Vector Correlations in the 225 nm Photodissociation of Co(CO)3NO.

Metal nitrosyls are fascinating compounds because they undergo significant geometry changes in the excited state. The volatile compound, Co(CO)3NO, is a model for understanding the excited-state behavior. In this experiment, Co(CO)3NO was photodissociated in a DC-sliced velocity mapping ion imaging apparatus with 1 + 1' resonance-enhanced multiphoton ionization (REMPI) detection of the nascent NO. Ion images were collected for different linear polarization of the dissociation and probe lasers to determine the vector correlations in the photodissociation. The fastest NO products arise from an excitation parallel to the dissociating Co-NO bond. The Co-NO bond bends in the excited state, producing an NO photoproduct with angular momentum that is also aligned in the laboratory frame. The µ-v-J vector correlations were measured and are consistent with the orientation µ||v⊥J caused by an excited-state Co-NO bend prior to dissociation. Slower NO photoproducts emerge with smaller vector correlations stemming from fragmentation, parent or fragment rotation, or intersystem crossing.

355 nm Photodissociation of N2O3 Revealed by Velocity-Mapped Ion Imaging.

Dinitrogen trioxide is proposed as a precursor to forming nitrous acid, an important source of hydroxyl radicals in the atmosphere. The spectroscopy and properties of N2O3 have been studied at high pressures or low temperatures, but there are no reports of its gas-phase photodissociation. This study investigates the 355 nm photodissociation of N2O3 in a DC-sliced velocity-mapped ion imaging apparatus using linearly polarized nanosecond pump and probe lasers. The N2O3 sample was generated by expanding NO and NO2 seeded in a He carrier gas. After photodissociation, a high fraction of the available energy ends up in translation of the products. Time-dependent density functional theory calculations confirm the parallel transition dipole assignment if the dissociation occurs from a nonplanar N2O3 conformation. The vector correlations are nearly at the physical limits for a system where µ||v⊥J. The DC-sliced velocity-mapped ion imaging technique is well-suited to investigate N2O3 photodissociation since it resolves product speeds and differentiates among the sources of NO+ in an expansion containing NO, NO2, HONO, and N2O3.

The photodissociation of N,N-dimethylnitrosamine at 355 nm: The effect of excited-state conformational changes on product vector correlations.

In a photodissociation experiment, the dynamics associated with creating reaction products with specific energies can be understood by a study of the product vector correlations. Upon excitation to the S1 state, N,N-dimethylnitrosamine (DMN) undergoes an excited-state geometry change from planar to pyramidal around the central N. The significant geometry change affects the vector correlations in the photoproducts. Using polarized lasers for 355 nm photodissociation of DMN and for NO photoproduct excitation in a velocity-mapped ion imaging apparatus reveals new vector correlation details among the parent transition dipole (µ), photofragment velocity (v), and photofragment angular momentum (j). The dissociation of DMN displays some µ-v correlation [ß02(20)=-0.2], little µ-j correlation [ß02(02)∼0], and, surprisingly, a v-j [ß00(22)] correlation that depends on the NO lambda doublet probed. The results point to the importance of the initial excited-state conformational change and uncover the presence of two photolysis channels.

A method for the determination of speed-dependent semi-classical vector correlations from sliced image anisotropies.

We present analytical expressions relating the bipolar moment ß(Q)(K)(k(1)k(2)) parameters of Dixon to the measured anisotropy parameters of different pump/probe geometry sliced ion images. In the semi-classical limit, when there is no significant coherent contribution from multiple excited states to fragment angular momentum polarization, the anisotropy of the images alone is sufficient to extract the ß(Q)(K)(k(1)k(2)) parameters with no need to reference relative image intensities. The analysis of sliced images is advantageous since the anisotropy can be directly obtained from the image at any radius without the need for 3D-deconvolution, which is not applicable for most pump/probe geometries. This method is therefore ideally suited for systems which result in a broad distribution of fragment velocities. The bipolar moment parameters are obtained for NO(2) dissociation at 355 nm using these equations, and are compared to the bipolar moment parameters obtained from a proven iterative fitting technique for crushed ion images. Additionally, the utility of these equations in extracting speed-dependent bipolar moments is demonstrated on the recently investigated NO(3) system.

UV photodissociation of η5-C5H5NiNO: an excited-state Jahn-Teller distortion produces a cartwheeling NO.

The 225 nm photodissociation of cyclopentadienylnickel nitrosyl was studied using velocity-mapped ion imaging with 1 + 1' REMPI detection of the NO (X (2)Π(1/2,3/2), v'' = 0) photofragment. The product recoil energy and angular distributions were measured for selected rotational states of NO. The NO product displays two speeds, a slow product peaked at the center of the ion image and a fast anisotropic product that has an inverted rotational population. In rotational states above J'' = 40.5, an even faster anisotropic NO photofragment appears, most likely because the metal-containing dissociation partner emerges in a lower electronic state, increasing the available energy. The µ-v-j vector correlations were measured and are consistent with the orientation µâˆ¥v⊥ j. The observed vector correlations arise from an excited-state Jahn-Teller distortion of the parent, a distortion that bends the Ni-NO coordinate prior to dissociation.

Determination of the v-j vector correlation in the photodissociation of nitrosobenzene at 305 nm.

The 305 nm photofragmentation dynamics of nitrosobenzene was measured using velocity-mapped ion imaging with polarized 1+1' resonance-enhanced multiphoton ionization probing of the NO (X, v"=0) fragment. The product recoil energy and angular distributions of selected rotational states of NO (X (2)Pi(1/2,3/2)) products have been measured. The recoil anisotropy of the NO photofragment is very small with beta(0)(2)(20)=0.03. The v-j vector correlation of NO shows a preference for v//j with an average value of beta(0)(0)(22)=+0.15 over the transitions studied. A mechanism involving dissociation from the ground state is used to explain the unusual propellerlike trajectory of the nascent NO.