I. Bar

Photodissociation of HOD (νOD=3): Demonstration of preferential O–D bond breaking

It has been predicted that photodissociation of vibrationally excited HOD may preferentially yield either OD+H or D+OH, depending on the vibrational mode and the dissociation wavelength. To date, only the former preference has experimentally been demonstrated. In the present work preferential O–D bond breaking has been achieved from the photolysis of HOD (νOD=3) at 193 nm. HOD was prepared in a specific rovibrational level of the second overtone of the O–D stretch via infrared excitation. The subsequent photolysis led to enhancement of both OH and OD production, the OH/OD branching ratio being 2.6±0.5. The results agree with the predictions of Imre and co‐workers [J. Phys. Chem. 93, 1840 (1989)] on the enhancement of νOD≥3 photodissociation but differ in the branching ratio obtained at the specific photolysis wavelength.

Rotational-state dependent selectivity in the bond fission of C2HD (5ν1)

Abstract We report the first demonstration of rotational-state selectivity in bond fission. C 2 HD is prepared in the 5 ν 1 vibrational-state and photodissociated by ∼ 243.1 nm photons that also probe the H/D fragments. The production of both H and D is greatly enhanced upon rovibrational excitation and is rotational-state dependent. The H/D branching ratio for the photodissociation of C 2 HD (5 ν 1 ) is 1.5 ± 0.2 for J ′ = 2 and 4.9 ± 0.5 for J ′ = 4. Possible mechanisms for the rotational dependence are discussed.

Photodissociation of rovibrationally excited C2H2: Observation of two pathways

C2H2 is prepared in the 2030000 (five quanta of C–H stretch) vibrational state and photodissociated by 243.135 nm photons that also probe the H photofragments via (2+1) resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The production of H atoms is greatly enhanced upon rovibrational excitation. The REMPI action spectrum shows the characteristic features of a Σu+–Σg+ band and mimics the absorption spectrum, except that the R(13) line intensity is an order of magnitude higher than that expected for a Boltzmann distribution. The maximum translational energy of the H atoms obtained from dissociation of the regularly distributed rotational states is 0.67±0.10, whereas for R(13) it is 1.34±0.10 eV. The observed intensities and linewidths indicate the existence of two photodissociation pathways following the preparation of C2H2, where the C2H fragment is produced in two different states. In the R(13) pathway an additional bent state is prepared, or an accidental coincidence...

Symmetry and geometry of the first two excited singlet states of dimethylnitrosoamine studied by vector correlations

A study of the geometry and symmetry of two different excited potential surfaces of the same molecule is demonstrated for the first time by analyzing the alignment and Λ doubling of the NO fragment resulting from the photodissociation of dimethylnitrosoamine irradiated at 363.5 nm, [S1(nπ*)←S0], and 250 nm, [S2(ππ*)←S0]. The values obtained for the alignment and Λ doubling indicate that (1) the fragmentation process is fast in both cases; (2) there is some planarity in the fragmentation process in both cases; (3) at 250 nm the transition dipole moment lies in the frame of the molecule, while at 363.5 nm it is perpendicular to the frame of the molecule; (4) the first excited state has an A‘ symmetry and the second excited state has an A’ symmetry.

Real time diagnostics of detonation products from lead azide using coherent anti‐Stokes Raman scattering

We report on the first example that applies coherent anti‐Stokes Raman scattering (CARS) to real time diagnostics of detonation products from a solid explosive. The supersonically expanding gaseous products, from the detonation of either lead azide pellets or powders, are studied, using broadband CARS. The density, and the rotational and vibrational temperatures of ground electronic state N2 molecules, N2(X), are monitored as a function of time at a fixed distance, 3.3 cm, from the azide surface. The rotational temperature is low, around 200–300 K, while the vibrational temperature is around 2000 K for delays of 8–12 μs, following the initiation of detonation. The density of N2(X), during this time interval, increases from ∼1×1017 to 1×1018 cm−3.

Control of fragment alignment via photodissociation from different types of parent rotation

The alignment of the OH fragment following dissociation of H2O (1,0,0) at 193 nm has been measured as a function of initially prepared parent rotations. The degree of alignment is almost perfect for an in‐plane rotation, but is reduced markedly for a mixture of in‐ and out‐of‐plane rotations.

Alteration of Cl spin–orbit branching ratios via photodissociation of pre-excited fundamental CH3 stretch of CH3CFCl2

Abstract The fundamental symmetric CH 3 stretch ( ν 2 ) in the ground electronic state of CH 3 CFCl 2 is efficiently prepared by stimulated Raman excitation. The excited molecules are photodissociated by ∼235 nm photons that further tag Cl( 2 P 3/2 ) and Cl( 2 P 1/2 ) [Cl * ] photofragments via (2+1) resonantly enhanced multiphoton ionization. The yield of both photofragments increases as a result of pre-excitation, indicative of energy flow from the excited C–H to the C–Cl bond and hence a better Franck–Condon overlap between the components of the wavefunction along the C–Cl coordinate. The Cl * /Cl branching ratio is 0.60±0.09 in ∼235 nm photolysis of vibrationally excited molecules and 0.40±0.07 in that of vibrationless ground state CH 3 CFCl 2 .

Overtone spectroscopy of methyl C–H stretch vibration in CH3CF2Cl and CH3CFCl2

Photoacoustic spectra of the second (3νCH), third (4νCH), and fourth (5νCH) overtones of the methyl C–H stretches in CH3CF2Cl and CH3CFCl2 were measured. The spectra are characterized by a multiple peak structure of partially resolved triplets and quartets with an anomalous linewidth decrease in the 4νCH region. The results are interpreted in terms of a simplified local mode model for C–H stretching vibrations, including also the stretch-deformation Fermi resonances. The model accounts for most spectral features and allows determination of the time scale for vibrational redistribution.

Films of chalcogenide glasses as perspective materials for optical information recording

Scalar and vectorial photoinduced processes in films of chalcogenide glasses (ChG) As-S-Se- Te and Ge-S-Se-Pb systems are considered. Both irreversible and reversible highly resolving materials for recording and storage of optical information, sensitive in a wide spectral range, including the near infra-red region, are developed. The photosensitivity of some films can be increased 103 - 104 times when excitation is accomplished by very short intense laser pulses. The polarization information recording (reversible and irreversible) is fulfilled for some ChG films and is based on the phenomena of photoinduced linear dichroism and polarized photodoping of the films by silver. The application of the ChG films in systems of optical information recording, in holography, and in integral optics is discussed.

Real-time measurement and control of particle-number density and size of the detonation products of lead azide

Time-resolved measurement and modeling of the number density and size of lead particles produced following the detonation of Lead Azide (LA) are presented. Particles expanding freely into vacuum through a supersonic nozzle or interacting with a barrier placed above the LA sample are monitored via attenuation of laser beams at 0.67, 1.3 and 10.6 µm. The attenuation depends on the conditions of expansion, but is always much more pronounced at 0.67 µm and 1.3 µm. From the ratio between the attenuations at 0.67 µm and 10.6 µm, the radius and number density of the particles are calculated applying Beers law and Mies theory. It is found that 20–90 µs following the detonation the attenuation at 32–36 mm above the LA sample is due to particles with radii of ≈0.9, ≈0.7 and ≈0.1 µm for free expansion into vacuum through the nozzle or near the barrier, respectively. Also, the expansion through the nozzle results in a transparent medium above the nozzle exit for the first few µs following the detonation. The effect of the nozzle is attributed to the fact that the velocity of the expanding detonation products is supersonic, which leads to compression and heating in the throat region, in contrast to the more familiar phenomenon of cooling at subsonic velocities. The dynamics of particles expanding under the different conditions and the mechanism of size reduction and elimination of particles is discussed.