24 MolecularSpectra

25 d l ln 2 d0 d ij ln 2 c 8kt m ij molecular

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: (VII.25) D L ln 2 D,0 D ij ln 2 c 8kT m ij !"#$%$ Molecular Spectra -3 Copyright © 2009 by Jerry M. Seitzman. All rights reserved. What we may need to know for the exam: How does the Doppler broadening scale? -scaling like cbar (average velocity) and a freq. How does the Collision broadening scale? -scaling like collision frequency School of Aerospace Engineering Line Broadening (con’t) • For isolated transitions of neutral molecules, If lines overlap significantly, can these are dominant effects get line “narrowing” effects – all occur simultaneously u du • Voigt Profile Vv Du D – convolution of indep. broadening effects V v a ij 2 ln 2 D a ln 2 e D,0 L D Molecular Spectra -4 Copyright © 2009 by Jerry M. Seitzman. All rights reserved. y y2 dy 2 ya 1 (VII.26) u ln 2 D2 a<< 1 a>>1 Doppler Lorentz (Collision) D L ij !"#$%$ 2 Ex. at 5 microns @ 2400 K what dominates? School of Aerospace Engineering at 10 atm, collision broad at 0.1 atm, 2400 K, Doppler like 3*bigger than Collision at 500 nm, 300 K, in b etween the two p ressures. a t 500 nm, high T, and P, they are comprable again Different broadenings b ecome important in d ifferent conditions Doppler vs. Collision Broadening • Typical values – collision broadening • 0.1-0.2 cm-1 at 1 atm, 300 K p c c , STP n T TSTP pSTP (cm-1) Shows the pressure influence. Facter of 10 change in pressure, Factor 0.1 atm 10 atm o f 10 change in broad. 300 K 0.01 1.0 2400 K , n ~ 0.5 0.0035 0.35 Factor 8 change in T, factor 3 change in broad – Doppler broadening D 7.1623 10 7 ij N2, (cm-1) 500 nm TK MW 300 K • Doppler tends to “dominate” at low p, high T, short Doppler Broad is small for longer wavelengths 5m 0.047 0.0047 2400 K 0.13 0.013 !"#$%$ Molecular Spectra -5 Copyright © 2009 by Jerry M. Seitzman. All rights reserved. A t high T (2400 K) there is a h uge difference for shoreter wavelengths. A t 5 microns, same huge d ifference but numbers are very small Wave number where we get the most a bsorption School of Aerospace Engineering • So far considered single transitions • But will normally have many radiative transi...
View Full Document

Ask a homework question - tutors are online