Atom Photofragment Research Articles

Structural Effects on the Energy Disposal and Atomic Photofragment Alignment for the Photodissociation of Alkyl Iodides at Excitation Wavelengths of 254 and 268 nm.

This work represents a step forward in the understanding of the widely studied photodynamics of alkyl iodides in the first absorption band. Ultraviolet (UV) photodissociation of several alkyl iodides (RI), specifically, a series of linear and ramified molecules with R = CnH2n+1, n = 1-4, at excitation wavelengths of 254 and 268 nm, which correspond to the maximum of the first absorption A-band, has been studied by combining resonance-enhanced multiphoton ionization (REMPI) detection of atomic photofragments I(2P3/2) and I*(2P1/2) and of pulsed slice imaging. Detailed examination of the total translational energy distributions of both atomic photofragments has been combined with stereodynamical information on the process obtained from the anisotropy β and alignment a02(∥) and Re[a12(∥, ⊥)] parameters to provide a description of the role played by the molecular structure of alkyl iodides in adiabatic and, especially, in nonadiabatic photodissociation dynamics through conical intersections or avoided crossings. The present results suggest that the linear structures couple more efficiently with the pure C-I reaction coordinate, whereas for the branched structures, the coupling with additional vibrational (bending) modes gains importance, showing the dissociation process a multidimensional character. In addition, a large degree of cofragment rotational alignment has been found for the small linear CH3I and C2H5I and, unexpectedly, for the branched t-C4H9I (C3v symmetry), whereas the rest of the alkyl iodides show low alignment parameters.

Ultraviolet Photodissociation Dynamics of the 1-Methylallyl Radical.

The ultraviolet (UV) photodissociation dynamics of the 1-methylallyl (1-MA) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the wavelength region of 226-244 nm. The 1-MA radicals were produced by 193 nm photodissociation of the 3-chloro-1-butene and 1-chloro-2-butene precursor. The 1 + 1 REMPI spectrum of 1-MA agrees with the previous UV absorption spectrum in this wavelength region. Quantum chemistry calculations show that the UV absorption is mainly attributed to the 3pz Rydberg state (perpendicular to the allyl plane). The H atom photofragment yield (PFY) spectrum of 1-MA from 3-chloro-1-butene displays a broad peak around 230 nm, while that from 1-chloro-2-butene peaks at ∼236 nm. The translational energy distributions of the H atom loss product channel, P (ET)'s, show a bimodal distribution indicating two dissociation pathways in 1-MA. The major pathway is isotropic in product angular distribution with β ∼ 0 and has a low fraction of average translational energy in the total excess energy, ⟨fT⟩, in the range of 0.13-0.17; this pathway corresponds to unimolecular dissociation of 1-MA after internal conversion to form 1,3-butadiene + H. The minor pathway is anisotropic with β ∼ -0.23 and has a large ⟨fT⟩ of ∼0.62-0.72. This fast pathway suggests a direct dissociation of the methyl H atom on a repulsive excited state surface or the repulsive part of the ground state surface to form 1,3-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03-0.08.

Imaging the Photodissociation Dynamics and Fragment Alignment of CH2BrI at 193 nm.

The photodissociation dynamics and photofragment alignment of bromoiodomethane (CH2BrI) have been studied at 193 nm using a double experimental and theoretical approach. In addition, the ultraviolet (UV)-vacuum ultraviolet (VUV) absorption spectrum of gas phase CH2BrI has been measured in the photon energy range of 5-11 eV using the VUV Fourier transform spectrometer (FTS) at the VUV beamline DESIRS of the synchrotron SOLEIL facility. The slice imaging technique in combination with resonance enhanced multiphoton ionization (REMPI) detection of the Br(2PJ) and I(2PJ) (with J = 3/2 and 1/2 for Br/I and Br*/I*, respectively) atomic photofragments have been used to produce experimental translational energy and angular distributions, which were analyzed to deliver, on one hand, the partitioning of the available energy among the different degrees-of-freedom of the photofragments and, on the other, the photofragment polarization in terms of aqk(p) alignment parameters. The experimental measurements were rationalized in terms of high-level ab initio calculations of vertical excitation energies, transition dipole moments and potential energy curves (PECs) along different reaction coordinates to provide a complete picture of the photodissociation dynamics. The results indicate that for excitation at 193 nm, prompt C-X cleavage (with X being either halogen atom, Br or I) competes with fast internal conversion and consequent stochastic dissociation in lower electronic states. In the case of the CH2Br + I(2P3/2)/I*(2P1/2) channels, the dynamics are greatly biased toward the stochastic dissociation process due to both the particular PECs landscape and the unfavored excitation of the CH2BrI ensemble with respect to the C-I molecular axis at this excitation energy. The ab initio PECs provide a tentative path for the fast dissociation process in either case. For the C-Br bond breakage, excitation to the 13A' electronic state and predissociation through the 11A'/11A″ or 12A'/12A″ states, leading to direct dissociation through the 10A'/9A″ states, appear as the most consistent dynamics. For the C-I channel, predissociation does not become a reliable possibility and a fast internal conversion may precede dissociation through the repulsive 6A'/6A″ and 4A'/4A″ states. The large content of rotational and vibrational excitation of the polyatomic cofragments is justified through the soft impulsive model and the geometrical changes produced along the dissociation pathway. Strikingly, the aqk(p) alignment parameters obtained for the Br(2P3/2) and I(2P3/2) photoproducts indicate that the rotational angular momentum of the CH2X (X = I or Br) cofragment appears highly constrained along the recoil direction. Finally, this work presents a highly plausible explanation for the branching ratio of secondary dissociation processes in the photodynamics of CH2BrI at 193 nm.

Photodissociation Dynamics of Vinoxy Radical via the B̃2A″ State: The H + CH2CO Product Channel.

Photodissociation dynamics of the jet-cooled vinoxy radical (CH2CHO) via the B̃2A″ state was studied in the near-ultraviolet (near-UV) region of 308-328 nm using high-n Rydberg H atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The vinoxy radical beam was produced by 193 nm photolysis of ethyl vinyl ether followed by supersonic expansion. The H + CH2CO product channel was observed directly in the H atom TOF spectra. The H atom photofragment yield (PFY) spectra were obtained by integrating the H atom TOF spectra and measuring the H atom REMPI signals, and showed several vibronic bands of the B̃2A″ state. The translational energy distributions of the H + CH2CO products, P(ET)'s, were obtained at several vibronic transitions. The P(ET) distributions were broad, peaking at a low energy of ∼3500 cm-1. The product translational energy release was moderate; the average translational energy release in the maximum available energy, ⟨fT⟩, was in the range of 0.24-0.27. The product angular distributions in this wavelength region were slightly anisotropic, with the β parameter in the range of 0.10-0.24. The near-UV photodissociation mechanism of the H + CH2CO product channel of the vinoxy radical is consistent with unimolecular dissociation on the electronic ground state (X̃2A″) following internal conversion from the B̃2A″ state to the Ã2A' state and then to the X̃2A″ state (although unimolecular dissociation from the first excited Ã2A' may also contribute).

Photodissociation Dynamics of H2O via the Ẽ' (1B2) Electronic State.

Photodissociation dynamics of H2O via the Ẽ'1B2 state were studied using the high-resolution H atom photofragment translational spectroscopy method, in combination with the tunable vacuum ultraviolet free electron laser (VUV FEL). The measured translational energy spectra allow us to determine the respective quantum state population distributions for the nascent OH(X2Π) and OH(A2Σ+) photofragments. Analyses of the quantum state population distributions show both the ground and electronically excited OH fragments to be formed with moderate vibrational excitation but with highly rotational excitation. Unlike the dissociation via the lower-lying electronic states, where OH(X) is the major fragment, the OH(A) products are predominant via the Ẽ' state. These products are mainly ascribed to a fast dissociation on the B̃1A1 state surface after nonadiabatic transitions from the initial excited Ẽ' state to the B̃ state. Meanwhile, another dissociation pathway from the Ẽ' state to the 1B2 3pb2 state, followed by coupling to the 1A2 3pb2 state, is also observed, which yields the OH(X) + H and O(3P) + 2H products.

Effects of Ring Fluorination on the Ultraviolet Photodissociation Dynamics of Phenol.

The dynamics of photoinduced O-H bond fission in five fluorinated phenols (2-fluorophenol, 3-fluorophenol, 2,6-difluorophenol, 3,4,5-trifluorophenol, and pentafluorophenol) have been investigated by H Rydberg atom photofragment translational spectroscopy following excitation at many wavelengths in the range 220 ≤ λ ≤ 275 nm. The presence of multiple fluorine substituents reduces the efficiency of O-H bond fission (by tunneling) from the first excited (11ππ*) electronic state, whereas all bar the perfluorinated species undergo O-H bond fission when excited at shorter wavelengths (to the 21ππ* state). As in bare phenol, O-H bond fission is deduced to occur by non-adiabatic coupling at conical intersections between the photoprepared "bright" ππ* states and the 11πσ* potential energy surface. In all cases, the fluorophenoxyl photoproducts are found to be formed in a range of vibrational levels, all of which include an odd number of quanta (typically one) in an out-of-plane (a″) vibrational mode; this product vibration is viewed as a legacy of the parent out-of-plane motions that promote non-adiabatic coupling to the dissociative 11πσ* potential. The radical products also show activity in in-plane vibrations involving coupled (both in- and out-of-phase) C-O and C-F wagging motions, which can be traced to the impulse between the recoiling O and H atoms and, in detail, are sensitive to the presence (or not) of an intramolecular F···H-O hydrogen bond. Upper limit values for the O-H bond dissociation energies are reported for all molecules studied apart from pentafluorophenol.

H atom Product Channels in the Ultraviolet Photodissociation of the 2-Propenyl Radical.

The H atom product channels in the ultraviolet photodissociation of 2-propenyl (CH2CCH3) radical were investigated in the wavelength region 224-248 nm using photofragment translational spectroscopy. The CH2CCH3 radicals were generated by 193 nm photodissociation of 2-chloropropene and 2-bromopropene precursors. The H atom photofragment yield spectra from both precursors revealed a broad feature peaking near 232 nm. The translational energy distributions of the H + C3H4 products peaked around 7-8 kcal/mol and extended close to the maximum excess energy. The fraction of the total available energy released as products' translation was nearly a constant (∼0.16 using the 2-chloropropene precursor and ∼0.18 using the 2-bromopropene precursor) in the wavelength range 224-248 nm. The angular distribution of the H atom product was isotropic. Quasi-classical trajectory (QCT) calculations were performed on the ground-state potential energy surface of CH2CCH3 for its decomposition at a 124 kcal/mol excitation energy (equivalent to 230 nm photolysis photon energy). The calculations yielded branching ratios for different dissociation product channels, 32% H + allene, 35% H + propyne, 0.5% H + cyclopropene, and 32% methyl + acetylene. The experimental and QCT translational energy distributions of the H atom loss channels qualitatively agreed, consistent with the main H atom product channels being the H + allene and H + propyne dissociations. The time scale of the 2-propenyl dissociation on the ground electronic state was calculated to be ∼2 ps, smaller compared to that of the overall UV photodissociation (≥10 ps, implied on the basis of the isotropic H atom product angular distribution). The mechanism of the UV photodissociation of 2-propenyl is consistent with unimolecular dissociation proceeding on the ground electronic state after internal conversion of the electronic excited states.

Stereodynamic Imaging of Bromine Atomic Photofragments Eliminated from 1-Bromo-2-methylbutane Oriented via Hexapole State Selector.

Both single-laser and two-laser experiments were conducted to look into the ion-imaging of Br*(2P1/2) and Br(2P3/2) photofragmented from 1-bromo-2-methylbutane in the range 232-240 nm via a detection scheme of (2+1) resonance-enhanced multiphoton ionization. The angular analysis of these photofragment distributions yields the anisotropy parameter β = 1.88 ± 0.06 for the Br* excited state which arises from a parallel transition, while β = 0.63 ± 0.09 for the Br ground state indicates the contribution from both a perpendicular transition and a non-adiabatic transition. When a hexapole coupled with an orienting field was implemented, the parent molecules are spatially oriented to yield an orientation efficiency |⟨cosθ⟩| of 0.15. Besides the χ angle between the recoil velocity v and the transition dipole moment μ, orienting molecules allows for the evaluation of the angle α between v and the permanent molecular dipole moment d. The angular analysis of Br* photofragment distribution yields χ = 11.5° and α in the range from 160° to 180° with weak dependency. In the two-laser experiments, the angular anisotropy of Br photofragment distribution was found to be smaller (0.38 ± 0.10) when the photolysis wavelength was red-shifted to 240 nm, suggesting the increasing contributions from perpendicular transitions.

Detection of the O2 A'3ΔU Herzberg III state by photofragment imaging.

Photofragment imaging is shown to provide a sensitive method for detection of the O2 A'3Δu Herzberg III state using a one-laser dissociation/O(1D) resonance enhanced multiphoton ionization (REMPI) scheme with a focused nanosecond dye laser beam tuned to 203.8 or 205.2 nm, combined with velocity map imaging of the atomic oxygen photofragment. O2 populated in the Herzberg states is generated by photodesorption at 250 nm of solid O2 ice held at 15 K and by an electric discharge in a pulsed molecular beam of pure O2. Ice photo-desorption results in Herzberg state products with higher translational, vibrational and rotational energy spreads, yielding the same signal as the discharge source but with lower velocity resolution. A clear signal with parallel character (β ∼ 0.9) assigned to dissociation of O2 A'3Δu(v = 0, 1 Ω = 1) was observed when using a pulsed electric discharge source under specific 'cold' conditions with O(1D) detection, driving one-photon dissociation around 205 nm. No products corresponding to O2 A'3Δu state dissociation were observed for 225.625 or 200.32 nm dissociation with O(3P2) detection, which implies that the O2 A'3Δu state dissociates exclusively to the third (O1D + O1D) dissociation limit. Dissociation is suggested to take place through the 21Πg upper state to the O1D + O1D limit where spin-orbit coupling of the A'3Δu state with the 11Πu state accesses the allowed parallel 1Πu → 1Πg transition. While the absence of a parallel-type photodissociation signal from the c1Σ-u state may be expected, the A3Σ+u should spin-orbit couple through the same pathway as the A'3Δu state. The fact that no clear A3Σ+u signal is observed suggests a faster deactivation process compared to the A'3Δu state in the discharge and ice desorption process.

Photofragment Translational Spectroscopy Studies of H Atom Loss Following Ultraviolet Photoexcitation of Methimazole in the Gas Phase.

The ultraviolet (UV) photodissociation of gas-phase methimazole has been investigated by H Rydberg atom photofragment translational spectroscopy methods at many wavelengths in the range of 222.5-275 nm and by complementary electronic structure calculations. Methimazole is shown to exist predominantly as the thione tautomer, 1-methyl-2(3 H)-imidazolinethione, rather than the commonly given thiol form, 2-mercapto-1-methylimidazole. The UV absorption spectrum of methimazole is dominated by the S4 ← S0 transition of the thione tautomer, which involves electron promotion from an a' (p y) orbital localized on the sulfur atom to a σ* orbital localized around the N-H bond. Two H atom formation pathways are identified following UV photoexcitation. One, involving prompt, excited-state N-H bond fission, yields vibrationally cold but rotationally excited methimazolyl (Myl) radicals in their first excited (Ã) electronic state. The second yields H atoms with an isotropic recoil velocity distribution peaking at low kinetic energies but extending to the energetic limit allowed by energy conservation given a ground-state dissociation energy D0(Myl-H) ∼24 000 cm-1. These latter H atoms are attributed to the unimolecular decay of highly vibrationally excited S0 parent molecules. The companion electronic structure calculations provide rationales for both fragmentation pathways and the accompanying product energy disposals and highlight similarities and differences between the UV photochemistry of methimazole and that of other azoles (e.g., imidazole) and with molecules like thiourea and thiouracil that contain similar N-C═S motifs.

Photodissociation dynamics of the ortho- and para-xylyl radicals.

The photodissociation dynamics of the C8H9 isomers ortho- and para-xylyl are investigated in a free jet. The xylyl radicals are generated by flash pyrolysis from 2-(2-methylphenyl)- and 2-(4-methylphenyl) ethyl nitrite and are excited into the D3 state. REMPI- spectra show vibronic structure and the origin of the transition is identified at 32 291 cm-1 for the para- and at 32 132 cm-1 for the ortho-isomer. Photofragment H-atom action spectra show bands at the same energy and thus confirm H-atom loss from xylyl radicals. To gain further insight into the photodissociation dynamics, velocity map images of the hydrogen atom photofragments are recorded. Their angular distribution is isotropic and the translational energy release is in agreement with a dissociation to products in their electronic ground state. Photodissociation of para-xylyl leads to the formation of para-xylylene (C8H8), while the data for ortho-xylyl agree much better with the isomer benzocyclobutene as the dominant molecular fragment rather than ortho-xylylene. In computations we identified a new pathway for the reaction ortho-xylyl → benzocyclobutene + H with a barrier of 3.39 eV (27 340 cm-1), which becomes accessible at the employed excitation energy. It proceeds via a combination of scissoring and rotational motion of the -CH2 and -CH3 groups. However, the observed rate constants measured by delaying the excitation and ionization laser with respect to each other are significantly faster than computed ones, indicating intrinsic non-RRKM behaviour. A comparably high value of around 30% of the excess energy is released as translation of the H-atom photofragment.

Dissociative Photoionization of the Elusive Vinoxy Radical.

These experiments report the dissociative photoionization of vinoxy radicals to m/z = 15 and 29. In a crossed laser-molecular beam scattering apparatus, we induce C-Cl bond fission in 2-chloroacetaldehyde by photoexcitation at 157 nm. Our velocity measurements, combined with conservation of angular momentum, show that 21% of the C-Cl photofission events form vinoxy radicals that are stable to subsequent dissociation to CH3 + CO or H + ketene. Photoionization of these stable vinoxy radicals, identified by their velocities, which are momentum-matched with the higher-kinetic-energy Cl atom photofragments, shows that the vinoxy radicals dissociatively photoionize to give signal at m/z = 15 and 29. We calibrated the partial photoionization cross section of vinoxy to CH3+ relative to the bandwidth-averaged photoionization cross section of the Cl atom at 13.68 eV to put the partial photoionization cross sections on an absolute scale. The resulting bandwidth-averaged partial cross sections are 0.63 and 1.3 Mb at 10.5 and 11.44 eV, respectively. These values are consistent with the upper limit to the cross section estimated from a study by Savee et al. on the O(3P) + propene bimolecular reaction. We note that the uncertainty in these values is primarily dependent on the signal attributed to C-Cl primary photofission in the m/z = 35 (Cl+) time-of-flight data. While the value is a rough estimate, the bandwidth-averaged partial photoionization cross section of vinoxy to HCO+ calculated from the signal at m/z = 29 at 11.53 eV is approximately half that of vinoxy to CH3+. We also present critical points on the potential energy surface of the vinoxy cation calculated at the G4//B3LYP/6-311++G(3df,2p) level of theory to support the observation of dissociative ionization of vinoxy to both CH3+ and HCO+.

The near ultraviolet photodissociation dynamics of 2- and 3-substituted thiophenols: Geometric vs. electronic structure effects.

The near ultraviolet spectroscopy and photodissociation dynamics of two families of asymmetrically substituted thiophenols (2- and 3-YPhSH, with Y = F and Me) have been investigated experimentally (by H (Rydberg) atom photofragment translational spectroscopy) and by ab initio electronic structure calculations. Photoexcitation in all cases populates the 11ππ* and/or 11πσ* excited states and results in S-H bond fission. Analyses of the experimentally obtained total kinetic energy release (TKER) spectra yield the respective parent S-H bond strengths, estimates of ΔE(A∼-X∼), the energy splitting between the ground (X∼) and first excited (A∼) states of the resulting 2-(3-)YPhS radicals, and reveal a clear propensity for excitation of the C-S in-plane bending vibration in the radical products. The companion theory highlights roles for both geometric (e.g., steric effects and intramolecular H-bonding) and electronic (i.e., π (resonance) and σ (inductive)) effects in determining the respective parent minimum energy geometries, and the observed substituent and position-dependent trends in S-H bond strength and ΔE(A∼-X∼). 2-FPhSH shows some clear spectroscopic and photophysical differences. Intramolecular H-bonding ensures that most 2-FPhSH molecules exist as the syn rotamer, for which the electronic structure calculations return a substantial barrier to tunnelling from the photoexcited 11ππ* state to the 11πσ* continuum. The 11ππ* ← S0 excitation spectrum of syn-2-FPhSH thus exhibits resolved vibronic structure, enabling photolysis studies with a greater parent state selectivity. Structure apparent in the TKER spectrum of the H + 2-FPhS products formed when exciting at the 11ππ* ← S0 origin is interpreted by assuming unintended photoexcitation of an overlapping resonance associated with syn-2-FPhSH(v33 = 1) molecules. The present data offer tantalising hints that such out-of-plane motion influences non-adiabatic coupling in the vicinity of a conical intersection (between the 11πσ* and ground state potentials at extended S-H bond lengths) and thus the electronic branching in the eventual radical products.

Stereodirectional images of molecules oriented by a variable-voltage hexapolar field: Fragmentation channels of 2-bromobutane electronically excited at two photolysis wavelengths.

The asymmetric-top molecule 2-bromobutane is oriented by means of a hexapole state selector; the angular distribution of the bromine atom photofragment, for the two fine-structure components, is acquired by velocity-map ion imaging. The molecular beam, spatially oriented along the time-of-flight axis, is intersected with a linearly polarized laser, whose polarization is tilted by 45° with respect to the detector surface. To obtain the mixing ratio of the perpendicular and parallel transitions, the fragment ion images and angular distributions can be appropriately simulated to give insight on the population mechanism of the specific electronic state involved at each selected excitation wavelength. The photofragment images obtained at 238.6 nm yielded an asymmetry factor β1 of 0.67, indicative of the extent of molecular orientation, and an anisotropy parameter β2 of 1.03, which is a signature of a prevailing parallel transition along the C-Br axis. When the photolysis wavelength is tuned to 254.1 nm, the corresponding angular distribution is less asymmetric (β1 = 0.24) and the obtained small value β2 = 0.12 is a characteristic of a predominantly perpendicular transition. The photofragment angular distributions are also affected by hexapole voltage, especially regarding the asymmetry factor, and this aspect provides information on the effect of molecular orientation.

Velocity map ion imaging and velocity-resolved action spectroscopy of H atom photofragments via Doppler-free multiphoton ionization

Hydrogen atom photofragments with high velocities in the laboratory frame lead to strong Doppler broadening of the (2+1) resonance-enhanced multiphoton ionization transition. Here, an alternative setup for Doppler-free ion imaging is demonstrated and explored for predissociation of methylamine and probing of H photofragments, at close wavelengths. This approach enables measurement of the entire velocity distributions in each laser pulse, enhances sensitivity and consequently provides acquisition of high quality H images and action spectra. These results shed light on dissociation dynamics and can be foreseen to be useful in revealing the dynamics in other molecular systems.

Probing Rotational Motion in 4-tert-Butylcatechol through H Atom Photofragmentation: Deviations from Axial Recoil.

The time-resolved photofragmentation dynamics of 4-tert-butylcatechol were studied following one photon excitation to the S1 (1(1)ππ*) state with ultraviolet radiation in the range 260 ≤ λ ≤ 286 nm. The preparation of an aligned molecular ensemble via photoexcitation leads to anisotropy in the H atom photofragments. These H atoms originate from the decay of the S1 state through coupling onto the S2 ((1)πσ*) state, which is dissociative along the nonintramolecular hydrogen bonded "free" O-H bond. The degree of anisotropy of these photogenerated H atoms decreases with increasing pump-probe time delay. This is attributed to rotational dephasing of the initially aligned molecular ensemble. The measured dephasing occurs on a time scale akin to the appearance time of these H atoms, which likely places an intrinsic lower bound on the dephasing lifetime. The present work demonstrates how a careful balance between the appearance time of the H atoms, determined by the S1 lifetime, and the rotational dephasing in 4-tert-butylcatechol provides an opportune window to probe rotational motion in real time.

A Multipronged Comparative Study of the Ultraviolet Photochemistry of 2-, 3-, and 4-Chlorophenol in the Gas Phase.

The S1((1)ππ*) state of the (dominant) syn-conformer of 2-chlorophenol (2-ClPhOH) in the gas phase has a subpicosecond lifetime, whereas the corresponding S1 states of 3- and 4-ClPhOH have lifetimes that are, respectively, ∼2 and ∼3-orders of magnitude longer. A range of experimental techniques-electronic spectroscopy, ultrafast time-resolved photoion and photoelectron spectroscopies, H Rydberg atom photofragment translational spectroscopy, velocity map imaging, and time-resolved Fourier transform infrared emission spectroscopy-as well as electronic structure calculations (of key regions of the multidimensional ground (S0) state potential energy surface (PES) and selected cuts through the first few excited singlet PESs) have been used in the quest to explain these striking differences in excited state lifetime. The intramolecular O-H···Cl hydrogen bond specific to syn-2-ClPhOH is key. It encourages partial charge transfer and preferential stabilization of the diabatic (1)πσ* potential (relative to that of the (1)ππ* state) upon stretching the C-Cl bond, with the result that initial C-Cl bond extension on the adiabatic S1 PES offers an essentially barrierless internal conversion pathway via regions of conical intersection with the S0 PES. Intramolecular hydrogen bonding is thus seen to facilitate the type of heterolytic dissociation more typically encountered in solution studies.

Infrared spectroscopy and 193 nm photochemistry of methylamine isolated in solid parahydrogen.

The in situ UV photolysis of a precursor molecule trapped in a parahydrogen (pH2) matrix is a simple method used to generate isolated radical photofragments that are well suited for infrared spectroscopic studies. However, for molecules that can dissociate via multiple pathways, little is known about how the pH2 matrix influences the branching among these open pathways. We report FTIR spectroscopic studies of the 193 nm photodecomposition of methylamine (MA, CH3NH2) isolated in pH2 quantum matrixes at 1.8 K. We observe single exponential decay of the MA precursor upon irradiation and the quantum yield for MA photodissociation is measured to be Φ = 0.26(2) consistent with a weak pH2 cage effect. By comparing to gas-phase results, we show the in situ photolysis results in greater production of molecular products (CH2═NH + H2) compared to radical products (CH3NH + H) consistent with the idea of partial caging of the H atom photofragments. The information gained in this work can be used to guide future photolysis studies in pH2 matrixes.

Communication: direct measurements of nascent O((3)P0,1,2) fine-structure distributions and branching ratios of correlated spin-orbit resolved product channels CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2) in VUV photodissociation of CO2.

We present a generally applicable experimental method for the direct measurement of nascent spin-orbit state distributions of atomic photofragments based on the detection of vacuum ultraviolet (VUV)-excited autoionizing-Rydberg (VUV-EAR) states. The incorporation of this VUV-EAR method in the application of the newly established VUV-VUV laser velocity-map-imaging-photoion (VMI-PI) apparatus has made possible the branching ratio measurement for correlated spin-orbit state resolved product channels, CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2), formed by VUV photoexcitation of CO2 to the 4s(10 (1)) Rydberg state at 97,955.7 cm(-1). The total kinetic energy release (TKER) spectra obtained from the O(+) VMI-PI images of O((3)P0,1,2) reveal the formation of correlated CO(ã(3)Π; v = 0-2) with well-resolved v = 0-2 vibrational bands. This observation shows that the dissociation of CO2 to form the spin-allowed CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel has no potential energy barrier. The TKER spectra for the spin-forbidden CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel were found to exhibit broad profiles, indicative of the formation of a broad range of rovibrational states of CO(X̃(1)Σ(+)) with significant vibrational populations for v = 18-26. While the VMI-PI images for the CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel are anisotropic, indicating that the predissociation of CO2 4s(10 (1)) occurs via a near linear configuration in a time scale shorter than the rotational period, the angular distributions for the CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel are close to isotropic, revealing a slower predissociation process, which possibly occurs on a triplet surface via an intersystem crossing mechanism.

Symmetry matters: photodissociation dynamics of symmetrically versus asymmetrically substituted phenols

We report a combined experimental (H (Rydberg) atom photofragment translational spectroscopy) and theoretical (ab initio electronic structure and vibronic coupling calculations) study of the effects of symmetry on the photodissociation dynamics of phenols. Ultraviolet photoexcitation to the bound S1((1)ππ*) state of many phenols leads to some O-H bond fission by tunneling through the barrier under the conical intersection (CI) between the S1 and dissociative S2((1)πσ*) potential energy surfaces in the R(O-H) stretch coordinate. Careful analysis of the total kinetic energy release spectra of the resulting products shows that the radicals formed following S1 ← S0 excitation of phenol and symmetrically substituted phenols like 4-fluorophenol all carry an odd number of quanta in vibrational mode ν(16a), whereas those deriving from asymmetrically substituted systems like 3-fluorophenol or 4-methoxyphenol do not. This contrasting behavior can be traced back to symmetry. Symmetrically substituted phenols exist in two equivalent rotamers, which interconvert by tunneling through the barrier to OH torsional motion. Their states are thus best considered in the non-rigid G4 molecular symmetry group, wherein radiationless transfer from the S1 to S2 state requires a coupling mode of a2 symmetry. Of the three a2 symmetry parent modes, the out-of-plane ring puckering mode ν(16a) shows much the largest interstate coupling constant in the vicinity of the S1/S2 CI. The nuclear motions associated with ν(16a) are orthogonal to the dissociation coordinate, and are thus retained in the radical products. Introducing asymmetry (even a non-linear substituent in the 4-position) lifts the degeneracy of the rotamers, and lowers the molecular symmetry to Cs. Many more parent motions satisfy the reduced (a'') symmetry requirement to enable S1/S2 coupling, the most effective of which is OH torsion. This motion 'disappears' on O-H bond fission; symmetry thus imposes no restriction to forming radical products with vibrational quantum number v = 0. The present work yields values for the O-H bond strengths in 3-FPhOH and 4-MeOPhOH, and recommends modest revisions to the previously reported O-H bond strengths in other asymmetrically substituted phenols like 3- and 2-methylphenol and 4-hydroxyindole.