Dynamics of the CH(A2Delta) product from the reaction of C2H with O2 studied by Fourier transform visible spectroscopy

The reaction C(2)H + O(2) --> CH(A(2)Delta) + CO(2) is investigated using Fourier transform visible emission spectroscopy. C(2)H radicals, produced by 193 nm photolysis of C(2)H(2), react with O(2) molecules at low total pressures to produce electronically excited CH(A(2)Delta). Observation of the CH(A(2)Delta-X(2)Pi) electronic emission to infer nascent rotational and vibrational CH(A(2)Delta) distributions provides information about energy partitioning in the CH(A(2)Delta) fragment during the reaction. The rotational and vibrational populations of the CH(A(2)Delta) product are determined by fitting the rotationally resolved experimental spectra with simulated spectra. The CH(A(2)Delta) product is found to be rotationally and vibrationally excited with T(rot) congruent with 1150 K and T(vib) congruent with 1900 K. The mechanism for this reaction proceeds through one of two five-atom intermediates and requires a crossing between electronic potential surfaces. The rotational excitation suggests a bent geometry for the final intermediate of this reaction before dissociation to products, and the vibrational excitation involves an elongation of the C-H bond from the compressed transition state to the final CH(A) state.     

Determining the vibrational pattern via overtone cold spectra: C-H methyl stretches of propyne

Vibrationally mediated photodissociation and photoacoustic (PA) spectroscopy were employed for studying the intramolecular dynamics of propyne initially excited to the first through fourth overtone of methyl C-H stretching modes. Room-temperature PA and jet-cooled action spectra, monitoring the absorption of the parent and the yield of the ensuing H photofragments, respectively, were obtained. The PA spectra exhibit mainly broad features, while the action spectra, due to inhomogeneous structure reduction, expose multiple peaks of recognizable shapes in the differing overtone manifolds. Symmetric rotor simulations of the band contours of the action spectra allowed retrieving of band origins and linewidths. The linewidths of the bands in each manifold enabled estimates for energy redistribution times out of the corresponding states to the bath states, the times ranging from 18+/-6 ps for two quanta of C-H excitation to subpicosecond for five quanta. The data were also analyzed in terms of a normal-mode model and a joint local-/normal-mode model. These models enabled determination of harmonic frequencies, anharmonicities, and interaction parameters reproducing the observed data in all monitored regions and provided spectral assignments. The measured Doppler profiles were well fitted by Gaussians with widths suggesting low average translational energies for the released H photofragments. These low energies and their similarities to those for dissociation of propyne isotopomers preexcited to acetylenic C-H stretches were ascribed to an indirect dissociation process occurring after internal conversion to the ground electronic state and isomerization to allene.     

Imaging the photodissociation of CH3SH in the first and second absorption bands: the CH3(X2A1)+SH(X2Pi) channel

The CH3(X2A1)+SH(X2Pi) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"<--X 1A' and second 2 1A"<--X1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (beta=-1+/-0.1) CH3(X2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X2Pi) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (beta=-0.8+/-0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X2A1)+S(X3P0,1,2).     

Two- and three-body photodissociation of gas phase I(3) (-)

The photodissociation dynamics of I3- from 390 to 290 nm (3.18 to 4.28 eV) have been investigated using fast beam photofragment translational spectroscopy in which the products are detected and analyzed with coincidence imaging. At photon energies < or = 3.87 eV, two-body dissociation that generates I- + I2(A 3Pi1) and vibrationally excited I2- (X 2Sigmau+) + I(2P(3/2)) is observed, while at energies > or = 3.87 eV, I*(2P(1/2)) + I2- (X 2Sigmau+) is the primary two-body dissociation channel. In addition, three-body dissociation yielding I- +2I(2P(3/2)) photofragments is seen throughout the energy range probed; this is the dominant channel at all but the lowest photon energy. Analysis of the three-body dissociation events indicates that this channel results primarily from a synchronous concerted decay mechanism.     

Photoisomerization and photodissociation of toluene in molecular beam

The photodissociation of isotope-labeled toluene C(6)H(5)CD(3) and C(6)H(5)(13)CH(3) molecules at 6.4 eV under collision-free conditions was studied in separate experiments by multimass ion imaging techniques. In addition to the major dissociation channels, C(6)H(5)CD(3) --> C(6)H(5)CD(2) + D and C(6)H(5)CD(3) --> C(6)H(5) + CD(3), the respective photofragments CD(2)H, CDH(2), and CH(3) and their heavy fragment partners C(6)H(4)D, C(6)H(3)D(2), and C(6)H(2)D(3) were observed from C(6)H(5)CD(3) dissociation. Photofragments (13)CH(3) and CH(3), and their heavy fragment partners C(6)H(5) and (13)CC(5)H(5), were also observed from C(6)H(5)(13)CH(3) dissociation. Our results show that 25% of the excited toluene isomerizes to a seven-membered ring (cycloheptatriene) and then rearomatizes prior to dissociation. The isomerization pathway competes with direct C-C bond and C-H bond dissociation. The significance of this isomerization is that the carbon atoms and hydrogen atoms belonging to the alkyl group are involved in an exchange with those atoms in the aromatic ring during isomerization. The dissociation rate of toluene at 193 nm is measured to be (1.17 +/- 0.1) x 10(6) s(-)(1).     

Electronic spectroscopy and photodissociation dynamics of the 1-hydroxyethyl radical CH3CHOH

The electronic spectroscopy and photodissociation dynamics of the CH3CHOH radical in the region 19,400-37,000 cm(-1) (515-270 nm) were studied in a molecular beam using resonance-enhanced multiphoton ionization (REMPI), photofragment yield spectroscopy, and time-of-flight (TOF) spectra of H and D fragments. The onset of the transition to the Rydberg 3s state, the lowest excited state, is estimated at 19,600 +/- 100 cm(-1). The 3s state dissociates fast, and no REMPI spectrum is observed. The origin band of the transition to the 3pz state, identified by 2 + 2 REMPI, lies at 32,360 +/- 70 cm(-1), and a vibrational progression in the C-O stretch is assigned. When exciting CH3CHOH near the onset of the unstructured absorption to the 3s state, only one peak is observed in the center-of-mass (c.m.) translational energy (Et) distribution obtained by monitoring H photofragments. The measured recoil anisotropy parameter beta = -0.7 +/- 0.1 is typical of a perpendicular transition. The O-H bond energy is determined to be D0 = 1.1 eV +/- 0.1 eV. At excitation energies >31,200 cm(-1) (3.87 eV) a second, low Et peak appears in the c.m. Et distribution with beta approximately 0. Its relative intensity increases with excitation energy, but its beta value does not change. In contrast, the beta value of the higher Et peak becomes monotonically less negative at higher excitation energies, decreasing to -0.2 +/- 0.1 at 35,460 cm(-1). By comparison of the TOF distributions of the isotopologs CH3CHOH, CH3CHOD, and CD3CHOH, it is concluded that two major product channels dominate the photodissociation, one leading to acetaldehyde and the other to vinyl alcohol (enol) products. There is no indication of isomerization to ethoxy. It appears that separate conical intersections lead to the observed channels, and the dynamics at the conical intersection and the exit channel deposit much of the available energy into internal energy of the products.     

H and D release in approximately 243.1 nm photolysis of vibrationally excited 3nu1, 4nu1, and 4nuCD overtones of propyne-d3

The photofragmentation of propyne-d(3), D(3)C-C[Triple Bond]C-H, following approximately 243.1 nm photodissociation of rovibrationally excited molecules promoted to the second (3nu(1)) and third (4nu(1)) acetylenic C-H overtone and to the third (4nu(CD)) methyl overtone has been investigated. The resulting H and D photoproducts were detected via (2+1) resonantly enhanced multiphoton ionization. The measured room-temperature photoacoustic and jet-cooled action spectra allowed derivation of the molecular parameters of the C-H overtones and the Doppler profiles revealed the translational energies associated with the H(D) photofragments and the H to D branching ratios. Propensities toward the latter were encountered, while the translational energy disposal in both photofragments was essentially identical for a given preexcitation. This behavior agrees with that found for the almost isoenergetic 193.3 nm photolysis of propyne [Qadiri et al., J. Chem. Phys. 119, 12842 (2003)], but contradicts previous findings. The bond fission of C-H and C-D is preceded by internal conversion to, and isomerization on, the ground-state potential energy surface (PES), followed by extensive intramolecular vibrational redistribution. For molecules preexcited to 3nu(1) and 4nu(1) an additional minor channel opens, where elimination of H occurs directly on the accessed excited PES, while that of D on the ground state.     

The vinyl + NO reaction: determining the products with time-resolved Fourier transform spectroscopy

We have studied the vinyl + NO reaction using time-resolved Fourier transform emission spectroscopy, complemented by electronic structure and microcanonical RRKM rate coefficient calculations. To unambiguously determine the reaction products, three precursors are used to produce the vinyl radical by laser photolysis: vinyl bromide, methyl vinyl ketone, and vinyl iodide. The emission spectra and theoretical calculations indicate that HCN + CH2O is the only significant product channel for the C2H3 + NO reaction near room temperature, in contradiction to several reports in the literature. Although CO emission is observed when vinyl bromide is used as the precursor, it arises from the reaction of NO with photofragments other than vinyl. This conclusion is supported by the absence of CO emission when vinyl iodide or methyl vinyl ketone is used. Prompt emission from vibrationally excited NO is evidence of the competition between back dissociation and isomerization of the initially formed nitrosoethylene adduct, consistent with previous work on the pressure dependence of this reaction. Our calculations indicate that production of products is dominated by the low energy portion of the energy distribution. The calculation also predicts an upper bound of 0.19% for the branching ratio of the H2CNH + CO channel, which is consistent with our experimental results.     

A study of the unimolecular dissociation of the 2-buten-2-yl radical via the 193 nm photodissociation of 2-chloro-2-butene

This work investigates the unimolecular dissociation of the 2-buten-2-yl radical. This radical has three potentially competing reaction pathways: C-C fission to form CH3 + propyne, C-H fission to form H + 1,2-butadiene, and C-H fission to produce H + 2-butyne. The experiments were designed to probe the branching to the three unimolecular dissociation pathways of the radical and to test theoretical predictions of the relevant dissociation barriers. Our crossed laser-molecular beam studies show that 193 nm photolysis of 2-chloro-2-butene produces 2-buten-2-yl in the initial photolytic step. A minor C-Cl bond fission channel forms electronically excited 2-buten-2-yl radicals and the dominant C-Cl bond fission channel produces ground-state 2-buten-2-yl radicals with a range of internal energies that spans the barriers to dissociation of the radical. Detection of the stable 2-buten-2-yl radicals allows a determination of the translational, and therefore internal, energy that marks the onset of dissociation of the radical. The experimental determination of the lowest-energy dissociation barrier gave 31 +/- 2 kcal/mol, in agreement with the 32.8 +/- 2 kcal/mol barrier to C-C fission at the G3//B3LYP level of theory. Our experiments detected products of all three dissociation channels of unstable 2-buten-2-yl as well as a competing HCl elimination channel in the photolysis of 2-chloro-2-butene. The results allow us to benchmark electronic structure calculations on the unimolecular dissociation reactions of the 2-buten-2-yl radical as well as the CH3 + propyne and H + 1,2-butadiene bimolecular reactions. They also allow us to critique prior experimental work on the H + 1,2-butadiene reaction.     

Molecular-beam experiments for photodissociation of propenal at 157 nm and quantum-chemical calculations for migration and elimination of hydrogen atoms in systems C3H4O and C3H3O

We investigated the dynamics of photodissociation of propenal (acrolein, CH(2)CHCHO) at 157 nm in a molecular beam and of migration and elimination of hydrogen atoms in systems C(3)H(4)O and C(3)H(3)O using quantum-chemical calculations. Compared with the previous results of photodissociation of propenal at 193 nm, the major difference is that the C(3)H(3)O fragment present at the 193-nm photolysis disappears at the 157-nm photolysis whereas the C(3)H(2)O fragment absent at 193 nm appears at 157 nm. Optimized structures and harmonic vibrational frequencies of molecular species with gross formula C(3)H(2-4)O were computed at the level of B3LYP/6-311G(d,p) and total energies of those molecules at optimized structures were computed at the level of CCSD(T)/6-311+G(3df,2p). Based on the calculated potential-energy surfaces, we deduce that the C(3)H(3)O fragment observed in the photolysis of propenal at 193 nm is probably CHCCHOH ((2)A") and/or CH(2)CCOH ((2)A") produced from an intermediate hydroxyl propadiene (CH(2)CCHOH) following isomerization. Adiabatic and vertical ionization potentials of eight isomers of C(3)H(3)O and two isomers of C(3)H(2)O were calculated; CHCCHOH ((2)A") and CH(2)CCOH ((2)A") have ionization potentials in good agreement with the experimental value of ～7.4 eV. We also deduce that all the nascent C(3)H(3)O fragments from the photolysis of propenal at 157 nm spontaneously decompose mainly to C(2)H(3) + CO and C(3)H(2)O + H because of the large excitation energy. This work provides profound insight into the dynamics of migration and elimination of hydrogen atoms of propenal optically excited in the vacuum-ultraviolet region.     

A complete look at the multi-channel dissociation of propenal photoexcited at 193 nm: branching ratios and distributions of kinetic energy

We observed fifteen photofragments upon photolysis of propenal (acrolein, CH(2)CHCHO) at 193 nm using photofragment translational spectroscopy and selective vacuum-ultraviolet (VUV) photoionization. All the photoproducts arise from nine primary and two secondary dissociation pathways. We measured distributions of kinetic energy of products and determined branching ratios of dissociation channels. Dissociation to CH(2)CHCO + H and CH(2)CH + HCO are two major primary channels with equivalent branching ratios of 33%. The CH(2)CHCO fragment spontaneously decomposes to CH(2)CH + CO. A proportion of primary products CH(2)CH from the fission of bond C-C of propenal further decompose to CHCH + H but secondary dissociation HCO → H + CO is negligibly small. Binary dissociation to CH(2)CH(2) (or CH(3)CH) + CO and concerted three-body dissociation to C(2)H(2) + CO + H(2) have equivalent branching ratios of 14%-15%. The other channels have individual branching ratios of ～1%. The production of HCCO + CH(3) indicates the formation of intermediate methyl ketene (CH(3)CHCO) and the production of CH(2)CCH + OH and CH(2)CC + H(2)O indicate the formation of intermediate hydroxyl propadiene (CH(2)CCHOH) from isomerization of propenal. Distributions of kinetic energy release and dissociation mechanisms are discussed. This work provides a complete look and profound insight into the multi-channel dissociation mechanisms of propenal. The combination of a molecular beam apparatus and synchrotron VUV ionization allowed us to untangle the complex mechanisms of nine primary and two secondary dissociation channels.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.     

IR laser manipulation of cis<-->trans isomerization of 2-naphthol and its hydrogen-bonded clusters

The cis<-->trans isomerization reaction has been carried out for 2-naphthol and its hydrogen (H) bonded clusters by infrared (IR) laser in the electronic excited state (S1) in supersonic jets. A specific isomer in the jet was pumped to the X-H stretching vibration in the S1 state, where X refers to C, O, or N atom, by using a stepwise UV-IR excitation, and the dispersed emission spectra of the excited species or generated fragments were observed. It was found that the isomerization occurs only in the H-bonded clusters but a bare molecule does not exhibit the isomerization in the examined energy region of Ev< or =3610 cm(-1), indicating a reduction of the isomerization barrier height upon the H bonding. The relative yield of the isomerization was observed as a function of internal energy. The isomerization yield was found to be very high at the low IR frequency excitation, and was rapidly reduced with the IR frequency due to the competition of the dissociation of the H bond within the isomer. Density-functional theory (DFT) and time-dependent DFT calculations were performed for estimating the barrier height of the isomerization for bare 2-naphthol and its cluster for electronic ground and excited states. The calculation showed that the isomerization barrier height is highly dependent on the electronic states. However, the reduction of the height upon the hydrogen bonding was not suggested at the level of our calculation.     

The photodissociation of NO(2) by visible and ultraviolet light

We present velocity map images of the NO, O((3)P(J)) and O((1)S(0)) photofragments from NO(2) excited in the range 7.6 to 9.0 eV. The molecule was initially pumped with a visible photon between 2.82-2.95 eV (440-420 nm), below the first dissociation threshold. A second ultraviolet laser with photon energies between 4.77 and 6.05 eV (260-205 nm) was used to pump high-lying excited states of neutral NO(2) and/or probe neutral photoproducts. Analysis of the kinetic energy release spectra revealed that the NO photofragments were predominantly formed in their ground electronic state with little kinetic energy. The O((3)P(J)) and O((1)S(0)) kinetic energy distributions were also dominated by kinetically 'cold' fragments. We discuss the possible excitation schemes and conclude that the unstable photoexcited states probed in the experiment were Rydberg states coupled to dissociative valence states. We compare our results with recent time-resolved studies using similar excitation and probe photon energies.     

Formation of the CH fragment in the 193 nm photodissociation of CHCl

The CH fragment from the 193 nm photodissociation of CHCl is observed in a molecular beam experiment. This fragment is formed in the higher-energy dissociation pathway, the lower pathway involving formation of CCl. Both the CHCl parent molecule and the CH fragment were detected by laser-induced fluorescence. The 193 nm CHCl absorption cross section was estimated from the reduction of the CHCl signal as a function of the photolysis laser fluence. The CH internal state distribution was derived from the analysis of laser-induced fluorescence spectra of the A-X Deltav=0 sequence. A modest degree of rotational excitation was found in the CH fragment; the most probable rotational level is N=1, but the distribution has a tail extending to N>25. Also observed is a slight preference for formation of Lambda-doublets of A(") symmetry, which appears to increase with increasing rotational angular momentum N. Vibrationally excited CH was observed, and the degree of vibrational excitation was found to be low. The energy available to the photofragments is predominantly released as translational excitation. The preferential formation of A(") Lambda-doublets suggests that dissociation occurs through a nonlinear excited state.     

Competitive C-H and O-D bond fission channels in the UV photodissociation of the deuterated hydroxymethyl radical CH2OD

Photodissociation studies of the CH2OD radical in the region 28,000-41,000 cm(-1) (357-244 nm), which includes excitation to the 3s, 3p(x), and 3p(z) states, are reported. H and D photofragments are monitored by using resonance-enhanced multiphoton ionization (REMPI) from the onset of H formation at approximately 30,500 cm(-1) to the origin band region of the 3pz(2A")<--1 2A" transition at 41,050 cm(-1). Kinetic energy distributions P(ET) and recoil anisotropy parameters as a function of kinetic energy, beta(eff)(ET), are determined by the core sampling technique for the channels producing H and D fragments. Two dissociation channels are identified: (I) D+CH2O and (II) H+CHOD. The contribution of channel II increases monotonically as the excitation energy is increased. Based on the calculations of Hoffmann and Yarkony [J. Chem. Phys. 116, 8300 (2002)], it is concluded that conical intersections between 3s and the ground state determine the final branching ratio even when initial excitation accesses the 3px) and 3pz states. The different beta(eff) values obtained for channels I and II (-0.7 and approximately 0.0, respectively) are attributed to the different extents of out-of-plane nuclear motions in the specific couplings between 3s and the ground state (of A' and A' symmetry, respectively) that lead to each channel. The upper limit to the dissociation energy of the C-H bond, determined from P(ET), is D0(C-H)=3.4+/-0.1 eV (79+/-2 kcal/mol). Combining this value with the known heats of formation of H and CH2OD, the heat of formation of CHOD is estimated at DeltaHf(0)(CHOD)=24+/-2 kcal/mol.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). II. Velocity map imaging studies

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ? CH(3)O (CD(2)OH ? CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).     

Product spin-orbit state resolved dynamics of the H+H2O and H+D2O abstraction reactions

The product state-resolved dynamics of the reactions H+H(2)O/D(2)O-->OH/OD((2)Pi(Omega);v',N',f )+H(2)/HD have been explored at center-of-mass collision energies around 1.2, 1.4, and 2.5 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the OH/OD radical products. The populations in the OH spin-orbit states at a collision energy of 1.2 eV have been determined for the H+H(2)O reaction, and for low rotational levels they are shown to deviate from the statistical limit. For the H+D(2)O reaction at the highest collision energy studied the OD((2)Pi(3/2),v'=0,N'=1,A') angular distributions show scattering over a wide range of angles with a preference towards the forward direction. The kinetic energy release distributions obtained at 2.5 eV also indicate that the HD coproducts are born with significantly more internal excitation than at 1.4 eV. The OD((2)Pi(3/2),v'=0,N'=1,A') angular and kinetic energy release distributions are almost identical to those of their spin-orbit excited OD((2)Pi(1/2),v'=0,N'=1,A') counterpart. The data are compared with previous experimental measurements at similar collision energies, and with the results of previously published quasiclassical trajectory and quantum mechanical calculations employing the most recently developed potential energy surface. Product OH/OD spin-orbit effects in the reaction are discussed with reference to simple models.     

UV-absorption spectra of the radical transients generated from the 193-nm photolysis of allene, propyne, and 2-butyne

The 193-nm photochemistry of allene (H2C=C=CH2), propyne (H3C-C[triple bond]CH), and 2-butyne (H3C-C[triple bond]C-CH3) has been examined, and the UV spectral region between 220 and 350 nm has been surveyed for UV-absorption detection of transient species generated from the photolysis of these molecules. Time-resolved UV-absorption spectroscopy was used for detection of transient absorption. Gas chromatographic/mass spectroscopic (GC/MS) analysis of the photolyzed samples were employed for identification of the final photodissociation products. An emphasis of the study has been on the examination of possibilities of formation of different C3H3 isomeric radicals, that is, propargyl (H2CCCH) or propynyl (H3CCC), from the 193-nm photolysis of these molecules. Survey of the UV spectral region, following the 193-nm photolysis of dilute mixtures of allene/He resulted in detection of a strong absorption band around 230 nm and a weaker band in the 320-nm region with a relative intensity of about 8:1. The time-resolved absorption traces after the photolysis event show an instantaneous rise, followed by a simple decay. The spectral features, observed in this work, following 193-nm photolysis of allene are in good agreement with the previously reported spectrum of H2CCCH radical in the 240- and 320-nm regions and are believed to originate primarily from propargyl radicals. In comparison, the spectra obtained from the 193-nm photolysis of dilute mixtures of HCCCH3/He and CH3CCCH3/He were nearly identical, consisting of two relatively broad bands centered at about 240- and 320-nm regions with a relative intensity of about 2:1, respectively. In addition, the time-resolved absorption traces after photolysis of propyne and 2-butyne samples, both in the 240 and 320 nm regions, indicated an instant rise followed by an additional slower absorption rise. The distinct differences between the results of allene with those of propyne and 2-butyne suggest the observed absorption features following 193-nm photolysis of these molecules are likely to be composite with contributions from a number of transient species other than propargyl radicals. Propyne and 2-butyne are structurally similar. The methyl (CH3) and propynyl (CH3C[triple bond]C) radicals are likely to be among the photodissociation products of 2-butyne, and similarly, propynyl is likely to be a photodissociation product of propyne. GC/MS product analysis of photolyzed 2-butyne/He mixtures indicates the formation of C2H6 (formed from the combination of CH3 radicals), and a number of C6H6 and C4H6 isomers formed from self- and cross reactions of C3H3 and CH3 radicals, including 1,5-hexadiyne and 2,4-hexadyine, that are potential products of combination reactions of propargyl as well as propynyl radicals.     

Internal state distribution of the CF fragment from the 193 nm photodissociation of CFCl and CFBr

The dynamics of the 193 nm photodissociation of the CFCl and CFBr molecules have been investigated in a molecular beam experiment. The CFCl and CFBr parent molecules were generated by pyrolysis of CHFCl2 and CFBr3, respectively, and the CFCl and the CF photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section of CFCl was determined from the reduction of the CF photofragment signal as a function of the photolysis laser fluence. The internal state distribution was derived from the analysis of laser fluorescence excitation spectra in the A 2Sigma+-X 2Pi band system. A very low degree of rotational excitation, with essentially equal A' and A" Lambda-doublet populations, and no vibrational excitation were found in the CF photofragment. The energy available to the photofragments is hence predominantly released as translational energy. The CF internal state distribution is consistent with the dissociation of a linear intermediate state. Considerations of CFCl electronic states suggest that a bent Rydberg state is initially excited.