Abneesh Srivastava

KINETICS OF MOLECULAR TRANSPORT ACROSS A LIPOSOME BILAYER

Abstract Transport of the organic molecule malachite green, across a bilayer of dioleoylphosphatidylglycerol liposome in aqueous solution, has been observed using a new method, namely second harmonic generation (SHG). Following the rapid mixing of liposome and malachite green solutions, a decay of the SHG signal in time is observed as the malachite green bound to the outer liposome surface crosses to the inner liposome surface. The effect of liquid-crystalline versus gel liposome structures on the transport across the bilayer is reported also.

Reactions at suprathreshold energy: Evidence of a kinematic limit to the internal energy of the products

In numerous state-to-state dynamics studies of atom transfer reactions at energies substantially in excess of the barrier height, population is not observed in many of the energetically accessible product quantum states. Frequently, the experimental state distributions end for internal energies well below the total energy limit. The lack of observable population in these high energy states has led us to conclude that not all of the total available energy is accessible to the rovibrational states of the products. To explain this behavior we describe a model in which the internal energy is constrained by kinematic factors. We use this model to show that the energy constraint is quantitatively determined by the skew angle for the reaction. We apply this model to the results of state-to-state experiments on a large number of reactions and get quantitative agreement between our predictions and actual observations.

Quantum state distributions of HCl from the ultraviolet photodissociation of HCl dimer

The rovibrational state distributions for the HCl product of the 193 nm photodissociation of (HCl)2, (HCl)2+hν→H+Cl+HCl, have been measured. The HCl dimer is prepared in a supersonic expansion of HCl in Ar, and its photoproduct detected by resonant multiphoton ionization under collisionless conditions. The state distributions are extremely “cold,” with very little of the available energy deposited in either rotation or vibration of the surviving HCl molecule. Only v′=0 product is observed, and linear rotational surprisal analysis yields an extremely large surprisal parameter, θr′ of 95(8). The results are in excellent agreement with theoretical predictions.

State-to-state dynamics of the H+c-C6H12→H2(v′,j′)+c-C6H11 reaction

The rovibrational state distributions for the H2 product of the H+c-C6H12→H2+c-C6H11 abstraction reaction have been measured at 1.6 eV collision energy under single-collision conditions. The state distributions are compared to those for the H2 product of the reaction of H atoms with CD3(CH2)4CD3 and n-C6H14 that we have measured previously. Abstraction of secondary H atoms from the cyclic C6 alkane produces H2(v′=0,j′) and H2(v′=1,j′) rotational state distributions that are significantly colder than those from the reaction of the linear C6 alkane. These results are interpreted within a simple phenomenological local reaction model that we have developed for these atom+polyatom reactions. The substantial differences between the state distributions for the reactions of the cyclic and linear alkanes indicate that reactions of cyclohexane are not representative of reactions at all secondary C–H.

State-to-state dynamics of the H+CD3(CH2)4CD3→H2+CD3((CH2)3CH)CD3 reaction: Dynamics of abstraction of secondary H in linear alkanes

Abstract Quantum state distributions for the H 2 product of the reaction of H atoms with CD 3 (CH 2 ) 4 CD 3 at 1.6 eV collision energy have been measured to characterize the dynamics of hydrogen atom abstraction at secondary sites in linear alkanes. The measured distributions are compared with those obtained previously for the H+ n -C 6 H 14 →H 2 +C 6 H 13 reaction. The two distributions are quite similar. The small differences seen between them are evaluated within a local reaction model developed for these types of reactions.

State-to-state dynamics of H + c-RH → H2(v', j') + c-R reactions: the influence of reactant stereochemistry

Abstract Rotational state distributions for the H 2 v ′ =0 product from the H+c-C 4 H 8 and H+c-C 5 H 10 abstraction reactions are reported for 1.6 eV collision energy. These results are compared to those previously obtained by us for the H+c-C 6 H 12 reaction. The H 2 ( v ′ =0, j ′ ) rotational state distributions from the H+c-C 5 H 10 and H+c-C 6 H 12 reactions are identical, within experimental uncertainty. In contrast, the H+c-C 4 H 8 reaction produces an H 2 ( v ′ =0, j ′ ) distribution that is much warmer. These results are interpreted through a model of the reaction dynamics that invokes the characteristic stereochemistry of the reactants.

State-resolved dynamics of 248 nm methyl-iodide fragmentation on GaAs(110)

The 248 nm initiated dissociation dynamics of methyl iodide physisorbed on GaAs (110) is investigated using 2+1 REMPI detection of the CH3 umbrella mode. The velocity and vibrational distributions of the photofragment are dependent on the adsorbate coverage and surface termination. Two translational energy distributions are measured for the CH3 fragment at high coverage, i.e., 25 ML, with maxima at 1 and 1.9 eV. These maxima are within 0.5–0.7 eV of those obtained in direct dissociation of gas-phase CH3I. At 1 ML one distribution is measured with its maxima at 0.3 eV. A modification of the surface with chemisorbed iodine reduces the yield of the CH3 at 1 ML but not at 25 ML. The dissociation dynamics observed at 1 ML is attributed to bond scission following electron attachment while that at 25 ML to direct photodissociation. At 3 ML coverage both the 1 and 25 ML components are observed. The measured fragment vibrational distribution at 1 ML has a 4 times lower v″=1/v″=0 ratio compared with that at 25 ML. ...