Richard Bersohn

Theory of the angular distribution of molecular photofragments

The theory of the anisotropy of the flux of molecular fragments produced by polarized light has been discussed previously for linear molecules by Zare, Jonah, and Busch and by Wilson but is here analyzed more systematically for an arbitrary molecule. Polarized light creates a cosine squared distribution of excited molecules and the distribution of fragments is given by f(θ) = (1/4π)[1 + βP2(θ)], where θ is the angle between the detection direction and the electric vector of the light. The parameter β depends on the orientation in the molecular framework of the transition dipole moment and the dissociation direction and on the time averaged correlation functions 〈 〈 Dm m′(2) [δ Ω (t)]〉 〉 of the rotating molecules. The correlation functions can be evaluated exactly for linear and spherical top molecules and can be expressed in terms of one dimensional integrals for symmetric tops using the techniques of St. Pierre and Steele. Correlation functions for asymmetric tops have not yet been evaluated. These corre...

Double‐Quantum Light Scattering by Molecules

Double‐quantum light scattering by a system of molecules is discussed in this paper. Expressions have been obtained for the scattered light intensity considering both the coherent and incoherent contributions. In that coherent contributions are also considered in this treatment, it goes beyond the scope of previous studies. It is shown that, for molecules of low symmetry, elliptically polarized light must be used in order to determine five independent quadratic forms in the 18 symmetric components (βijk+βikj). According to the present results, the apparent discrepancy between the observed value of ⅓ for the depolarization ratio for CCl4 and the value to be expected from theory may be due to the fact that the coherent contribution had been neglected in previous theoretical considerations. In general, orientational correlation is essential if there is to be appreciable contribution from coherent scattering. For macromolecules, this constitutes a major difference between single‐ and double‐quantum scattering...

Photodissociation of molecular beams of aryl halides

A molecular beam photodissociation study has been made of a number of simple aryl iodides and bromides. The angular distribution of photofragments is characterized by an anisotropy parameter β which was measured for each molecule. The values of β depend on the lifetime of the excited state compared to the rotational correlation times of the molecule as well as on the orientation of the transition dipole with respect to the C–X bond. Knowing these orientations we can extract excited state liftimes as follows: methyl iodide, 0.07 picoseconds (psec); iodobenzene, 0.5 psec; α‐iodonaphthalene, 0.9 psec; and 4‐iodobiphenyl, 0.6 psec. It is concluded that methyl iodide directly dissociates but that the aryl compounds predissociate. The analogous aryl bromides have small anisotropy parameters and it is estimated that they live in the excited state perhaps two orders of magnitude longer than the aryl iodides. The data suggest a mechanism for predissociation in which S2 undergoes intersystem crossing to a triplet s...

Effect of Partial Deuteration and Temperature on Triplet‐State Lifetimes

The lifetime of the triplet state of aromatic molecules increases when deuterons are substituted for its protons. For naphthalene, our experiments and others show that the decay rate is linear with the number of protons and independent of the position of substitution. This result is shown to follow from Lins theory of nonradiative decay of excited states.

Photodissociation of molecular beams of methylene iodide and iodoform

Molecular beams of CH2I2 and CHI3 have been dissociated by polarized ultraviolet light and the angular distribution of the fragments has been measured. The photofragment mass spectra show that the lower lying excited states of CH2I2 and CHI3 dissociate to give an iodine atom and the corresponding radical. The angular distribution fitted the form 1+βP2(ϑ), where ϑ is the angle between the detection and polarization directions. β was 0.90±0.16 for CH2I2 and 0.46±0.16 for CHI3. These values prove that the molecule dissociates in a time short compared to its period of rotation. Moreover, the first two absorption bands in CH2I2 are shown to be of A1→B1 type polarized along the I–I vector, and in CHI3 they are of A1→E type polarized perpendicular to the threefold axis. A crude exciton model amounts for the number and symmetries of the energy states.

Measurement of the relative populations of I(2P01/2) and I(2P03/2) by laser induced vacuum ultraviolet fluorescence

The ground (2P03/2) and first excited (2P01/2) states of iodine atoms can absorb two photons at 304.7 and 306.7 nm, respectively, to reach 2D05/2 and 2D03/2 states. The excited atoms fluoresce twice, emitting an IR and then a VUV quantum. This is the basis of a new method for measuring the relative quantum yields of the two fine structure states at very short times after the atoms are formed. Quantum yields for I* production are reported for a number of alkyl halides and HI upon photodissociation.

Photodissociation of aryl and aryl–alkyl halides at 193 nm: Fragment translational energy distributions

Molecular beams of a number of aryl and aryl–alkyl halide molecules were photodissociated using an excimer laser at 193 nm and fragment translational energy distributions measured. Previous work had shown that dissociation of aryl halide molecules takes place by a spin–orbit dependent crossing from an intermediate (π, π*) delocalized state to a triplet (σ, σ*) state localized on the C–X bond (X=Cl,Br,I). The vibrational energy of the intermediate state remains in the aromatic framework; thus the fragment translational energy distribution reflects only the electronic but not the vibrational energy of the intermediate state. This fact is exploited in identifying the electronic nature of this state. Present work indicates that both phenyl and naphthyl halide molecules dissociate at 193 nm to produce vibrationally hot aryl radicals. The actual dissociation pathway depends on the competition between intersystem crossing and internal conversion which are functions of both ring size and halogen substituents. Iod...

Dissociation of methanol and ethanol activated by a chemical reaction or by light

When energized sufficiently either vibrationally or electronically, ROH (where R is methyl or ethyl) can dissociate to form H atoms and RO radicals. We have determined the translational energy release (〈ETr 〉=0.82Eavl ) and angular distribution (β=−0.60±0.03) from the laser induced fluorescence spectra of H atoms produced in the 193 nm photodissociation of CD3OH. We have also determined that the quantum yield for producing H from CD3OH is 0.86±0.10. In contrast, the reaction of O(1D)+CH4 which produces vibrationally excited CH3OH, has a quantum yield for producing H atoms of roughly 0.25 with only 22% of the available energy released as translation. We conclude that although the total available energy is the same in both cases, the dissociation of photoexcited methanol is prompt whereas the dissociation of chemically activated methanol shows some degree of internal vibrational equilibration.

The state distribution of OH radicals photodissociated from H2O2 at 193 and 248 nm

Photodissociation of H2O2 at 248 and 193 nm yields largely vibrationless OH radicals in their ground electronic state. At 248 nm, on the average about 64 and 3 kcal/mol of energy are relased as translational and rotational energy, respectively. At 193 nm the corresponding quantities are 92 and 8 kcal/mol. The distribution of the OH radicals over K″ peaks near K″=5 when dissociated at 248 nm and near K″=6 at 193 nm but the latter distribution is somewhat broader. Doppler width anisotropy data imply that at 248 nm a single upper state is reached but that at 193 nm several upper surfaces may be responsible for the absorption. It is concluded that the upper state potential functions(s) may be respresented by the sum of a large repulsive term depending only upon the distance between centers of mass of the OH radicals and a small angularly dependent term which generates the rotational excitation.

Energy Distribution of the Fragments Produced by Photodissociation of CS2 at 193 nm

Gaseous CS2 was dissociated at 193 nm into CS and S. The translational and internal energy distributions of the CS fragments were measured. There is now overwhelming evidence that the upper electronic state S3 is predissociative. In fact there are three upper states of importance, the initially excited S3, a state which dissociates to CS(X 1Σ) and S(1D) and a triplet state which dissociates to CS(X 1Σ) and S(3P). The CS fragments were rotationally excited with an average rotational energy ∼3.5 kcal/mole. The vibrational populations were also strongly inverted for both the 1D and the 3P dissociations and their surprisal plots were linear. CS fragments were found with v?7. Of the dissociations resulting in CS fragments with v?6, 80±10% of the S atoms are produced in the 1D state and 20±10% in the 3P state.