Threshold Energy Measurements Research Articles

Molecular high-Rydberg fragments from CH4 and C2H6 by electron impact dissociation

In this work we report the observation of polyatomic high-Rydberg molecules which survive for several hundred microseconds. Methane and ethane were excited by electron impact to produce high-Rydberg states of their fragments: CH3 from methane and CH3, C2H4, and C2H5 from ethane. These identifications are proven by measurements of threshold energies and by comparisons to fragmentation patterns for dissociative ionization. Sharp threshold peaks in the excitation functions show the occurrence of direct high-l excitation. The absence of parent CH4 or C2H6 in high-Rydberg states remains unexplained, but may be related to a strong interaction between the Rydberg electron and the core.

A study of the metastable and long-lived Rydberg fragments resulting from electron impact on NH3

The production of long-lived, excited fragments following electron-impact dissociation of ammonia has been investigated using the technique of translational spectroscopy. Over an impact energy range from threshold to 200 eV, the dominant species observed were hydrogen 2s and Rydberg atoms. Through extensive time-of-flight and excitation function measurements of these fragments, five dissociation channels were found to contribute to the production of the H(2s) atoms, while the H(RYD) signal was found to consist of hydrogen atoms resulting from at least four distinct processes. Threshold and kinetic energy measurements show that for most of these processes a substantial amount of the bombarding electron’s energy is channeled through the unobserved product or products of the dissociation. In addition to the hydrogen features, a low intensity group of fragments which we believe are long-lived nitrogen Rydberg atoms was identified in the time-of-flight spectra. Excitation function measurements indicate that two dissociation channels contribute to the production of these fragments. When possible, the present results are compared to previous measurements of the dissociative excitation and ionization of ammonia.

Low-lying states in112In

Low-lying states below 500 keV excitation in112In have been investigated via the112Cd(p, nγ) reaction. New levels have been established atEx=206.5keV and 456.1 keV from the measuredγ-ray excitation functions,γ−γ coincidences and the precision measurements of the (p, n) threshold energy of the ground state and of the 206.5 keV state of112In. Spins and parities of the 206.5 keV state (2+) and the 456.1 keV state (3+) and multipolarities and mixing ratios of the deexcitationγ-rays have been determined from the angular distributions and linear polarizations of the deexcitation γ-rays as well as the excitation functions of the residual levels. Possible configurations of the newly-found levels are discussed. Half-lives of two states have been remeasured:T1/2=15.2±0.1 min for the ground state andT1/2=20.9±0.1 min for the 156.4 keV (4+) state. The ground stateQ-value for the112Cd(p, n)112 In reaction has been measured to be −3.376±0.006 MeV.

The level structure of 76As by the 76Ge(p, nγ) reaction

The level scheme of 76As up to an excitation energy of 785 keV has been obtained from the measurement of threshold energies of γ-transitions following the 76Ge(p, n) 76As reaction. The (p, n) cross sections corresponding to the individual residual states have been extracted from the γ-ray measurement. The results have been compared with the cross sections calculated from the statistical theory of compound nuclear reactions. Spin assignments were allowed by this comparison to eight excited states below 236 keV. Systematics of the level structures of doubly odd As isotopes are interpreted in terms of shell-model configurations.

The dissociative ionization in oxygen

Dissociative ionization in O 2 has been examined by means of a retarding potential lens combined with a quadrupole mass spectrometer. This particular arrangement made possible detailed analysis of the O + ions kinetic energy distribution and the measurement of threshold energies of O + ions with known kinetic energy. These two types of informations bring new experimental evidence, besides direct dissociative ionization of O 2, for predissociation of the B 2Σ g − state, the 2Π u state and the c 4Σ u − state of O 2 +. A new predissociation of the O 2 +( 2Δ g) state has been found. At low ion-energy the predissociation of the 2Φ u state or an autoionizing predissociation is observed.

Precision time-of-flight system for measurement of the beam energy of the Munich Tandem accelerator

A time-of-flight system measuring accelerator beam energies within time intervals of about 10 ms with an absolute accuracy of 1.5:10 5 has been developed and successfully put into operation. The system improves the accuracy obtainable in the measurement of nuclear reaction Q-values, resonance and threshold energies by about an order of magnitude in many cases. In addition it can be used to correct any experimental data for Tandem energy fluctuations slower than 100 Hz. In this way the effective energy resolution of the beam analysing system is improved to about 0.5 keV which is sufficient for the demands of the high resolution Q3D-magnet spectrograph.

A study of the level scheme of 74As by the 74Ge(p, nγ) 74As reaction

A level scheme of 74As up to about 700 keV excitation energy has been determined by threshold energy measurements on the reaction 74Ge(p, nγ) 74As. For eleven levels below 465.3 keV, spins and parities were assigned by using transition multipolarities obtained from the observed internal conversion coefficients and by comparing relative cross sections for the (p, n) reactions leading to various residual levels with those predicted by Hauser-Feshbach theory.

Photodissociation of NCN3 in the Vacuum-Ultraviolet Production of CN B2Σ and NCN A 3Π

The photodissociation of NCN3 in the vacuum ultraviolet has yielded CN B2Σ and NCN A 3Π. No NCN 1Π was detected. The fluorescence intensity of the CN violet emission, If / I0, was measured as a function of incident wavelength, λ. The If / I0−vs−λ curve shows structure, indicating that the process NCN3 → lim hvCN B2Σ + N3, is predissociative. The yield at 1216 Å is estimated to be approximately 2.5% The threshold wavelength of incident photons to yield CN B2Σ is 1685 ± 20 Å. The production of NCN A 3Π is attributed to the formation of N2 A 3Σ, NCN3→ lim hνNCN X3Σ + N2 A 3μ, with the threshold wavelength of 1915 ± 30 Å followed by the sensitized reaction, N2 A 3Σ + NCN3 → NCN A 3Π + 2N2. Bond dissociation energies obtained from threshold energies are D(NC–N3) = 4.2 ± 0.1 eV (= 96 ± 2 kcal mole−1) and D(NCN–N2) = 0.3 ± 0.1 eV (= 7 ± 2 kcal mole−1) where the error limit indicates an experimental uncertainty of the threshold energy measurements. Estimated heats of formation are ΔHf°(NCN3) = 4.7 ± 0.2 eV (= 108 ± 5 kcal mole−1 and ΔHf°(NCN) = 5.0 ± 0.2 eV (= 115 ± 5 kcal mole−1) from which D(N–CN) of 4.3 ± 0.2 eV (= 99 ± 5 kcal mole−1) is obtained. The absorption coefficient of NCN3 in the region 1200 to 2000 Å has been measured. A comparison is made of bond energies of several azide compounds.

Radiative Recombination in Annealed Electron-Irradiated GaAs

Broad-band emission peaking near 1.39 eV appears in Zn-doped GaAs in addition to the narrow bound-exciton emission at 1.49 eV. Electron irradiations at energies between 0.6 and 2.0 MeV for time-integrated fluxes of 1.3\ifmmode\times\else\texttimes\fi{}${10}^{18}$ e/${\mathrm{cm}}^{2}$ quench all luminescence. For bombardment energies below a critical value, which lies between 1.0 and 1.5 MeV, isochronal annealing near 190\ifmmode^\circ\else\textdegree\fi{}C causes the reappearance of the broad-band emission, which now is found to possess structure. The structure consists of three no-phonon lines at 1.4433, 1.4415, and 1.4402 eV which are repeated at intervals of the transverse acoustic phonon energy. The emission intensity is greatest for temperatures near 220\ifmmode^\circ\else\textdegree\fi{}C. Further annealing at higher temperatures diminishes the intensity of the fine structure and the associated broad-band emission; complete disappearance occurs above 260\ifmmode^\circ\else\textdegree\fi{}C. The emission is not regained upon annealing when the bombardment energy exceeds the critical value. It is suggested that direct lattice displacement occurs at the critical bombarding energy. This would correspond to a threshold displacement energy of 50-60 eV. It is also suggested that previous measurements of threshold energy (10-20 eV) in GaAs and other experimental observations may be due to known large concentrations (\ensuremath{\approx}${10}^{19}$ ${\mathrm{cm}}^{3}$) of defects in as-grown GaAs, in the form of Ga-As interstitial pairs.

The character of oscillation spikes during quasi-continuous operation of a ruby laser

Quasi-continuous operation of a ruby laser at room temperature was performed with a crystal placed in the spherical cavity with threshold energy 48 J. The duration of the impulse of stimulated emission was 2700μsec. The character of spikes was observed and it was found that it is far from sinusoidal. The results of threshold energy measurements for other pumping configurations are discussed and compared with those obtained by other authors.