Broadening of rotational lines

Gue] combined analysis of IR and MW data, absolut calibration of vibrational frequencies. [90Sch] combined analysis of MW and IR data on C 0 to obtain molecular parameters from which MW or IR transition frequencies can be calculated for calibration purposes. [87Nol] gives frequency list of pure rotational transitions up to J=35-34. Pressure broadening of rotational lines measured in [87Sem] and compared with other results. 

G Millot, B Lavorel, JI Steinfeld. Collisional broadening of rotational lines in the stimulated Raman pentad Q-branch of CD4. J Quant Spectrosc Radiat Transfer 47 81-90, 1992. 

Although the rotation barrier is chiefly created by the high-frequency modes, it is necessary to consider coupling to low-frequency vibrations in order to account for subtler effects such as temperature shift and broadening of tunneling lines. The interaction with the vibrations q (with masses and frequencies m , tu ) has the form... 

Frenkel D., Gravestein D. J., van der Elsken J. Non-linear density dependence of rotational line-broadening of HC1 in dense argon, Chem. Phys. Lett. 40, 9-13 (1976). 

It is important to note that the alignment induced by most orienting systems is due to the existence of very weak interactions between the macromolecule and the media itself. Because of the weak character of these interactions, the rotational tumbling of the molecule is not impaired or restricted, and therefore there is no additional broadening of resonance lines due to relaxation phenomena [37]. 

Infrared spectroscopy 100-1500 Intensity 1 of rotational lines of light molecules Boltzmann factor for rotational levels related to I Also Doppler line broadening useful, principal applications to plasmas and astrophysical observations, proper sampling, lack of equilibrium, atmospheric absorption often problems... 

The appearance of the IR spectrum of a compound depends somewhat on the sample s phase. Under high resolution, gas-phase IR bands consist of closely spaced lines—the rotational fine structure however, IR bands of liquids and solids very rarely show rotational fine structure. In most solids, the molecules are held in fixed lattice positions and are not free to rotate. In liquids, the high rate of intermolecular collisions and the substantial intermolecular interactions cause random shifts in the rotational energies, thereby broadening the rotational lines of a band sufficiently to merge them into one another, and eliminate the rotational fine structure. (Broadening of fine structure lines is also observed in gas-phase spectra when the pressure is increased.)... 

Figure 2. Rotationally resolved REMPI spectra of the 6 and 6 1 vibronic bands of the benzene-Ar complex. No broadening of the lines in the 6q1 band is observed showing that even for an excess energy of 1444 cm-1, which is more than four times the binding energy, the complex is stable on the nanosecond time scale. (Taken from Ref. 29.)...

The infrared spectra of phosgene in the presence of HX (X = Cl or Br) or DX (X = Cl or Br) have been recorded in liquid argon, krypton and xenon the shifts (to lower frequency) in i>(HX) and i>(DX) indicate the presence of weak hydrogen bonding [1461]. Moreover, the presence of phosgene broadens the rotational lines in the vibrational spectrum of HF [1905]. 

The slow rotational diffusion of the particles leads to a certain broadening of resonance lines and side bands (Fig. 10), finally resulting in a total loss of the MAS effect when the correlation time r reaches the same order of magnitude as the reciprocal spinning velocity 1/cUr (Fig. 10, bottom). The sideband integrals follow a pattern which is predicted by the method of Herzfeld and Berger." ... 

Although flames are convenient sources of MOH molecules, they suffer from serious drawbacks for spectroscopic and dynamical studies. The high temperature ( 2000 K) of flames causes numerous vibrational and rotational levels to be populated resulting in very dense spectra. The high pressure (1 atm) broadens the rotational lines (>0.1 cm ) and increases the overlap of the lines. In addition, resonant laser-induced fluorescence is difficult to detect because of quenching and the overwhelming presence of nonresonant fluorescence caused by rapid collisional energy transfer. The luminescence of the flame itself also interferes with measurements. 

The energy required lo cause i change in rotational level is quite small amt corresponds to radiation of n UM) cm (A UMi pm). Because roiaiional levels arc quantized, absorption in gases in this far-IR region is characlen/ed b> discrete, well-dctinod lines. In liquids or solids, intramolecular collisions and interactions cause broadening of ihc lines into a continuum. 

Fig. 1-2. Mechanisms of normal and resonance Raman scattering S, Stokes A, anti-Stokes. The dashed lines represent the virtual state. The shaded areas indicate the broadening of rotational-...

The experimental lineshapes of the carboxyl and methyl carbon resonances of fully enriched L-Alanine have been studied in detail at different MAS frequencies and decoupling field strengths. Complex lineshapes at intermediate spinning speeds were explained by the joint effect of off rotational resonance and coherent CSA-dipolar cross-correlation. It was found that coherent CS A-dipolar cross-correlation introduces either a differential intensity and/or a differential broadening of the lines of the J-multiplet. The conditions that lead to such effects were explained and experimentally verified. Additional simulations showed that these effects can be expected over a wide range of static magnetic fields and are not restricted to L-Alanine. 

Isotropic liquids usually show only one absorption maximum in their relaxation spectrum. Broadening of such lines may be sufficient to allow the relaxation to be resolved into multiple processes corresponding to whole molecule rotation and the internal rotation of a flexible group within the molecule. 

Molecular two-photon spectroscopy can also be applied in the infrared region to induce transitions between rotational-vibrational levels within the electronic ground state. One example is the Doppler-free spectroscopy of rotational lines in the V2 vibrational bands of NH3 [258]. This allows the study of the collisional properties of the V2 vibrational manifold from pressure broadening and shifts (Vol. 1, Sect. 3.3) and Stark shifts. 

Each vibrational transition of vapor-phase molecules is accompanied by rotational transitions. In the liquid state the effect of molecular collisions is to broaden the rotational lines so that they cannot be resolved. This is why one observes IR bands as opposed to lines in solution spectra. In the vapor phase, however, it is possible to resolve the rotational structure in a vibrational transition. 

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