Vibrationally mediated photodissociation of ammonia: the influence of N-H stretching vibrations on passage through conical intersections

Velocity map ion imaging of the H atoms formed in the photodissociation of vibrationally excited ammonia molecules measures the extent of adiabatic and nonadiabatic dissociation for different vibrations in the electronically excited state. Decomposition of molecules with an excited symmetric N-H stretch produces primarily ground state NH(2) along with a H atom. The kinetic energy release distribution is qualitatively similar to the ones from dissociation of ammonia excited to the electronic origin or to several different levels of the bending vibration and umbrella vibration. The situation is very different for electronically excited molecules containing a quantum of antisymmetric N-H stretch. Decomposition from that state produces almost solely electronically excited NH(2)*, avoiding the conical intersection between the excited state and ground state surfaces. These rotationally resolved measurements agree with our previous inferences from lower resolution Doppler profile measurements. The production of NH(2)* suggests that the antisymmetric stretching excitation in the electronically excited molecule carries it away from the conical intersection that other vibrational states access.     

A comparison of the decomposition of electronically excited nitro-containing molecules with energetic moieties C-NO2, N-NO2, and O-NO2

Decomposition of electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) moieties has been intensively investigated over the past decades; however, their decomposition behavior has not previously been compared and contrasted. Comparison of their unimolecular decomposition behavior is important for the understanding of the reactivity differences among electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) bond connections. Nitromethane (NM), dimethylnitramine (DMNA), and isopropylnitrate (IPN) are used as model molecules for C-NO(2), N-NO(2), and O-NO(2) active moieties, respectively. Ultraviolet lasers at different wavelengths, such as 226, 236, and 193 nm, have been employed to prepare the excited states of these molecules. The decomposition products are then detected by resonance enhanced two photon ionization (R2PI), laser induced fluorescence (LIF) techniques, or single photon ionization at 10.5 eV. NO molecules are observed to be the major decomposition product from electronically excited NM, DMNA, IPN using R2PI techniques. The NO products from decomposition of electronically excited (226 and 236 nm) NM and IPN display similar rotational (600 K) and vibrational distributions [both (0-0) and (0-1) bands of the NO molecule are observed]. The NO product from DMNA shows rotational (120 K) and vibrational distributions (only (0-0) transition is observed) colder than those of NM and IPN. At the 193 nm excitation, electronically excited NO(2) products are observed from NM and IPN via fluorescence detection, while no electronically excited NO(2) products are observed from DMNA. Additionally, the OH radical is observed as a minor dissociation product from all three compounds. The major decomposition pathway of electronically excited NM and IPN involves fission of the X-NO(2) bond to form electronically excited NO(2) product, which further dissociates to generate NO. The production of NO molecules from electronically excited DMNA is proposed to go through a nitro-nitrite isomerization pathway. Theoretical calculations show that a nitro-nitrite isomerization for DMNA occurs on the S(1) surface following a (S(2)/S(1))(CI) conical intersection (CI), whereas NO(2) elimination occurs on the S(1) surface following the (S(2)/S(1))(CI) conical intersection for NM and IPN. The present work provides insights for the understanding of the initiation of the decomposition of electronically excited X-NO(2) energetic systems. The presence of conical intersections along the reaction coordinate plays an important role in the detailed mechanism for the decomposition of these energetic systems.     

IR and visible luminescence studies in the infrared multiphoton dissociation of 1,2-dibromo-1,1-difluoroethane

The infrared multiphoton dissociation of 1,2-dibromo-1,1-difluoroethane gives rise to IR and visible luminescence. Vibrationally excited parent molecules dissociate via two primary channels yielding bromine and vibrationally excited HBr. The strong visible emission observed between 350 to 750 nm has been assigned to electronically excited carbene CF₂Br H.     

Two-photon excitation of mercury atoms by photodissociation of mercury halides

Photodissociation of HgCl₂, HgBr₂, and Hgl₂ with an ArF laser at 193 nm produces strong fluorescence from highly excited Hg atoms. The experiments indicate that single photon dissociation of the mercuric halide, HgX₂, is followed by the two-photon dissociation of HgX (X² Σ) molecules to produce electronically excited Hg atoms.     

Generation and characterization of highly vibrationally excited molecular beam

A simple method to generate and characterize a pure highly vibrationally excited azulene molecular beam is demonstrated. Azulene molecules initially excited to the S4 state by 266-nm UV photons reach high vibrationally excited levels of the ground electronic state upon rapid internal conversion from the S4 electronically excited state. VUV laser beams at 157 and 118 nm, respectively, are used to characterize the relative concentrations of the highly vibrationally excited azulene and the rotationally and vibrationally cooled azulene in the molecular beam. With a laser intensity of 34 mJ/cm2, 75% of azulene molecules absorb a single 266-nm photon and become highly vibrationally excited molecules. The remaining ground-state azulene molecules absorb two or more UV photons, ending up either as molecular cations, which are repelled out of the beam by an electric field, or as dissociation fragments, which veer off the molecular-beam axis. No azulene without absorption of UV photons is left in the molecular beam. The molecular beam that contains only highly vibrationally excited molecules and carrier gas is useful in various experiments related to the studies of highly vibrationally excited molecules.     

Proton-coupled electron transfer originating from excited states of luminescent transition-metal complexes

Proton-coupled electron transfer (PCET) is of fundamental importance for small-molecule activation processes, such as water splitting, CO(2)-reduction, or nitrogen fixation. Ideally, energy-rich molecules such as H(2), CH(3)OH, or NH(3) could be generated artificially using (solar) light as an energy input. In this context, PCETs originating directly from electronically excited states play a crucial role. A variety of transition-metal complexes have been used recently for fundamental investigations of this important class of reactions, and the key findings of these studies are reviewed in this article. The present minireview differs from other reviews on the subject of PCET in that it focuses specifically on reactions occurring directly from electronically excited states.     

Photodissociation dynamics of the 2-methylallyl radical

Time- and frequency-resolved photoionization of the hydrogen atom product from a jet-cooled electronically excited 2-methylallyl radical, C4H7, provides information on the dissociation dynamics. The measured dissociation rates and kinetic energy release of 2-methylallyl and its isotopologue CD3C3H4 combined with high level ab initio calculations suggests unimolecular dissociation with methylenecyclopropane and hydrogen as the major C-H bond fission channel with no evidence for nonstatistical behavior in dissociation. Other possible dissociation and isomerization pathways are discussed based on the results of the coupled-cluster ab initio calculations.     

Effect of diabatic correlation on the reactions of O(2 3P, 2 1D) with N2 O and CO2

Although correlation diagrams based upon the application of spin and angular momentum conservation have been shown to be a useful device in interpreting the chemistry of electronically excited atoms, experimental observations suggest that a more complete understanding of such chemical processes requires some insight into the electronic structure of the collision complex. In the absence of such information, it is possible to consider the role of diabatic correlations on the energetics of elementary processes with a view toward analyzing the behavior of the reactants along the reaction coordinate. Here, the aeronomically interesting reactions of ground state and electronically excited oxygen atoms with N₂ O and CO₂ are analyzed and the effects of low-lying molecular excited states on the reactivity of these molecules assessed.     

State and velocity distributions of Cl atoms produced in the photodissociation of ICI at 237 nm

Photofragment spectroscopy of ICI molecules photodissociated at 237 nm is studied by 2 + 1 resonance-enhanced multi-photon ionization and time of flight techniques. Doppler profiles of the chlorine atom fragments in two spin—orbit states show that chlorine atoms in the ground state, ²P_3/2, are produced from a perpendicular dissociative transition, and chlorine atoms in the excited state, ²P , arise from a parallel transition. The possible electronically excited states leading to dissociation in both the perpendicular and parallel cases are considered.     

Excited atoms and molecules in physicochemical processes and diagnostics of nonequilibrium plasma

The role of metastable excited atoms and molecules in the mechanisms of physicochemical processes and diagnostics of quasi-equilibrium and nonequilibrium atomic and molecular electric-discharge plasmas is analyzed. The consideration is focused on the mechanism of excitation of electronic and vibronic states, as emission from these levels form optical spectra used for plasma diagnostics. The contribution of metastable excited species to other physicochemical processes: dissociation, chemical reactions involving atoms and molecules, ionization, ion-electron recombination, and ion conversion, is briefly discussed. It is shown that the participation of metastable species should be taken into account before application of spectral methods of plasma diagnostics, especially, at elevated (atmospheric and higher) pressures.     

Dynamics at conical intersections: the influence of O-H stretching vibrations on the photodissociation of phenol

Comparing the recoil energy distributions of the fragments from one-photon dissociation of phenol-d(5) with those from vibrationally mediated photodissociation shows that initial vibrational excitation strongly influences the disposal of energy into relative translation. The measurements use velocity map ion imaging to detect the H-atom fragments and determine the distribution of recoil energies. Dissociation of phenol-d(5) molecules with an initially excited O-H stretching vibration produces significantly more fragments with low recoil energies than does one-photon dissociation at the same total energy. The difference appears to come from the increased probability of adiabatic dissociation in which a vibrationally excited molecule passes around the conical intersection between the dissociative state and the ground state to produce electronically excited phenoxyl-d(5) radicals. The additional energy deposited in electronic excitation of the radical reduces the energy available for relative translation.     

Vibrationally excited molecules and mechanisms of chemical and physical processes in non-equilibrium plasmas

The results of theoretical and experimental investigations of some physical and chemical processes caused by collisions between vibrationally excited molecules (such as molecular dissociation, molecular electronic excitation, ionization and ion conversion, gas heating) under non-equilibrium plasma conditions of low-pressure electrical discharges are presented. It is shown that the role of vibrationally excited molecules in these processes is very large for nitrogen molecules, in which the probability of V—T exchange is much smaller than in other molecules for low gas temperatures.     

Collisional Activation Mass Spectrometry—A New Probe for Determining the Structure of Ions in the Gas Phase

Organic ions with high translational energy colliding inelastically with neutral atoms or molecules become excited electronically at the expense of their translational energy. The excitation energy enables a wide range of dissociation reactions to occur and the intensity relationships yield information about the structure of the ions concerned and also permit conclusions to be drawn about the mechanism of their formation.     

Relaxation of highly vibrationally excited molecules

The rate constant for V-V relaxation of diatomic homonuclear molecules is determined from collisions of unexcited molecules with molecules near the dissociation threshold. It is shown that a quasi-resonant transition through several levels dominates in this process so that the energy difference between the initial and final states of the excited molecule is approximately equal to the transition energy from the zero level to the first one. The relaxation kinetics of excited molecules is studied. Absorption of IR radiation by polyatomic molecules is discussed taking into account collisions. A criterion for the negligibility of energy loss is obtained, and the dissociation rate of molecules under the action of IR laser irradiation found. The computational results are compared with experimental data. A self-consistent procedure is formulated for a gas irradiated by a quasi-continuously operating IR laser, in order to determine simultaneously the dissociation rate, dissipation energy flux and temperature. The existence of an optimum radiation region for dissociation is found.     

CHEMICAL MECHANISMS OF CHEMI- AND BIOLUMINESCENCE. REACTIONS OF HIGH ENERGY CONTENT ORGANIC COMPOUNDS

Abstract— The chemistry of high energy content molecules was investigated. In particular, the mechanisms by which these compounds rearrange to generate electronically excited states was probed. A mechanism for chemiexcitation involving chemically initiated electron transfer is described. It is concluded this mechanism is applicable to many chemi- and bioluminescent processes.     

Formation and Excitation of CN Molecules in He–CO–N<Subscript>2</Subscript>–O<Subscript>2</Subscript> Discharge Plasmas

The emission by electronically excited CN molecules is a prominent feature in the spectrum of active nitrogen containing traces of carbonaceous species. A large amount of experimental work has been devoted to an investigation of CN emission. However, up to now the plasma chemistry of CN radicals and processes leading to the excitation of CN electronic states are still poorly understood. The results of experimental measurements and numerical simulations are compared in order to establish the role of various channels of CN creation in the ground and excited states.     

Advanced chemistry of dioxetane-based chemiluminescent substrates originating from bioluminescence

A class of high-energy molecules, the 1,2-dioxetanes, have received a great deal of attention because of their unique ability to decompose thermally into electronically excited carbonyl products. Their chemistry originates from studies on molecular mechanisms of bioluminescence and has a history of over 30 years. However, the luminescent efficiency of the dioxetanes could hardly be compared with that for bioluminescence until a dioxetane bearing an easily oxidized aryl group was found to afford efficient chemiluminescence by the intramolecular chemically initiated electron exchange luminescence (CIEEL) mechanism. Nowadays, the CIEEL-type dioxetanes are being applied to modern biochemical and biomedical analyses. In this review, the advanced chemistry of the CIEEL-type dioxetanes as highly efficient chemiluminescent substrates is described, focusing on their molecular design and synthesis. Singlet chemiexcitation processes for the intramolecular CIEEL decay of dioxetanes and triplet chemiexcitation processes for the thermolysis of dioxetanes are also discussed.     

Fundamentals of tandem mass spectrometry: a dynamics study of simple C−C bond cleavage in collision-activated dissociation of polyatomic ions at low energy

The loss of methyl radical in collision-activated dissociation (CAD) of acetone and propane molecular ions has been studied at low energy using a tandem hybrid mass spectrometer. Although the two processes are very similar chemically and energetically, very different dynamical features are observed. Acetyl ions from acetone ion are predominantly backward-scattered, with intensity maxima lying inside and outside the elastic scattering circle, confirming our previous observation that electronically excited states are important in low-energy acetone CAD. Ethyl ions from propane ion show a forward-scattered peak maximum at a nonzero scattering angle, which is consistent with generally accepted models for vibrational excitation and redistribution of energy before dissociation. Both processes demonstrate that CAD at low energy proceeds via small-impact-parameter collisions with momentum transfer. Comparison of the present results with higher energy CAD dynamics studies and earlier work leads to some tentative general conclusions about energy transfer in these processes.     

Dynamics and prototropic reactivity of electronically excited states in simple surfactant aggregates

Excitation of a molecule from the ground state to an electronically excited state can cause changes in its geometry, dipole moment, acidity or basicity, redox potentials and solvation. Bimolecular quenching of the excited state of the probe by other molecules present in the medium can be used to determine the mobilities of molecules and estimate microviscosities and encounter probabilities in the medium. Differences in excited state acidity or basicity relative to the ground state can be employed to investigate the dynamics of ultrafast proton transfer reactions. Three areas of current interest where fluorescent probes have served to elucidate important dynamic processes of molecules in simple self-aggregating surfactant systems such as aqueous micelles and reverse micelles are considered: (a) bimolecular quenching of excited states; (b) the dynamics of solvation of excited states and (c) ultrafast intermolecular excited state proton transfer (ESPT) reactions.