Spectroscopy
Table of Contents
1. Time-resolved spetroscopy
First proposed by Ahmed H. Zewail
Mechanical shutter was used back in 1960. They work up to few milisecond to microsecond. Electronic control caps at Gigahertz. Now use pulse-lazer upto attosecond resolution.
1.1. Pump-probe method
The pump–probe method is a basic technique in time-resolved spectroscopy for watching very fast processes, often on femtosecond to picosecond timescales.
- A short, intense pump pulse hits the sample first.
- It excites the system, for example promoting an electron to a higher energy state.
- After a controlled delay, a second weaker probe pulse arrives.
- The probe checks how the system has changed after that delay.
- By repeating the experiment with different time delays, build up a picture of the dynamics in time.
Pump–probe spectroscopy can reveal:
- Excited-state lifetimes
- Relaxation and energy transfer
- Vibrational motion
- Chemical reaction dynamics
- Electron transfer
Depending on the method, the probe may:
- measure absorption changes,
- induce fluorescence,
- cause stimulated emission,
- or produce ionization.
1.1.1. Laser-Induced Fluorescence (LIF)
The pump excites the molecule, and the detected fluorescence gives information about the excited state and its decay.
1.1.2. Resonance two-photon ionization (R2PI)
One photon excites the molecule to an intermediate state, and another photon ionizes it. This is useful for sensitive detection of specific species and states.
Use pulsed nozzle to spary the sample, and the timeing is matched with the YAG laser-dye laser
Monitored using
- Etalon: Fabry-Perot Cavity
- I₂ LIF
- Wavelength monitor
2. Absorption Spectroscopy
2.1. Cavity Ring-down Spectroscopy
- CRDS
- Cavity Ring-down Laser Absorption Spectroscopy (CRLAS)
Laser is resonated within a cavity causing absorption by the sample (normally, in gas phase) within the cavity.
High-finesse optical catity is used, which effectively create tens of kilometers of optical path length. Due to the long path length, this is used in detection of miniscule concentration.
The signal is produced after the laser is turned off, and it decays exponentially \[ I(t) = I_0 e^{\frac{t}{\tau(\lambda)}}. \] Broadband laser is used for broader spectral coverage. The mean lifetime is used to measure the concentration.
American company Piccaro Technology is manufacturing this commercially.
3. Raman Spectroscopy
- IR은 공간 분해능이 높지 않지만 Raman은 높다 (300 nm).
- Raman은 간접적인 빛을 측정하기 때문에 직접적인 빛을 측정하는 IR에 비해 해석이 어렵다. 따라서 고분자에서는 IR이 더 유용해진다.
- 라만은 경험이 있어야 다룰 수 있다.
Sotkes Raman scattering First excites into virtual energy state (not stable state) and falls back to a higher vibrational state. Stokes shift occurs that elongates the wavelenth
The virtual state here means the transitional non-stable state obtained from perturbation theory as a superposition of energy eigenstates, although the true state would be an eletron-photon state.
Anti-Stokes Raman scattering First excites into virtual energy state and falls back to a lower vibrational state. Stokes shift occurs that shorten the wavelenth
The strength of Raman signal is extremely weak (1 in 10 million) compared to Rayleigh scattering.
Manufacturers
- IDEX Health and Science
- Semrock (optical fibers)
- Horiba (Raman Spectroscopy)
- Park System
3.1. Raman Efficiency
\[ I(\omega_s) = \frac{\omega_s^4}{12\pi\varepsilon_0c^3}Q_0^2|A|^2 \left| \frac{\delta \alpha}{\delta Q} \right|^2 \]
3.2. Depth of Penetration
High frequency penetrates less.
3.3. Surface Enhanced Raman Spectroscopy
- SERS
Molecules are absorbed on rough metal surfaces or by nanostructures such as plasmonic-magnetic silica nanotubes.
The signal is amplified 10⁶ to 10¹² times on those special surface.
3.4. Tip-Enhanced Raman Scattering
- TERS
- Variant of SERS
Synchronized with AFM (atomic force miscroscope)
SiN may be used as the tip. The concentrated field at the tip generates much higher signal.
3.5. Resonance Raman Scattering
- RRS
The excited state is chosen to be a real state. Amplifying the signal thousands to tens of thousands times.
4. Detector
4.1. CCD
CCD sensor is placed after a grater. Full vertical binning (FVB) may be used to sum across vertical pixels, and show the spectrogram.