F. J. Comes

Dynamics of H2O2 photodissociation: OH product state and momentum distribution characterized by sub‐Doppler and polarization spectroscopy

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...

Doppler spectroscopy of OH in the photodissociation of hydrogen peroxide

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 dissocia...

Photofragmentation dynamics of hydrogen peroxide: Analysis of two simultaneously excited states

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 electro...

Influence of H2O2 internal motion on scalar and vector properties of OH photofragments

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≂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 an...

Photodissociation of room-temperature and jet-cooled water at 193 nm

Abstract The photodissociation dynamics of water in its first absorption band has been studied in detail by photolyzing room-tempera-ture and jet-cooled H 2 O with an ArF excimer laser at 193 nm. The fate of the ejected OH(X 2 Π) photofragments was probed by laser-induced fluorescence. The excess energy is transferred almost exclusively into translational motion of the products, ∂ t = 0.97. The rotational distribution depends strongly on the initial temperature. For warm water ( T = 300 K), the rotational distribution can be described by a Boltzmann distribution with a temperature parameter of 400 K. No significant difference between the two Λ components, probed via Q and R, P lines, was observed. In the case of jet-cooled H 2 O the rotational distribution of the Π − component of the Λ doublets can be described by a temperature parameter of 330 K; that of the Π + component strongly deviates from a Boltzmann distribution. The Λ doublet population shows an increasing inversion with increasing J OH . The dissociation process does not distinguish between the two spin-orbit states and the spin is only a spectator in the dissociation process of H 2 O at 193 nm. These results are compared with observations of the photolysis of water at 157 nm.

SPIN SELECTIVITY IN THE ULTRAVIOLET PHOTODISSOCIATION OF PHOSGENE

The ultraviolet photodissociation of phosgene in its first absorption band 1A2←1A1 was investigated by resonance enhanced multiphoton ionization and time of flight techniques. Nascent atomic chlorine fragments were observed and their state specific kinetic energy distributions were determined. Of the chlorine atoms 15% are produced in the excited 2P1/2 spin–orbit state with a mean kinetic energy of 3200 cm−1 compared to a value of 1500 cm−1 for the mean kinetic energy of the ground state chlorine atoms. The analysis of the kinetic energy spectra yielded evidence for a concerted three‐body decay. The formation of intermediate COCl is of minor importance in the dissociation process, the formation of a stable final COCl product can be excluded. Competing pathways on the upper potential energy surface are discussed. A significant excitation of the carbon monoxide CO fragments is predicted.

Joint product state distribution of coincidently generated photofragment pairs

The joint product state distribution of coincident fragment pairs formed in the same elementary photodissociation process has been determined. This correlation between quantum state populations of two molecular products has been measured by high resolution Doppler spectroscopy in conjunction with level‐specific detection of the ejected photofragments. One product molecule, formed in a specific quantum state, is excited by laser induced fluorescence and the wing of the corresponding Doppler profile is analyzed to determine the product state distribution of the partner fragment. In the photodissociation of jet‐cooled hydrogen peroxide at 193 nm two OH partner radicals are formed with comparable angular momenta. For a specific rotation of one hydroxyl radical the product state distribution of the partner fragment is centered at about the same rotation with a width of only a few rotational quanta. The orbital angular momentum is of the order of 1 ℏ and the impact parameter is extremely small. Experimentally o...

PHOTOFRAGMENT ENERGY DISTRIBUTION AND ROTATIONAL ANISOTROPY FROM EXCITATION OF HN3 AT 266 nm

Abstract The photodissociation dynamics of hydrazoic acid at 266 nm has been analyzed by high-resolution Doppler spectroscopy. The NH fragment carries most of the available energy as translation, f T (NH) = 0.32, and only 3% as rotation; N 2 is mainly internally excited, f int (N 2 ) = 0.48 with fairly low translational motion, f T (N 2 ) = 0.17. The rotational distribution is Gaussian-like, peaking at J NH ≈5–6. Previously published rotational state distributions are perturbed by rotational relaxation. The Λ doublets are populated statistically. The alignment of NH rotation versus the transition dipole moment of the parent is low ( A 0 (2) ⩽ 0.19). The vector correlation between the translational (ν NH ) and rotational ( J NH ) motion of the NH fragment is positive, indicating a preferential parallel alignment of ν NH and J NH .

Vector correlations in the photofragmentation of HN3

Hydrazoic acid was excited to its lowest electronic excited state A 1A‘ and the fragments were analyzed by high resolution Doppler spectroscopy. The NH fragment is rotationally cold, while N2 is strongly internally excited fint(N2)=0.48. The Λ doublets are populated statistically. The alignment of NH rotation vs the transition dipole moment of the parent is low (βμJ≤0.14). The vector correlation between the translational (vNH) and rotational (JNH) motion of the NH fragments is positive and increases with increasing JNH, indicating a preferential parallel alignment of vNH and JNH(βvJ≂0.40). The observed correlation between the transition dipole moment of the parent and the NH(1Δ) recoil velocity is negative at low NH rotations [βμv(JNH=2)=−0.4] and increases to positive values with increasing JNH. The HN3 distorts from a nonplanar configuration after excitation of a linear‐bent electronic transition in the NNN framework, resulting in a strong N2 rotation and relatively weak NH rotation. The upper potentia...

Photodissociation of hydrogen peroxide at 157 nm: rotational distribution of nascent OH(2Σ+, υ′, N′)

Abstract Photodissociation of H 2 O 2 at 157.8 nm yields electronically excited OH radicals. The state distribution of nascent fragments, OH( 2 Σ, υ′, N′ ), has been obtained by spectrally resolved resonance transitions of the OH (A → X) fluorescence. The OH(A) rotational excitation shows a distribution with a maximum at N′ = 21 with 43% of the total available energy (14040 cm −1 ) appearing in rotation of the electronically excited fragment. In contrast to rotation, the OH(A) vibrational mode is cold. Only 3% of the OH(A) radicals are in the OH(υ′ = 1) state. The fragments are formed statistically between the two spin components of OH( 2 Σ + ). The rotational excitation cannot be explained by a simple repulsion of the two fragments. The experimental results strongly suggest that the vibrational OH torsion mode of the parent H 2 O 2 is promoted into rotation of the fragments, which is also supported by theoretical arguments. According to these considerations the excess energy is mainly partitioned as rotational energy among the two OH product radicals.