PN: Doppler ltd abs and Fl spectroscopy Flashcards
what is spectral res usually limited by
Rresplving power of the dispersing spectrometer
only with large and expensive isntruments can the doppler limit be reached
what is the detection sensitivity
The min abs power than can still be detected
limited usually by intensity fluctuations in source and etector nose
List some adv of lasers of bradband light in spectroscopy
-no monochromator is required since the absorption coeff alpha(q) can be measured from the tranmitted intensity
-Detector noise generally negligible and power fluctuations of laser can be limited giving great sensitivity
-spectral detectrion sensitivity incr when spectral resolution increases (if laser bandwidht is small)
-line profile of absorption line dw can be measured if laser width Dw «_space;dw
-multiple relfections can create long absop paths (enable low absorption measuemets)
-tunable single mode lasers cna measure line prfoiles of molecule transitions
- pump probe and study excited states
What do you need to reach high detection sesntitivity for absorbing molcules
if alpha small and Nground»Nexcited:
low - bg detector noise Pn
High - path length, xsection, incident power P0
list 3 sources of error in abs spec
Source power fluc
random fluc in densitiy of absorbing molecules (e.g. turb air)
detectro noise
most NB is laser stability
detector noise usually very small
What is ICLAS
Intra Cavity Laser Abs Spec
absorbing sample is placed inside a cavity with the laser (or outside) can this can enhance the sensitivity
you measure
1) the fluorescne from the sample
2) the laser power that is missing givien gabs coeff
List the 4 ways ICLAS gives amplified sensititivity in abs Spec
1) many round trips -> high chance for smaple to interact with laser phtoon -> many transitions -> q times amplification, high 99999% refl give very high q amplifying factors
2)single mode laser -> operate just above laserthreshold and then small changes give big impact measure w/0 sample and then w/ sample and the difference will give alpha (still high amplf Q factor)
3) multimode laser -> mode comp in dye lasers
how does mode comp in dye lasers help in ICLAS
assume laser oscil sim on N modes with = gain and losses
as the Y is tunes across abs spect of sample one of the oscil modes could be tuned to resonance of the modlecule
that mode gets depleted
pop inv in that mode gets less depleted and then the adjacent N-1 modes participate in the gain that wk gave up
this will deplet gain agian wk and decrease the I of the resonant mode until none left
CRDS
cavity ring down spectroscopy
-send laser pulse into cavity (containg the sample) with P0
-oscil back and forth high R mirros
time delay between pulses in round trip known but very quick so the output pulses arrive too fast for slow detector
-detector picks up exp decay (conv on indicvidual pulses)
- wihtout the sample the exp decay will be slower (P0 decreased less as the cavity leaks)
1/T1 - 1/T2 = c-alphha
Similarities and diff between ICLAS and CRDS
same as ICLAS -> inc eff abs path lenth -> here: a det from time measurement (decay time of the ringing cavity , Increased path length -> increased a -> more effect on exp decay out
while in ICLAS the gain competition between diff resonator modes is used as an ehancement of alpha
Exp setup for CRDS
Tunable pulsed laser with lenses used to mode match, sent into ringdown cavity (sph mrirors) into detector
mode matching done to get TEM00 mode
if several resoantor modes within laser bandwidth then beat signals get imposed onto the exp decay curve, can still extract alpha
2 conditions for CRDS to be achieved
1) laser bandwidth of all excited cavity modes < bandwidth of absorptionlines
imples that laser pulse width in freq smaller than absorpiton line
2) relaxation time Tr of the cavity > Texc (the excited molecules)
