cavity ring down spectroscopy 14 February 2012

1. CE 540 cavity ring down spectroscopy 14 February 2012

2. history: • first work in early 1980’s to determine mirror reflectivities in laser applications • operated using red laser line 632nm (e.g. laser gyroscopes) • mirrors were getting so good (>99.99% efficiency) that measuring them was hard • first work Herbelin et al (1980)App. Op. 19, 144 • Anderson et al 1984, Appl. Optics 23, 1238  pulsed dye laser (many wavelengths) • development of high quality dielectric dichroic mirrors

3. CRD Optics

10. CSU thesis

13. cavity intensity as a function of time: I(t) = Io exp(-t/to) [assuming index of refraction n = 1], simple exponential decrease I(t) / Io = exp(-t/to) for cavity ring down time of t = to  time for intensity to drop by 1/e for an empty cavity, the decay constant is dependent on the loss mechanisms within the cavity = mainly mirror reflectivity. If L is the cavity length and c the speed of light, then R = = exp{-L/(cto)} where (cto)/L # cell reflections to 1/e intensity  ln (R) = L/(cto)  to = L/(c ln(R)) but ln(R) = 1 – R for R near 1  to = L / {c(1 – R)} [sec] L = 50cm, decay time to = 30µs  R = 0.99994 and # round trips ~10,000  total path length 10km

14. now add a molecular absorption: t= L / {c(1 – R + aL)} [sec] { from I(t) / Io = exp(-t/to – act) and ct = L, total cavity length at time t } where a is the absorption coefficient [/cm] and goes like concentration x cross section Then the relationship between empty cavity decay time (to) and the decay time with an absorber present (t) is: 1/t = 1/to + ca since ca is 1/time added for the absorber The t are a function of wavelength and at each wavelength are fitted with an exponential decay curve to get the decay time. Knowing to allows calculation of a and from that the concentrations can be determined over a full spectral line using Beer’s law using differential spectroscopy.

15. 750 ppm loss  1 – 750 x 10-6 = 0.99925 = 0.0075% O’Keefe et al www.lgrinc.com/ publications/ acs.pdf best we can do with the NASA system upstairs is about 0.015%, about 2x worse

16. some specs: • mirror reflectivity > 99.995% in near UV/vis are available • fractional absorbance detectable < 10-7/pulse using mirrors at uncertainty in to of 1% • example: • L = 50 cm  • decay time ~ 30µsec for reflectivity 99.994% • gives ~ 10,000 round trips during the decay time •  a detectable light pulse every 3 nsec = 2 x L/c with laser pulse length < shorter • than cavity round trip achievable absorbance example: 10-7 absorbance. NO2 example with L = 900m = 90000 cm (as in HW problem) I/Io = 0.9999999 = exp(-L (2.5 x 10-19)N)  N = 4.5x106 molecules/cm3 = 0.2 pptv

17. Advantages of CRDS: • everything is fractional, so not affected by fluctuations in light source intensity – ring • down time does not depend on intensity • very sensitive due to long path length (km) due to multiple mirror reflections • spectral range is usually good since dichroic laser mirrors are somewhat broad • Disadvantages: • laser is nearly monochromatic  must build up the spectrum • need particular laser wavelengths for different molecules • high reflectance mirrors (>>99.99% reflectivity) • expensive • absorbances must be < a few % or don’t get enough bounces • source bandwidth must be < absorption bandwidth • frequencies within the laser BW must be reflected equally by the mirrors • excited state lifetime must be < laser roundtrip time in the cavity – so once a photon • is absorbed during a particular laser pass, the excited specie has time to relax • before the next laser pass • laser scan time limits the time resolution of spectra • dirt in system  decreased R. I guess this gets calibrated out when you run the cell • w/o the molecule you want to measure in the mixture. But you are sucking outside • air into the cell all the time and it is dirty