Photoelectron Energy Spectra Research Articles

Stabilization in relativistic photoionization with circularly polarized light.

Relativistic ionization rates and related phenomena are calculated for ground-state hydrogen atoms in the presence of a circularly polarized electromagnetic field. A Dirac formalism is used, with spin effects fully included. A primary purpose of this work is to explore the effects of relativity on atomic strong-field stabilization. A wide range of frequencies is covered, and in all cases relativistic effects first dampen the stabilization phenomenon as the field intensity increases, and then strongly enhance it in the intensity domain where the ponderomotive potential exceeds the electron rest energy. Relativistic effects can produce major changes in the photoelectron energy spectrum, but perhaps the most easily observable effect is the major shift that can occur in the photoelectron angular distribution.

Electrochemical preparation of microelectrodes modified with non-stoichiometric mixed-valent molybdenum oxides

A blue, conductive, compact, mixed-valent Mo(VI, V) oxide film was grown on carbon fiber (CF) microelectrode surface by cycling the potential between + 0.20 and −0.70 V vs. SCE in a fresh prepared Na 2MoO 4 solution (pH = 2, H 2SO 4). The thickness of the oxide film was controlled by charge passing through. X-ray photoelectron energy spectra (XPS) confirmed that the products of molybdenum oxide film coating on the CF electrode surface are mainly a mixture of Mo 6+ and Mo 5+ complex, meanwhile a little of Mo 4+ exists in the film, but the Mo 3+ peak was not observed in XPS. Well-defined redox transitions have been attributed to the reductive formation of hydrogen molybdenum bronzes [MoO 3−x(OH) x] and reoxidation in acidic media. The coating is easily permeable to ions. The molybdenum oxide film catalyses the electroreduction of chlorate to chloride and bromate to bromide in H 2SO 4.

Plateau in above threshold ionization spectra.

We present photoelectron energy spectra for the rare gas atoms in strong 40 fs, 630 nm laser pulses. A new property in the above threshold ionization distribution is described, namely, a plateau. Numerical calculations using one- and three-dimensional models suggest that at least in part this is a one-electron effect. All rare gas atoms investigated show similar behavior, indicating that the plateau in above threshold ionization is a universal phenomenon. We discuss a simple mechanism possibly responsible for the plateau.

Above-threshold multiphoton photoelectric effect of a gold surface

A discrete structure consisting of peaks separated by hv photon energy was found experimentally in the photoelectron energy spectrum, when illuminating a gold target surface by relatively long light pulses (approximately nanoseconds) and using moderate (approximately 120 MW/cm 2 ) intensity of Nd:glass lasers (λ = 1.06 μm). This structure can be considered as the above-threshold photoeffect (ATP) of metal surfaces. Its occurrence at surprisingly low laser intensities [which are four orders of magnitude less than the typical intensity range of the analogous above-threshold ionization (ATI) of atoms] was observed when the space charge was considerably eliminated.

Theory of atomic excitation and ionization by ultrashort laser pulses.

An alternative momentum-space representation (``impulse representation'') well adapted to the evaluation of converged nonperturbative solutions of the nonrelativistic time-dependent Schr\odinger equation for atoms subject to intense, ultrashort laser pulses is described. Exact expressions for survival, excitation, and ionization probabilities and photoelectron energy spectra are presented for the case of a hydrogen atom struck by a \ensuremath{\delta}-function pulse. A method for the extension of such calculations to realistic ultrashort pulses is described, based on a Sturmian basis expansion of the time-dependent Schr\odinger wave function.

Two-photon dissociation of HD+ with one intermediate resonance in the presence of two different laser fields

The time-dependent dissociation probabilities, line shapes of dissociation rate, and photofragment kinetic-energy distribution have been investigated for resonant two-photon dissociation of HD+ from the 1sσg(v=0, J=0) state in the presence of two radiation fields of different frequencies ω1 and ω2. Simultaneous dissociation in two different electronic states at two different final energies occur through the absorption of either ω1+ω2 or 2ω2 photons. Only one of the two fields causes a near-resonant coupling either with (v=6, J=1) through absorption of a photon of frequency ω1 or with the (v=14, J=1) state through absorption of a photon of frequency ω2. Dissociation probabilities have been calculated as functions of the resonant and off-resonant field amplitudes and times. The dissociation probability at fixed times show interesting variation with the field amplitudes. Conditions for time-dependent interference oscillations in dissociation probabilities are discussed and these oscillations demonstrated. Intensity-dependent line shapes for dissociation rate are obtained for various other combinations of fields. Long-time kinetic-energy distribution of photofragments is similar to the energy spectrum of photoelectrons obtained from atomic autoionization. The photofragment spectra for two energies in two final electronic continuum states are compared for different values of the field amplitudes and detunings.

Resonant above-threshold ionization of atomic hydrogen.

An all-order multiphoton resonant theory for above-threshold ionization is presented within the resolvent formalism. An analytical expression displaying all the contributions to the electron yield is discussed. The theory is applied to the calculation of photoelectron energy spectra for ionization of H by picosecond and subpicosecond pulses of linearly polarized light at 608 nm, with laser intensities in the range ${10}^{13}$--${10}^{14}$ W/${\mathrm{cm}}^{2}$. The role of intensity, interaction time, and transition couplings, in the resonantly enhanced electron yield coming from each (nl) intermediate state, is discussed. The modifications which affect the shape of the spectrum as the laser peak intensity increases are described in great detail. An inversion of the relative amplitudes of the resonant structures is observed. This effect is attributed to the relative dimensions of the effective interaction volumes in which the field reaches sufficient intensities to give rise to the resonances. Our results reproduce the main features of recently measured spectra [H. Rottke et al., Phys. Rev. Lett. 64, 404 (1990)].

Energy partition among fragments and electrons in high field dissociation.

We present an experimental investigation for chlorine molecules and their atomic fragments in which high field molecular dissociation is followed by fragment ionization. It is shown that the intensity dependence of the photoelectron and atomic photofragment energy spectra provides a direct measure of the fragments' ac-Stark shift. Furthermore, we observe an unusually strong above-threshold ionization photoelectron series associated with the two-photon ionization of the excited chlorine fragment. This implies that the fragments are experiencing field strengths in excess of the ``low field'' saturation. These results are compared to a single-channel quantum-defect calculation.

Photoelectrons from resonant ionization of Na 2: angular and non-Franck—Condon energy distributions

We present energy and angular distributions of photoelectrons from the resonant two-photon, two-color ionization of sodium dimer through particular rovibronic levels of the B 1Π u and A 1Σ + u state. The vibrational state distributions of the ion core produced from the B state do not correspond to Franck—Condon distributions, according to the photoelectron energy spectra. Non-Franck—Condon distributions of final states were observed at two ionizing wavelengths, and through all the four intermediate rovibrational levels of the B state studied here. Ionization through the A state showed a Franck—Condon-like distribution of final states. At best, plausible conjectures can be suggested to account for the non-Franck—Condon behavior of the B state.

Space-charge effect on the energy spectrum of photoelectrons produced by high-intensity short-duration laser pulses on a metal.

We consider the photoelectron spectra produced by a high-intensity short-duration laser pulse on a metal and present the results of a computer simulation of the electron-energy variations in a diode and beyond it, taking into account the space-charge effect. We show that the energy increase in the diode is indeed responsible for the unexpectedly high-electron-energy spectrum observed in a recent experiment by Farkas and Toth [Phys. Rev. A 41, 4123 (1990)].

Mass spectra and photodetachment of sodium fluoride negative ion clusters

A flowing afterglow discharge ion source with NaF vaporized into the flow produces the expected stable anions F −(NaF) n , as well as the unusual species Na(NaF) − n and (NaF) − n ( n ⩽ 17). The dominance of (NaF) − 6 suggests that it has a particularly stable structure. A fixed-frequency laser has been used to photodetach electrons from mass-selected ion beams extracted from the flowing afterglow. Photoelectron energy spectra, vertical detachment energies and estimates of adiabatic electron affinites are reported for the (NaF) − n and Na(NaF) − n ions. The F(NaF) − n ions are found to be too stable for photodetachment at 488 nm.

Influence of space charge on photoelectron angular distributions and energy spectra in resonance multiphoton ionization

The influence of space charge on photoelectron energy spectra and angular distributions has been investigated experimentally for the two photon resonance three photon ionization of Xe via the 4f [3/2], J = 2 states. Experimental conditions under which space charge does not affect the photoelectron angular distribution are discussed.

Multiphoton detachment of H- and the applicability of the Keldysh approximation.

We report results of calculations of photodetachment rates and photoelectron energy spectra for the negative hydrogen ion irradiated by a low-frequency field. We have carried out calculations in both one-electron and two-electron models (with electron-electron correlation included in the initial channel in the two-electron model). Our main focus is on exploring the differences between various approximations that arise from different treatments of the electron-field interaction, rather than from different treatments of the electron-electron interaction. Thus, for the most part, we describe the active electron by the one-electron model. Within this framework, the Keldysh theory yields results in remarkably good agreement with those obtained using Floquet theory. Furthermore Keldysh theory reproduces well the minima and ``discontinuous'' slopes of the rate at thresholds. The accuracy of the Keldysh approximation can be understood by examining the parameters that characterize the strength of the electron-field interaction. The Faisal-Reiss theory also gives good agreement with Floquet theory (unless the intensity is very high); the reason for this is more technical, having to do with the short range of the one-electron atomic potential. We have generalized the Keldysh and Faisal-Reiss theories to include electron-electron correlation by representing the ground-state ion with an accurate two-electron wave function. Correlation leads to a long-range interaction of the active electron with the atomic core, and thereby to more significant discrepancies between the Keldysh and Faisal-Reiss theories. We compare results obtained using the two-electron Keldysh theory with results, obtained by others, using two-electron perturbation theory and two-electron Floquet theory.

An Extended Model of Double Optical Resonance

Abstract A ladder-type model of double optical resonance, extended by allowing some internal structure for the middle level, is considered and its classical analogue is given. Ionization and photoelectron energy spectra from such a model with singlet, doublet and triplet middle-level structures are evaluated numerically for a wide range of laser pulse intensities and durations. The topics of most interest are peak narrowing by extremely strong upper-level ionization damping, reversal of the peak-height asymmetry by increasing lower-resonance strength at non-zero upper-resonance detuning, and number-of-peak reduction and flattening of the spectrum by shortening of the atom-photon interaction time. When applied to natural barium, this model gives results in good agreement with experiment.

Nonadiabatic transitions between degenerate opposite-parity states of hydrogen in an intense field.

We analyze the photoelectron energy spectrum that results from near-threshold ionization of atomic hydrogen, initially in the 2s state, by a sharply turned on intense field. We demonstrate the existence of resonance structure close to threshold. The structure derives from transitions between degenerate ns and np states; opposite parity, degenerate states in the hydrogen atom are mixed by the nonadiabatic turn-on; at higher energies further structures in the photoelectron energy spectrum are generated through the action of counterrotating terms in the atom-laser coupling. We confirm the existence of a Rydberg series structure and investigate the possibility of q reversals in the line shapes.

Energy spectrum of photoelectrons produced by picosecond laser-induced surface multiphoton photoeffect.

Photoelectrons were produced from a gold surface using strong picosecond Nd:glass laser pulses of ${10}^{10}$-W/${\mathrm{cm}}^{2}$ intensity. Unexpectedly high (up to 600 eV) energies were observed in the electron energy spectrum only when space-charge effects were completely eliminated. This result is compared with the well-known electron energy spectra of the multiphoton ionization of gases.

Multiphoton processes in an intense laser field: III. Resonant ionization of hydrogen by subpicosecond pulses.

We have calculated photoelectron energy spectra for ionization of H(1s) by subpicosecond pulses with wavelengths 608 and 616 nm and with maximum intensity 1.2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W/${\mathrm{cm}}^{2}$. The main features of the recently measured spectrum (at 608 mm) are reproduced.

Photoelectron spectroscopy of vibrationally excited H2 (E,F 1 Sigma g+).

Photoelectron energy spectra following (2+1)-multiphoton ionization of ${\mathrm{H}}_{2}$ via the double-well E,${\mathit{F}}^{1}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}^{+}$ state have been measured using a magnetic-bottle electron spectrometer. The range of vibrational levels studied, ${v}_{E}$,F=3--9, includes states localized at the bottom of the E well, and also states that span both wells. The branching among the vibrational ionization channels is governed by the strong R dependence of the electronic wave function and the degree of localization of the vibrational wave function in the E or the F well. Theoretical analysis confirms that at least two mechanisms contribute to the observations: a direct process involving only the E,F state and the ${\mathrm{H}}_{2}^{+}$ ionization continuum, and an indirect process involving the $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ autoionizing state. Mass spectroscopic measurements carried out for ${v}_{E}$,F=9, J=1 show that about 25% of the ions produced were ${\mathrm{H}}^{+}$. Qualitative arguments suggest that most of these protons arise from dissociative ionization to the ${\mathrm{H}}_{2}^{+}$ continuum.

Giant Stark shifts in multiphoton ionization.

Photoelectron energy spectra from multiphoton ionization, when taken with ultrashort pulses (<ps), have been shown1 to reveal the resonant character of the transitions, while this effect remains hidden with longer pulses. The resonances are induced by ac Stark shifts and produce structures in the electron energy spectra. The amplitudes of the structures depend on the pulse peak power but not on their positions (in energy), which depend only on the static detuning of the resonance and the various Stark coefficients.

Anomalies in above-threshold ionization observed in H2 and its excited fragments.

The photoelectron energy spectra resulting from a multiphoton process in ${\mathrm{H}}_{2}$ with high laser intensities is, in general, determined by the character of the intermediate resonance state. Above-threshold ionization (ATI) is observed in the ionization of the molecule ${\mathrm{H}}_{2}$ as well as of its excited atomic fragments, and even of atomic hydrogen in the ground state. ATI is observed with five, three, and two extra photons for three different wavelengths \ensuremath{\lambda}=532 nm, \ensuremath{\lambda}=355 nm, and \ensuremath{\lambda}=266 nm, respectively. In the \ensuremath{\lambda}=532 nm spectra radiative coupling between dressed states is observed. A shift to higher values of the center of the vibrational distribution of the ion by seven vibrational quanta is observed between ATI peaks which differ by four photon energies. A calculation, taking into account a coupling of the ${\mathit{B}}^{1}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$ state to the dressed $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}^{+}$(2p${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$${)}^{2}$ state, and of the X $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}^{+}$ state by the dressed 2p${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$ state, shows the trend of the observed ionization spectrum. The calculation is performed to describe the effect on the average of the vibrational distribution and not on the distribution itself. The structure of the ATI spectrum in the \ensuremath{\lambda}=355 nm and \ensuremath{\lambda}=266 nm is nearly a reproduction of the normal ionization spectrum, repeated several times at higher electron energies. In the \ensuremath{\lambda}=266 nm spectra both ATI of the molecule, as well as ATI of the atomic fragments, are observed.