Sensors

Laser- based optical sensors

Laser principle: Absorption, spontaneous emission, and stimulated emission.

Ground state means - lowest possible energy.

When excited with addition of energy through photons the electron goes form ground state to and excited upper state.

BO Approximation

Etot = Eel + Evib + Erot ( treat seperatly)

Eel due to electronic configration of electrons

Evib due to periodic displacment of atoms abt mean positions

Erot due to ratation of atoms abt common axis passing through center of mass

Seperation between  Eel > Evib >  Erot

Allowed transition is by planck's law

Wavlength in UV = Electronic transition occuring

Mainly:

• 3kind of energies Eel, Evib, Erot
• QM : energy levels are discrete Quantised and trsnsitions coour between only specific energy levels

LIGHT MATTER INTERACTION

1. Scattering

2. Absorption

3 Emission

Lines : Erot        Bands: Evib    System: Eelec

Erot J     

Lines: Single transition 

Molecules with permanent dipole and can interact with microwave termed as microwave active (H2O, HCl )and those who does not have a permanent electric dipole are microwave inactive (N2, H2)

Erot = (1/2) IW2

Evib V

Bands: Vibrations levels asscoiated with rotational levels. 

Evib =( u+ 1/2) hw (h here is hcross=h/2*pi)

Eel 

System : Transition between 2 electronic levels

Beer Lambert law: Ratio between intial intensity from Laser source to transmitted intensity by photodetector.

Quantitative way of applying absorbtion specta.

ROTATIONAL SPECTRA 

B= Rotational Constant = (28* 10^-40) / I

Erot= BJ(J+1)hc

F(J)= BJ(J + 1) cm^-1

Applications

1.Power plant emissions (sensing gases)

2. Engine efficiency monitoring

3. Sensing benzene in environment

4. Ammonia sensor