Prabhakar Misra

Thermal Effects Associated with the Raman Spectroscopy of WO3 Gas-Sensor Materials

Metal oxides are suitable for detecting, through conductive measurements, a variety of reducing and oxidizing gases in environmental and sensing applications. Metal-oxide gas sensors can be developed with the goal of sensing gases under specific conditions and, as a whole, are heavily dependent on the manufacturing process. Tungsten oxide (WO3) is a promising metal-oxide material for gas-sensing applications. The purpose of this paper is to determine the existence of a correlation between thermal effects and the changes in the Raman spectra for multiple WO3 structures. We have obtained results utilizing Raman spectroscopy for three different structures of WO3 (monoclinic WO3 on Si substrate, nanopowder, and nanowires) that have been subjected to temperatures in the range of 30-160 °C. The major vibrational modes of the WO3:Si and the nanopowder samples, located at ~807, ~716, and ~271 cm(-1), correspond to the stretching of O-W-O bonds, the stretching of W-O, and the bending of O-W-O, respectively; these are consistent with a monoclinic WO3 structure. However in the nanowires sample only asymmetric stretching of the W-O bonds occurs, resulting in a 750 cm(-1) band, and the bending of the O-W-O mode (271 cm(-1)) is a stretching mode (239 cm(-1)) instead, suggesting the nanowires are not strictly monoclinic. The most notable effect of increasing the temperature of the samples is the appearance of the bending mode of W-OH bonds in the approximate range of 1550-1150 cm(-1), which is related to O-H bonding caused by humidity effects. In addition, features such as those at 750 cm(-1) for nanowires and at 492 and 670 cm(-1) for WO3:Si disappear as the temperature increases. A deeper understanding of the effect that temperature has on the Raman spectral characteristics of a metal oxide such as WO3 has helped to extend our knowledge regarding the behavior of metal oxide-gas interactions for sensing applications. This, in turn, will help to develop theoretical models for the identification of specific metal oxide-gas relationships.

Laser excitation and emission spectroscopy of the methoxy radical in a supersonic jet

Abstract Laser-induced excitation and wavelength-resolved emission spectra of the methoxy (CH 3 O) radical have been obtained in a supersonic jet environment. Fluorescence in the near ultraviolet from several vibronic bands belonging to the A 2 A 1 - X 2 E electronic system of CH 3 O has been dispersed by a 0.6 m monochromator with a resolution of 0.3 nm. A complete set of vibrational frequencies with all assignments for the X 2 E state of CH 3 O has been obtained. To the best of our knowledge, the dispersed spectra of CH 3 O for the 2 2 0 and 2 1 0 3 1 0 bands are presented here for the first time. The vibrational constants determined by a least-squares fit for the ν 3 mode are ω″ e = 1071 cm −1 and ω″ e x ″ e = 8.4 cm −1 . For the Jahn-Teller active ν 6 mode, the vibrational and anharmonic constants ω″ e = 786 cm −1 and ω″ e x ″ e = 55 cm −1 , respectively, have been obtained for the first time.

Laser optogalvanic wavelength calibration with a commercial hollow cathode iron-neon discharge lamp

Abstract-351 optogalvanic transitions have been observed in the 337-598 nm wavelength region using an iron-neon hollow cathode discharge lamp and a pulsed tunable dye laser. 223 of these have been identified as transitions associated with neon energy levels. These optogalvanic transitions have allowed, in conjunction with interference fringes recorded concomitantly with an etalon, the calibration of the dye laser wavelength with 0.3 cm-’ accuracy. INTRODUCTION Wavelength calibration of tunable lasers is important in experimental laser spectroscopy. An iodine absorption cell can be used for precise calibration of the spectral region 50&675 nm in the visible, since a good atlas exists for that region. ’ However, in the blue and near U.V. regions of the electromagnetic spectrum, there is a paucity of suitable species for wavelength calibration of tunable lasers. Tellurium has been employed by Miller.’ However, its spectral atlas is not easily accessible. The optogalvanic (OG) effect provides a good solution for wavelength calibration inadequacies in the visible and near U.V. regions of the electromagnetic spectrum. The OG effect, discovered by Penning3 in 1928, is the impedance change of a neon discharge tube when illuminated by a second neon discharge tube. In 1976, Green et al4 observed the OG effect with a tunable dye laser and measured the changes in voltage across a low-pressure gas discharge tube irradiated by a laser beam that was tuned to the wavelength of transition of a species present in the discharge. These authors reported optogalvanic signals for transitions involving lithium, sodium, calcium, barium, uranium, neon, and helium. It was pointed out4 that the OG effect might have significant impact in optical spectroscopy and other areas of applied physics and chemistry. Extensive work has since been carried out to understand the OG effect in some detail and to use it in laser spectroscopy. Precise wavelength calibration of tunable lasers is an important application of the OG effect. OG lines have been observed in case of various species sputtered from hollow cathodesk9 and from fill gases such as neon and argon. 4~‘S’9 A larger discharge current (> 20 mA) is needed for exciting OG transitions associated with species sputtered from cathodes as compared to the signal from the gas atoms. Uranium, for example, is a good candidate for producing the OG effect, because it is an excellent wavelength standard. In addition, uranium gives rise to spectral lines that are narrow and do not possess hyperfine structure over a wide spectral region extending from the infrared to the ultraviolet. However, the OG signals arising from discharge gases are usually much stronger than those from cathode elements. OG transitions from argon have been observed in the wavelength regions 367422 nm,19 415-670 nm,” 425-700 nrn,j4 420-740 nm,16 555-575 nm,” 727-772 nm,13 360-740 nm,‘* 244&2780 nm.” OG signals with commercial neon lamps, which are

LASER MEDIATED RELEASE OF DYE FROM LIPOSOMES

Liposomes made from phospholipids and containing sulforhodamine dye (1–50 mM) have been irradiated with nanosecond and picosecond laser pulses. Individual liposomes were locally heated by laser absorption of dye dimers during a single laser pulse, and heating was sufficient to release the liposome contents. The extent of dye release produced by a single laser pulse was shown to be quantitatively dependent on several interdependent variables, including dye concentration, liposome size, laser excitation parameters and initial temperature of the dye‐liposome system. Fluorescence lifetime data having three components have been obtained and analyzed in terms of three dye environments. Quantitative estimates support a photo‐induced thermal mechanism for liposome lysis and release of its contents. These results may be useful for laser induced delivery of therapeutic agents or other applications of lasers in biological systems.

Laser-induced excitation and dispersed fluorescence spectra of the ethoxy radical

Extensive laser excitation and dispersed fluorescence spectra of the ethoxy radical have been recorded in a supersonic jet expansion. Neon transitions have been used to calibrate both the wavelength of the excitation dye laser and the optical multichannel analyser system used to record the wavelength-resolved emission spectra. Both the excitation and dispersed spectra are characterized by prominent progressions involving the C - O stretch vibrational mode. Seven vibrational frequencies for the excited electronic state and eight for the ground state have been assigned. To the best of our knowledge, 10 of these 15 assigned frequencies are reported here for the first time. Vibrational and anharmonic constants for the C - O stretch mode have been determined via least-squares fits for the X state and the B state .

Rotationally-resolved excitation spectroscopy of the methoxy radical in a supersonic jet

Abstract Laser-induced fluorescence excitation spectra of the methoxy radical have been obtained under sufficiently high resolution in a supersonic jet expansion. The rotational structure associated with its origin band has been identified in the midst of strong overlapping rotational transitions due to the hydroxyl radical in the 31490–31700 cm−1 spectral region. Rotationally-resolved A 2A1 - X 2E spectra of the 00 0 band of methoxy have been explicitly assigned using the nomenclature for prolate symmetric top transitions in doublet states.

Femtosecond laser-induced breakdown spectroscopy of surface nitrate chemicals.

Ultrashort laser-induced breakdown spectroscopy was used to detect the emission radiation from the breakdown of surface contaminants by a femtosecond laser pulse. This study focused on the detection of visible to near-infrared radiation signatures from molecular fragments of the nitro (NO(x)) group present in the breakdown plasma, where target chemicals of potassium nitrate (KNO(3)) and sodium nitrate (NaNO(3)) were used. Spectral signatures at a wavelength region around 410 nm were observed for both KNO(3) and NaNO(3), and were identified as the fluorescence transitions of the NO(x)-molecular structures. The signatures obtained were systematically analyzed and studied as functions of laser parameters. It is shown that for laser parameters used in this study, laser pulse durations ≥1 ps were not as effective as shorter pulses in generating these signatures. A visible wavelength NO(x) signature and the extended high-intensity propagation of a femtosecond laser could be advantageous to detecting nitro-group energetic materials at standoff distances.

Polarity of Laser Excited Optogalvanic Transitions in Neon

Abstract Irradiation of a gaseous discharge with a tunable laser produces observable voltage changes at wavelengths that correspond to electronic transitions for species within the discharge. Optogalvanic (OG) transitions of neon have been investigated in the visible and near UV regions by axially irradiating a hollow cathode discharge with an excimer-pumped dye laser. A variety of OG transitions that originate from metastable states have been recorded. A digital oscilloscope was used to record the waveforms of the OG transitions. The polarities of 29 OG transitions recorded in the near UV and visible are explained in terms of processes that affect the population of neon atoms in metastable states.

Nanomaterials in Nanomedicine

Nanomedicine refers to the applications of nanotechnology to the field of medicine. Nanomaterials have led to the development of novel devices for the early detection of malignant tumors, as well as significant enhancements in efficient drug, gene and protein delivery mechanisms to targeted sites in the human body. As nanoparticles become increasingly smaller in size, they also present the potential for harming certain organs of the body. Safety issues involving nanoparticles need to be solved using in vivo techniques. Research in nanomedicine has improved biological therapies, such as vaccination, cell therapy and gene therapy. A particular kind of colloidal nanoparticle, called the liposome, which has properties similar to a red blood cell, has viscoelastic properties that make it extremely useful for a variety of applications in the pharmaceutical and consumer product sectors of the global market. Liposomes have been clinically established as efficient nanosystems for targeted drug delivery. Their efficacy has been demonstrated in reducing systemic effects and toxicity, as well as in attenuating drug clearance. The Maxwell Spring-Dashpot model has been reviewed for liposomes and the viscoelastic exponential equation shown to fit the liposome data. The relevance of this study is to the increasing use of viscoelastic characteristics of liposomes for efficient drug delivery and targeted destruction of malignant tumors. Nanobiotechnology has the potential to facilitate the integration of diagnostics with therapeutics, and in turn lead to personalized medicine tailored for a specific individual.

Development of an Earth and Space Science-Focused Education Program at Howard University

We have developed a curriculum at Howard University that educates and prepares students, especially underrepresented minorities, for careers in Atmospheric, Earth and Space Sciences. One of the main objectives of the current initiative has been to provide introductory Earth & Space Science courses at the undergraduate level through the Department of Physics & Astronomy. These courses enhance student learning by including astronomical observing and laboratory demonstration opportunities at the Howard University Observatory. Intermediate-level courses in Atmospheric and Space Science have also been designed, and will serve as a bridge toward the graduate-level courses currently being offered by the M.S./Ph.D.-granting Howard University Program in Atmospheric Sciences (HUPAS). HUPAS is the first and only program at a Historically Black College or University (HBCU) that offers a terminal graduate degree (M.S. or Ph.D.) in Atmospheric Sciences. Currently, a total of 23 graduate students are enrolled in HUPAS, which include 14 African-Americans and 6 Hispanic-Americans, and 3 international students from Mexico, Barbados and Madagascar; and the gender breakdown is 13 females and 10 males. To the best of our knowledge, this is the largest number of African/Hispanic students enrolled in a graduate atmospheric science program in the U.S.