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Protein unfolding is a ubiquitous process responsible for the loss of protein functionality (denaturation), which, in turn, can be accompanied by the death of cells and organisms. The nature of enthalpy–entropy compensation (EEC) in the kinetics of protein unfolding is a subject of debate. In order to investigate the nature of EEC, the “completely loose” transition state (TS) model has been applied to calculate the Arrhenius parameters for the unfolding of polyglycine dimers as a model process. The calculated Arrhenius parameters increase with increasing dimer length and demonstrate enthalpy–entropy compensation. It is shown that EEC results from the linear correlations of enthalpy and entropy of activation with dimer length, which are derived directly from the properties of the transition state. It is shown that EEC in solvated (hydrated, etc.) proteins is a direct consequence of EEC in proteins themselves. The suggested model allows us also to reproduce and explain “exotic” very high values of the pre-exponential factor measured for the proteins unfolding, which are drastically higher than those known for unimolecular reactions of organic molecules. A similar approach can be applied to analyzing the nature of EEC phenomena observed in other areas of chemistry.

The formation of singlet oxygen 1O2 provided by the photoexcitation of the encounter complexes of isoprene with oxygen (C5H8-O2) in the gas phase within the spectral region 253.5-355 nm has been observed at the elevated pressure of oxygen. Singlet oxygen has been detected with its NIR luminescence centered near 1.27 μm. The photogeneration of 1O2 is found to be a one-photon process. In the UV-C region (253-278 nm) the quantum yield of 1O2 is measured. This yield of 1O2 is governed mainly by photoexcitation of O2 molecules to the Herzberg III (3Δu) state via enhanced absorption by C5H8-O2 collision complexes. So excited triplet O2 gives rise to singlet oxygen because of triplet-triplet annihilation in the collisions with unexcited O2 molecules. In the UV-B (308 nm) region the appearance of 1O2 is attributed to the excitation of a double spin-flip (DSF) transition in complex C5H8-O2. In the UV-A region (355 nm) besides DSF the O2-assisted T1 ← S0 excitation of isoprene to the triplet state takes place, which is a sensitizer of 1O2 formation. The contribution of the encounter complexes C5H8-O2 to the production of singlet oxygen and to the lifetime of isoprene in the Earth's troposphere are estimated.

Photodissociation of the van der Waals complex Ar-I

In the Comment the authors Y. Nosaka and A. Y. Nosaka present arguments against the mechanism of singlet oxygen production which was suggested in the earlier published paper (Demyanenko et al., J. Phys. Chem. C 2019, 123, 2175) to explain the experimental data for the Near IR (NIR)-luminescence observed to appear after laser excitation at 355 nm of TiO2 suspension in air-saturated solvents CCl4 and H2O. The main arguments of the Comment are directed against Assignment of the NIR Emission and Electron Photodetachment Mechanism suggested by Demyanenko et al.. Particularly the Comment claims that the red shift (near 30 nm) and bandwidth (50-60 nm) of the NIR luminescence observed in paper by Demyanenko et al. are too large to be assigned to the luminescence of singlet oxygen 1O2. In the Reply it is shown that several arguments of the Comment are based on the incorrect statements. The current interpretation of the nature of the red shift and bandwidth of the luminescence of singlet oxygen governed by interact...

UV photodissociation of tungsten hexacarbonyl W(CO)6 has been studied in the molecular beam conditions using time-of-flight mass spectrometry and velocity map imaging. Irradiation of W(CO)6 by puls...

An automated apparatus is presented that allows a user to measure the mass spectrum or velocity map images of photofragments during tuning of the dye-laser radiation wavelength and its second harmonic, which interact with an investigated substance in a molecular beam. The NI LabVIEW environment was used for automation. The apparatus was successfully tested when studying the resonant ionization of oxygen molecules. The automation software makes it possible to extend the possibilities of measuring spectral dependences by creating additional modules without modifying the existing program code.

A broadband near-IR photodetector based on a large-area G12180-250A InGaAs photodiode (Hamamatsu) with a 5-mm-diameter photosensitive area was developed and tested. It is designed for detecting the IR luminescence of singlet oxygen. A circuit for signal amplification is assembled on the basis of broadband low-noise operational amplifiers with the subsequent filtering of a signal with an adjustable passband f varied from 15 to 600 kHz, thus allowing one to obtain a time resolution of better than 1 µs, a sensitivity level of 107–109 V/W, and a noise equivalent power NEP [W/Hz1/2] ≈ 1.4 × 10–14λ–1[µm] f [kHz] in a frequency band of 15–600 kHz.

Features of the near-infrared phosphorescence of singlet oxygen 1O2 photosensitized by TiO2 in solutions have been investigated. The short-lived phosphorescence of 1O2 with spectral maximum at approximately 1300 nm is observed to follow excitation by pulsed laser radiation at 355 nm of TiO2 suspended in CCl4 and water H2O. The shorter lifetime and spectral shift of this phosphorescence, as compared with phosphorescence of 1O2 sensitized by phenalenone in these solvents, are attributed to the adsorbed state of emitting singlet oxygen. The observed laser pulse energy dependence indicates the two-photon and three-photon nature of this phosphorescence initiation in CCl4 and H2O, respectively. The formation of 1O2 in CCl4 is assumed to be provided by the sequence of one-photon excitation of TiO2, giving rise to superoxide anion O2– and one-photon photodetachment of electron from O2–. The photodetachment of electron from O2– in water H2O is a two-photon process because of higher solvation energy of superoxide a...

The formation of singlet oxygen

Photodissociation of the van der Waals complex Ar–I2 after excitation into the Rydberg states of I2 has been investigated with velocity map imaging of photofragments. Formation of the translationally hot ions of argon Ar+ with three modes in kinetic energy distribution has been revealed. The measured dependence of the kinetic energy of Ar+ on the pumping photon energy indicates the appearance of Ar+ from three channels of the photodissociation of the linear intermediate Ar+–I–I− containing chemically bound argon. These channels are (1) dissociation into Ar++ I2−; (2) three-body dissociation into (Ar+)* + I* + I−, with (Ar+)* and I* being the 2P1/2 states of the species; and (3) two-body electron photodetachment, giving rise to Ar+ + I2 + e. Three indicated channels are similar to those established for the photodissociation of trihalide anions. This similarity confirms the conclusion on the formation of the Ar+–I–I− intermediate, which is isoelectronic to the trihalide anion Cl–I–I−. The mechanism of the Ar+–I–I− formation involves two-photon excitation of the complex Ar–I2 into the Rydberg state of I2 converted into the ion-pair state and further electron transfer from Ar to I+ of the ion-pair state. The self-assembling of the structure making the formation of the Ar+–I–I− intermediate energetically accessible is confirmed by modeling the dynamics in the excited linear complex Ar–I2. Photoexcitation of the van der Waals complexes of noble gases with halogens into the ion-pair states of halogen is supposed to be a promising approach for generating the new chemical compounds of noble gas atoms.

UV photodissociation of tungsten hexacarbonyl W(CO)

The UV-photodissociation at 266 nm of a widely used TiO2 precursor, titanium tetraisopropoxide (Ti(OiPr)4, TTIP), was studied under molecular-beam conditions. Using the MS-TOF technique, atomic titanium and titanium(II) oxide (TiO) were detected among the most abundant photofragments. Experimental results were rationalized with the aid of quantum chemical calculations (DLPNO–CCSD(T) and DFT). Contrary to the existing data in the literature, the new four-centered acetone-elimination reaction was found to be the primary decomposition process of TTIP. According to computational results, the effective activation barrier of this channel was ∼49 kcal/mol, which was ∼13 kcal/mol lower than that of the competing propylene elimination. The former process, followed by the dissociative loss of an H atom, was a dominating channel of TTIP unimolecular decay. The sequential loss of isopropoxy moieties via these two-step processes was supposed to produce the experimentally observed titanium atoms. In turn, the combinati...

The one-laser two-color resonance enhanced multiphoton ionization REMPI [(1 + 1′) + 1] and velocity map imaging have been applied to investigate formation of molecular oxygen in excited singlet O2( a 1 Δ g ) and ground O2( X 3 Σ g - ) states in the photodissociation of van der Waals complex isoprene-oxygen C5H8-O2. These molecules were found to appear in the different rotational states with translational energy varied from a value as low as ∼1 meV to a distribution with temperature of about 940 K. The observed traces of electron recoil in the images of photoions reveal participation of several ionization pathways of the resonantly excited intermediate states of O2.

Ab initio calculations have been carried out to investigate interaction of titanium dioxide TiO 2 with oxygen O 2 in ground triplet and excited singlet states. On a singlet potential energy surface (PES) formation of a stable compound of titanium peroxide TiO 4 is revealed which should appear in reaction of TiO 2 with singlet oxygen without activation barrier. This peroxide is lower in energy than the ground state of two individual molecules TiO 2 + 3O 2 by 34.6 kcal/mol. Location of conical intersection between triplet and singlet PESs of TiO 2 O 2 is also investigated.

The UV-photodissociation at 266 nm of a widely used TiO

The desorption of excited rubidium (Rb) atoms off the surface of helium (He) nanodroplets is studied in detail using femtosecond time-resolved photoion and photoelectron imaging spectroscopy in combination with quantum wave packet simulations. The good agreement of the measured time-dependent velocity distributions with the simulation when exciting the Rb dopant atoms into the 6p-state supports the pseudo-diatomic model (PDM) for the Rb-He droplet interaction, even on the level of quantum wave packet dynamics. Time-resolved photoelectron spectra reveal the partitioning of excitation energy into the dopant and the droplet degrees of freedom.

The photodissociation of van der Waals complexes of iodine X-I

Using femtosecond pump-probe ion imaging spectroscopy, we establish the key role of I(+) + I(-) ion-pair (IP) states in the predissociation dynamics of molecular iodine I2 excited to Rydberg states. Two-photon excitation of Rydberg states lying above the lowest IP state dissociation threshold (1st tier) is found to be followed by direct parallel transitions into IP states of the 1st tier asymptotically correlating to a pair of I ions in their lowest states I(+)((3)P2) + I(-)((1)S0), of the 2nd tier correlating to I(+)((3)P0) + I(-)((1)S0), and of the 3rd tier correlating to I(+)((1)D2) + I(-)((1)S0). Predissociation via the 1st tier proceeds presumably with a delay of 1.6-1.7 ps which is close to the vibrational period in the 3rd tier state (3rd tier-mediated process). The 2nd tier IP state is concluded to be the main precursor for predissociation via lower lying Rydberg states proceeding with a characteristic time of 7-8 ps and giving rise to Rydberg atoms I(5s(2)5p(4)6s(1)). The channel generating I((2)P3/2) + I((2)P1/2) atoms with total kinetic energy corresponding to one-photon excitation is found to proceed via a pump - dump mechanism with dramatic change of angular anisotropy of this channel as compared with earlier nanosecond experiments.

The strong enhancement of H-atom UV-photogeneration is revealed in van der Waals complexes of unsaturated hydrocarbons RH (RH ethylene C2H4, propylene C3H6, butadiene C4H6, butene–2 C4H8) with oxygen RH–O2 as compared with the ‘free’ RH molecules. The results obtained indicate enhancement of H atom yield to be due to one-quantum photoprocess 1RH–3O2 + hν → 3RH∗ + 3O2(1O2) providing triplet RH molecules with excitation level sufficient for dissociation.

The mechanism and spectral dependence of the quantum yield of singlet oxygen O(2)(a (1)Δ(g)) photogenerated by UV radiation in gaseous oxygen at elevated pressure (32-130 bar) have been experimentally investigated within the 238-285 nm spectral region overlapping the range of the Wulf bands in the absorption spectrum of oxygen. The dominant channel of singlet oxygen generation with measured quantum yield up to about 2 is attributed to the one-quantum absorption by the encounter complexes O(2)-O(2). This absorption gives rise to oxygen in the Herzberg III state O(2)(A' (3)Δ(u)), which is assumed to be responsible for singlet oxygen production in the relaxation process O(2)(A' (3)Δ(u), υ) + O(2)(X (3)Σ(g)(-)) → O(2)({a (1)Δ(g)}, {b (1)Σ(g)(+)}) + O(2)({a (1)Δ(g), υ = 0}, {b (1)Σ(g)(+), υ = 0}) with further collisional relaxation of b to a state. This mechanism is deduced from the analysis of the avoiding crossing locations on the potential energy surface of colliding O(2)-O(2) pair. The observed drop of the O(2)(a (1)Δ(g)) yield near spectral threshold for O(2) dissociation is explained by the competition between above relaxation and reaction giving rise to O(3) + O (O + O + O(2)) supposed in literature. The quantum yield of O(2)(a (1)Δ(g)) formation from encounter complex N(2)-O(2) measured at λ = 266 nm was found to be the same as that for O(2)-O(2).

The channel of singlet oxygen O2((1)Δg) photogeneration from van der Waals complexes of oxygen X-O2 has been investigated to discriminate between two possible mechanisms based on charge-transfer (CT) or double spin-flip (DSF) transitions. The results obtained in this work for complexes with X = ethylene C2H4, 1,3-butadiene C4H6, deuterated methyl iodide CD3I, benzene C6H6 and water H2O and for those investigated previously indicate the DSF mechanism as a source of singlet oxygen. The formation of O2((1)Δg) is observed only when the energy of exciting quantum is sufficient for DSF transition. Universally detected low vibrational excitation of O2((1)Δg) arising in the photodissociation of van der Waals complexes X-O2 indicates the DSF mechanism as its source. For complex of ethylene C2H4-O2ab initio calculations of vertical energy ΔE(vert) for DSF and CT transitions have been carried out. The positive results of singlet oxygen formation from C2H4-O2 can be explained by the DSF but not by the CT mechanism.

The kinetic parameters for the “inversion substitution” reactions of Br-containing fire suppressants CH3Br and CF3Br with molecular oxygen CX3Br + O2 → CX3O2 + Br (X = H, F) have been obtained on the basis of computational study carried out by quantum chemistry methods and transition state theory (TST). The DFT/B3LYP approach has been used to obtain molecular structures and vibrational frequencies and CCSD(T) method with additive corrections at MP2 and SODFT levels have been applied in order to calculate reaction enthalpies and activation barriers. Activation barrier for the substitution reaction CH3Br + O2 → CH3O2 + Br was found to be (53.3 kcal/mol) close to the endoergicity (51.2 kcal/mol) of bimolecular abstraction CH3Br + O2 → CH2Br + HO2 which means that this “inversion substitution” should be considered as a competing process in the oxidation initiation of bromine containing substances. The results of these calculations together with the earlier results have allowed us to build the correlation of Evans–Polanyi type between the activation barrier and reaction enthalpy for “inversion substitution” reactions CH3X + O2 → CH3O2 + X. For bromotrifluoromethane CF3Br (Halon-1301) the barrier of “inversion substitution” was found to be of 86 kcal/mol that is higher by about 15 kcal/mol as compared with unimolecular decay CF3Br → CF3 + Br barrier. This makes negligible the role of bimolecular substitution reaction for kinetics of CF3Br in oxygen environment. The TST calculations of the rate constants and their temperature dependence for the direct and reverse “inversion substitution” reactions have been carried out within the temperature interval of 273–2000 K.

The photogeneration of singlet oxygen O 2 ( a 1 Δ g ) resulted from the photoexcitation at 248 nm of gaseous oxygen at elevated pressure (3–80 atm) has been investigated. Singlet oxygen monitored by its IR-luminescence at 1.27 μm was found to appear via two processes which are identified to be due to absorption of individual O 2 molecules and encounter complexes O 2 –O 2 , respectively. The one-quantum absorption of complexes gives rise to appearance of oxygen in Herzberg III state O 2 ( 3 Δ u ) which is supposed to be a triplet sensitizer responsible for singlet oxygen O 2 ( a 1 Δ g ) production observed in our experiments.

The predissociation dynamics of lithium iodide (LiI) in the first excited A-state is investigated for molecules in the gas phase and embedded in helium nanodroplets, using femtosecond pump-probe photoionization spectroscopy. In the gas phase, the transient Li(+) and LiI(+) ion signals feature damped oscillations due to the excitation and decay of a vibrational wave packet. Based on high-level ab initio calculations of the electronic structure of LiI and simulations of the wave packet dynamics, the exponential signal decay is found to result from predissociation predominantly at the lowest avoided X-A potential curve crossing, for which we infer a coupling constant VXA = 650(20) cm(-1). The lack of a pump-probe delay dependence for the case of LiI embedded in helium nanodroplets indicates fast droplet-induced relaxation of the vibrational excitation.

The channel of singlet oxygen O2(1Δg) photogeneration from van der Waals complexes of oxygen X–O2 has been investigated to discriminate between two possible mechanisms based on charge-transfer (CT) or double spin-flip (DSF) transitions. The results obtained in this work for complexes with X = ethylene C2H4, 1,3-butadiene C4H6, deuterated methyl iodide CD3I, benzene C6H6 and water H2O and for those investigated previously indicate the DSF mechanism as a source of singlet oxygen. The formation of O2(1Δg) is observed only when the energy of exciting quantum is sufficient for DSF transition. Universally detected low vibrational excitation of O2(1Δg) arising in the photodissociation of van der Waals complexes X–O2 indicates the DSF mechanism as its source. For complex of ethylene C2H4–O2ab initio calculations of vertical energy ΔEvert for DSF and CT transitions have been carried out. The positive results of singlet oxygen formation from C2H4–O2 can be explained by the DSF but not by the CT mechanism.

Results of 7 nights of CCD VR photometry of the intermediate polar 1RXS J062518.2+733433 obtained at the Korean 1.8m telescope are reported. The corrected ephemeris for the orbital minimum is BJD (Orb.min)

Probabilistic methods allow to evaluate nuclear plant safety level systemically during plant lifetime and to develop recommendations on foreground directions in improving technical measures and operational procedures. Probabilistic assessment of plant safety level requires developing and introduction of appropriate methods and software. During last years JSC “Afrikantov OKBM” has developed different methods and software allowing to perform probabilistic study of nuclear plants safety and has implemented substantial amount of safety analysis and safety monitoring studies for different purposed small modular reactors. The paper describes methods and software developed by JSC “Afrikantov OKBM” and some results of its utilizing for small modular reactors.

Velocity map imaging of the photofragments arising from two-photon photoexcitation of molecular iodine in the energy range 73 500-74 500 cm(-1) covering the bands of high-lying gerade Rydberg states [(2)Π1/2]c6d;0g (+) and [(2)Π1/2]c6d;2g has been applied. The ion signal was dominated by the atomic fragment ion I(+). Up to 5 dissociation channels yielding I(+) ions with different kinetic energies were observed when the I2 molecule was excited within discrete peaks of Rydberg states and their satellites in this region. One of these channels gives rise to images of I(+) and I(-) ions with equal kinetic energy indicating predissociation of I2 via ion-pair states. The contribution of this channel was up to about 50% of the total I(+) signal. The four other channels correspond to predissociation via lower lying Rydberg states giving rise to excited iodine atoms providing I(+) ions by subsequent one-photon ionization by the same laser pulse. The ratio of these channels varied from peak to peak in the spectrum but their total ionic signal was always much higher than the signal of (2 + 1) resonance enhanced multi-photon ionization of I2, which was previously considered to be the origin of ionic signal in this spectral range. The first-tier E0g (+) and D(')2g ion-pair states are concluded to be responsible for predissociation of Rydberg states [(2)Π1/2]c6d;0g (+) and [(2)Π1/2]c6d;2g, respectively. Further predissociation of these ion-pair states via lower lying Rydberg states gives rise to excited I(5s(2)5p(4)6s(1)) atoms responsible for major part of ion signal. The isotropic angular distribution of the photofragment recoil directions observed for all channels indicates that the studied Rydberg states are long-lived compared with the rotational period of the I2 molecule.

Ab initio calculations at the MP2 level have been carried out for two most stable neutral isomers of van der Waals dimer (CH 3 I) 2 and for their ionized forms. It was found within MP4(SDTQ)//MP2 approach that neutral head-to-tail HT isomer (dimerization energy −778 cm −1 ) is more stable than head-to-head HH form (dimerization energy −699 cm −1 ). Comparison of calculated ionization potentials and earlier experimentally observed appearance potential of dimer ion allowed us to conclude that ionization of the more stable neutral HT form is followed by conversion into the ionic HH + form which is more stable than HT + by 0.97 eV.

The temperature dependence of the rate constant of the inversion substitution reactions CH3X + O2 → CH3O2• + X• (X = SH, NO2), can be expressed as k = 6.8 × 10–12(T/1000)1.49exp(–62816 cal mol–1/RT) cm3 s–1 (X = SH) and k = 6.8 × 10–12(T/1000)1.26 × × exp(–61319 cal mol–1/RT) cm3 s–1 (X = NO2), as found with the use of high-level quantum chemical methods and the transition state theory.

The ns photodissociation of perfluoroethyliodide C2F5I at 266 nm has been studied by using the resonant two-photon ionization (R2PI) technique. Recoil anisotropy parameters as well as average translational energy of the I atoms in the fine structure states P-2(1/2) and P-2(3/2) have been determined. The main contribution (99%) to the absorption at 266 nm was found to be caused by a parallel transition to the (3)Q(0) state which gives mainly excited-state atoms I(P-2(1/2)). The ground-state atoms I(P-2(3/2)) were found to appear mainly (88%) from the primarily excited (3)Q(0) state via curve-crossing (3)Q(0)-(1)Q(1) and to a lesser extent (12%) from direct absorption by a perpendicular transition to the (1)Q(1) and (3)Q(1) states. The fs pump-dump technique in combination with ns R2PI probing of the fragments I(P-2(1/2)) and I(P-2(3/2)) and time-of-flight mass spectrometry have been applied to probe the early stage dynamics of the C2F5I molecule on the excited state (3)Q(0) potential energy surface (PES). The evolution time of the excited molecule to the point where the energy gap between the excited state (3)Q(0) and the ground-state potential energy surfaces drops to a value of about 12 440 cm-1 was found to be 52 +/- 13 fs. This time corresponds to about 0.8 Angstrom extension of the C-I bond distance. The molecular dynamics simulation with DFT calculated ground-state PES and (3)Q(0) PES with the shape calculated for methyl iodide found in the literature gives reasonable agreement with the experimental result for the evolution time. (C) 2001 American Institute of Physics. (Less)

The rate constant of self-reaction of silyl radicals, SiH3 + SiH3 → products (1), was measured at 300 K over an extended buffer gas pressure range (1−100 bar, He) using excimer laser pulsed photoly...

Oxalyl chloride, (COCl)2, has been characterized as a photolytic source of chlorine atoms for kinetic studies. UV absorption spectrum of oxalyl chloride, as well as the photodissociation yield of chlorine atoms resulting from the photodissociation of oxalyl chloride at 193 nm (ArF excimer laser) in helium as a bath gas at pHe = 1, 10, and 100 bar, were measured using laser pulsed photolysisUV−vis transient absorption spectroscopy. The photodissociation yield of chlorine atoms was determined from the measurements of photodepletion of oxalyl chloride via its absorption at 210.0 nm as well as via 270−390 nm UV absorption build-up caused by molecular chlorine formed in the recombination of chlorine atoms. The photodissosiation yield of chlorine atoms (the number of Cl atoms appeared per one dissociated oxalyl chloride molecule) is independent of helium pressure over the pressure range 1−100 bar. The photodissociation yields of chlorine atoms at 298 K were measured as 2.01 ± 0.08 and 1.97 ± 0.12 at pHe = 10 ba...

Resonant two-photon ionization R2PI has been applied to the measurements of the quantum yields ϕ of iodine atoms in the fine structure states 2 P 1/2 and 2 P 3/2 arising in the UV-photodissociation process ( λ =266 and 355 nm) of the perfluoroalkyliodides C 2 F 5 I, n -C 3 F 7 I and i -C 3 F 7 I as well as of methyl iodide CH 3 I ( λ =266 nm). The measured values of the quantum yields of the excited state ( 2 P 1/2 ) of the I atoms are found to be ϕ 266 ∗ =0.921±0.010 and ϕ 355 ∗ =0.178±0.025 for C 2 F 5 I, ϕ 266 ∗ =0.919±0.011 and ϕ 355 ∗ =0.296±0.029 for n -C 3 F 7 I, ϕ 266 ∗ =0.900±0.015 and ϕ 355 ∗ =0.396±0.034 for i -C 3 F 7 I, and ϕ 266 ∗ =0.71±0.03 for CH 3 I. The results obtained show that the quantum yield ϕ 266 ∗ of the excited state atoms I( 2 P 1/2 ) in the photodissociation process of the perfluoroalkyliodides differs noticeably from unity which has previously been assumed or measured for these molecules.

The direct time-resolved measurements of the energy dependent rate constant k(E*) have been carried out for dissociation of allyl iodide (AI) vibrationally excited via C–H overtones (v=6). Resonant two-photon ionization (R2PI) technique has been used for the detection of atomic iodine I(2P3/2) arising from the dissociation of photoexcited AI molecules. For R2PI detection a method with narrow-band vacuum ultraviolet radiation (VUV) was used. VUV radiation was generated by means of nonresonant frequency tripling of visible dye-laser radiation in gaseous xenon. Measured k(E*) values were found to be in excellent agreement with those calculated within the microcanonical version of the statistical Rice–Ramsperger–Kassel–Marcus theory in its “phase space” or “loose” transition state limit. The canonical version of the same model is also in good agreement with experimental data from the literature on the dissociation of allyl iodide under thermal heating conditions where collisional excitation takes place.

The speed and angular distribution of O atoms arising from the photofragmentation of C(5)H(8)-O(2), the isoprene-oxygen van der Waals complex, in the wavelength region of 213-277 nm has been studied with the use of a two-color dissociation-probe method and the velocity map imaging technique. Dramatic enhancement in the O atoms photo-generation cross section in comparison with the photodissociation of individual O(2) molecules has been observed. Velocity map images of these "enhanced" O atoms consisted of five channels, different in their kinetic energy, angular distribution, and wavelength dependence. Three channels are deduced to be due to the one-quantum excitation of the C(5)H(8)-O(2) complex into the perturbed Herzberg III state ((3)Δ(u)) of O(2). This excitation results in the prompt dissociation of the complex giving rise to products C(5)H(8)+O+O when the energy of exciting quantum is higher than the complex photodissociation threshold, which is found to be 41740 ± 200 cm(-1) (239.6±1.2 nm). This last threshold corresponds to the photodissociation giving rise to an unexcited isoprene molecule. The second channel, with threshold shifted to the blue by 1480 ± 280 cm(-1), corresponds to dissociation with formation of rovibrationally excited isoprene. A third channel was observed at wavelengths up to 243 nm with excitation below the upper photodissociation threshold. This channel is attributed to dissociation with the formation of a bound O atom C(5)H(8)-O(2) + hv → C(5)H(8)-O(2)((3)Δ(u)) → C(5)H(8)O + O and/or to dissociation of O(2) with borrowing of the lacking energy from incompletely cooled complex internal degrees of freedom C(5)H(8)*-O(2) + hv → C(5)H(8)*-O(2)((3)Δ(u)) → C(5)H(8) + O + O. The kinetic energy of the O atoms arising in two other observed channels corresponds to O atoms produced by photodissociation of molecular oxygen in the excited a (1)Δ(g) and b (1)Σ(g)(+) singlet states as the precursors. This indicates the formation of singlet oxygen O(2)(a (1)Δ(g)) and O(2)(b (1)Σ(g)(+)) after excitation of the C(5)H(8)-O(2) complex. Cooperative excitation of the complex with a simultaneous change of the spin of both partners (1)X-(3)O(2) + hν → (3)X-(1)O(2) → (3)X + (1)O(2) is suggested as a source of singlet oxygen O(2)(a (1)Δ(g)) and O(2)(b (1)Σ(g)(+)). This cooperative excitation is in agreement with little or no vibrational excitation of O(2)(a (1)Δ(g)), produced from the C(5)H(8)-O(2) complex as studied in the current paper as well as from the C(3)H(6)-O(2) and CH(3)I-O(2) complexes reported in our previous paper [Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. The formation of O(2)(a (1)Δ(g)) from C(5)H(8)-O(2) was observed at λ(pump) = 213-277 nm with the yield going down towards the long wavelength edge of this interval. This spectral profile is interpreted as the red-side wing of the band of a cooperative transition (1)X-(3)O(2) + hν → (3)X(T(2))-(1)O(2)(a (1)Δ(g)) in the C(5)H(8)-O(2) complex.

Van der Waals binding energies for the X-O(2) complexes (X=Xe, CH(3)I, C(3)H(6), C(6)H(12)) are determined by analysis of experimental velocity map imaging data for O((3)P(2)) atoms arising from UV-photodissociation of the complex [A. V. Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. Several dissociation pathways have been observed, we focus on the channel corresponding to prompt dissociation of X-O(2) into X+2O((3)P) fragments, which is present for complexes of O(2) with all partners X. Our method is based on analysis of the kinetic energy of all three photofragments, where the O atom kinetic energy was directly measured in the experiment and the kinetic energy of the X partner was calculated using momentum conservation, along with the measured angular anisotropy for O atom recoil. We exploit the fact that the clusters are all T-shaped or nearly T-shaped, which we also confirm by ab initio calculations, along with knowledge of the transition dipole governing radiative absorption by the complex. The effect of partitioning the kinetic energy between translation along the X-O(2) and O-O coordinates on the angular anisotropy of the O atom recoil direction is discussed. Van der Waals binding energies of 110±20 cm(-1), 280±20 cm(-1), 135±30 cm(-1), and 585±20 cm(-1) are determined for Xe-O(2), CH(3)I-O(2), C(3)H(6)-O(2), and C(6)H(12)-O(2) clusters, respectively.

Photodissociation of the (HI)(2) van der Waals dimers at 248 nm and nearby wavelengths has been studied using time-of-flight mass spectrometry and velocity map imaging. I(2)(+) product ions with a translational temperature of 130 K and "translationally hot" I(+) ions with an average kinetic energy of E(t) = 1.24 +/- 0.03 eV and angular anisotropy beta = 1.92 +/- 0.11 were detected as dimer-specific ionic photofragments. Velocity map images of the I(2)(+) and I(+) species were found to be qualitatively similar to those observed in the case of photoexcitation of the (CH(3)I)(2) dimer (J. Chem. Phys. 2005, 122, 204301). As in the case of the (CH(3)I)(2) dimer, the absence of neutral I(2)-specific features in the ionic species images from (HI)(2) allows us to eliminate neutral molecular I(2) as a precursor of I(+) and I(2)(+). Similar to the case of (CH(3)I)(2), we deduce that the observed I(2)(+) ions are produced in their (2)Pi(3/2,g) ground electronic state as a result of photodissociation of the ionized dimer (HI)(2)(+) + h nu --> I(2)(+) + .... The formation of "translationally hot" I(+) ions is attributed to photodissociation of nascent vibrationally excited I(2)(+) with an average vibrational energy of 1.05 +/- 0.10 eV. This vibrational excitation is explained by the nonequilibrium initial I-I distance in I(2)(+) arising in photodissociation of (HI)(2)(+) after prompt release of the light H atoms. On the basis of our ab initio calculated value for the I-I distance of (3.17 A) in the (HI)(2)(+) precursor dimer, the vibrational excitation of I(2)(+) is expected to be 1.02 eV, which is in quantitative agreement with our experimentally deduced value. The interpretation of our results was supported by ab initio calculations of the structure and energy of neutral and ionized dimers of HI at the MP4(SDTQ)//MP2 level.

A new class of reactions of molecular oxygen O2 + ZH3I → O2ZH3 + I (Z = C, Si) proceeding by the mechanism of “inversion substitution” was investigated by quantum chemistry methods and the transition state theory (TST). The profiles of the potential energy surfaces (PES) along the reaction coordinate and the characteristics of transition states were calculated using the DFT approach with the B3LYP hybrid functional and the DZVP basis set. The characteristics of the transition states were then used for TST calculations of the rate constants for the direct and reverse “inversion substitution” reactions and their temperature dependences in the temperature interval 273–2000 K. The activation barriers to the substitution reactions under study were found to be substantially lower than the barriers to the abstraction reactions O2 + ZH3I → ZH2I + HO2 (by 16.3 kcal mol−1 for Z = C and by 7.2 kcal mol−1 for Z = Si). The results obtained show that the “inversion substitution” reactions dominate over the abstraction reactions in the interaction of molecular oxygen with carbon- and silicon-centered iodides as well as (probably) many other substrates.

The formation of neutral I2 by the photodissociation of the methyl iodide dimer, (CH3I)2, excited within the A band at 249.5 nm is evaluated using velocity map imaging. In previous work [J. Chem. Phys. 122, 204301 (2005)], we showed that the formation of I2+ from photodissociation of the methyl iodide dimer takes place via ionic channels (through the formation of (CH3I)2+). It is thus not possible to detect neutral I2 by monitoring I2+. Neutral I2 is detected in this study by monitoring I atoms arising from the photodissociation of I2. Iodine atoms from I2 photodissociation have a characteristic kinetic energy and angular anisotropy, which is registered using velocity map imaging. We use a two-color probe scheme involving the photodissociation of nascent I2 at 499 nm, which gives rise to I atoms that are ionized by (2+1) resonance enhanced multiphoton ionization at 304.67 nm. Our estimate of the yield of nascent I2 is based on the comparison with the signal from I2 at a known concentration. Using molecular beams with a small fraction of CH3I (1% in the expanded mixture) where smaller clusters should prevail, the production of I2 was found to be negligible. An upper estimate for the quantum yield of I2 from (CH3I)2 dimers was found to be less than 0.4%. Experiments with a higher fraction of CH3I (4% in the expanded mixture), which favor the formation of larger clusters, revealed an observable formation of I2, with an estimated translational temperature of about 820 K. We suggest that this observed I2 signal arises from the photodissociation of several CH3I molecules in the larger cluster by the same UV pulse, followed by recombination of two nascent iodine atoms is responsible for neutral I2 production.

The CH(3)I A-state-assisted photofragmentation of the (CH(3)I)(2) van der Waals dimer at 248 nm and nearby wavelengths has been revisited experimentally using the time-of-flight mass spectrometry with supersonic and effusive molecular beams and the "velocity map imaging" technique. The processes underlying the appearance of two main (CH(3)I)(2) cluster-specific features in the mass spectra, namely, I(2)(+) and translationally "hot" I(+) ions, have been studied. Translationally hot I(+) ions with an average kinetic energy of 0.94+/-0.02 eV appear in the one-quantum photodissociation of vibrationally excited I(2)(+)((2)Pi(32,g)) ions (E(vib)=0.45+/-0.11 eV) via a "parallel" photodissociation process with an anisotropy parameter beta=1.55+/-0.03. Comparison of the images of I(+) arising from the photoexcitation of CH(3)I clusters versus those from neutral I(2) shows that "concerted" photodissociation of the ionized (CH(3)I)(2)(+) dimer appears to be the most likely mechanism for the formation of molecular iodine ion I(2)(+), instead of photoionization of neutral molecular iodine.

The photodissociation of (56)FeO was studied by means of the velocity map imaging technique. A molecular beam of iron atoms and iron monoxide molecules was created using an electrical discharge with an iron electrode in a supersonic expansion of molecular oxygen. The ground state iron atom Fe((5)D(4)) and FeO concentrations in the molecular beam have been estimated. The dissociation energy of the FeO X (5)Delta ground electronic state was found to be D(0) (0)(FeO)=4.18+/-0.01 eV. The effective absorption cross section of FeO at 252.39 nm (vac), leading to the Fe((5)D(4))+O((3)P) dissociation channel, is approximately 1.2 x 10(-18) cm(2). A (1+1) resonantly enhanced multiphoton ionization spectrum of (56)FeO in the region 39 550-39 580 cm(-1) with rotational structure has been observed, but not assigned. Angular distributions of Fe((5)D(4)) and Fe((5)D(3)) products for the channel FeO--Fe((5)D(4,3))+O((3)P) have been measured at several points in the 210-260 nm laser light wavelength region. The anisotropy parameter varies strongly with wavelength for both channels.

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Time-resolved dynamics of the photodissociation of molecular oxygen, O(2), via the (3)Sigma(u) (-) ion-pair state have been studied with femtosecond time resolution using a pump-probe scheme in combination with velocity map imaging of the resulting O(+) and O(-) ions. The fourth harmonic of a femtosecond titanium-sapphire (Ti:sapphire) laser (lambda approximately 205 nm) was found to cause three-photon pumping of O(2) to a level at 18.1 eV. The parallel character of the observed O(+) and O(-) images allowed us to conclude that dissociation takes place on the (3)Sigma(u) (-) ion-pair state. The 815 nm fundamental of the Ti:sapphire laser used as probe was found to cause two-photon electron photodetachment starting from the O(2) ion-pair state, giving rise to (O((3)P)+O(+)((4)S)) products. This was revealed by the observed depletion of the yield of the O(-) anion and the appearance of a new O(+) cation signal with a kinetic energy E(transl)(O(+)) dependent on the time delay between the pump and probe lasers. This time-delay dependence of the dissociation dynamics on the ion-pair state has also been simulated, and the experimental and simulated results coincide very well over the experimental delay-time interval from about 130 fs to 20 ps where two- or one-photon photodetachment takes place, corresponding to a change in the R(O(+),O(-)) interatomic distance from 12 to about 900 A. This is one of the first implementations of a depletion scheme in femtosecond pump-probe experiments which could prove to be quite versatile and applicable to many femtosecond time-scale experiments.

The effect of a local environment on the photodissociation of molecular oxygen is investigated in the van der Waals complex X-O(2) (X=CH(3)I, C(3)H(6), C(6)H(12), and Xe). A single laser operating at wavelengths around 226 nm is used for both photodissociation of the van der Waals complex and simultaneous detection of the O((3)P(J),J=2,1,0) atom photoproduct via (2+1) resonance enhanced multiphoton ionization. The kinetic energy distribution (KED) and angular anisotropy of the product O atom recoil in this dissociation are measured using the velocity map imaging technique configured for either full ("crush") or partial ("slice") detection of the three-dimensional O((3)P(J)) atom product Newton sphere. The measured KED and angular anisotropy reveal a distinct difference in the mechanism of O atom generation from an X-O(2) complex compared to a free O(2) molecule. The authors identify two one-photon excitation pathways, the relative importance of which depends on IPx, the ionization potential of the X partner. One pathway, observed for all complexes independent of IPx, involves a direct transition to the perturbed covalent state X-O(2)(A'(3)Delta(u)) with excitation localized on the O(2) subunit. The predominantly perpendicular character of this channel relative to the laser polarization detection, together with data on the structure of the complex, allows us to confirm that X partner induced admixing of an X(+)-O(2) (-) charge transfer (CT) state is the perturbing factor resulting in the well-known enhancement of photoabsorption within the Herzberg continuum of molecular oxygen. The second excitation pathway, observed for X-O(2) complexes with X=CH(3)I and C(3)H(6), involves direct excitation into the (3)(X(+)-O(2) (-)) CT state of the complex. The subsequent photodissociation of this CT state by the same laser pulse gives rise to the superoxide anion O(2) (-), which then photodissociates, providing fast (0.69 eV) O atoms with a parallel image pattern. Products from the photodissociation of singlet oxygen O(2)(b (1)Sigma(g) (+)) are also observed when the CH(3)I-O(2) complex was irradiated. Potential energy surfaces (PES) for the ground and relevant excited states of the X-O(2) complex have been constructed for CH(3)I-O(2) using the results of CASSCF calculations for the ground and CT states of the complex as well as literature data on PES of the subunits. These model potential energy surfaces allowed us to interpret all of the observed O((3)P(J)) atom production channels.