels that differ by their excitation enables the compounding of logic operations on one molecular frame.

From above threshold ionization to statistical electron emission: the laser pulse-duration dependence of C60 photoelectron spectra

The photoelectron spectra of C60 ionized using a 790 nm laser with pulse durations varying from 25 fs to 5 ps have been determined. For 25 fs pulses, in the absence of fragmentation, the ionization mechanism is direct multiphoton ionization with clear observation of above threshold ionization. As the pulse duration is increased, this becomes dominated by a statistical ionization due to equilibration among the electronic degrees of freedom. For pulse durations on the order of a ps coupling to the vibrational degrees of freedom occurs and the well-known phenomenon of delayed (&mgr;s) ionization is observed.

A new experimental method in the study of ionic Rydberg states by resonance ionization mass spectroscopy

We report a new method for detecting highly excited ions. With a strong constant electric field ionization following the extracting of ions by a pulsed electric field in a time-of-flight mass spectrometer, the direct multi-photon ionization background can be eliminated in laser multi-step resonance ionization mass spectroscopy of ionic Rydberg states. A high detection efficiency can be obtained for low members of ionic Rydberg states by use of this method. We have applied the method in the study of the spectroscopy of ionic Rydberg states in ytterbium.

Constant-electric-field ionization mass spectroscopy in laser-excited Yb^+ Rydberg states

Using a new method of constant-electric-field ionization of Rydberg states of ions, we have eliminated the influence of the direct multiphoton ionization background in multistep excitation of Yb+ Rydberg states and obtained the pure spectrum of np(n = 17−86) Rydberg levels of Yb+.The precision of the value of the Yb ii ionization potential has been greatly improved to 98 231.75(20) cm−1.

Pulsed visible laser photolyis of B(C2H5)3, Al2(CH3)6, Ga(CH3)3, and In(CH3)3: Multiphoton ionization spectra of Al, Ga, and In atoms

Multiphoton dissociation/ionization of the title compounds by focused visible laser pulses (390<λ<650 nm, 107–1010 W/cm2) has been investigated by use of a low-pressure (≂1 Torr) ionization cell. Al2(CH3)6, Ga(CH3)3, and In(CH3)3 fragment in the laser pulse to produce ground state and excited neutral metal atoms, which were detected by resonance-enhanced two- or three-photon ionization within the photolysis laser pulse. The observed MPI spectra are thus characteristic of the metal atoms, and carry information on the relative abundances of ground and excited state metal atoms produced in the fragmentation process. Analysis of these spectra provided new information on the np1 2PJ and nf1 2FJ Rydberg series of Al(I), Ga(I), and In(I). Evidence is presented to indicate that under relatively high visible laser flux conditions B(C2H5)3 undergoes a direct multiphoton ionization process to form the molecular ion.