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Notes: The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser-molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin-orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr. Analysis of the product translational energy distributions for the dissociation products indicates that the IBr is formed electronically excited and that the halogen atom products are spin-orbit excited. Excitation at 210 nm, of the transition assigned as n(Br)→σ*(C–Br) based on comparison with CH3Br, results in selective breaking of the stronger C–X bond in the molecule, the C–Br bond, and no fission of the C–I bond. Some concerted elimination of IBr also occurs; the IBr velocity distribution indicates it is probably formed electronically excited as in photolysis at 193.3 nm. The selective breaking of the C–Br bond over the weaker C–I bond is discussed in contrast to previous photolysis studies of polyhalomethanes.

Notes: We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products' velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).

Notes: We present a study of the dissociation of CH3I on coupled repulsive electronic potential energy surfaces by the technique of polarized emission spectroscopy. We excite CH3I at 266 nm and disperse the photons emitted from the dissociating molecule by both frequency and angular distribution with respect to the polarization direction of the excitation laser. We thus measure the polarization of the first 12 C–I stretching emission features, corresponding to the spectral region between 266 and 317 nm. We also obtain the rotational envelope of selected emission features in higher resolution scans and model the lineshapes with parameters derived from the polarization results. The polarization measurements show the emission into the first few low-lying C–I stretching vibrational levels is via a transition moment parallel to the absorbing one, consistent with excitation to and emission from the 3Q0(2A1) repulsive surface. Emission to higher C–I stretching overtones shows an increasing contribution from emission via a transition moment perpendicular to the absorbing one, consistent with emission from a repulsive surface of E symmetry following excitation to the 3Q0(2A1) state.We extract from the data the fraction of photons emitted via a perpendicular transition for each of the C–I stretch emission features. The analysis includes the derivation of analytic expressions for the angular distribution of the photons, with and without integration over the rotational contour, when the detector has a finite acceptance angle. We discuss the results in relation to a simple model where photoabsorption excites the molecule to the 3Q0(2A1) repulsive surface (parallel transition moment) and amplitude develops on the 1Q1(3E) repulsive surface as the molecule dissociates through a curve crossing. The changes in amplitude of the molecular wavefunction on the A1 vs the E repulsive surfaces during dissociation is thus probed. We outline a crude classical quasidiatomic approximation for roughly extracting from our data the electronic energy at which the "curve crossing'' occurs. This derived energy is compared to that given in model and ab initio calculations of the excited electronic potential energy surfaces. Finally, we discuss the results in relation to the simple quasidiatomic Landau–Zener crossing model utilized by other workers, a model which does not fully explain the collection of experimental results over the last decade on the iodoalkane curve crossing.

Notes: Vibrationally mediated photodissociation is a means of studying the spectroscopy of bound vibrational overtone states and of probing the electronic photodissociation dynamics of highly vibrationally excited molecules. In these experiments, a highly vibrationally excited hydrogen peroxide molecule prepared by initial excitation in the region of the third (4νOH) or fourth (5νOH) overtone of the OH stretching vibration absorbs an additional photon to dissociate to OH fragments whose individual quantum state populations are measured by laser induced fluorescence. This technique is a means of obtaining excitation spectra for bound highly vibrationally excited states and confirms the accuracy of a model that incorporates the role of the torsional vibration in the vibrational overtone spectroscopy. The photodissociation dynamics of highly vibrationally excited molecules are substantially different from those observed for dissociation by single photons of comparable or greater energy. Approximately 11% of the OH fragments formed in the vibrationally mediated photodissociation through 4νOH are vibrationally excited as compared to an unobservable amount (≤2%) in the single photon ultraviolet dissociation.

Notes: We present a study of the fragmentation dynamics of H2S excited at 193.3 nm using the technique of emission spectroscopy of dissociating molecules. The emission spectrum measured extends from near 50 000 cm−1 to 35 000 cm−1, corresponding to emission to vibrational levels in the ground electronic state with seven quanta of S–H stretching vibration. We assigned the emission to previously unobserved vibrational levels of H2S using the calculated energies of Halonen and Carrington [J. Chem. Phys. 88, 4171 (1988)]. The early part of the spectrum contains progressions in both symmetric stretching and in symmetric/antisymmetric combination bands, as well as stretching progressions in combination with one quantum in the bend. In the latter part of the spectrum, which reflects the latter part of the dissociation dynamics, emission occurs selectively to vibrational levels characterized best in a local mode basis as having the vibrational energy localized in one of the two S–H bonds. We interpret emission to these local mode vibrations as evidencing the evolution of the dynamics into dissociative motion of one of the S–H bonds with little HSH bending or vibrational excitation of the other S–H bond, which correlates to the SH fragment. The emission spectrum thus reveals the progression of the dynamics, providing a crucial link between the absorption spectrum and the final observed SH product quantum state distributions.

Notes: These experiments investigate the photodissociation of methyl vinyl ether at 193 nm in a crossed laser-molecular beam apparatus. We observe two C–O bond fission channels, a minor channel producing CH3+CH2CHO (X˜ 2A″) and the major channel yielding CH3+CH2CHO (A˜ 2A′). Some of the neutral A˜ state vinoxy product undergoes secondary dissociation to produce ketene+H. These experiments on the photodissociation of methyl vinyl ether, which produce nascent vinoxy and methyl radicals cleanly in a one-to-one ratio, serve two purposes. First, using the measured photofragment velocities and product branching we calibrate the relative sensitivity of mass spectrometric detection to the methyl and vinoxy polyatomic radical products at the m/e=15 daughter ion, taking into account the loss of neutral vinoxy to ketene+H formation. This relative mass spectrometric sensitivity calibration factor is determined to be 0.116±0.022. Knowledge of this factor allows us to extract the product branching ratio between the two major competing primary product channels from the O(3P)+ethylene reaction, a branching ratio that has been much disputed in the literature. Our results give a CH3+HCO/H+CH2CHO product branching ratio of R=0.61±.11, corresponding to 38% (±5%) branching to the CH3+HCO channel. Second, we use the result that the channel producing A˜-state vinoxy dominates over the formation of ground-state vinoxy to test propensity rules being developed to help predict what product channels may be suppressed by electronically nonadiabatic effects in chemical reactions. These propensity rules discriminate between channels that are "electronically facile" and "electronically difficult/prohibitive." We find that a qualitative analysis of the changes in electronic configuration along the reaction coordinates for the photodissociation of methyl vinyl ether correctly predicts the dominant channel to be the production of excited state (A˜) vinoxy, the electronically facile channel. © 2001 American Institute of Physics.

Notes: These experiments investigate the competition between C–C and C–Br bond fission in bromoacetone excited in the 1[n(O),π*(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C–C fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-of-flight technique identifies the primary photofragmentation channels at 308 nm. The time-of-flight spectra evidence two dissociation channels, C–Br fission and fission of one of the two C–C bonds, BrH2C–COCH3. The distribution of relative kinetic energies imparted to the C–Br fission and C–C fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl- and bromopropionyl chloride.In the second part of the work we focus on the marked conformation dependence to the branching between C–C fission and C–Br fission. Photofragment angular distribution measurements show that C–Br fission occurs primarily from the minor, anti, conformer, giving a β of 0.8, so C–C fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C–Br reaction coordinate and qualitative consideration of the conical intersection along the C–C reaction coordinate. The resulting model proposes that C–C bond fission cannot compete with C–Br fission in the anti conformer because the dissociation samples regions of the phase space near the conical intersection along the C–C fission reaction coordinate, where nonadiabaticity inhibits C–C fission, while from the gauche conformer C–C fission can proceed more adiabatically and dominate C–Br fission. A final experiment confirms that the branching ratio changes with the relative conformer populations in accord with this model.

Notes: Previous crossed laser-molecular beam experiments on bromoacetyl chloride investigated the influence of electronic nonadiabaticity on the branching between C–Cl and C–Br fission upon 1[n(O),π*(C=O)] excitation at 248 nm. This note identifies an additional contribution to the data from a minor primary C–C fission channel. Using the distribution of kinetic energies imparted to C–C fission products in bromoacetone following 1[n(O),π*(C=O)] excitation as a characteristic one, we reanalyze the Br+ and Cl+ time-of-flight spectra from bromoacetyl chloride at 248 nm to identify any contribution from C–C bond fission. We find that a small portion of the Br+ signal results from CH2Br fragments from C–C fission. Although this in no way alters the major conclusions from this previous work, it does slightly alter the P(ET) derived for C–Br fission and the C–Cl:C–Br branching ratio from bromoacetyl chloride.

Notes: This study uses emission spectroscopy of H2S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck–Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH→HS+H reaction. Excitation wavelengths from 199–203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm−1 above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm−1 for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces.This emission, into local mode eigenstates such as 00+1, 11+0, 11+1, 21+0, 21+1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0+0 progression for n≥4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch resonance in the Franck–Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers' measurements of the SH product vibrational state distribution shows that v=0 products are strongly favored at excitation wavelengths near the conical intersection.