Question

of 20-16. (a) In the cavity ring-down measurement at the opening this chapter, absorbance is given by ?(1 where L is the length of the triangular path in the cavity, c is the speed of light, ? is the ring-down lifetime with sample in the cavity. and To is the ring-down lifetime with no sample in the cavity. Ring- down lifetime is obtained by fitting the observed ring-down signal intensity I to an exponential decay of the form le", where fo is the initial intensity and t is time. A measurement of CO, is made at a wavelength absorbed by the molecule. The ring-down lifetime for a 21.0-cm-long empty cavity is 18.52 ?s and 16.06 ?s for a cav- ity containing CO2. Find the absorbance of CO2 at this wavelength (b) The ring-down spectrum below arises from 13CH, and CH4 from 1.9 ppm (vol/vol) of methane in outdoor air at 0.13 bar. The spectrum arises from individual rotational transitions of the ground vibrational state to a second excited C-H vibrational state of the molecule. (i) Explain what quantity is plotted on the ordinate (y-axis). (ii) The peak for 12CH, is at 6 046.954 6 cm-1.What is the wavelength of this peak in nm? What is the name of the spectral region where this peak is found? 350 13CH 300 384 382 380 250 200 376 CH 374 150 6 049.04 6 049.12 6 049.20 6 049.28 50 6 048.0 Wavenumber (cm) 6 047.0 6 049.0

Answer 1

(a) Gver*- L. = 21.oc Loin 8ample -der Nouo u5e the selation, 乙 13 S2 3 xio'Ox 2.302f Loaye numbes 6646.9544 cm' L