Rotation rotational temperature

Dotan I and Viggiano A A 1993 Temperature, kinetic energy, and rotational temperature dependences for the reactions of Ar ( l i.,.2)with Oj and CO Chem. Phys. Lett. 209 67-71... 

The rotational temperature is defined as the temperature that describes the Boltzmann population distribution among rotational levels. For example, for a diatomic molecule, this is the temperature in Equation (5.15). Since collisions are not so efficient in producing rotational cooling as for translational cooling, rotational temperatures are rather higher, typically about 10 K. 

The three bands in Figure 9.46 show resolved rotational stmcture and a rotational temperature of about 1 K. Computer simulation has shown that they are all Ojj bands of dimers. The bottom spectmm is the Ojj band of the planar, doubly hydrogen bonded dimer illustrated. The electronic transition moment is polarized perpendicular to the ring in the — Ag, n — n transition of the monomer and the rotational stmcture of the bottom spectmm is consistent only with it being perpendicular to the molecular plane in the dimer also, as expected. 

A remarkable feature of these spectra is the resolution of individual rotational lines in such large molecules. [Note that the expanded specttum in, for example. Figure 9.47(a) covers only 5000 MFIz (0.17 cm )]. This is due partly to the very low rotational temperature (3.0 K for aniline and 2.2 K for aniline Ar), partly to the reduction of the Doppler broadening and partly to the very high resolution of the ring dye laser used. 

Secondly, due to the smallness of the rotational temperature for the majority of molecules (only hydrogen and some of its derivatives being out of consideration), under temperatures higher than, say, 100 K, we replace further on the corresponding summation over rotational quantum numbers by an integration. We also exploit the asymptotic expansion for the Clebsch-Gordan coefficients and 6j symbol [23] (JJ1J2, L > v,<0... 

The H2O molecules are cooled in a supersonic expansion to a rotational temperature of 10K before photodissociation. The evidence for pathway competition is an odd-even intensity alteration in the OH product state distribution for rotational quantum numbers V = 33 45. This intensity alternation is attributed to quantum mechanical interference due to the N-dependent phase shifts that arise as the population passes through the two different conical intersections. 

Section III.C). Using a rotational temperature to characterize an ion source can be misleading, as the reactions used to form the ions of interest can be quite exothermic, producing vibrationally and even electronically excited ions. These degrees of freedom are more difficult to cool than rotations. Transitions from vibrationally excited molecules provide very useful information, if they can be identified and analyzed. Hot FeO (produced using 3% N2O in helium) has a... 

Figure 4. Experimental laser-induced fluorescence, upper plot, and calculated spectra, lower plot, of the linear He P Cl feature in the ICl B—X, 3-0 region. An P Cl(X,v" = 0) rotational temperature of 0.19 K was measured for the experimental spectrum, and a temperature of 0.20 K was used in the calculations. Adapted from Ref. [51].

Figure 8. The relative peak intensities of the T-shaped and linear He I Cl fluorescence excitation features at 17,831 and 17,842cm , respectively, are plotted as a function of reduced distance along the expansion. The l Cl(A, v" = 0) rotational temperature determined at each distance is shown on the top abscissa. Taken with permission from Ref. [67].

Action spectra were recorded at varying downstream distances and with different He backing pressures to investigate if the populations of the conformers of the higher order Rg XY complexes could also be manipulated. The Av = — 2 action spectra of the He l Cl complexes recorded at two distances downstream from the nozzle orifice, Z = 8.8 and 19.1, corresponding to monomer rotational temperatures of 2.34(3) K and 1.09(10) K, respectively, are shown in Fig. 9a and b [62]. It is evident that the intensity of the feature... 

Figure 9. Action spectra acquired in the F Cl B—X, 3-0 spectral region and with the probe laser tuned to the F Cl E—B, 9-1 transition. Both spectra were recorded using the same source conditions, but with the lasers intersecting the expansion at Z = 8.8, (a), and Z = 19.1, (b). Monomer rotational temperatures of 2.34(3) K and 1.09(10) K were measured at the two distances [62].

The model [39] was developed using three assumptions the conformers are in thermodynamic equilibrium, the peak intensities of the T-shaped and linear features are proportional to the populations of the T-shaped and linear ground-state conformers, and the internal energy of the complexes is adequately represented by the monomer rotational temperature. By using these assumptions, the temperature dependence of the ratio of the intensities of the features were equated to the ratio of the quantum mechanical partition functions for the T-shaped and linear conformers (Eq. (7) of Ref. [39]). The ratio of the He l Cl T-shaped linear intensity ratios were observed to decay single exponentially. Fits of the decays yielded an approximate ground-state binding... 

For linear molecules that lack a center of symmetry, Eq. (58) is applicable and at temperatures significantly greater than the rotational temperature , 9 s= h2/%n2lk, the sum in Eq. (58) can be replaced by an integral. This... 

The time-of-flight spectrum of the H-atom product from the H20 photodissociation at 157 nm was measured using the HRTOF technique described above. The experimental TOF spectrum is then converted into the total product translational distribution of the photodissociation products. Figure 5 shows the total product translational energy spectrum of H20 photodissociation at 157.6 nm in the molecular beam condition (with rotational temperature 10 K or less). Five vibrational features have been observed in each of this spectrum, which can be easily assigned to the vibrationally excited OH (v = 0 to 4) products from the photodissociation of H20 at 157.6 nm. In the experiment under the molecular beam condition, rotational structures with larger N quantum numbers are partially resolved. By integrating the whole area of each vibrational manifold, the OH vibrational state distribution from the H2O sample at 10 K can be obtained. In... 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

The rotational temperature of H2O in the molecular beam is quite low, about 10 K. As in the hydrogen molecule, the water molecule has para and ortho rotational levels with nuclear spin-statistics of 1 3 respectively. Since the para and ortho rotational levels have different nuclear wavefunc-tions, the conversion between the para and ortho levels is extremely slow, as in the hydrogen molecule. In H2O, the nuclear spin-statistics for the lowest rotational levels are as follows ... 

Consequently, even at the lowest rotational temperature both the Ooo and 1qi levels will be populated, with a ratio of 1 3 respectively. The... 

Figure 3.7 Simulated spectrum of CO with a rotational temperature of 40 K. Reproduced by with permission of Albert Nummelin, Chalmers University of Technology...

The relative transition intensities of the spectrum are a good measure of the local temperature of the molecule. This raises the interesting question of whether the relative intensities of R- and P-branch transitions have the same rotational temperature and whether molecules in the same parts of space tell the same temperature. 

Some simple algebra allows the concentration of H2+ to be established within the steady-state approximation. The simple model for the conditions in GL2136 has a temperature of 10 K determined from the rotational temperature of CO (which... 

Fig. 4. Laser-induced nuorescence detected TPD for NO/Pt(l 11) for 0 , = 0.40 (saturation coverage). NO(i = 0 F, J = 4.S, n(A ) , s=40cm ) was probed. The instantaneous rotational temperatures obtained for 20 K intervals in are plotted against 7. The solid line corresponds to full rotational accommodation, Tf = Tj.

Population distributions obtained either by integrating state-resolved LIF-TPD data across each of the two major NO desorption features or within ATs = 20 K increments were always Boltzmann. The latter results were characterized by rotational temperatures 90 5% of the surface temperature. 

Fig. 5. Rotational temperatures ofNO desorbing from Pt(l 11). The data are representative of data published for (x) neat thermal desorption , ( +) thermal desorption in the presence of coadsorbed C0 ° (solid squares) and (solid triangles) trapping/desorption in molecular beam scattering, (open triangle) reaction limited desorption from NO-NHj complexes, (open circle) and (open square) NHj oxidation reactions. The solid line is for full accommodation. The dashed curve represents results for translational energy measurements in direct inelastic scattering ...

Two components were observed in the LID TOP spectra. The slower component was found to have a cosine angular flux distribution and an average kinetic energy 250 K, approximately the value of T . This component was also characterized by a rotational population distribution described by a rotational temperature T t = 170 20K. These results are comparable to the thermal LID results for Pt(lll), Pd(lll) and Pt(foil) discussed in the previous sections. 

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