1、PhotodissociationPhotodissociation dynamics of the methyl dynamics of the methyl radical at 212.5nm:radical at 212.5nm:Effect of parent internal excitationEffect of parent internal excitationContentsIntroduction Experiment Results and Discussions Conclusions ReferencesIntroductionIntroductionFree Ra
2、dicals Important in atmospheric,combustion and interstellar chemistry Much harder to study because they are unstable in nature Difficult to generate WWays to Generate Radicals Photolysis(Photodissociation)Discharge Pyrolysis(Heating)Chemical Activation(Reactions)These methods are often used in studi
3、es of radical spectroscopy and dynamics studied in cell experiments!T To Generate Radical Beams Nozzle tip photolysis Discharge near nozzle tip Pyrolysis right before expansion(Peter Chen Source)Advantage:Radicals are cooled down in the expansion Disadvantage:Radicals are always accompanied by the p
4、recursors or other radical species Way to generate CH3 Photodissociation of CH3I at low photolysis energy provides a good source for CH3 radicals CH3I +hv CH3 +I In a photodissociation experiment,the CH3 radical can be separated with the I atom in space,therefore making a pure radical beam of CH3 sh
5、ould not be very difficult.N Newton Diagram for CHewton Diagram for CH3 3I PhotodissociationI Photodissociation P Product Kinetic Energy Distribution at 266 nm Parker et al.CH3 Absorption Spectrum near 210 nmCallear and Metcalfe et al.Experiment The H Atom Rydberg“Tagging”Technique by Welge et al.J.
6、Chem.PhysJ.Chem.Phys.92,7027(1990)Results and Discussions1.Determination of CH3 Photodissociation Channel2550751000500100015002000Signal(counts)Time of Flight(s)0o 90o=-0.8CH3+212.5nm CH2(X 3B1)+H(2S)(1)CH3+212.5nm CH2(1A1)+H(2S)(2)(Eavl=8710 cm-1)Energetic Limit:5800 cm-1(Eavl=5563 cm-1)2.Effect of
7、 parent internal excitation01k2k405060708001k2k01k2k3k90o0o(b)Signal(counts)90o0o(c)Time of Flight(s)90o0o(a)v2=0vv2=0v2=1v2=110203040501020304002000400060008000102030 0o 90o(a)6.50 s Delay(b)6.65 s Delay P(E)(arb.unit)Translational Energy(cm-1)(c)6.80 s Delay Energetic LimitEnergetic LimitEnergetic
8、 LimitEnergetic Limit:6400 cm-1The vibrational frequency of the umbrella mode:606 cm-1CH3 v2=13.Anisotropy ParameterDelay Time (s)6.35 6.50 6.65 6.80 -0.8-0.71 smaller6.35 s:a fast photodissociation process&/C36.50 s:effect of the rotatoinal excitation of CH36.50,6.80s:effect of the vibrational exci
9、tation of CH3Product 3D Contour Plot90o0oC Conclusions Molecular beam photodissociation of hydrocarbon halides(CH3)can be used to produce pure radical sources for photodissociation studies using HRTOF method.This opens a new direction for studies of photochemistry of many hydrocarbon radicals.CH3 photodissociation at 212.5 nm from the vibrational ground state is very fast predissociation dissociation,while vibrational excitation of CH3 has a significant effect on the dynamics of dissociation.
