Laser Spectroscopy for Chemists Anindya Datta Department of - - PowerPoint PPT Presentation

Laser Spectroscopy for Chemists

Anindya Datta Department of Chemistry, IIT Bombay

31‐07‐2019

Properties of laser

• Intensity
• Monochromaticity
• Pulsed operation
• Coherence

Ordinary Spectrometer Resolution: 0.03 cm‐1

31‐07‐2019 Laser Spectroscopy 3

High resolution spectroscopy

Spectrum of ICl

Physical Chemistry, McQuarrie and Simon Pp. 613

Laser Spectrometer Resolution: 3 × 10‐5 cm‐1

Attractive PES Reactant should not vibrate

31‐07‐2019 Laser Spectroscopy 4

Control of reaction path

Molecular beams

Physical Chemistry, Atkins and dePaula 10th ed. Pp. 907‐913

Repulsive PES No vibration, no reaction

31‐07‐2019 Ultrafast Dynamics 5

Chemical Kinetics

How fast does the reaction proceed? Rate constant, k What is mechanism? Order, n H2 + Br2 = 2 HBr

Physical Chemistry, Atkins, 10th ed.

 dx dt  k a  x

 

n

 d[H2] dt  k[H2][Br

2] 3 2

[Br

2] k'[HBr]

31‐07‐2019 Ultrafast Dynamics 6

Temperature dependence: Arrhenius

Origin: van’t Hoff equation (thermodynamics) Potential Energy profile Activated Complex Molecular Reaction Dynamics

Physical Chemistry, Atkins, 10th ed.

Fastest reaction: k = 6  1012 s‐1  = 1/k = 170  10‐15 s‐1 Ultrafast Dynamics: Femtochemistry

k  AeEa RT k  kT

h Q* QAQB eE0 RT

31‐07‐2019 Ultrafast Dynamics 7

The most fundamental chemical process:

Atomic motion : 1 km s‐1 Bond length: 1 Å Time taken? 100 fs How much time does it take?

Bond breaking

Fastest reaction: k = 6  1012 s‐1  = 1/k = 170  10‐15 s‐1 Ultrafast Dynamics: Femtochemistry