Richard Bersohn

Bio: Richard Bersohn is an academic researcher from Columbia University. The author has contributed to research in topics: Photodissociation & Excited state. The author has an hindex of 39, co-authored 128 publications receiving 4488 citations. Previous affiliations of Richard Bersohn include Tel Aviv University & Hokkaido University.

Double‐Quantum Light Scattering by Molecules

Abstract: 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...

Theory of the angular distribution of molecular photofragments

Abstract: 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...

A nomenclature for Λ‐doublet levels in rotating linear molecules

Abstract: 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.

Photodissociation of molecular beams of methylene iodide and iodoform

Abstract: 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

Abstract: 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.

Activation of c-h bonds by metal complexes

Abstract: ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

Abstract: The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and ...

Perturbation theory of the non-linear optical polarization of an isolated system

Abstract: The non-linear optical polarization of an isolated atom or molecule is treated, giving careful consideration to secular and resonant terms in the perturbation expansion. The Method of Averages introduced by Bogoliubov and Mitropolsky is used. The case where resonance-induced excited state populations are negligible, which is relevant to a wide range of non-linear optical experiments, is examined in detail for polarizations through third order in the perturbing fields. This yields concise expressions which are valid for any combination of applied field frequencies, including static fields.

A Hierarchical and Comparative Kinetic Modeling Study of C1 − C2 Hydrocarbon and Oxygenated Fuels

Abstract: A detailed chemical kinetic mechanism has been developed to describe the oxidation of small hydrocarbon and oxygenated hydrocarbon species. The reactivity of these small fuels and intermediates is of critical importance in understanding and accurately describing the combustion characteristics, such as ignition delay time, flame speed, and emissions of practical fuels. The chosen rate expressions have been assembled through critical evaluation of the literature, with minimum optimization performed. The mechanism has been validated over a wide range of initial conditions and experimental devices, including flow reactor, shock tube, jet-stirred reactor, and flame studies. The current mechanism contains accurate kinetic descriptions for saturated and unsaturated hydrocarbons, namely methane, ethane, ethylene, and acetylene, and oxygenated species; formaldehyde, methanol, acetaldehyde, and ethanol.