| Details: |
Colliding particles at low temperatures (few-Kelvin to milli-Kelvin range) reveals quantum nature of
interaction between any atoms or molecules. At these temperatures, the de Broglie wavelength of the collisional
system becomes comparable to the effective range of the interaction and the collisions are dominated by a few
partial waves. Quantum effects such as shape resonances become observable at this regime and lead to sharp
increase in collision cross section. These resonance states can be formed by two different mechanisms. Sharp, long
lived states arise due to tunneling through a potential barrier whereas quantum reflection leads to a formation of
short-lived resonances located above a potential barrier. In this work, we report the first experimental identification
of these two mechanisms.
We use merged beam method [1] in combination with velocity map imaging technique [2] to probe the
dynamics of He*-D2 collisions in the range 1.0 to 50.0 K. In our studies, we are able to distinguish between the
two mechanisms by comparing the elastic and Penning ionization cross sections (see Fig. 1a). Our results show
resonances at 2.0, 4.0 and 8.0 K in the elastic cross section whereas only the 2.0 K resonance is visible in the
Penning ionization spectra. Shape resonances appear as sharp peaks in both processes whereas quantum reflection
resonances only appear in the elastic cross section since the probability of finding particles at short separation,
where ionization takes place, is small.
References
[1] A. B. Henson, S. Gersten, Y. Shagam, J. Narevicius and E. Narevicius, Science 338, 234 (2012)
[2] A. Eppink and D.H. Parker, Rev.Sci. Instrum. 68, 3477 (1997)
[3] E. Lavert-Ofir, Y. Shagam, A. B. Henson, S. Gersten, J. Klos, P. S. Zuchowski, J. Narevicius and E.
Narevicius, Nat. Chem. 6, 332, (2014) |