Two-step Photoionization Research Articles

A tunable laser-atomic beam photoelectron source with sub-milli-electron-volt resolution: design, operation and application to threshold electron attachment studies

A novel tunable laser-atomic beam photoelectron source with sub-milli-electron-volt resolution, producing energy-variable free electrons (current 10-12 A) by resonant two-step laser photo-ionization of metastable Ar*(4s 3P2) atoms in a collimated beam, is described. Operation and application of the source are demonstrated by a series of laser photoelectron attachment experiments, in which the process e-(E)+SF6 to SF6- is investigated in the electron energy range 0<E<200 meV, both under nearly field-free conditions (electric stray fields 0.03-0.05 V m-1) and in defined static electric fields up to 650 V m-1. In essential agreement with the results of model calculations, the effective electron attachment yield is characteristically reduced near threshold (E approximately=0) with increasing electric field strength. The field also affects the contribution due to electron transfer from Rydberg atoms, either excited below the Ar+(3P5 2P32/) ionization threshold (E<0) or close to the Ar+(3P5 2P12/) threshold (E=177.5 meV), where stabilization of auto-ionizing Ar**(nl') Rydberg states by l-mixing occurs. The possible influence of such electric field effects on previous threshold photoelectron spectroscopy for attachment data is examined. Prospects of future experiments involving the described or more intense laser photoelectron sources, based on resonant two-step photo-ionization of alkali atoms, are discussed.

Photoionization of the DX centers in Te-doped AlxGa1-xAs: Absence of two-step photoionization of DX centers.

Photoionization studies have been performed for Te-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As to investigate the properties of the theoretically predicted intermediate neutral relaxed state of the DX center, i.e., ${\mathit{DX}}^{0}$. The expected effect of the ${\mathit{DX}}^{0}$ state on the photoionization process in the neutral charge region, where the recapture of an electron by ${\mathit{DX}}^{0}$ is possible, was calculated from the rate equations for a two-electron system. The photoionization measurements were made in both the depletion region and the neutral charge region. It is found that the photoionization rates of DX centers in the two charge regions are identical, indicating no significant effect of the ${\mathit{DX}}^{0}$ state on the photoionization of DX(Te) centers. Thus, the intermediate ${\mathit{DX}}^{0}$ state, if it indeed exists, must be highly unstable, with an activation energy of less than 20 meV. These results contradict an earlier report which claimed to find direct evidence for a two-step photoionization of DX(Te) centers. The results of the present study, together with a detailed analysis of the experimental method used in the earlier study, indicate that the earlier claim is unsubstantiated.

Persistent photoconductivity and photoionization of deep electron traps in Ga-doped Cd1-xMnxTe.

Strong persistent photoconductivity (PPC) has been observed in bulk n-type Cd 1-x Mn x Te (x=0.03) crystals heavily doped with Ga at low temperatures (below ∼150 K). The kinetics of the PPC effect were investigated by means of photoconductivity and photo-Hall measurements. The observed photo-conductivity transients displayed a strongly nonexponential behavior. We use the observed transient behavior to show that the transfer of electrons from the ground state of defects, responsible for the PPC, to the conduction band proceeds in two steps, via an intermediate state of the defects. Numerical simulations based on the two-step photoionization model are in excellent agreement with the experimental data

Laser desorption laser photoionization time-of-flight mass spectrometry of dyes

The mass spectra of a number of dye species have been investigated using two-step desorption laser photoionization time-of-flight mass spectrometry (L 2 TOFMS). A pulsed CO 2 laser, wavelength 10.6 μm, was used to vaporize the samples as intact neutral molecules which were photoionized using either 193- or 266-nm laser radiation. Four classes of dyestuffs have been examined: azo, antraquinone, phthalocyanine, and coumarin. Significant differences were found in the fragmentation patterns and ionization efficiencies at these two laser wavelengths particularly for the azo dyes

Two-step selective photoionization of rubidium by using a Ti:sapphire laser

A pulsed Ti:sapphire laser (TSL) is applied for what is, to our knowledge, the first time to laser isotope separation. A collimated beam of a rubidium atom with a natural isotope composition ((85)Rb, 72%; (87)Rb, 28%) is excited selectively from the ground state to the 5(2)P(3/2) state by irradiation of the single-longitudinalmode TSL, and is photoionized by successive UV radiation. Ion spectra of each isotope indicate that each isotope is selectively photoionized.

Two-step single-colour photoionization spectroscopy of atomic uranium

The single-colour photoionization spectrum of atomic uranium is recorded in the spectral region from 366 to 371 nm. Most of the 43 observed resonances are found to be associated with the two-step photoionization process. Based on known energy levels seven new transitions have been proposed. Three of these transitions originate either from the ground state or the lowest metastable state at 620 cm−1.

Comment on ‘‘Complete fragmentation pattern for two-step double photoionization in Xenon’’

Comment on ‘‘Complete fragmentation pattern for two-step double photoionization in Xenon’’

Resonance ionization in a gas cell: a feasibility study for a laser ion source

A laser ion source based on resonance photo-ionization in a gas cell is proposed. The gas cell, filled with helium, consists of a target chamber in which the recoil products are stopped and neutralized, and an ionization chamber where the atoms of interest are selectively ionized by the laser light. The extraction of the ions from the ionization chamber through the exit hole-skimmer setup is similar to the ion-guide system. The conditions to obtain an optimal system are given. The results of a two-step one-laser resonance photo-ionization of nickel and the first results of laser ionization in a helium buffer gas cell are presented.

Photoelectric studies of two-step photoionization of Ti3+ ions in oxide crystals

In photoconductivity studies in Al2O3:Ti3+ and Y3Al5O12:Ti3+ (YAG:Ti3+) a nearly quadratic dependence of photocurrent on excitation intensity is experimentally observed. Spectral and kinetics measurements show that the Ti3+ photoionization under visible and UV excitation in the t → e transition region takes place as a result of a two-step process via the intermediate e state. The experimentally estimated energy between this e level of Ti3+ and the conduction band bottom is in Al2O3 larger and in YAG smaller than the zero-phonon e-t line (ZPL) energy. This determines the essential difference in persistent hole-burning in the ZPL of Al2O3:Ti3+ and YAG:Ti3+[1] and in the efficiency of e level optical pumping.

Measurement of yields for electron emission from surfaces upon impact of laser-excited Ar*(4p) and Kr*(5p) atoms

A laser method for the determination of the electron emission coefficient gamma for the impact of slow laser-excited rare gas atoms Rg*(mp5(m+1)p 3D3) on surfaces is described. It is based on controlled partial depletion of metastable Rg*(mp5(m+1)s 3P2) atoms in a collimated beam by two-step laser photoionization in combination with saturated laser excitation of the metastable (3P2) atoms to the (3D3) level at the surface. The method simultaneously yields the coefficients gamma for the metastable Rg*(3P2) and the laser-excited Rg*(3D3) atoms. As an application, the authors report gamma values corresponding to the impact of Ar*(3P2, 3D3) and Kr*(3P2, 3D3) atoms on several gas-covered metal surfaces. The data show that the electron emission process involves Auger de-excitation and can be interpreted as Penning ionization of adsorbed molecules. Comparison of the gamma values for the four excited rare gas species shows a complex excitation energy dependence.

Complete fragmentation pattern for two-step double photoionization in xenon.

An extensive study of the 4${\mathit{f}}_{5/2}$ photoionization in xenon is presented which allows, through the observation of the coincident ${\mathit{N}}_{5\mathrm{\ensuremath{-}}}$${\mathrm{O}}_{23}$${\mathrm{O}}_{23}$ $^{1}$${\mathit{S}}_{0}$ Auger electron, the establishment of the first complete fragmentation pattern for two-electron emission with information on the momenta and the spin projections of both electrons.

Laser pulse sequence effects on selective ionization in three-level systems driven by coherent radiation fields

Using the time-dependent Schrodinger equation we investigate the effect of different pulse sequences of two lasers on selective ionization in three-level systems driven by coherent radiation fields. It is assumed that inhomogeneous broadening as well as the radiative decay can be neglected during the laser pulses. We demonstrate to what an extent the selectivity given by the absorption or relaxation process can be increased by coherent interaction during the first step and different timing of the incoherent interaction during the second step of a two-step photoionization. A comparison is made with the rate-equation results. We could show that utilization of coherent excitation for high ionization selectivity is most efficient if the two laser pulses do not overlap.

A method of detecting the rare isotopes 85Kr and 81Kr by means of collinear laser photoionization of atoms in an accelerated beam

A method is suggested for detecting rare krypton isotopes, based on the collinear laser photoionization of atoms in an accelerated beam at the exit from a mass separator. The results of investigations into the two-step collinear laser photoionization of metastable krypton atoms in an accelerated beam are presented.

Direct evidence for two-step photoionization of DX (Te) centers in AlxGa1-xAs.

A detailed analysis of the photoionization process of DX(Te) centers in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As (0.25x0.55) for different temperatures and light intensities revealed, for the first time, that the process goes through two steps. The only possible interpretation of the phenomenon is by means of a negative-U character of the defect; i.e., the center binds two electrons in the ground state, forming a ${\mathit{DX}}^{\mathrm{\ensuremath{-}}}$ energy state. An intermediate state for the process is a one-electron localized state of the defect.

Two-step photoionization of hydrogen atoms in interplanetary space

Photoionization is one of the key processes which determine the properties of fluxes of neutral atoms in interplanetary space. A new two-step channel (called indirect) of photoionization of hydrogen atoms is proposed. Hydrogen atoms are at first excited to states with principal quantum number n > 2, then decay to metastable H(2S) states, where they can be photoionized. Competing processes due to the interaction with solar wind plasma and solar radiation are considered and the photoionization rate through the proposed indirect channel is calculated. This rate depends on distance from the Sun as ∞ 1 R 4 at large distances ( tR > 1–2 a. u.) and as ∞ 1 R 2 at close approaches, where it is higher than the rate of direct photoionization.

An analysis on the efficient laser photoionization of atoms

The basic characteristics of the two-step three-level laser photoionization of atoms with statistical broadening were analyzed by using the Bloch equation. It followed from numerical results that in most practical cases, the minimum laser fluence in terms of the efficiency of ionization is obtained when the Rabi frequency of the first excitation step is larger than the second excitation step. The minimum fluence is dependent on the ionization probability and the degree of broadening, while there is little dependence on the interaction time.

Influence of the 3d 23P 0 and 3d 23P 2 valence states on the stark spectrum of the rydberg state of Ca

The Stark spectrum of the Ca atom has been investigated in the vicinity of the n = 10–13 transitions by using two-step excitation and photoionization methods with an applied electric field up to 8 kV/cm. The results indicate that the 3 d 2 3 P 0 and 3 d 2 3 P 2 valence states are repelled by the n = 12 Stark manifold. Interference has been observed for the 3 d 2 3 P 0 state and the Stark states and is enhanced as the electric field is increased. The crossing and anticrossing structures of levels among the zero-field non-degenerate states, valence states, and Stark states are also discussed.

Resonant two-step photoionization of molecules: Effects of pulse sequence, pulse duration and delay time of two lasers on ionization probability and selectivity

Using rate equations the effect of different pulse sequences, pulse durations, and delay times of two lasers on ionization probability and selectivity in resonant two-step photoionization of molecules is investigated. The results demonstrate to what extent a given selectivity of the absorption or relaxation of the intermediate electronic, state is influenced by a special combination of these parameters.

Measurement of vibrational frequencies of the H3 molecule using two-step photoionization

A two-step photoionization scheme is used to determine vibrational frequencies of the n=3 Rydberg states of triatomic hydrogen. Symmetric-stretch frequencies in orthotrihydrogen of 3212.1 and 3168 cm−1 are measured for the N=1 levels of the states 3s 2A′1 and 3d 2E″, respectively. Transitions to bending-mode excited levels in the 3d states and overtone levels of the 3p 2 E′ state are observed also.

Optical-absorption and photoionization measurements from the excited states of Ce3+:Y3Al5O12.

The optical transitions in ${\mathrm{Ce}}^{3+}$:${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ from the lowest 5d excited state of the ${\mathrm{Ce}}^{3+}$ ion to the host conduction band have been investigated. Two spatially overlapped and time-sequenced laser pulses were used to observe directly the excited-state absorption (ESA) and the two-step photoionization of the cerium ions. The peak cross section of the ESA transition is (1.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$ at 700 nm. The ESA spectrum was measured from 600 to 880 nm and is used to construct an energy-level diagram for the ${\mathrm{Ce}}^{3+}$ ions relative to the energy-band states of the ${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ host. The energy gap between the lowest relaxed 5d state and the conduction-band edge is estimated to be 10 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Strong-probe saturation measurements indicate a slow and inefficient back-transfer of excitation to the cerium ions following the ESA. Photoconductivity measurements were used to confirm the two-step photoionization of the cerium ions.