Molecular Lifetimes Research Articles

Lifetimes of local and hyperspherical vibrational resonances of ABA molecules

The complete spectrum of vibrationally excited ABA* molecular resonance lifetimes is evaluated using the simple Rosen–Thiele–Wilson model of coupled Morse oscillators. Two complementary methods are used: First, unimolecular dissociative resonance wave functions are propagated in time by the Fourier method, where the initial wave functions are obtained as an approximation by linear combinations of symmetry-adapted products of Morse functions. Second, bimolecular reaction S matrices are propagated along the hyperspherical radius of the system giving the diagonalized lifetime matrix, which is analyzed for resonance lifetimes and energies. The resulting uni- and bimolecular resonance energies agree within ±0.002 eV and the lifetimes within ±30%. Uni- and bimolecular assignments of gerade (+) and ungerade (−) ABA* symmetries agree perfectly. On the average, the unimolecular decay times decrease as the resonance energies increase from the ABA*→A+BA to about 3/4 of the A+B+A dissociation threshold; even more highly excited resonances tend to be slightly more stabilized. Superimposed on this overall nonmonotonous energy dependence is a strong, 1–2 orders of magnitude variation of lifetimes, indicating substantial mode selectivity for the decay of individual resonances, irrespective of the excitation energy. The mode selectivity is investigated for hyperspherical mode resonances with lobes extending across the potential valleys, in contrast with local mode resonances with frontier lobes pointing towards the valleys. On the average, hyperspherical mode resonances decay at a slower rate than local mode resonances. This conclusion agrees with our previous analysis of low energy ABA* resonances, and with Hose and Taylor’s analysis of the Hénon–Heiles system. However, these correlations are also violated by several important exceptions: the ABA* system has many slowly, but also a few rapidly, decaying hyperspherical resonances.

A combined time‐resolved and stereospecific deuterium labelling study of the elimination of methanol from the molecular ion of methoxycyclohexane

AbstractIt is shown by field ionization kinetics in combination with both site‐specific and stereospecific D‐labelling that the loss of a molecule of methanol from the molecular ion of methoxycyclohexane can occur via 1,4‐ and 1,3‐eliminations. The 1,4‐elimination predominates at molecular ion lifetimes of ≥10−10.1 s. It is found that ∼19% of this reaction channel corresponds to a stereospecific cis‐elimination, whereas the remaining 81% is only site‐specific. At molecular ion lifetimes of between 10−10 and 10−9 s, a very sudden increase of the 1,3‐elimination is observed at the expense of the 1,4‐elimination. A stereospecific loss of methanol, however, is not observed at all for the 1,3‐elimination within the limits of error. Possible intermediates and reaction pathways, which can account for the observations made, are discussed.

Anti-inflammatory activity of superoxide dismutases: inhibition of carrageenan induced edema in rats.

Eighteen different superoxide dismutases from procaryote, plant, fish, bird and mammalian species have been tested for anti-inflammatory activity in the rat paw pad carrageenan-induced inflammation model. Very large differences in activity are observed. Homologous rat Cu-SOD is not active and indeed shows slight pro-inflammatory activity. The different SODs have different iso-electric values, different metals (Cu, Mn or Fe) at the active centre, different molecular weights and different circulation lifetimes. Biological activity is a function of amino acid sequence rather than of such secondary parameters.

An NQR study of the effects on the torsional oscillations of a molecular crystal due to uniaxial pressures

This work reports the experimental results of the uniaxial pressure dependence at various temperatures of the spin-lattice relaxation time, T 1, in paradichlorobenzene single crystals. The directionality of the pressure dependence, together the temperature dependence ot T 1 near room temperature, give us some information about the dependence of the molecular torsional frequencies and lifetimes of the oscillations ground levels with the intermolecular crystalline potential.

Determination of Atomic and Molecular Excited-State Lifetimes Using an Optoelectronic Cross-Correlation Method

A new optoelectronic cross-correlation technique has been used to measure excited-state lifetimes of several organic fluorophores in solution and of sodium atoms in both air-acetylene and methane-air flames. Capable of time resolution in the picosecond range, the new method was validated by the excellent agreement between measured and literature values for fluorescence lifetimes of the organic species. Lifetime values for Na atoms in flames are among the most precise ever reported and agree closely with values calculated from the known flame gas composition, temperature, and the quenching cross-sections of the two dominant quenching species in the flame, N2 and CO2. From the Na excited-state lifetimes (0.72 ± 0.07 ns for air-acetylene and 0.48 ± 0.08 ns for methane-air flames), quantum efficiencies for atomic fluorescence were calculated.

Laser-activated chemisorption of SF6molecules on a hot surface

Activated chemisorption of vibrationally excited SF6 molecules was observed on the surface of tungsten at high temperatures. A quadrupole mass spectrometer was used to study the channels for negative surface ionization of a molecular beam excited by CO2 laser radiation. Two types of adsorption centers with different molecular lifetimes were observed on the surface and laser excitation tended to reduce the potential barrier between these. A qualitative theory of the effect was developed. The measured temperature dependences were compared with the calculated ones.

Present State of the Calculation of Radiative Lifetimes of Molecules

The calculation of molecular radiative lifetimes are reviewed. The equations which determine the radiative lifetimes are given and the theoretical methods which have been used to solve these equations are discussed. The results of the limited number of calculations available in the literature are summarized and attempts are made to access their accuracy. Some new results are presented for the radiative lifetimes of the A1Π state of CO.

The formation of [C9H10]+˙ ions by loss of a molecule of water from molecular ions of 3‐phenylpropanol with lifetimes of 10−11 to 10−5 s

AbstractField ionization kinetic experiments in conjunction with deuterium labelling have been shown that the molecular ions of 3‐phenylpropanol with lifetimes as short as 10−11s lose a molecule of water via a specific 1,3 elimination. At times > 10−11s two distinct hydrogen interchange processes in the molecular ions appear to complete with this reaction. One of the intechange processes involves the benzylic and hydroxylic hydrogen atoms and starts to complete with the elimination of water at shorter molecular ion lifetimes than the other interchange process in which the ortho hydrogen atoms also participate. Decomposing [C9H10]+˙ ions generated by elimination of water from the molecular ions of 3‐phenylpropanol or by direct ionization of various isomeric C9H10 compounds could not be distinguished adequately, illustrating isomerization either to a common ion structure or to a set of ions with rapidly interconverting structures. A consideration of the energetics of the elimination of water from 3‐phenylpropanol suggests that at threshold energies 1‐phenylpropene or indane type structures can be formed. Arguments for the latter have been obtained from the observation that a labile fluorine atom is present in the [M – H2O]+˙ ions generated from 3‐pentafluoro‐phenylpropanol.

Tropospheric ozone: 2. Variations along a meridional band

We present summaries of 703 previously unanalyzed ozonesonde launches at six stations distributed along a meridional band near 75°W between 9° and 53°N. Altitude and seasonal variations are examined to assist understanding the patterns and relative roles of injection, transport, loss, and synthesis of ozone in the middle troposphere. By comparison with other North American stations we conclude that a longitudinal gradient of 10–20% is likely, increasing eastward. The data are generally consistent with conventional understanding of tropospheric ozone in terms of increased injection from the stratosphere during spring at high latitudes, southward transport with molecular lifetimes of several months, and a principal sink at the ground, but details of the altitude and seasonal behavior can be interpreted ambiguously as evidence either for shorter-scale transport or for ozone synthesis within the troposphere.

An efficient method for measuring atomic and molecular lifetimes using a modulated or deflected CW dye laser beam

By pulse modulating or deflecting a CW dye laser beam, cascade-free lifetime determinations at high spectral resolution can be performed with the decayed-coincidence technique using measuring times of about 1 minute. The method is illustrated by lifetime measurements for the 7 2P 3 2 1 2 levels of cesium.

Sharp Resonances in Radiationless Transitions

We have observed extremely sharp Doppler-free resonances in the measurement of molecular lifetimes of ex-cited states in the low pressure gas-phase of naphthalene. These resonances are seen at extremely low magnetic fields and have a width of only 3 Gauss. Their extreme sharpness of 2.8 × 10−4 cm−1 is within a factor of 2 of the natural width of the radiationless transition, and some 130 fold below the Doppler width of the state considered. Such sharp resonances should provide unique insights into the states and dynamics of radiationless processes. They are probably due to sharp field dependent changes in the non-radiative coupling strength of the spin-orbit transition into the triplet.

Effect of refractive index on molecular radiative lifetimes

The effect of the refractive index of the medium ( n) on the fundamental photophysical parameters τ R = 1 k f has been investigated, using the highly-fluorescent compound 9,10-diphenylanthracene (DPA). Experiments show that for DPA in isopentane solution, reducing the temperature from room temperature to −160°C (whereupon n increases from 1.351 to 1.439) causes the experimental fluorescence lifetime to decrease from 8.8 to 7.5 ns. These data and behavior of the integrated extinction coefficient are consistent with an n −2 to n −3 dependence of the radiative lifetime. Radiative lifetimes for the thermalized gas phase are compared with those in solution for naphthalene and pyrene and are also consistent with an n −2 to n −3 dependence of the radiative lifetime. It is concluded that extrapolations of lifetimes data from one set of conditions to another should not be made unless absorption and/or quantum yield data are known under both conditions. When these data are not available, an n −2 correction factor applied to τ R probably is accurate to within ten percent.

Morse Franck-Condon Factors and R-Centroids for Some Lyman Bands of H2, HD and D2

Abstract Using new rotational and vibrational constants, Morse Franck-Condon factors and r-centroids are computed for 108 Lyman bands of astrophysical importance H2, HD and D2 molecules. Experimental oscillator strengths conjoint with Morse Franck-Condon Factors for six Lyman bands of molecular hydrogen yield radiative lifetimes for v = 0–5 levels of the B 1σu + state that are inconsistent with the experimental lifetime measurements. The limitation of analytic Morse function in the region of small internuclear separation for the B 1σu + state of H2 is discussed. The r-centroids (rv′v″) are found to increase with frequencies of the Lyman bands of H2, HD and D2. For a sequence, Δr = rv′+1,v″+1 - rv′v″ is constant. A comparison of rv′v″ values of the Lyman bands shows that rv′vv″ of the band (v′v″) increases in going from H2 to D2.

Photometry of the Cometary Atmosphere: A Review

Photometry and polarimetry of the cometary heads still constitute one of the most important sources of information about the physical processes in comets. For instance, most of the present estimates of molecular lifetimes are based on the observed distribution of molecules in the cometary head and the assumption of a particular kinematical behaviour of the matter in the cometary atmospheres.The study of kinematics and dynamics of cometary heads and tails has been based upon the analysis of the forms and apparent motions of well-defined envelopes, halos, knots in tails and streams. Direct inspection of a large number of photographs (or drawings from the last century) of several bright comets demonstrates that the cometary head is generally a complicated object. The heads consist of nearly circular diffuse patterns with superposition of different features, particularly of curved streams. This is illustrated by the Atlas of the Cometary Forms compiled by Rahe, Donn and Wurm (1970).

Design of an Apparatus for High Resolution Measurements of Atomic and Molecular Lifetimes by the High Frequency Deflection Technique

The High Frequency Deflection technique introduced at this laboratory a couple of years ago has proved to be a very versatile general method for measuring optical lifetimes. Thus lifetime measurements have been performed in a large number of atoms, ions, molecules and ion-molecules with much greater efficiency than other methods permit and at 20-50 times higher spectral resolution than what is normally achieved. The present version of the apparatus associated with the High Frequency Deflection technique is described in this paper from a technical point of view and its performance is discussed.

The measurement of molecular lifetimes by the photon-photon delayed coincidence method

The photon-photon delayed coincidence method has been used to measure the average lifetime of the a 3 Sigma g+ state of hydrogen and the value (10.45+or-0.25) ns has been found. This type of coincidence technique has previously only been used for the measurement of excited states of atoms, but it is shown that the counting rates and accuracy can be much higher in the case of molecular excited states.

High Resolution Measurements of Atomic and Molecular Lifetimes Using the High Frequency Deflection Technique

The basic principles of the High Frequency Deflection (HFD) technique for the determination of atomic and molecular lifetimes as well as the first experimental tests were presented a couple of years ago. In principle this technique is a further refinement of the conventional multichannel delayed coincidence technique with periodic electron excitation of free molecules. The differences are mainly that the HFD technique utilizes a continuous electron beam from a high-power gun operating in the kilovolt range and that the periodic excitation is accomplished by sweeping the beam at an optimal rate, i.e. with a period equal to 3-10 times the lifetime to be studied. With these two refinements the HFD technique enables lifetime measurements in all kinds of atoms, molecules, ions and ion-molecules with several orders of magnitude higher efficiency than other methods permit, thus opening possibilities for high resolution work in the absence of Doppler broadening. Thus a number of lifetimes have been determined at a spectral resolution of 0.1 Å FWHM or better, which is 20-50 times higher a resolution than what is normally achieved in lifetime measurements. The variable sweep frequency and the multichannel registration make possible measurements over a large lifetime range (up to about 104 ns) which in turn offers unique possibilities for analysing multi-exponential decay curves. With a time scale of arbitrary precision given by the sweep frequency, lifetime measurements have so far been performed with a total error down to 0.5% in favourable cases.The HFD technique is now currently in use at this laboratory and several hundreds of lifetimes have been studied and reported in a number of publications. In this paper we shall discuss more in detail the technique itself, its properties and possibilities compared to other methods for lifetime measurements. A review of some of the experimental results obtained so far, with particular emphasis on abundance determinations of atoms and molecules of fundamental astrophysical interest, is also included as well as a critical discussion of given error limits in lifetime measurements in general.

He+ afterpulses in photomultipliers: Their effect on atomic and molecular lifetime determinations

Determination of atomic and molecular lifetimes by delayed coincidence photon counting with photomultipliers two or more years old may lead to erroneous results caused by afterpulses and even direct photodecay in perfused helium. Spurious lifetimes as low as 15 nsec for the B2Σu+(v′=0) state of N2+ and the C3Πg(v′ = 0) state of N2 have been observed.

Gas excitations using high frequency deflected electron beams: A convenient method for determinations of atomic and molecular lifetimes

As pointed out earlier, gas jets excited with a monoenergetic electron beam in the keV, mA region yield intense atomic and molecular spectra. It has also been shown that high-frequency sweeping of the electron beam and fast-coincidence techniques convert this light source to a convenient method for determining atomic and molecular lifetimes. An improved version of such an apparatus has now been completed, being designed for 2–20 keV, 300 mA electrons and a variable sweep frequency. The apparatus has just been tested on a great number of lifetimes in the range 1–10 000 ns in atoms and molecules such as H, He, N, O, C 2, CH, CH +, CO 2 +, N 2, N 2 +, and O 2 +. Good agreement is obtained in most cases where measurements using other techniques are available. Presumably this high frequency deflection technique will find its most important applications in determining lifetimes in molecules of vital astrophysical interest including transitions in the infrared region.

A high-luminosity technique for determinations of atomic and molecular lifetimes

Numerous spectra and lifetimes in atoms and molecules such as H, He, N, Ne, O, Xe, C 2, CH, CO 2, H 2O, N 2, NH, NO, O 2 and OH have been measured by HF sweeping of energetic electrons from a high power gun and fast coincidence technique. Applications of this universal technique are discussed.