Predicting intense-field photoionization of atoms and molecules from their linear photoabsorption spectra in the ionization continuum

We report a new approach to intense-field photoionization that is based on the ad hoc assumption that m photons of energy arriving within a typical electronic response time are effectively equivalent to a single photon of energy . The heuristic model contains no adjustable parameters and unifies apparent multiphoton and field aspects. Moreover, nonsequential, suppressed and above-threshold ionization phenomena become readily understandable. Predicted ionization intensities are in satisfactory agreement with available experimental data ranging from C₆H₆ to Ne^{3 +} , from femtosecond to nanosecond laser pulses, and from ultraviolet to infrared laser radiation.Received: 20 January 2004, Published online: 17 August 2004PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Wr Other multiphoton processes - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift     

Effects of the blackbody radiation on the harmonically bound electron-energy level shifts and consistency conditions

The influence of the vacuum radiation field on the harmonically bound electron (frequency ₀) is considered. The electron is minimally coupled to the blackbody radiation field. The dynamics of the system is exactly solvable. The high (k _B T₀) and low (k _B T₀) temperature expansions of the kinetic and potential energy are given. In the high temperature regime theT ²-dependent dynamic Stark shift is found whereas in the low temperature regime there is no temperature dependent shift. The position correlation function of the electron shows in the low temperature regime a unclassical algebraic decay (t ^–4,t/k _B T).     

Nonlinear behaviour of atomic fluorescence in mercury vapours following double-resonance laser excitation

When mercury atoms, present with a buffer gas (N₂ or Ar) in a quartz cell or in a graphite furnace are excited into the 7³ S ₁ level by means of two pulsed dye lasers tuned at 253.652 nm (6¹ S _o 6³ P ₁) and 435.835 nm (6³ P ₁ 7³ S ₁), a laser-like, collimated emission is observed along the axis of the cell at the green mercury line (7³ S ₁ 6³ P ₂), 546.074 nm). This radiation exhibits a pronounced non-linear dependence upon the number density of the mercury atoms in the cell. This behaviour is interpreted as being due to amplified spontaneous emission (ASE), which occurs as a result of the transient population inversion between the 7s ³ S ₁ and 6p ³ P ₂ ^o levels. The parameters governing the population inversion can be deduced by applying a rate equation analysis to the atomic system. A time-resolved observation of both ASE and spontaneous fluorescence signals confirms the totally different characteristics of the two emission processes.     

Quantum fields in 1+1-dimension carrying a true ray representation of the Poincaré group

In 1+1 spacetime dimensions there are genuine ray representations of the Poincaré group P ₊ ^sup . We shall construct a free relativistic quantum field theory, such that the fields _d are covariant under P ₊ ^sup and transform with a nontrivial infinitesimal exponent d.     

Photon echo in the presence of a magnetic field: New prospects for data accumulation and processing

The possibility for echo signal switching-off and the switching of echo polarization between the ±45° positions is demonstrated for the photon echo (PE) and stimulated photon echo (SPE) generated in Yb vapor at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition by two pulses of the resonant linearly polarized radiation at appropriate experimental parameters in the weak magnetic field limit. The experimental data are in qualitative agreement with the theoretical analysis and the calculations made for the 1 ? 0 transition. The strong magnetic field limit leads to unpolarized PE and SPE signals generated by linearly polarized radiation pulses. The possibility of the generation of a long-lived echo in ytterbium vapor due to the magnetic field induced mixing of the upper working level with the metastable level is discussed. The results can be employed for the optical data storage and processing.     

Positivity of energy inn-dimensional Lorentzian spaces

The spinor approach of Witten and Nester is used to show that in ann-dimensional Lorentzian spaceV _n , the conserved total energy-momentum vectorP is nonspace-like. It is shown thatP may be expressed in terms of an integral of an antisymmetric tensorE over ann – 2-dimensional subspace at space-like infinity inV _n .E is expressed in terms of a spinor field inV _n and its covariant derivatives. This tensor is a generalization of that used in the discussion of five-dimensional Lorentzian spaces.     

Resonance-enhanced multiple-photon ionisation spectroscopy of excited sulphur atoms

Sulfur atoms are produced in the excited 3s ²3p ^{4 1} D ₂ and 3s ²3p ^{4 1} S ₀ levels by two-photon photodissociation of CS₂ in the gas phase in the region 285–305 nm. These excited atoms are detected by three-photon (two to resonance) ionisation at selected laser wavelengths. Many new transitions have been observed which have not been reported previously. The new dark states that have been accessed by two-photon absorption have been characterised and the energy levels with respect to the ground ³ P ₂ state have been determined. Configuration interaction between 6p ³ P ₁, 6p ⁵ P ₁, and 4p¹ P ₁ states, and also between 6p ³ P ₂, 6p ⁵ P ₂, and 4p¹ D ₂ states has been observed. It is found that intermediate states reached from the ¹ S ₀ level through two-photon absorption lie above the first ionisation potential of S⁺(⁴ S ⁰). It is proposed that autoionisation into the ⁴ S ⁰+e^– continuum is a dominant mechanism in the decay of these levels, although ionisation by a further photon absorption is not ruled out.     

The applications of the stark effect to the stabilization of the CO2 laser and the submillimeter laser

Methods based on the Stark effect are described for dither-free frequency stabilization of the optically pumped submm laser. The CO₂ pump laser was stabilized using a Stark Lamb dip signal of the submm lasant in an external Stark cell. An estimated frequency stability (f/f) better than ±1.4×10^–8, for one hour recording, was obtained by this method. The frequency of the submm laser was stabilized using the d.c. and a.c. Stark effects for a metal-dielectric rectangular waveguide laser. An estimated frequency stability of ±6×10^–8 was obtained for 119 m line of CH₃OH laser for one hour recording.     

Off-shell photon light-cone wave functions with odd chirality

Based on a more reasonable simulation method for dealing with the Λ-parameter characterizing the pole form for the form factor (the Fourier transform of the instanton zero mode), and a unified way for regularizing the integrals appearing in the expressions of the light-cone photon wave functions, the transverse photon wave function (u, P ²) at the leading twist with the on-shell or off-shell momentum have been re-examined in the effective low-energy theory derived from the instanton vacuum, and the twist-two parts of the other two photon wave functions, h ^{(s), twisttwo} _{γ ||} (u, P ²) and h ^{(t), twisttwo} _{γ ||} (u, P ²), with odd chirality are calculated based on the Wandzura-Wilczek-like relations as well. A brief discussion of the dependence of the coupling f _γ ^⊥(P ²) and the light-cone photon wave functions with respect to P² and the end-point behavior of the photon wave functions are given.     

Measurement of dijet production at low <Emphasis Type="Italic">Q</Emphasis><Superscript>2</Superscript> at HERA

Triple differential dijet cross sections in interactions are presented in the region of photon virtualities 2 < Q ² < 80 GeV², inelasticities 0.1 < y < 0.85, jet transverse energies E ^* _{T 1} > 7 GeV, E ^* _{T 2} > 5 GeV, and pseudorapidities . The measurements are made in the centre-of-mass frame, using an integrated luminosity of 57 pb^-1. The data are compared with NLO QCD calculations and LO Monte Carlo programs with and without a resolved virtual photon contribution. NLO QCD calculations fail to describe the region of low Q ² and low jet transverse energies, in contrast to a LO Monte Carlo generator which includes direct and resolved photon interactions with both transversely and longitudinally polarised photons. Initial and final state parton showers are tested as a mechanism for including higher order QCD effects in low E _T jet production.Received: 13 January 2004, Revised: 21 July 2004, Published online: 18 August 2004     

Ultrafast optical responses of three-level systems in β-carotene: Resonance to a high-lying n 1 A g − excited state

Ultrafast optical phenomena in all-trans-β-carotene have been investigated by femtosecond absorption and fluorescence spectroscopy. Following a resonant pump pulse, both fluorescence and absorbance changes have a decay time of 150 fs. The signals are assigned to the lowest optically allowed singlet excited state, 1¹ B _u ⁺ . Transmittance changes induced by nonresonant pump pulses depend on the pump photon energy. They are interpreted in terms of the ac Stark effect in three-level systems and two-photon absorption of the pump and probe pulses generating a high-lying n ¹ A _g ^? state.     

A study of the ϱ0 meson polarization in π− p reaction at 5 GeV/c

Experimental data on ⁰ meson polarization in ^– p reaction at 5 GeV/c are presented. Change in ⁰ meson polarization for mesons produced at smallP ² and atP ² 0·3 or produced in backward direction in CMS is demonstrated. Natural explanation of these phenomena is one pion exchange mechanism and mechanism of quark-antiquark annihilation as observed for ⁰ meson production in¯pp reactions.The authors are indebted to Ján Piút, Richard Lednický, V. S. Rumiancev and N. K. Koutsidi for discussions and critical comments.     

Determination of the Concentration of Electrons in the Laser Plasma of Helium

The dependence of the Stark width of the spectral line of He II P 468.6 nm on the electron concentration in the laser plasma of helium for the range of electron densities N _e = (1–10)·10²³ m^–3 and electron temperatures of the order of 60 kK has been measured. The results obtained correspond well to Griem's theoretical data. An empirical relation is suggested which makes it possible to reliably determine the electron concentration from measured halfwidths in the investigated range of N _e.     

Depolarizing collisions in ytterbium vapor: Isotropic and anisotropic relaxation

Isotropic depolarizing collisions are studied using a stimulated photon echo with a specific polarization of the excitation radiation pulses in a mixture of ytterbium with krypton for the J = 1 ? J = 0 transition of ¹⁷⁴Yb. The difference between the relaxation rates of orientation and alignment γ _b ⁽²⁾ ? γ _b ⁽¹⁾ of the ³ P ₁(6s6p) ¹⁷⁴Yb level is measured as a function of the krypton pressure. The collision photon echo at the J = 1 ? J = 0 transition induced by the anisotropic relaxation is studied for the Yb + Xe mixture. The power of the collision echo increases from zero with the addition of a buffer gas to ytterbium, reaches an optimal level, and decreases with an increase in the buffer gas pressure. The polarization of this collision-induced echo differs from the polarization of the conventional echo. The experimental results are in qualitative agreement with theoretical predictions.     

Random walk with persistence

The exact analytic result is obtained for the Fourier transform of the generating functionF(R,s)= _n=0 s ⁿ P(R,n), whereP(R,n) is the probability density for the end-to-end distanceR inn steps of a random walk with persistence. The moments R ²(n), R ⁴(n), and R ⁶(n) are calculated and approximate results forP(R,n) and R ^–1(n) are given.     

Mesoscopic transport through toroidal carbon nanotubes threaded with a THz magnetic flux

We have investigated the quantum transport through mesoscopic systems with a toroidal carbon nanotube coupled with two metal leads (N-TCN-N) threaded with an ac magnetic flux. The energy shifting takes place by applying the magnetic flux, and this shifting arises from both the dc and ac components of magnetic flux. The dc magnetic flux induces the periodic variation of energy gap E _g of the TCN, and the ac magnetic flux component always increases the energy gap. As the photon energy is larger than the energy gap , the electrons in the valence band can jump to the conductance band at zero temperature, and the tunneling current appears for , ( ). The differential conductance and tunneling current display clear effect of ac flux by modifying the current oscillation structures. The photon-assisted tunneling current exhibits stair-like I-V characteristics, and it shows different behaviors for different TCN systems. The magnitude of the current is suppressed by the applied ac flux. We also present the time-dependent current evolution, which is contributed by the oscillating current components.Received: 31 May 2004, Published online: 3 August 2004PACS: 73.40.-c Electronic transport in interface structures - 73.63.Fg Nanotubes - 73.61.Wp Fullerenes and related materials - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

Zero magnetic field splitting and anomalies of intensities in the Zeeman spectra of La(Pr)Cl3-crystals

Optical absorption and Zeeman effect measurements on La(Pr)Cl₃ show a zero magnetic field splitting of the degenerate Pr³⁺ levels and . The site symmetry of the Pr³⁺ ions is lowered fromC _3h toC _s . The resultant splittings of the doublet levels are:I ³ H ₄:2|T _ak |=(0.12±0.05)cm^–1;a ³ P ₁:2|T _al |=(0.10±0.05)cm^–1.The intensities of the optical transitions induced by the deformation decrease with increasing magnetic field. All observations are described by a first order perturbation calculation using a low symmetric part of the crystal field and Zeeman energy as simultaneous perturbations on a Pr³⁺ ion in a crystal field with site symmetryC _3h .Project of the Sonderforschungsbereich Festkörperspektroskopie SFB65, Darmstadt—Frankfurt, supported by the Deutsche Forschungsgemeinschaft.     

The loop expansion for the effective potential in theP(φ)2 quantum field theory

We study the loop expansion for the effective potential, defined as the Fenchel transform (convex conjugate) of the pressure in an external field, in theP()₂ quantum field theory. For values of the classical fielda for which the classical potentialU ₀(a)=P(a)+1/2m ² a ² equals its convex hull and has nonvanishing curvature we prove that the 1-PI loop expansion is asymptotic as 0. We also give an example of a double well classical potential for which the 1-PI loop expansion fails to be asymptotic, and find the true asymptotics.This paper is a condensed version of the author's Ph.D. thesis for the Department of Mathematics, University of British Columbia, Vancouver, B.C., Canada V6T 1Y4     

Collisionless excitation of NH3 molecules via one- and two-photon transitions by multimode radiation of TEA CO2-laser

The excitation of ammonia by one- and two-photon transitions by TEA CO₂-laser radiation has been studied under collisionless conditions. It has been shown that the experimental results of two-photon excitation of ¹⁴NH₃ molecules in the 2v ₂ a(1, 1) a(1, 1) transition by the 10P(24) CO₂-laser line, the frequency offset of which is 0.02cm^–1, can be described well by modelling the laser radiation as a broadband chaotic field. The experimental results of one-photon excitation in ¹⁴NH₃ and ¹⁵NH₃ [4], with the offset values exceeding the CO₂-laser linewidth, can be also described well with the use of a chaotic field with a continuous spectrum. In the case of a nearly exact one-photon resonance, however, the excitation efficiency of molecules depends critically on the real mode structure of the CO₂-laser radiation.     

Continuous-wave frequency mixing and UV generation in sodium vapor

Resonant nonlinear four-wave mixing processes have been studied in sodium vapor. The generation of cw uv radiation and the upconversion of =10.8 m light is reported. The coefficientC=P ₄/P₁P₂P₃ obtained was on the order of 10^–2 W^–2, the pump spectral width being 12GHz. Resonant atomic nonlinearities are shown to be used for effective cw frequency conversion.