Table of contents

Classical and quantal derivations are given of the key formula of the binary-encounter approximation.

Direct experimental schlieren observation of self focused filaments is reported. Of two mechanisms shown to be possible the preferred one is due to cubic polarizability of excited atoms produced in the electron cascade due to the laser beam.

Results are quoted for the 'physical' three-body transition operator yielding the volume-independent three-body reaction coefficient, in terms of which one computes the three-body elastic scattering rate when three initially free independently moving particles collide under the influence of short-range forces.

Cross sections calculated from a semi-quantal treatment are presented for the ionization of hydrogen and for the excitation of the second quantum level of hydrogen by collision with hydrogen- atoms in the ground state. Comparison with the cross sections derived from Born's approximation shows excellent agreement.

Heisenberg's correspondence principle for non-relativistic matrix elements has been generalized: quantal transition amplitudes between strongly coupled states are expressed as Fourier components of integrals over classical trajectories. The new theory reduces to Heisenberg's correspondence principle in particular cases. The theory is applicable whenever the change in quantum number is small compared with the initial quantum number. In these situations it is more comprehensive than both quantal first order perturbation theory and the sudden approximation. Furthermore, compared with standard quantal methods, the evaluation of amplitudes is particularly simple. The theory is worked out for one- and three-dimensional separable systems and the generalization to a system of arbitrary dimension is indicated.

For an N-fermion system, using M one-particle basic functions to represent the wave function, it is already well known that the case M=N leads to a single determinant (or Hartree-Fock) form for the wave function. In a recent article, two of the authors showed by numerical experiment that the case M=N+2 also leads to a particularly simple and elegant form of the N-fermion wave function, one which is a certain linear combination of only (¹/₂N+1) ((p) identical to integral part of p) Slater determinants. The present paper gives an analytical derivation of this useful result.

The recent thermal diffusion factor alpha _T, data for the system CO₂-CH₃Cl, CO-CH₃Cl, He-CH₃Cl and Ar-CO₂ have been compared with the available theories which take into consideration inelastic collisions. The results indicate limited success of the theories. Many features of thermal diffusion phenomenon in polyatomic gases have yet to be understood satisfactorily, and for this purpose further theoretical and experimental work is necessary.

For pt.I. see abstr. A14168 of 1970. The pseudo-state modification of the close-coupling expansion is applied to the 2s and 2p excitation of atomic hydrogen by electron impact. Pseudo-states are used which ensure the implicit inclusion of all important excited state polarizabilities. A detailed comparison is made with results obtained from other modifications of the close-coupling expansion and the eigenphase minimum principle is used to determine the best result for each partial cross section. Comparison of the theory and experiment in the first electron volt above the n=2 threshold shows very good agreement in the ratio Q(1s-2s)/Q(1s-2p), but a 20% discrepancy exists between the individual cross section magnitudes when experiment is normalized to the Born approximation at higher energies.

The total ionization cross sections of He, N₂, H₂ and O₂ due to electron impact have been calculated by integrating the expressions given by Khare for the ionization cross sections per unit energy range. The results are in satisfactory agreement with the experimental data. Comparison has also been made with other theoretical values.

The influence of the inclusion of the full interaction of an incoming ion with the various multipoles induced in the ground state of atomic hydrogen is investigated by considering the collisional excitation of the 2s and 2p states of hydrogen by protons and helium ions with impact velocities v in the range 0.4<or=v<or=5 a.u. While the most significant change in the cross sections arises from the ion-dipole distortion, the ion-quadrupole term produces a relatively large effect that cannot be ignored in an accurate theoretical description of ion-atom collisions. The 2s cross section is increased by as much as 50% from the normal four-state treatment, while the excitation of the 2p magnetic substates is more severely affected. The polarization fraction of the emitted Lyman- alpha radiation is enhanced except at the lower incident energies, when rotational coupling between the magnetic substates becomes markedly evident.

It has been shown previously that there is an exact relation between scattering amplitude in the wave treatment and the probability amplitude in the impact parameter method. The analysis of the derivation of this relation enables us to modify the usual impact parameter formulae for excitation cross section of atom by electron impact to include the difference of initial and final velocities. The modified formula provides close agreement with the quantum- mechanical cross sections. The normalization of cross sections and dipole approximation for electron-atom interaction both in impact parameter and the wave treatment are discussed and compared. Results are given for electron excitation of H1s-2p, H1s-2s, Na3s-3p, He3³S-3³P, He2³S-3³P and Ca4s²-4p².

The cross section for charge exchange in the reaction He⁺ +H^- to He+H has been measured over a range of relative velocities from 2.2 to 14.0*10⁷ cm s^-1(0.2 to 8.0 keV barycentric energy). The technique of modulated inclined ion beams developed for the observation of H⁺-H^- collisions was used. The cross section falls from a value of 127*10^-16 cm² at 2.2*10⁷ cm s^-1 to 9.7*10^-16 cm² at 14.0*10⁷ cm s^-1 and exhibits prominent structure at intermediate velocities. The results are compared with those previously obtained for the H⁺-H^- system.

A beam attenuation technique developed previously in this laboratory has been used to obtain cross sections sigma ₀₁ and sigma _0*1 for fast ground state and metastable helium atoms in helium, neon, argon, krypton and nitrogen at impact energies within the range 60-350 keV. Values have also been obtained for targets of hydrogen and helium in the range 10-40 keV. Measured cross sections sigma _0*1 are shown to be in good agreement with values calculated recently by Bates et al. for targets of H₂, Ne, Ar and Kr using a classical impulse approximation. The observed energy dependence of sigma ₀₁ in the case of targets of hydrogen and helium is also found to be in good agreement with that predicted by the recent calculations of Bell et al. based on the first Born approximation.

An apparatus has been developed for the measurement of the drift velocities of electrons in compressed gases. Velocities have been measured in nitrogen, argon and carbon dioxide at pressures from approximately 3 atm up to pressures of 70, 100 and 17 atm respectively. The measurements have established that, to within the experimental error, the electron drift velocities in nitrogen and argon are independent of gas density and depend only on the ratio of the the electric field to gas particle density (E/N) up to the highest pressure investigated. In carbon dioxide, at pressures of approximately 10 atm and above, it has been found that the behaviour of electrons appears to be fundamentally different from their behaviour at low pressures; in particular, the drift velocity was found to decrease with increasing pressure for a fixed value of the ratio E/N, and a slow decay of current occurred at the end of the electron pulses.

The drift velocities of ions created in nitrogen and carbon dioxide have been measured using groups of Townsend avalanches at pressures up to 3 atm. The velocities were invariant with gas pressure, or density, and the results indicate that in both gases the ions studied were predominantly of one type, which were the same in each case as those studied in other investigations at high pressure. Discrepancies with earlier low- pressure measurements are discussed.

An experimental method for the determination of the electron impact excitation coefficients is described. This requires the measurement of the absolute intensities of spectral lines emitted by a plasma in the extreme ultraviolet and the determination of the electron temperature of the plasma by a laser scattering technique. Results are presented for five transitions in four times ionized nitrogen. These are corrected for the effect of stepwise excitation and opacity and then compared with values that have been calculated theoretically. The final results are in agreement with the theory to within the experimental uncertainty which is +or-20% for relative values and +or-50% for absolute values.

180 spectral lines at wavelengths between 240 and 750 AA are classified. They arise from transitions from singly and doubly excited levels in Na VII-IX, Mg VI-X, Al VII-XI, Si VII-XII, P X-XIII, S XI-XIV, Cl XII-XV, K XIII and Ca XIV, In addition 59 emission lines of chlorine XII-XV are classified in the region between 17 aa and 40 aa. The spectra were emitted from plasmas generated through focusing the output of a 750 MW neodymium Q switched laser onto solid targets. Some of the magnesium and silicon lines are present in solar spectra. Fe XIX-Fe XXIV solar flare line classifications are confirmed as a result of more accurate extrapolations along isoelectronic sequence which depend on new data reported in this paper.

Rotational analysis of four absorption bands of the weak A ³ pi (1_u) from x¹ Sigma _g⁺ red system of ⁷⁹Br⁷⁹Br has been made. Accurate wave numbers for the band origins have been combined with earlier data for ⁷⁹Br⁸¹Br to obtain the vibrational numbering of the A ³ Pi (1)_u state. It is necessary to increase Brown's provisional numeration for this state by 7 units.

The number F of ²³⁸Pu L X-rays per alpha disintegration of ²⁴²Cm has been determined using fast coincidence techniques. The result F=(9.70+or-0.14)% is in accord with the one value previously reported. The disagreement between the measured values for the L₂ subshell yields in ²⁴⁰Pu and ²³⁸Pu is therefore not resolved and an explanation is suggested for the observed discrepancies.

A detailed study of the L emission spectrum of europium has been made with the help of a focusing spectrograph of the transmission (Cauchois) type, having a mica crystal bent to a radius of curvature of 40 cm. A new diagram line at 1563.7 X.U. corresponding to the quadrupole transition L_IN_IV,V( gamma ₁₁) in europium has been observed for the first time. The presence of two more quadrupole lines corresponding to the L_IIN_VI,_VII( gamma ₈') and the L_IIIN_VI,VII( beta ₇') transitions has been established.

Using a 40 cm curved-crystal spectrograph of the transmission type the L emission spectra of some rare earths have been investigated. The high-frequency satellite L gamma ₁' in the spectra of ⁶⁹Tm, ⁶⁸Er, ⁶⁷Ho, ⁶⁶Dy, ⁶⁵Tb and ⁶⁴Gd has been reported for the first time at wavelengths of 1303.5, 1353.0, 1406.5, 1458.6, 1516.4 and 1577.7 X.U. respectively. The origin of L gamma ₁' has been ascribed to the transition L_IIM_III to M_IIIN_IV on the basis of the multiple ionization theory. The measured values of the wavelength of L gamma ₁, have been found to be in good agreement with those calculated on the aforesaid transition.

The (0,0) band of the C₃ system of the SbF molecule has been photographed at a reciprocal dispersion of 0.3 AA mm^-1 on a two metre plane grating spectrograph and the rotational analysis has been carried out. The system has been ascribed to an electronic transition of the type ¹ Pi to ¹ Sigma ⁺. The nature of three different lower states of the molecule have been discussed.

The L spectrum of tantalum 73 was re-investigated with special reference to weak lines. As a result of this, the existence of the so far unconfirmed transitions L_IO_II, L_IIM_II, L_IIM_III, L_IIN_II, L_IIN_V, L_IIN_VI, L_IIO_II, L_IIIM_II and L_IIIM_III has been confirmed. A more satisfactory value of the wavelength corresponding to the transition L_IIN_II was obtained. However, the transitions L_IN_I, L_IO_I, L_IN_VI,VII and L_IN_V could not be confirmed.

Absolute band oscillator strengths, measured in absorption, are presented for the v'=0-3, v"=0; v'=2-5, v"=1 bands of the Schumann-Runge System of O₂.

Recently made absolute absorption coefficient measurements of v"=0 progression O₂ Herzberg bands are used to place on an absolute basis relative band strengths for this system derived from the emission measurements of Degen and Nicholls.