Self-assembling of the neutral intermediate with chemically bound argon in photoexcited van der Waals complex Ar-I2.

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.

Singlet Oxygen Generation via UV-A, -B, and -C Photoexcitation of Isoprene-Oxygen (C5H8-O2) Encounter Complexes in the Gas Phase.

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.

Tungsten Isotope-Specific UV-Photodecomposition of W(CO)6 at 266 nm.

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 pulsed laser radiation at 266 nm results in the appearance of singly and doubly charged tungsten ions. The isotope composition of these ions deviates essentially from natural abundance with deviation being pulse energy-dependent. The velocity map images of the tungsten ions indicate proceeding of several, more than two, parallel channels (sequences of the one-photon processes) of photodissociation, giving rise to tungsten atoms. Isotope effect is assigned to appear in a one-photon bound-bound transition in W(CO) intermediate followed by its predissociation. In the model suggested, the final state of this transition is a vibronic state with excited vibrational mode of W-C stretching vibration. This vibrational excitation is responsible for isotopic shift in the location of the final state. The suggested model fits the observed isotopic composition quantitatively.

Decomposition Pathways of Titanium Isopropoxide Ti(OiPr)4: New Insights from UV-Photodissociation Experiments and Quantum Chemical Calculations.

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 combination of these reactions with propylene elimination can lead to another detected species, TiO. These results indicate that the existing mechanisms of TTIP thermal and photoinitiated decomposition in the chemical-vapor deposition (CVD) of titanium dioxide should be reconsidered.

Photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) via charge-transfer state: A velocity map imaging investigation.

The photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) excited via Charge-Transfer (CT) band has been studied with the velocity map imaging technique. Photodissociation of both complexes gives rise to translationally "hot" molecular iodine I2 via channels differing by kinetic energy and angular distribution of the recoil directions. These measured characteristics together with the analysis of the model potential energy surface for these complexes allow us to infer the back-electron-transfer (BET) in the CT state to be a source of observed photodissociation channels and to make conclusions on the location of conical intersections where the BET process takes place. The BET process is concluded to provide an I2 molecule in the electronic ground state with moderate vibrational excitation as well as X molecule in the electronic excited state. In the case of X = I2, the BET process converts anion I2- of the CT state into the neutral I2 in the repulsive excited electronic state which then dissociates promptly giving rise to a pair of I atoms in the fine states 2P1/2. In the case of C2H4-I2, the C2H4 molecules appear in the triplet T1 electronic state. Conical intersection for corresponding BET process becomes energetically accessible after partial twisting of C2H4+ frame in the excited CT state of complex. The C2H4(T)-I2 complex gives rise to triplet ethylene as well as singlet ethylene via the T-S conversion.

Singlet oxygen photogeneration from X-O2 van der Waals complexes: double spin-flip vs. charge-transfer mechanism.

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.

Predissociation of high-lying Rydberg states of molecular iodine via ion-pair states.

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.

Photodissociation of (SO2⋯XH) Van der Waals complexes and clusters (XH = C2H2, C2H4, C2H6) excited at 32,040-32,090 cm(-1) with formation of HSO2 and X.

We studied photodecomposition dynamics of (SO2⋯XH) Van der Waals' (VdW) complexes and clusters in gas phase, with X = C2H, C2H3, and C2H5. SO2 was excited by frequency-doubled radiation of a tunable dye laser and resonance-enhanced multiphoton ionization was used to detect the C2H (m/z 25), C2H3 (m/z 27), and C2H5 (m/z 29) ions by time-of-flight mass spectroscopy. Spectra obtained at higher nozzle pressures (P0 > 2.5 atm) indicate formation of clusters. Detailed studies of the VdW complex structure were carried out by analyzing the rotational structure of the respective action spectra. We also performed ab initio theoretical analysis of structures of the VdW complexes and transitional states leading to photodecomposition. We find that the structure of the transition state is significantly different as compared to the equilibrium ground-state structure of the respective complex. The photodecomposition mechanism depends on the hydrocarbon molecule bound to SO2.

Quantum yield and mechanism of singlet oxygen generation via UV photoexcitation of O2-O2 and N2-O2 encounter complexes.

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).

Photochemical reaction dynamics in SO(2)-acetylene complexes.

The dynamics of photoinduced reactions between electronically excited SO(2) molecule (A (1)A(2)<--X (1)A(1)) and acetylene molecule (X (1)Sigma(g) (+)) in the SO(2)-acetylene van der Waals (vdW) complexes (clusters) was studied. The SO(2) molecule was excited by frequency-doubled radiation of a tunable dye laser, and resonance enhancement multiphoton photoionization of the produced photofragments was induced by ArF (193 nm) laser radiation or by frequency-doubled radiation of a second tunable dye laser to observe the C(2)H radical. The HOSO radical was detected by its IR emission. We found that the main photodecomposition channel of the vdW complexes (clusters) involves the SO(2) ( *)+C(2)H(2)-->HOSO+C(2)H reaction. Indeed, the analysis of the action spectra of the excitation laser radiation showed that the photofragments emerging in our experimental conditions (SO(2), 5%; C(2)H(2), 5%; and Xe; P(0)=2 atm) originate from the SO(2)...C(2)H(2) vdW complex (cluster). We analyzed the structure of this vdW complex theoretically, obtaining C(s) symmetry, with the acetylene molecule located above the OSO plane. The resonance-enhanced multiphoton photoionization action spectra of the C(2)H (A<--X) photofragmentation and the IR emission spectra of the HOSO radical allowed the authors to probe the energy distribution between the photofragments formed. The reaction that involves transition of the acetylene H atom to the SO(2) oxygen should be the primary step of the process considered, followed by nonstatistic dissociation of the vdW complex (cluster), with the C(2)H radical formed in its vibrationless state and excited both rotationally and translationally, and the HOSO radical excited vibrationally, rotationally and translationally. The proposed reaction mechanism was discussed, employing transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) approaches. The kinetics of photofragment formation was investigated, yielding characteristic radical build-up time of 0.64 micros.

Ionic pathways following UV photoexcitation of the (HI)2 van der Waals dimer.

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.

State dynamics of acetylene excited to individual rotational level of the V1(2)K1(0,1,2) subbands.

The dynamics of the IR emission induced by excitation of the acetylene molecule at the 3(2) Ka2, A1Au<--4(1) la1, X1Sigmag+ transition was investigated. Vibrationally resolved IR emission spectra were recorded at different delay times after the laser excitation pulse. The observed IR emission was assigned to transitions between vibrational levels of the acetylene molecule in the ground state. Values of the relaxation parameters of different vibrational levels of the ground state were obtained. The Ti-->Tj transition was detected by cavity ring-down spectroscopy in the 455 nm spectral range after excitation of the acetylene molecule at the same transition. Rotationally resolved spectra of the respective transition were obtained and analyzed at different delay times after the laser excitation pulse. The dynamics of the S1-->Tx-->T1-->S0 transitions was investigated, and the relaxation parameter values were estimated for the T1 state.

Excited-state dynamics of acetylene excited to individual rotational level of the V04K01 subband.

Dynamics of the IR emission induced by excitation of the acetylene molecule using the (3(2)K(a) (0,1,2),A (1)A(u)<--4(1)l(a) (1),X (1)Sigma(g) (+)) transition was investigated. The observed IR emission was assigned to transitions between the ground-state vibrational levels. Acetylene fluorescence quenching induced by external electric and magnetic fields acting upon the system prepared using the (3(4)K(a) (1),A (1)A(u)<--0(0)l(a) (0),X (1)Sigma(g) (+)) excitation was also studied. External electric field creates an additional radiationless pathway to the ground-state levels, coupling levels of the A (1)A(u) excited state to the quasiresonant levels of the X (1)Sigma(g) (+) ground state. The level density of the ground state in the vicinity of the excited state is very high, thus the electric-field-induced transition is irreversible, with the rate constant described by the Fermi rule. Magnetic field alters the decay profile without changing the fluorescence quantum yield in collisionless conditions. IR emission from the CCH transient was detected, and was also affected by the external electric and magnetic fields. Acetylene predissociation was demonstrated to proceed by the direct S(1)-->S(0) mechanism. The results were explained using the previously developed theoretical approach, yielding values of the relevant model parameters.

UV photodissociation of the van der Waals dimer (CH3I)2 revisited: pathways giving rise to ionic features.

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.