K1.3 experimental methods

lifetime of a species - symbol + definition

lifetime for first order reactions

graph showing half life and lifetime for first order reaction

techniques used to measure lifetimes of minutes/hours

conventional techniques - titrations, conductivity, pressure, volume change

techniques used to measure lifetimes of 10s of seconds

automated conventional techniques (titrations, conductivity, pressure, volume change)

techniques used to measure lifetimes of seconds

mixing problems - monitoring requires fast response, spectroscopic techniques

techniques used to measure lifetimes of milliseconds

discharge flow or flash photolysis

techniques used to measure lifetimes of microseconds

flash photolysis - spectroscopic monitoring

techniques used to measure lifetimes of nanoseconds - femtoseconds

specialised flash photolysis - approaching timescales of bond formation

relationship between concentration and conductivity for first order kinetics and how this is useful

discharge flow labelled diagram including discharge and what is injected

how is reaction time changed?

by moving the injector back and forward (vary the distance and t=d/v)

how is flow velocity calculated and what does this allow you to find

v (cm s^-1) is calculated from volumetric flow rate (cm³ s^-1)/flow tube area (cm²)

allows t to be calculated (t=d/v)

how is the reaction monitored

measure the OH signal (usually detected by laser-induced fluorescence spectroscopy) and hence monitor the reaction via the loss of OH

phase and kinetics for discharge flow method

gas phase reaction. most of the mixture is an unreactive buffer gas, eg He/Ar/N₂ and the total pressure is a few Torr

make reaction pseudo-first-order such that d[OH]/dt = -k’[OH] and k’ =k[C₆H₁₂]

[C₆H₁₂] >> [OH]

explain the laser-induced fluorescence detection of OH

Laser induces electronic excitation of OH at 308nm

There is then spontaneous emission (like radioactive decay) and the intensity of this is proportional to OH concentration.

This is detected by a sensitive detector called a photomultiplier (PMT)

how is the bimolecular rate coefficient obtained from pseudo-first-order data

discharge flow possible detection methods and what they are used to detect

• laser-induced fluorescence spectroscopy (LIF) for OH, H, CN, IO

• resonance fluorescence (RF, using a lamp) for H, N, O, Br, Cl, F

• mass spectrometry

• laser-magnetic resonance (older method), ³CH₂

• resonance-enhanced multi-photon ionisation, CH₃, HCl

pros of discharge flow method

• relatively cheap

• flexible method of radical generation

• fast timescale possible

• variety of detection methods available

discharge flow cons

• limited range of timescales (10^-4s) and pressures (only up to a few Torr)

• Gas-phase only

• Surface reactions can change kinetics

• Temperature control is challenging

what technique is used for ultrafast reactions

• what timescale is this

• give an example equation

• how are the reactants generated

• how is the reaction monitored

• what type of technique is this

when half life < 10^-4 s flow methods are too slow as reagents don’t move along the tube before they have reacted and mixing times are long

flash photolysis is used instead: A(radical) + B(molecule) → products

premix reactant B and something A can be made from then generate A radicals with a flash of light

monitor radical A as it reacts with B and decays in time

example of a pump and probe technique

flash photolysis with absorption spectroscopy diagram and example graph