P.N. Bajaj

Photodissociation Dynamics of Phosphorus Trichloride (PCl3) at 235 nm Using Resonance Enhanced Multiphoton Ionization (REMPI) with Time-of-Flight (TOF) Mass Spectrometry

The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersonic beam by resonance enhanced multiphoton ionization (REMPI), using time-of-flight (TOF) mass spectrometry. The study is focused on the nascent state of the primary chlorine atom, formed on excitation of the (n, sigma*) transition of the molecule around 235 nm. Dissociation of PCl(3) and the REMPI detection of chlorine atoms are performed, using the same laser around 235 nm. The photofragments, namely, Cl((2)P(3/2)) and Cl*((2)P(1/2)), are probed, using the 2+1 REMPI scheme in the 234-236 nm region. We have determined the photofragment speed distribution, the recoil anisotropy parameter beta, and the spin-orbit branching ratio for chlorine atom elimination channels. Polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies. The anisotropy parameters for Cl and Cl* are characterized by values of 0.0 +/- 0.05 and 0.20 +/- 0.05, respectively. Two components, namely, the fast and the slow, are observed in the speed distribution (P(v)) of Cl and Cl* atoms, formed from different potential energy surfaces. The average translational energies for the Cl and Cl* channels for the fast component are 29.7 and 30.6 kcal/mol, respectively. Similarly, for the slow component, the average translational energies for the Cl and Cl* channels are 9.5 and 9.1 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of an impulsive model, for the fast component, and a statistical model, for the slow component. Apart from the chlorine atom elimination channel, molecular chlorine (Cl(2)) elimination is also observed in the photodissociation of PCl(3). The observation of the molecular chlorine in the dissociation process and the bimodal translational energy distribution of the chlorine atom clearly indicate the existence of a crossover mechanism from the initially prepared state to the ground state.

Dynamics of Cl (2Pj) atom formation in the photodissociation of fumaryl chloride (ClCO - CH = CH - COCl) at 235 nm: a resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) study.

The photodissociation dynamics of fumaryl chloride (ClCO-CH═CH-COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl ((2)P(3/2)) and Cl* ((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, β(i). The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(E(T)), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.

Infrared diode laser diagnostic of supersonic free jets

Semiconductor diode laser absorption has been used to measure the rotational temperature of pure SF6 and SF6 diluted by Ar in free supersonic expansions. Intensity ratios of Q-branch band heads were found to be a sensitive probe of rotational temperature. We did not observe any convincing evidence of condensation and found that rotational freezing took place within 50 nozzle diameters.

Relaxation in pure and seeded supersonic jets of SF6

Abstract Nozzle expansion of pure SF 6 and SF 6 seeded in carrier gases e.g. Ar, He and N 2 , is used to study relaxation of the various degrees of freedom. The diode laser absorption spectrum of the Q branch of the ν 3 mode of SF 6 is used to derive the rotational temperature ( T rot ). Time-of-flight studies determine the translational temperature ( T tg ) while the vibrational temperature ( T vib ) is estimated from the energy balance equation. Rotational and translational temperatures follow each other closely while the vibrational temperature lags behind considerably. The effective specific heat ratio (γ) and the collisional effectiveness parameter ( e ) for SF 6 are determined from these measurements. Seeded-beam data prove argon to be a better refrigerant than He or N 2 . Cooling in seeded beams at moderate stagnation pressures is not substantially enhanced even at very high dilution.

Photodissociation Dynamics of Halogenated Thiophenes at 235 nm: A Resonance Enhanced Multiphoton Ionization-Time-of-Flight (REMPI-TOF) Study

The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2-bromo-5-chlorothiophene, has been studied in a supersonic molecular beam around 235 nm, using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary halogen atoms. A single laser has been used for excitation of halothiophenes, as well as for the REMPI detection of photoproducts, namely, chlorine and bromine atoms, in their spin-orbit states X((2)P(3/2)) and X*((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio, for chlorine and bromine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies, β(ι). The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the β is well characterized by a value of ~0.0, within the experimental uncertainties. For 2-chlorothiophene, we have observed two components for the Cl and only one component for the Cl* atom elimination channel in the translational energy distributions. The average translational energies for the fast and the slow components of the Cl channel are 3.0 ± 1.0 and 1.0 ± 0.5 kcal/mol, respectively. For Cl*, the average translational energy is 3.5 ± 1.0 kcal/mol. For 2-bromo-5-chlorothiophene, we have observed only one component for Cl, Cl*, Br, and Br* in the translational energy distributions. The average translational energies for the Cl and Cl* channels are 3.5 ± 1.0 and 5.0 ± 1.0 kcal/mol, respectively, whereas the average translational energies for the Br and Br* channels are 2.0 ± 1.0 and 3.5 ± 1.0 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The ΔH(f)(298) value for 2-chlorothiophene has been estimated theoretically to be 23.5 kcal/mol.

Associative ionisation of laser-excited uranium with molecular oxygen

Abstract Associative ionisation of resonantly excited ( 5 L 6 0 to 7 M 7 ) atomic uranium with molecular oxygen has been investigated in a crossed beam configuration. The laser-enhanced associative ionisation cross section (5×10 −14 cm 2 ) for the formation of UO 2 + ions has been found to be three orders of magnitude higher as compared to the ground state atom reaction cross section.

Condensation of SF6 in seeded supersonic jets

Fractional condensation in seeded supersonic free jets of SF 6 has been determined at different locations along the jet axis and at different stagnation pressures using a diode laser absorption technique. Rotational temperature and relative number densities of SF 6 have been determined from Q-branch absorption of the v 3 band. The fractional condensation increases with increased stagnation pressure as well as with increased dilution. The onset pressure for condensation of SF 6 in pure SF 6 jets is above 2000 torr, whereas in seeded jets, it is much lower (900-1200 torr). Calculated dimer mole fractions do not account for the observed degree of condensation. Experimentally observed local onset number density can be reasonably correlated with the trimer critical model of Fisher.

Thermal stability and IR-laser-driven selective photochemistry of a volatile uranyl compound at natural abundance

Abstract Molecular beam mass spectrometric measurements have been carried out to investigate the IR-laser chemistry of bis(1,1,1,5,5,5,-hexafluoropentane-2,4-dionato)dioxouranium(VI) tetrahydrofuran [UO 2 [(CF 3 CO) 2 CH] 2 ] · THF. The compound has been characterized and shown to be stable in the gas phase up to 95°C. An isotope selectivity, factor for 238 U species as high as three was obtained in on-line dissociations of this compound (with natural abundance of uranium isotopes) using the 10P(6) line of a TEA CO 2 laser IR-laser dissociation studies have also been carried out with the vapour in static or flowing condition in a cell. Negligible isotope selectivity is observed in the cell experiments.