Optimization of soft x-ray line emission from laser-produced carbon plasma with laser intensity

Absolute measurement for He-α resonance (1s^{2 1}S₀?1s2p¹ P ₁, at 40.2 Å) line emission from a laser-produced carbon plasma has been studied as a function of laser intensity. The optimum laser intensity is found to be ≈1.3×10¹² W/cm² for the maximum emission of 3.2 × 10¹³ photons sr^?1 pulse^?1. Since this line lies in the water window spectral region, it has potential application in x-ray microscopic imaging of biological sample in wet condition. Theoretical calculation using corona model for the emission of this line is also carried out with appropriate ionization and radiative recombination rate coefficients     

Two-photon photoemission from Ni (1 1 0) and (1 1 1) surfaces

Photoemission was observed when the samples were irradiated with photons in the energy range from 2.5 to 3.3 eV from a tunable dye laser with an intensity of 10⁸Wcm^?2. The emission shows a quadratic intensity dependence. The variation with angle of incidence and polarization is different for the two surfaces. The result obtained from the (1 1 0) surface is discussed with help of the band structure as a two-photon surface photoelectric effect.     

Photon and electron emission during halogen adsorption on sodium

The adsorption of chlorine, bromine and iodine on UHV evaporated sodium films is accompanied by the emission of photons and electrons. Although the emission probabilities are small (～10^?7 and ～10^?5 per adsorbed molecule respectively), the effect provides a sensitive means of monitoring the adsorption process. For moderate exposures, the photon and electron yields are proportional to the pressure. At high exposures, due to the formation of a halide film on the surface, the photon yield decreases rapidly, eventually becoming proportional to the square root of the pressure. The high sensitivity allows use of the light signal to measure diffusion through the halide layer. The spectral distributions of the observed photons are characterized by wide (～1 eV) emission bands in the range 4000–8000 A. The spectra are influenced by surface composition, as demonstrated most clearly in the Na + I₂ spectrum, where a peak occurs on the high energy side of the main band when the NaI film starts to form. The origin of the spectra, their comparison with gas phase chemiluminesccnce and a model of the reaction kinetics are discussed.     

The temperature dependence of optical gap and photoconductivity threshold in undoped hydrogenated amorphous silicon films

We report on the temperature dependences of the optical gap E_o and the photoconductivity threshold (?ω)_o for undoped hydrogenated amorphous silicon films. When increasing the temperature, both E_o and (?ω)_o are seen to linearly decrease at respective rates β= 3.5 10^?4 eV K^?1 (temperature range 290 K–460 K) and γ= 5.2 10^?4 eV K^?1 (temperature range 220 K – 360 K). At higher temperatures E_o decreases at the rate β = 14.3 10^?4 ev K^?1. Our results are discussed in terms of conduction in extended states. We show there is no physical reason in relating the temperature dependence of the activation energy and that of the gap as generally assumed. From optical absorption we deduce a minimum metallic conductivity σ_min the value of which agrees with Mott's predictions. On the contrary, σ_min measured from dark conductivity is nearly two orders of magnitude lower. A discussion is proposed infering band bending at the film substrate interface.     

On the enhancement of the nonlinear frequency transformation in periodic media

It is theoretically shown that an enhancement of the nonlinear frequency transformation in a periodic medium occurs under definite conditions if it is accompanied by light diffraction in this medium. The enhancement of the phase-matched frequency transformation occurs if the harmonic frequency ω approaches the stop band frequency ω_e. In the nondepleted pump approximation the corresponding increase of the harmonic intensity is proportional to ω_e(ω?ω_e)^?1 and is restricted by a quantity proportional to the fourth power of the sample thickness. The formulas for the enhancement of second harmonic generation are presented.     

Spontaneous emission of photons and electrons during chemisorption of chlorine on sodium

Photons and electrons are emitted when Cl₂ molecules react on a Na surface prepared by UHV evaporation. The emission yield per reacting molecule is 10^?7-10^?6 for photons and approximately 10^?5 for electrons. The dominating light emission band has a maximum at hv = 2.15 eV (width 0.6 eV). A less intense u.v. band has a maximum at about 4.7 eV. A drastic decrease in the photon and electron emission at a Cl₂ exposure of about 5.10^?3 torr·sec, is attributed to the formation of a continuous NaCl film on the Na surface.     

Fast Intrinsic Emissions of Wide-Gap Oxides Under Electron Irradiation

The emission spectra have been measured in the range of 1.6–9.0eV under irradiation of wide-gap oxides by single electron pulses (3 ns, 300kV). A fast (τ < 3 ns) continuous and temperature-independent emission, connected mainly with the transitions of hot holes between the levels of the valence band of oxides, can be separated in these spectra at 300–600 K, when the inertial emissions (5–7eV) of localized excitations undergo a strong thermal quenching. It is suggested that a drastic decrease of the intensity of this so-called hole intraband luminescence (IBL) in a short-wavelength spectral region is caused by the lowering of the density of states at the edges of the valence band and, therefore, supplies information on the width of an anion valence band E^v. The drastic decrease of the IBL intensity takes place at 6.4–8.6eV in BaMgAl₁₀O₁₇, SrAl₂O₄, MgAl₄O₇, MgO and BeO, that agrees satisfactorily with the values of E^v in these systems obtained by other methods.     

Optical absorption in a heavily doped semiconductor with a narrow forbidden band

The coefficient of interband absorption in a heavily doped, strongly compensated semiconductor with a narrow forbidden band was calculated for emission frequencies ?ω which are smaller than the width? _g of the forbidden band. If the width of the forbidden band is smaller than the characteristic energy w₀ of the random field produced by randomly distributed impurities, the optical absorption gap is missing and the coefficient of absorption increases with the frequency, with the increase being basically proportional toω.     

Shape of the IR bands of CH4: The CH4-Kr system in different phase states

The IR absorption spectrum of mixtures of CH₄ with Kr in the range 800–6000 cm^?1 is studied in the region of the gas-liquid and liquid-crystal phase transitions. It is found that, as the system density increases, the ratio of the ν₄ band intensity to that of the ν₃ band becomes larger and the measured values of the second spectral moment of the ν₄ band become smaller than the corresponding calculated values. The quantum counterpart of the generalized J-diffusion model was used to describe the shape of the ν₃ and ν₄ bands, and the rotational relaxation times of CH₄ in solutions were obtained. It was shown that, as the density increases, a broad 1470-cm^?1 band appears in the region of the “forbidden” (induced by a Coriolis interaction) ν₂ band (ν_Q=1535 cm^?1) detected in a low-density gas. The intensity of this band is appreciably higher than that of the ν₂ band.     

Diffusion and compressibility of sodium on the (110) plane of tungsten

The surface diffusion parameters and the compressibility of sodium on the (110) plane of tungsten have been measured using the field emission fluctuation method for sodium coverages from 0.2 to 3 × 10¹⁴ atoms cm^?2 and for temperatures from 170 to 500 K. Two temperature regimes can be defined. In the high temperature regime (? 300 K) the diffusion is essentially normal with an activation energy ranging from 0.28 to 0.58 eV and a preexponential coefficient D₀ from 10^?8.1 to 10^?2.7 cm² s^?1. In this regime the compressibility increases with temperature indicating an effective repulsive adatoms interaction. In the low temperature regime (? 300 K) the diffusion coefficient decreases with temperature at high coverage and slowly increases with temperature at lower coverage. The transition between both regimes appears on the compressibility versus temperature curve as an inflection point. The comparison of the present results with slow electron diffraction results furnishes strong evidence that the observed transition corresponds to a continuous short-range order-disorder transition.     

Dependence of cooperative emission spectra of complex organic molecules on exciting photon energy

The dependences of cooperative emission, superfluorescence, and lasing spectra on the wavelength of pumping radiation were studied for highly concentrated ethanol solutions of organic dye molecules. The relative intensity of cooperative emission was found to decrease, while the intensities of superfluorescence and lasing were found to increase with the energy of exciting photons. The increase in the dephasing rate of quantum states is supposed to be primarily responsible for the change in proportion between these intensities. On increasing the dephasing rate, with all other factors being the same, the concentration of the excited molecules with the phased quantum states decreases, while that with the dephased states increases. The ratio between the cooperative emission and dephasing rates determines the proportion between the intensities of cooperative emission, superfluorescence, and lasing. The onset of lasing at high concentrations (above >10¹⁹ cm^?3) in the case of pumping by high-energy photons occurs due to the high dephasing rates of quantum states resulting from a more rapid redistribution of vibrational energy.     

Energy transfer and sensitised luminescence in single crystals of CaF2:(Ce+Mn) system

The measurement of absorption, luminescence, and relative intensity of cerium and manganese emission during fluorescence, on single crystals of CaF₂:(Ce+Mn), have been reported in this paper. The occurence of the absorption bands have been explained in the following manner: (i) 250 mu band due to the transfer transition from F^? to²D, the excited state of Ce³⁺, (ii) 305 mμ band due to the characteristic transition of Ce³⁺ and (iii) 335 mμ band due to the perturbed level of the lattice. The emission bands have been explained like this, (i) 320 and 340 mμ bands due to the transitions of²D to²F_5/2,7/2 levels of Ce³⁺ (ii) 380 and 440 mμ bands corresponding to the perturbed levels and (iii) 520 mμ band due to transfer of energy from cerium centres to manganese centres. The results of the present investigation indicate that energy transfer in this system occures not only from cerium to manganese centres but also from cerium to cerium and cerium to perturbed levels of the lattice. It has also been observed that energy transfer process is temperature dependent in this case. An energy level scheme is proposed to explain the transfer mechanism.     

Multiphoton generation of electron-hole pairs involving free carriers in an indirect-gap crystal

The paper reports on a study of nonlinear absorption of high-intensity laser radiation in the case where the photon energy is less than one half of the indirect-gap width in a crystal. The deficiency in the energy needed for two-photon excitation of the electron-hole pair is made up by the kinetic energy of free electrons, which was acquired in intraband nonlinear absorption of light. The probabilities of Auger-type indirect two-photon interband transitions involving free electrons have been calculated by perturbation theory. It is shown that, for a free-carrier concentration in the conduction band n _c ? 10¹⁵ cm^?3, and the radiation intensity range of interest for the experiment, j ～ 3–10 GW/cm², the calculated probabilities of such transitions exceed by several orders of magnitude those of “conventional” direct and indirect (involving phonons) multiphoton transitions which can take place in the system considered.     

Emission and laser-excitation spectra of the ≈A2Π ⇐ ≈X2Π transition of rotationally cooled bromochloroacetylene cation in the gas phase

Rotationally cooled bromochloroacetylene cations have been studied in the ≈X²Π_ω and ≈A²Π_ω (ω = $\text{3}\text{2}$, $\text{1}\text{2}$) states in the gas phase by two complementary spectroscopic techniques. The ≈A²Π ? ≈X²π band system was excited in emission by electron impact of a seeded supersonic helium free jet and as fluorescence by laser excitation of cations formed by Penning ionisation in a cooled environment. Comparison of the two sets of data locates the spin—orbit component bands and the origins of the systems, and allows the main spectral features to be vibronically assigned. The vibrational frequencies of most of the fundamentals are thus obtained to within ±2 cm^?1 for this cation in both the states.     

极紫外阿秒脉冲在高次谐波产生过程中引起的非偶极效应

当红外强激光和极紫外(XUV)阿秒脉冲共同作用于原子分子时，电离出去的电子通常会吸收和辐射激光光子而发生能量扩展.讨论了由于XUV阿秒脉冲的短波长与扩展后的电子波包尺度可相比拟时在高次谐波产生过程中引起的非偶极效应.采用H⁺₂作为模型分子，并把分子轴置于激光场的传播方向，通过解二维含时薛定谔方程并比较考虑非偶极效应和采用偶极近似两种方法计算得到的结果，两者相比，前者的谐波强度降低，谐波频率向低级次稍有移动，电子能谱的能带内出现了更多的光电子峰.在相同的光电子能量处，两种方法计算得到的信号强度相差2—5倍.并且这种非偶极效应随着红外基频光光强的增大而增强，随阿秒脉冲波长的增大而减弱. 关键词： 非偶极效应 光场空间不均匀性 阿秒脉冲 高次谐波产生     

Collision-induced vibrational polarizability and mixed second refractivity virial coefficients B ab R: an experimental study of the SF6—rare gas mixtures

Mixed second refractivity virial coefficients B ^ab _R (ω) are determined in the infrared spectral range from the analysis of the buffer gas-induced intensity redistribution in the ν₃ absorption band at ω ? 930 cm^?1 of ³⁴SF₆ perturbed by the rare gases Ar, Kr, and Xe at relative densities up to 140 Amagat. The values of the refractivity virial coefficients are found to be several orders of magnitude greater than those observed in the spectral regions far removed from single-photon resonances. The undamped dynamic dipole—induced-dipole (DID) model is shown to qualitatively correctly reproduce the observed unusual dispersion of the B ^ab _R (ω) functions. A new estimate of the integrated band intensity S(ν₃) of SF₆ is also obtained.     

Characteristic structure in core electron spectra of metals due to the electron-plasmon coupling

The coupling of metallic core electrons to the density fluctuations of the conduction electrons is studied. Due to the strong electron-plasmon coupling there is a characteristic satellite structure in the core electron spectrum, starting at the plasma energy?ω _p below the quasiparticle level and with a maximal spectral weight at (1.6–2.5)×?ω _p below the same level. The total spectral weight in the satellite band is 50–100 percent of the quasiparticle weight, the actual value dependent on the density of the conduction electrons. The possible implications on X-ray photoemission, soft X-ray emission and absorption, and inelastic scattering of electrons are drawn. Particularly, a close correspondence with the location of the fine structure of the L_2,3 absorption spectrum of Al is found. The relation to the cohesive energy is also considered.     

Resonant tunneling and a nonlinear response in RF fields

The problem of resonant tunneling through a double-barrier nanostructure in a strong alternating electric field is solved completely. To this end, a perturbation method is proposed. Electron wavefunctions and a nonlinear response are obtained in analytical form over wide ranges of field frequencies and amplitudes, using the perturbation method and the semiclassical approximation. The semiclassical expression for the current allows for contributions of all orders with respect to the field, i.e., electron transitions with the emission and absorption of any number of photons. This enables one to find the limits of resonant current and output power. The case of ?ω?Γ is considered, where ? is the rationalized Planck constant, ω is the field frequency, and Γ is the resonance level width. It is established that the maximum resonant current is approximately as high as half the resonant constant current. For the quantum regime of oscillation, the output power can be 10⁶–10⁷ W/cm² at ω=10¹³ s^?1 and the output power rises with ω, in contrast to the well-known classical regime, where the power decreases rapidly.     

Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.