PN: Vibrational spectroscopy

Question 1

Molecule DOF

Answer 1

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

Question 2

Spectrum vs molecular movement

Answer 2

VIS - electronic trans
Rotational - IR
Vibration -THZ??

Question 3

Common vibrational spec (VS) units

Answer 3

WAVEnumber cm^-1
number of waves per unit length

Question 4

Actual vs imagined potnential

Answer 4

Harmonic is imagined
anharmonic is real (Morse-type potential function)

Question 5

Fingerpringtin in IR

Answer 5

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

Question 6

parts of FTIR

Answer 6

FTIR is a frequency domain measurement
parts:
Movable and fixed mirrors
Beam splitter
Smaple, detector
HeNe used to track OPL diff Time

Question 7

FTIR data analysis

Answer 7

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.)

Question 8

Why THz a time domain spectroscopy measurement

Answer 8

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

Question 9

why is Thz useful

Answer 9

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

Question 10

Disadvatage of Thz-TDS

Answer 10

Water nd water vapour absorb strongly in Thz region

Question 11

WHy is it hard to perform time domain measurement in the VIS

Answer 11

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

Question 12

Ft

Answer 12

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)

Question 13

WHat does real and imaginary part of E field give you in THZ

Answer 13

Real part related to absorption coeff (amplitude)
a~ Re(Esample/Eref)

Im (phase) part related ot the refractive index
n ~ Im(Esample/Eref)

Question 14

How is a thz pulse produced

Answer 14

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

Question 15

How is THz pulse produced NLO

Answer 15

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

Question 16

How is a THz pulse detected

Answer 16

Either by 1) photoconductive antennas (PCAs) or by 2) electro optic sampling

Question 17

Explain how PCAs work

Answer 17

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

Question 18

How does electro optic smapling work in THz TDS

Answer 18

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)