Photodetachment studies of negatively charged and metastably excited helium nanodroplets

Cheolkyu Kim, University of Rhode Island

Abstract

Beams of negatively charged or metastably excited helium nanodroplets were produced by fragmentation of a low entropy supersonic expansion followed by electron bombardment. After a flight time of 4 ms, optical properties of these droplets were studied by crossing them with an infrared light beam above a detector. For negative droplets, light induced electron detachment was observed whose spectrum showed a broad peak centered at $1.5\mu$m. Weaker light induced signals were also seen at longer wavelengths $(2.5\sim6\mu$M), but no spectral resolution was possible. Spontaneously detached electrons were observed as well, which suggests that the negative ion, while long lived, is inherently metastable. Photo-detached electrons came predominantly from larger droplets, while spontaneously detached electrons came preferentially from smaller ones. Both varieties had energies below 1eV, with photo-detached electrons the more energetic. These and other observations are consistent with the bubble model which presumes that an excess electron resides in a cavity near the center of the helium droplet.^ Light induced detachment signals also were observed with neutral metastably excited droplet beams. We interpret them as arising from the photo induced release from these droplets of metastable particles which subsequently are detected when they release secondary electrons at surfaces. The photo-detachment spectrum shows that the long-lived excitation is the $\rm He\sb2(a\Sigma\sb{u}\sp+)$ molecule. The rotational lines of the $(\nu = 0)\ a\Sigma\sb{u}\sp+\to(\nu = 0)\ c\Sigma\sb{g}\sp+$ electronic transition are found to be blue shifted from their gas phase values by about 0.3 nm with a FWHM of $\sim$1.2nm, indicating that the molecule is weakly bound on the surface. Moreover, the rotational structure of the a $\to$ b and a $\to$ c transitions indicates that only the high J states and the J = 1 state are populated. We believe this is a consequence of rotational relaxation of the molecule on the surface which leads to its detachment, and a thus the transit time of 4ms is suggested as a lower limit of the relaxation time for J $\ge$ 13. ^

Subject Area

Chemistry, Physical|Physics, Condensed Matter|Physics, Atomic

Recommended Citation

Cheolkyu Kim, "Photodetachment studies of negatively charged and metastably excited helium nanodroplets" (1997). Dissertations and Master's Theses (Campus Access). Paper AAI9812221.
http://digitalcommons.uri.edu/dissertations/AAI9812221

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