Unimolecular Reaction Dynamics from Kinetic Energy Release Distributions. 7. Average Translational Energy Release

Lorquet, Jean-Claude

doi:10.1021/jp000044u

Cited by 10 publications

(21 citation statements)

References 42 publications

(103 reference statements)

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“…Lorquet (2000) has shown that the relationship between the average kinetic energy release U 0 and the internal energy measured in excess of the dissociation threshold, ∆E, is not linear. It contains information about the density of vibrationalrotational states.…”

Section: Kinetic Energy Release (Heating) Of the Ionic Fragmentsmentioning

confidence: 99%

Destruction of formic acid by soft X-rays in star-forming regions

Boechat-Roberty¹,

Pilling

Santos

2005

A&A

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Abstract. Formic acid is much more abundant in the solid state, both in interstellar ices and cometary ices, than in the interstellar gas (ice/gas ∼ 10 4 ) and this point remains a puzzle. The goal of this work is to experimentally study ionization and photodissociation processes of HCOOH (formic acid), a glycine precursor molecule. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons from toroidal grating monochromator TGM) beamline (200-310 eV). Mass spectra were obtained using photoelectron photoion coincidence (PEPICO) method. Kinetic energy distributions and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Photoionization and photodissociation cross sections were also determined. Due to the large photodissociation cross section of HCOOH it is possible that in PDRs regions, just after molecules evaporation from the grain surface, formic acid molecules are almost totally destroyed by soft X-rays, justifying the observed low abundance of HCOOH in the gaseous phase. The preferential path for the glycine formation from formic acid may be through the ice phase reaction.

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Section: Kinetic Energy Release (Heating) Of the Ionic Fragmentsmentioning

confidence: 99%

Destruction of formic acid by soft X-rays in star-forming regions

Boechat-Roberty¹,

Pilling

Santos

2005

A&A

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“…If metastable ions are produced with a distribution of internal energies, the above KERD should be averaged over the fragmentation probability function, P ( E , J 0 ), that defines the probability that a precursor ion with energy E and angular momentum J 0 will dissociate in the FFR. However, the correction introduced by averaging the KERD over P ( E , J 0 ) is less than 3%47 and can be safely neglected in most cases. Finally, the KERD in Eqn (22) is averaged over the initial rotational distribution of the precursor ion.…”

Section: Theoretical Approachesmentioning

confidence: 99%

Kinetic energy release distributions in mass spectrometry

2001

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Kinetic energy releases (KERs) in unimolecular fragmentations of singly and multiply charged ions provide information concerning ion structures, reaction energetics and dynamics. This topic is reviewed covering both early and more recent developments. The subtopics discussed are as follows: (1) introduction and historical background; (2) ion dissociation and kinetic energy release: kinematics; potential energy surfaces; (3) the kinetic energy release distribution (KERD); (4) metastable peak observations: measurements on magnetic sector and time-of-flight instruments; energy selected results by photoelectron photoion coincidence (PEPICO); (5) extracting KERDs from metastable peak shapes; (6) ion structure determination and reaction mechanisms: singly and multiply charged ions; biomolecules and fullerenes; (7) theoretical approaches: phase space theory (PST), orbiting transition state (OTS)/PST, finite heat bath theory (FHBT) and the maximum entropy method; (8) exit channel interactions; (9) general trends: time and energy dependences; (10) thermochemistry: organometallic reactions, proton-bound clusters, fullerenes; and (11) the efficiency of phase space sampling.

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“…At still higher internal energies, F keeps on decreasing but is found to level off or even re-increase, say at E ≈ 2.5 eV, where it reaches its lowest value, which is roughly of the order of 50% for the investigated reactions. This behaviour has been observed in a number of cases: loss of a Br atom from C 2 H 3 Br + [40], loss of I from C 2 H 5 I + [48], HCN loss from the pyridine ion [42] and, very recently, loss of an acetylene molecule from C 6 H 6 + ions. 1 As the internal energy increases, the lifetime decreases whereas the volume of phase space to be sampled increases enormously.…”

Section: A Very Interesting Question Concerns the Energy-dependence Omentioning

confidence: 83%

“…A MEM analysis reveals that the average kinetic energy release does not necessarily increase linearly with the internal energy E [48]. In fact, it is determined by two quantities.…”

Section: Average Translational Energy Releasementioning

confidence: 97%

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Analysis of kinetic energy release distributions by the maximum entropy method

Leyh¹,

Gridelet²,

Locht³

et al. 2006

International Journal of Mass Spectrometry

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Energy is not always fully randomized in an activated molecule because of the existence of dynamical constraints. An analysis of kinetic energy release distributions (KERDs) of dissociation fragments by the maximum entropy method (MEM) provides information on the efficiency of the energy flow between the reaction coordinate and the remaining degrees of freedom during the fragmentation. For example, for barrierless cleavages, large translational energy releases are disfavoured while energy channeling into the rotational and vibrational degrees of freedom of the pair of fragments is increased with respect to a purely statistical partitioning. Hydrogen atom loss reactions provide an exception to this propensity rule. An ergodicity index, F, can be derived. It represents an upper bound to the ratio between two volumes of phase space: that effectively explored during the reaction and that in principle available at the internal energy E. The function F(E) has been found to initially decrease and to level off at high internal energies.For an atom loss reaction, the orbiting transition state version of phase space theory (OTST) is especially valid for low internal energies, low total angular momentum, large reduced mass of the pair of fragments, large rotational constant of the fragment ion, and large polarizability of the released atom. For barrierless dissociations, the major constraint that results from conservation of angular momentum is a propensity to confine the translational motion to a two-dimensional space. For high rotational quantum numbers, the influence of conservation of angular momentum cannot be separated from effects resulting from the curvature of the reaction path.The nonlinear relationship between the average translational energy and the internal energy E is determined by the density of vibrational-rotational states of the pair of fragments and also by non-statistical effects related to the incompleteness of phase space exploration.The MEM analysis of experimental KERDs suggests that many simple reactions can be described by the reaction path Hamiltonian (RPH) model and provides a criterion for the validity of this method.Chemically oriented problems can also be solved by this approach. A few examples are discussed: determination of branching ratios between competitive channels, reactions involving a reverse activation barrier, nonadiabatic mechanisms, and isolated state decay. Statistical methods in mass spectrometryFor the last 50 years, mass spectrometrists have made great efforts to rationalize fragmentation patterns and breakdown graphs by various statistical models that are all based on a common assumption. The internal energy deposited in a molecular ion is expected to be completely randomized before dissociation [1][2][3][4]. When this is the case, the reaction is declared ergodic.Equivalently, the energetically available phase space is said to be uniformly sampled within the lifetime of the molecular ion. In still more esoteric terms, the molecular ion is said to have reached a stat...

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Unimolecular Reaction Dynamics from Kinetic Energy Release Distributions. 7. Average Translational Energy Release

Cited by 10 publications

References 42 publications

Destruction of formic acid by soft X-rays in star-forming regions

Destruction of formic acid by soft X-rays in star-forming regions

Kinetic energy release distributions in mass spectrometry

Analysis of kinetic energy release distributions by the maximum entropy method

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