Photofragment Translational Spectroscopy Research Articles

Infrared multiphoton dissociation of 2-chloro-1,1,1,2-tetrafluoroethane in a molecular beam

Mechanism and dynamics of the infrared multiphoton dissociation of 2-chloro-1, 1,1,2-tetrafluoroethane have been studied using a photofragmentation translational spectroscopy. The molecule dissociates competitively through three-centered elimination of HCl and C–Cl bond rupture. The HCl elimination reaction accounts for 74% of the total primary dissociation yields. The center-of-mass translational energy distribution for the HCl elimination indicates that an exit barrier of several kcal/mol exists along the reaction coordinate on the potential energy surface. The infrared multiphoton dissociation of CF3CF produced by the HCl elimination from CF3CHClF also occurs as a secondary process through its dissociation into two CF2 molecules. The average excitation energy of dissociating CF3CHClF has been determined to be about 20 kcal/mol above the C–Cl dissociation threshold of the molecule by comparing the observed center-of-mass translational energy distribution for the C–Cl bond rupture reaction with that calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory.

Photofragment Translational Spectroscopy

Photofragment translational spectroscopy (PTS) is a technique for studying the mechanisms and the dynamics of photodissociation processes under collision-free conditions. After a brief introduction to the methodology of PTS, a high-performance apparatus developed in our laboratory is described. Various applications are then presented to demonstrate the versatility of this technique. Examples include studies of the photodissociation dynamics of small molecules and the investigation of decomposition mechanisms in atmospherically important molecules.

Photodissociation of dichlorocarbene CCl 2 at 248 nm in a cold beam

The photodissociation of the dichlorocarbene radical CCl 2 at 248 nm has been studied by photofragment translational spectroscopy. An intense beam of cold CCl 2 was generated from CHCl 3 by a pulsed molecular beam source equipped with a flash pyrolysis nozzle. The photolysis proceeds exclusively via CCl bond fission yielding CCl+Cl. The photofragment recoil distributions imply that more than 60% of the available energy is channeled into the internal degree of freedom of CCl and that the angular distribution is almost isotropic.

Excess Energy and Structural Dependence of the Rate of Energy Redistribution during the Photodissociation of Iodotoluenes

Abstract : The ortho (2-), meta (3-1), and para (4-) iodotolunes (C7H7I) photodissociated at 266 nm and 30.4 nm and studied with state-selective one- dimensional photo-fragment translational spectroscopy with state-selective velocity, and translational energy distributions are obtained for the ground I(2P3/2) and spin-orbit excited state I*(2P1/2) iodine atoms produced photodissociation. As has been observed in iodobenzene, the ground state I channel observed in each isomer of iodotoluene exhibits both a prompt alkyl iodide-like dissociation channel following a parallel excitation to the alkyl iodide 30(zero)(n, sigma*) repulsive state and curve crossing to the 1Q1(n, sigma*) state, and a slower, indirect dissociation channel following a competitive excitation to aromatic charge transfer (pi, pi*) predissociate excited states at both 266 nm and 304 nm. The I* channel observed at both 266 nm and 304 nm for each isomer results from prompt dissociation resulting from parallel absorption to an alkyl iodide type 3Q(zero)(n, sigma*) state. The rapid I and I* dissociative channels observed for each isomer are found to exhibit strong dependence on the excess excitation. This is discussed in terms of the increase in the density of the vibrational states of the toluene ring. Dissociation times and rates of internal energy redistribution (IER) from the slow dissociative channel (Beta) are estimated for each isomer. For 2- iodotoluene, the rate of IER for the slow I channel increases from 20 kcal/mol- ps at 304 nm to 30 kcal/mol-ps at 266 nm, while the rates of IER for 3- and 4- iodotoluene (16 kcal/mol-ps and 15 kcal/mol-ps, respectively) remain unchanged as the photon energy is increased

Near ultraviolet photolysis of phosphine studied by H atom photofragment translational spectroscopy

We report time-of-flight measurements of the H atom fragments arising in the photodissociation of phosphine at three near ultraviolet excitation wavelengths—218.2 nm, 221.6 nm, and 228.0 nm. For these wavelengths, energetic considerations dictate that ground state [Formula: see text] fragments must be the partner product of the primary photolysis event. The measured H atom kinetic energy distributions indicate that internal excitation of these partner fragments accounts for the major part [Formula: see text] of the available energy, in excess of that required to break the H—PH2 bond, provided by the photon. Inspection of the poorly resolved structure evident near the peak of the H atom kinetic energy distribution suggests the presence of substantial bending excitation and a-axis rotational excitation in the PH2 partner fragment. The H atom kinetic energy distribution also shows clear evidence of atoms arising from secondary photolysis of the primary [Formula: see text] fragments. This secondary photo-dissociation is observed to show a marked anisotropy, consistent with a perpendicular photo-excitation of the [Formula: see text] fragment. Arguments are presented which indicate that the secondary photolysis leads both to the formation of PH(X) fragments, carrying high levels of rotational excitation, and to the three-body atomisation process.

Photofragment Translational Spectroscopy of n-Butyl Iodide at 277 and 304 nm: Polarization Dependence and Energy Partitioning

Photodissociation of n-C 4 H 9 I at excitation wavelengths of 277 and 304 nm has been studied utilizing state-selective photofragment translational spectroscopy. The quantum yields of I * ( 2 P 1/2 ) at these wavelengths are determined to be 0.61 and 0.14, respectively. The anisotropy parameters observed at 277 nm are β(1)=1.6±0.1 for the ground-state I( 2 P 3/2 ) and β(I * )=0.9±0.1 for the excited-state I * ( 2 P 1/2 ) which substantially differs from the limiting value

Photofragment Translational Spectroscopy of Fluorinated Halomethanes

AbstractThe primary and secondary dissociation processes of CF2Br2 and CF2BrI have been studied by photofragment translational spectroscopy in a pulsed molecular beam. The photolysis of CF2Br2 at 193 nm produces mainly the fragment pair CF2Br + Br and, in a competing primary process, CF2 + Br2. The CF2Br radicals are formed with sufficient internal energy to overcome the barrier of the unimolecular secondary dissociation to CF2+ Br. The photodissociation of CF2BrI at 248 nm also proceeds along two decay pathways involving the fission of the weak C‐I bond and the stronger C‐Br bond, respectively. The former reaction produces spin‐orbit excited iodine atoms I(2P1/2) and stable CF2Br radicals, whereas the latter reaction produces unstable CF2I radicals. The results of this study are compared with those of other workers, and an account of the ultraviolet photochemistry of fluorinated halomethanes is given.

The Photodissociation of Pyridine at 193 nm

AbstractUsing photofragmentation translational spectroscopy, the primary processes involved in the photodissociation of pyridine are investigated at 193 nm. Upon absorption of a single photon, HCN + C4H4, CH3 + C,H2N, C3H3 + C2H2N, and H + C5H4N are the observed primary pathways, with relative yields of 59%, 21%, 11%, and 9%, respectively. This paper reports the experimental details and results of this study. Comparison of the results of this study with a previous study in our laboratory investigating the photolysis of benzene serves as a direct measure of the strong influence substitution of a heteroatom in an aromatic ring can have on a molecule's photochemistry.

Observation of the correlated O 3P j1 , 3P j2 state distribution from the predissociation of O 2 B 3Σ −u

A fast beam photofragment translational spectroscopy study of the predissociation of the O 2 B 3Σ − u state is presented. A 5 keV beam of vibrationally excited O 2 X 3Σ − g is prepared via photodetachment of O − 2. The B 3Σ − u (gn′=7)←X 3Σ − g (ν″=4) transition of the Schumann-Runge band is then excited, resulting in predissociation to two O 3P j atoms. We determine the photofragment kinetic energy angular distribution using a time- and position-sensitive detector. Our energy resolution (10 meV) is sufficient to resolve the energy splittings of the O atom spin-orbit levels, enabling us to determine the correlated ( j 1, j 2) fine structure distribution for the photofragments. These results do not appear to be consistent with predictions based on a recent model of O 2 B 3Σ − u state predissociation.

Photodissociation of nitric acid in a cold molecular beam at 193 nm

The photodissociation of HONO 2 at 193 nm has been investigated by photofragment translational spectroscopy. Two competing primary dissociation processes were found: (1) HONO 2→OH + NO 2 (60%) and (2) HONO 2→HONO + O ( 1D) (40%). The translational energy distribution of reaction (1) shows two components which correspond to formation of stable and unstable (hot) NO 2 photofragments with a ration of 1:2. The decay of hot NO 2 produces the secondary fragments NO + O( 3P). The recoil anisotropy parameters associated with the respective dissociation processes β(1) =−0.6±0.1 and β(2) =−0.6±0.2 are consistent with an electronic transition dipole moment μ of the excitation which is oriented parallel to the line connecting the two terminal O atoms in the parent molecule.

Photodissociation of CF 2BrCl at 193 nm investigated by photofragment translational spectroscopy

The photodissociation of the halon CF 2BrCl has been studied by the time-of-flight crossed-laser molecular-beam method at 193 nm. Two primary decays are operative, the dominant one yielding CF 2Cl+Br ( D (1) 0)=63.5±3 kcal/mol) with Br( 2P 3/2): Br( 2P 1/2)≈2:1 and the second one CF 2Br+Cl ( D (3) 0=78±4 kcal/mol). The large internal energy release gives rise to efficient (≥50%) secondary decays CF 2Cl→CF 2+Cl ( D (4) 0)=51.5±3 kcal/mol) and CF 2Br→CF 2+Br ( D (5) 0=37±5 kcal/mol) and allows assessment of the pertinent dissociation energies D 0.

Photodissociation of methyl nitrate in a molecular beam

The photodissociation of methyl nitrate, CH30N02, has been investigated by photofragment translational spectroscopy. At the photolysis wavelength of 193 nm the predominant primary decay (-70%) involves the fission of the weak CH3O-NO2 bond (Di - 41 kcal/mol) to yield CH3O + NO2 fragment pairs which show a bimodal translational energy distribution. The component consisting of the fast fragment pairs has an average translational energy (ET) = 41 kcal/mol and a recoil anistropy 6 = 0.9, while the slow component is characterized by (ET) = 19 kcal/mol and 0 = 0. A significant portion of the NO2 fragments of the slow component is subject to a unimolecular decay to NO + O(3P). An additional primary decay route leads to the formation of methyl nitrite, CH3ON0, and atomic oxygen O(lD) with (ET) = 6 kcal/mol and /3 = 1.2. This competing reaction requires fission of the strong N-0 bond (@,I - 118 kcal/mol) and its occurrence indicates an initial localization of the photoexcitation on the NO2 moiety in the parent molecule. At higher laser fluence secondary photodissociation of the primary fragments CH3ONO was observed. Photolysis at 248 nm is shown to produce CH30 + NO2 fragment pairs exclusively, with (ET) = 17 kcal/mol. Nitric oxide (NO) and nitrogen dioxide (NOz), collectively denoted as NO,, play a central role in the catalytic and photochemical processes of the atmo~phere.'-~ The most important precursor and product species of atmospheric NO, are nitric acid (HN03), nitrate ion (NOj) in aerosols, and peroxyacetic nitric anhydride (PAN). These compounds and NO, account for most of what is often referred to as reactive oddnitrogen, NO,. Recent atmospheric measurements3q4 indicate that in some cases 742% of the NO, consists of relatively stable organic species, most notably the alkyl nitrates RON02, R = CH3, CzH5, etc. These compounds are formed as minor products in the reaction of organic peroxy radicals with NO and are removed both by reaction with OH radicals and by photolysis with light in the near-UV region. The relative importance of the two destruction mechanisms depends on the atmospheric conditions and on the substituent group R, with the photolytic decay route being most important in the case of methyl nitrate, CH30N02.3 From this point of view the photodissociation mechanism of alkyl nitrates merits attention.

The photofragmentation of methyl hydroperoxide CH 3OOH at 193 and 248 nm in a cold molecular beam

The photodissociation of methyl hydroperoxide (CH 3OOH) has been studied in a molecular beam by photofragment translational spectroscopy. As CH 3OOH rapidly decomposes upon contact with metal, a new metal-free molecular beam source was designed, which is also suited to the generation of beam pulses of corrosive gases. After excitation at 193 and 248 nm, CH 3OOH decays exclusively into CH 3O+OH. At 193 nm, the average kinetic energy of the fragments 〈 E t〉=67 kcal/mol, corresponding to ≈ 65% of the available energy and implying an average internal energy 〈 E int〉=37kcal/mol. A very similar partitioning was observed at 248 nm with 〈 E t〉=48 kcal/mol (≈67%) and 〈 E int>=23 kcal/mol. According to these data, the secondary decay CH 3O→CH 2O+H is expected to be efficient (≈50%) only for λ exc=193 nm.

The near ultraviolet photodissociation dynamics of azomethane

The photodissociation of azomethane following absorption of a single 351 nm photon was studied using the method of molecular beam photofragment translational spectroscopy. The dissociation was observed to proceed via cleavage of both C–N bonds to yield N2 and two methyl radicals. The measured time-of-flight spectra show evidence that the two methyl radicals possess unequal velocities in the azomethane center of mass suggesting that the dissociation is not symmetric. The angles between the asymptotic center-of-mass velocities for all three fragments are strongly correlated, implying that the methyldiazenyl radical (CH3N2) intermediate decomposes within a fraction of its rotational period. We conclude, therefore, that the dissociation is concerted, not stepwise as was inferred from recent time-resolved experiments. The overall translational energy distributions for all the photofragments in the azomethane center of mass reveal that an average of 60% of the total available energy appears as translation. A possible mechanism, consistent with the experimental findings, will be proposed and discussed.

Photodissociation of CFCl 3 at 193 nm investigated by photofragment translational spectroscopy

The collisionless photodissociation of CFCl 3 at 193 nm has been studied with the method of crossed laser and molecular beams. The primary dissociation is shown to produce exclusively the fragment pairs CFCl 2 + Cl with a non-thermal translational energy distribution. The average kinetic energy of 32 kcal/mol corresponds to 42% of the available energy and implies an average internal energy of CFCl 2 of 43 kcal/mol. From the recoil anisotropy parameter β = +0.74 ± 0.15 the photoexcitation is inferred to proceed via an E ← A 1 transition polarized perpendicular to the C–F bond. At higher laser fluence a fraction of the nascent CFCl 2 radicals absorbs a further photon and dissociates into CFCl + Cl; the cross section for this secondary process is estimated to be approximately twice as large as that of the primary photodissociation.

Isotope specific photodissociation of SO 2 at 193.3 nm

The 193 nm photodissociation of the isotopomers SO 2, SO * 2 and SOO * (with O *= 18O) was studied by high-resolution photo-fragment translational spectroscopy. The translational energy distributions P( E T) of the fragment pairs SO+O, SO *+O *, SO+O * and SO *+O were determined from systematic analysis of the time-of-flight distributions obtained with various degrees of O * substitution and were found to be significantly different due to a different ro-vibrational distribution of the diatomic fragments. In the case of the symmetric isotopomers SO 2 and SO * 2 the difference of the P( E T) distributions is attributed to a slightly different state excitation in the C̃ electronic band of the parent molecule. The SO and SO * bond fission of the asymmetric SO0 * have equal probabilities but lead to different energy distributions in the SO and SO * fragments which reflect subtle differences in the dissociation dynamics of the two reaction modes.

Photofragmentation of CF2I2. Competition between radical and three-body dissociation

The photodissociation of CF2I2 following excitation into the first UV absorption band system has been studied by photofragment translational spectroscopy. Time-of-flight and angular distributions of the dissociation products measured at the photolysis wavelengths 351, 337, 308, and 248 nm reveal the competition between a radical channel that forms the fragment pairs CF2I+I and a three-body dissociation that yields CF2+I+I. These processes are found to occur via B1←A1 type electronic transitions. For the iodine atoms produced in the radical channel the spin–orbit branching ratio I*(2P1/2)/I(2P3/2) increases from I*/I=0.02 at 351 nm to I*/I=0.2 at 337 nm and reaches I*/I=2 at 308 nm. This wavelength-dependent selectivity of the I*/I ratio suggests that the absorption band system is composed of at least two overlapping B1←A1 transitions. Although energetically accessible at 351 nm (where CF2I from the radical channel decays spontaneously to CF2+I), the three-body dissociation is observed first at 308 nm as a minor decay mode and becomes the exclusive decay at 248 nm. The dissociation energy for the formation of CF2I+I(2P3/2) was determined to be D0I≤51.3±2 kcal/mol; in the case of the reaction forming CF2+I(2P3/2)+I(2P3/2) the dissociation energy is D0I+D0II≤63.3±2 kcal/mol.

Photodissociation dynamics of CO2 at 157.6 nm by photofragment-translational spectroscopy

The photodissociation of CO2 at 157 nm was studied by the photofragment-translational spectroscopy technique. Product time-of-flight spectra were recorded and center-of-mass translational energy distributions were determined. Two electronic channels were observed—one forming O(1D) and the other O(3P). With previously determined anisotropy parameters of β=2 for the O(3P) channel and β=0 for the O(1D) channel, an electronic branching ratio of 6%±2%O(3P) was obtained, consistent with previous results. The translational energy distribution for the CO(v)+O(3P) channel was very broad (over 30 kcal/mol) and appeared to peak near CO(v=0). The value of β=2 for the O(3P) channel was confirmed by comparing Doppler profiles, derived from our measured translational energy distribution, with previously measured Doppler profiles. This suggests that the O(3P) channel arises from a direct transition to an excited triplet state. The O(1D) channel had a structured time-of-flight which related to rovibrational distributions of the CO product. The influence of the excitation of the CO2(ν2) bending mode was investigated and shown to have a small but not negligible contribution. Based upon a comparison of our data with a previous vacuum-ultraviolet (VUV) laser induced fluorescence study, we obtain as our best estimate of the vibrational branching ratio, CO(v=0)/CO(v=1)=1.9, for the CO(v)+O(1D) channel.

Primary product channels in the photodissociation of methane at 121.6 nm

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH4(CD4) at 121.6 nm. Contrary to the previous consensus view, we find simple C–H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH3 fragments are formed with very high levels of internal excitation: Some (∼25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H+H2+CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH4(Ã 1T2) molecules undergo rapid internal conversion (promoted by the Jahn–Teller distortion of this excited state) to high vibrational levels of the ground state prior to fragmentation. The realization that CH4 photodissociation at 121.6 nm yields CH3 (and CH) fragments, rather than methylene radicals, will necessitate some revision of current models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.

Translational spectroscopy of H(D) atom fragments arising from the photodissociation of H2S(D2S): a redetermination of D00(S–H)

The technique of H/D atom photofragment translational spectroscopy has been used to further investigate the collision-free photodissociation of H2S and D2S molecules both in the near ultraviolet (at 218.2 and 221.6 nm) and in the vacuum ultraviolet (at 121.6 nm). Measurements of the H/D atom photofragment angular distributions confirms that the near UV dissociation occurs promptly, following a perpendicular photo-excitation. More than 99% of the resulting SH/SD fragments are formed in their ground vibronic level, with a ca. 3 : 2 preference in favour of the lower (2Π3/2) spin–orbit component. Product rotation accounts for ca. 1% of the available energy in the case of H2S photolysis at these near UV wavelengths (ca. 2% in the case of D2S dissociation). The groundstate SH/SD photofragments can also be photolysed at these near UV excitation wavelengths. Simulations of the kinetic energy distribution of the resulting H/D atomic fragments show that the secondary photolysis also involves a perpendicular transition, and that the partner S atoms are formed in all three 3PJ spin–orbit states. The product energy disposal following 121.6 nm photolysis of D2S closely parallels that deduced in an earlier study of H2S photodissociation at this same wavelength (Schnieder et al., J. Chem. Phys., 1990, 92, 7027). The D-atom kinetic energy spectrum shows clear evidence for the formation of rovibrationally excited SD(A 2Σ+) fragments amongst the primary products, and also suggests an important role for the three-body dissociation process leading to D + D + S(1D) atoms.Given D00(HS–H)= 31 480 ± 40 cm–1, the present results provide a refined value for the S–D bond strength in the D2S molecule; D00(DS–D)= 32 030 ± 50 cm–1; for the SH and SD radical bond dissociation energies; D00(S–H)= 29 300 ± 100 cm–1 and D00(S–D)= 29 700 ± 100 cm–1, and an improved expression for the potential-energy function for the A 2Σ+ state of the mercapto radical.