Bibliothek

Notizen: The formation of ground state OH(X) radicals from the photolysis of jet cooled H2O2 at 193 nm is studied by Doppler and polarization spectroscopy. The features of the process are characterized by a complete analysis of the scalar and vector properties of the fragments. In the dissociation process 85% of the available energy is released into fragment translation. The remaining part emerges as rotational excitation that performs a narrow Gaussian-like distribution peaking at N=12 with a FWHM of ΔN(approximately-equal-to)5. The vector correlations between the transition dipole moment μ of the H2O2 and recoil velocity v as well as angular momentum J of the products were evaluated in terms of four bipolar moments. The observed 〈μ ⋅ v〉 correlation was used to determine the state specific contribution of both the A˜ 1A and B˜ 1B dissociative states to the overall product rotational distribution. On the average, 65% of the OH fragments are formed via the 1A state. A comparison of data obtained from the photolysis of room temperature and jet cooled H2O2 molecules indicates that transfer of parent rotation causes a symmetric broadening of the product distribution and a small increase in the 〈v ⋅ J〉 correlation [βvJ(T(approximately-equal-to)20 K)=0.5, βvJ(T=300 K)=0.7] of the fragments. In order to describe the influence of initial parent motion on the product state distribution and on vector correlations a model is used where the formation of two OH radicals in the same microscopic event is considered.

Notizen: It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A') and Λ(A‘), e.g., Π(A') and Π(A‘) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A') for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A‘). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A')/Λ(A‘) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules.

Notizen: The dynamics of the photodissociation of hydrogen peroxide has been analyzed by a complete characterization of the scalar and vectorial properties of the OH fragment using Doppler and polarization spectroscopy. When hydrogen peroxide is optically excited at 193 nm the hydroxyl radicals are formed exclusively in the X 2Π3/2,1/2 ground state with 84% of the available energy (Eav=417 kJ/mol) being released as OH recoil translation. The remaining energy is transferred in product rotation showing a strongly inverted rotational state distribution peaking at N‘=12. Vector correlations between the transition dipole moment of the parent H2O2 and the OH product rotational and translational motions were observed by Doppler broadened spectral lines and evaluated in terms of four bipolar moments. The quantitative contribution of two different electronic excited states in the dissociation process could be determined by analyzing the vector properties of the fragment. 62% of the OH products evolve from the A˜ 1A electronic excited state while 38% of the fragments are formed via the B˜ 1B state when hydrogen peroxide is excited at 193 nm. The OH rotational state distributions when produced from the A˜ 1A and the B˜ 1B state show no remarkable difference. The vector correlation of the recoil velocity vOH and the rotation JOH is strongly positive and increases with increasing JOH indicating a strong preference towards vOH and JOH being parallel to one another. The major part of product rotation is caused by a strong dependence on the torsion angle of the two upper potential surfaces.

Notizen: Hydrogen peroxide has been optically excited at a wavelength of 266 nm and the OH photofragment completely characterized by Doppler and polarization spectroscopy using the laser-induced fluorescence technique. The entire internal state distribution (vibration, rotation, spin, and Λ components), translational energy, angular distribution, rotational alignment, and vector correlations between rotational and translation motions of OH products is measured. The hydroxyl radicals are formed in the X 2Π3/2,1/2 ground state with 90% of the available energy (248 kJ/mol) being released as OH recoil translation. The angular distribution is nearly a sin2 θ distribution about the electric vector of the photolysis laser. The internal motion of OH is vibrationally cold (no vibrationally excited OH was found) while the rotational excitation in v‘=0 can be described by a Boltzmann distribution with a temperature parameter of Trot=(1530±150) K. The two spin states are found to be populated nearly statistically, in contrast to the Λ components which show an increasing inversion with increasing OH rotation. The observed profiles of recoil Doppler broadened spectral lines are strongly dependent on the nature of the transition, the excitation–detection geometry, and the relative polarizations of the dissociating and analyzing laser light. However, the line intensities show only a minor dependence on geometry and polarization indicating a low alignment of OH photofragments (A(2)0≤0.1). For the first time the vector correlation between product rotational and translational motions was analyzed and evaluated in terms of the three bipolar moments β20(20), β00(22), and β20 (22). The bipolar moment β20(20) corresponds to the conventionally defined anisotropy parameter β=2β20(20) =−0.71. The angular distribution peaks in the direction perpendicular to the electric vector of the dissociating laser light, indicating the predominant electronic excited state in H2O2 being of 1A symmetry. The moment β00(22) increaseswith JOH showing a bias towards vOH and JOH being parallel to one another. The moment β20(22) is a measure of the mutual correlation of the fragment translational and rotational vectors and the transition dipole vector μ in the parent molecule. The positive value of this moment [β20(22)=0.11] indicates that the expectation value of 〈J2y〉 should be very small when μ is parallel to the z axis and vOH perpendicular to μ (x axis). 〈J2x〉 originates in the torsional motion in the H2O2 parent molecule, while 〈J2z〉 reflects the bending vibration of nearly planar H2O2 where the H atoms are in the trans position.

Notizen: The photofragmentation of hydrogen peroxide has been investigated at an excitation wavelength of 266 nm using polarized laser beams. Doppler spectroscopy in connection with the laser induced fluorescence technique was applied to determine the nascent translational distribution of OH products. The hydroxyl radicals are formed in their electronic ground state, X 2Π3/2,1/2. The released energy is transformed almost exclusively in translation of these photoproducts. The Q1(4) transition in OH(X 2Π3/2→A 2Σ+) was used to study the anisotropic angular distribution of recoiling OH photofragments and analyzed to obtain information about the lifetime and symmetry of the excited dissociative state. The Doppler profiles for different directions of the electric field vectors of dissociation and probe laser relative to the detector axis have been determined for two different geometries. The angular distribution measured by Doppler spectroscopy, peaks in the direction perpendicular to the electric vector of the dissociating laser light, indicating that the predominant electronic excited state in H2O2 must be of 1A symmetry. An upper limit for the lifetime τ of the excited H2O2 parent molecule before dissociation is τ≤6×10−14 s.

Notizen: The correlation between angular momenta of the coincident product pairs formed in an elementary dissociation process has been investigated. Doppler shift measurements are used in connection with state-specific detection of scattered photofragments by laser-induced fluorescence. The method is demonstrated in the photodissociation of H2O2 and D2O2 at 193 and 266 nm where Doppler profiles of OH (OD) absorption lines are used to determine the (average) rotational energy of the OH (OD) partner molecule coincidently formed in the same elementary fragmentation process. At high product rotations the partner molecules are generated with comparable angular momenta, while at low rotational excitation a more complex correlation is observed. In addition to the partner fragment distribution, dissociation energies were determined to be 197 kJ/mol for HO–OH and 204 kJ/mol for DO–OD, which are significantly lower than the previously recommended values.