Search NASA⌕ Search

SEARCH · Search NASA

Results for “Photodissociation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Core-to-valence spectroscopic detection of the CH{sub 2}Br radical and element-specific femtosecond photodissociation dynamics of CH{sub 2}IBr

Element-specific single photon photodissociation dynamics of CH{sub 2}IBr and core-to-valence absorption spectroscopy of CH{sub 2}Br radicals are investigated using femtosecond high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy. Photodissociation of CH{sub 2}IBr along both the C–I or C–Br reaction coordinates is observed in real-time following excitation at 266 nm. At this wavelength, C–I dissociation is the dominant reaction channel and C–Br dissociation is observed as a minor pathway. Both photodissociation pathways are probed simultaneously through individual 4d(I) N{sub 4/5} and 3d(Br) M{sub 4/5} core-to-valence transitions. The 3d(Br) M{sub 4/5} pre-edge absorption spectrum of the CH{sub 2}Br radical photoproduct corresponding to the C–I dissociation channel is characterized for the first time. Although the radical's singly occupied molecular orbital (SOMO) is mostly localized on the central carbon atom, the 3d(Br) → π{sup *}(SOMO) resonances at 68.5 eV and 69.5 eV are detected 2 eV below the parent molecule 3d(Br) → σ{sup *}(LUMO) transitions. Core-to-valence XUV absorption spectroscopy provides a unique probe of the local electronic structure of the radical species in reference to the Br reporter atom. The measured times for C–I dissociation leading to I and I{sup *} atomic products are 48 ± 12 fs and 44 ± 4 fs, respectively, while the measured C–Br dissociation time leading to atomic Br is 114 ± 17 fs. The investigation performed here demonstrates the capability of femtosecond time-resolved core-level spectroscopy utilizing multiple reporter atoms simultaneously.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Transition state region in the A-Band photodissociation of allyl iodide—A femtosecond extreme ultraviolet transient absorption study

Femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy based on a high-harmonic generation source is used to study the 266 nm induced A-band photodissociation dynamics of allyl iodide (CH{sub 2} =CHCH{sub 2}I). The photolysis of the C—I bond at this wavelength produces iodine atoms both in the ground ({sup 2}P{sub 3/2}, I) and spin-orbit excited ({sup 2}P{sub 1/2}, I*) states, with the latter as the predominant channel. Using XUV absorption at the iodine N{sub 4/5} edge (45–60 eV), the experiments constitute a direct probe of not only the long-lived atomic iodine reaction products but also the fleeting transition state region of the repulsive n{sub I}σ{sup ∗}{sub C—I} excited states. Specifically, three distinct features are identified in the XUV transient absorption spectrum at 45.3 eV, 47.4 eV, and 48.4 eV (denoted transients A, B, and C, respectively), which arise from the repulsive valence-excited nσ{sup ∗} states and project onto the high-lying core-excited states of the dissociating molecule via excitation of 4d(I) core electrons. Transients A and B originate from 4d(I) → n(I) core-to-valence transitions, whereas transient C is best assigned to a 4d(I) →σ{sup ∗}(C—I) transition. The measured differential absorbance of these new features along with the I/I* branching ratios known from the literature is used to suggest a more definitive assignment, albeit provisional, of the transients to specific dissociative states within the A-band manifold. The transients are found to peak around 55 fs–65 fs and decay completely by 145 fs–185 fs, demonstrating the ability of XUV spectroscopy to map the evolution of reactants into products in real time. The similarity in the energies of transients A and B with analogous features observed in methyl iodide [Attar et al. J. Phys. Chem. Lett. 6, 5072, (2015)] together with the new observation of transient C in the present work provides a more complete picture of the valence electronic structure in the transition state region. The results provide a benchmark for theoretical calculations on the nature of core-excited states in halogenated hydrocarbons, especially in the transition state region along the C—I reaction coordinate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Photodissociation dynamics of the methyl perthiyl radical at 248 and 193 nm using fast-beam photofragment translational spectroscopy

The photodissociation dynamics of the methyl perthiyl radical (CH{sub 3}SS) have been investigated using fast-beam coincidence translational spectroscopy. Methyl perthiyl radicals were produced by photodetachment of the CH{sub 3}SS{sup −} anion followed by photodissociation at 248 nm (5.0 eV) and 193 nm (6.4 eV). Photofragment mass distributions and translational energy distributions were measured at each dissociation wavelength. Experimental results show S atom loss as the dominant (96%) dissociation channel at 248 nm with a near parallel, anisotropic angular distribution and translational energy peaking near the maximal energy available to ground state CH{sub 3}S and S fragments, indicating that the dissociation occurs along a repulsive excited state. At 193 nm, S atom loss remains the major fragmentation channel, although S{sub 2} loss becomes more competitive and constitutes 32% of the fragmentation. The translational energy distributions for both channels are very broad at this wavelength, suggesting the formation of the S{sub 2} and S atom products in several excited electronic states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C 3 H 7 X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel

The photodissociation dynamics of astrophysically relevant propyl derivatives (C 3 H 7 X; X = CN, OH, HCO) at 157 nm exploiting an ultracompact velocity map imaging (UVMIS) setup has been reported. The successful operation of UVMIS allowed the exploration of the 157 nm photo dissociation of six (iso)propyl systems - n/i-propyl cyanide (C 3 H 7 CN), n/i-propyl alcohol (C 3 H 7 OH), and (iso)butanal (C 3 H 7 CHO) – to explore the C 3 H 7 loss channel. The distinct center-of-mass translational energy distributions for the i-C 3 H 7 X (X= CN, OH, HCO) could be explained through preferential excitation of the low frequency C-H bending modes of the formyl moiety compared to the higher frequency stretchings of the cyano and hydroxy moieties. Although the ionization energy of the n-C 3 H 7 radical exceeds the energy of a 157 nm photon, C 3 H 7 + was observed in the n-C 3 H 7 X (X= CN, OH, HCO) systems as a result of photoionization of vibrationally "hot" n-C 3 H 7 fragments, photoionization of i-C 3 H 7 after a hydrogen shift in vibrationally "hot" n-C 3 H 7 radicals, and/or two-photon ionization. Our experiments reveal that at least the isopropyl radical (i-C 3 H 7 ) and possibly the normal propyl radical (n-C 3 H 7 ) should be present in the interstellar medium and hence searched for by radio telescopes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1985

Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region

A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.

Tielens, A. G. G. M.↗
DOE OSTI2014

Quantum dynamical investigation of the simplest Criegee intermediate CH{sub 2}OO and its O–O photodissociation channels

The singlet electronic potential energy surfaces for the simplest Criegee intermediate CH{sub 2}OO are computed over a two-dimensional reduced subspace of coordinates, and utilized to simulate the photo-initiated dynamics on the S{sub 2} (B) state leading to dissociation on multiple coupled excited electronic states. The adiabatic electronic potentials are evaluated using dynamically weighted state-averaged complete active space self-consistent field theory. Quasi-diabatic states are constructed from the adiabatic states by maximizing the charge separation between the states. The dissociation dynamics are then simulated on the diabatically coupled excited electronic states. The B ← X electronic transition with large oscillator strength was used to initiate dynamics on the S{sub 2} (B) excited singlet state. Diabatic coupling of the B state with other dissociative singlet states results in about 5% of the population evolving to the lowest spin-allowed asymptote, generating H{sub 2}CO (X {sup 1}A{sub 1}) and O ({sup 1}D) fragments. The remaining ∼95% of the population remains on repulsive B state and dissociates to H{sub 2}CO (a {sup 3}A″) and O ({sup 3}P) products associated with a higher asymptotic limit. Due to the dissociative nature of the B state, the simulated electronic absorption spectrum is found to be broad and devoid of any vibrational structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Communication: Ultraviolet photodissociation dynamics of the simplest Criegee intermediate CH{sub 2}OO

The velocity and angular distributions of O {sup 1}D photofragments arising from UV excitation of the CH{sub 2}OO intermediate on the B {sup 1}A′←X {sup 1}A′ transition are characterized using velocity map ion imaging. The anisotropic angular distribution yields the orientation of the transition dipole moment, which reflects the π*←π character of the electronic transition associated with the COO group. The total kinetic energy release distributions obtained at several photolysis wavelengths provide detail on the internal energy distribution of the formaldehyde cofragments and the dissociation energy of CH{sub 2}OO X {sup 1}A′ to O {sup 1}D + H{sub 2}CO X {sup 1}A{sub 1}. A common termination of the total kinetic energy distributions, after accounting for the different excitation energies, gives an upper limit for the CH{sub 2}OO X {sup 1}A′ dissociation energy of D{sub 0}≤ 54 kcal mol{sup −1}, which is compared with theoretical predictions including high level multi-reference ab initio calculations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Velocity map imaging of O-atom products from UV photodissociation of the CH{sub 2}OO Criegee intermediate

UV excitation of jet-cooled CH{sub 2}OO X{sup 1}A′ to the excited B{sup 1}A′ electronic states results in dissociation to two spin-allowed product channels: H{sub 2}CO X{sup 1}A{sub 1} + O {sup 1}D and H{sub 2}CO a{sup 3}A″ + O {sup 3}P. In this study, the higher energy H{sub 2}CO a{sup 3}A″ + O {sup 3}P channel is characterized by velocity map imaging and UV action spectroscopy, in both cases utilizing 2 + 1 resonance enhanced multiphoton ionization detection of O {sup 3}P products, which complements a prior experimental study on the lower energy H{sub 2}CO X{sup 1}A{sub 1} + O {sup 1}D channel [Lehman et al., J. Chem. Phys. 139, 141103 (2013)]. Anisotropic angular distributions indicative of rapid dissociation are obtained at 330 and 350 nm, along with broad and unstructured total kinetic energy distributions that provide insight into the internal excitation of the H{sub 2}CO a{sup 3}A″ co-fragment. A harmonic normal mode analysis points to significant vibrational excitation of the CH{sub 2} wag and C–O stretch modes of the H{sub 2}CO a{sup 3}A″ fragment upon dissociation. At each UV wavelength, the termination of the kinetic energy distribution reveals the energetic threshold for the H{sub 2}CO a{sup 3}A″ + O {sup 3}P product channel of ca. 76 kcal mol{sup −1} (378 nm) and also establishes the dissociation energy from CH{sub 2}OO X{sup 1}A′ to H{sub 2}CO X{sup 1}A{sub 1} + O{sup 1}D products of D{sub 0} ≤ 49.0 ± 0.3 kcal mol{sup −1}, which is in accord with prior theoretical studies. The threshold for the H{sub 2}CO a{sup 3}A″ + O {sup 3}P channel is also evident as a more rapid falloff on the long wavelength side of the O {sup 3}P action spectrum as compared to the previously reported UV absorption spectrum for jet-cooled CH{sub 2}OO [Beames et al., J. Am. Chem. Soc. 134, 20045 (2012)]. Modeling suggests that the O {sup 3}P yield increases uniformly from 378 to 300 nm.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium

The impact of the photodissociation of HCN and HNC isomers is analyzed in different astrophysical environments. For this purpose, the individual photodissociation cross sections of HCN and HNC isomers have been calculated in the 7–13.6 eV photon energy range for a temperature of 10 K. These calculations are based on the ab initio calculation of three-dimensional adiabatic potential energy surfaces of the 21 lower electronic states. The cross sections are then obtained using a quantum wave packet calculation of the rotational transitions needed to simulate a rotational temperature of 10 K. The cross section calculated for HCN shows significant differences with respect to the experimental one, and this is attributed to the need to consider non-adiabatic transitions. Ratios between the photodissociation rates of HCN and HNC under different ultraviolet radiation fields have been computed by renormalizing the rates to the experimental value. It is found that HNC is photodissociated faster than HCN by a factor of 2.2 for the local interstellar radiation field and 9.2 for the solar radiation field, at 1 au. We conclude that to properly describe the HNC/HCN abundance ratio in astronomical environments illuminated by an intense ultraviolet radiation field, it is necessary to use different photodissociation rates for each of the two isomers, which are obtained by integrating the product of the photodissociation cross sections and ultraviolet radiation field over the relevant wavelength range.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Mechanisms for the near-UV photodissociation of CH{sub 3}I on D{sub 2}O/Cu(110)

The system of CH{sub 3}I adsorbed on submonolayer, monolayer, and multilayer thin films of D{sub 2}O on Cu(110) has been studied by measuring the time of flight (TOF) distributions of the desorbing CH{sub 3} fragments after photodissociation using linearly polarized {lambda} = 248 nm light. For multilayer D{sub 2}O films (2-120 ML), the photodissociation is dominated by neutral photodissociation via the 'A-band' absorption of CH{sub 3}I. The polarization and angle dependent variation in the observed TOF spectra of the CH{sub 3} photofragments find that dissociation is largely via the {sup 3}Q{sub 0} excited state, but that also a contribution via the {sup 1}Q{sub 1} excitation can be identified. The photodissociation results also indicate that the CH{sub 3}I adsorbed on D{sub 2}O forms close-packed islands at submonolayer coverages, with a mixture of C-I bond axis orientations. For monolayer and submonolayer quantities of D{sub 2}O we have observed a contribution to CH{sub 3}I photodissociation via dissociative electron attachment (DEA) by photoelectrons. The observed DEA is consistent with delocalized photoelectrons from the substrate causing the observed dissociation- we do not find evidence for an enhanced DEA mechanism via the temporary solvation of photoelectrons in localized states of the D{sub 2}O ice.

36 MATERIALS SCIENCE↗
NASA NTRS1993

A new method for the calculation of photodissociation cross sections

A time-independent quantum mechanical approach to the calculation of photodissociation cross sections is developed. The method is based on the use of a discrete variable representation and the application of absorbing boundary conditions. Both total photodissociation cross sections and partial cross sections (hence product state distributions) are obtained using the same basic technique, but the calculation is particularly efficient when only the former quantity is required. The method is applied to the photodissociation of HCl(+) for which accurate potential energy curves and dipole moment functions are available, to the photodissociation of ClCN which is a direct process, involving a single excited electrosonic state, and to the photodissociation of ICN, which involves several strongly coupled excited electronic states.

Seideman, Tamar↗
NASA NTRS2002

Photodissociation of Peroxynitric Acid in the Near-IR

Temperature-dependent near-IR photodissociation spectra were obtained for several vibrational overtone transitions of peroxynitric acid (HNO4) with a tunable OPO photolysis/OH laser-induced-fluorescence system. Band-integrated photodissociation cross-sections (definity integral of sigma(sub diss)), determined relative to that for the 3nu(sub 1), OH stretching overtone, were measured for three dissociative bands. Assuming unit quantum efficiency for photodissociation of 3nu(sub 1), we find 2nu(sub 1) + nu(sub 3)(8242/cm) = (1.21 x 10(exp -20) (independent of temperature), 2nu(sub 1) (6900/cm) = 4.09 x 10(exp 18) * e(sup (-826,5/T)) (295 K greater than T greater than 224 K), and nu(sub 1) + 2nu(sub 3) (6252/cm) = 1.87 x 10(exp -19) * e(sup (- 1410.7/T)) (278 K greater than T greater than 240 K) sq cm/molecule cm. The photodissociation cross-sections are independent of pressure over the range 2 to 40 Torr. Temperature-dependent quantum yields (phi) for these transitions were obtained using integrated absorption cross-sections (definity integral of sigma(sub abs)) of HNO4 overtone vibrations measured with a FTIR spectrometer. In the atmosphere, photodissociation in the infrared is dominated by excitation of the first overtone of the OH stretching vibration (2nu((sub 1)). Inclusion of all dissociative HNO4 overtone and combination transitions yields a daytime IR photolysis rate of approximately 1 x 10(esp -1)/s. This process significantly shortens the estimated lifetime of HNO4 in the upper troposphere and lower stratosphere.

Roehl, Coleen M.↗
DOE OSTI2020

Nonequilibrium Solvent Effects during Photodissociation in Liquids: Dynamical Energy Surfaces, Caging and Chemical Identity

In the gas phase, potential energy surfaces can be used to provide insight into the details of photochemical reaction dynamics. In solution, however, it is unclear what potential energy surfaces, if any, can be used to describe even simple chemical reactions such as the photodissociation of a diatomic solute. In this paper, we use mixed quantum/classical (MQC) molecular dynamics (MD) to study the photodissociation of $Na$ $^+_2$ in both liquid Ar and liquid tetrahydrofuran (THF). We examine both the gas-phase potential surfaces and potentials of mean force (PMF), which assume that the solvent remains at equilibrium with the solute throughout the photodissociation process and show that neither resemble a nonequilibrium dynamical energy surface that is generated by taking the time integral of work. For the photodissociation of $Na$ $^+_2$ in liquid Ar, the dynamical energy surface shows clear signatures of solvent caging, and the degree of caging is directly related to the mass of the solvent atoms. For $Na$ $^+_2$ in liquid THF, local specific interactions between the solute and solvent lead to changes in chemical identity that create a kinetic trap that effectively prevents the molecule from dissociating. Here, the results show that nonequilibrium effects play an important role even in simple solution-phase reactions, requiring the use of dynamical energy surface to understand such chemical events.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗