Rotationally elastic and inelastic dynamics of NO(X2Π, v = 0) in collisions with Ar

A combined theoretical and experimental study of the depolarization of selected NO(X(2)Π, v = 0, j, F, ɛ) levels in collisions with a thermal bath of Ar has been carried out. Rate constants for elastic depolarization of rank K = 1 (orientation) and K = 2 (alignment) were extracted from collision-energy-dependent quantum scattering calculations, along with those for inelastic population transfer to discrete product levels. The rate constants for total loss of polarization of selected initial levels, which are the sum of elastic depolarization and population transfer contributions, were measured using a two-color polarization spectroscopy technique. Theory and experiment agree qualitatively that the rate constants for total loss of polarization decline modestly with j, but the absolute values differ by significantly more than the statistical uncertainties in the measurements. The reasons for this discrepancy are as yet unclear. The lack of a significant K dependence in the experimental data is, however, consistent with the theoretical prediction that elastic depolarization makes only a modest contribution to the total loss of polarization. This supports a previous conclusion that elastic depolarization for NO(X(2)Π) + Ar is significantly less efficient than for the electronically closely related system OH(X(2)Π) + Ar [P. J. Dagdigian and M. H. Alexander, J. Chem. Phys. 130, 204304 (2009)].

O+OH-->O(2)+H: A key reaction for interstellar chemistry. New theoretical results and comparison with experiment

We report extensive, fully quantum, time-independent (TID) calculations of cross sections at low collision energies and rate constants at low temperatures for the O+OH reaction, of key importance in the production of molecular oxygen in cold, dark, interstellar clouds and in the chemistry of the Earth's atmosphere. Our calculations are compared with TID calculations within the J-shifting approximation, with wave-packet calculations, and with quasiclassical trajectory calculations. The fully quantum TID calculations yield rate constants higher than those from the more approximate methods and are qualitatively consistent with a low-temperature extrapolation of earlier experimental values but not with the most recent experiments at the lowest temperatures.

Nd:YAG-CO(2) double-pulse laser induced breakdown spectroscopy of organic films

Laser-induced breakdown spectroscopy (LIBS) using double-pulse irradiation with Nd:YAG and CO(2) lasers was applied to the analysis of a polystyrene film on a silicon substrate. An enhanced emission signal, compared to single-pulse LIBS using a Nd:YAG laser, was observed from atomic carbon, as well as enhanced molecular emission from C(2) and CN. This double-pulse technique was further applied to 2,4,6-trinitrotoluene residues, and enhanced LIBS signals for both atomic carbon and molecular CN emission were observed; however, no molecular C(2) emission was detected.

Rotationally inelastic collisions of CN(A(2)Pi) with small molecules

An optical-optical double resonance technique has been utilized to investigate rotational energy transfer of selected rotational/fine-structure levels of CN(A(2)Pi, v = 3) in collisions with the molecular partners CO(2) and CH(4). Total removal rate constants for several rotational/fine-structure levels were measured for both collision partners. State-to-state relative rate constants were determined for several initial levels. These show a strikingly strong collisional propensity to conserve the fine-structure/Lambda-doublet label, akin to our previous observations with the N(2) collision partner. The possible origin of this propensity is discussed.

Dependence of elastic depolarization cross sections on the potential: OH(X 2Pi)-Ar and NO(X 2Pi)-Ar

Elastic tensor and depolarization cross sections are computed for the collision of two exemplary diatomic molecules with (2)Pi electronic ground states-OH and NO-with argon. The interaction of a diatomic molecule in a Pi state with a spherical collision partner must be described by two potential energy surfaces (PESs), corresponding to the two asymptotically degenerate electronic states, of A(') and A(") symmetry. Quantum scattering calculations are most naturally based on the average (V(sum)) and half-difference (V(dif)) of these two PESs. When V(dif) is neglected, the OH(X (2)Pi)-Ar depolarization cross sections are found to be significantly reduced in magnitude, while the NO(X (2)Pi)-Ar cross sections are relatively unaffected. In addition, treating the molecules as closed-shell (1)Sigma(+) species with a corresponding rotational level structure and using (V(sum)) to model the PES, we predict depolarization cross sections which differ significantly from those based on full inclusion of the electronic degeneracy and fine structure of these (2)Pi molecules. This indicates that any single-PES-based simulation of the collisional depolarization of these two molecules would be subject to significant error.

Experimental and theoretical study of rotationally inelastic collisions of CN(A2pi) with N2

Optical-optical double resonance was employed to study rotational energy transfer in collisions of selected rotational/fine-structure levels of CN(A2pi, v = 3) with N2. The CN radical was generated by 193 nm photolysis of BrCN in a slow flow of N2 at total pressures of 0.2-1.4 Torr. Specific fine-structure lambda-doublet levels of CN(A2pi, v = 3) were prepared by pulsed dye laser excitation on isolated lines in the CN A-X (3,0) band, while the initially excited and collisionally populated levels were observed after a short delay by laser-induced fluorescence in the B-A (3,3) band. Total removal rate constants for specified rotational/fine-structure levels involving total angular momentum J from 4.5 to 12.5 were determined. These rate constants decrease with increasing J, with no obvious dependence on the fine-structure/lambda-doublet label. State-to-state relative rate constants were determined for several initial levels and show a strikingly strong collisional propensity to conserve the fine-structure/lambda-doublet label. Comparison is made with the results of quantum scattering calculations based on potential energy surfaces averaged over the orientation of the N2 molecule. Reasonable agreement is found with experimentally determined total removal rate constants. However, the computed state-to-state rate constants show a stronger propensity for fine-structure and lambda-doublet changing transitions. These differences between experiment and theory could be due to the neglect of the N2 orientation and the correlation of the CN and N2 angular motions.

Comparison of laser-induced breakdown spectra of organic compounds with irradiation at 1.5 and 1.064 microm

A comprehensive investigation of laser-induced breakdown spectroscopy (LIBS) at 1.500 microm of residues of six organic compounds (anthracene, caffeine, glucose, 1,3-dinitrobenzene, 2,4-dinitrophenol, and 2,4-dinitrotoluene) on aluminum substrates is presented and compared with LIBS at the Nd:YAG fundamental wavelength of 1.064 microm. The overall emission intensities were found to be smaller at 1.500 microm than at 1.064 microm, and the ratios of C(2) and CN molecular emissions to the H atomic emissions were observed to be less. Possible reasons for the observed differences in LIBS at 1.064 microm versus 1.500 microm are discussed.

Further investigation of the photodissociation dynamics of dichlorocarbene near 248 nm

A further investigation of the 248 nm photodissociation of CCl(2), which expands upon our original study of this process [S. K. Shin and P. J. Dagdigian, Phys. Chem. Chem. Phys. 8, 3446 (2006)], is presented. The CCl(2) parent molecule and the CCl photofragment were detected by laser fluorescence excitation in a molecular beam experiment. From the dependence of the CCl(2) signals on the photolysis laser fluence, attenuation cross sections of the 0(0), 1(1), and 2(1) vibrational levels were determined; the cross sections for the excited vibrational levels were found to be significantly smaller than those for the ground vibrational level. The previously observed fragment CCl bimodal rotational state distribution was found to arise from the photolysis of more than one parent molecule. At low CHCl(3) mole fractions in the gas supplied to the pyrolysis beam source, it was concluded that CCl(2) is the photolysis precursor for both low-J and high-J CCl fragments. On the basis of the dependence of the CCl signals on the photolysis laser fluence, ground and vibrationally excited CCl(2), respectively, were assigned as the precursors to these two classes of fragments. The photofragment excitation spectra for low-J and high-J CCl fragments from the photolysis of CCl(2) were recorded in the wavelength range around 248 nm; both were found to be structureless. The 248 nm photodissociation dynamics of CCl(2) is discussed in light of our experimental observations and quantum chemical calculations of the CCl(2) excited electronic states.

Effect of photochemistry on molecular detection by cavity ringdown spectroscopy: case study of an explosive-related compound

Explosives and explosive-related compounds usually have dissociative excited electronic states. We consider the effect of excited-state dissociation upon an absorption event on the UV cavity ringdown spectroscopy (CRDS) detection of these molecules. A change in the photon decay lifetime with increasing laser energy is demonstrated with vapors of 2,6-dinitrotoluene in the open atmosphere. The magnitude of the effect is modeled with coupled equations describing the time-dependent light intensity and molecular concentration within the cavity. The light intensities required within this model to explain the observed changes in the photon decay lifetimes are consistent with the light intensities expected within the cavity under our experimental conditions. It was also found that the slow diffusion of the molecules in static air can magnify the effect of photochemistry on UV CRDS trace detection of molecules with dissociative excited states.

Comparison of IR and UV cavity ring-down spectroscopy detection of transient intermediates: pyrolysis of methyl azide to form methyleneimine

Mid-infrared cavity ring-down spectroscopy (CRDS) has been employed in this study to examine the hydride stretching region of methyl azide and its pyrolysis product methyleneimine. The absorption spectrum of methyl azide over 2835-3085 cm(-1) was recorded, and the integrated absorption cross section was determined. The pyrolysis of methyl azide and subsequent production of methyleneimine was observed at various wavenumbers. Using IR CRDS, we were able to observe vibrational transitions of methyleneimine without interference from the methyl azide precursor. Our previous UV CRDS study showed that electronic transitions of methyleneimine overlapped with those of methyl azide. IR CRDS should thus be useful for the detection of polyatomic transient intermediates without interference from precursors.

Internal state distribution of the CF fragment from the 193nm photodissociation of CFCl and CFBr

The dynamics of the 193 nm photodissociation of the CFCl and CFBr molecules have been investigated in a molecular beam experiment. The CFCl and CFBr parent molecules were generated by pyrolysis of CHFCl2 and CFBr3, respectively, and the CFCl and the CF photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section of CFCl was determined from the reduction of the CF photofragment signal as a function of the photolysis laser fluence. The internal state distribution was derived from the analysis of laser fluorescence excitation spectra in the A 2Sigma+-X 2Pi band system. A very low degree of rotational excitation, with essentially equal A' and A" Lambda-doublet populations, and no vibrational excitation were found in the CF photofragment. The energy available to the photofragments is hence predominantly released as translational energy. The CF internal state distribution is consistent with the dissociation of a linear intermediate state. Considerations of CFCl electronic states suggest that a bent Rydberg state is initially excited.

Detection of vapors of explosives and explosive-related compounds by ultraviolet cavity ringdown spectroscopy

The detection of vapors of dinitrobenzenes and dinitrotoluenes by UV cavity ringdown spectroscopy (CRDS) was investigated. Absorption cross sections at 248 nm were estimated by measurements on saturated vapors and compared with solution-phase values. The computed subparts per 10(9) detection sensitivity with no effort at preconcentration was demonstrated through measurements on diluted flows. The factors affecting measurements on 1 atm total pressure were considered, and it was demonstrated that Rayleigh scattering by air will reduce the detection sensitivity by 5%-10%. The UV absorption spectra of these compounds are broad, resulting in a relatively poor selectivity for single-wavelength measurements with the UV CRDS technique.