Detection of OH radical in laser induced photodissociation of tetrahydrofuran at 193 nm

On excitation at 193 nm, tetrahydrofuran (THF) generates OH as one of the photodissociation products. The nascent energy state distribution of the OH radical was measured employing laser induced fluorescence technique. It is observed that the OH radical is formed mostly in the ground vibrational level, with low rotational excitation (approximately 3%). The rotational distribution of OH (v"=0,J) is characterized by rotational temperature of 1250+/-140 K. Two spin-orbit states, 2Pi3/2 and 2Pi1/2 of OH are populated statistically. But, there is a preferential population in Lambda doublet levels. For all rotational numbers, the 2Pi+(A') levels are preferred to the 2Pi-(A") levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 17.4+/-2.2 kcal mol-1, giving an fT value of approximately 36%, and the remaining 61% of the available energy is distributed in the internal modes of the other photofragment, i.e., C4H7. The observed distribution of the available energy agrees well with a hybrid model of energy partitioning, predicting an exit barrier of approximately 16 kcal mol-1. Based on both ab initio molecular orbital calculations and experimental results, a plausible mechanism for OH formation is proposed. The mechanism involves three steps, the C-O bond cleavage of the ring, H atom migration to the O atom, and the C-OH bond scission, in sequence, to generate OH from the ground electronic state of THF. Besides this high energy reaction channel, other photodissociation channels of THF have been identified by detecting the stable products, using Fourier transform infrared and gas chromatography.     

On the titanium oxide neutral cluster distribution in the gas phase: Detection through 118 nm single-photon and 193 nm multiphoton ionization

Titanium oxide clusters are generated in a supersonic expansion by laser ablation of the metal and reaction with oxygen (0.1-6%) in He expansion gas. Mass spectra of the titanium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Only the 118 nm (10.5 eV) light can ionize Ti(m)O(n) neutral clusters without fragmentation. Both the 193 nm (6.4 eV) and 355 nm (3.5 eV) multiphoton ionization cause fragmentation of the neutral clusters during the ionization process and, thus, can complicate the determination of the stable neutral Ti(m)O(n) gas-phase species. Employing 118 nm single-photon ionization and line-width data, the Ti(m)O(2m) and Ti(m)O(2m+1) series are found to be the most stable neutral cluster species for high oxygen content in the expansion gas. Fragmentation during the multiphoton ionization process for 193 nm light yields the cluster ions Ti(m)O(2m-1,-2)+. These ions are formed by the loss of one or two oxygen atoms from Ti(m)O(2m,2m+1) neutral species. The dominant cluster growth process is suggested to be through the addition of TiO2 species. For low oxygen content (<2%) in the expansion gas, oxygen-deficient clusters of the form Ti(m)O(2m-1,-2) are also observed. These latter series are not fragmented by the 193 nm ionization process.     

On the iron oxide neutral cluster distribution in the gas phase. I. Detection through 193 nm multiphoton ionization

Iron oxide (FemOn) neutral clusters are generated in the gas phase through laser ablation of the metal and reaction with various concentrations of O2 in He. The mixture of expansion gas and neutral FemOn cluster species is expanded through a supersonic nozzle into a vacuum system, in which the clusters are ionized by an ArF excimer laser at 193 nm, and the ions are detected and identified in a time-of-flight mass spectrometer. In this report, the experimental parameters that influence the observed cluster distributions, such as ablation laser power, expansion pressure, vacuum system pressure, and 193 nm ArF ionization laser power, are explored. In the second paper in this series, the effect of the ionization laser wavelength (355 nm, 193 nm, 118 nm) on the observed cluster ion distribution is explored. The cluster ion distribution observed employing 193 nm laser ionization, is sensitive to the neutral cluster distribution as evidenced by the change in the observed time-of-flight mass spectra with changes in laser power, growth conditions, and expansion conditions. The thermodynamically stable neutral clusters for saturated O2 growth conditions are suggested to be of the forms FemOm, FemO(m+1), and FemO(m+2); which one of these series of neutral clusters is most stable depends on the size of the cluster. For m < 10, FemOm is the most stable neutral cluster series, for 10 < or = m < or = 20, FemO(m+1) is the most stable neutral cluster series, and for 21 < or = m < = 30, FemO(m+2) is the most stable neutral cluster series. Some neutral cluster fragmentation is clearly present for 193 nm ionization due to multiphoton absorption in both the neutral and ionic cluster species.     

Formation of hot hexafluorobenzene in the 193 nm photolysis

Time-resolved absorption spectra of hexafluorobenzene vapor have been observed with ArF laser (193 nm) excitation. The initial intermediate is postulated to be due to HFB2(S₀) (hot hexafluorobenzene, with internal energy of 639 kj/mol) because the transient spectrum can be simulated as part of the S₃(¹E_1u) ← S₀ transition at 3050 K.     

193 nm photolysis of CHCl3: Probe of the CH product by CRDS

The CH radical production induced by 193 nm two-photon photolysis of CHCl₃ has been measured for the first time via the cavity ring-down absorption spectroscopy of its A–X bands, using a commercial nanosecond pulsed dye laser. The range of pressure and laser intensity, as well as the time window detection, have been carefully chosen to ensure a constant CH number density during the measurement and to avoid post-photolysis reactivity. Internal energy distribution of the CH(X²II) fragment has been derived from population distribution simulations, leading to an average vibrational temperature T_vib = 1900 ± 50 K and rotational temperature T_rot = 300 ± 20 K. Two competing mechanisms can be invoked for the CH production channel: either two-photon absorption via resonant excited states of CHCl₃ leading to dissociation of excited CHCl₃, or two-photon sequential dissociation via the formation of the vibrationally excited CHCl₂ fragment. The latter mechanism is proposed to be the prominent process for CH formation.     

Dynamics of OH radical generation in laser-induced photodissociation of tetrahydropyran at 193 nm

Tetrahydropyran (THP) undergoes photodissociation on excitation with ArF laser at 193 nm, generating OH radical as one of the transient photoproducts. Laser-induced fluorescence technique is used to detect the nascent OH radical and measure its energy state distribution. The OH radical is formed mostly in the ground vibrational level (v"=0), with low rotational excitation. The rotational distribution of OH (v"=0,J) is characterized by a temperature of 433+/-31 K, corresponding to a rotational energy of 0.86+/-0.06 kcalmol. Two Lambda-doublet levels, 2Pi+(A') and 2Pi-(A"), and the two spin-orbit states, the 2Pi(3/2) and 2Pi(1/2), of OH are populated statistically for all rotational levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 21.9+/-3.2 kcal mol(-1), from the Doppler-broadened linewidth, giving an ft value of approximately 43%, and most of the remaining 57% of the available energy is distributed in the internal modes of the other photofragment, C5H9. The observed distribution of the available energy is explained well, using a hybrid model of energy partitioning, with an exit barrier of 40 kcal mol(-1). The potential-energy surface of the reaction channel was mapped by ab initio molecular-orbital calculations. Based on experimental and theoretical results, a mechanism for OH formation is proposed. Electronically excited THP relaxes to the ground electronic state, and from there, a sequence of reactions takes place, generating OH. The proposed mechanism first involves C-O bond scission, followed by a 1,3 H atom migration to O atom, and finally, the C-OH bond cleavage giving OH.     

The three-body dissociation dynamics of Cl2O at 248 and 193 nm

The photodissociation of Cl₂O has been studied at 248 and 193 nm using photofragment translational spectroscopy (PTS) experiments with tunable VUV photoionization detection. The sole products observed were Cl, O and ClO fragments. Based on the derived translational energy distributions for the ClO and Cl photofragments we conclude that at 248 nm 15% of Cl₂O excitation results in three-body dissociation. At 193 nm no Cl₂ fragments are observed and we conclude that the oxygen atoms arise solely from three-body dissociation. Dissociation geometries derived from forward convolution fitting suggest two qualitatively distinct three-body dissociation pathways: asymmetric concerted dissociation and symmetric concerted dissociation in agreement with recent theoretical predictions.     

On the copper oxide neutral cluster distribution in the gas phase: detection through 355 nm and 193 nm multiphoton and 118 nm single photon ionization

The distribution of neutral copper oxide clusters in the gas phase created by laser ablation is detected and characterized through time-of-flight mass spectroscopy (TOFMS). The neutral copper oxide clusters are ionized by two different approaches: Multiphoton absorption of 355 and 193 nm radiation; and single photon absorption of 118 nm radiation. Based on the observed cluster patterns as a function of experimental conditions (e.g., copper oxide or metal sample, ablation laser power, expansion gas, etc.) and on the width of the TOFMS features, one can uncover the true neutral cluster distribution of CumOn species following laser ablation of the sample. Ablation of a metal sample generates only small neutral CumOn clusters for m less, similar 4 and n approximately 1, 2. Ablation of copper oxide samples generates neutral clusters of the form CumOm (m < or = 4) and CumO(m-1) (m > 4). These clusters are directly detected without fragmentation using single photon, photoionization with 118 nm laser radiation. Using 355 and 193 nm multiphoton ionization, the observed cluster ions are mostly of the form Cu2mOm+ for 4 < or = m < or = 10 (193 nm ionization) and CumO1,2 (355 nm ionization) for copper oxide samples. Neutral cluster fragmentation due to multiphoton processes seems mainly to be of the form CumO(m,m-1) --> CumO(m/2,m/2+1). Neutral cluster growth mechanisms are discussed based on the cluster yield from different samples (e.g., Cu metal, CuO powder, and Cu2O powder).     

Photodissociation of isobutene at 193 nm

The collisionless photodissociation dynamics of isobutene (i-C(4)H(8)) at 193 nm via photofragment translational spectroscopy are reported. Two major photodissociation channels were identified: H + C(4)H(7) and CH(3) + CH(3)CCH(2). Translational energy distributions indicate that both channels result from statistical decay on the ground state surface. Although the CH(3) loss channel lies 13 kcal mol(-1) higher in energy, the CH(3):H branching ratio was found to be 1.7 (5), in reasonable agreement with RRKM calculations.     

The kinetics and mechanism of n-butyraldehyde photolysis in the vapor phase at 313 nm

The photolysis was investigated at 313 nm wavelength, 253–529 K temperatures, and 4 × 10^?11-2 × 10^?9 mol·photon/cm²·sec light intensities by determining the quantum yields of 20 reaction products. Primary quantum yields for the seven primary processes and rate constant ratios, rate constants, and Arrhenius parameters for secondary processes were derived on the basis of the suggested reaction scheme. The dependence of the quantum yields of the four major primary processes on experimental conditions was established.     

Photodissociation of propargyl chloride at 193 nm

The photodissociation of propargyl chloride (C3H3Cl) has been studied at 193 nm. Ion imaging experiments with state-selective detection of the Cl atoms and single-photon ionization of the C3H3 radicals were performed, along with measurements of the Cl + C3H3 and HCl + C3H2 recoil kinetic energy distributions, using a scattering apparatus with electron bombardment ionization detection to resolve the competing Cl and HCl elimination channels. The experiments allow the determination of the Cl (2P3/2) and Cl (2P1/2) (hereafter Cl) branching fractions associated with the C-Cl bond fission, which are determined to be 0.5 +/- 0.1 for both channels. Although prior translational spectroscopy studies by others had concluded that the low velocity signal at the Cl+ mass was due to daughter fragments of the HCl elimination products, the present work shows that Cl atoms are produced with a bimodal recoil kinetic energy distribution. The major C-Cl bond fission channel, with a narrow recoil kinetic energy distribution peaking near 40 kcal/mol, produces both Cl and Cl, whereas the minor (5%) channel, partitioning much less energy to relative kinetic energy, produces only ground spin-orbit state Cl atoms. The maximum internal energy of the radicals produced in the low-recoil-kinetic-energy channel is consistent with this channel producing electronically excited propargyl radicals. Finally, in contrast to previous studies, the present work determines the HCl recoil kinetic energy distribution and identifies the possible contribution to this spectrum from propargyl radicals cracking to C3+ ions in the mass spectrometer.     

Gas phase photolysis of 1-pentene at 147 nm (8.4 eV)

The photolysis of gaseous 1-pentene was carried out in a static system using the xenon resonance line at 147 nm (8.4 eV) at pressures in the range 0.5 – 400 Torr (0.7 – 533 hPa). Only decomposition processes were studied and no attempt was made to establish the pattern of free radical reactions. The major dissociation products observed were ethylene, allene, propylene, 1,3-butadiene, acetylene and propyne. The minor products included methane, ethane, propane, some C₅H₈ and C₄H₆ hydrocarbons, and 1-butene. The radical species were identified using scavengers such as oxygen, H₂S and HI. The pressure dependence of the yields of the major radicals (C₃H₅, CH₃, C₂H₅, C₂H₃ and C₃H₇) was established. The C₂H, C₄H₅, C₄H₇, CH₂ and C₃H₃ radicals were found to be unimportant.The primary decomposition channels are established. The main processes are the cleavage of a CH bond with a yield φ of 0.45 – 0.48 and the cleavage of a CC bond with a yield φ of 0.47. The allylic CC bond appears to be the only CC bond which undergoes primary rupture. All four primary intermediates, i.e. C₅H₉, C₃H₅, C₂H₅ and H, are energized. The radicals either decompose (isomerization prior to decomposition is possible in some cases) or undergo collisional stabilization; some hydrogen atoms add to the double bond prior to thermalization. Some details of the secondary processes are established but the overall mechanism is too complex to be fully interpreted.     

On the zirconium oxide neutral cluster distribution in the gas phase: detection through 118 nm single photon, and 193 and 355 nm multiphoton, ionization

Zirconium oxide clusters are generated in the gas phase by laser ablation of the metal into a flow of ca. 5% O2/95% He at 100 psig and supersonic expansion into a vacuum chamber. Mass spectra of neutral gas phase zirconium oxide clusters are obtained through photoionization at three different laser wavelengths: 118, 193, and 355 nm. Ionization of the clusters with 118 nm laser radiation is through a single photon ionization mechanism, while ionization by 193 and 355 nm laser radiation is through a multiphoton (three or more photon) mechanism. Fragment ion features are observed in the mass spectra of ZrmOn+ for only the 193 nm and 355 nm ionization schemes. The true neutral ZrmOn cluster distribution is obtained only through 118 nm single photon ionization, as verified by mass spectral peak linewidths and calculations of the cluster binding energies, ionization energies, and fragmentation rates. The neutral cluster distribution consists mainly of the series ZrmO2m and ZrmO(2m+1) for m = 1,..., approximately 30.     

Photodissociation Dynamics of Benzene at 193 nm

Photodissociation of benzene at 193 nm has been investigated using the photofragment translational spectroscopy (PTS) technique. H atom elimination channel for benzene at 193 nm is from a one‐photon dissociation process, while H₂ and CH₃ elimination channels come from a two‐photon excitation process.     

The photodissociation of phenylsilane at 193 nm

Molecular hydrogen is observed to be one of the major primary products in the 193 nm photodissociation of phenylsilane. A two-channel dissociation mechanism is proposed, yielding PhSiH+H₂ and SiH₂ +PhH with the former predominant. The implications of this observation for experiments which utilise phenylsilane as a precursor for SiH₂ radicals are discussed.     

Gas phase photolysis of 1-butene at 123.7 nm (10.0 eV)

The photolysis of 1-butene was carried out in a static system using the krypton resonance line at 123.7 nm (10.0 eV) at pressures in the range 15–500 Torr (2–66.5 kPa). The major products observed were ethylene, acetylene, 1,3-butadiene, allene, n-butane and propylene. Identification of the radical species was made by the use of scavengers such as oxygen and H₂S. Evidence is presented for the occurrence of nine primary processes to which quantum yields have been ascribed. The main processes are fragmentation giving C₃H₅ radicals with a yield φ of 0.29 and the formation of C₄H₆ hydrocarbons with a yield φ of 0.23. A hydrogen atom mechanism, involving the occurrence of hot hydrogen atoms that have an excess energy as high as 0.6 eV, was proposed to account for the pressure dependence of propylene. Dissociation of excited radicals contributes to the formation of ethylene.     

Molecular elimination in photolysis of fluorobenzene at 193 nm: internal energy of HF determined with time-resolved Fourier-transform spectroscopy

Following photodissociation of fluorobenzene (C(6)H(5)F) at 193 nm, rotationally resolved emission spectra of HF(1相似文献   

Kinetics of Si2H2 produced by the 193 nm photolysis of disilane

Si₂H₂ species were produced through the 193 nm excimer laser photolysis of Si₂H₆. Time‐resolved photoionization mass spectrometry was employed to study the reaction kinetics of Si₂H₂. The lower limit of self‐reaction of Si₂H₂ was estimated, k(Si₂H₂ + Si₂H₂) ≥ (1.7 ± 0.5) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, through analysis of the decay traces. This rate constant was independent of a total pressure on the entire pressure range (p_total = 1–7 Torr). No reaction of Si₂H₂ with H₂, CH₄, SiH₄, and Si₂H₆ was confirmed. The decay rates of Si₂H₂ reacting with O₂, NO, and HCl exhibit negative dependence on the total pressure. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 136–141, 2001     

Photodissociation dynamics of benzyl alcohol at 193 nm

Photodissociation dynamics of benzyl alcohol, C(6)H(5)CH(2)OH and C(6)H(5)CD(2)OH, in a molecular beam was investigated at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed, including OH elimination and H(2)O elimination from the ground electronic state, H atom elimination (from OH functional group), and CH(2)OH elimination from the triplet state. The dissociation rate on the ground state was found to be 7.7 × 10(6) s(-1). Comparison to the potential energy surfaces from ab initio calculations, dissociation rate, and branching ratio from Rice-Ramsperger-Kassel-Marcus calculations were made.     

Detection of OH Radical in the Photodissociation of p-Aminobenzoic Acid at 266 nm

Photodissociation of p-aminobenzoic acid at 266 nm was investigated by probing the nascent OH photoproduct employing the laser-induced fluorescence technique. It was found that the nascent OH radical was vibrationally cold and its rotational state distribution conformed to be a Boltzmann behavior, characterized by a rotational temperature of 1040±110 K. The rotational energy of OH was determined to be 8.78±0.84 kJ/mol. Between the two spinorbit states of OH, ^2Ⅱ3/2 and ^2Ⅱ1/2, the former was found to be preferentially populated. The distribution of the II（A＇） state for the A-doublet was dominant. Finally, a probable mechanism for the formation of OH produced from the photodissociation of p-aminobenzoic acid is discussed.