Rosario C. Sausa

Potential energy surfaces for the interaction of CH(X 2Π,B 2Σ−) with Ar and an assignment of the stretch‐bend levels of the ArCH(B) van der Waals molecule

New multireference, configuration‐interaction potential energy surfaces are reported for the interaction of Ar with the CH radical in its ground (X 2Π) and second excited (B 2Σ−) electronic states. These potential energy surfaces are then used in an adiabatic analysis of the rovibronic levels of the ArCH(X) and ArCH(B) van der Waals complexes. A qualitative discussion of the expected features in the B←X electronic spectrum of ArCH is presented, and these are compared with the experimental spectrum reported earlier by Lemire et al. [J. Chem. Phys. 99, 91 (1993)].

Trace detection of explosives with low vapor emissions by laser surface photofragmentation–fragment detection spectroscopy with an improved ionization probe

Trace explosive residues are measured in real time by surface laser photofragmentation-fragment detection (SPF-FD) spectroscopy at ambient conditions. A 248-nm laser photofragments the target residue on a substrate, and a 226-nm laser ionizes the resulting NO fragment by resonance-enhanced multiphoton ionization by means of its A-X (0, 0) transitions near 226 nm. We tested two probes on selected explosives and modeled their electric field in the presence of a substrate with an ion optics simulation program. The limits of detection range from 1 to 15 ng/cm2 (signal-to-noise ratio of 3) at 1 atm and 298 K and depend on the electrode orientation and mechanism for NO formation.

Trace Detection of Nitrocompounds by ArF Laser Photofragmentation/Ionization Spectrometry

A new method for detecting trace vapors of NO2-containing compounds near atmospheric conditions has been demonstrated with the use of one-color-laser photofragmentation/ionization spectrometry. An ArF laser is employed to both photolytically fragment the target molecules in a collision-free environment and ionize the characteristic NO fragments. The production of NO is hypothesized to result from a combination of two NO2 unimolecular fragmentation pathways, one yielding NO in its X2II electronic ground state and the other in its A2Σ+ excited state. Ionization of ground-state NO molecules is accomplished by resonance-enhanced multiphoton ionization processes via its A2Σ+ ← X2II (3, 0), B2II ← X2II (7, 0) and/or D2Σ+ ← X2II (0, 1) bands at 193 nm. The analytical utility of this method is demonstrated in a molecular beam time-of-flight apparatus. Limits of detection range from the parts-per-million (ppm) to parts-per-billion (ppb) level for NO, NO2, CH3NO2, dimethylnitramine (DMNA), ortho- and meta-nitrotoluene, nitrobenzene, and trinitrotoluene (TNT). Under effusive beam experimental conditions, discrimination between structural isomers, ortho-nitrotoluene and meta-nitrotoluene, has been demonstrated with the use of their characteristic photofragmentation/ionization mass spectra.

Bound—bound ArNO AX vibronic transitions. New mass-resolved 1+1 REMPI observations

Abstract The mass-resolved 1+1 REMPI spectra associated with bound—bound ArNO AX vibronic transitions has been reinvestigated. The current results provide a basis for assigning the vibrationless ArNO AX band origin, and establishing a refined upper limit for the A state dissociation energy ( D 0 ⩽43cm −1 ). Assignments for bands associated with vibrationally excited A state levels are also proposed.

Investigation of the gas‐phase B̃–X̃ electronic spectra of CH–Ar by laser‐induced fluorescence

Gas‐phase methyidyne–argon (CH–Ar) van der Waals complexes have been detected spectroscopically by laser‐induced fluorescence (LIF) in the region of the CH B 2Σ−–X 2Πr (0,0) and (1,0) bands near 363.5 and 388.9 nm, respectively. They are formed by a supersonic free‐jet expansion of argon gas seeded with CH radicals generated from the 248 nm photolysis of CHBr2Cl. The excitation spectra reveal a number of rovibronic bands which are assigned to various stretching and/or bending motions of the CH–Ar complex. From the excitation spectra, lower limits for the ground and exited state binding energies are estimated. Rotational analysis based on combination differences and computer simulations of eight of the rovibronic bands yields an average ground state value of B‘av = 0.174 ± 0.004 cm−1 and excited state constants ranging from B’=0.086–0.116 cm−1. This indicates that the CH–Ar van der Waals bond is lengthened considerably upon electronic excitation. A splitting of the ground state rotational energy levels, re...

Spectral differentiation of trace concentrations of NO 2 from NO by laser photofragmentation with fragment ionization at 226 and 452 nm: quantitative analysis of NO–NO 2 mixtures

Laser-induced photofragmentation with fragment ionization is used to detect and spectrally differentiate trace concentrations of NO(2) from NO in NO-NO(2) mixtures. A laser operating near 226 or 452 nm ionizes the target molecules, and the resulting electrons are collected with miniature electrodes. NO is detected by (1 + 1) resonance-enhanced multiphoton ionization by means of its A (2)?(+) ? X (2)? (0, 0) transitions near 226 nm, whereas NO(2) is detected near 226 nm by laser photofragmentation with subsequent NO fragment ionization by means of both its A (2)?(+) ? X (2)? (0, 0) and (1, 1) transitions. The NO fragment generated from the photolysis of NO(2) is produced rovibrationally excited with a significant population in the first vibrational level of the ground electronic state (X (2)?, upsilon? = 1). In contrast, ambient NO has a room-temperature, Boltzmann population distribution favoring the lowest ground vibrational level (X (2)?, upsilon? = 0). Thus discrimination is possible when the internal energy distributions of both fragment NO and ambient NO are probed. We also demonstrate this approach using visible radiation, further simplifying the experimental apparatus because frequency doubling of the laser radiation is not required. We measured up to three decades of NO-NO(2) mixtures with limits of detection (signal-to-noise ratio of 3) in the low parts per billion for both NO and NO(2) for a 10-s integration time using both ultraviolet or visible radiation.

Trace Analysis of NO2 in the Presence of NO by Laser Photofragmentation/Fragment Photoionization Spectrometry at Visible Wavelengths:

Trace concentrations of NO and NO2 molecules are differentiated spectrally by using a visible dye laser and a simple flow cell with a pair of miniature electrodes for ion detection. NO is detected near 452 nm by (2+2) resonance-enhanced multiphoton ionization via its A2∑+-X2II (0,0) transitions, while NO2 is detected by laser photofragmentation with subsequent fragment NO ionization via the A2∑+-X2II (0,0) and (1,1) transitions. Spectral differentiation is possible since the internal energy of the NO photofragment differs from that of “ambient” NO. Measurement of vibrationally excited NO via its A2∑+-X2II (0,3) band is also demonstrated at 517 nm. Rotationally resolved spectra of NO and fragment NO are analyzed by using a multiparameter computer program based on two-photon energy level expressions and line strengths for A2∑+-X2II transitions. Boltzmann analysis of the P12 + O22 branch of the (0,0) band reveals that the rotational temperature of fragment NO is approximately 500 K compared to room-temperature NO. Limits of detection [signal-to-noise (S/N) = 3] of NO and NO2 are in the 20–40-ppbv range at 449.2, 450.7, and 452.6 nm for a 10-s integration time. The limit of detection of NO2 at 517.5 nm is 75 ppbv. The analytical utility of the technique for ambient air analysis is evaluated and discussed.

Laser-induced fluorescence, mass spectrometric, and modeling studies of neat and NH3-doped H2/N2O/Ar flames

Abstract A combined experimental and modeling study of neat and NH 3 -doped (Φ = 1), 30 Torr flames is reported. The major species concentrations are measured by molecular beam mass spectrometry (MB/MS), whereas the minor species OH, NH, and O-atom concentrations are measured by laser-induced fluorescence (LIF). The species NO is measured both by LIF and MB/MS, and O 2 by MB/MS. The flame temperatures are measured both by OH and NH LIF and by thin wire thermometry. The flames are modeled with PREMIX using the temperature profiles and several detailed chemical mechanisms as input. The mechanisms include the GRI 2.11, SSLA, and their derivatives. The SSLA mechanism was developed previously in our laboratory from a critical literature review. Calculations using all the mechanisms predict fairly well the profiles of the major species for both neat and doped flames. However, both the SSLA and GRI 2.11 calculations fail to predict the postflame O 2 concentration in the neat flame, the drop in the O 2 concentration with the addition of NH 3 , and the NH 3 decay in the doped flame. Sensitivity analyses suggest refinements to the SSLA and GRI 2.11 mechanisms. The experimental results are predicted rather well using a modified SSLA mechanism in which the NH + NO = N 2 O + H reaction rate is decreased and the N 2 O + M = N 2 + O + M reaction rate and/or H 2 O third body efficiency is increased to the limit of their uncertainty. Rate analyses performed on the modeled calculations reveal the reactions important to NO, O 2 , NH, OH, and O-atom production and consumption and NH 3 consumption. These reactions are presented and discussed.

Observation and analysis of rotationally resolved B—X electronic transitions of CD—Ar

Abstract The CD—Ar van der Waals complex has been generated in the gas phase by the 248 nm photolysis of CDBr 3 entrained in a supersonic jet of argon. Rotationally resolved spectra associated with the complex have been observed by laser-induced fluorescence in the vicinity of the CD B 2 Σ − -X 2 Π (1, 0) band near 366 nm. Bands at 27310.7 and 27327.8 cm −1 are rotationally analyzed using computer simulations, and assigned as (1, 0 0 , 0) and (1, 1 p , 0), respectively. The vibronic structure of the CD—Ar excited states has also been analyzed using a model based on hindered internal rotation. The results are compared to those reported for CH—Ar.

Nanomechanics of RDX Single Crystals by Force-Displacement Measurements and Molecular Dynamics Simulations.

Nanoenergetic material modifications for enhanced performance and stability require an understanding of the mechanical properties and molecular structure-property relationships of materials. We investigate the mechanical and tribological properties of single-crystal hexahydro-1,3,5-trinitro-s-triazine (RDX) by force-displacement microscopy and molecular dynamics (MD). Our MD simulations reveal the RDX reduced modulus (Er) depends on the particular crystallographic surface. The predicted Er values for the respective (210) and (001) surfaces are 26.8 and 21.0 GPa. Further, our simulations reveal a symmetric and fairly localized deformation occurring on the (001) surface compared to an asymmetric deformation on the (210) surface. The predicted hardness (H) values are nearly equal for both surfaces. The predicted Er and H values are ∼33% and 17% greater than the respective experimental values of 0.798 ± 0.030 GPa and 22.9 ± 0.7 GPa for the (210) surface and even larger than those reported previously. Our experimental H and Er values are ∼19% and 9% greater than those reported previously for the (210) surface. The difference between the experimental values reported here and elsewhere stems in part from an inaccurate determination of the contact area. We employ the parameter √H/Er, which is independent of area, as a means to compare present and past results, and find excellent agreement, within a few percent, between our predicted and experimental results and between our results and those obtained from previous nanoindentation experiments. Also, we performed nanoscratch simulations of the (210) and (001) surfaces and nanoscratch tests on the (210) surface and present values of the dynamic coefficient of deformation friction.