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Negative Ion Photoelectron Spectroscopy: Testing the Limits of Normal Mode Analysis Public Deposited

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  • Negative ion photoelectron spectroscopy has been used to study the furanide anion (C4H4O-), dihalomethyl anions (CHX2-, where X = Cl, Br, and I), the cyanopolyyne anions HC4N- and HCCN-, propadienylidenide (H2CCC-), and propargylenide (HCCCH-). Using this experimental technique in combination with calculations and Franck-Condon simulations, we learn about the electronic and vibrational structures of these molecules.

    Furanide is an ideal anion to interrogate using photoelectron spectroscopy. The five-membered ring structure of furanide constrains it to a relatively small geometry change upon photodetachment. Thus, there is substantial Franck-Condon overlap between the wavefunctions of the ground vibrational state of the anion and of the ground vibrational state of the neutral. A prominent origin peak is observed in the photoelectron spectrum, from which we measure its electron affinity (EA). Our standard Franck-Condon analysis, which assumes uncoupled and harmonic normal modes, reproduces the observed photoelectron spectrum. The excellent agreement between simulation and experiment enables the identification of individual vibronic transitions that give rise to the peaks in the spectrum. With peak assignments, we measure the frequencies of several active vibrational modes.

    In sharp contrast to the rigid furanide anion, the dihalomethyl anions undergo a large geometry change upon photodetachment. When an electron is removed, the pyramidal anion becomes nearly planar, exciting multiple large-amplitude vibrations. As a result of the large geometry change between the anion and the neutral, the best Franck-Condon overlap occurs with high vibrational levels of the neutral — where mode-coupling and anharmonicity become important. Our standard Franck-Condon analysis breaks down under these circumstances, and the origin peak is unobservable. Only by applying sophisticated theoretical methods can we interpret the structure of the photoelectron spectra of the dihalomethyl anions.

    The cyanopolyyne anions HC4N- and HCCN- are also challenging to investigate via photoelectron spectroscopy. The bent anions become quasilinear upon photodetachment to the 3A" ground states of these neutral cyanopolyynes. However, unlike the spectra of the dihalomethyl anions, the origin peaks of HC4N and HCCN have observable intensity in their photoelectron spectra. The geometries of the 1A' excited states are very similar to that of their respective anions, leading to short vibrational progressions and intense origin peaks of the excited states.

    Propadienylidenide and propargylenide, both m/z 38, display very different photoelectron spectra. The rational synthesis made possible by the flowing-afterglow anion source allows us to selectively prepare the different isomers by choosing the appropriate precursor. Reacting O- with allene produces primarily H2CCC-, which is a relatively rigid molecule that exhibits modest, resolved vibrational progressions with an intense origin peak. Reacting O- with propyne yields a mixture of both H2CCC- and HCCCH-; we subtract the spectrum of H2CCC- to obtain the spectrum of HCCCH-. Unlike its isomer, HCCCH- undergoes a significant geometry change when an electron is detached, and the photoelectron spectrum of the ground state of propargylene is characterized by an extended vibrational progression. Again, because the best Franck-Condon overlap occurs with higher vibrational levels of the neutral, assignment of the origin peak is not straightforward, and our standard Franck-Condon simulations are of no help.

    In this work, we investigate several floppy molecules using negative ion photoelectron spectroscopy. It is particularly challenging to elucidate the photoelectron spectra of these molecules because they test the limits of the normal mode analysis that is typically applied to these spectra. These species illustrate the difficulties involved in probing the electronic and vibrational structure of floppy molecules, as well as the theoretical methods that are required to understand their complex spectra.

Date Issued
  • 2012-01-01
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  • 2020-02-04
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