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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: This work investigates how molecular dissociation induced by local 1[n(O),π*(C=O)] electronic excitation at a carbonyl functional group can result in preferential fission of an alpha bond over a weaker bond beta to the functional group and how nonadiabaticity in the dynamics drives the selectivity. The experiment measures the photofragment velocity and angular distributions from the photodissociation of acetyl chloride and bromoacetyl chloride at 248 nm, identifying the branching between bond fission channels and the mechanism for the selectivity. The anisotropic angular distributions measured shows dissociation occurs on a time scale of less than a rotational period, resulting in primary C–X (X=Cl, Br) bond fission, but no significant C–C bond fission. While the selective fission of the C–Cl over the C–C alpha bond can be predicted from the adiabatic correlation diagram for this special class of Norrish type I cleavage, the preferential fission of the C–Cl alpha bond over the C–Br bond beta to the carbonyl group would not be predicted on the adiabatic potential energy surface. In bromoacetyl chloride, fission of the C–Cl and C–Br bonds occurs with a branching of 1.0:1.1 (approximately 1.0:0.5 from the 1nπ* transition) compared with a predicted statistical branching ratio of 1:30. This preferential α-bond fission is attributed to a dissociation mechanism on the coupled [n,π*(C=O)] and [n(X),σ *(C–X)] electronic states, a model consistent with the lack of C–C fission and the measured kinetic energy and angular distributions.The selectivity results from the relative strengths of the electronic coupling between the initially excited [n,π*(C=O)] bound configuration and the two [n(X),σ *(C–X)] states, the weaker coupling inhibiting the adiabatic crossing over the barrier to C–Br bond fission. The results demonstrate the need to go beyond the Born–Oppenheimer approximation to gain predictive ability in any reactive system where the electronic configuration changes along the reaction coordinate, particularly at barriers due to configuration crossings. In addition, the Cl product angular distribution determines the orientation of the 1[n(O),π*(C=O)] transition dipole moment and shows it is governed by the C2v symmetry of the localized carbonyl electronic orbitals and not by the asymmetric substitution at the carbonyl group. Spectra of the Br atoms from direct dissociation at 193 nm help separate the contribution from the overlapping nσ *(C–Br) transition at 248 nm.

Notes: This work measures the change in branching between the CN+I(2P3/2) and the CN+I(2P1/2) product channels when one photodissociates vibrationally excited rather than cold ICN at 248.5 nm. The crossed-laser molecular beam experiment tests a model for the dependence of branching at a conical intersection on the amplitude of the dissociative wave function at bent geometries. The results provide a critical comparison between the long-standing empirical surfaces for ICN photodissociation and more recent ab initio potential energy surfaces for ICN's first absorption band. In the experiment, we observe an increase in branching from 49% to 58% I(2P3/2) products when the temperature of the ICN parent photodissociated is increased in going from a 100 °C vs a 400 °C nozzle expansion. We analyze the angular distributions of the photofragments to eliminate the possibility that the change in branching is due to an increased contribution from direct absorption to the electronic state correlating with I(2P3/2) products. We discuss the relationship between these studies, which mediate the effective off-diagonal potential coupling by forcing the nuclear dynamics to sample more strongly coupled regions of the intersecting potential surfaces, with intramolecular electron transfer studies which examine how the coupling depends on the particular conformer.

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: We investigate the origin of the observed fission of the stronger S–H bond over the weaker C–S bond in CH3SH excited at 193 nm using the complementary techniques of mass-resolved photofragment time-of-flight spectroscopy and emission spectroscopy. The velocities and angular distributions of the CH3S and SH photofragments show that both C–S and S–H bond fission occur on a subpicosecond timescale and impart considerable energy to relative product translation. The dispersed emission from photoexcited CH3SH molecules in a flow cell evidences a progression in the CH3 umbrella mode and combination bands with one quantum in the C–S stretch, but no progression with S–H stretch. Examination of the results with reference to previous ab initio calculations of the excited state surfaces reveals the importance of nonadiabatic coupling in the dissociation dynamics. This is a clear example of selective bond fission upon excitation of an electronic state that is not repulsive in the bond that breaks. We discuss the implication of the work with respect to using the Born–Oppenheimer approximation in reactive collisions near a saddle point along the reaction coordinate.