Photodissociation Of Polyatomic Molecules Research Articles

Conical intersections in solution: formulation, algorithm, and implementation with combined quantum mechanics/molecular mechanics method.

The significance of conical intersections in photophysics, photochemistry, and photodissociation of polyatomic molecules in gas phase has been demonstrated by numerous experimental and theoretical studies. Optimization of conical intersections of small- and medium-size molecules in gas phase has currently become a routine optimization process, as it has been implemented in many electronic structure packages. However, optimization of conical intersections of small- and medium-size molecules in solution or macromolecules remains inefficient, even poorly defined, due to large number of degrees of freedom and costly evaluations of gradient difference and nonadiabatic coupling vectors. In this work, based on the sequential quantum mechanics and molecular mechanics (QM/MM) and QM/MM-minimum free energy path methods, we have designed two conical intersection optimization methods for small- and medium-size molecules in solution or macromolecules. The first one is sequential QM conical intersection optimization and MM minimization for potential energy surfaces; the second one is sequential QM conical intersection optimization and MM sampling for potential of mean force surfaces, i.e., free energy surfaces. In such methods, the region where electronic structures change remarkably is placed into the QM subsystem, while the rest of the system is placed into the MM subsystem; thus, dimensionalities of gradient difference and nonadiabatic coupling vectors are decreased due to the relatively small QM subsystem. Furthermore, in comparison with the concurrent optimization scheme, sequential QM conical intersection optimization and MM minimization or sampling reduce the number of evaluations of gradient difference and nonadiabatic coupling vectors because these vectors need to be calculated only when the QM subsystem moves, independent of the MM minimization or sampling. Taken together, costly evaluations of gradient difference and nonadiabatic coupling vectors in solution or macromolecules can be reduced significantly. Test optimizations of conical intersections of cyclopropanone and acetaldehyde in aqueous solution have been carried out successfully.

Parent–molecule rotational depolarization of photofragment angular momentum distributions: diatomic and polyatomic molecules

We extend the A(q)(k) polarization-parameter model, which describes product angular momentum polarization from one photon photodissociation of polyatomic molecules in the molecular frame [J. Chem. Phys., 2010, 132, 224310], to the case of rotating parent molecules. The depolarization of the A(q)(k) is described by a set of rotational depolarization factors that depend on the angle of rotation of the molecular axis γ. We evaluate these rotational depolarization factors for the case of dissociating diatomic molecules and demonstrate that they are in complete agreement with the results of Kuznetsov and Vasyutinskii [J. Chem. Phys., 2005, 123, 034307] obtained from a fully quantum mechanical approach of the same problem, showing the effective equivalence of the two approaches. We further evaluate the set of rotational depolarization factors for the case of dissociating polyatomic molecules that have three (near) equal moments of inertia, thus extending these calculations to polyatomic systems. This ideal case yields insights for the dissociation of polyatomic molecules of various symmetries when we compare the long lifetime limit with the results obtained for the diatomic case. In particular, in the long lifetime limit the depolarization factors of the A(0)(k) (odd k), Re(A(1)(k)) (even k) and Im(A(1)(k)) (odd k) for diatomic molecules vanish; in contrast, for polyatomic molecules the depolarization factors for the A(0)(k) (odd k) reduce to a value of 1/3, whereas for the Re(A(1)(k)) (even k) and Im(A(1)(k)) (odd k) they reduce to 1/5.

Photofragment angular momentum distributions in the molecular frame. III. Coherent effects in the photodissociation of polyatomic molecules with circularly polarized light

We extend the a(q) (k)(s) polarization parameter model [T. P. Rakitzis and A. J. Alexander, J. Chem. Phys. 132, 224310 (2010)] to describe the components of the product angular momentum polarization that arise from the one-photon photodissociation of asymmetric top molecules with circularly polarized photolysis light, and provide a general equation for fitting experimental signals. We show that the only polarization parameters that depend on the helicity of the circularly polarized photolysis light are the A(0) (k) and Re[A(1) (k)] (with odd k) and the Im[A(1) (k)] (with even k); in addition, for the unique recoil destination (URD) approximation [for which the photofragment recoil v arises from a unique parent molecule geometry], we show that these parameters arise only as a result the interference between at least two dissociative electronic states. Furthermore, we show that in the breakdown of the URD approximation (for which the photofragment recoil v arises from a distribution of parent molecule geometries), these parameters can also arise for dissociation via a single dissociative electronic state. In both cases, the A(0) (k) and Re[A(1) (k)] parameters (with odd k) are proportional to cosΔφ, and the Im[A(1) (k)] parameters (with even k) are proportional to sinΔφ, where Δφ is the phase shift (or average phase shift) between the interfering paths so that Δφ can be determined directly from the A(q) (k), or from ratios of these A(q) (k) parameters. Therefore, the determination of these A(q) (k) parameters with circularly polarized photolysis light allows the unambiguous measurement of coherent effects in polyatomic-molecule photodissociation.

A new adiabatic approach to the photodissociation of polyatomic molecules

An adiabatic theory of polyatomic photodissociation is developed that explicitly treats the interaction between the relative and internal motion and leads to an expression for the nuclear wavefunction of the final dissociative state which differs considerably from the zeroth-order distorted-wave approximation. The vibrational frequencies ..omega../sub i/ of the fragments are found to depend on the interfragment distance. In the evaluation of the matrix element needed to describe the energy distribution of the fragments, it is found necessary to use values of ..omega../sub i/ different from the frequencies of the free fragments. These values of ..omega../sub i/ can be expressed in terms of the force constants of the precursor system. Inverse vibrational distributions are obtained from this theory in accord with recent experimental data.

A self-consistent eikonal treatment of photodissociation by visible radiation

A theoretical treatment of photodissociation of polyatomic molecules by visible radiation is developed, starting with an electron-field representation, where the electromagnetic field is quantized. The dissociation dynamics are described with a generalized eikonal treatment that accounts for the self-consistent coupling of quantal electronic transitions with classical nuclear motions, and that provides dissociation times. A simple model of C-I bond breaking is developed to illustrate the theory. Results are presented for position, momenta, and electronic probabilities versus time, for two examples of photodissociation.

Polarized fluorescence of polyatomic fragments produced through photodissociation of polyatomic molecules in the gas phase

A combined theoretical and experimental study is carried of the polarized emission of polyatomic products produced through photodissociation of polyatomic molecules. A general approach, based on the formalism of dissociation kernels and orientational correlation functions, is developed to predict anisotropy of the fluorescence of photoproducts. We consider the most general case of asymmetric top parent and product molecules. The rotational predissociation effect is taken into account. Various kinds of photoreactions are studied: those when fragments after dissociation are in the electronically excited states and those when fragments are in the ground electronic states so that additional laser pulse is necessary to excite their fluorescence. Particular attention is concentrated on those practically important extreme cases, when predissociation times and lifetimes of the electronically excited states of photoproducts are short or long as compared to the averaged period of free rotation. The steady state polarized fluorescence of radicals produced through dissociation of several disulfides into two identical radicals is measured. The results are interpreted in the framework of the free recoil model (FRM). In this model, photoproducts are assumed to experience no torque and fly apart freely, so that the only origin of the fragment rotation is rotation of the parent molecule. Predictions of the impulsive model (IM), in which fragments are supposed to suffer instantaneous torque due to the rupture of the chemical bonds of the parent molecule, are demonstrated to disagree strongly with our experimental data. This gives an additional confirmation of the validity of the FRM in describing dissociation of polyatomic molecules into polyatomic fragments. The FRM can therefore be invoked to estimate interrelation between the characteristic times, governing the processes of dissociation and emission, and the averaged period of free molecular rotation. Also, the FRM can be used for the determining orientations of the absorption and emission dipole moments in the reference frames of the parent and product molecules.

Kinematic distribution function to calculate rotational populations of photofragments from photodissociation of triatomic molecules

A general procedure to evaluate the rotational state population distributions of the nascent photofragments from the photodissociation of polyatomic molecules has been implemented with the use of the kinematic distribution function developed by Chen and Pei [Chem. Phys. Lett. 124, 365 (1986)]. Numerical evaluations of rotational state population distributions of diatomic photofragments from photodissociation of the general class of triatomic molecules are presented. The calculated rotational state population distributions are compared with the most recent experimental data on OH and SH photofragments to obtain the information on the kinematic aspects of the photodissociating H2O and H2S molecules.

Time-dependent rotational state distributions in direct photodissociation

We analyze the time dependence of rotational excitation in the photodissociation of polyatomic molecules using classical trajectories. The systems studied are the photodissociation of H 2O in the first and the second absorption band and of H 2O 2.

Coherence effects in the polarization of photofragments

The general quantum-mechanical treatment of coherence effects in the polarization of the fluorescence, as well as in the orientation of photofragments, is reviewed. They are the result of simultaneous excitation of dissociative electronic states with different symmetry correlating to the same limit. The coherent excitation produces interferences which depend both on the photoabsorption probability amplitudes as well as on the relative phases of the continuum vibrational wavefunctions at large distances. Oscillations in the polarization degree of photofragments fluorescence as a function of the photon energy are predicted. Examples are presented in the case of direct dissociation of diatomic molecules. The case of slow predissociation is also analysed. Finally, possible extensions to the photodissociation of polyatomic molecules are considered.

Rotational Distributions in Direct Molecular Photodissociation

Photodissociation of polyatomic molecules through the absorption of UV photons is an important and interesting topic of physical chemistry. Although it has been amply studied in the past, photodissociation is still an active field of current research. It combines aspects of absorption, molecular motion in electronically excited states, dissociation, and uni ... molecular reactions. A photodissociation process is usually envisioned to proceed in two, more or less separate steps according to

Some Mechanisms in the Photodissociation of Polyatomic Molecules

AbstractThe various mechanisms promoting predissociation in polyatomic molecules are reviewed, and illustrated by the spectroscopy of various electronic states of HNO, DNO, NH3 and ND3. The quantum number dependences of the predissociation rates are used to discuss the particular mechanism(s) operating in each case. Recent observations using H‐atom time‐of‐flight spectroscopy of the energy disposal in the photodissociation of NH, through its à state show that a conical intersection in the exit channel has a massive influence on the dissociation dynamics.

Bending contribution to rotational distributions in the photodissociation of polyatomic molecules

Bending contribution to rotational distributions in the photodissociation of polyatomic molecules

Fragment angular distributions from photodissociation of polyatomic molecules

Abstract We present results of quantum calculations for the photofragment angular distributions of linear triatomic molecules. Deviations from axial recoil are predicted and explained (even for high energy dissociations) when (a) the diatomic fragment rotational angular momentum differs from the initial total angular momentum of the triatomic, and when (b) the initial triatomic bending degrees of freedom are excited.

Measurement of water vapor in the stratosphere by photodissociation with Ly α (1216 Å) light

Photodissociation of polyatomic molecules by vacuum-uv light often results in the formation of electronically excited diatomic molecular fragments. Based on this, instruments that measure mixing ratios or densities of selected polyatomic species in the stratosphere and higher troposphere can be constructed. This is demonstrated by an instrument to detect and measure stratospheric water mixing ratios utilizing H(2)O photodissociation by Ly alpha (1216 A) light and detecting OH(A(2)J(+)X(2)Pi) emission. Also, detectors for H(2)O(2), HNO(3), and NO(2) are discussed.

Internal Recombination during Photo-dissociation of Polyatomic Molecules

IT has been generally admitted that, in the primary decomposition of a molecule by absorption of light, free atoms or unsaturated radicals are produced. In an extensive investigation upon the photodecomposition of carbonyl compounds, Dr. R. G. W. Norrish and his co-workers1 have advanced the view that an aldehyde molecule can be disrupted into a saturated hydrocarbon and a carbon monoxide molecule in one process. This is equivalent to a recombination of the liberated hydrogen atom and alkyl radical at the moment they leave the remainder of the molecule.