Photodetachment of metastable helium molecules from helium clusters

Chih-Ching Hu, University of Rhode Island


Metastable helium molecules ( a3S+u ) were created by electron bombardment of helium nanodroplets and were found to reside on droplet surfaces. Upon absorbing infrared light they became detached from the droplets. Secondary electrons were ejected when the detached molecules impacted on the nearby metal surfaces and were detected with a channel electron multiplier (CEM). The CEM signal was measured as a function of wavelength. This photo-induced detachment process resulted from the electronic excitation of the metastable helium molecule. We utilized this mechanism to develop a new method, which we call “molecular detachment spectroscopy”, to probe the superfluid helium droplet. ^ We observed several interesting features in the high-resolution ro-vibrational spectrum of c3S+g←a3 S+u electronic transition: (1) the spectrum showed a highly non-thermal rotational distribution. Rotational states with N = 1 and 11 ≤ N ≤ 29 were populated, but intermediate N states were missing. (2) All spectral lines were slightly blue shifted by about 0.22 nm relative to the free molecular transitions. This suggested that the metastable molecules were weakly bound on the droplet surfaces. (3) The line shapes of transitions from the N = 1 rotational state were narrow and symmetric. The FWHM of line profiles was measured to be ∼0.01 nm. (4) The R(1) spectral line splits into three peaks while the P(1) line has only one. The effect originates from the splitting of the different :Nz: levels of the excited molecule by the surface interaction. (5) All high rotational lines have asymmetric profiles with red shoulders as well as long structured blue shoulders. (6) Higher vibrational state transitions were also found in the spectrum. Their R(1) peaks showed triplet splitting similar to the ground vibrational state. ^ The time-of-flight spectra of the detached molecules showed two different characteristic energies, one thermal (with typical energy ∼7 K) and one much larger (∼0.1 eV), a significant fraction of the absorbed photon energy. Further examination of the bombarding electron energy threshold suggested that the precursor of the low vibrational states of the a3S+u -state molecule was the atomic 23P state. ^

Subject Area

Chemistry, Physical|Physics, Molecular|Physics, Fluid and Plasma

Recommended Citation

Chih-Ching Hu, "Photodetachment of metastable helium molecules from helium clusters" (1999). Dissertations and Master's Theses (Campus Access). Paper AAI9960027.