Initial Excitation Energy Research Articles

Study of the total and partial fragmentation dynamics of Ar–HCl after uv photodissociation

The uv photolysis of the Ar–HCl cluster is studied applying an exact time-dependent wave packet method in three dimensions, assuming zero-total angular momentum. The photodissociation process is found to occur via two different fragmentation mechanisms, depending on the initial excitation energy of the cluster. One mechanism leads to total dissociation of the complex, producing three fragments, Ar–HCl+hν→H+Ar+Cl. The fragmentation dynamics in this case is governed by resonance states at relatively low energies of the cluster, in which the H atom collides a number of times with Ar and Cl before dissociating. Manifestations of these collisions are found in the final kinetic energy distribution of the photofragments, which is redshifted in the case of the H fragment, and blueshifted in the Ar and Cl cases. The second type of mechanism consists of a fast and direct photodissociation of the hydrogen, leading to a partial fragmentation of Ar–HCl into hot H fragments and bound Ar–Cl radical molecules. This mechanism dominates at higher energies, which are those mostly populated by the wave packet initially prepared in the present calculations. The experimental implications of the results are discussed.

Breakup temperature of target spectators in 197Au +197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction 197Au + 197Au at 1000 MeV per nucleon. Pairs of 3,4He and 6,7Li isotopes and pairs of 3,4He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decayat relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at τlab = 150°, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage.

Ion Desorption Reaction of Surface Admolecules Induced by Synchrotron Radiation.

Studies of surface photochemical reactions induced by core excitation have developed with the advance of synchrotron radiation technique over a range extending from vacuum ultraviolet to soft X-ray. Studies of the photon stimulated ion desorption (PSID) have been carried out mainly on the basis of the initial excitation energy dependence of the desorption yield. In particular, site-specific reactions have been found in PSID following core electron excitation. This is very interesting from the viewpoint of the control of chemical reactions using monoenergetic photo-excitation. The Auger stimulated ion desorption (ASID) mechanism is widely proposed as a model of PSID. As the Auger final state is directly related to the ion desorption in this model to elucidate the PSID mechanism, it is essential to get the information about the Auger final state. In this article, we have described about PSID of formic acid induced by carbon core excitation using ion TOF method and about PSID of condensed H2O induced by oxygen core excitation using Auger electronphotoion coincidence (AEPICO) method.

Breakup temperature of target spectators in 197Au + 197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction Au + Au at 1000 MeV per nucleon. Pairs of $^{3,4}$He and $^{6,7}$Li isotopes and pairs of $^{3,4}$He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the observed universality of the spectator decay at relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at $\theta_{lab}$ = 150$^{\circ}$, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage. PACS numbers: 25.70.Mn, 25.70.Pq, 25.75.-q

The dependence of fission time-scales on fragment mass asymmetry in the 20Ne+ 165Ho reaction at 50 A MeV

The pre-scission and post-scission multiplicities of alpha particles were determined in coincidence with fission fragments for four windows of exit channel mass asymmetry in the reaction 20Ne+ 165Ho at 50 A MeV. The technique used employed a kinematic analysis of the energy spectra of alpha particles as a function of the emission angle with respect to the fission axis. Using a procedure of fitting the experimental energy spectra with those resulting from a Monte Carlo simulation program, the total multiplicity of alpha particles was separated into pre-scission and post-scission components. Using the pre-scission and post-scission multiplicities, neutron multiplicity measurements and other empirical observations, both the initial excitation energy and the excitation energy at scission of the compound nucleus were determined. The pre-scission times in the de-excitation of a highly excited 178W compound nucleus with initial excition energy of 570 MeV were evaluated using statistical model calculations. Only a small decrease in scission time was derived for the most asymmetric events studied. The possible contribution of very damped deep inelastic processes is discussed.

A new mechanism for synthesis of superheavy elements

A dynamical theory is proposed for nuclear reactions leading to residues of superheavy elements. Fusion and fission processes are treated consistently by a diffusion equation which describes a time-dependent probability distribution in the collective coordinate or deformation space. The potential energy in the equation is time-dependent, because cooling due to particle evaporation gradually restores the shell correction energy which gives rise to a potential pocket essential for the stabilization of the superheavy elements around Z = 114 and N = 184. It is shown that there is an optimum initial excitation energy or incident energy of reactions as the result of a compromise between two conflicting requirements; higher energies which favour larger fusion probabilities and lower energies which favour larger residue probabilities or a quicker restoration of the shell-correction energy. A promising experimental direction is suggested.

Intramolecular energy transfer rates for vinyl bromide and deuterium-substituted vinyl bromides from power spectrum line splittings

Continuous frequency modulated (CFM) line splittings are used to determine the energy transfer rate coefficients for the local C–Br and C=C vibrational modes in vinyl bromide and the C–H stretching modes in doubly deuterium-substituted vinyl bromides. The global potential developed by Abrash et al. is employed in all calculations. Energy transfer rate coefficients are extracted from the fine structure spacing of the numerically computed power spectrum of the bond coordinates. The consistency of the averaged individual rate coefficients is evaluated by comparison with results obtained from local mode energy decay curves. It is found that the total intramolecular vibrational relaxation (IVR) rate coefficients for all modes investigated are large relative to the unimolecular decomposition rate. However, previous studies show that IVR is not globally rapid so statistical behavior of the unimolecular reaction is not expected. It is shown that near overlapping resonances in the power spectrum make it difficult to accurately extract CFM line splittings. This limitation effectively precludes the use of power spectra to investigate IVR rates for some modes. For the specific case of vinyl bromide, it is demonstrated that the C–Br and C=C stretching modes have sufficiently isolated bands that IVR rates out of these modes can be determined from the line splittings. However, the superposition of the three C–H stretching fundamentals makes it essentially impossible to investigate these modes in vinyl bromide. For the case of doubly deuterium-substituted vinyl bromides, the C–H stretching fundamental is well isolated so that IVR relaxation rates can be easily obtained from the power spectrum line splittings. The consistency of the IVR rate coefficients obtained from line splittings is investigated by calculation of these coefficients from the envelopes of bond energy decay curves. The differences between the two results varies from 15% for the C=C stretch to 43% for one of the C–H stretching modes. The average deviation is 30% which is in accord with the accuracy of the method (±25%) previously estimated by Agrawal et al. The effect of initial local excitation energy on the line splittings and associated rate coefficients is investigated for the C–Br stretching mode. The results show that the line splitting and rate coefficients are nearly independent of excitation energy below 0.8 eV. Above this energy, both the line splittings and the IVR rate coefficients increase rapidly. This is interpreted as being due to increased intermode coupling at higher energies produced by the greater vibrational anharmonicity. It is concluded that CFM line splittings can be effectively used as a probe of energy transfer rates in six-atom molecules provided the modes under examination have reasonably isolated bands in the power spectrum.

Dilepton and photon decay of giant resonances built on excited states of56Ni

Nuclear states in {sup 56}Ni, at an initial excitation energy E{sup {asterisk}}=70 MeV, were populated via the isospin T=0 reaction {sup 16}O+{sup 40}Ca. The dilepton- (e{sup +}e{sup {minus}}) pairs and photon-decay yields were measured simultaneously. Statistical decay calculations describe well both measurements on an absolute scale. The isospin mixing parameter extracted from these measurements ({alpha}{sup 2}=0.065) is consistent with other studies of the isospin symmetry in highly excited compound nuclei. The 1{sigma} upper limit for the observed monopole strength is 4.3 nb, which is 3 times higher than the one sum rule value calculated with the statistical model for the decay of the compound nucleus. {copyright} {ital 1997} {ital The American Physical Society}

Application of the methylenology principle to substitution reactions. A theoretical study

The substitution reaction of chloride with methyl chloride, 2-chloroethyl radical and allyl chloride has been investigated with a number of different ab initio theoretical methods. Depending on the theoretical method chosen, the intrinsic barrier for the SN2′ reaction in allyl chloride is 7–11 kcal mol–1 higher than the barrier for the SN2 reaction in methyl chloride. The reaction of chloride with the 2-chloroethyl radical proceeds through formation of an intermediate complex, which can best be characterized as an ethylene fragment flanked by a resonating chloride anion/chloride radical pair. The overall process has been termed ‘SRN2c’, as nucleophilic substitution occurs in this open shell system with overall ‘cine’ regiochemistry. The intrinsic barrier for the SRN2c reaction is approximately 10 kcal mol–1 lower than that for the SN2 reaction. The differential barrier heights in these three substitution reactions have been rationalized using the valence bond curve crossing model. The high SN2′ barrier is due to a larger initial excitation energy as compared to the SN2 reaction and also to a smaller transition state resonance energy. The very low barrier and the formation of an intermediate in the SRN2c reaction is the consequence of a low lying electronic state, in which homolytic C–Cl bond cleavage has occurred and a C–C double bond is formed. This ‘double bond’ state descends low enough to cross with the Lewis curves used to describe bond breaking and bond making in nucleophilic substitution reactions in general. Only a small initial excitation is necessary to reach the ‘double bond’ state from the electronic ground state. This small initial excitation is the origin of the low barrier for the SRN2c substitution process.

Dynamics of geminate charge separation in liquid methylcyclohexane studied by the photoassisted ion pair separation technique

A laser photoassisted ion pair separation technique has been developed to study the kinetics of geminate charge recombination in the picosecond domain in hydrocarbon liquids. Recombination of geminate pairs in liquid methylcyclohexane has been studied by this technique. The electrons were created by two-photon ionization of the anthracene and tetramethyl-p-phenylenediamine (TMPD) admixture using a picosecond UV laser pulse at 270 or 360 nm and then were excited by an IR laser pulse at 1080 nm. The diffusion model of photostimulated ion pair dissociation has been extended to the picosecond time regime in order to describe the observed dependence of the IR enhanced photoconductivity of the solutions on the time delay between the UV and IR pulses. The average initial separation between a geminate trapped electron and its sibling cation was obtained from the experimental decay curves of the IR enhanced photocharge. The separation in the geminate electron-cation pair was found to increase with initial excess excitation energy, ΔE, of the solute above its ionization threshold. It is concluded that the hot electron, the precursor of the trapped geminate electron, is initially injected into the liquid with kinetic energy that increases with increasing the ΔE energy.

Possible mechanisms for T-phase generation

Recent extensions to the wave-number-integration approach [Schmidt et al., J. Acoust. Soc. Am. 98, 465–472 (1995) and H. Schmidt, J. Acoust. Soc. Am. 97, 3316(A) (1995)] have made it possible to apply spectral methods to stepwise range-dependent elastic problems. Using these methods, it is possible to study the potential mechanisms of T-phase generation in oceanic waveguides. Such a study has applications in the prediction of tsunamigenisis from T-phase observations. The general environment considered is a canonical sound-speed profile and sloping bottom. The seismoacoustic field is determined via the above spectral methods, and then a modal decomposition is applied. The initial modal excitation and subsequent energy exchange from coupling is examined. The energy that becomes trapped in the water column is indicative of what might be observed on a distant hydrophone. The inclusion of elastic effects in these range-dependent scenarios is likely to be critical in order to fully model the T-phase generation. For a sloping elastic bottom, the lower modes may actually be directly excited by the shear waves, whereas energy injected from compressional waves in the bottom must couple down through the higher modes. The strong presence of low-order modes is consistent with observations. This study considers the importance of a variety of parameters, including slope, source-depth, frequency and bottom characteristics. Both rift and slope bathymetries are considered. [Work supported by ONR.]

Photocarrier Recombination in Microcrystalline Silicon Studied by Light Induced Electron Spin Resonance Transients

AbstractTo get information on the density of states distribution and the photocarrier recombination in microcrystalline silicon (μc-Si:H), samples with various amounts of n- and p-type doping are studied with electron spin resonance (ESR) and stationary and time-resolved light induced ESR. The intensity of the dark ESR signals from dangling bonds (DB) and conduction electrons (CESR) is investigated as a function of the doping level. The DB signal has a flat distribution over a wide doping range while the CESR signal strongly increases with n-type doping. Upon illumination with white or infrared light both resonances are enhanced with an intensity that depends on the doping level. The decay of the light induced signal and the dependence in time and intensity of the residual signal on different initial excitation energies and dark/light -sequences is studied. The results are discussed with a schematic band diagram for μc-Si:H. The existence of a potential barrier is proposed which spatially separates photogenerated carriers. A large band-offset between crystalline and disordered regions is further suggested.

Dilepton decay from excited states in 28Si populated via different isospin entrance channels.

Nuclear states in $^{28}\mathrm{Si}$, with an initial excitation energy E*=50 MeV, were populated via the isospin T=0 reaction $^{4}\mathrm{He}$${+}^{24}$Mg and the mixed-isospin $^{3}\mathrm{He}$${+}^{25}$Mg reaction. In both reactions the dilepton (${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) and photon decay yields were measured concurrently, up to transition energies of 30 MeV. Both the dilepton and the photon yields are well described by a modified version of the statistical decay model, which includes both the photon and dilepton decays of giant resonances built on excited nuclear states. An excess of counts in the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ spectrum, over the converted photon yield, is observed in the energy region 17--22 MeV in the $^{3}\mathrm{He}$ induced reaction, possibly indicating an entrance channel effect. \textcopyright{} 1996 The American Physical Society.

Limiting temperatures of neutron rich nuclei: A possible interpretation of data from isotope yield ratios.

The recent ALADIN report of limiting temperatures for nuclear disassembly, derived from measurements of isotopic ratios for He and Li nuclei, is discussed. It is suggested that the entire excitation energy dependence which is observed may result from the fact that limiting temperatures for the onset of Coulomb instability are being measured for progressively lighter neutron rich nuclei as the excitation energy per nucleon increases. While the basic observation of plateauing in the intermediate excitation energy range remains valid, the higher excitation results may not signal entry into the vapor phase. The ALADIN result for A\ensuremath{\approxeq}125, when combined with lower energy data, indicates a plateau temperature near 6.5 MeV over the range of 3--11 MeV/nucleon initial excitation energy.

Dynamical effects in the population of compound nuclei

Evidence for the influence of reaction dynamics on the population of compound nuclei will be presented. Large dissipation in certain heavy-ion reactions leads to long compound nucleus formation times which can change the initial angular momentum and excitation energy distribution of the compound nucleus. This effect can be observed by measuring high energy γ-rays from the giant dipole resonance. The overall neutron multiplicity, however, is not a sensitive observable. The long formation times might even influence the final spin distribution of evaporation residues, which could possibly explain the entrance channel dependent population of superdeformed bands.

Emission of photons in spontaneous fission of 252Cf.

High energy photon emission accompanying the spontaneous fission of $^{252}\mathrm{Cf}$ is measured for different mass splits. The photon yields up to an energy of 20 MeV are obtained at several angles relative to the fission direction. Statistical model calculations are used to interpret the data. The photon yield is found to be very sensitive to the initial excitation energy sharing among the daughter nuclei and to their level density parameters. Using experimentally extracted level densities obtained from neutron evaporation measurements, the photon yield is well described by calculations for all mass splits.

Dilepton decay of giant resonances built on excited states of 28Si.

Nuclear states in ${}^{28}\mathrm{Si}$ were populated at an initial excitation energy ${E}^{*}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}50$ MeV via the isospin $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ reaction ${}^{16}\mathrm{O}{+}^{12}\mathrm{C}$. The dilepton- and photon-decay yields were measured simultaneously for the first time up to transition energies of 30 MeV. Statistical decay calculations describe well both measurements on an absolute scale. The measured absolute dilepton cross section, integrated over the transition energy region 15--30 MeV, amounts to $\ensuremath{\sigma}({e}^{+}{e}^{\ensuremath{-}})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}84\ifmmode\pm\else\textpm\fi{}6$ nb. Implications for measuring a possible monopole strength are discussed.

Onset of vaporization for the Ar+Ni system

Using the 4π multidetector INDRA, collisions between 36Ar and 58Ni have been investigated over a broad bombarding energy range, from 32 to 95 AMeV. The onset for complete vaporization of the system into neutrons, H and He isotopes as well as the evolution with energy of the isotopic composition of the vaporization events were determined. Initial excitation energy needed for vaporization is discussed.

Lifetimes in the decay of 40Ca and 47V studied by crystal blocking.

The lifetimes in the decay of compound nuclei formed in the fusion reactions {sup 12}C + {sup 28}Si {r_arrow} {sup 40}Ca and {sup 19}F + {sup 28}Si {r_arrow} {sup 47}V are measured to be (5--10){times}10{sup {minus}18} s using the blocking of evaporation residues in Si {l_angle}100{r_angle} single crystal. Similar measurements using low energy alpha particles evaporated in the decay of {sup 40}Ca yield lifetimes in the same range. However, the shape of the blocking pattern for the high energy {alpha} particles in the same decay is similar to that for the elastically scattered particles indicating a lifetime shorter than 10{sup {minus}18} s. These results, along with our earlier measurements in {sup 44}Ti and those reported by others, show that the lifetimes for the later stages of the compound nuclear decay are nearly the same for nuclei in the mass range 40--160 and are relatively insensitive to the initial excitation energy.

Laboratory studies of 3.3 micron emission from naphthalene induced by 193 and 248 nanometer excitation

view Abstract Citations (41) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Laboratory Studies of 3.3 Micron Emission from Naphthalene Induced by 193 and 248 Nanometer Excitation Williams, Richard M. ; Leone, Stephen R. Abstract Laboratory studies of infrared emission from gas-phase naphthalene in the 3.3 micrometer region following ultraviolet laser excitation are used to interpret the unidentified infrared bands observed in many astronomical objects. A time-resolved Fourier transform infrared emission technique acquires the time and spectrally resolved data. Two excitation wavelengths are employed: 193 nm and 248 nm. The infrared emission features are strongly dependent on the initial excitation energy. Wavelength-resolved spectra recorded 6.8 microseconds after the laser pulse show a 45/cm redshift from the gas-phase absorption spectra for 193 nm excitation and 25/cm for 248 nm excitation. We hypothesize that a series of sequence bands originating from the highly vibrationally excited ensemble of molecules is responsible for the observed shift. As collisional and radiative deactivation removes energy from the highly vibrationally excited molecules, the maximum in the emission profile gradually approaches the customary absorption maximum. This indicates that the amount of redshift is strongly dependent on the amount of internal vibrational energy in the molecule at the time of the vibrational transition. Publication: The Astrophysical Journal Pub Date: April 1995 DOI: 10.1086/175559 Bibcode: 1995ApJ...443..675W Keywords: Infrared Spectra; Interstellar Matter; Laser Induced Fluorescence; Molecular Excitation; Naphthalene; Red Shift; Ultraviolet Lasers; Vibrational States; Fourier Transformation; Molecular Collisions; Radiative Transfer; Astrophysics; INFRARED: ISM: LINES AND BANDS; LINE: IDENTIFICATION; MOLECULAR PROCESSES full text sources ADS |