Laurie J. Butler

Evolution of molecular dissociation through an electronic curve crossing: Polarized emission spectroscopy of CH3I at 266 nm

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 vi...

The influence of parent bending motion on branching at a conical intersection in the photodissociation of CX3I (X=H,D,F)

This work measures the change in branching between the CF3+I(2P3/2) and I(2P1/2) product channels when one photodissociates vibrationally excited rather than cold CF3I at 248.5 nm. The experiment tests a model for the dependence of branching at a conical intersection on the amplitude of the dissociative wave function at bent geometries, a model which we propose here to explain previously observed differences in branching between the I(2P1/2) and I(2P3/2) channels at 248 nm for CH3I versus CD3I. In the CF3I experiment, we observe an increase in the branching from 13% to 17% I(2P3/2) products when the temperature of the CF3I parent is increased from 100 to 400 °C, in agreement with the qualitative prediction of the model. 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.

Nonadiabaticity and the competition between alpha and beta bond fission upon 1[n,π*(C=O)] excitation in acetyl‐ and bromoacetyl chloride

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 carbon...

Selective bond fission in methyl mercaptan at 193 nm via radial derivative coupling between the 2 1A‘ and 1 1A‘ adiabatic electronic states

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 impli...

Polarized emission spectroscopy of photodissociating nitromethane at 200 and 218 nm

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 spec...

Molecular dissociation dynamics of H2S at 193.3 nm studied via emission spectroscopy

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 ...

Competing isomeric product channels in the 193 nm photodissociation of 2-chloropropene and in the unimolecular dissociation of the 2-propenyl radical

This paper presents product translational energy spectroscopy measurements of the primary photofragmentation channels of 2-chloropropene excited at 193 nm and of the unimolecular dissociation of the 2-propenyl radical. Tunable vacuum ultraviolet (VUV) photoionization of the products allows us to distinguish between the various product isomers formed in these processes. The data show evidence for three significant primary reaction channels in the dissociation of 2-chloropropene: An excited-state C–Cl fission channel producing fast Cl atoms, a C–Cl fission channel producing slow Cl atoms, and HCl elimination. A minor C–CH3 fission channel contributes as well. The measured branching of the major primary product channels is: [fast C–Cl]:[slow C–Cl]:[HCl elimination]=62%:23%:15%. The experiments also allow us to resolve selectively the product branching between the unimolecular dissociation channels of the 2-propenyl radical, a high energy C3H5 isomer; we measure how the branching ratio between the two competi...

Resonance emission spectroscopy of predissociating SO2 C̃(1 1B2): Coupling with a repulsive 1A1 state near 200 nm

This work investigates the predissociative C(11B2) state of SO2 by cooling the SO2 in a pulsed molecular beam and dispersing the emission upon resonant excitation into several different vibronic absorption features in the C state between 197 and 212 nm. Unlike at the lower excitation energies, the dispersed emission spectra at the higher excitation energies are dominated by progressions with odd quanta in the antisymmetric stretch mode ν3 and combination bands with up to six quanta in the bending mode ν2. The formidable intensity for emission into vibrational states with odd quanta in the antisymmetric stretch of the jet-cooled molecule suggests that the intermediate state at high energies in the excited state is of mixed electronic character at nonsymmetric geometries, so the operative components of the transition moment for excitation and emission may be different. We discuss our results by considering the avoided crossing seam and coupling between the C(11B2) state and two dissociative states potent...

Dissociation dynamics of CH3SH at 222, 248, and 193 nm: An analog for probing nonadiabaticity in the transition state region of bimolecular reactions

These experiments use molecular photodissociation of CH3SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH3+SH→CH3S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments’ velocities and angular distributions determine the branching between the CH3+SH and the CH3S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S–H bond over the weaker C–S bond. Fission of the C–S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C–S bond fission and S...

Distance Dependence of Nonadiabaticity in the Branching Between C–Br and C–Cl Bond Fission Following 1[n(O),π∗(C=O)] Excitation in Bromopropionyl Chloride

These experiments on bromopropionyl chloride investigate a system in which the barrier to C–Br fission on the lowest 1A‘ potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C–Br fission dramatically reduces the branching to C–Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C–Br fission, further suppressing C–Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH2)2COCl, determining the branching ratio between the competing primary C–Br and C–Cl fission pathways and detecting a minor C–C bond fission pathway. While the primary C–Cl:C–Br fission branching ratio is 1:2, the distribution of relative...