PN: Vibrational spectroscopy Flashcards
Molecule DOF
N atoms -> 3N DOF
3 for translation along xyz
3 for rotation around xyz
3N-6 for vibrabration
many vibrational energy levels in a potneital ontop of roatioal sublevels of elect transitiosn
Spectrum vs molecular movement
VIS - electronic trans
Rotational - IR
Vibration -THZ??
Common vibrational spec (VS) units
WAVEnumber cm^-1
number of waves per unit length
Actual vs imagined potnential
Harmonic is imagined
anharmonic is real (Morse-type potential function)
Fingerpringtin in IR
Observation of sepctral features ina certain region is often indicative of specific chemical functional group in a molecule
many tables wehere one or more abs bands ina given IR spectrum corresp directly to vibrational modes with a certain functional gorup
these modes can be used to dist one molcule from another -> fingerprent bands
Each molecule has a unquiw IR spectrum -> computer sysetems of libraries to comapre
parts of FTIR
FTIR is a frequency domain measurement
parts:
Movable and fixed mirrors
Beam splitter
Smaple, detector
HeNe used to track OPL diff Time
FTIR data analysis
Get interferogram of ref (w/o sample) and with sample
-aphodization (FT smoothing waveforms to reduce ripples)
-phase corrections (The interferogram contains out-of-phase elements which are introduced by optical path differences in the instrument. The data must be phase corrected or the resulting spectrum will not be photometrically accurate.)
Why THz a time domain spectroscopy measurement
in normal UVVIS spec you detect in freq domain/wavelength regime, analysis easy but only gives amplitude info.
Time domain - E field of radiation is recorded E(t) directly so you can get A and phase out
why is Thz useful
1)bridges gap between optical and elctronic regime
2) many materials transparent in Thz - polymersw, clothing, plastic, btu some have unique fingerprints so absorption can be used for ID
3) advatange sinc elow photon energy, non ionizing (unlike Xray) can be used for biological
Disadvatage of Thz-TDS
Water nd water vapour absorb strongly in Thz region
WHy is it hard to perform time domain measurement in the VIS
High E filed oscillation frquency
in Thz region you can use fs pulses to perform the measurment and get spectral info by doing a FT
Ft
Math operation that transform f(t) -> f(w).
Any f(t) can be decomposed as sum of infinite # sin and cos functions with diff frequenccies and amplitudes
Ft gives these A and w, and phases of the sinusoidal functions
Irl (discrete -> FFT)
WHat does real and imaginary part of E field give you in THZ
Real part related to absorption coeff (amplitude)
a~ Re(Esample/Eref)
Im (phase) part related ot the refractive index
n ~ Im(Esample/Eref)
How is a thz pulse produced
fs (mayeb Ti saph?) in NIR (800nm) sent into GaAs substrate with applied voltage
charges interact and recombine and form a THz pulse due to transient screening of bias field by photo injected charge
also X2
How is THz pulse produced NLO
a nonlinear X2 medium used to produce single cycle pico s THz pulse
Difference frequency mxiing (optical rect) generate a broadband Thz pulse
-shorter fs pulse -> broader spectra -> borader Thz pulse
-X2 second order processs NL
- ZnTe provides phase mathcing for 800nm TiSapp (Menlo system used fibre laser) light power emitted by 2 phtoon absorption 1600 nm then diff freq to get a lower energy phton out
How is a THz pulse detected
Either by 1) photoconductive antennas (PCAs) or by 2) electro optic sampling
Explain how PCAs work
Photoconductie antennas used to detect THz pulses
- the E field is determined by the relative arrival times of the optical and THz pulse
a) opticla pulse arrives BEFORE Thz pulse - by delaying the arrival of optical optical we can get it to
b) arrive at SAME TIME ( maybe at the top and btm of the THZ peak)
c) scan over to map out the THZ pulse in time
records many data pts as it the path length changes so several datpts to form one pulse through the smaple
How does electro optic smapling work in THz TDS
As in the video
- BS and path diff in the probe timin gbeam.
gen)THz pulse arrives on ZnTe either before, same time, after the probe smapling pulse from BS
1a) intial pusle split by BS one arm to delay stage one arm to crystal and smaple
1b) goes through electro optic medium (ZnTe) and sample
2) gets polarized elliptically (direction of ellipse axes determined by over times of THz and sampling pulse)
3) sent through a polarizing BS which splits the elliptically polarize light into 2 sep beams
4) sent into balanced photo diodes which give current out syaing +,0,- depending on icnident polarization states (overlap of two pulses)
