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.

To predict the branching between energetically allowed product channels, chemists often rely on statistical transition state theories or exact quantum scattering calculations on a single adiabatic potential energy surface. The potential energy surface gives the energetic barriers to each chemical reaction and allows prediction of the reaction rates. Yet, chemical reactions evolve on a single potential energy surface only if, in simple terms, the electronic wavefunction can evolve from the reactant electronic configuration to the product electronic configuration on a time scale that is fast compared to the nuclear dynamics through the transition state. The experiments reviewed here investigate how the breakdown of the BornOppenheimer approximation at a barrier along an adiabatic reaction coordinate can alter the dynamics of and the expected branching between molecular dissociation pathways. The work reviewed focuses on three questions that have come to the forefront with recent theory and experiments: Which classes of chemical reactions evidence dramatic nonadiabatic behavior that influences the branching between energetically allowed reaction pathways? How do the intramolecular distance and orientation between the electronic orbitals involved influence the nonadiabaticity in the reaction? How can the detailed nuclear dynamics mediate the effective nonadiabatic coupling encountered in a chemical reaction?

The dissociation of nitromethane following the excitation of the π* ← π transition at 193 nm has been investigated by two independent and complementary techniques, product emission spectroscopy and molecular beam photofragment translational energy spectroscopy. The primary process is shown to be cleavage of the C–N bond to yield CH3 and NO2 radicals. The translational energy distribution for this chemical process indicates that there are two distinct mechanisms by which CH3 and NO2 radicals are produced. The dominant mechanism releasing a relatively large fraction of the total available energy to translation probably gives NO2 radicals in a vibrationally excited 2B2 state. When dissociated, other nitroalkanes exhibit the same emission spectrum as CH3NO2, suggesting little transfer of energy from the excited NO2 group to the alkyl group during dissociation for the dominant mechanism. This conclusion is supported by the apparent loss of the slow NO2 product in the molecular beam studies to unimolecular dissociation to NO+O, which will occur for NO2 with 72 kcal/mol or more internal energy. Evidence is presented which suggests that the NO2 produced via the minor mechanism, which releases a smaller fraction of the available energy to translation, has a large cross section for absorbing an additional photon via a parallel transition and dissociating to NO+O.

This paper presents the first experimental investigation under collisionless conditions of the competing photodissociation channels of methylamine excited in the first ultraviolet absorption band. Measurement of the nascent photofragments' velocity distributions and preliminary measurements of some photofragments' angular distributions evidence four significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H2 elimination. The data, taken on photofragments from both methylamine and methylamine-d2, elucidate the mechanism for each competing reaction. Measurement of the emission spectrum of methylamine excited at 222 nm gives complementary information, evidencing a progression in the amino wag (or inversion) and combination bands with one quantum in the methyl (umbrella) deformation or with two quanta in the amino torsion vibration. The emission spectrum reflects the forces in the Franck-Condon region which move the molecule toward a ciscoid geometry. The photofragment kinetic energy distributions measured for CH3ND2 show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation. The reaction coordinates for N-H and C-N fission are analyzed in comparison to that for ammonia dissociation from the A state and with reference to ab initio calculations of cuts along the excited state potential energy surface of methylamine which show these reactions traverse a small barrier in the excited state from a Rydbergkalence avoided crossing and then encounter a conical intersection in the exit channel. The measured kinetic energy distribution of the C-N bond fission photofragments indicates that the NH2 (NDz) product is formed in the A 2A1 state; the C-N fission reactive trajectories thus remain on the upper adiabat as they traverse the conical intersection. The mechanism for C-H bond fission is less clear; most of the kinetic energy distribution indicates the reaction evolves on a potential energy surface with no barrier to the reverse reaction, consistent with dissociation along the excited state surface or upon internal conversion to the ground state, but some of the distribution reflects more substantial partitioning to relative translation, indicating that some molecules may dissociate via a repulsive triplet surface. In general, the photofragment angular distributions were anisotropic, but the measured p -0.4 f 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation. We end with analyzing why in methylamine three other primary dissociation channels effectively compete with N-H fission while in CH30H and CH3SH primarily 0-H and S-H fission, respectively, dominate.

This study photolytically generates, from 2-bromoethanol photodissociation, the 2-hydroxyethyl radical intermediate of the OH + ethene reaction and measures the velocity distribution of the stable radicals. We introduce an impulsive model to characterize the partitioning of internal energy in the C(2)H(4)OH fragment. It accounts for zero-point and thermal vibrational motion to determine the vibrational energy distribution of the nascent C(2)H(4)OH radicals and the distribution of total angular momentum, J, as a function of the total recoil kinetic energy imparted in the photodissociation. We render this system useful for the study of the subsequent dissociation of the 2-hydroxyethyl radical to the possible asymptotic channels of the OH + ethene reaction. The competition between these channels depends on the internal energy and the J distribution of the radicals. First, we use velocity map imaging to separately resolve the C(2)H(4)OH + Br((2)P(3/2)) and C(2)H(4)OH + Br((2)P(1/2)) photodissociation channels, allowing us to account for the 10.54 kcal/mol partitioned to the Br((2)P(1/2)) cofragment. We determine an improved resonance enhanced multiphoton ionization (REMPI) line strength for the Br transitions at 233.681 nm (5p (4)P(1/2) <-- 4p (2)P(3/2)) and 234.021 nm (5p (2)S(1/2) <-- 4p (2)P(1/2)) and obtain a spin-orbit branching ratio for Br((2)P(1/2)):Br((2)P(3/2)) of 0.26 +/- 0.03:1. Energy and momentum conservation give the distribution of total internal energy, rotational and vibrational, in the C(2)H(4)OH radicals. Then, using 10.5 eV photoionization, we measure the velocity distribution of the radicals that are stable to subsequent dissociation. The onset of dissociation occurs at internal energies much higher than those predicted by theoretical methods and reflects the significant amount of rotational energy imparted to the C(2)H(4)OH photofragment. Instead of estimating the mean rotational energy with an impulsive model from the equilibrium geometry of 2-bromoethanol, our model explicitly includes weighting over geometries across the quantum wave function with zero, one, and two quanta in the harmonic mode that most strongly alters the exit impact parameter. The model gives a nearly perfect prediction of the measured velocity distribution of stable radicals near the dissociation onset using a G4 prediction of the C-Br bond energy and the dissociation barrier for the OH + ethene channel calculated by Senosiain et al. (J. Phys. Chem. A 2006, 110, 6960). The model also indicates that the excited state dissociation proceeds primarily from a conformer of 2-bromoethanol that is trans across the C-C bond. We discuss the possible extensions of our model and the effect of the radical intermediate's J-distribution on the branching between the OH + ethene product channels.

Advances in the study of photodissociation dynamics over the past 30 years are reviewed. An overview of experimental techniques that have been developed to extract photofragment energy and angular distributions is presented, followed by a discussion on several current topics of interest in the field of photodissociation.

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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.