Photolysis Wavelength Research Articles

The dynamics of OH channel in the 266 and 355 nm photodissociation of 2-nitrophenol

Photodissociation of gaseous 2-nitrophenol at 266 and 355 nm was investigated by probing the nascent OH products with the laser-induced-fluorescence (LIF) technique. Rotational state distributions of the OH photofragments were measured and characterized by Boltzmann temperatures of 900 ± 40 K and 1070 ± 80 K at 266 and 355 nm, respectively. The OH photoproducts were found to be vibrationally cold at both photolysis wavelengths. Preferential populations of the OH F 1 (Π 3/2) spin–orbit state and Π + Λ-doublet state were observed at both photolysis wavelengths. We have proposed a mechanism for OH production involving the photoisomerization to form aci-nitro isomers, which then dissociate into OH fragments.

“Roaming” Dynamics in CH3CHO Photodissociation Revealed on a Global Potential Energy Surface

We present a quasiclassical trajectory study of the photodissociation of CH3CHO to molecular and radical products, CH4 + CO and CH3 + HCO, respectively, using global ab initio-based potentials energy surfaces. The molecular products have a well-defined potential barrier transition state (TS) but the dynamics exhibit strong deviations from the TS pathway to these products. The radical products are formed via a variational TS. Calculations are reported at total energies corresponding to photolysis wavelengths of 308, 282, 264, 248 and 233 nm. The results at 308 nm focus on a comparison with experiment [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] and the elucidation of the nature and extent of non-TS reaction dynamics to form the molecular products, CH4 + CO. At the other wavelengths the focus is the branching ratio of these products and the radical products, CH3 + HCO.

OH Fragment from Benzoic Acid Monomer Photolysis: Threshold and Product State Distribution

Photodissociation dynamics of benzoic acid monomer (BAM) at different ultraviolet excitation wavelengths (280-295 nm) has been investigated. The nascent OH product state distributions were measured using the laser-induced fluorescence (LIF) technique. The rotational state distributions, the Lambda-doublet-state ratio, and spin-orbit state distributions of the OH fragment were also measured at 280-294 nm. The OH fragments are vibrationally cold, and their rotational state distributions are peaked at J'' = 3.5 at each photolysis wavelength. No LIF signal of OH fragments was observed at 295 nm. The photodissociation threshold is determined to be 102.5-103.9 kcal/mol for OH channel. The dissociative state and mechanism have been discussed for OH produced from the photodissociation of BAM.

Photoinitiated Cationic Polymerization of p-methylstyrene by Methoxy Trityl Salts

Photoinitiated cationic polymerization of p-methylstyrene has been investigated in dichloromethane at 25°C, using stable, soluble, and nonhygroscopic di, tri and pentamethoxy trityl carbocationic salts, having non-nucleophilic anions such as SbF6 −, AsF6 −, PF6 −. The reactivity of these salts falls in the order: trimethoxy > dimethoxy > pentamethoxy. The effects of counter ion structure, salt concentration, photolysis time and photolysis wavelength on the polymerization rate are presented.

Imaging studies of the photodissociation of H2S+ cations. I. Illustrations of the role of nuclear spin

Ion imaging methods have been used to study the dynamics of H(2)(D(2)) molecular elimination from H(2)S(+)(D(2)S(+)) cations following photoexcitation to the A(2)A(1) state in the wavelength range 300<lambda<360 nm. Ground (X (2)B(1)) state parent ions were formed by multiphoton ionization of a jet cooled H(2)S(D(2)S) sample, resonance enhanced at the two photon energy by the v=0 level of the (1)A(2)(... 2b(1) (1)4pb(2)(1)) Rydberg state. This Rydberg excited state predissociates sufficiently slowly that the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum shows resolved rovibronic structure, thereby allowing full quantum state selectivity at this intermediate stage of the cation preparation process. Analysis of the S(+) ion images following one photon excitation of the resulting H(2)S(+)(D(2)S(+)) cations shows that these fragments are formed in their ground (4)S state, and that the H(2)(D(2)) cofragments are formed predominantly (if not exclusively) in rotational states with either odd or even J rotational quantum number--depending on the chosen REMPI preparation wavelength. This striking specificity for forming ortho- or para-H(2)(D(2)) products can be traced back to the state selectivity introduced in the REMPI preparation step. In the case of H(2)S, therefore, the nuclear spin symmetry of the two equivalent H nuclei in ortho-H(2)S (and H(2)S(+)) carries through into ortho-H(2) products, and para-H(2)S molecules map into para-H(2) fragments, surviving photoionization of the Rydberg state, photolysis of the resulting parent cation, and two subsequent radiationless transitions during the evolution from the photoexcited (A(2)A(1) state) cation through to S((4)S)+H(2) products. We identify two distinct fragmentation pathways. One, which we term route I, involves nonadiabatic (Renner-Teller) coupling to the X state at near linear configurations and subsequent (spin-orbit induced) coupling to the repulsive (4)A(2) potential energy surface (PES) at smaller bond angles. This process operates throughout the photolysis wavelength range investigated and yields rotationally "cool" and vibrationally "cold" H(2) products. The second (route II) shows a long wavelength threshold lambda approximately 335 nm, and gradually becomes dominant as the photolysis wavelength is reduced. Route II dissociation involves vibronically facilitated nonadiabatic transfer from the A to the B(2)B(2) state, followed by spin-orbit induced transfer to the (4)A(2) PES; the route II fragmentation dynamics results in H(2) products carrying higher levels of rotational and vibrational excitation.

Wavelength Dependence of Nitrate Radical Quantum Yield from Peroxyacetyl Nitrate Photolysis: Experimental and Theoretical Studies

The photolysis wavelength dependence of the nitrate radical quantum yield for peroxyacetyl nitrate (CH(3)C(O)OONO(2), PAN) is investigated. The wavelength range used in this work is between 289 and 312 nm, which mimics the overlap of the solar flux available in the atmosphere and PAN's absorption cross section. We find the nitrate radical quantum yield from PAN photolysis to be essentially invariant; Phi(NO3)(PAN) = 0.30 +/- 0.07 (+/-2sigma) in this region. The excited states involved in PAN photolysis are also investigated using ab initio calculations. In addition to PAN, calculations on peroxy nitric acid (HOONO(2), PNA) are performed to examine general photochemical properties of the -OONO(2) chromophore. Equation of motion coupled cluster calculations (EOM-CCSD) are used to examine excited state energy gradients for the internal coordinates, oscillator strengths, and transition energies for the n --> pi* transitions responsible for the photolysis of both PNA and PAN. We find in both molecules, photodissociation of both O-O and O-N bonds occurs via excitation to predissociative electronic excited states and subsequent redistribution of that energy as opposed to directly dissociative excitations. Comparison and contrast between experimental and theoretical studies of HOONO(2) and PAN photochemistry from this and other work provide unique insight on the photochemistry of these species in the atmosphere.

UV photodissociation of N-methylpyrrole: The role of 1πσ ∗ states in non-hydride heteroaromatic systems

The photophysics of jet-cooled N-methylpyrrole molecules following excitation to their first excited singlet state (the 1A 2 state, arising from a 3s/σ ∗←π electron promotion) has been investigated by resonance enhanced multiphoton ionisation spectroscopy, by measurements of wavelength resolved ‘action’ spectra for forming CH 3 photoproducts, and by velocity map imaging studies of these CH 3 products (in their v = 0 and v 2 = 1 vibrational levels). CH 3 products are observed at all excitation wavelengths within the NMP ( 1 A 2 ← X ∼ 1 A 1 ) absorption band. Direct dissociation on the 1A 2 potential energy surface (PES) yields ‘fast’ CH 3 fragments, with an average total kinetic energy release (TKER) of ∼6500 cm −1, but this product channel is only observed in a narrow wavelength range near the absorption band origin. All of the measured CH 3 images also show a broad component, peaking at lower TKER (∼1700 cm −1); this component extends beneath the ‘fast’ feature in images recorded at wavelengths near the 1 A 2 ← X ∼ 1 A 1 origin, and accounts for all of the CH 3 products observed at shorter photolysis wavelengths. These products are attributed to decay of highly vibrationally excited ground state molecules formed by radiationless transfer from the 1A 2 state. Similarities and differences with the results of previous studies of the H + pyrrolyl products arising in the UV photodissociation of pyrrole are discussed in terms of the likely nuclear motions on the relevant ground and excited PESs (along R N – CH 3 / R N – H ), and the possible couplings between these surfaces. The present study confirms that the proposed model of 1πσ ∗ state induced bond fission in heteroaromatic molecules [A.L. Sobolewski, W. Domcke, Chem. Phys. 259 (2000) 181] is also applicable to non-hydride substituted heteroaromatics, but that mass effects can have an important influence on the subsequent nuclear dynamics.

Speed dependent rotational angular momentum polarization of the O2 (aΔg1) fragment following ozone photolysis in the wavelength range 248–265nm

The translational anisotropy and rotational angular momentum polarization of a selection of rotational states of the O2 (a 1Deltag; v=0) photofragment formed from ozone photolysis at 248, 260, and 265 nm have been determined using the technique of resonance enhanced multiphoton ionization in combination with time of flight mass spectrometry. At 248 nm, the dissociation is well described as impulsive in nature with all rotational states exhibiting similarly large, near-limiting values for the bipolar moments describing their angular momentum alignment and orientation. At 265 nm, however, the angular momentum polarization parameters determined for consecutive odd and even rotational states exhibit clear differences. Studies at the intermediate wavelength of 260 nm strongly suggest that such a difference in the angular momentum polarization is speed dependent and this proposal is consistent with the angular momentum polarization parameters extracted and reported previously for longer photolysis wavelengths [G. Hancock et al., Phys. Chem. Chem. Phys. 5, 5386 (2003); S. J. Horrocks et al., J. Chem. Phys. 126, 044308 (2007)]. The alternation of angular momentum polarization for successive odd and even J states may be a consequence of the different mechanisms leading to the formation of the two O2 (a 1Deltag) Lambda doublets. Specifically, the involvement of out of plane parent rotational motion is proposed as the origin for the observed depolarization for the Delta- relative to the Delta+ state.

A directly-dissociative stepwise reaction mechanism for gas-phase peroxyacetic acid

The dissociation of peroxyacetic acid (PAA, CH 3C(O)OOH) was studied by UV laser photolysis in the gas phase under collision-free conditions. Three pathways are proposed for PAA dissociation that ultimately produce CH 3 + CO 2 + OH. Two pathways are stepwise with sequential bond dissociation and the other pathway is concerted with simultaneous fission of the same bonds. The laser-induced fluorescence (LIF) spectrum of OH was analyzed after 266 and 240 nm photolysis of PAA. OH is produced in the 2Π ground electronic state with no vibrational energy ( v ″ = 0 ) and rotational distributions that peak at N″ ≈ 3–4 and extend to N″ = 11 at both photolysis wavelengths. Spin-orbit population ratios of the ground electronic state were slightly above unity for both studies. The average OH Λ-doublet ratio was 1.07 ± 0.14 after 266 nm photolysis, which indicates that OH is produced by torsional forces. At 240 nm PAA photolysis, the Λ-doublet ratio increased from unity at N″ = 4 to a statistical value of ∼2 at N″ = 11. Average OH translational energies determined from Doppler profiles were 34 ± 2 and 43 ± 2 kcal mol −1 at 266 and 240 nm photolysis, respectively. The average speed of OH is used to determine the total translational energy of products for the proposed stepwise and concerted pathways. The calculated total translational energies are compared to thermochemical available energy calculations for each pathway to determine the viability of the reactions. Experimental results indicate that dissociation occurs from an excited state, so a comparison of 266 and 240 nm photolysis data is made to discover the direct versus indirect nature of the mechanism. The UV spectra of PAA, acetic acid, hydrogen peroxide, and methylhydroperoxide are reviewed in light of the results. This study concludes that PAA photolysis occurs through initial stepwise O O bond dissociation by a directly-dissociative mechanism from a highly-repulsive electronic state like the hydroperoxides and an indirect mechanism from an exit channel barrier similar to acetic acid is not accessed at these wavelengths.

State-selective photodissociation dynamics of formaldehyde: Near threshold studies of the H+HCO product channel

The laser-induced photodissociation of formaldehyde in the wavelength range 309<lambda<330 nm has been investigated using H (Rydberg) atom photofragment translational spectroscopy. Photolysis wavelengths corresponding to specific rovibronic transitions in the A (1)A2<--X (1)A1 2(0)(1)4(0)(3), 2(0)(2)4(0)(1), 2(0)(2)4(0)(3), 2(0)(3)4(0)(1), and 2(0)(1)5(0)(1) bands of H(2)CO were studied. The total kinetic energy release spectra so derived can be used to determine partial rotational state population distributions of the HCO cofragment. HCO product state distributions have been derived following the population of various different N(Ka) levels in the A (1)A2 2(2)4(3) and 2(3)4(1) states. Two distinct spectral signatures are identified, suggesting competition between dissociation pathways involving the X (1)A1 and the a (3)A2 potential energy surfaces. Most rovibrational states of H(2)CO(A 1A(2)) investigated in this work produceH+HCO(X (2)A') photofragments with a broad kinetic energy distribution and significant population in high energy rotational states of HCO. Photodissociation via the A (1)A2 2(2)4(3) 1(1,1) (and 1(1,0)) rovibronic states yields predominantly HCO fragments with low internal energy, a signature that these rovibronic levels are perturbed by the a (3)A2 state. The results also suggest the need for further careful measurements of the H+HCO quantum yield from H(2)CO photolysis at energies approaching, and above, the barrier to C-H bond fission on the a (3)A2 potential energy surface.

UV Photodissociation Dynamics of HN3 in 190-248 nm

The H+N3 channel in the ultraviolet photodissociation of HN3 has been investigated from 190 nm to {248 nm} using the high-n Rydberg H-atom time-of-flight technique. Product translational energy distributions as well as product angular anisotropy parameters were determined for the H+N3 channel at different photolysis wavelengths. N3 vibrational state distribution has also been derived from the product translational energy distribution at these wavelengths. Above photolysis wavelength 225 nm, HN3 predominantly dissociate through the repulsive state. Below 225 nm, a new slow channel starts to appear at 220 nm in addition to the existing channel. This channel is attributed to a ring closure dissociation channel to produce the cyclic N3 product. As photolysis energy increases, this new channel becomes more important.

Laser-induced fluorescence study of the hydrogen atom formation dynamics in the 248 nm gas-phase photodissociation of vibrational state selected water (H2O (|04−〉))

The vibrationally-mediated H2O gas-phase photodissociation was studied at a photolysis wavelength of 248 nm. Single rotational states of the |03−〉|2〉 and |04−〉 H2O overtone vibrations were prepared by laser photoexcitation around 720 nm. H atoms formed in the photodissociation of the H2O (|04−〉 $$J'_{K_a K_c } $$ = 313) were detected by Lyman-α laser-induced fluorescence spectroscopy with sub-Doppler resolution to determine their translational energy. The present result confirms that in the dissociation process the major part (ca. 93%) of the available energy is released as relative translational energy of the nascent H + OH photofragments, in agreement with earlier complementary measurements (R. L. Vander Wal, J. L. Scott and F. F. Crim, J. Chem. Phys. 94, 1859 (1991)), where the internal excitation of the OH product radical was investigated at different photolysis wavelengths.

Probing the O2 (aΔg1) photofragment following ozone dissociation within the long wavelength tail of the Hartley band

The technique of resonance enhanced multiphoton ionization (REMPI) has been used in conjunction with time-of-flight mass spectrometry (TOFMS), to investigate the dynamics of ozone photolysis in the long wavelength region of the Hartley band (301-311 nm). Specifically, both the translational anisotropy and the rotational angular momentum orientation of the O(2) (a (1)Delta(g); nu=0, J=16-20) fragments have been measured as a function of photolysis wavelength. Within this region, the thermodynamic thresholds for the formation of these products in combination with O ((1)D(2)) are approached and passed, and consequently these studies have allowed an investigation into the effects on the dynamics of slowing fragment recoil velocities and the increasing importance of vibrationally mediated photolysis. The determined beta parameters for all the J states probed follow a similar trend, decreasing from a value typical for the initial (1)B(2)<--(1)A(1) excitation responsible for the Hartley band [for example, beta=1.40+/-0.12 for the O(2) (a (1)Delta(g); J=18) fragment], to a much lower value beyond the thermodynamic threshold for the fragment's production (for example, beta=0.63+/-0.19 for the J=18 fragment following photolysis at 311 nm). This trend, similar to that observed when probing the atomic fragment in a previous set of experiments, [Horrocks et al., J. Chem. Phys. 125, 133313 (2006); Denzer et al., Phys. Chem. Chem. Phys. 16, 1954 (2006)] is consistent with the photodissociation of vibrationally excited ozone molecules beyond the threshold wavelengths and we estimate approximately 1/3 of this to be from excitation in the nu(3) asymmetric stretching mode. These observations are substantiated by the values of the beta(0) (2)(2,1) orientation moment measured, which for photolysis at 301 nm are negative, indicating that a bond opening mechanism provides the key torque for the departing O(2) fragment. The orientation moment becomes positive again for photolysis beyond threshold, however, as the increasing impulsive dissociation again begins to dominate the nature of the rotation of the departing molecular fragment. In addition, a (2+2) REMPI scheme has been utilized to probe the O(2) (a (1)Delta(g)) "low" J fragments, where the majority of the population resides following photolysis within this region. The REMPI-TOFMS technique has been used to confirm the rotational character of a spectral feature through examination of the signal line shapes obtained using different experimental geometries. The dynamical information subsequently obtained, probing the "low" J O(2) (a (1)Delta(g)) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O ((1)D(2)) atomic fragment with data for the O(2) (a (1)Delta(g); J=16-20) fragments.

Imaging the photodissociation of CH3SH in the first and second absorption bands: The CH3(X̃A12)+SH(XΠ2) 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).

Dependence of the nascent vibrational distribution of NO(v) on the photolysis wavelength of NO2 in the range λ = 266–327 nm measured by time-resolved Fourier transform infrared emission

Time-resolved FTIR has been used to study the photodissociation of NO2 at photolysis wavelengths of lambda = 266, 282, 295, 308, 320 and 327 nm. Nascent vibrational distributions of the NO(v) fragment have been determined at all wavelengths: 266 nm photolysis populates NO(v = 1-7) and shows a distribution that is inverted at v = 5, whereas 327 nm photolysis populates NO(v = 1-3) and is inverted at v = 2. Surprisal plots were performed on the nascent distributions presented in this work and on all data sets available in the literature in the range 266-370 nm in order to parameterise the wavelength dependence of the nascent distribution of NO(v) from NO2 photolysis. In general, the surprisal parameter was found to be a linear function of wavelength and was used to generate a simple model for the wavelength dependence of the nascent distribution of NO(v) in the region lambda = 265-372.5 nm. The results give a more accurate parameterisation of the formation of NO(v) of use in atmospheric modelling.

Fully state-selected VMI study of the near-threshold photodissociation of NO2: variation of the angular anisotropy parameter

Velocity-map ion imaging (VMI) has been used to study the angular distribution of the NO fragment generated in the photodissociation of NO(2) at a variety of photolysis wavelengths. Images were recorded for the channels NO (2)Pi(1/2) (v = 0, J= 3/2, 11/2 and 21/2) + O ((3)P(2,1)), for excitation energies ranging from the onset (E(avl)/hc = 0 cm(-1)) to E(avl)/hc approximately 900 cm(-1). The angular anisotropy parameter beta was obtained as a function of available energy. Photofragment multiphoton ionization (PHOMPI) spectra were also recorded in the energy range E(avl)/hc = 0-300 cm(-1) for each of these channels. Large fluctuations of beta as a function of E(avl) were found in all observed dissociation channels. These variations are discussed in terms of the lifetimes of the originally photoexcited overlapping resonances in the A(2)B(2) state of NO(2), the dynamics of which are strongly influenced by nonadiabatic coupling with the X[combining tilde](2)A(1) state. The potential use of this photolysis process for production of cold oxygen atoms is discussed.

The 1-bromoheptane photodissociation near 234 nm

The photodissociation of n-C 7H 15Br, near the red wing of the absorption A-band, has been investigated utilizing the velocity mapped ion imaging technique. Both the speed and angular distributions of Br ∗( 2P 1/2) and Br( 2P 3/2) fragments were determined. The experimental data indicate that the photodissociation in the investigated range originates from adiabatic and non-adiabatic transitions of the three low-lying dissociative electronic states. The fragments kinetic energies and the angular distributions show that about 70% energy is deposited into the internal energy of the fragments. These results can be explained using soft impulsive model. The experiment also shows that the anisotropy parameter for Br ∗ is sensitive to the photolysis wavelength.

Vector properties of the O(D21) fragment produced from the photolysis of ozone in the wavelength range of 298to320nm

The speed averaged translational anisotropy and electronic angular momentum polarization of the O(1D2) atomic fragment formed from the photodissociation of ozone in the atmospherically important long wavelength region of the Hartley band (298 to 320 nm) have been measured using resonance enhanced multiphoton ionization time of flight mass spectrometry. The translational anisotropy parameter, beta, is found to decline from 1.1 for photolysis at 300 nm to a minimum value of 0 at 310 nm which is the threshold for production of O(1D2) in conjunction with the O2(a 1Deltag v = 0) molecular cofragment. For photolysis wavelengths greater than 310 nm, O(1D2) is formed from the dissociation of internally excited ozone molecules. The corresponding beta parameters are markedly lower than for atomic fragments produced with the same speed from the photolysis of ground state ozone molecules. This result is consistent with two different pathways contributing to the photolysis of internally excited ozone at the longest wavelengths studied corresponding to initial internal excitation either in the symmetric or asymmetric stretching vibration. In addition, the polarization of the atomic angular momentum has been determined with the incoherent polarization parameters a0(2)(||) and a0(2)(_|) increasing from values of -0.53 and -0.62 at 300 nm to -0.37 and -0.19 at 317 nm, consistent with the increasing contribution from the photolysis of internally excited ozone as the dissociation wavelength lengthens. Evaluation of these alignment parameters allows the populations of the magnetic substrates, mj, to be determined. For example, for a photolysis wavelength of 303 nm the populations of mj = 0, +/- 1, +/- 2 are in the ratio of 0.36: 0.56: 0.08 and this ratio is essentially independent of the photolysis wavelength. The coherent contribution to the atomic polarization is quantified by the Re{a1(2)(||, _|)} and Im{a1(1)(||, _|)} parameters and these are found to vary from -0.21 and 0.21 at 300 nm to -0.04 and 0.24 at 313 nm, respectively.

Microsecond UV flash photolysis of Co 2(CO) 8 in solution: Wavelength dependence of the Co(CO) 4/Co 2(CO) 7 branching ratio

Time-resolved step-scan Fourier transform infrared spectroscopy is used to observe, for the first time in room-temperature solution, the coordinatively unsaturated species Co(CO) 4 and Co 2(CO) 7, the products of UV flash photolysis of Co 2(CO) 8. There appears to be minimal interaction between Co(CO) 4 and the solvent, but significant solvation of ‘Co 2(CO) 7’. The Co 2(CO) 7:Co(CO) 4 ratio depends very strongly on the photolysis wavelength: photolysis in n-heptane at 266 nm or 308 nm produces [Co 2(CO) 7] and [Co(CO) 4] in a ratio of ∼2:1, but the ratio following 355 nm photolysis is ∼1:4. The wavelength dependence of the branching ratio is discussed in terms of the electronic spectrum of Co 2(CO) 8.

Photodissociation of Mg+–XCH3 (X=F, Cl, Br, and I) complexes. II. Fragment angular and energy distributions

Angular and energy distributions of photofragments from Mg+-XCH3 (X=F, Cl, Br, and I) were deduced from time-of-flight (TOF) profiles measured by rotating the polarization direction of the dissociation laser with respect to ion beam direction. The TOF profiles of ICH3+ and MgI+ fragment ions produced from Mg+-ICH3 complex with 266 and 355 nm photons showed clear but opposite recoil anisotropy to each other. In addition, BrCH3+ formed by a dissociation of the Mg+-BrCH3 complex at a photolysis wavelength of 266 nm also showed an anisotropic distribution in the TOF profile which had the same behavior as the profile of ICH3+. For Mg+-FCH3 complex, CH3+ and MgF+ formed with a 266 nm photon had also spatial anisotropy, in which the TOF profile of MgF+ was almost opposite to that of MgI+. These anisotropic distributions were explained by (1) local excitation on the Mg+ ion, (2) rapid dissociation compared with a rotational period of the parent complex, and (3) geometrical structures of the parent complexes. Anisotropy beta parameter values were determined to be +1.30(ICH3+), -0.50(MgI+), +0.74(BrCH3+), and +0.75(CH3+ and MgF+). This dependence on the halogen atom observed in beta values was qualitatively explained by both the geometrical parameters and classical rotational periods of parent complexes. In the product energy distribution, 46%, 40%, 21%, 16%, and 16% of available energies were found to be transferred into translational energies of ICH3+, MgI+, BrCH3 +, CH3+, and MgF+, respectively. These values were compared with energy distributions estimated by a statistical prior distribution and a nonstatistical impulsive model. For ICH3+ and MgI+, the translational energies determined from the measurement had values between those estimated from statistical and nonstatistical models. On the other hand, the energy partitioning for the product ions of BrCH3+, CH3+, and MgF+ was found to be almost statistical. From these considerations, we concluded that nonstatistical processes were more important in the dissociation of Mg+-ICH3 than in other systems.