# TWO PHOTON SPECTROSCOPY OF DISSOCIATIVE STATES.

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 Title: TWO PHOTON SPECTROSCOPY OF DISSOCIATIVE STATES. Creators: Sander, Robert K.; Oldman, Richard J.; Mahoney, Richard T.; Wilson, Kent R. Issue Date: 1969 Publisher: Ohio State University Abstract: Using a technique we have developed. Photodissociation Recoil Spectroscopy, in isolated molecules we have been able to observe electronic transitions which involve two photons and lead to dissociative states. We cross a molecular beam of the parent molecules with an intense pulse of polarized laser light and observe the distribution of the recoiling photodissociation fragments as a function both of angle (measured with respect to the electric vector of the light) and of translational energy (measured by flight time to a mass-spectrometer detector). We have illuminated $Br_{2}$ and $I_{2}$ with second harmonic neodymium laser light $(\sim 18,830 cm^{-1})$ which for both $Br_{2}$ and $I_{2}$ falls in the one photon discrete region of $^{3}\Pi_{o_{u}^{+}} \leftarrow ^{1}\Sigma_{o_{g}^{+}}$ (although other $u \leftarrow g$ transitions are also probably present). For $I_{2}$, a one photon peak due to ground state $^{2}P_{3/2}$ atoms from predissociation and from direct excitation of an underlying continua is found, and at high photon fluxes a peak at larger translational energy appears which matches the energy for a two photon process yielding two excited $^{2}P_{1/2}$ atoms. There is only one g state correlating with two $^{2}P_{1/2}$ atoms, a $^{1}\Sigma_{o_{g}^{+}}$, and its predicted energy makes it accessible to our observation. In the experiments with $Br_{2}$, we observe a peak in the energy distribution corresponding to the absorption of two photons, although the state of the atoms produced is not yet certain. As with IR (one photon) and Raman (two photon) spectroscopy, the one photon ($u \leftarrow g$ and $g \leftarrow u$) and two photon ($g\leftarrow g$ and $u \leftarrow u$) electric dipole transitions of molecules with inversion symmetry from mutually exclusive sets. Thus two photon spectroscopy offers a means to observe otherwise forbidden transitions. We are currently studying the angular distributions of the two photon processes to determine both the symmetries of the states involved and whether the processes are resonant or non-resonant. Description: Author Institution: Department of Chemistry, University of California URI: http://hdl.handle.net/1811/15652 Other Identifiers: 1969-B-10