Vibrational Relaxation in Nitrous-Oxide Gas: Motional Narrowing of Raman Q-Branches
The theory of motional narrowing of spectral lines is approached from the standpoint of coherence time of the radiation signal rather than, as heretofore, by examining the autocorrelation function. It is found that, if an oscillator undergoes Markovian frequency modulation due to interaction with a thermal bath, then the linewidth of its power spectrum is inversely proportional, for rapid enough relaxation, to the thermal relaxation rate of the bath state. Measurements have been made of the Raman spectra of the (upsilon)(,1) and (upsilon)(,3) vibrational modes in N(,2)O gas. The evolution of the Q-branch lineshapes is followed as pressure rises from one to 50 atm. Evidence is found of motional narrowing, which counteracts the usual collision broadening. This behaviour is explained as the result of an "indirect" vibrational dephasing process: molecular collisions, without affecting vibrational behaviour directly, nonetheless cause vibrational frequency changes indirectly by inducing transitions on the rotational quantum numbers with which the vibration is coupled. When the rotational transition probabilities are calculated in accordance with the Gordon J-diffusion model, the vibrational lineshapes predicted by the indirect-dephasing theory display the requisite motional narrowing, and closely resemble the measured profiles at lower pressures. At higher pressures, ordinary collision broadening appears to dominate the experimental lineshapes.
- Pub Date:
- Physics: Molecular