Kieran F. Lim

Trajectory simulations of collisional energy transfer in highly excited benzene and hexafluorobenzene

Quasiclassical trajectory calculations of the energy transfer of highly vibrationally excited benzene and hexafluorobenzene (HFB) molecules colliding with helium, argon and xenon have been performed. Deactivation is found to be more efficient for HFB in accord with experiment. This effect is due to the greater number of low frequency vibrational modes in HFB. A correlation between the energy transfer parameters and the properties of the intramolecular potential is found. For benzene and HFB, average energies transferred per collision in the given energy range increase with energy. Besides weak collisions, more efficient ‘‘supercollisions’’ are also observed for all substrate–bath gas pairs. The histograms for vibrational energy transfer can be fitted by biexponential transition probabilities. Rotational energy transfer reveals similar trends for benzene and HFB. Cooling of rotationally hot ensembles is very efficient for both molecules. During the deactivation, the initially thermal rotational distribution heats up more strongly for argon or xenon as a collider, than for helium, leading to a quasi‐steady‐state in rotational energy after only a few collisions.

Analytical methodology for the determination of urea: current practice and future trends

The determination of urea is important in a wide range of fields, including clinical diagnostics, environmental monitoring and food science. Numerous analytical techniques have been developed for the determination of urea, with no single technique dominant in all areas because of the diversity of applications. An overview of the existing analytical methodologies for urea is presented, and some new approaches are discussed, particularly those based on chemiluminescence detection to improve the sensitivity and the selectivity for the determination of this important analyte.

The detection of latent fingermarks on porous surfaces using amino acid sensitive reagents: A review

The introduction of ninhydrin treatment as a chemical technique for the visualisation of latent fingermarks on porous surfaces revolutionised approaches to forensic fingermark examination. Since then, a range of amino acid sensitive reagents has been developed and such compounds are in widespread use by law enforcement agencies worldwide. This paper reviews the development and use of these reagents for the detection of latent fingermarks on porous surfaces. A brief overview is provided, including an historical background, forensic significance, and a general approach to the development of latent fingermarks on porous surfaces. This is followed by a discussion of specific amino acid sensitive treatments.

Quasiclassical trajectory study of collisional energy transfer in toluene systems. I. Argon bath gas: Energy dependence and isotope effects

Experimental studies of collisional energy transfer from highly vibrationally excited toluene to various bath gases have recently been reported [Toselli and Barker, J. Chem. Phys. 97, 1809 (1992), and references therein]. A quasiclassical trajectory investigation for toluene in argon bath gas at 300 K for initial internal energies E’=41 000, 30 000, and 15 000 cm−1 is reported here. Collisional energy transfer is almost linearly dependent on E’. Predictions of energy transfer quantities are very sensitive to the average well depth of the assumed individual pairwise potentials, but is less sensitive to the detailed shape. Qualitative and quantitative agreement with experiment is obtained where the overall well depth is physically realistic. Isotope studies using 40Ar and pseudohelium (4Ar) bath gases indicate that energy transfer is independent of the mass of the bath‐gas collider, but perdeuteration increases 〈ΔE2〉1/2 by 13% over the undeuterated values.

Flow analysis based on a pulsed flow of solution: theory, instrumentation and applications.

The increased demands placed on solution propulsion by programmed flow systems, such as sequential injection analysis, lab-on-value technology, bead injection and multi-commutation, has highlighted the inability of many conventional pumps to generate a smooth, consistent flow. A number of researchers have examined ways to overcome the inadvertent, uncontrolled pulsation caused by the mechanical aciton of peristaltic pumps. In contrast, we have developed instruments that exploit the characteristics of a reproducible pulsed flow of solution. In this paper, we discuss our instrumental approaches and some applications that have benefited from the use of a reproducible pulsed flow rather than the traditional linear flow approach. To place our approach in the context of the continuously developing field of flow analysis, an overview of other programmed flow systems is also presented.

The a priori calculation of collisional energy transfer in highly vibrationally excited molecules: The biased random walk model

An a priori calculation of collisional energy transfer has been carried out, based on an extension of Gilbert’s ‘‘biased random walk’’ model [J. Chem. Phys. 80, 5501 (1984)]. The model assumes that energy migration during the collision is random except for certain physical and statistical constraints. It is shown that the probability of energy transfer can be obtained accurately from a relatively small number (10–50) of trajectories using a Smoluchowski equation and generalized Langevin equation approach. Calculations for the azulene/argon system, employing realistic inter‐ and intramolecular potentials, show excellent agreement with the experimental results of Rossi, Pladziewicz, and Barker [J. Chem. Phys. 78, 6695 (1983)] and Hippler, Lindemann, and Troe [J. Chem. Phys. 83, 3906 (1985)]. This suggests that the extended model may be reliably and economically used to calculate appropriate energy transfer quantities. Moreover, a number of general trends seen in experimental results can be rationalized with...

Lawsone: a novel reagent for the detection of latent fingermarks on paper surfaces

Lawsone (2-hydroxy-1,4-naphthoquinone) reacts with latent fingermark deposits on paper surfaces to yield purple-brown impressions of ridge details which are also photoluminescent; this compound represents the first in a completely new class of fingermark detection reagents.

Quasiclassical trajectory study of collisional energy transfer in toluene systems. II. Helium bath gas: energy and temperature dependences, and angular momentum transfer

The collisional deactivation of highly vibrationally excited toluene‐d0 and toluene‐d8 by helium bath gas has been investigated using quasiclassical trajectory simulations. Collisional energy transfer was found to increase with initial toluene internal energy, in agreement with the experiments of Toselli and Barker [J. Chem. Phys. 97, 1809 (1992), and references therein]. The temperature dependence of 〈ΔE2〉1/2 is predicted to be T(0.44±0.10), in agreement with the experiments of Heymann, Hippler, and Troe [J. Chem. Phys. 80, 1853 (1984)]. Toluene is found to have no net angular‐momentum (rotational‐energy) transfer to helium bath gas, although 〈ΔJ2〉1/2 has a temperature dependence of T(0.31±0.07). Re‐evaluation of earlier calculations [‘‘Paper I:’’ Lim, J. Chem. Phys. 100, 7385 (1994)] found that rotational energy transfer could be induced by increasing the mass of the collider, or by increasing the strength of the intermolecular interaction: in these cases, angular‐momentum transfer depended on the initi...

Pulsed flow chemistry: a new approach to solution handling for flow analysis coupled with chemiluminescence detection

Novel instrumentation for flow analysis, coupled with chemiluminescence detection, and based on pulsed flow chemistry is described. The pulsed flow chemistry manifold allows for rapid and efficient mixing of chemiluminescent reagent with sample immediately in front of the detector, thus giving high precision (less than 1% RSD) and detectability, particularly for rapid, short-lived emissions. The instrument is also characterised by a compact size, high sampling frequency (120–300 samples per hour) and low reagent consumption (less than 500 μl per cycle). Other flow analysis modes (e.g., flow injection, continuous flow and stopped-flow) are available by simply altering software settings. The suitability of the system was shown by using chemiluminescence reactions of analytical significance, namely the oxidation of luminol with hexacyanoferrate(III) in the presence of 4-hydroxy-3-methoxybenzeneacetic acid (homovanillic acid), and the reduction of acidic potassium permanganate by morphine in the presence of polyphosphates. The latter reaction was used to determine morphine in aqueous process samples and the results obtained with the pulsed flow instrument displayed good agreement (mean difference of 1.8%) with those of a previously validated flow injection analysis method.

Modeling collisional energy transfer in highly excited molecules

Data from classical trajectory simulations of the collision of a highly excited molecule with a monatomic bath gas are used to test the validity of the precepts used in the biased‐random‐walk (BRW) model for collisional energy transfer. This model assumes that energy migration during the collision is pseudorandom except for the constraint of microscopic reversibility, and leads to a simple displaced Gaussian form for the energy‐transfer probability distribution. The BRW assumptions are shown to be of acceptable validity to exact classical trajectory simulations. A simple analytical approximation to the mean‐square energy transfer per collision is obtained which reproduces the trajectory data to within an average of ±20%, and also gives acceptable accord with experimental data. The model shows that the magnitude of the average energy transferred per collision is governed by the time taken to traverse the overall interaction potential in and out from the appropriate collision diameter, by the internal energ...