K.V.S. Rama Rao

Laser isotope separation of 13C: A comparative study

IR laser chemistry of (CF3Br/Cl2) mixture and neat CF2HCl are examined in the context of 13C enrichment. Decomposition extent, enrichment factor and energy absorbed are measured for both systems at their respective optimum conditions. A direct comparison is obtained by keeping extraneous factors such as laser, its pulse duration, cell, irradiation geometry etc. the same. The halogen scavenged CF3Br MPD requires lower fluence compared to neat CF2HCl irradiation. Overall throughput for a product with 60–65% 13C content in a single stage is the same for both systems requiring a similar amount of energy. However, at lower enrichment levels, CF2HCl MPD is better than (CF3Br/Cl2) photolysis in terms of both product yield and energy absorption.

Real-time observation of :CF2 formation in the infrared multiple-photon dissociation of fluoroform-d

Abstract :CF 2 radicals are directly detected by transient absorption at 249 nm in the TEA CO 2 laser-induced decomposition of CDF 3 . Vibrationally hot :CF 2 radicals, T v ≈725 K, are produced in a 4–5 μs time scale. Second-order decay of :CF 2 radicals is also observed, yielding the dimerisation rate constant K 2 = 4.2 × 10 7 M −1 s −1 .

UV absorption spectrum and decay kinetics of CF2Br in CO2 laser-induced photodissociation of CF2Br2

Abstract A transient absorption spectrum has been observed in the pulsed CO 2 laser photodissociation of CF 2 Br 2 . This has been assigned to the intermediate CF 2 Br radical. The spectrum shows a broad band in the region of 220–320 nm with a maximum around 260 nm. The absorption cross section of CF 2 Br at 262 nm and its dimerization rate constant are reported.

Tea CO2 laser driven oxidation of tetrafluoroethene and decafluorocyclopentane with molecular oxygen. Evidence for the dioxetane mechanism

Abstract Time-resolved emission spectra of the TEA CO 2 laser induced oxidation of tetrafiuoroethene and decafluorocyclopentane with molecular oxygen indicate that the oxidation of tetrafluoroethene occurs by at least two processes. The dioxetane mechanism represents a faster reaction compared to the oxidation of transiently formed difluorocarbene.

Real-time observation of concerted benzene formation in the infrared multiphoton dissociation of 1,4-cyclohexadiene

Abstract The infrared-multiphoton dissociation of 1,4-cyclohexadiene opens up two dissociation channels from its ground electronic state. The major dehydrogenation channel ( > 95%) gives benzene and hydrogen whereas the retro-Diels—Alder reaction channel produces 1,3-butadiene and acetylene. The temporal evolution of benzene as a photoproduct is monitored. From the real-time formation of benzene, a unimolecular dissociation rate for 1,4-cyclohexadiene has been found to be (1.0±0.15) × 10 6 s −1 . The UV spectrum of nascent benzene reveals a red-shift of 8 nm. The effects of laser energy and substrate pressure on dissociation yield are also investigated.

Isotope selective ir-laser multffhoton dissociation of CF3Cl and selectivity enhancement with substrate pressure

Abstract Photolysis of CF 3 Cl with a 9.6 μm CO 2 laser has indicated, that carbon-13 selectivity increases when the excitation frequency is slightly red-shifted from the v 1 band centre at R(18). A selectivity enhancement with substrate pressure was also observed if the P(18) line, which is far red-shifted, is used at low fluence.

Sensitized multiphoton dissociation of UF6 IN SF6-UF6 mixtures by a tea CO2 laser

UF 6 undergoes decomposition in the presence of SF 6 when mixtures of both are irradiated with a TEA CO 2 laser. The mechanism for UF 6 decomposition may involve vibrational energy transfer from excited SF 6 and laser absorption from the same laser pulse by excited UF 6 in its vibrational quasi-continuum.

PHOTOIONIZATION OF CRYSTAL VIOLET IN AQUEOUS SOLUTION

Abstract— Laser flash photolysis studies were carried out on a triphenylmethane dye, crystal violet (CV+), at 248 nm in aqueous solutions. The results show that CV+ undergoes photoionization and the resulting transients CV‐2+, hydrated electrons (e‐aq) and CV+ radical formed by the reaction of e‐aq with CV+ have been characterized. Studies using suitable scavengers were done to support the characterization of the transient species. Laser intensity effects show that the ionization is biphotonic. Two mechanisms are proposed to explain the observed photoionization involving higher excited singlet state and/ or another long‐lived excited state of the dye.

Reduction of triphenyl methane dyes by the diphenyl ketyl radical and hydrated electrons: a time-resolved study

Abstract The reduction of four triphenyl methane (TPM) dyes (brilliant green (BG + ), malachite green (MG + ), crystal violet (CV + ) and methyl green (MEG 2+ )) by diphenyl ketyl radicals (BPH . ) and hydrated electrons (e aq − ) was studied by pulse radiolytic kinetic spectrophotometry. The rate constants for the reaction of BPH . with the cationic dyes are (8.0±1.0) × 10 8 , (1.5±0.2) × 10 9 , (4.0±0.5) × 10 8 and (1.0±0.15) × 10 10 dm 3 mol −1 s −1 respectively and for the reaction with e aq − are (9.5±1.0) × 10 9 , (9.3±1.0) × 10 9 , (4.0±0.7) × 10 10 and (3.7±0.4) × 10 10 dm 3 mol −1 s −1 respectively. The transient spectra resulting form the reduction of the dyes were characterized. The time-resolved spectra and pH variation indicate that the dye radical generated from the e aq - reaction is subsequently protonated. The dye radicals decay by a second-order process without giving rise to bleach recovery.

Four-centered concerted ethane elimination in the IR and UV laser photolysis of dimethyldisulfide. Real-time observation of S2 and CH3S radicals

Abstract CO 2 and KrF laser-induced dissociation of dimethyl disulfide opens two major primary dissociation channels. The moleculer elimination channel produces S 2 and C 2 H 6 via a four-centered transition state whereas the radical channel generates two CH 3 S radicals. The time-resolved UV absorption spectra of the transients CH 3 S and S 2 are reported. From the real time formation of CH 3 S radical, a unimolecular dissociation rate constant for dimethyl disulfide has been found to be 6.8 × 10 5 s −1 and ⩾ 1 × 10 7 s −1 in the IR and UV laser photolysis, respectively. In SF 6 -sensitised dissociation of dimethyl disulfide, a time lag between the laser pulse and the onset of dissociation is observed, which is ascribed to intermolecular energy transfer from vibrationally excited SF 6 to the substrate.