Femtosecond filamentation in transparent media

Measurements of the vacuum-ultraviolet (<80-nm) radiation produced by intense ultraviolet (248-nm) irradiation (1015–1016 W/cm2) of rare gases have revealed the copious presence of both harmonic radiation and fluorescence from excited levels. The highest harmonic observed was the seventeenth (14.6 nm) in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen from ions of Ar, Kr, and Xe, with the shortest wavelengths observed being below 12 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4d95s5p → 4d105s manifold of Xe7+ at ~17.7 nm, was detected. These experimental findings, in alliance with other studies concerning multielectron processes, give evidence for a role of electron correlations in a direct nonlinear process of inner-shell excitation.

Studies of multiphoton production of vacuum-ultraviolet radiation in the rare gases

Since the appearance of the title paper, a number of new developments have occurred which need to be included in that body of material. We present additional remarks and clarifications which supplement and update numerous aspects of the Bethe theory discussed in the earlier paper. We also bring the bibliography up to date. Plasma stopping power, the ${z}^{3}$ effect, and stopping power for particles at extreme relativistic energies are among the new topics included. We make several comments on Fano's earlier review article, Ann. Rev. Nucl. Sci. 13, 1 (1963).

Inelastic Collisions of Fast Charged Particles with Atoms and Molecules-The Bethe Theory Revisited

High-energy 20-fs pulses generated by a Ti:sapphire laser system were spectrally broadened to more than 250 nm by self-phase modulation in a hollow fiber filled with noble gases and subsequently compressed in a broadband high-throughput dispersive system. Pulses as short as 4.5 fs with energy up to 20-microJ were obtained with krypton, while pulses as short as 5 fs with energy up to 70 microJ were obtained with argon. These pulses are, to our knowledge, the shortest generated to date at multigigawatt peak powers.

/pdf/compression-of-high-energy-laser-pulses-below-5-fs-4xiizf2tqa.pdf

Compression of high-energy laser pulses below 5 fs

Inelastic collisions of fast charged particles with atoms and molecules the Bethe theory revisited

Consideration is given to the interacting collisional and radiative processes occurring in a plasma. A statistical theory describing the general loss mechanism, for which the name collisional-radiative recombination is proposed, is described. This theory enables the collisional-radiative recombination coefficient a to be determined knowing the relevant spontaneous transition probabilities and the rate coefficients for radiative recombination and collisional excitation and ionization. Detailed calculations are carried out on hydrogen-ion plasm as which are optically thin. It is found that α is an increasing function of the number density of free electron n(c) the increase being especially marked if the electron temperature T is low; for example, if T is 250 °K α becomes almost 20 times as great as the radiative recombination coefficient (which describes the loss in a very tenuous plasma) when n(c) is only about 10 8 /cm 3 , whereas if T is 64 000°K a does not become as great as this until n(c) is about 10 18 /cm 3 . From a similar investigation in which the ground level of the hydrogen atom is made inaccessible (in crude representation of an alkali atom) it is inferred that the value of a is probably not very sensitive to the species of singly charged ion involved. Recombination of electrons with bare nuclei of charge Ze to form hydrogenic ions is similarly treated for an optically thin plasma. It is shown that to a close approximation the reduced coefficient α/ Z is a function of a reduced tem perature T/Z 2 and a reduced number density n(c)/Z 7 only. The values of the reduced coefficients are of com parable m agnitude and have a similar dependence of the reduced temperature and density as the coefficients for hydrogen ion plasmas. The variation of the recombination coefficient α with Z in the same plasma (i.e. same n(c) and T ) is investigated. It may be expressed in the form a α ∝ Z z where the index z depends on n(c) and T . Though z is generally positive as would be expected, it is negative if n(c) and T are very high. A physical explanation of this is presented.

Recombination between electrons and atomic ions, II. Optically thick plasmas

A method is suggested by which the refractive index and Verdet constant of an atomic system may be derived theoretically It is applied to atomic hydrogen and to the inert gases and a comparison is made with experimental data The Verdet constant of neon is not anomalous The origin of the suggestion appears to be an underestimate of the experimental error The analysis yields values of th e polarizabilities of th e inert gases which are respectively He, 1-384; Ne, 2-663; Ar, 11-080; K r, 16-734; X e, 27-292 in units of α 3 0

The refractive indices and Verdet constants of the inert gases

It is shown that the cross sections for excitations of hydrogen by proton impact. given by the customary second Born approximation, are not reliable because of the neglect of a large term of the fourth order in the interaction energy. The expression for the cross section obtained by including only terms to the third order in the interaction energy was employed to calculate the cross sections for the proton impact excitation of the 2s state of atomic hydrogen. Allowance was made for distortion and polarization due to the 1s, 2s, and 2p/sub 0, plus or minus 1/ intermediate states. When polarization was ignored by neglecting all terms involving the 2p states, excellent agreement was found with the distortion approximation calculation of sates. (auth)

The 1s-2s excitation of hydrogen atoms by proton impact

The expression for the cross section obtained from the second Born approximation by including only terms to the third order in the interaction en ray was employed to th calculate cross sections for the electron impact excitation of the 2s level of aiomic hydrogen. Allowance was made for distortion and polarization due to the 1s, 2s, and 2p/sub 0, plus or minus 1/ intermediate states. These cross sections are compared with the available experimental data. (auth)

A. E. Kingston

Papers

Recombination between electrons and atomic ions, II. Optically thick plasmas

The refractive indices and Verdet constants of the inert gases

The 1s-2s excitation of hydrogen atoms by proton impact

The 1s-2s Excitation of Hydrogen Atoms by Electron Impact