Molecular Lifetimes Research Articles

Vibrational lifetimes in liquids

For the first time the vibrational population lifetime of simple normal modes of polyatomic molecules in liquids has been directly measured, according to the experimenters, Alfred Laubereau, Dietrich von der Linde and Wolfgang Kaiser (Munich Technical University). Earlier the vibrational dephasing lifetime in both liquids and solids had been measured by the Munich group, who studied polyatomic molecules in liquids and lattice vibrations in diamond, and by Robert Alfano and Stanley Shapiro (General Telephone and Electronics), who measured lattice vibration lifetimes in calcite and molecular vibrational lifetimes in liquid nitrogen. Some observers believe that the technique opens a new field for the study of molecular relaxations in liquids and optical phonon relaxations in solids.

Molecular Autoionization Lifetimes and Cross Sections for Penning Ionization: Numerical Results for He* (1s2s 3S) + H(1s 2S)

The width Γ (or lifetime ℏ/Γ) for autoionization of He* (1s2s 3S) + H(1s2S) has been calculated as a function of internuclear distance, and cross sections for Penning and associative ionization (He*+H → He+H++e−, HeH++e−) have been determined for low collision energies. Associative ionization is 22% of the total ionization cross section in the limit of zero collision energy; this fraction decreases with increasing energy, being ·18% at a collision energy corresponding to 300°K. The distribution in energy of the ionized electron is also calculated, and it is seen that measurement of this quantity should lead to a good estimate of the well depth of the He*–H potential. Comparison of these results with those obtained by an orbiting model shows that the model (suitably scaled) is adequate in predicting the total ionization cross section, but is less accurate for the more detailed collision properties.

A new electron-photon coincidence method for the measurement of lifetimes in atoms and molecules

A new method of measurement of atomic and molecular lifetimes is described, in which delayed coincidences are recorded between inelastically scattered electrons and the subsequent decay photons. The main advantage of this method is that sources of systematic error, such as cascading, spectral line blending, radiation trapping and collisional de-excitation, can often be eliminated. The method is described in detail, together with measurements on three helium states: 3 3D(13.4+or-0.5)ns, 4 3S(63.5+or-1.2)ns and 4 1S(75+or-5)ns.

Vacancy cascade spectroscopy: A convenient method of studying highly ionized atoms and molecules

Vacancy cascades have been created in gas jets of He, N 2, O 2, CO 2, Cl 2 and Xe by bombardment with a monoenergetic beam of 2–10 keV electrons. The resulting spectra, observed at 500 – 2500 Å are comparable to beam-foil spectra, but have several orders of magnitude higher intensity, with a broad distribution of ionization stages. This technique can be used to measure atomic and molecular lifetimes, and other applications are discussed as well.

Lifetime measurements by an electron photon coincidence method

A new method is described for determining neutral atomic and molecular lifetimes, by measuring delayed coincidences between inelastically scattered electrons and the subsequent decay photons. The method has been applied to three states in helium (3 3D, 4 3S and 4 1S), to the C 3Π u state in nitrogen and the a 3Σ g + state in hydrogen. Apart from the 4 1S lifetime, all the above measurements are thought to be free from cascade and other systematic errors.

A Cold Cathode Invertron

A new gaseous discharge source has been developed for use in measurement of atomic and molecular electronic state lifetimes where the transitions occur in the range 1000–12 000 Å. The source is a coaxial aluminum cold cathode-anode device utilizing fast pulse circuits, magnetic field breakdown enhancement, and a flowing gas supply. Excitation of gaseous species is achieved by electron collisions in the discharge. The cold cathode device can be used with any gaseous species since its source of electrons is not a heated oxide-coated cathode. Excitation fall times of 2 nsec are obtained.

An electron-photon coincidence method for measuring atomic and molecular lifetimes

A new method is described for determining neutral atomic and molecular lifetimes by measuring delayed coincidences between inelastically scattered electrons and the subsequent decay photons. The lifetime of the 4 1S state of helium, as measured by this method, is 73± 2 nsec.

The Physics and Chemistry of Interstellar Grains. I. Crystal Growth in Space

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Physics and Chemistry of Interstellar Grains. I. Crystal Growth in Space Donn, Bertram Abstract Experimental and theoretical developments since about 1945 in (1) nucleation of condensing vapors, (2) mechanism of crystal growth, (3) chemical reactions at low temperatures including surface reactions of free radicals, and (4) radiation chemistry of solids, now permits much more reliable analyses of grains to be carried out than were previously possible. We consider the simplest case of growth of pure crystals without reactions. Molecular impact frequencies, surface lifetimes and diffusion rates are calculated. Growth rates and shapes are estimated according to which of the two crystal growth mechanisms is appropriate, (1) trapping with little mobility, producing nearly spherical particles, or (2) dislocation trapping yielding nonspherical grains. Process 2 requires a significant diffusion distance. At 500K all abundant metals except aluminum, iron, and nickel would diffuse and tend to develop irregular particles. At 20~K process (1) would apply to all metals. Molecules of hydrogen with carbon, oxygen, and nitrogen would probably diffuse. At 5 K only molecules more volatile than CO2 undergo diffusion. ~latelets permit sufficient mass for water, ammonia, methane, or graphite grains to produce interstellar extinction. At 20~K the expected surface diffusion of hydrogen compounds is about 1 mm in 10 yr. Carbon atoms on the hexagonal graphite face appear to be similar. Trapping occurs on crystal edges, and platelets should tend to form. A small number of screw dislocations would lead to highly irregular whisker arrays. Exponential growth rates occur. For platelets with N (H) = 1/cm3, r reaches I0-~ cm in about 3x108 yr or 3x107 yr if N (H) =10. Inclusion of chemical reactions, heterogeneous composition, and radiation chemical effects should make growth less favorable. Also recent extinction measurements in the ultraviolet cannot be explained by uniform grains. Consideration should be given to extinction by Platt-type grains. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109645 Bibcode: 1965AJ.....70S.320D full text sources ADS |