Chemical Applications of Infrared Laser Photochemistry

Citation

Wight, Charles Albert (1982) Chemical Applications of Infrared Laser Photochemistry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/gskp-8c88. https://resolver.caltech.edu/CaltechTHESIS:05172018-114924220

Abstract

Infrared multiphoton electron detachment reactions of gas phase carbanions are investigated using the techniques of ion cyclotron resonance spectroscopy. Ions are spatially confined in a low pressure environment for up to several seconds during irradiation with the unfocused output of a line tunable continuous wave CO₂ laser. Analysis of the photodetachment yields as a function of irradiation time, neutral gas pressure, laser intensity and laser wavelength have revealed details of the multiphoton excitation process as well as radiative and collisional relaxation mechanisms. The emphasis for many of these studies is on the wavelength dependence of multiphoton electron detachment yields since the resulting infrared spectra are sensitive probes of molecular structure for distinguishing and identifying isomeric anions. An introduction to low power infrared photochemistry of ions is given in Chapter I.

In Chapter II this technique is used to obtain infrared multiphoton electron detachment spectra of two C₇H₇^- isomers, benzyl and cycloheptatrienyl anions. In the first known example where infrared spectra have been used to probe mechanistic details of an ion-molecule reaction, deprotonation of norbornadiene by CH₃O^- is shown to yield a mixture of benzyl and cycloheptatrienyl anions.

A closer examination of multiphoton electron detachment kinetics is the subject of Chapter III. The results are interpreted in terms of a kinetic model which is developed to describe low power multiphoton excitation and vibrational relaxation. Radiative relaxation of benzyl anion does not appear to be significant at laser intensities greater than 10 W /cm² and collisions with neutral molecules are only moderately effective in deactiviating vibrationally excited anions.

Identification of cis and trans butenyl anions in Chapter IV demonstrates the sensitivity of this technique for differentiating molecular structures. Deprotonation of cis-2-butene by NH₂^- forms the cis anion while deprotonation of trans-2-butene results in a mixture of cis and trans C₄H₇^-. The multiphoton electron detachment spectrum of a third isomer, 2-methylallyl anion, is also presented.

Vibrational relaxation of allyl anion, C₃H₅^- is probed by infrared laser photodetachment techniques in Chapter V. In contrast to the results for benzyl anion in Chapter III, the radiative relaxation of 53 s^-1 for allyl anion shows this to be the dominant mechanism for relaxation at pressures below 10^-5 torr.

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