David P. Baldwin

Reaction Product Imaging: The H + D2 Reaction

The differential cross section for the H + D2 → HD + D reaction has been measured using a technique called reaction product imaging. In this experiment, a photolytically produced beam of hydrogen (H) atoms crossed a beam of cold deuterium (D2) molecules. Product D atoms were ionized at the intersection of the two particle beams and accelerated toward a position-sensitive detector. The ion images appearing on the detector are two-dimensional projections of the three-dimensional velocity distribution of the D atom products. The reaction was studied at nominal center-of-mass collision energies of 0.54 and 1.29 electron volts. At the lower collision energy, the measured differential cross section for D atom production, summed over all final states of the HD(v,J) product, is in good agreement with recent quasi-classical trajectory calculations. At the higher collision energy, the agreement between the theoretical predictions and experimental results is less favorable.

Photodissociation of acetylene: Determination of D00 (HCC–H) by photofragment imaging

Acetylene cooled in a He supersonic expansion is photodissociated by excitation in the 201–216 nm region of the A 1Au −X 1∑+g transition. Subsequent ionization of the H‐atom fragments by 2+1 (243 nm) REMPI, and mass‐selected ion imaging allows analysis of the velocity distribution of H‐atoms from the HCCH hν→ C2H+H process. Measurement of the maximum velocity for H atoms from this channel produced by photodissociation of acetylene through the A 1Au −X 1∑+g  V70K10, 110V40K10, 210V50K10 and V50K10 vibronic transitions gives a value for D00 (HCC–H) of 131±1 kcal/mol. Other channels producing hydrogen atoms (including HC2 hν→ C2+H and HCCHhν→ HCCH+ hν→ C2H++H) are detected at all photon fluxes used. These multiphoton channels produce hydrogen atoms with higher translational energy and therefore obscure measurement of the maximum velocity of H atoms produced by single‐photon dissociation of acetylene. Reduction of photon flux by more than two orders of magnitude to ∼5×106 J/cm2 gives a background, multiph...

Application of ion imaging to the atom-molecule exchange reaction : H+HI→H2+I

The bimolecular abstraction reaction H+HI→H2+I has been investigated in a neat molecular beam of HI using ion imaging to detect the H2 (v=1,J=11,13) products. Images obtained determine the laboratory‐frame product velocity distribution and show evidence for reaction with fast and slow H atoms arising from the I (2P3/2) and I*(2P1/2) channels in the photolysis of HI, as well as formation of I and I* in the reaction product channel.

Aerosol Mass Measurement and Solution Standard Additions for Quantitation in Laser Ablation-Inductively Coupled Plasma Atomic Emission Spectrometry

A new approach for quantitation in laser ablation-inductively coupled plasma atomic emission spectrometry (LA-ICPAES) is presented. A portion of the laser-ablated sample aerosol is diverted to an aerosol mass monitor to measure variations in the amount of sample ablated and transported to the ICP torch. This provides a normalization for variations in laser ablation efficiency due to changes in laser power and focus at the sample and variations in material transport out of the ablation cell and into the ICP torch. During the laser ablation sampling process, solution standards are nebulized and the aerosol is added to the laser-ablated aerosol to generate a standard addition curve for the analyte being determined. The standard addition procedure corrects for potential plasma-related matrix effects in the ICP emission signal resulting from the ablated sample. The precision of this method, for triplicate analyses for the determination of 16 elements in four glass samples, and the accuracy of this method relative to the nominal glass compositions are both approximately 10%. 19 refs., 4 figs., 4 tabs.

Time-resolved studies of particle effects in laser ablation inductively coupled plasma-mass spectrometry : Part 1. Investigation of nanosecond and femtosecond pulse width lasers and devices for particle size selection

Transient signal responses for ablated samples as a function of particle size and laser parameters are characterized. Data are acquired with time resolution of 5 or 6 ms per data point. Large positive spikes in signal are observed and increase in both amplitude and frequency with increasing particle size. Particle sizes are selected using a differential mobility analyzer. Spikes in the signal also increase with decreasing laser rastering rates. A comparison of lasers with pulse widths of 370 fs and 5 ns shows that shortening the pulse width significantly reduces the frequency and amplitude of positive spikes in signal. These large positive spikes are attributed to the vaporization, atomization, and ionization of individual large intact particles, which are considered to be a major cause of fractionation in laser ablation ICP-MS.

Photodissociation dynamics of doubly excited Rydberg states of molecular hydrogen

We have applied photofragment ion imaging to investigate the dissociation dynamics of low‐lying, doubly excited states of molecular hydrogen. A doubly excited electronic state is one in which both of the hydrogen electrons reside in excited molecular orbitals. Two‐step, two‐color multiphoton excitation of H2, first via 201.8 nm, two‐photon excitation into the E, F 1Σ+g(vE=0, J=1) state, followed by ∼563 nm, 1+m (m=1, 2) excitation through the B‘ 1Σ+u(v=0, J=0, 2), D 1Πu(v=2, J=1, 2), and B’ 1Σ+u(v=4, J=0, 2) states provides a ready means of populating several low‐lying doubly excited states of H2 at increasing internuclear separations. From these doubly excited repulsive states, both dissociation and autoionization processes are possible. Because the excitation energy remains relatively constant as each intermediate state is accessed, differences in the photodissociation dynamics via each state can be ascribed directly to the effects of changing internuclear separation and electronic symmetry of the inter...

Analysis of glass fragments by laser ablation-inductively coupled plasma-mass spectrometry and principal component analysis.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is used to differentiate glass samples with similar optical and physical properties based on trace elemental composition. Laser ablation increases the number of elements that can be used for differentiation by eliminating problems commonly associated with dissolution and contamination. In this study, standard residential window and tempered glass samples that could not be differentiated by refractive index or density were successfully differentiated by LA-ICP-MS. The primary analysis approach used is Principal Component Analysis (PCA) of the complete mass spectrum. PCA, a multivariate analysis technique, provides rapid analysis of samples without time-consuming pair-wise comparison of calibrated analyses or prior knowledge of the elements present in the samples. Probabilities for positive association of the individual samples are derived from PCA. Utilization of the Q-statistic with PCA allowed us to distinguish all samples within the set to a certainty greater than the 99% confidence interval.

The effect of firearm muzzle gases on the backspatter of blood

Injuries caused by gunshots can produce what bloodstain pattern analysts know as “backspatter.” Observations about the presence or absence of backspatter on an individual may be used in court as evidence of guilt or innocence. The discharge of three firearms (.22 caliber revolver, .38 caliber revolver, and .308 caliber rifle) and the resulting impact of bullets on a blood source were recorded using high-speed digital video imaging. Blood droplets, firearm muzzle gases, and ballistic shock waves were visualized using standard reflected light and shadowgraphy imaging techniques. A significant interaction between air currents, muzzle gases, and particulate material emanating from the firearms upon discharge with backspattered blood was observed. Blood droplets, initially spattered back toward the firearm and the shooter, were observed to change direction under the influence of firearm-induced air currents and were blown forward toward and beyond their original source location. Implications for experts testifying in court and for bloodstain pattern instructors are discussed.

In situ determination of uranium in soil by laser ablation-inductively coupled plasma atomic emission spectrometry

The concentration of uranium in soil has been determined for 80 sites in an area suspected to have uranium contamination by in situ laser ablation - inductively coupled plasma atomic emission spectrometry (LA-ICPAES), utilizing a field-deployable mobile analytical laboratory. For 15 of the 80 sites analyzed, soil samples are collected so that the field LA-ICPAES results could be compared to laboratory-determined values. Uranium concentrations determined in the field by LA-ICPAES for these 15 sites range from <20 parts per million (ppm) by weight to 285 ppm. The uncertainty in the values determined, however, is large relative to the uranium concentrations encountered at this site. The 95% confidence interval (CI) values are approximately 85 ppm. The uranium concentrations determined by laboratory LA-ICPAES analysis range from <20 to 102 ppm (95% CI of approximately 50 ppm); microwave dissolution and subsequent standard addition determination of uranium by solution nebulization ICPAES using an ultrasonic nebulizer yields 19-124 ppm uranium (95% CI of approximately 10 ppm). For 11 of the 15 samples, the field- and laboratory-determined uranium concentrations agree, within the uncertainty of the determined values. 19 refs., 5 figs., 3 tabs.

Application of ion imaging to the study of unimolecular and bimolecular reactions

We use the photofragment ion imaging technique to investigate the 266 nm photodissociation dynamics of hydrogen iodide. We show the quantitative features of ion imaging by comparing determinations of photofragment recoil velocity distributions, product branching ratios and the HI bond dissociation energy with previously published results. Excellent agreement with previous experimental and theoretical results is obtained. The H atoms produced from this process are then used as reactants in the reaction of H + HI and H + D2. Imaging techniques are used to measure the velocity distributions of the products of these reactions.