Bradley Arnold

DETERMINATION OF EQUILIBRIUM CONSTANTS FOR WEAKLY BOUND CHARGE-TRANSFER COMPLEXES

Abstract— The evaluation of the equilibrium constants for charge‐transfer complex formation has been of interest for five decades. During this time, absorption spectroscopy using Benesi‐Hildebrand, or related methods, has been used to obtain the equilibrium constants. These methods require relatively high concentrations of donor or acceptor to be present in solution when weakly bound complexes are studied, conditions that lead to the formation of higher order complexes and inconsistent determinations of these constants. A new method is presented that allows weakly bound charge‐transfer complexes to be studied under low concentration conditions and the equilibrium constants to be determined accurately for the first time. Using this method, the equilibrium constant for the formation of 1, 2, 4, 5‐tetracyanobenzene/pentamethylbenzene charge‐transfer complex was found to be KCT= 6.8 ± 0.3 M−1 with an extinction coefficient at 400 nm of εCT= 150 ± 30cm−1M−1.

Standoff detection using coherent backscattered spectroscopy

Intense laser pulses may be used for standoff detection of energetic materials. Coherent backscattered spectroscopy offers a tremendous advantage over other spectroscopic detection techniques in that it uses stimulated or amplified spontaneous emission from the sample to produce a minimally divergent, directional beam back to the detection platform. The characteristics of the backscattered beam depend largely on the intensity and pulse width of the laser source as well as the concentration and photo-physical characteristics of the target molecule. Different target molecules will exhibit different backscattered emission signals, allowing differential detection of energetic materials in the vapor phase. Because of the highly directional nature of the coherent backscattered beam, detection limits in the vapor of less than 1 ppm at ranges up to 100 meters can be anticipated.

Characterization of near-infrared low energy ultra-short laser pulses for portable applications of laser induced breakdown spectroscopy

We report on the delivery of low energy ultra-short (<1 ps) laser pulses for laser induced breakdown spectroscopy (LIBS). Ultra-short pulses have the advantage of high peak irradiance even at very low pulse energies. This opens the possibility to use compact, rare-earth doped fiber lasers in a portable platform for point detection applications using LIBS for elemental analysis. The use of low energy ultra-short pulses minimizes the generation of a broad continuum background in the emission spectrum, which permits the use of non-gated detection schemes using very simple and compact spectrometers. The pulse energies used to produce high-quality LIBS spectra in this investigation are some of the lowest reported and we investigate the threshold pulse requirements for a number of near IR pulse wavelengths (785-1500 nm) and observe that the pulse wavelength has no effects on the threshold for observation of plasma emission or the quality of the emission spectra obtained.

Association constants for 1,2,4,5‐tetracyanobenzene and tetracyanoethylene charge‐transfer complexes with methyl‐substituted benzenes revisited

The association constants for charge-transfer (CT) complex formation of a series of methylated benzene donors with 1,2,4,5-tetracyanobenzene and tetracyanoethylene as acceptors were measured. In several cases the values determined previously using standard analysis techniques, such as Benesi–Hildebrand or related methods, were shown to be incorrect and a new method for determining the association constants for weak complexes is presented. A systematic error occured in the determination of these constants when standard analysis was carried out on weakly bound complexes. In general, the thermodynamic stabilities have been underestimated and the extinction coefficients for the CT absorptions overestimated. Furthermore, it was demonstrated that the ground-state stabilization of the complexes studied here is due primarily to non-bonded interactions and that the ion-pair contributions are minor in the ground state. A notable exception may be the tetracyanobenzene–hexamethylbenzene complex where preliminary evidence points to a significant contribution of the ion-pair state to the ground-state stability. This study raises significant questions about what is currently known concerning the thermodynamics of CT complexes because much of what is believed may be based on incorrectly determined constants. Copyright

Standoff detection of nitrotoluenes using 213-nm amplified spontaneous emission from nitric oxide

A method of standoff detection based on the observation of laser-induced fluorescence–amplified spontaneous emission (LIF-ASE) is described. LIF-ASE generates uniaxial intensity distributions of the observed fluorescence with the majority of intensity propagating along the excitation axis in both the forward and backward directions. The detection of bulk vapor at significant standoff distances is readily achieved. This method was used to detect NO directly and as a photoproduct after 213-nm excitation of 2-, 3-, and 4-nitrotoluene. The NO LIF-ASE spectra were studied as a function of buffer gas. These studies showed that the emission from different vibrational states was dependent upon the buffer gas used, suggesting that the populations of vibrational states were influenced by the environment. A similar sensitivity of the vibrational populations was observed when the different nitroaromatic precursors were used in nitrogen buffer gas. Such sensitivity to environmental influences can be used to distinguish among the different nitroaromatic precursors and facilitate the identification of the bulk vapor of these analytes.

AN APPLICATION OF TIME-RESOLVED LINEAR DICHROISM SPECTROSCOPY : THE EXCITED SINGLET STATE OF 1,2:5,6-DIBENZANTHRACENE

Abstract Time-resolved linear dichroism spectroscopy allows the relative transition moment vectors of the S1→ Sn, transitions to be obtained. This information can be used to aide in the assignment of the observed excited state and ground state transitions. The transition moment directions are constrained by the symmetry of the absorbing chromophore such that structural changes on the excited state surfaces can also be observed. The time dependence of the dichroism is related to the solution dynamics through the measurement of rotational correlation times. The spectroscopy of l,2:5,6-dibenzanthracene is presented as an example of these measurements.

Acousto-optical imagers for chemical and biological detection: growth and characterization of Hg2Cl2-xBrx crystals

We studied the growth and design of solid-solution crystal of mercurous chloride (Hg2Cl2) and mercurous bromide (Hg2Br2). The lattice parameters of the mixtures obey Vagard’s law in the studied composition range. The study demonstrates that properties are very anisotropic with crystal orientation, and performance achievement requires extremely careful fabrication to utilize theoretical AO figure of merit. In addition, some parameters such as crystal growth fabrication, processing time, resolution, field of view and efficiency will be described for imagers based on novel solid solution materials. It was predicted that very similar to the mercurous chloride and mercurous bromide solid solutions also have very large anisotropy, and acousto-optic figure of merit decreases significantly as function of the crystal orientation.

ROTATIONAL DYNAMICS OF EXCITED PROBES: THE ANALYSIS OF EXPERIMENTAL DATA

ABSTRACT The time course of molecular rotational motion can be observed directly using partially oriented samples produced by photoselection and fast laser techniques. The magnitude of the observed absorption dichroism or fluorescence anisotropy is related to the relative directions of the excitation and observation transition moment vector as well as the degree of orientation. The duration of the observed effect depends on the shape of the probe and the solvent viscosity. It has been the accepted procedure to assume isotropic rotational dynamics of the probe when calculating rotational time constants. This assumption is too severe. It has been shown that the assumption of isotropic rotational dynamics leads to incorrectly determined rotational time constants using simulated data. Systematic errors in these constants render them inappropriate for evaluating rotational dynamics theories such as Debye-Stokes-Einstein theory or extensions thereof.

Analysis of Charge-Transfer Complex Absorption Spectra and the Applicability of the Mulliken two-State Model

Abstract Complete absorption spectra, free from interference due to an unbound acceptor and donor, of several charge-transfer complexes have been examined. These spectra serve as a significant test to the applicability of the Mulliken two-state model for this series of complexes. the appearance of multiple new absorption bands and the trend in the observed oscillator strength of the CT transitions and the localized excitation within the complex spectrum require significant expansions of Mullikens simple two-state model. Multiple new CT absorption bands are frequently encountered and their appearance has been explained in terms of multiple ion-pair states participating in CT interactions. the relative positions of these bands are predicted based on the gas-phase ionization potentials of the donors and the electron affinity of the acceptor in excellent agreement with the observed spectra achieved. the integrated absorption intensities of the localized bands within each CT complex spectrum have been measured and used as an indicator of the extent on intensity borrowing by the CT transition. These measurements show that for the better donors used in this study, i.e. hexamethylbenzene and pentamethylbenzene, intensity borrowing is not significant. However, for the poorer donors used, an increasing contribution of the LE within the CT absorption bands is observed. a multi-state model, which includes the localized excited state of the acceptors well as additional ion-pair states, is the minimum required to describe these complexes.

Surface modification at nanoscale; Nanoparticle-nanowire transition (Conference Presentation)

This study provides excellent method to create large surface area and morphologies which can be used in drug delivery and for absorption of drugs. In addition provides knowledge about morphological transition