Recoil Anisotropy Parameter Research Articles

Photodissociation dynamics of tetrahydrofuran at 193 nm.

Tetrahydrofuran (THF), a cyclic ether with the chemical formula C4H8O, can be considered the simplest analog of the deoxyribose backbone component of deoxyribonucleic acid (DNA). As such, it provides a useful model for probing the photochemistry of such biomolecular motifs. We present a velocity-map imaging study into the ultraviolet dissociation of THF at a wavelength of 193 nm. Excitation to the S1 state occurs via a 3s ← n transition involving a lone-pair electron on the oxygen atom, and has been shown by other authors to result in rapid ring opening via cleavage of one of the C-O bonds to form a ring-opened C4H8O diradical, followed by C-C bond cleavage over a longer timescale to form either OCH2 + C3H6 products (Channel 1a), HOCH2 + C2H5 products (Channel 1b), or OCH2CH2 + C2H4 products (Channel 2). The C2H4O products formed via Channel 2 are unstable on the timescale of our experiment and dissociate further to form CH3 and CHO. We also observe a number of minor products resulting from H or H2 loss from the primary photofragments. The speed distributions observed for all photofragments are broad, indicating excitation of a range of rotational and vibrational states of the products. The angular distributions of the photofragments show an interesting speed dependence: the slowest products have almost isotropic angular distributions, but the magnitude of the recoil anisotropy increases monotonically with photofragment speed. The fastest products exhibit highly anisotropic angular distributions, with the recoil anisotropy parameter β approaching its limiting value of -1 (-0.75 for Channel 1 and -0.5 for Channel 2). This behaviour is attributed to the range of timescales over which the diradical intermediate dissociates into the observed photofragments. Rapid dissociation leads to fast photofragments which retain the correlation between the transition dipole moment for the S1 ← S0 excitation (which lies perpendicular to the ring) and the photofragment velocities (which lie predominantly in the plane of the ring). Slow dissociation results in a high degree of energy redistribution into internal modes, slower photofragments, and loss of correlation between the photofragment velocities and the transition dipole. The higher barrier associated with dissociation via Channel 2 suggests somewhat longer lifetimes for the diradical intermediate and is consistent with a corresponding reduction in the maximum observed value for β.

A quantum wavepacket study of state-to-state photodissociation dynamics of HOBr/DOBr

Photodissociation of HOBr is an important step in the reaction network of the depletion of ozone in stratosphere. Here, we report the first three-dimensional potential energy surfaces for the lowest three singlet states for HOBr, based on high level multi reference configuration interaction calculations. Quantum dynamics calculations are performed with a real wavepacket method, yielding not only absorption spectra but also internal state and angular distributions of the photodissociation fragments. Our results agree quantitatively with the measured total absorption cross sections of HOBr in the ultraviolet region and reproduce well the observed vibrationally cold and rotationally hot OH/OD fragments via photodissociation of HOBr/DOBr at 266 nm. In addition, we predict that the recoil anisotropy parameters for OH/OD are close to the limiting value of a parallel transition, suggesting a rapid dissociation process at 266 nm following an in-plane transition from the ground state (11A′) to the 21A′ state. This is consistent with the experimental conclusion derived from the measured rotational alignment. However, spin and electronic angular momenta need to be taken into account in the future to achieve a more quantitative agreement with experiment. Our work is expected to motivate further experimental investigations for this benchmark system.

Resonance enhanced multiphoton ionisation (REMPI) detection of Cl(2Pj) atom in the photodissociation of halogenated pyrimidines at 235 nm: role of triplet states

ABSTRACTThe dynamics of chlorine atom (2Pj) formation in the photodissociation process of halogen substituted pyrimidines, namely, 2,4,6-trichloropyrimidine and 5-chloro-2,4,6-trifluoropyrimidine have been studied around 235 nm using Resonance Enhanced Multiphoton Ionisation Time-of-Flight Mass Spectrometry technique. For the chlorine atom dissociation channel, we have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio. In both the molecules, the TOF profiles for Cl (2P3/2) and Cl* (2P1/2) are found to be independent of laser polarisation suggesting a zero value for β, within the experimental uncertainties. For 2,4,6-trichloropyrimidine, the average translational energies for Cl and Cl* elimination channels are determined to be 6.0 ± 1.2 and 7.0 ± 1.5 kcal/mol, respectively. Similarly, for 5-chloro-2,4,6-trifluoropyrimidine, the average translational energies for Cl and Cl* elimination channels are determined to be 6.5 ± 1.2 and 7.9 ± 1.6 kcal/mol, respectively. Computational calculations are performed to generate the potential energy curves along the dissociating C-Cl bond using equation of motion coupled cluster with single and double excitations (EOM-CCSD) method. Computational studies suggest the role of triplet states in the photodissociation process forming the Cl atom.

Imaging the Photodissociation Dynamics of Nitrous Acid (HONO): The Role of Torsion.

We present the findings of the first imaging study of trans-HONO and cis-HONO photodissociation through the [Formula: see text] band and [Formula: see text] band of the Ã1A″-X̃1A' transition. The NO photofragment was probed by (1 + 1) resonance-enhanced multiphoton ionization and studied using the direct-current slice imaging technique with our finite slice reconstruction method. The NO state-specific translational energy distributions show some rotational structure corresponding to the internal state distribution in the OH cofragment. All images showed a perpendicular angular distribution with a recoil anisotropy parameter from ca. -0.6 to -0.8. In both bands, cis-HONO showed greater anisotropy than trans-HONO. Deviation from the limiting value expected for a pure perpendicular dissociation is ascribed to deviation of the transition moment from normal to the heavy atom plane owing to OH torsion, rather than lifetime effects assumed in the large body of previous work.

Resonance enhanced multiphoton ionization − time of flight (REMPI-TOF) detection of Br ( 2 P j ) atoms in the photodissociation of 4-bromo-2,3,5,6-tetrafluoropyridine at 234 nm: Effect of low-lying πσ* states

Abstract The photodissociation dynamics of 4-bromo-2,3,5,6-tetrafluoropyridine (BTFP) have been studied in a supersonic molecular beam around 234 nm, which prepares the molecule in its ππ* state. The dynamics of the C Br bond dissociation have been investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS) by detecting the nascent spin orbit states of the primary bromine atoms, namely, Br (2P3/2) and Br*(2P1/2). State specific polarization dependent TOF profiles were obtained, from which the translational energy distributions and recoil anisotropy parameters, βi, were extracted using forward convolution method. A strong polarization dependence of TOF profiles suggests anisotropic distributions of the Br and Br* fragments. Two components, namely, the fast and the slow, are observed in the translational energy distribution of Br and Br* atoms, formed from different potential energy surfaces. The average translational energies released into the Br and Br* channels for the fast component are 17.3 ± 2.0 and 11.1 ± 2.0 kcal/mol, respectively. Similarly, for the slow component, the average translational energies imparted into the Br and Br* channels are 2.6 ± 1.0 and 1.5 ± 1.0 kcal/mol, respectively. The relative quantum yields of Br and Br* are 0.87 ± 0.15 and 0.13 ± 0.06. The anisotropy parameters for Br and Br* are characterized by a similar value of 0.60 ± 0.05. The energy partitioning into the translational modes is interpreted with the help of various models. The experimental studies revealing the nature of translational energies distribution for Br and Br* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and Multi Configuration Quasi Degenerate second order Perturbation Theory (MCQDPT2) suggest that the initially prepared ππ* state crosses over to a nearby σ* repulsive state along the C Br bond, mainly, πσ* state from where the dissociation takes place. The results also indicate the process of fast internal conversion to ground state from the initially prepared ππ* state, which eventually forms slow Br atoms after C-Br bond dissociation process on the ground state. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section. Non-adiabatic curve crossing plays an important role in the C Br bond dissociation of BTFP.

Chlorine atom formation dynamics in the dissociation of halogenated pyridines after photoexcitation at 235 nm: A resonance enhanced multiphoton ionization-time of flight (REMPI-TOF) study

The photodissociation dynamics of halogen substituted pyridines, namely, 3-chloropyridine (ClPy) and 3-chloro-2,4,5,6-tetrafluoropyridine (ClFPy), has been studied around 235nm by detecting chlorine atoms in their spin orbit states Cl(2P3/2) and Cl∗(2P1/2) using the REMPI-TOF technique. We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio, for chlorine atom elimination channels. The TOF profiles for Cl and Cl∗ are found to be independent of laser polarization suggesting a zero value for β, within the experimental uncertainties. For 3-chloropyridine, the average translational energies for Cl and Cl∗ elimination channels are determined to be 3.7±1.0 and 7.0±1.5kcal/mol, respectively. Similarly, for 3-chloro-2,4,5,6-tetrafluoropyridine, the average translational energies for Cl and Cl∗ elimination channels are determined to be 8.0±1.5 and 9.0±1.5kcal/mol, respectively. The theoretical calculation suggests that the fluorine substitution increases the possibility of cross over to the π–σ∗ state from the initially prepared π–π∗ state.

Dynamics of Cl(2Pj) formation in the photodissociation of halogenated thiadiazole at 235 nm: A resonance enhanced multiphoton ionization-time of flight (REMPI-TOF) study

Abstract In one colour experiment, the photodissociation dynamics of halogen substituted thiadiazole, namely, 3,4-dichloro-1,2,5-thiadiazole, has been studied in a supersonic molecular beam around 235 nm, which prepares the molecules in its π–π* state. The study is conducted using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary chlorine atoms in their spin orbit states Cl(2P3/2) and Cl*(2P1/2). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio, for chlorine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least squares fitting method, taking into account the fragment anisotropies, βi. The TOF profiles for Cl and Cl* are found to be independent of laser polarization, i.e., the β is well characterized by a value of ∼0.0, within the experimental uncertainties. The translational energy distributions is well explained with only one component for both Cl and Cl* atom elimination channel. The average translational energies for Cl and Cl* channel are 5.2 ± 1.5 and 7.9 ± 1.5 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The experimental studies revealing the nature of translational energies distribution for Cl and Cl* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and ab initio molecular orbital (MO) theory suggest that the initially prepared π–π* state crosses over to a nearby σ* repulsive state along the C Cl bond, from where the dissociation takes place. The results also indicates the role of fast internal conversion to ground state from the initially prepared π–π* state, from where various dissociation channels occurs. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section.

Resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) study of tetrachloroethylene photodissociation at 235 nm: Role of bound [formula omitted] state

In one-color REMPI-TOF experiment, the photodissociation dynamics of tetrachloroethylene has been studied by probing the chlorine atom photofragments, namely, Cl (2P3/2) and Cl∗ (2P1/2), using 2+1 REMPI scheme in the 234–236nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter (β), and the spin–orbit branching ratio for chlorine atom elimination channels. The β is well characterized by a value of ∼0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 17.6±1.9 and 14.0±1.7kcal/mol, while, that for the slow components are 2.2±1.0 and 3.2±1.0kcal/mol, respectively. To understand the nature of the dissociative potential energy surface involved in the chlorine atom formation channel, detailed theoretical calculations are performed using Time-dependent Density Functional Theory (TD-DFT) method.

Photoexcitation of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) to repulsive surface nσ∗(C–Br) at 234 nm: Dynamics of C–Br and C–Cl bond rupture

The photodissociation dynamics of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) have been studied near 234nm, in a molecular beam environment, employing resonance-enhanced multiphoton ionization with time-of-flight mass spectrometer (REMPI-TOF-MS). Both bromine and chlorine atoms are detected in both the ground and spin–orbit excited states. The C–Br bond scission is observed predominantly due to direct σ∗(C–Br)←n(Br) transition, with translational energy distribution described with a soft impulsive model. The recoil anisotropy parameter (β) of 0.6±0.2 for this channel vindicates the impulsive nature of dissociation. The diabatic crossing from the nσ∗(C–Br) surface to nσ∗(C–Cl) diabatic surface is responsible for chlorine formation via the C–Cl bond fission, which is supported by the excited state molecular orbital calculations. The velocity distribution of bromine atom has one component, however, that of chlorine atom is bimodal. Both fast and slow chlorine atom channels, with the β value of 0.3±0.1, are produced impulsively from the nσ∗(C–Cl) repulsive surface.

Photodissociation Dynamics of Halogenated Thiophenes at 235 nm: A Resonance Enhanced Multiphoton Ionization-Time-of-Flight (REMPI-TOF) Study

The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2-bromo-5-chlorothiophene, has been studied in a supersonic molecular beam around 235 nm, using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary halogen atoms. A single laser has been used for excitation of halothiophenes, as well as for the REMPI detection of photoproducts, namely, chlorine and bromine atoms, in their spin-orbit states X((2)P(3/2)) and X*((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio, for chlorine and bromine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies, β(ι). The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the β is well characterized by a value of ~0.0, within the experimental uncertainties. For 2-chlorothiophene, we have observed two components for the Cl and only one component for the Cl* atom elimination channel in the translational energy distributions. The average translational energies for the fast and the slow components of the Cl channel are 3.0 ± 1.0 and 1.0 ± 0.5 kcal/mol, respectively. For Cl*, the average translational energy is 3.5 ± 1.0 kcal/mol. For 2-bromo-5-chlorothiophene, we have observed only one component for Cl, Cl*, Br, and Br* in the translational energy distributions. The average translational energies for the Cl and Cl* channels are 3.5 ± 1.0 and 5.0 ± 1.0 kcal/mol, respectively, whereas the average translational energies for the Br and Br* channels are 2.0 ± 1.0 and 3.5 ± 1.0 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The ΔH(f)(298) value for 2-chlorothiophene has been estimated theoretically to be 23.5 kcal/mol.

Resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) study of phosphorous oxychloride (POCl3) dissociation at 235 nm: Dynamics of Cl(2Pj) formation

In one-color REMPI-TOF experiment, the photodissociation dynamics of POCl3 has been studied by photolyzing POCl3 and probing the chlorine atom photofragments, namely, Cl(2P3/2) and Cl∗(2P1/2) using 2+1 REMPI scheme, in the 234–236nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter, and the spin–orbit branching ratio for chlorine atom elimination channels. The anisotropy parameters for Cl and Cl∗ are the same, and characterized by a value of 0.0±0.05. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 12.5±1.5 and 16.8±1.5kcal/mol, while, for the slow components, the average translational energies are 1.5±1.0 and 2.5±1.0kcal/mol, respectively. Apart from the chlorine atom elimination channel, Cl2 elimination is also observed in the photodissociation of POCl3.

Dynamics of C[sbnd]Cl bond fission in photodissociation of 2-furoyl chloride at 235 nm

The photodissociation dynamics of 2-furoyl chloride at 235nm has been investigated, employing resonance-enhanced multiphoton ionization technique and time-of-flight mass spectrometry. Both the Cl fragments, Cl(2PJ=3/2, relative quantum yield 0.85±0.11) and Cl∗(2PJ=1/2), have the recoil anisotropy parameter (β) value close to zero, and show bimodal translational energy distributions. The branching ratio of the high kinetic energy CCl bond scission to the low energy CCl scission is 0.78/0.22. The dominant high kinetic energy channel arises mainly because of electronic pre-dissociation. But, the low energy channel results from the ground electronic state of 2-furoyl chloride, formed subsequent to non-radiative relaxation of the initially prepared excited state.

Time-slice velocity-map ion imaging studies of the photodissociation of NO in the vacuum ultraviolet region

The time-slice velocity-map ion imaging and the resonant four-wave mixing techniques are combined to study the photodissociation of NO in the vacuum ultraviolet (VUV) region around 13.5 eV above the ionization potential. The neutral atoms, i.e., N((2)D(o)), O((3)P(2)), O((3)P(1)), O((3)P(0)), and O((1)D(2)), are probed by exciting an autoionization line of O((1)D(2)) or N((2)D(o)), or an intermediate Rydberg state of O((3)P(0,1,2)). Old and new autoionization lines of O((1)D(2)) and N((2)D(o)) in this region have been measured and newer frequencies are given for them. The photodissociation channels producing N((2)D(o)) + O((3)P), N((2)D(o)) + O((1)D(2)), N((2)D(o)) + O((1)S(0)), and N((2)P(o)) + O((3)P) have all been identified. This is the first time that a single VUV photon has been used to study the photodissociation of NO in this energy region. Our measurements of the angular distributions show that the recoil anisotropy parameters (β) for all the dissociation channels except for the N((2)D(o)) + O((1)S(0)) channel are minus at each of the wavelengths used in the present study. Thus direct excitation of NO by a single VUV photon in this energy region leads to excitation of states with Σ or Δ symmetry (ΔΩ = ±1), explaining the observed perpendicular transition.

Ultraviolet photodissociation dynamics of trichloroethylene at 235 nm

The photodissociation dynamics of trichloroethylene was investigated near 235 nm, (π,π*) transition, by detecting the nascent products, Cl (2P3/2) and Cl* (2P1/2), via 2 + 1 resonance enhanced multiphoton ionization. The photofragment speed distribution, the recoil anisotropy parameter β and the spin–orbit branching ratio for chlorine atom elimination channels were determined from time of flight profiles in polarization experiments. Based on the dynamical information, the features of potential energy surface along the reaction coordinate were discussed.

Conformer specific dissociation dynamics of iodocyclohexane studied by velocity map imaging

The photodissociation dynamics of iodocyclohexane has been studied using velocity map imaging following excitation at many wavelengths within its A-band (230 ≤ λ ≤ 305 nm). This molecule exists in two conformations (axial and equatorial), and one aim of the present experiment was to explore the extent to which conformer-specific fragmentation dynamics could be distinguished. Ground (I) and spin-orbit excited (I∗) state iodine atom products were monitored by 2 + 1 resonance enhanced multiphoton ionization, and total kinetic energy release (TKER) spectra and angular distributions derived from analysis of images recorded at all wavelengths studied. TKER spectra obtained at the longer excitation wavelengths show two distinct components, which can be attributed to the two conformers and the different ways in which these partition the excess energy upon C-I bond fission. Companion calculations based on a simple impulsive model suggest that dissociation of the equatorial (axial) conformer preferentially yields vibrationally (rotationally) excited cyclohexyl co-fragments. Both I and I∗ products are detected at the longest parent absorption wavelength (λ ∼ 305 nm), and both sets of products show recoil anisotropy parameters, β > 1, implying prompt dissociation following excitation via a transition whose dipole moment is aligned parallel to the C-I bond. The quantum yield for forming I∗ products, Φ(I∗), has been determined by time resolved infrared diode laser absorption methods to be 0.14 ± 0.02 (at λ = 248 nm) and 0.22 ± 0.05 (at λ = 266 nm). Electronic structure calculations indicate that the bulk of the A-band absorption is associated with transition to the 4A(') state, and that the (majority) I atom products arise via non-adiabatic transfer from the 4A(') potential energy surface (PES) via conical intersection(s) with one or more PESs correlating with ground state products.

Photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol and 2-(bromomethyl) hexafluoro-2-propanol at 234 nm: Resonance-enhanced multiphoton ionization detection of Br (2P j)

The photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol (BTFP) and 2-(bromomethyl) hexafluoro-2-propanol (BMHFP) have been studied at 234 nm, and the C-Br bond dissociation investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS). Br formation is a primary process and occurs on a repulsive surface involving the C-Br bond of BTFP and BMHFP. Polarization dependent time-of-flight profiles were measured, and the translational energy distributions and recoil anisotropy parameters extracted using forward convolution fits. A strong polarization dependence of time-of-flight profiles suggest anisotropic distributions of the Br((2)P(3/2)) and Br((2)P(1/2)) fragments with anisotropy parameter, β, of respectively 0.5 ± 0.2 and 1.2 ± 0.2 for BTFP, and 0.4 ± 0.1 and 1.0 ± 0.3 for BMHFP. The measured velocity distributions consist of a single velocity component. The average translational energies for the Br((2)P(3/2)) and Br((2)P(1/2)) channels are 9.2 ± 1.0 and 7.4 ± 0.9 kcal/mol for BTFP, and 15.4 ± 1.8 and 15.1 ± 2.0 kcal/mol for BMHFP. The relative quantum yields of Br((2)P(3/2)) and Br((2)P(1/2)), which are 0.70 ± 0.14 and 0.30 ± 0.06 in BTFP and 0.81 ± 0.16 and 0.19 ± 0.04 in BMHFP, indicate that the yield of the former is predominant. The measured anisotropy parameters for the Br((2)P(3/2)) and Br((2)P(1/2)) channels suggest that the former channel has almost equal contributions from both the parallel and the perpendicular transitions, whereas the latter channel has a significant contribution from a parallel transition. Non-adiabatic curve crossing plays an important role in the C-Br bond dissociation of both BTFP and BMHFP. The estimated curve crossing probabilities suggest a greater value in BTFP, which explains a greater observed value of the relative quantum yield of Br((2)P(1/2)) in this case.

Dynamics of Cl (2Pj) Atom Formation in the Photodissociation of Fumaryl Chloride (ClCO − CH = CH − COCl) at 235 nm: A Resonance Enhanced Multiphoton Ionization (REMPI) Time-of-Flight (TOF) Study

The photodissociation dynamics of fumaryl chloride (ClCO-CH═CH-COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl ((2)P(3/2)) and Cl* ((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, β(i). The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(E(T)), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.

Photodissociation Dynamics of Phosphorus Trichloride (PCl3) at 235 nm Using Resonance Enhanced Multiphoton Ionization (REMPI) with Time-of-Flight (TOF) Mass Spectrometry

The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersonic beam by resonance enhanced multiphoton ionization (REMPI), using time-of-flight (TOF) mass spectrometry. The study is focused on the nascent state of the primary chlorine atom, formed on excitation of the (n, sigma*) transition of the molecule around 235 nm. Dissociation of PCl(3) and the REMPI detection of chlorine atoms are performed, using the same laser around 235 nm. The photofragments, namely, Cl((2)P(3/2)) and Cl*((2)P(1/2)), are probed, using the 2+1 REMPI scheme in the 234-236 nm region. We have determined the photofragment speed distribution, the recoil anisotropy parameter beta, and the spin-orbit branching ratio for chlorine atom elimination channels. Polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies. The anisotropy parameters for Cl and Cl* are characterized by values of 0.0 +/- 0.05 and 0.20 +/- 0.05, respectively. Two components, namely, the fast and the slow, are observed in the speed distribution (P(v)) of Cl and Cl* atoms, formed from different potential energy surfaces. The average translational energies for the Cl and Cl* channels for the fast component are 29.7 and 30.6 kcal/mol, respectively. Similarly, for the slow component, the average translational energies for the Cl and Cl* channels are 9.5 and 9.1 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of an impulsive model, for the fast component, and a statistical model, for the slow component. Apart from the chlorine atom elimination channel, molecular chlorine (Cl(2)) elimination is also observed in the photodissociation of PCl(3). The observation of the molecular chlorine in the dissociation process and the bimodal translational energy distribution of the chlorine atom clearly indicate the existence of a crossover mechanism from the initially prepared state to the ground state.

Ultraviolet photodissociation of C2F5I with a small and simple photofragment translational spectrometer

Photodissociation dynamics of C(2)F(5)I near 280 and 304 nm has been investigated on a small and simple time-of-flight photofragment translational spectrometer (PTS). On this new PTS, the photolyzed and ionized fragments, not accelerated by electric field, travel freely for a short flight path (<50 mm) and are detected by microchannel plates. In the spectra of the I(*)((2)P(1/2)) channel at 281.73 and 304.02 nm, vibrational peaks with spacing of approximately 350 cm(-1) are partially resolved, indicating the preferential excitation of CF(2) wag mode (nu(11)=366 cm(-1)) of C(2)F(5) photofragment. The fraction of the available energy disposed into the internal energy is higher than 50% for both I(*) channel and I channel, showing the high excitation of vibration in the C(2)F(5) fragments. The fragment recoil anisotropy parameter beta(I(*)), determined to be 1.70 at 281.73 nm and 1.64 at 304.02 nm, reveals that I(*) atoms are produced predominantly from the parallel (3)Q(0) <-- N transition. The anisotropy parameter beta(I), determined to be 1.25 at 279.71 nm and 0.88 at 304.67 nm, implies that I atoms are produced from two excited states, i.e., direct dissociation via the perpendicular (3)Q(1) <-- N transition, and indirect dissociation via the parallel (3)Q(0) <-- N transition then curve crossing to the (1)Q(1) potential energy surface. Analysis on the recent studies with vibrational state resolution in the photodissociation of alkyl iodides in the A band reveals that the "symmetric bending" mode on alpha-carbon of alkyl iodides is the preferential vibrational excitation mode, which can be explained by the classic impulsive model.

Direct Observation of the Primary and Secondary C−Br Bond Cleavages from the 1,2-Dibromopropane Photodissociation at 234 and 265 nm Using the Velocity Map Ion Imaging Technique

Photodissociation dynamics of 1,2-dibromopropane has been investigated at 234 and 265 nm by using the velocity map ion imaging method. At both pump energies, a single Gaussian-shaped speed distribution is observed for the Br*((2)P(1/2)) fragment, whereas at least three velocity components are found to be existent for the Br((2)P(3/2)) product. The secondary C-Br bond cleavage of the bromopropyl radical which is energized from the ultrafast primary C-Br bond rupture should be responsible for the multicomponent translational energy distribution at the low kinetic energy region of Br((2)P(3/2)). The recoil anisotropy parameter (beta) of the fragment from the primary C-Br bond dissociation is measured to be 0.53 (0.49) and 1.26 (1.73) for Br((2)P(3/2)) and Br*((2)P(1/2)), respectively, at 234 (265) nm. The beta value of Br((2)P(3/2)) from the secondary C-Br bond dissociation event at 265 nm is found to be 0.87, reflecting the fact that the corresponding Br((2)P(3/2)) fragment carried the initial vector component of the bromopropyl radical produced from the primary bond dissociation event. Density functional theory has been used to calculate energetics involved both in the primary and in the secondary C-Br bond dissociation dynamics.