This artist’s conception symbolically represents complex organic molecules, known as polycyclic aromatic hydrocarbons, seen in the early universe. These large molecules, comprised of carbon and hydrogen, are among the building blocks of life. NASA’s Spitzer Space Telescope is the first telescope to see polycyclic aromatic hydrocarbons so early–10 billion years further back in time than seen previously. Credit: NASA/JPL-Caltech/T. Pyle (SSC)
DIBs  (diffuse interstellar bands) are a collection of about 400 absorption bands that show up in spectra of light that reaches the earth after traversing interstellar medium. Indications exist they may arise from the presence of large hydrocarbon molecules. Recent experiments lend novel credibility to this hypothesis.
Among the hydrocarbons that are possible carriers of the DIBs, polycyclic aromatic hydrocarbons (PAHs) are considered promising. The linewidths of the DIBs = lifetimes of the excited states that are involved in the absorption process, are often considered as an argument that speaks against the PAHs. But the new experiment shows the lifetimes of excited states of small to medium-size PAHs are consistent with the linewidths that are observed for the DIBs.
METHOD: a series of small to medium-size PAH molecules (naphthalene, anthracene, pyrene and tetracene, containing 2-4 benzene-like aromatic rings), were ionized by an ultrashort extreme-ultraviolet (XUV) laser pulse. As a result of electron correlation, the absorption of an XUV photon not only led to removal of 1 of the electrons, but also electronic excitation of the molecular ion left behind.
~The lifetimes of these excited cationic electronic states were monitored by probing the ions with a moderately strong, time-delayed infrared (IR) laser pulse. When ions are formed, the electronic excitation is at its highest, and only one or a few IR photons are needed to remove a 2nd electron. However, a little later, when ion relaxes and energy is transferred from the electronic to the vibrational degrees of freedom, more IR photons are needed to remove the second electron. ie monitoring the formation of doubly-charged ions as a function of the time delay between the XUV and IR laser pulses allowed extraction of the lifetimes of the states formed by the XUV ionization process.
>> ie these lifetimes of a few 10s of femtoseconds are well within the range of what is required for potential carriers of the DIBs.
The new experiments have implications for the further development of attosecond science. eg observation of charge migration, i.e. ultrafast (attosecond to few-femtosecond) motion of an electron or hole through a molecular structure. It has been proposed that charge migration may provide new opportunities for control of chemical reactivity, a goal that is as old as the chemical research itself. The PAH molecules represent the largest molecular species yet to which ultrafast XUV-IR pump-probe spectroscopy has been applied. PAH molecules are also ideal candidates for observing attosecond to few-femtosecond timescale charge migration. Such experiments will therefore be attempted next. http://www.alphagalileo.org/ViewItem.aspx?ItemId=155622&CultureCode=en
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