Function Of Rotational Level Research Articles

Molecular Hydrogen Optical Depth Templates for FUSE Data Analysis

ABSTRACTThe calculation and use of molecular hydrogen optical depth templates to quickly identify and model molecular hydrogen absorption features longward of the Lyman edge at 912 Å are described. Such features are commonly encountered in spectra obtained by the Far Ultraviolet Spectroscopic Explorer and also in spectra obtained by the Space Telescope Imaging Spectrograph, albeit less commonly. Individual templates are calculated containing all the Lyman and Werner transitions originating from a single rotational state (J′′) of the zeroth vibrational level (v′′) of the ground electronic state. Templates are provided with 0.01 Å sampling for Doppler parameters ranging from 2≤b≤20 km s−1 and rotational states 0≤J′′≤15. Optical depth templates for excited vibrational states are also available for select Doppler parameters. Each template is calculated for a fiducial column density of log [N(cm -2)] = 21 and may be scaled to any column less than this value without loss of accuracy. These templates will facilitate the determination of the distribution of molecular hydrogen column density as a function of rotational level. The use of these templates will free the user from the computationally intensive task of calculating profiles for a large number of lines and allow concentration on line‐profile or curve‐of‐growth fitting to determine column densities and Doppler parameters. The templates may be downloaded freely from http://www.pha.jhu.edu/∼stephan/h2ools.html.

Collisional quenching of high rotational levels in A 2Σ+ OH

Collisional removal of the v′=0 level of the A 2Σ+ state of the OH radical has been studied as a function of rotational level N′ at room temperature. OH in high rotational levels of the X 2Πi state were created by 193 nm photolysis of HNO3 and excited to A 2Σ+ by a tunable dye laser. Time decays of fluorescence at varying pressures were measured. For O2 and H2, the quenching cross section σQ decreased with increasing N′ until N′∼10; for higher N′ it appears to remain approximately constant. Xe behaves the same way except that the decrease continues to N′=15. For Kr, σQ appears to decrease to within experimental error of zero at N′=10; and for N2 it was within error of zero above N′=10. These results have implications for laser-induced fluorescence atmospheric monitoring of OH and combustion temperature determinations, as well as a fundamental understanding of collisional quenching. Quenching of OH, N′∼1, by HNO3 was found to be 81±8 Å2.

Collisional quenching of highly rotationally excited NH (A 3Πi)

Collisional removal of the v′=0 level of the A 3Πi state of the NH molecule has been studied as a function of rotational level with particular attention to high N′. Multiphoton dissociation of NH3 at 193 nm produces highly rotationally excited ground state NH (to N″=30), which is subsequently electronically excited with a pulsed dye laser, and quenching rate coefficients determined by the pressure dependence of the time-resolved fluorescence. Colliders investigated are NH3, CO, CH4, H2, and D2. The quenching rates at first decrease with increasing N′, but become nearly constant at highest N′. This is consistent with dynamic effects on an attractive, anisotropic surface. Radiative rates for A 3Πi were also determined and found to decrease with increasing N′ at a rate in excellent agreement with recent ab initio calculations.

Rotational level dependence of predissociation in the v′=3 level of OH A 2Σ+

Using laser-induced fluorescence in a low-pressure flame, relative predissociation rates have been determined as a function of rotational level in the v′=3 level of the A 2Σ+ state of the OH molecule. A Boltzmann analysis of an excitation scan in the (3,0) band yields an apparent temperature of 970 K in a 1670 K flame, showing that kpre increases rapidly with rotational level. The F1 levels predissociate slightly faster than F2 for the same N′, agreeing with previous multichannel scattering predictions. The results can be placed on an absolute basis using recent linewidth measurements in a flow tube.

Direct inelastic scattering of N2 from Ag(111). I. Rotational populations and alignment

The rotational state populations and the quadrupole and hexadecapole alignment moments of N2 scattered off clean Ag(111) are determined by resonance enhanced multiphoton ionization (REMPI). The scattered N2 is found to be highly aligned with its rotational angular momentum vector J parallel to the surface. The degree of alignment is found to increase with increasing rotational excitation. We see less than perfect alignment at intermediate J values indicating that the surface is not completely flat. The alignment is relatively insensitive to incident energy, incident angle, or surface temperature Ts. However, the rotational state population distributions show pronounced rainbows for higher incident energy and/or more grazing exit angle. The rotational state distributions are found to depend strongly on the final scattering angle at low Ts; this effect is markedly reduced at higher Ts. Time-of-flight measurements are used to determine the average velocity of the scattered N2 as a function of rotational level. It is found that higher rotational excitation correlates with lower average velocity and that the incident molecules lose 20%–30% of their translational energy to the solid. No correlation is found between velocity and alignment. A comparison is made with published results for the NO/Ag(111) system and a variety of theoretical models found in the literature.

Rotational dynamics and electronic energy partitioning in the electron-stimulated desorption of NO from Pt(111)

Using laser resonance ionization, we have examined the rotational dynamics of neutral NO molecules desorbed by electron impact from a cold (80 K) Pt(111) surface. Previously [A. R. Burns, E. B. Stechel, and D. R. Jennison, Phys. Rev. Lett. 58, 250 (1987)], we observed ‘‘hot’’ rotational temperatures in the range 481–642 K for the ν=0, 1, 2, and 3 vibrational levels. We now present more detailed data which do not reveal shifts or broadening in the rotationally selected NO velocity distributions as a function of rotational level J in the range J=2.5–29.5. The rotationally independent velocity distributions, peaked at 0.05 eV, as well as the rotational temperature can be understood within the framework of ‘‘electronically stimulated adsorbate rotation’’ (Burns et al., above). The model also predicts complete rotational alignment in a plane normal to the surface. In the laboratory, no alignment is observed, but this is most likely due to the presence of stray magnetic fields. We will also discuss our measurements which show an equal partitioning between the two lowest NO electronic states (2Π1/2 and 2Π3/2), and a preference for the π level which is perpendicular (antisymmetric) to the plane of rotation.

On the use of various scaling theories in the deconvolution of rotational relaxation data: Application to pressure-broadened linewidth measurements

The inversion of pressure-broadened spectral linewidth data to yield state-to-state rotationally inelastic rate constants is considered. Deconvolution procedures are presented based upon the rate relationships given by the information theory (IT), infinite order sudden (IOS), and dynamic-coupling theory (DCT) formalisms. In addition, a statistical analysis for the IOS and DCT inversion methods is developed. The three scaling theories are applied to the inversion of experimental data for the CO–He, CO–Ne, HCl–He, and HCl–Ne systems. The results indicate that the IT formalism is inappropriate for deconvoluting linewidth data. For the IOS method the extreme sensitivity of the rates to uncertainties in the measurements can often make the extracted rates unreliable. The DCT inversion procedure is shown to be stable and to yield reliable rates. The application of the DCT method to simulated linewidth data for the He–HCN system yields rates in excellent agreement with ab initio calculated values and correctly predicts the propensity for even rotational changes in this near-homonuclear system. The necessary input for an inversion is the widths as a function of rotational level. We conclude that accurate linewidth measurements contain a wealth of detailed state-to-state collisional information which can now be obtained directly by use of the DCT inversion technique.