Lutfur R. Khundkar

Picosecond monitoring of a chemical reaction in molecular beams: Photofragmentation of R–I→R‡+I

Studies of chemical reactions by molecular beam techniques and spectroscopic methods have provided extensive information on the nature of the intermediates involved and on the energy distribution in products.(1-6) Until now, however, there is no report of direct picoseconds time resolution of a chemical reaction in molecular beams.

Picosecond photofragment spectroscopy. III. Vibrational predissociation of van der Waals’ clusters

This paper, last in this series, reports on the picosecond dynamics of vibrational predissociation in beam‐cooled van der Waals’ clusters. Reaction rates have been measured for clusters (1:1) of phenol and cresol (p‐methylphenol) with benzene by the picosecond pump–probe photoionization mass‐spectrometry technique. Dissociation to form phenol (cresol) and benzene takes place from vibrational levels of the S_1 state of phenol (cresol) prepared by the pump laser. The predissociation rates were measured for a number of different excess energies upto ∼2500 cm^(−1), and the reaction threshold was found to be 1400 cm^(−1) above the S_1 origin for phenol–benzene and ∼1795 cm^(−1) for cresol–benzene, respectively. For phenol–benzene, the predissociation rates, following excitation of ring‐type modes, vs excess energy vary more or less smoothly. Cresol–benzene exhibits biexponential decay, with the fast component becoming more dominant at higher energies. A non‐RRKM model involving division of the vibrational phase space is discussed to explain this observation.

Exciton and vibronic effects in the spectroscopy of bianthracene in supersonic beams

Excitation and dispersed fluorescence spectra of 9,9’‐bianthracene in a supersonic expansion are reported. The spectra are anthracene‐like, indicating that the rings are weakly coupled. Exciton effects are considered in the interpretation of the spectra. The torsional potential in S_1 is modeled as a double‐well (Gaussian perturbation on a one‐dimensional harmonic oscillator) with barriers to perpendicularity and planarity of ∼30 and ∼1100 cm^(−1), respectively. The S_0 torsional potential shows negative anharmonicity which is modeled as a quartic perturbation. Anthracenic modes in S_1 and S_0 are also assigned. Finally, measurements of S_1 fluorescence lifetimes up to ∼6000 cm^(−1) excess energy in the excited state show no evidence of charge transfer.

Picosecond photofragment spectroscopy. IV. Dynamics of consecutive bond breakage in the reaction C2F4I2→C2F4+ 2I

Picosecond photofragment spectroscopy of the ultraviolet (UV)photodissociation of 1,2‐diiodotetrafluoroethane reveals consecutive breaking of the two C–I bonds. Spin–orbit excited (I^*) atoms show a prompt rise, in agreement with a direct mode dissociation of the first bond. Ground‐state (I) atoms show a biexponential buildup, one component being fast (≤1 ps) while the other component is slow (30–150 ps depending on total energy), characteristic of the second bond breaking. The transient behavior of I atoms changes with the available energy. These results are interpreted in terms of a two step model involving a weakly bound radical. Simulations of transient behavior of I atoms, based on estimated internal energy distributions from the primary step and a model for dissociation rates as a function of energy, suggest that surface crossings are relevant to the dynamics and that the quantum yield of I atoms varies with excitation energy.

Picosecond mass spectrometry of a collisionless photodissociation reaction

We wish to report on the direct observation (in real time) of a photodissociation reaction under collisionless conditions. This is achieved by the technique of picosecond mass spectrometry in skimmed molecular beams.

Unimolecular reaction rates in solution and in the isolated molecule: Comparison of diphenyl butadiene nonradiative decay in solutions and supersonic jets

The recent study of diphenyl butadiene (DPB) in supersonic jets and in solution by Shepanski et al.(1) and by Courtney and Felming(2), respectively, provides an opportunity to compare the isomerization rates measured in the isolated molecule (jet) with those measured at very low viscosity in solution. These comparisons should shed light on the vibrational energy flows between “optical” and “reactive” modes in the isolated molecule and on the connection between activated, friction dependent, models of barrier crossing in solution,(3-5) and statistical RRK (or RRKM) theories of gas phase unimolecular reactions(6).